Before setting out to explore the Universe's development and history since the first moments after the Big Bang, we want to pay homage to what this writer (and many, many others) consider the greatest scientific instrument yet devised by mankind - the first of the Great Observatories: the Hubble Space Telescope (which we will often refer to as HST). No other instrument has advanced our knowledge of astronomy and the Universe as much as this splendid observatory in outer space. Perhaps no other astronomical observatory has captured the public's imagination, with its numerous sensational pictures, as has the Hubble. HST has provided many extraordinary views of stars, galaxies, dust clouds, exploding stars, and interstellar and intergalactic space, extending our view to the outermost reaches of the Universe. HST has brought about a revolution in our understanding of Astronomy and Cosmology. One good reason for placing this HST review on this second page is simply that many of the subsequent illustrations of the Cosmos used in this Section were made by this telescope.
This, the most versatile optical telescope up to the present and perhaps the penultimate remote sensing system, receives its name to honor Edwin Hubble, the man who confirmed much about the existence, distribution, and movement of galaxies, leading to the realization of an expanding Universe. Here he is at work in the 1920s on the 100-inch Palomar telescope:
Prior to the 1990s, surveying and studying stars and galaxies as visible entities required the use of optical telescopes at ground-based locations. This ground photo shows the Kitt Peak observatory complex in Arizona, one of the premier observatories in North America.
But such telescopes were hampered by adverse effects contributed by the atmosphere. Even when placed away from cities and on high mountains, the effects of the atmosphere, smog, any nearby lights, etc. degraded these images. The multinational Observatories atop Mauna Loa on the Big Island of Hawaii illustrates high quality terrestrial conditions for earth-based astronomy.
As the space programs developed, astronomers dreamed of placing the telescopes in space orbit where viewing conditions were optimized. HST is the outgrowth of a concept
first suggested in 1946 by Lyman Spitzer who argued that any telescope placed
above Earth's atmosphere would produce significantly better imagery from outer
space. (Spitzer has been honored for this idea by having the last of the Great Observatories named after him; see page 20-4.) The HST was launched from the Space Shuttle in April of 1990 after 20 years of dedicated efforts by more than 10000 scientists and engineers to get this project funded and the spacecraft built. Here is the HTS in the Bay of the Shuttle:
And this is a photo of the HST in orbit, as seen from the Shuttle: A general description of the Hubble Space Telescope and its mission is given in this review by the Space Telescope Science Institute. This cutaway diagram shows the major features and components of the HST: But, as scientists examined the first images they were dismayed to learn that these were both out of focus and lacked expected resolution. HST proved unable to deliver quite the sharp pictures
expected because of a fundamental mistake in grinding the shape of its primary (2.4 m)
mirror. The curvature was off by less than 100th of a millimeter but this error prevented focusing of light to yield sharp images. Astronomers and engineers put their heads together to solve this egregious problem and designed optical hardware that could restore a sharp focus. In December of 1993 the Hubble telescope was revisited by the Space Shuttle. (This mission to salvage the HST is a definitive answer to the critics of manned space missions - only human intervention could remedy an otherwise lost cause.) At that time 5 astronaut spacewalks succeeded in installing corrective mirrors and servicing other sensors. The package was known as COSTAR (Corrective Optics Space Telescope
Axial Replacement). After the first servicing mission, the striking improvement in optical and electronic response is evident in the
set of images below made by the telescope, which show the famed M100 (M denotes
the Messier Catalog number) spiral galaxy viewed by the Wide Field Planetary Camera
before (bottom left) and after (bottom right) the correction. For an indication of how much HST improves astronomers' views of distant astronomical bodies, one of the best earth-based telescope images, from Kitt Peak, is shown at the top:
Another way to judge the improvement that HST provides by being above the atmosphere is to compare absorption spectra for Hydrogen in the Visible and Ultraviolet coming from a quasar source as recorded by a ground based telescope and HST. The increased sensitivity of the HST instrumentation, unimpeded by atmospheric absorption, provides more detected Hydrogen lines in both the UV and Visible regions of the EM spectrum. In some respects, the HST shares remote sensing features found on Landsat. HST has filters that narrow the wavelengths sensed. The filters range through part of the UV, the Visible, and the Near-IR. This permits individual chemical elements to be detected at their diagnostic wavelengths. The resulting narrow band images can then be combined through filters to produce the multicolored imagery that has made many Hubble scenes into almost an "abstract art" form - one of the reasons that the general public has taken so positively to this great instrument. As an example, here is an HST multifilter image of the Crab Nebula in which the blue is assigned to radiation from neutral hydrogen, the green relates to singly ionized oxygen, and the red doubly ionized oxygen. HST images can be combined with those made from other space observatories that sense at wavelengths outside the visible. This provides information on chemical composition as well as temperatures and the types of radiation involved. Consider this example: These multiwavelength images give rise to one technique for picking out galaxies that are located at various great distances from Earth - the so-called Deep Field galaxies (page 20-3a) that formed early in the Universe's history. These galaxies are moving away at ever greater velocities. The redshift method (see page 20-10) of determining distance relies on the Doppler effect in which motion relative to the observer reduces the frequency (lengthens the wavelength towards/to/past red) of light radiation as the galaxies move away from Earth as a result of expansion. Those ever farther away, moving at progressively greater velocities, experience increasing redshifts. A galaxy emitting light at some maximum frequency can be imaged through, say, a narrow bandpass blue filter. This frequency translates to a specific redshift and hence a particular distance. A galaxy farther away has its redshift toward/to the green and will appear brightest through a green filter. Filters passing longer wavelengths will favor detection of greater redshifts - thus galaxies still more distant from Earth. Younger/closer galaxies may not even shine bright enough at shorter wavelengths to be detectable in filters whose bandpass cutoffs exclude those wavelengths. Information on both original Hubble instruments and those added later appears in this site prepared by the Space Telescope Science Institute. The history of HST in terms of instrument placements and servicing missions, from the early days to the present and a look to the future is given in this chart prepared by the Space Telescope Institute: The original 5 instruments onboard HST were: the FOC (Faint Object Camera); FOS (Faint Object Spectrograph) GHRS (Goddard High Resolution Spectrograph); HSP (High Speed Photometer) and WFPC1 (Wide Field Planetary Camera); added since (by subsequent visits using the Space Shuttle) are NICMOS (Near Infrared Camera and MultiObject Spectrometer); STIS (Space Telescope Imaging Spectrograph); ACS (Advanced Camera Surveyor); FGS (Fine Guidance Sensor); and WFPC2; future additions (by Shuttle flights) may be the COS (Cosmic Origins Spectrograph) and WFPC3. During this repair mission the NICMOS (Near Infrared Camera and Multi-Object Spectroscope) sensor, out of working order for nearly three years, was repaired and upgraded. This pair of images, ACS on the left and NICMOS on the right, shows the improved quality of imaging of part of the Cone Nebula, bringing out more details of the dust that dominates this feature: Many of the most informative HST images can be viewed on the Space Telescope Science Institute's (Baltimore, MD) Home Page .
HST has imaged numerous galaxies at different distances - almost to the edge of space - from Earth
that are therefore also at different time stages in the general evolution of
the Universe. The following illustration shows both spiral and elliptical
galaxies (but not the same individuals) at 2, 5, 9, and 14 billion years after
the Big Bang in a sequence that represents different stages in this development.
The Hubble Space Telescope has had a remarkable impact on the study of the Universe. In its honor, the Astronomy Picture of the Day (APOD) web site, in celebration of its 10th anniversary, has compiled a collage of a variety of the more spectacular images acquired by HST, supplemented with a few images made by other instruments. This is reproduced here; be on the lookout for many of the individual embedded images in this montage elsewhere in this Section. However, technology and design are allowing ground-based telescopes to "catch up" with the HST, at least for those galaxies that are relatively close to Earth. The resolution and clarity of some recently activated ground telescopes are on a par with their Hubble counterparts, at least within the depth range (lookback time) of a few billion light years. This results from better detectors, improved optics, the ability of a ground telescope to dwell on the target for much longer time spans (allowing buildup of the incoming radiation to generate a bright image), and, for some location on high mountain tops, above most of the atmosphere. This is illustrated with this pair of images which show a Highton Compact Group galaxy (HCG87) imaged by ESO's southern hemisphere telescope (left) and by the Hubble ST (right): Thus, the need now is to have a more powerful and sophisticated telescope in space as the eventual HST replacement. NASA and the astronomical community always seem to have new telescopes on the drawing boards. The big follow-up being planned by The Space Telescope Institute and Goddard Space Flight Center is NGST which stands for the Next Generation Space Telescope. In 2002, this telescope was formally renamed the James Webb Space Telescope (JWST), to honor the second NASA Administrator for his many accomplishments in galvanizing the space program, including his role in the Moon program. Final decisions as to its components and the contractor(s) to build it have not yet been made but a launch date has been set for no sooner than 2013. It will move far from Earth to "park" at a Lagrangian point (about 1000000 miles away, where the Earth's and the Sun's gravitys balance out). A separate launch will place a big heat shield to block out the Sun's rays to keep the sensors at about 20° K above absolute zero.
The principal scientific goal of JWST is to obtain improved information about the Universe's history between about 1 million and 2 billion years. The telescope main sensor will concentrate on the infrared region of the spectrum, with a range between 0.6 and 28 µm. Because of the spectral wavelength redshift that results from the expansion of space (see page 20-9), the visible light from these early moments in the Universe's history will have now, as received, extended into the infrared. (For further information, check out Goddard's NGST site.) This diagram summarizes the current and anticipated status of space telescopes' ability to see back in time towards the earliest events following the Big Bang: The HST has the detection capability and resolving power to look back to about the half billion years whereas the JWST will be able to detect and image events taking place about 300,000 years after the BB. Earlier than that will be difficult to examine by visual means because of the opacity of the Universe prior to that time. A significant number of other space telescopes have been placed in orbit; most have instruments that cover other parts of the EM spectrum beyond the visible. Among these we mention here: SWIFT (gamma-ray bursts) the Chandra Telescope (X-ray region), XMM-Newton (X-ray region), FUSE and Galex (UV), and the Spitzer Space Telescope (Infrared). Many of those telescopes operating in various parts of the spectrum as described on page 20-4. For a comprehensive listing of nearly all space telescopes launched or planned consult the space observatories website. And, click to see a list of the largest ground telescopes.
However, the Hubble Space Telescope remains the premier astronomical instrument - in many opinions, the finest instrument of any kind yet made - in the stable of space observatories. But, being complicated in its electronics and optics, like any fine instrument it has a finite lifetime. Being out in space, it is not easy to repair the HST whenever a major failure occurs. This happened, for example, in January of 2007 when the Advanced Camera Surveyor (ACS) experienced what amounts to a short-circuit that has rendered it useless (unless NASA engineers can somehow fix it remotely). It is unlikely that a repair or replacement of this instrument can be made during the Shuttle visit described in the next paragraph.
Hubble has been visited four times (1993; 1997; 1999; 2002) already for repair and upgrade. However, its components are now well beyond their planned lifetime and will likely fail in the next few years. Following the Columbia disaster, the perils of space travel for humans caused NASA to decide against another servicing mission that could be too dangerous at the higher altitude in which HST orbits. This raised a storm of protest and expressions of dismay from both the scientific community and an involved public. Sensitive to this outcry, the current NASA Administrator, Michael Griffin, ordered a "rethink" of that decision and on October 31, 2006 he announced that with the resumption of the Shuttle program now showing success the HST is scheduled to be visited by astronaut-repairmen in the Fall of 2008 to rescue it from eventual failure. The principal tasks will be carried out in 5 (dangerous and challenging) EVAs: EVA-1: Installation of three rate sensing units (six gyros) and one battery module (three batteries) EVA-2: Installation of the Cosmic Origins Spectrograph and the second battery module
EVA-3: Installation of the Wide Field Camera 3 and insulation repairs EVA-4: Space Telescope Imaging Spectrograph repair and installation of a cooling system EVA-5: Installation of a replacement Fine Guidance Sensor. The ACS, which has had some periodic problems, apparently failed totally in January, 2007. Because it is located deep inside the telescope, it cannot be conveniently repaired or replaced during the EVA (unless NASA engineers find a way). May the best happen to this most supreme of instruments! Note: This Section cannot be a complete textbook on Astronomy. Some topics covered in a course are not included here. This review of the HST may have piqued your curiosity about "telescopes" and how they work. The main function of a telescope is to gather photons from a source, concentrate them (focus) so as to improve detectability, and display them as discrete images or numerical data sets. If interest is aroused, try these two websites for a primer on optical telescopes: Wikipedia, and How Stuff Works.
On the first page of this Section, we stated that stars are a fundamental unit making up the visible Universe. We repeat the definition of a star given in the Overview on that page: A star is defined as a massive, spherical astronomical body that is undergoing (or has undergone) burning of nuclear fuels (initially Hydrogen; as it evolves elements of greater atomic number are consumed as well) so as to release energy in large amounts of both luminous and non-luminous radiation (over a wide range of the EM spectrum). The vast majority of stars are found within or close to collections of stars called galaxies.
We concentrate on this page on the inception, evolution, and demise of individual stars. A helpful Web Site that supplements the content of this page has been put online by Prof. Nick Strobel of Bakersfield College (Note: his original online Astronomy Notes have disappeared off the Internet; this link is to a mirror site in Denmark) . Also recommended is the University of Oregon site cited in the Preface (especially relevant are Lectures 15-18, and 20). Another source of information is at the WikepediaStar web site. Before reading the next two pages, it may be profitable for you to get an overview of Star Formation by reading relevant pages from the just-cited Oregon lectures.
The number of stars in the Universe must be incredibly huge - a good guess is 100 billion galaxies each containing on average 100-200 billion stars or (1011 times 1011, which calculates as 1022). More recently, an independent estimate using other means states the value as 7 x 1022 stars; that survey made by Simon Driver of the Australian National University is said by him to still be a conservative underestimate. To make his point, here is a view of stars near us that indicates the densities just within a part of the Milky Way - our own galaxy, the host of our parent star, the Sun.
Yet on a very clear, moonless night in dry air (say, in the western Great Plains of the United States), one sees at any one location, without using a telescope or binoculars, only about 2500 "star" points within the Milky Way in the northern hemisphere and a similar number in the southern hemisphere. Under normal (unaided eye) viewing conditions, outside the Milky Way band only a few individual stars can be seen by eye alone (less than 200 in a typical suburban [which cuts down detection because of city lights] setting with humid air; this number increases to more than a thousand in a rural area with an arid climate). Many of these are close to Earth (first few hundred light years) in the halo of the Milky Way, but several are planets, and a small number are nearby galaxies (the unaided eye can discern a galaxy only as an apparent single light source). The brightest stars are generally those closest to the Sun (around 1 to 100 light years away). Only when powerful telescopes are used does the astronomer realize by estimate or extrapolation that billions of galaxies exist; by inference we deduce that these probably contain stars in numbers similar to those that can be roughly counted in the Milky Way (in many tens of billions). Wherever you happen to be on the Earth's globe, as you look up into the sky you will see a myriad of hundreds to a few thousand stars (depending on atmospheric conditions) that extend from directly above to the horizon in any, and all, directions. Some groupings of stars form distinct patterns that the ancients imagined were signs from the gods telling humans how to live their lives. These were given names and were called constellations (discussed later on this page). Here is a "star map" with some common constellations (left unnamed):
Let us talk a bit more about constellations. That in itself is not truly a scientific topic but which remains useful to astronomers as a convenient way to locate stars by reference to "Sky Maps". Such maps contain the Constellations - patterns of certain visible stars (a few were actually galaxies but this was not known at the time) that the ancients imaginatively discerned in looking at individual stars within narrow patches of the celestial hemisphere which seemed to be distinctive and readily recognized. These arrangements were given fanciful names, of gods, animals, and other descriptors from their everyday experiences. This began with the Babylonians in Mesopotamia, and the system was expanded to 88 named constellations by the Greeks and Romans some 2000 years ago. (Astrologers, Psychics and Fortune Tellers have used constellations as "signs" and for horoscopes for several millenia.) This next pair of illustrations shows some of the major constellations in the Northern hemisphere, plotted in two half circle fields, one for summertime, the other for winter; the look direction is towards the north: The constellations visible from the Southern hemisphere are different: In each Hemisphere the stars and constellations shift positions with the seasons. To illustrate this, we repeat here an illustration first included on page 19-2: To the ancients, the stars were all equidistant on the Celestial Sphere and varied only in brightness. Modern astronomers now know that individual stars/galaxies in the light points that make up the constellation pattern are actually located at various distances from Earth, which together with diifferences in size account for their different brightnesses. Most of the defining stars in a constellation are located in our galaxy, the Milky Way. As we shall see next, the groups of constellations also shift with the seasons and with the place on Earth where the stargazer is positioned (also, different constellations are seen by those in the Southern Hemisphere of the Earth than those in the Northern Hemisphere [lookers at the Equator will see some of the constellations visible in each Hemisphere]). More about the constellations can be found at the Star Map Internet site. How do astronomers - or anyone - locate a particular star among the plethora that can be seen? You can imagine that the pinpoints of light, which can be stars, galaxies, planets, asteroids, comets, and large spacecraft, are located over a hemisphere of infinite depth that extends from your local horizon to the zenith point directly vertical to your location (latitude). Although you cannot see it, there is a complimentary hemisphere either to your south (if you are in the northern terrestrial hemisphere) or to your north (for southern hemisphere dwellers). These two hemispheres constitute the Celestial Sphere. On any given night, the various heavenly bodies will seem to travel across the sky in arcuate paths, owing to the rotation of the Earth. The Moon follows a definite path on any given night and the Sun goes from horizon to horizon on a path a few degrees displaced from the Moon; these too shift with seasons and location. Some insight into this sphere as it relates to a person on Earth is given by this Celestial Sphere plastic globe:
How does one locate any point of light on the Celestial Sphere? By consensus, the Earth's North and South Pole (through which pass its rotational axis) are made to coincide with the North and South Poles of the Celestial Sphere. In the northern hemisphere a star called Polaris - the North Star - coincides almost exactly with the extension of the Earth's rotational pole onto the Sphere.) Here is one way to determine the location of any light point (e.g., star) on this sphere:
In this figure, the Celestial Sphere is held fixed (does not rotate). Here, the frame of coordinate reference is a great circle plane (equatorial) passing through the horizon as defined by true N(orth), S, E, and W. A second great circle contains the N & S directions and two points directly above the observer at any location on Earth - the zenith - and below - the nadir. At some given moment, the star's position on the Sphere can be specified by fixing its azimuth in degrees with respect to North or South and its altitude in degrees from 0 to 90°. (These are equivalent to longitude and latitude in the geographic coordinate system.) If one were looking straight up overhead while standing on the North Pole, the relative locations of the constellation at that moment would constitute a reference Celestial (Hemi-)Sphere (same for South Pole but with different star patterns). Locating a given star would be done using the above azimuth-altitude spherical geometry. But globally the location is complicated by the three major types of motion involved in the real world: 1) on any given night the stars will move in arcs across the sky (centered on the Celestial Pole) in arcuate patterns; thus each star shifts position such that at different times it will lie at different points along its arc; 2) as the Earth revolves around the Sun (giving the different seasons), the positions of the stars will shift; 3) at any specific point on the Earth's sphere away from a Pole, the particular portion of the Celestial Sphere that is visible will differ (different hemispherical segments will be visible at different localities); The effect of the Earth's rotation on the stars can be illustrated by a simple experiment: point a wide-angle lens camera up towards the hemisphere and keep its shutter open for hours. This would be the result: Various constellations seemingly occupy different positions in the sky as the months and seasons pass during an earth year. Actually, the Earth's rotational axis retains its direction in space relative to the fixed stars (around the celestial poles) as it makes its annual revolution around the stars. There is an apparent shift of the constellations owing to the position of the Earth at each season:
As the seasons progress, the Sun has different locations within the constellations on each seaons' starting date at the time of its daily zenith when crossing the ecliptic, as follows: Vernal Equinox(Spring) = Aries; Summer Solstice = Cancer; Autumnal Equinox = Libra; Winter Solstice = Capricorn. Because the altitude and azimuth of a star are constantly changing in response to Earth's motion and to one' position on the terrestrial sphere, it is not always useful to rely on the above horizontal coordinate system to catalog the positions of stars. A more convenient coordinate system for cataloging purposes is one based on the celestial equator and the celestial poles and defined in a similar manner to latitude and longitude on the surface of the Earth. This location system takes the hours of observation into account. This means that one must consider both the specific time (usually relative to Greenwich Mean Time) when the observation is to be made (say, through a telescope in an Observatory) and the geographic location of the Observatory. Examine this diagram, in which the Celestial Pole is tilted 23° to accommodate the tilt of the Earth's rotational pole:
In this system, known as the Equatorial Coordinate system, the analog of latitude is the declination, δ. The declination of a star is its angular distance in degrees measured from the celestial equator along the meridian through the star. It is measured north (as +) and south (-) of the celestial equator and ranges from 0° at the celestial equator to 90° at the celestial poles, being taken to be + when north of the celestial equator and - when south. The zero point chosen on the celestial sphere is the first point of the constellation Aries, γ, and the angle between it and the intersection of the meridian through a celestial object such as a star and the celestial equator is called the Right Ascension (RA) of the star. RA is sometimes denoted by the Greek letter α and is measured from 0h to 24h along the celestial equator eastwards from the first point of Aries, i.e., in the opposite direction to that in which hour angle is measured.
Because of the rotation of the Earth, a reference called the hour angle
(HA) increases uniformly with time, going from 0° to 360° in 24 hours. Defining the observer's meridian as the arc of the great circle which passes from the north celestial pole through the zenith to the south celestial pole, the hour angle of a star - which changes with time of day - is measured from the observer's meridian westwards (for both northern and southern hemisphere observers) to the meridian through the star (from 0° to 360°). The hour angle of a particular object is therefore a measure of the time since it crossed the observer's meridian - hence the name. For this reason it is often measured in hours, minutes and seconds of time rather than in angular measure (just like longitude). The hour angle is referenced to the hour angle of the Constellation Aries at Vernal Equinox. In practice, if one wishes to locate a particular star, galaxy, or planet at some moment, the main steps are to look up declination δ and RA in a star catalog or Ephemeris, read the time using a sidereal clock, and adjust the telescope settings to the hour angle τ and the δ value, taking into account location and time. (This time is usually sidereal time, exactly 24 hrs earth time. The so-called 24-hr solar day is not really precisely 24 hours because the Earth has moved along its orbital path a specific distance during a full rotation, so that it is 3 minutes and 56 seconds longer that the sidereal day,) The relationship between the stars and Earth's time and space location proved a means of marine navigation even for the ancients. But precise determination of latitude and longitude requires two important capabilities: 1) fixing the location as declination of a star, and 2) establishing accurately the time relative to some place of reference (Greenwich). The development of the sextant (shown below) and of a Chronometer made this possible. The above is only a part of the story about the celestial sphere and ways to find individual planets, stars, and galaxies. Additional treatment of how to navigate the sky at night is found on the aforementioned
University of Oregon website. Perusal of this review is highly recommended.
Interesting, but back to Astronomical Science. The standard model for a star is, of course, our Sun. The Sun is typical of most stars; as we shall note shortly, these stellar bodies vary from about 0.1 to 100 times the mass contained in the Sun. Without a telescope, under exceptional viewing conditions (using binoculars), about 9000 individual stars can be seen in the wide celestial band that is the central disc of the Milky Way (M.W.) galaxy. Others elsewhere in the celestial hemisphere make up about 2000 points of stellar light that can be seen (in clear air, away from urban light contamination) by the naked eye. Some are nearby within our galaxy and are not particularly large, while others are mostly stars of the Giant/Supergiant types in the halo (see below) around the Milky Way. Still others (a minority) are galaxies that lie in intergalactic space beyond the Milky Way but mostly within a billion light years from Earth. The solar planets are interspersed with these cosmic bodies. Telescopes can resolve countless more stars in the M.W., can recognize millions of galaxies, and can pick out some individual stars in nearby galaxies.
A degree of luminosity of an object in the sky (galaxy; star; glowing clouds; planet) can be represented by its apparent magnitude - a measure of how bright it actually appears as seen by the telescope or other measuring device. This magnitude is a function of 1) the intrinsic brightness which varies as a function of size, mass, and spectral type (related to star's surface temperature) and 2) its distance from Earth. (Magnitude as applied to a galaxy, which seldom shows many individual stars unless they are close [generally less than a billion years away], is an integrated value for the unresolved composite of glowing stars and gases within it.) The brightness of a star can be measured photometrically (at some arbitrary wavelength range) and assigned a luminosity L (radiant flux). For two stars (a and b) whose luminosities have been determined, this relationship holds:
from which can be derived:
To establish a numerical scale, some reference star(s) must be assigned an arbitrary value. Initially, the star chosen, Polaris, was rated at +2.0 but when it was later found to be a variable star, others were selected to be the 0 reference value for m. The magnitude scale ranges from -m (very bright) to +m (increasingly faint) values. The very brightest objects have larger negative numbers. The more positive the number, the fainter is the object (planet; star; galaxy); very distant galaxies, even though these may be extremely luminous, could have large positive apparent magnitudes because of the 1/r2 decrease in brightness with increasing distance. The Sun has the value - 26.5; the full Moon is -12.5; Venus is -4.4; the naked eye can see stars brighter than + 7; Pluto has a magnitude of +15; Earth-based telescopes can pick out stars visually with magnitudes down to ~+ 20 (faintest) and with CCD integrators to about +28, and the HST to about +30. Thus, the trend in these values is from decreasing negativity to increasing positivity as the objects get ever less luminous as observed through a telescope. Each change in magnitude by 1 unit represents an increase/decrease in apparent brightness of 2.512; a jump of 3 units towards decreasing luminosity, say from magnitude +4 to +7, results in a (2.512)3 = 15.87 decrease in brightness (the formula for this is derivable from the above equations, such that the ratio of luminosities is given by this expression: 10(0.4)(mb - ma). Below is a simple linear graph that shows various astronomical objects plotted on the apparent magnitude scale: Absolute magnitude (M) is the apparent magnitude (m) a star would have if it were relocated to a standard distance from Earth. Apparent magnitude can be converted to absolute magnitude by calculating what the star's or galaxy's luminosity would appear to be if it were conceived as being moved to a reference distance of 10 parsecs (10 x 3.26 light years) from Earth. The formula for this is:
where r is the actual distance (in parsecs; 1 pc = 3.26 light years = 206,265 A.U. = 3.086 x 10 16 meters) of the star from Earth. Both positive and negative values for M are possible. The procedure envisions all stars of varying intrinsic brightnesses and at varying distances from Earth throughout the Cosmos as having been arbitrarily relocated at a single common distance away from Earth. Both luminosity and magnitude are related to a star's mass (which is best determined by applying Newton's Laws of motion to binary stars [a pair; see below for a discussion of binaries]). The graph below, made from astrometric data in which mass is determined by gravitational effects, expresses this relationship; in the plot both mass and luminosity are referenced to the Sun (note that the numbers are plotted in logarithmic units on both axes): There is a relationship between absolute magnitude (here given by L for luminosity) and mass (given by the conventional letter M; which accounts for replacing the absolute magnitude M with L). Here is one expression: In the above, both L and M for a given star are ratioed to the values determined for the Sun. Note the two different power exponents. It seems that some stars obey a fourth power, others a 3, and a few are just the square of the mass. The most general expression in use is given as L = M3.5. There are relatively few stars with mass greater 50 times the Sun. Very rarely, we can find a star approaching 100s solar mass, but these are so short-lived that nearly all created before the last million years have exploded, with their mass being highly dispersed, and thus ceasing to send detectable radiation. If the Sun were envisioned as displaced outward to a distance of 32.6 l.y., its apparent magnitude as seen from Earth would be -26.5; its absolute magnitude would be changed to +4.85. A quasar, which is commonly brighter than a galaxy, has an absolute brightness of - 27 (note that in the absolute scale increasingly negative values denote increasing intrinsic brightness).
One classification of stars is that of setting up categories of star types in a series of decreasing sizes and luminosities (see also the discussion below of the Hertzsprung-Russell [H-R] diagram). These are the Luminosity/Type Classes: Ia, Ib: Extreme Supergiants (Hypergiants); II: Supergiants (Betelgeuse); III: Giants (Antares); IV: Subgiants; V: Dwarfs (Sun): VI: Subdwarfs (metal poor); VII: White Dwarfs (burned out stars). The oddity in this classification is the omission of a category of "Normal"; a star is either a Giant or a Dwarf. The diagram below relates this hierarchy to star brightnesses (magnitudes):
Another classification is based on density. Starting with the least dense and progressing to the most dense, this is the sequence: Supergiant; Red Giant; Main Sequence ; Red Dwarf; Brown Dwarf; White Dwarf; Neutron Star; and Black Hole. This is also ranked according to (decreasing) size (diameter). Each of the above bold-faced types are described in some detail on this two-part page.
Supergiants are dimensionally wide, and also luminous. NGC 3603 is the largest star in the Milky Way, being 116 times the mass of the Sun: The brightest star in the northern hemisphere of the sky is Sirius, an A type star (see the H-R plots below and accompanying paragraphs which explain the letter designation of stars) of apparent magnitude -1.47 that lies 8.7 light years away. Here is how it appears through a telescope: Closest to the Sun is the red dwarf Proxima Centauri, being 4.2 light years away. Just slightly farther away (4.4 l.y.) are Alpha Centauri A and B (visible in southern hemisphere), stars similar to the Sun that are among the brightest in the heavens.
Here is a telescope view of Alpha Centauri A: The map below is a plot of the distances from Earth (outer circle is 13.1 light years in radius) of the 25 nearest individual or binary stars or local clusters in our region of the Milky Way Galaxy. Many of these stars are red dwarfs (see next page): (Information Bonus: Just beyond this map's edge is the star Vega (27 light years away). It has two claims to fame: 1) it alternates with Polaris as the North Star used in navigation; the Earth's precession brings Vega into this position every 11000 years, and 2) It was the nearby star used as the host for an extraterrestrial civiliation in Carl Sagan's extraordinary science fiction novel "Contact" (later made into the movie of the same name "starring" Jodie Foster; in the film contact was made with a planet near Vega as a signal picked up by the Socorro, NM radio telescope array - as initially interpreted that signal consisted of a string of prime numbers [those divisible only by themselves and 1]). Referring to the above diagram, the following is extracted verbatim from the caption accompanying this image that was displayed on the Astronomy Picture of the Day Website for February 17, 2002: "What surrounds the Sun in this neck of the Milky Way Galaxy? Our current best guess is depicted in the above map of the surrounding 1500 light years constructed from various observations and deductions. Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars. The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange. The Gum Nebula, shown in green, is a region of hot ionized Hydrogen gas. Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC. Future observations should help astronomers discern more about the local Galactic Neighborhood and how it might have affected Earth's past climate."
The largest star so far measured in the Milky Way is Mu Cephi (in the galactic cloud IC1396), seen as the orange disc (also called Herschel's Garnet star) near top center of this HST image. Located about 1800 light years from Earth, it is almost 2500 times the diameter of the Sun. This is an example of a rare type of star known as a Hypergiant (see next page). Another even bigger star (2800 times the solar diameter; 2.4 billion miles) is Epsilon Aurigae (in the constellation Auriga, the Charioteer), residing in the Milky Way about 3300 light years from Earth. This star, also known as Al Maaz (Arabic for he-goat) and visible to the naked eye) is considered by many astronomers to be the "strangest" star in the firmament. Every 27 years this star (magnitude 3.2) undergoes a diminishing of brightness (about 60000 times greater than the Sun) lasting about 2 years. The last such event was in 1983; the next in 2010. It is thus one of a class called "eclipsing stars". The cause of this regular pattern of luminosity change is still uncertain; some astronomers think it is caused by the passage of a second massive star across Epsilon Aurigae's face but that binary is so far undetected, leading to the hypothesis that the drop in luminosity occurs when a cloud of dark material (dust) orbiting the star as a clump obscures Epsilon Aurigae each time it moves through the line of sight to the Earth. Most stars bigger than the Sun are not as huge as Mu Cephi or Epsilon Aurigae. The majority are no larger than about 100x the diameter of the Sun. This diagram illustrates the relative size of some common stars (setting the Sun's diameter as 1), which establishes our star as rather ordinary in the size scheme within the Milky Way: One of the largest stars whose size can be accurately determined is VY Canis Majoris, in the Milky Way about 5000 l.y. from Earth. Best estimate of its diameter is about 2100 times that of the Sun. VY Canis Majoris is a Hypergiant star. Here is the best telescope view of this massive stellar body. This star is a superb example of how one view within one segment of the spectrum gives a specific impression of an apparently simple visualization but is misleading in that a different spectral region discloses a much different appearance. VY Canis Majoris is actually surrounding by a much larger cloud of gas of varying composition, as evident in this pair of HST images. What is going on is that the star, because of its huge size, is destined to be short-lived and is already unstable, throwing off much of its mass. It is likely to be destroyed in less than another 100000 years as a supernova. Another appropriate way to distinguish stars by size is to rank them according to mass. In the Milky Way, the Arches cluster contains the most massive single star (about 130 solar masses) found yet; theorists think this is a reasonable upper limit throughout the Universe: More than half of the stars in a typical galaxy are also tied locally to a second star as a companion (the mutual interrelation of two stars is referred to by the term 'binary'), such that each of the pair orbits around a common center in space determined by their mass-dependent mutual gravitational attraction. This arrangement is exemplified by the image made by the HST Faint Object Camera (FOC) of the Persei 56 group:
Some stars are grouped into more than one companion; ternary groupings (three stars orbiting about a common center of gravity) are fairly common. Here is an image of four stars orbiting as a unit about a gravity center in the galaxy M73. Binary star systems are recognized by three means: 1) visual, through a telescope (as in the above two images); 2) by periodic drops in brightness caused by passage of one star across another (eclipse; an uncommon observation condition); and 3) by measuring spectral characteristics in which both a Doppler shift towards the red and the blue occur as one star moves away and the other towards Earth (and the reverse) along pathways of their mutual orbits.
To demonstrate the second means, examine this diagram which shows the brightness levels (and magnitude variations) for a binary star in which one is larger and brighter than the other (thus there are different decreases in brightness when the brighter star passes in front of the less luminous star, and vice versa). Incidentally, this method is also used to hunt for and verify planets associated with stars. Spectral line shifts are used to study the motions of binary stars. We will treat stellar spectroscopy in detail on page 20-7
As a preview, the spectral method can be illustrated by looking at a pair of spectral strips for two similar stars that are mutually orbiting: Bright lines for Hydrogen appear in the top and bottom (dark background) strips. This fixes a reference location for excited Hydrogen in the rest state. The two center spectral strips include the same Hydrogen lines, the first strip acquired from one and the second the other star. Note that the lines in one have moved to the left and the other to the right of the reference lines position. The spectrum on the bottom center has been blueshifted (see page 20-9) towards shorter wavelengths; the spectrum at the top center has been redshifted towards longer wavelengths. This is explained thusly: The bottom star is in motion towards the observing system on Earth whereas the top star is moving away from the telescope. This would occur when the two stars are aligned sideways to the line of sight and are moving in opposite directions around a common center of gravity. For some visual binaries, movements over time can be observed and plotted, such as illustrated here for the star Mizar (in the Ursa Major constellation), which is resolvable into Mizar A and Mizar B. The Chandra X-ray Observatory has imaged a close binary pair in the M15 Galaxy. Prior to obtaining this image, the object was thought to be a single star, but at X-ray wavelengths, it is now resolved into a faint blue star and a nearby companion believed to be a neutron star giving off high energy radiation. Thus: Most binary stars exist as two separated entities. But in 2008 an observation of two binaries that actually are enjoined (like Siamese twins) by shared solar matter were reported from a galaxy 13 million l.y. away. This is an artist's enhancement of one of these peanut-shaped pairs:
The Birth, Life and Death of Stars
From Unsold & Baschek, The New Cosmos, 4th Ed.
From Unsold & Baschek, The New Cosmos, 4th Ed.
La/Lb = (2.512)mb - ma
mb - ma = 2.5 log(La/Lb)
From Nick Strobel's Astronomy Notes.
M = m + 5 - 5log10r,
Turning now to stellar evolution, to preview what will be examined in some detail later on this page and the next page, the pattern of a star's history follows a pathway that, depending on its total mass, eventually splits into one of two branches (>
or \>), as it leaves what is known as the Main Sequence.
This is: 1) Development of a large cloud of denser gas made up of predominantly molecular Hydrogen (H2) + dust --> Protostar --> T-Tauri Phase --> Main Sequence (if mass less than 8 solar masses)--> Red Giant --> Planetary Nebula --> White Dwarf; or 2) (if mass greater than 8 solar masses) Main Sequence --\> Supernova --\> Neutron Star or Black Hole (if mass [size] is greater than 50-100 solar masses).
The star types are categorized into Spectral Classes which are defined on the basis of certain chemical elements that become excited at different temperatures and give off characteristic radiation at specific wavelengths. The classes are designated by the letters (O, B,...etc.) assigned to each group. Here are spectra for some of the different classes (this is treated in some detail on page 20-7).
Both star classification and evolution can be summarized in a graphlike chart that consists of a plot of luminosity (vertical axis) or, alternatively the related magnitude parameter, versus star surface temperature which is expressed also by (correlated with) the star's visual color (note also the Spectral Type designations at the top). This is known as the Hertzsprung-Russell (H-R) Diagram. (The masses of the stars in the diagram increase to the left on the abscissa; Red Giants are big but have low mass densities [many less than the Sun, since mass was lost in evolving to that state].) Most known stars lie along the Main Sequence; they describe a stage in which a protostar reaches some fixed size and mass and commences burning of most of its Hydrogen before changing to some other star type off the sequence. Here is a H-R diagram:
The Sun (also called Sol) is a G type star on the Main Sequence. Very hot stars on the M.S. include the Blue White stars. Red Dwarfs are M stars. Large luminous but low temperature stars form several Giant classes. Small, still luminous and very hot (at surface) stars make up the White Dwarfs. The above H-R plot also shows along the right ordinate the relative sizes of each star compared with the Sun. As far as we now know, stars do not completely vanish, but survive as dwarfs or Black Holes (but the latter in principle can disappear by evaporation as Hawking radiation).
The same diagram, expressed differently, shows examples of actual stars on the Main Sequence and off located by their luminosities and relative masses. Among the off-Main Sequence evolved star groups are four types of Giants (Sub; Red; Bright; Super), T Tauri. These are discussed again on this page or elsewhere in this Section. Not shown among the Dwarfs is the recent designation of LT for Brown Dwarfs. Note that the letters at the bottom include some like B0 and B5 or K0-K5; this denotes subdivision of each class into temperature subclasses (0 being hottest and 5 coolest in a class). Temperature ranges (in °K) are: O class = greater than 30000; B = 11000 - 30000; A = 7500 - 11000; F = 6000 - 7500; G = 5000 - 6000; K = 3500 - 5000; M = less than 2500. Colorwise, the first three are all "blue-white" stars, F is bluish to white; G is white to yellow; K is yellow orange; and M is red. A star on the Main Sequence will follow some pathway during its subsequent history. To illustrate this progression, look first at this evolution diagram for a star the mass of the Sun. The first diagram extends the history of a F star by showing the sequence of star stages from its very inception as a nebular mass that grows into a protostar, then to the M.S., next, as it burns most of its , off the M.S. as a Red Giant, followed by an explosion, and subsequent evolution into a final dwarf state.: This second diagram follows the history of a G star (which is the path to be followed by the Sun in about 5 billion years) after it leaves the Main Sequence:
The key steps in the progression are 1) exhaustion of the main nuclear fuel; 2) change to a Red Giant, with shedding of some mass; 3) explosion to the Planetary Nebula phase, dispersing much of the star's mass into interstellar space; 4) survival of a central core as a White Dwarf star.
The pathways of protostars to the Main Sequence depend on their mass (in multiples of a solar mass) at the stage when they commence proceeding to the M.S and initiate Hydrogen fusion. The times involved in this transition will vary systematically with mass; thus, a 15 solar mass protostar takes only about 10000 years to reach the M.S. whereas a 2 solar mass star may require up to 10,000,000 years for the process to begin fusion:
This next diagram shows the evolutionary history of three stars of differing mass at the upper, central, and lower ends of the Main Sequence after they leave the M.S.:
These pathways are somewhat generalized. A Sun-sized star eventually becomes a Red Giant and then a White Dwarf. A smaller star can evolve directly into a White Dwarf. A much larger star will destroy itself as a Supernova that yields a planetary nebula (the gaseous remnants from the explosion; page 20-2a) but may retain some of its mass as a Neutron Star. When the details are plotted, the path of a star of 5 solar mass size from the M.S. to a Red Giant can be more complex, as shown in this 14 step example (starting at ZAMS on the Main Sequence):
The largest number of individual stars in galaxies fall in a narrow range between just under 1 solar mass to about 10 solar masses. During their evolution to Red Giants, they follow this internal history of burning (fusion) of the initial Hydrogen:
As these burn their Hydrogen fuel into helium, they start to contract and begin to burn that helium and further brighten, cast off some of the outer Hydrogen, and become luminous (for stars under a solar mass of 2.3, there is a short-lived large increase in luminosity known as the helium flash phase). Then, as the helium burns to carbon (which organizes into a core of degenerate carbon and some O(oxygen); see page 20-7), such stars follow what is known as the asymptotic giant branch (AGB) pathway which begins with a second Red Giant state.
A star's precise position along the Main Sequence depends on its total mass
of H fuel that collects during the formative phase into the gas ball. Some stars (e.g., Type M) have masses as low as 1/20th
of the Sun (1 solar mass is the standard of reference as is the luminosity of
the Sun, also set at 1), whereas others fall within a range of greater masses
that may exceed 50 solar masses (Type O). The high mass stars on the Main Sequence are
brighter and bluer whereas those at the lower end of the M.S. tend to be yellow to orange. The initial quantity of mass in a star is the prime determinant of its life expectancy, which also depends on its evolutionary history and final
fate. As a general rule, small stars may take more than 50 billion years to burn out completely, stars in the size range of the Sun live on the order of 5 to 15 billion years, and much bigger stars carry their cycle to completion in a billion or less years. Stars whose masses are similar to the Sun's actually will burn about 90% of their Hydrogen during their stay on the Main Sequence. Stars with greater than 50 solar masses may complete their M.S. burning in just 20-30 million years.
The lifetime spent on the Main Sequence is approximately proportional to the inverse cube of the star's mass (this is true for most stars, especially massive ones; stars less than a solar mass have lifetimes closer to the inverse 4th power). O and B blue-white stars may last only a few million years. Red Dwarfs can potentially last a trillion years or more.
The relation between size (mass) and age is shown in this next diagram (check the values on the curve itself, not the abscissa/ordinate values); the most massive stars have the shortest lifetimes. The history (from onset in the nebular phase) and fate of stars (at the end of their history) of all sizes (and different masses) can be conveniently summarized in this Evolution diagram; the various pathways depend on starting mass:
From B.C. Chaboyer, p. 53, Scientific American, May 2001
A generalized and simpler version of this shows the pathways followed by stars about the size of the Sun and stars that are much more massive:
Of special interest are the end products of each evolutionary path. After burnout or explosion, small stars end up as White Dwarfs; intermediate stars as Neutron Stars; and the largest stars as Black Holes.
Now to a more detailed discussion of the history of stars as expressed in the above diagrams. We shall begin by zeroing in on several of the common modes by which stars are born.
Stars develop within galaxies in nebulae (also called Giant Molecular Clouds [GMC] composed mostly of H2)
by progressive sub-fragmentation, aggregation and contraction of gas and dust
into centers of higher density. (More on GMCs is found on page 20-3; also do not confuse this type of nebula with "planetary nebula", the dispersed matter that makes up the residue from explosive destruction of a star, as described on the next page [20-2a].) These nebulae represent localized concentration of gases brought about by several processes such as the driving force of shock waves from supernova explosions and intergalactic magnetic fields. The clouds turn very slowly but this helps to develop "seed" locations - internal denser regions that bring the gases toward them because of greater gravitational attraction. The H-He atoms in these denser local regions
assemble into gas balls (the stars) and dust clouds by collisions and gravitational forces
at initially low temperatures (100's of ºK) in a turbulent process of condensation,
generating heat (in large part dissipated as thermal radiation). Thus, molecular Hydrogen clouds are the regions of gas where most new stars are born.
These clouds (GMC) are usually "photogenic" and hence many breathtaking images have been shown to the public. Let's start with an example of a mature galaxy in which star formation is continuing. A case in point is NGC 604, about 1500 l.y. wide, at the edge of galaxy M33; thus, this is the most active region in an already formed, but still primitive, galaxy in which Hydrogen gas has concentrated and is collapsing into new stars. We will take four looks at different scales. Here is the galaxy, which is 2.7 million light years away, with the reddish (from Hydrogen excitation) NGC 604 in its upper right: Seen through a telescope at the Kitt Peak (Arizona) Observatory, the GMC appears to consist of excited gases and stars seemingly associated with it (but some may actually be at different distances in the foreground): As viewed by the Hubble Space Telescope, NGC 604 now shows some of the details of gases being moved about in a very irregular pattern, with stars forming as bright dots. A later HST image of the central part of NGC 604 shows a characteristic feature, the development of a large number of small starbursts within the central part of the circulating gas medium. This next image shows a GMC in which there are both old (red) and new (blue) stars:
A parenthetical note: GMCs resemble planetary nebula but the latter would be depleted in Hydrogen and, for some, enriched in heavier elements.
Now, consider an example of localized individual star formation. Stellar object 07427-2400 is a young forming massive star about 100000 years old located 20000 light years from Earth. It has a huge protostellar disc (GMC) of accreting molecular Hydrogen that is spiraling into its massive central star (now about 100 times the luminosity of the Sun). In the process, shock waves are produced that move against the disk, making it luminous also by exciting the Hydrogen and ionized iron. The IRAS Observatory has produced this image
One way to study GMCs is to plot the distribution of excited carbon monoxide (CO) dispersed within the molecular Hydrogen. In this state CO produces two prominent emission lines at 1.3 and 2.6 mm in the near radio wave segment of the EM spectrum. (H2 does not emit strong signals in the radio region.) Here is the CO pattern that occurs in the Orion Nebula, a GMC which also contains regions of strong HII, i.e., ionized H (see below).
Outside the clouds, H and He also
are dispersed, at much lower densities, as the principal elements distributed
in interstellar space. The density of free H (mostly neutral) in that space is estimated to be between
3 and 8 atoms per cubic meter. This atomic Hydrogen when excited but not ionized is detectable by its signature at a 21 cm wavelength as determined through radio telescopy, representing photon radiation given off when excited Hydrogen reverts to its lowest energy state. But, in spiral galaxies most atomic Hydrogen gas has been rearranged in long streamers between arms of existing stars, as seen in this 21-cm radio telescope image of the Milky Way.
When GMCs heat up to temperatures above about 5000° K, the Hydrogen can be further ionized (see Page 20-7 for a discussion of the different ionized states of Hydrogen and their characteristic spectral lines). This gives rise to strongly emitting clouds that are referred to as HII Regions (Atomic Hydrogen is denoted by HI; singly ionized [loss of one electron] by HII). One prominent line used to image and study HII regions is Hα, whose line lies at 0.656 µm - the N3 --> N2 transition in Balmer series. These clouds are photogenic and deserve several examples here. First, an emission nebula as imaged by a telescope used in the 2Mass project (inventory of stellar objects in the Visible-Near IR): We follow this with an image of part of M16 (Eagle Nebula) that has heated to the HII temperature range, with colors chosen to indicate that HII clouds also are bright in the Near-IR: Note this image which contains an emission cloud (pink) and two smaller reflection clouds (molecular Hydrogen) (blue): In both of the above examples, the cloud contains a multiplicity of stars. In this next case, a cloud similar to the Eagle Nebula example contains just one star. This is typical, but is hard to observe in galaxies beyond the Milky Way. As will be discussed on page A-11, as a star begins to burn and send out strong illuminating radiation in the visible, the remaining gas and dust will become lit up as a distinct enshrouding
cloud. In the image below a single star in the nearby Large Magellanic Cloud (a cluster of stars within the Milky Way's influence) is responsible for illuminating the irregular gas/dust cloud that has not yet (if ever) organized into a disk or ring but is likely to dissipate in part by further infall into its parent star. 
Before organizing into an galaxy or after a galaxy has formed, the initial nebulae can have irregular shapes. Some nebulae appear dominated by dark dust, mixed with Hydrogen. These may have elongated shapes, some of which are described as "pillars". Part of the Eagle nebula contains such dark dust concentrations, as seen here:
A close view of one of these pillars (said by many as the most fascinating image yet obtained by the HST) is shown on page 20-11. Another type of dark dust-rich clot, with sharp boundaries, of star-forming material is called a "Bok Globule" (see several examples on Page 20-4), which commonly produces a large number of massive O-type stars, the brightest on the Main Sequence, that have short life times. Here is a typical grouping of dark patches that belong to the Bok Globule category:
A pair of Bok Globules in IC 2944 appear to be merging in this HST close-up: Now look at part of the Keyhole nebula, some 9000 light years from our own galaxy in the halo of the Milky Way. Its size is about 200 l.y. in diameter. It is classed as a dark nebula, but in this rendition computer processing brings out its rich colors. (Note: the term nebula, derived from the Latin for "cloud", has multiple meanings. In the early 20th century, the word was applied to bright objects in the sky that Hubble and others showed to be galaxies. Now, the term is restricted to any collection of Hydrogen gas and dust that may occur outside of a galaxy, as intragalactic material, or as remnants of exploding stars. A good review of the types of nebulae is found at The Web Nebula). One of the largest nebulae is the Carina Nebula, seen only from Earth's southern hemisphere. It is a bright nebula about 7500 l.y. away that lies just before the Keyhole nebula (4th image above). When imaged in the Infrared by the Spitzer Space Telescope (next page), the dust clouds and pillars of this nebula are revealed to contain newly forming stars, probably caused by shock wave compression of Hydrogen gas related to star flaring or bursting events. This is one mode of star formation that has been confirmed in other nebulae. Within the Carina nebula is the very bright star Eta Carina, which displays two tear-drop plumes of gases and dust being ejected in opposition.
The star, first discovered by Herschel in 1677, began to flare up to brighter magnitudes in the early 1700s, faded, flared to a lesser extent in the 1800s, and became less bright by 1900. This HST view shows the gaseous material ejected in two directions; the star however is still present, thus this is not a supernova (page 20-6) but may be a nova.
The HST Wide Field Camera has recently imaged a small cluster of stars in an
early stage of their organization. This is in the Small Magellanic Cloud, about
200,000 light years away in the Milky Way galaxy halo. This "cloud" (almost 10 light years wide) consists
of glowing Hydrogen gas within which numerous stars are embedded. At least 50
of those that can be resolved appear to be young, massive stars. As time continues,
these stars will enlarge as gravity pulls in the surrounding nebular material.
Because of their large sizes, their destiny is to rapidly burn up their Hydrogen
fuel, and eventually explode as supernovae (see below), many ending as neutron
stars. As a large number of stars develop from a nebula, and become luminous as Hydrogen-burning ensues, processes including radiation pressure from starlight will allow the stars to be seen through the diminishing dust and gas. The nebula may continue to produce more new stars if it draws more Hydrogen from beyond its boundaries, but generally nebulae tend to use up available H2 and may deactivate. Stars may then form elsewhere as new clouds develop and reach conditions favoring stellar generation. Individual stars develop along fairly well known blueprints. A central clot of mainly gas organizes and is surrounded by an envelope usually enriched in dust. As the protostar heats up, some of its material is ejected by magnetic forces as jets, such as in these two examples: The expulsion of these high speed gases and charged particles can cause parts of the surrounding nebular masses to be excited and glow in luminescent patches. This phenomenon is known as Herbig-Haro (HH) Objects. Here is one example:
Gas jets are often developed during the HH phase. The jet from the nascent star HH211-480 contains discernible water (in its spectra). Beyond it (to the right) the gases have collected in a luminous "cloud". Instead of expelled jets, some stars have a "tail" analogous to that of a comet, produced by ablation as a star moves through space. Beta-Mira-Head-C is an example:
Sometimes the formation process during the HH stage produces an effect known as a "space tornado". The gases and dust involved seem to organize in a swirl like the winds of a terrestrial tornado, but on a huge scale. Magnetic lines of force, and electrical currents, may be involved. Here is an example:
The emergence of these objects at two opposing sides (bipolar) of the protostar is typical. The next HST view shows this HH effect in a glowing "cloud" which is located near the end of a jet (bright hemisphere, to the right) passing through it.
In the nascent star phase, the dust and gases form a very large volume of organizing material called a "globule" (at least some of these are Bok Globules; see above). A globule in the inner Milky Way, designated DC303.8-14.2 shown below, was first detected by ESA's IRAS satellite. In this trio of images, the left image of the globule, obtained during the Digital Sky Survey observations, shows the extent of the nebular mass seen in visible red light. The center image made by Kimmo Lehntinen's team using VLT ANTU telescope at ESO's Observatory in Chile, is the inset of the left image shown here in color from several infrared band images on this telescope; it shows a distinct ring of gases and dust that emits strongly in the infrared. The right image (of the inset of the center image) indicates several jets of the Herbig-Haro type involved in the early stages of formation of the eventual star.
From the above discussion we conclude that the dominant behavior during the pre-Main Sequence history of a protostar is marked by light gases continuing to inflow and build up the star's mass and size. Much of the dust remains as a thick disk outside the star, such as this example:
As will be further explained on page 20-11, disks like this are the potential conditions that lead to planet formation. Meanwhile, the star approaches pressure-temperature levels capable of initiating Hydrogen fusion, as described in the next paragraph.
When more matter accrues within a growing nebula, its internal gravity continues to increase and draw in still more gases. Gravitationally-driven collapse into forming stars induces compression and further
heat rise. The protostar phase is reached as temperatures rise to 2000 - 3000°
K. At ~10,000° K, the H begins to ionize (electrons stripped away) and,
in the process, loses some heat energy by radiation which tends to slow or counter
the compression. Over time, the cloud eventually reaches a density that requires it to then undergo local clumping of gases into clots that grow into still denser concentrations to become
stars (these smaller clots can exist for much of the galaxy's life but are the sites of further star formation). Here is a Hubble Space Telescope false color view of the central Orion nebula, which appears to be in an early stage of organization into stars (hence, a younger nebula). (See page page 20-3 for other Orion images, treated on that page from a galaxy formation viewpoint.)
This is another false color image of a different area of the gas cloud in the Orion nebula, which lies within the Milky Way at a distance of 1500 l.y., as seen by HST's Wide Camera; on the right is the same area imaged in the IR in which a bright small star "shines through" as a protostar.
The message given by this example is that anyone - including those who are not professional scientists - can participate in the exploration of the Cosmos. In July, 2003 a report was released stating that the Orion nebula contains the hottest stars yet discovered in the Universe. Temperatures were obtained using Chandra X-ray data. The 3 hottest were supermassive stars shown in the right panel below: The single hottest of these stars reaches a surface temperature of 60 million degrees Centigrade (108,000,000° F), more than double the value of the previous record holder. As the early stages of star formation proceeds, the cloud tends to gather around the star in a more isolated manner, removed from neighboring gas and dust nebula. It may then enter the T Tauri phase at which the growing star starts to generate strong stellar winds. The cloud disk still can exceed 150 A.U. in dimension. This telescope image shows the glowing cloud (rendered here in blue, but actually of a different color) around the incipient, still poorly organized central star (a binary pair).
Here are two more T Tauri stars, the one on the left showing the nebular shield that masks the bright growing star and the one on the right showing another T Tauri star as seen in the infrared: The star now rapidly contracts as it passes through the Hayashi phase. This relies on the proton-proton nuclear reaction which releases radiation energy that causes a notable increase in luminosity. However, hydrostatic equilibrium (see below) is not yet reached as the growing star continues to experience disruptive convection. This next view shows a star after most of its accretionary disk material has been incorporated into its mass, as it nears the stage where it will be on the Main Sequence. When a star has finally organized into its Hydrogen-burning sphere, it may eject and dissipate its remaining nebular material as shown in this image of what is now known as McNeil's nebula (named after its initial discover, an amateur astronomer): For stars of masses near that of the Sun, it takes about 10 million years to work through the protostar phase and another 20 million years to join the Main Sequence. More massive stars reach the Main Sequence more rapidly. Below is a view taken through the Japanese Suburu Telescope of S106, which has a mass density twenty times that of the Sun, that began to burn only about 100,000 years ago. This star, 2000 l.y. from Earth, still is showing dust and gas flowing into the central body. An early stage of another massive star, AFGL2591, 10 times the size of the Sun, has been viewed in infrared light by the newly operational Gemini North Telescope on Mauna Kea, Hawaii. Some 3000 l.y away in the Milky Way (located against the backdrop of the Constellation Cygnus), the central region of the forming star is still disorganized. Infalling material continues its growth but also sets off a return outflow of gas and dust. After a star has moved onto the Main Sequence, the history of its life cycle there will
be a continuous (somewhat oscillating) "contest" between contractive heating
during stages of gravitational collapse and expansive cooling by thermal radiation
outbursts whenever rising temperatures increase Hydrogen ionization. Generally,
an evolving star tends to seek out a balance [hydrostatic equilibrium] between
inward gravitational forces and outward radiation pressure developed from the burning of the star's nuclear fuel. This is illustrated in this simplistic diagram:
In its early life,
the contraction phase ultimately dominates, so that a star's deep interior temperature
eventually will be raised above 107 K (varies with star size), at
which stage a fundamental nuclear reaction within the Hydrogen gas commences.
This involves thermonuclear fusion: p + p => H2 + e+
+ neutrinos (H2 or deuterium is a single proton and a neutron and e+ is a positron
[emitted]). That change of state results in thermal energy release which contributes
to continual rises in temperature. Deep within the star, an alternate but dominant fusion process involves melding of 4 single protons into a single helium nucleus consisting of two protons and two neutrons. As temperatures increase further, some protons,
neutrons, deuterium (and minute amounts of tritium [H3]) combine
(in a three step process) into helium (He4 nuclei [2p, 2n]) which
migrates into the star's interior towards its core. In these reactions, some of the mass is converted to energy (E = mc2) which radiates outward as the source of the star's luminosity and which produces the outward pressure that counteracts inward forces owing to gravitational contraction. Luminosity varies as the fourth power of a star's mass (thus a star with twice the mass of the Sun shines 16 times brighter).
Helium remains stable until
temperatures approach 100 million° K, at which state it reacts with more protons
and neutrons to transmute into other elements of higher mass numbers (see below). More massive Main Sequence stars can generate Carbon; some of this element may be in the star initially if it is formed from previous gases and particles that contain carbon produced in earlier star generations. This carbon-enriched star, as its temperature rises and interior pressure increases, can go through another fuel-burning process known as the CNO. Through a series of steps as reactions of Carbon with Hydrogen protons take place, first C12 is converted to isotopes of Nitrogen or O15 but reaction with He4 will lead to C12 again plus energy released as positrons and neutrinos.
When the H => He process reaches a steady state, gravitational contraction
no longer dominates (attains a balance called hydrostatic equilibrium)), the star's total radiant (EM) energy
output per second (defined as its luminosity; also referred to as brightness) becomes constant, and the star
reaches a stable state on the Main Sequence (M.S.), populated by stars that
are primarily in the Hydrogen-burning stages. This equilibrium - in which inward directed gravity forces are more or less countered by outward radiation pressure - is maintained during most of the star's life on the Main Sequence. These stars spend up to
90% of their total lives on the Main Sequence.
To re-enforce these statements about a star's early history, review this chart:
This next view of part of the Orion Nebula is inserted here to make a special point. While Hubble, Chandra and other space telescopes, and some of the large ones on Earth usually seem to provide the most spectacular images, small telescopes operated by "amateur astronomers", if used effectively, can yield their own superb views. The image below was made through a 14 inch "backyard telescope" by Russell Croman using filters and exposing on a tracked target for 7 hours. (Check out his web site for many other astronomical photos taken by him.) The color output rivals some HST images. Red in the image highlights sulphur-rich parts of the nebula; green show Hydrogen enrichment, and blue singles out Oxygen.
The page on Stars is continued on page 20-2a, accessed by the Next button at the bottom here or at the top of this page.
