Let us take a worthy diversion to learn some of the characteristics of a single typical Main Sequence star: our Sun (also called Sol, its Latin name). You can start with an overview by running through the U. of Oregon Solar Astronomy page. Marshall Space Flight Center hosts a Solar Physics site where you can pick up much useful information by clicking on topics in the left margin. Another overview is found on the Wikipedia website.
The importance of conducting intense studies of our Sun, underlying the reasons for the attention given it below, is that it is by far the best accessible example of a typical star whose properties and functions can thus be examined in detail. This favors generalization of solar characteristics to all stellar bodies.
We start by giving this internal density and temperature profile of the Sun, as seen in this diagram. It indicates that in the core the temperature will have risen to levels that permit (require) fusion of Hydrogen: The Sun, whose alternate name is Sol, is a Spectral Type G star on the M.S. and, like most stars
of similar or lower mass, began as a T-Tauri protostar, which started
out as a distended cloud of dense dust and molecular gas that took 10 - 100
m.y. to contract into the Hydrogen-burning stage. The Sun is an average
star on the Main Sequence. Its size is approximately 1,392,000 km (865,000 miles)
in diameter (109 Earth diameters) (it is 130,000 times the volume of Earth). Temperatures in its central
core are estimated to reach about 15,000,000° K; the dominant Hydrogen within the Sun exists in the plasma state (is ionized). The density of the core (which is still gas at this high temperature) is about 160 gm/cubic centimeter; the core is enriched in Helium.
At the solar surface (the photosphere)
the temperature has decreased to about 5600° C. (5770° K; 10,400 ° F;. It rotates on an average of 35 earth days at its equator. Here is a diagram showing the principal features associated with the Sun (and by implication, most Main Sequence stars):
The temperature variations within and above the Sun are shown in this diagram: The temperatures from the chromosphere through the corona follow this general profile or gradient:
The diagram shows that the surface layer or photosphere is actually a thin shell of hot gas. Beyond, the chromosphere is part of the "atmosphere" of the Sun. As that layer thins, it heats up considerably to form the broader corona layer, which consists of very tenuous gas dominated by Hydrogen whose ions can move over much longer free paths causing the abrupt rise in temperatures that has been designated as a Transition Zone.
Beyond the Corona, the Sun still has influence, mainly through the presence of outward streaming particles making up the solar wind (see below). This action extends beyond Pluto, in a roughly spherical distribution known as the Heliosphere.
The elevated interior temperatures combine with the Sun's magnetism to drive materials from below its surface into the solar atmosphere and well beyond. The Sun act as a huge bipolar magnet similar to the conventional bar magnet, as shown by these two illustrations:
The extent of the field is huge, as one might expect from a body more than 1.6 million kilometers (1000000 miles) in diameter. The field's effects extend to the outer reaches of the Solar System. However, on a local scale involving a small part of the solar surface the emerging lines of force act almost as though the Sun had many small poles (see next diagram). The lines of force exert a close control over the various types of Solar Prominences (jets and flares of Hydrogen gas and plasma) described below, with one example previewed here.
The solar "atmosphere", largely a plasma of ionized gases, extends outward from the photosphere, through the chromosphere, a transitions zone, and then to its outer atmosphere, or corona. The corona is very thin (much lower density than Earth's atmosphere), very hot (up to 4,000,000° K), and made up mainly of Hydrogen and Helium ions moving at high velocities (hence, the high kinetic temperatures; these result from the motions of individual ions that build up speeds because there are much fewer collisions in the tenuous gaseous envelope). Both shock waves and magnetic lines of force impel the particles in all directions into the Solar System. The corona is hard to see under ordinary viewing conditions. But, it stands out when photographed during solar eclipses (left) or through an instrument called the coronagraph that masks out the Sun itself (right). The heights reached by the corona gases vary considerably over time, related in part to solar storms.
Between
the corona and the photosphere is a gaseous atmosphere called the chromosphere (brighter inner zone) in which kinetic temperatures hover around 5000 to 6000 °K. The next image, made by the SOHO satellite launched in 1996, shows the photospheric surface, with its distinctive convective patches known as granules and sunspots. Superimposed around this image is a telephoto of the Sun's chromosphere taken during the solar eclipse of February 1998.
SOHO's EIT instrument has produced an even more detailed image of the roiling surface of the Sun, caused by convective transfer of hot gases:
This is a close look at the chromosphere taken through a ground helioscope, using a green filter and blocking out the Sun's surface. At a height of its upper limit (the Transition Zone), from 10000 to 16000 km (6000 to 10000 miles), temperatures may be as high as 1,000,000° K.
As early as 1610, Galileo and others first noted dark areas on the Sun's bright photosphere, its apparent surface. These were named, aptly, sunspots and refer to large (often several times the Earth's diameter) individual or clustered features on the Sun's surface that are several thousand degrees cooler than their surroundings. Here is a helioscope image of sunspots in March, 2001.
This black and white image also shows great detail within a sunspot, which can be as much as twice the diameter of Earth: Sunspots are cooler than their surroundings (around 3400° C compared with the 5600° C for the Sun's average surface temperature) They tend to cluster around a band extending from the solar equator. They are caused by churning up of Hydrogen gas by the strong solar bipolar magnetic field. In effect, they are manifestations or products of solar magnetic "storms". The mechanism involved is shown in this diagram:
On average, sunspots come and go on an 11 year cycle (min-max-min) but that cycle is further extended to 22 years because of a polarity change (north-south-north) during the full interval. Sunspots are associated with increases in expulsion of charged particles in the solar wind and thus can produce interference in radio broadcast signals on Earth. Below it is a plot of sunspot frequency measured over the last 400 years. This montage, made from SOHO images, shows representative appearances of the Sun during the 11 year cycle: Seen in a close-up, sunspots have a distinctive appearance as seen in this solar telescope photo; the surrounding surface contains small, irregular bright areas called granules which are the upwelled part of numerous local convection currents that carry hot Hydrogen gas to the photosphere (dark areas represent the inward return paths of this circulating gas). A similar view, made through a Swedish solar telescope, creates the impression of "bumps" or irregularities - these are the granules - on the Sun's photospheric surface - a convincing depiction that this surface is not strictly smooth.
Various turbulent flow patterns (given descriptive non-technical names based on human experience analogs) develop in and around sunspots, as depicted in these ground-based telescope pictures: One of the closest views yet of a sunspot and its neighborhood has been made by the Swedish Solar Telescope. The Sun (and by extrapolation, stars in general) undergoes considerable variations in exterior activities, along with some changes in energy and solar wind output, over time periods of months to a few years. The term "Sun storm" is applied to one type of such phenomena. The magnetic field generated by the Sun varies in this instance. On Earth, these variations can notably affect radio and TV signal transmissions (satellites, and even astronauts in space, are also influenced). The Sun goes through different cycles of varying activity; these show characteristic periodicity. Here are three solar images showing the appearance of the Sun over a half cycle that goes from a solar minimum to a maximum in 3 1/2 years (part of the seven year cycle): The smallest of the ejections of solar material beyond the Photosphere are solar prominences. These gas and particle jets almost look like "fountains" or upwelled clouds. Here is two notable examples of this feature: Other small disturbances are called spicules. These are plasma jets that are typically about 300 km (200 miles) wide and can extend out to about 4800 km (3000 miles). They are short-lived, lasting about 5 to 10 minutes: Solar flares extending well beyond the photosphere occur during Sun storms (and, by inference, this is likely to happen also on many, perhaps most, stars). Here are two TRACE (Transition Region and Coronal Explorer) images acquired in the summer of 2000 that shows some spectacular solar flaring; these loop in patterns controlled by local magnetic lines of force.
The base of a solar flare, imaged by SOHO, might remind one of an active lava surface in a volcanic caldera, except in these examples the solar surface is very much hotter and incandescent gases rise much higher.
SOHO is capable of time-lapse imagery so that the history or growth sequence of a flare can be displayed as a series. This image shows the succession of growth and dissipation of a prominence as observed with the LASCO sensor on SOHO. Flares can last for days. The instrument also provides temperature data, giving an average of 107,000 °K for the gases that make up the feature.
The strongest solar flare recorded in history occurred in early November of 2003, during which several other prominent flares were observed. Much disruption of communications was inflicted. This is a SOHO image of this massive flare:
Using its Extreme Ultraviolet Photometer System (XPS) instrument, SORCE (Solar Radiation and Cloud Experiment), a new solar observing satellite launched in early 2003, caught the series of these strong flares during this October-November, 2003 period. Rapid monitoring of flare activity is imporntant on Earth since these can cause severe radio disturbances in the terrestrial atmosphere and can damage orbiting satellites. A SOHO Extreme UV (EIT) image of the Sun's face is shown in this diagram along with the plot of solar flares during this span of intense activity:
A somewhat different picture of the Sun's activity is gained by converting X-radiation into an image; the one shown here was obtained by the Japanese Yohkoh satellite:
SOHO is equipped to image the Sun at various wavelengths in the Ultraviolet. Images at 1710, 1950, and 840 nanometers (blue, green, red respectively) are combined in this false color composite which emphasizes UV emission variations on the Sun's surface.
In the Extreme Ultraviolet wavelength region, here is a view taken from the above image that is colored blue to accentuate one of the invisible wavelengths in the UV. Plumes of very hot gases enter the chromosphere from discrete locations out of the Sun. One type of megaplume, discovered only 30 years ago, is called Coronal Mass Ejection (CME), in which ionized Hydrogen gas is expelled considerable distances from the Sun's surface, following helical pathways outward; this SOHO image captured that effect: The CME, although more violent and extensive than solar flares, usually lasts only a few hours. The start and final stage of a CME, covering a 3 hour time period, is followed in these images made using earth-based and Solar Maximum Mission (SMM) inputs. The largest of the CMEs is a prominence known as a Magnetic Flux Rope (MFR), seen here in this image.
Being prominent, albeit infrequent, the MFR has been observed over several centuries. Here is a ground telescope view of an MFR made in 1964 (this rendition smears out the distinct ropy patterns)
The twisting of the ejected hydrogen gas is caused by twists in the spiraling magnetic lines of force, as depicted in this diagram:
The TRACE satellite recorded this CME in 1999. Look very carefully for ripples extending along the coronal surface. These have been termed solar "tsunamis" (that term refers to giant waves set in motion by upheaval of water above the epicenter of a marine earthquake), but in this case the explosive disturbance involved with the CME generates sonic waves that make ripples in the corona as they propagate like "sonar pulses" from the CME site.
Another feature within the solar atmosphere, associated with loops (often twisted) and curved flares, has been dubbed "moss" because of its spongy light-dark appearance owing to variations in temperature within the 10000 to 1000000° K range. Here it is imaged by TRACE in ultraviolet light:
The twisting can be involved - almost chaotic - as indicated in this TRACE image:
It should be no surprise that the Sun (and by inference all stars) undergoes various physical movements and displacements in and near the photosphere. Some of these result from violent events that send seismic-like waves within the surficial gases. A whole field - helioseismology - has evolved to study these phenomena. A broad review of this subject is found at this Stanford University site. Helioseismology deals mainly with systematic oscillations detectable at the surface and waves that can be used to probe the Sun's interior structure and dynamics.
Here is a generalized map of the Sun's interior in terms of sonic velocities that are sensitive to temperature differences. Red refers to hotter zones in which acoustic speeds are slowed; blue to faster - hence cooler - zones.
Surficial disturbances, including explosions, also produce acoustic waves (largely at low frequencies); these are sometimes referred to as "sunquakes" but are not equivalent to terrestrial seismic quakes. The illustration below shows a sound wave associated with the emergence of a solar flare which radiates outward in the gas much as a ripple moves when a stone is cast in water.
One of the consequences of these techniques for penetrating the solar interior, making it as though "transparent", is the ability to see some broad features on the surface of the Sun not facing the observing instrument. The Michelson Doppler Imager on SOHO used sonic waves to create this pair of images, the top showing sunspots on the visible solar face and the bottom detecting features on the opposite face.
Several satellites have been placed in space to get more information on the Heliosphere - which pertains mainly to the solar environment beyond the Sun's corona. Our understanding of this huge feature - it consists of the Sun, the planets, and the solar cavity through which flows the solar wind - is summarized in this diagram:
The Heliosphere's magnetic field is responsible for deflecting extrasolar particles, such as makes up some of cosmic rays' constituents, from neighboring stellar sources. There is continuous emission of charged particles, mainly Hydrogen ions in a plasma, from the corona outward into the heliosphere and beyond. This is the solar wind which through particle expulsion at high velocities causes these ions to escape the Sun's gravitational field and travel great distances outward through the Solar System. These particles along with galactic cosmic rays (stellar winds, similar to the solar winds, some part of which is derived from stars that expel gamma rays and energetic ions of many atomic species) continuously bombard the Earth; the Van Allen Belts provide major protection from the solar and cosmic influx that otherwise would affect (or even prevented the inception of) life on our planet. Solar flares and magnetic storms occur randomly (no established interval) but periodically on the Sun during which the wind is intensified. Here is a view which shows particles being pushed, mostly in a zone around the solar equator, beyond the Sun's gaseous envelope. Where the solar (or more generally, stellar, since this phenomenon is presumed to occur around most/all stars) wind meets particles in outer space (essentially beyond solar limits), a bow shock is often created. On Earth, a similar bow shock results from solar wind interaction with Earth's magnetic field. Bow shocks have been observed associated with individual stars, such as LL Ori in the Orion constellation: A major mission, Genesis, to study solar wind composition was launched in 2001 and was returned to Earth on September 8, 2004. The result was an apparent catastrophic failure but has instead been salvaged as at least a partial success. The story of Genesis is another fascinating example of NASA "Can Do", and deserves telling here. But because the write-up is long, we have chosen to let you elect to read it on a separate page, accessed by clicking on page 20-2b. Ulysses, a joint ESA-NASA program managed out of JPL, was launched on October 6, 1990 to monitor several aspects of the heliosphere environment. A good review of this project is found at the Ulysses Home Page. Traveling far into the outer reaches of the Solar System, Ulysses makes a complete orbit around the Sun once every 6 years. Here is a plot of the data on the Solar Wind achieved during two cycles, one during a solar minimum and the other during a maximum (these occur during magnetic pole reversals over an eleven year period) : Another Ulysses instrument, called DUST, provides data on the density and motions of the myriads of interplanetary dust (also called "stardust") moving around the Sun. This stardust is mostly very small particles in sizes of less than a millimeter. The distribution appears to be controlled in part by solar wind maxima and minima, as seen in this diagram. During the minimum the highest dust concentration collects below the plane of the ecliptic (defined by planetary orbits) but reverts to that plane during a maximum. Moving at an average speed of 26 km/sec, the transit time for a given dust particle to orbit around the Sun has been determined to be about 20 years. Using these data and extrapolating from other information, this diagram depicts the location of major dust concentrations around the Sun and extending to the local stars in our part of the galaxy: Knowledge of stardust distribution and behavior is relevant to a better understanding of the history of planetary bodies in dust clouds around stars, as discussed on page 20-11. The latest solar probe now studying the Sun is STEREO, launched in October, 2006. As its name suggests, this spacecraft has the capability of imaging solar features in 3-D or stereo. Here is a 2-D STEREO image of the full Sun in the UV: 
P.C. Frisch, University of Chicago
As mentioned near the beginning of this page, a prime reason to study the Sun in detail is that it is the only star that is really close. Of the billions in our galaxy, the Milky Way, few others show any of the details described above for the Sun. An exception is this telescope view of a star 300 light years away that was imaged by the Gemini Observatory (a very large planet orbits this star [upper left]):
The strong magnetic field associated with the Sun is believed to form around stars in general, particularly those still burning their elemental fuel. Sometimes the materials ejected from stars will follow magnetic field lines that organize the escaping gases and accompanying non-gaseous elements into prominences and loops. This is beautifully displayed in the Hourglass planetary nebula (see below) that has developed around a Red Giant (see below) that is evolving into an eventual White Dwarf. At the Red Giant stage, the star can have a magnetic field between 50 and 500 Gauss, strong enough to induce the Zeeman effect. This effect, associated with strong magnetic fields, alters the spectrum of excited molecules; in the Hourglass case, excited water molecules are one component of the substance giving off the light associated with the loops.
This survey of the Sun ends with a look at both its past and its future. The Sun is expected to last about 5 billion years more. Here is a visual review of its stages of life:
With this in-depth examination of one star, the Sun, let us return to the evolution of stars in general. Stars undergo a series of changes as they form and then pass through their fuel burning cycle. They can be further categorized
in terms of their end products at completion of the burning. The type of star
that evolves depends on the initial mass of H gas as it reaches its early stage
of burning. Stars can also be classed according to their relative ages into
Population I and Population II types. Type I stars are generally
younger (that is, were formed much later in cosmic time), and continue to form even today. In Spiral galaxies, they are most
common in the arms. They contain a larger proportion of the heavier elements which,
as we will show later, are largely produced during late stages in an earlier large star's history. The star's life ended in its explosive destruction that spewed out these elements into the gas-dust nebular debris from which present type I stars have formed). Type II stars are older (formed earlier), having burned much of their fuel, and generally reside near the galactic core in Spirals or are the dominant stars in Elliptical galaxies. Type II's are deficient in heavier elements which means that they developed when the Universe was younger from raw materials that had not yet accumulated these elements; many are small enough to have lived long lives (but are too small to form heavier elements; see page 20-7). These chemical differences are evident in the spectra characteristic of each type: this pair of spectral strips shows the upper one, from a Population II star in the Milky Way, to display almost exclusively Hydrogen lines whereas a Population I star (the Sun) in the same galaxy (lower strip) has those lines plus many others representing different excited elements beyond helium in atomic number.
Star formation is a continuous process in galaxies, even those that formed early in Universe time. A recent estimate based on several decades of observations of the rate of new star formation in the Milky Way (which itself began about 7 billion years ago) indicates an average of about 6 stars per year begin their nuclear fuel burning (an arbitrary time of birth). Older galaxies tend to have used up much of the remaining Hydrogen gas dispersed in intergalactic space, so that the rate and number of new stars will be less (in general, new births diminish with time). Some galaxies as observed today show large numbers of new stars formed over intervals of hundreds of millions of years. These so-called "starburst galaxies" display numerous areas of light blue-white representing many stars that formed during a narrow span of time. NGC3310, seen below, contains several hundred clots of young stars, each clot containing up to a million stars (estimated):
The H-R and Evolution diagrams on the preceding page show several classes of stars whose initial mass lies below that of the Sun. In total numbers they greatly exceed that of stars on the H-D diagram of solar mass and larger. Of particular interest are the Red and Brown Dwarfs. This diagram shows the relative sizes of Red and Brown Dwarfs:
Although hard to discern because of their small sizes, the Red Dwarfs have now been deduced to be the most abundant stars in the Milky Way galaxy - and by inference in most galaxies, especially the spiral types. Here is where the Red Dwarfs fit on the Hertzsprung-Russell diagram:
These M stars have masses ranging from about 0.1 to 0.4 that of the Sun; their surface brightness is less than 1/2000th of the Sun. Their surface temperatures are around 3000° K, producing light that is distinctly red. They do burn their Hydrogen fuel but are too small to develop a helium core; the limited new helium that is produced (besides some primordial helium) is redistributed throughout the star by convection. Because of very slow fusion rates, the Red Dwarfs are capable of very long lifetimes (up to 100 billion years). Even though abundant within galaxies, they do not contribute much to the total galactic mass. Proxima Centauri, which is the second closest to the Sun (4.1 light years), is a Red Dwarf.
Barnard's Star is a Red Dwarf that is 5.9 l.y from the Sun. It is notable because its proper motion (relative to the Sun) is 140 km/sec, the fastest known in the Milky Way (probably not strictly true since motions of more distant stars are difficult to measure). Here is a view of the star (superimposed on a steady background) showing the shift in location over just 6 years.
Red Dwarfs are almost certainly the most numerous type of luminous stars within our Universe. But because of their low luminosities, they are hard to see even in Hubble Space Telescope images. Here is an exception HST view of a small part of intergalactic space in which larger stars are almost absent. The red blotches are Red Dwarfs:
The next image is of an as yet unnamed Red Dwarf 7.2 l.y. from the Sun, shown here as an Artist's conception based on observatonal data. This approximates how the Red Dwarf would look if a high resolution image could be made of such a small star.
Below is an actual image of GL623, a Red Dwarf with an even smaller companion (possible White Dwarf - see below):
Most Red (and Brown) dwarfs exist as "singles", that is, they are not commonly paired as binaries (a binary star combo is more common in stars within the middle segment of the H-R diagram, in part because these are in sizes large enough to be resolved). There is some evidence that Red Dwarfs may have associated planets but none yet has been discovered directly. Once again, we reiterate that these two Dwarf types are likely to be numerically the most abundant stars in the Universe.
The Brown Dwarfs have masses lower than 0.08 that of the Sun [arbitrarily set at 1; Black Dwarfs have even less mass]). (Another class of small stars, the White Dwarf, is the end product of stages of expansion or explosion of large stars.) A recent HST infrared image (right) of the Orion Nebula (shown on the left below in visible light) has revealed that Brown Dwarfs (the brownish-orange spots in the IR view) are widespread throughout the region here imaged.
This next image shows more than 30 Brown Dwarfs (the most ever observed in one small region) in the vicinity of the rho Ophiuchi Cloud (some 570 l.y. away). These are each only a small fraction of the Sun's mass and are less than 1 million years old. In the image, made in the infrared by ESA's Infrared Space Explorer, are several bright, very massive stars.
Although generally hard to find because of their small size, several nearby Brown Dwarfs have been imaged at moderately high magnifications. Here is what a pair looks like:
The closest Brown Dwarf to Earth found so far is 12.7 light years distant, in the constellation Pavo (Earth's southern hemisphere). It is catalogued as SCR1845-6357AB. It orbits a small M Dwarf star, being 4.5 A.U. from its parent. Its mass is about 65x greater than Jupiter. Its estimated surface temperature is 750°C. Here is a view made by the Very Large Telescope at the ESO facility in Chile:
The Brown Dwarfs do produce internal energy from limited fusion of deuterium but never reach the Hydrogen fusion stage of larger stars (for this reason they have been called "failed stars). Their surface temperatures fall below 2600° C, insuffient to fuse lithium in the outer layers (Li spectral lines become an indicator of this type of dwarf). These have low luminosities and are hard to detect even in our galaxy, the Milky Way. Yet they may be very abundant within galaxies (one estimate holds them to approach all luminous stars in number), accounting for a considerable fraction of the total mass of stellar bodies. Since the Dwarfs burn their Hydrogen at very low rates, they will be long-lived. The smallest of the Dwarfs are not much larger than some giant planets, into which they grade (a planet does not produce a significant output of radiation through nuclear processes). In October 2000, astronomers reported spherical objects smaller than Brown Dwarfs, ranging in mass from 5 to 15 times that of Jupiter, that appear to "free float" (do not orbit stars). They are not hot enough to initiate any nuclear burning. They may be incipient "dwarfs" that could grow larger into eventual stars. What we presently know about brown dwarfs is neatly summarized in this diagram:
Both Brown and Red Dwarfs are primary, that is, they are not the final product of a multi-step stellar evolution. They are simply clots of gas that did not accrue enough mass to allow Hydrogen to burn efficiently to higher atomic number elements prior to being forced to leave the Main Sequence by inflationary or explosive means. As said before, taken together Red and Brown Dwarf stars are by far the most abundant types in the Universe.
The smallest stellar object considered to be a star is the Neutron Star. This is the surviving core of a Red Giant that has exploded as a supernova. Its size is only about 20 km in diameter on average. It contains about 1.5 to 2 solar masses, hence it is extraordinarily dense (yet, much less than Black Hole densities). Here is a model for a Neutron Star's interior:
Being so small, Neutron Stars are undetectable from Earth. But, like Black Holes, they attract matter which accretes around them and if excited glows like a normal star, as seen here. More about Neutron Stars on later pages.
Turning now to the other size extreme, near the top left of the H-R diagram, above the Main Sequence, is a region containing the most massive of stars: the Blue Giants and Supergiants (not named in the plot). These stars simply grew into masses that carried them beyond the upper limits assigned to the Main Sequence. Typically, a Blue Giant is more than 40000 times more massive than the Sun, has a diameter at least 8 times greater, and has surface temperatures exceeding 20000° K. It tends to have a short life span (~100 million years) but can go into a Red Giant phase (see below). One of the brightest stars in the sky is Rigel, in the Constellation Orion, a Blue Supergiant (B type), as seen here:
Among the brightest of the Main Sequence stars are the B types with surface temperatures in excess of 11000° K. Perhaps the best known are a small cluster of blue-white stars known as the Pleiades (the Seven Sisters) which lie in the Milky Way only about 375 light years from Earth.
Such clusters of very massive, bright (blue-white) stars are much less common, since stars of this size tend to burn out far more rapidly. Here is an unnamed cluster of large stars that are probably not a starburst.
The HST has found a star, in the Pistol Nebula, that is currently the brightest known in the Milky Way, being ten million times more luminous than the Sun, and 100 times more massive. The star, only 25000 l.y. away, is estimated to have begun its Hydrogen burning only about 1-3 million years ago. In this view, the red "clouds" around the central star may, according to one interpretation, be Hydrogen gas and other material shedding from the star perhaps as it enters a destructive phase, or, less likely, are still involved in continuing collapse onto that star. This view is in the infrared; in visible light the star is shrouded with opaque dust.
The Wolf-Rayet star is one type of very massive O star which has a surface temperature of around 50000° K. It is short-lived after reaching the Main Sequence and before destroying itself it sheds much of its mass by expulsion of Hydrogen driven away by its stellar winds. This next image shows a Wolf-Rayet (WR) star (black arrow) - NGC2359 - imaged in the Visible; below it is WR124 (in the constellation Sagittarius) imaged in the near infrared. Both show the extent to which the gases are expelled even as the parent star remains intact (this separates the WR types from planetary nebulae (see below) in which the central star has exploded.
WR stars are rare. Less than 200 have been detected in the Milky Way Galaxy over the last 150 years.
Much more common are Red Giants which are stars with a hot contracted core but cooler outer envelope of greatly expanded (up to 100x the normal star diameter) diffuse gases emitting surface radiation in the visible red. Our Sun is scheduled to become a Red Giant in about 5 billion years when its final fuel material will expand as a hot gas out to roughly the limits of the present Earth orbit).
As it ages, the Sun is likely to behave like a class of smaller stars known as Mira types. The inner part of an expanded Red Giant in this class would, using our Solar System for scale, expands as shown below. The outer part of a Mira star contains molecular layers of tenuoous gases and may be cool enough to permit water to form from Hydrogen and Oxygen molecules spewed out in the expansion. Red Giants develop as the Hydrogen in
the deep interior (core) is finally depleted and the helium derived from it
tries to fuse (burn) to carbon. The core shrinks even as fusion continues in
the outer regions of the star. Energy is rapidly lost, so that hydrostatic equilibrium
is disturbed, allowing for expansion driven by stellar winds. The energy density
of the star's surface, being lower, shifts emerging light wavelengths from bluish
towards red. A Red Giant, sometimes described as a "bloated" star, can exist up to 500,000 million years. Below is a typical
Red Giant, Betelguese (actually classed as a Red Supergiant). It is present in the constellation Betelguese and can easily be seen by a small telescope) as seen by the HST. Among other well known Red Giants are Arcturus (below) and Aldebaran (in the Constellations Bootes and Taurus respectively).
Another Red Giant in the Mira star group shows a pronounced asymmetry of its outer envelope, as imaged in the UV: This next HST image shows the globular cluster M10. It is notable for the large number of Red Giants and some Blue Giants, besides smaller stars on the Main Sequence. What happens to a star after Hydrogen and helium fuel is consumed depends on its size. Smaller stars (Spectral types A through M) end up as the surviving cores of Red Giants which are greatly reduced in size to the White Dwarf stage. Larger stars (O and B) undergo a different process that involves explosive shedding of nearly all their remaining gaseous matter and synthesized elements in a supernova (see next page). Type O stars (8-10 solar masses) follow a sequence that involves a small supernova which ends with a White Dwarf. More massive stars which explode leave a small stellar body known as a Neutron star (see above). A Red Giant star in the late phase of shedding its now largely consumed gases has been observed by the Chandra Space Telescope at this stage. Only some Helium is left that will then be expelled as the star, Beta Ceti, moves towards its White Dwarf end state. After the bulk of the mass is shed from the outer envelope of a Red Giant whose initial
mass was less than 10 solar masses, it will have lost nearly all its remaining nuclear
fuel, shrinking rather abruptly (over a few thousand years) to a radius much
less than Sun size (some as small as the Earth) and ending up as a dense, hot
core (~1.4 solar masses) that becomes a very hot, luminous White
Dwarf (surface temperatures as high as 170,000° K). A White Dwarf is, as
the name suggests, small (but it differs from the Brown Dwarf group described above by having started out with a mass greater than 1): A star not much larger than the Sun shrinks to a size
comparable to Earth but with a density of about 1,000,000 g/cubic cm. Its core
mass is said to consist of degenerate matter, i.e., owing to quantum effects its pressure no longer depends
on temperature, i.e., can vary independently, - in this case close-packed electrons
are degenerate (a state in very dense matter in which the pressure in a very hot gas or plasma depends on density but is independent of temperature) but not protons or neutrons. The White Dwarf nevertheless is
still hot and bright. The ultimate fate of a White Dwarf is to cool and fade away. Both White and Brown Dwarfs can eventually lose any fusionable fuel and become Black Dwarves (star "cinders") that are no longer luminous but continue to radiate heat away.
Although White Dwarf stars are small, they still shine early in their history. The HST has succeeded in detecting these stellar "midgets" amidst nearby stars. The image below show seven tiny bright dots which are actually White Dwarfs: We finish this survey of star types by this discussion of Variable or Pulsating stars. This group is characterized by a systematic, cyclic increase-decrease-increase is apparent brightness over a period of hours to days. The luminosity or star magnitude thus changes in a periodic way. Generally, there is a close correspondence between the star size and the luminosity such that the frequency of this brightening-diminishing can be correlated with the magnitude. This magnitude serves as a "standard candle" for approximating the distance of the star from Earth: once established, the brightness will assume a value that is a function of how far away it is. This subject of Cosmic Distance is examined more fully on page 20-9. For now, we shall only consider variable stars in a general sense. These fall into two types: Cepheids and RR Lyrae. Both are offshoots from the Main Sequence of the H-R distribution, as shown here: The next diagram shows that Variable Stars are characterized by changes in magnitude (ordinate) on a regular periodicity of hours (RR Lyrae) to days or weeks (Cepheids)
This next view is a radio telescope image of Betelguese; on the right is an analysis of the temperature profile in its outer shell:
Here are light curves - periodicity-controlled changes in luminosity over a time scale of days - for several Cepheids.
This information is used as input to derive these generalized plots of luminosity versus periodicity for two types of Cepheids and for RR Lyrae. Note that RR Lyrae have almost constant luminosity but some variability in Period during a single day.
What is happening physically is an alternating expansion and contraction of the star's outer shells and atmosphere. With expansion comes surface cooling and a drop in brightness, which reverses when the star soon after contracts.
A paragraph in the Wikipedia entry for Cepheids is reproduced here to explain why stars can be variable and pulsating: "Various instabilities can cause a star to pulsate. The most common type of instability is related to oscillations in the degree of ionization in outer, convective layers of the star. Suppose the star is in the swelling phase. Its outer layers expand, causing them to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the gas more transparent, and thus makes it easier for the star to radiate its energy. This in turn will make the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression (swelling and shrinking) is maintained. The pulsation of cepheids is known to be driven by oscillations in the ionization of helium (from He++ to He+ and back to He++)."
We have now covered the main types of stars in the heavens. Before summing up with a treatment of star evolution, this is a logical place to discuss a category of stellar end product - called a planetary nebulae - that is, in effect, a dispersed star. (This designation is a misnomer in that planets are not the end product; early observers (the first being Wm. Herschel, discoverer of Uranus) once thought that, at the poorer spatial resolution of their telescopes, the objects they saw with a torus- or disk-like appearance resembled an early stage of a planet's formation.) Some of the most spectacular images of Planetary Nebulae obtained by ground and space telescopes are those associated with Red Giant evolution. One or more rings or shells often represent the shedding of matter in the final ejection phase around a Red Giant as it becomes a White Dwarf star. This stage is comparatively rapid, taking 10000 - 20000 years for the rings to disperse. The scale of expansion places the edge of these gaseous envelopes at diameters up to a 1000 times that of our Solar System. In the images that follow, you should note that many of these nebulae resemble the Giant Molecular Clouds displayed on the previous page; the two types are distinguished in that GMC contain much small amounts of heavier elements than do the Planetary Nebulae.
The important thing to remember about planetary nebulas (or, in Latin, nebulae) is that they are the expelled atmospheres of dying stars whose masses are less than about 8 solar masses (and have reached the carbon stage of fusion production). After expanding into the Red Giant phase, the stars finally throw off the remaining gas envelope in one or more bursts at high speeds into surrounding space. We see these nebulae after they have been pushed out huge distances from their parent star which usually becomes a White Dwarf (some larger stars that end up as Neutron stars may also shed their remaining gases). These lower mass stars end their lives in spurts of ejection spread over thousands of years, in contrast to Supernovae (page 20-6) or Black Holes, which are the death throes of stars of mass greater than 8 Suns that explode just once, almost instantaneously, and disperse their debris at high speeds over shorter time periods. In some respects, supernova debris resembles planetary nebula but since Supernovae are much more violent, their gas and debris can expand outward very much more rapidly.
This and the next paragraph were copied directly (as italicized) from the Wikepedia website labeled "Planetary Nebulae": Many stars become unstable and may explode as the Helium stage of star evolution winds down. Helium fusion reactions are extremely temperature sensitive. This means that just a 2% rise in temperature more than doubles the reaction rate. This makes the star very unstable - a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space. The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.
The Hubble Space Telescope has proved the ideal tool to image planetary nebulae - as a class these may be the most photogenic of all astronomical objects. A classic example of a planetary nebula with a fairly regular shape is M57, the Ring Nebula, as seen
by HST (red tones represent excited Hydrogen; green is associated with ionized
Oxygen).
Similar in its shape and stage of development is this expanding debris ring making up the Helix Nebula; the colors were assigned to different IR wavelengths as sensed by the Spitzer Space Telescope: This next image, of the Retina nebula, at first sight appears very anomalous in shape. Thus: But, in fact the apparent rectangular shape is an artifact of the image display. What you are seeing is a side view of a ring nebular. The outer ends of the HST image are a dark red that is difficult to perceive, so that the curved endings are not there to divulge the ring shape. The youngest planetary nebula yet found, the Stingray Nebula, first appeared about 20 years ago. It shows both a central ring and opposing lobes, the hallmarks of these nebulae, formed as explained below. It is near the Earth; all (or most) planetary nebulae observed in the detail shown in the following images were formed within the Milky Way. At first (when telescope acuity was limited), the planetary nebulae were thought to be normally spherical, resulting when strong winds blew out the remaining gases equally in all directions. Now, only about 10% remain that way as expansion continues. Some nebulae, such as Abell 39, are still nearly spherical having possibly not yet broken up into streamers:
Another apparently spherical nebula is nicknamed the Owl Nebula (NGC 3587) for its obvious resemblance to an owl's face. Located in the Milky Way ~2000 l.y. from Earth, this nebula contains three distinct layers: a faint dark blue outer ring consisting of now dispersed gases expelled in the early stages; a medium blue middle ring driven by superwinds, and an inner light blue ring, plus a purplish central filling that represents material that has migrated inward:
A typical elliptical form is associated with IC418, which shows a delicate lacing of gas streamers within:
The more common expression of a planetary nebula shows the gases (which appeared colored in the images below because of excitation into ionic states from UV radiation emanating from the star remnant) to distribute in a wide array of shapes. Rings (torus), ellipsoids, bi-polar lobes, and streamers are shape components of the nebulae. One to several (combination) factors account for this diversity: winds of different speeds coming off the surviving star remnant; accretion tails, binary stars (the star not exploded influences the dispersal of the nebula), and complex twisting of magnetic lines of force all are likely to play roles.
In the first stages of development, the gas expulsions have been called proto-nebulae. A famed example is known whimsically as "Gomez' Hamburger, named after its discover, Arturo Gomez using a telescope at the Cerro Toledo Observatory in Chile. The "buns" are gas clouds that glow here in visible reflected light; the "meat" appears to be thick obscuring dust:
Another example is this early stage of proto-nebula development around an exploding Red Giant, as seen by the HST WFC:
Striking views of planetary nebulae in later development stages are taken as ultraviolet images because the gases expelled from a dying star are excited by ultraviolet radiation during the process. Here is a UV image (GALEX satellite) of NGC 7293:
In the next image, the Cats-Eye nebula (NGC6543), some 3000 l.y. away, appears to be a later stage
of the outward propulsion of gases around a Red Giant (possibly one of a binary
pair, with the second star a possible dwarf) in which several rings are made
luminous through excitation by expelled particles. This is the color scheme most often used for the nebula.
A recent observation made using the Nordic Optical Telescope (ground-based) has revealed that the Cats-Eye nebula is surrounded by a halo ring and filaments extending beyond that. This stunning image shows the full Cats-Eye complex in a false color rendition in which the red is due to excited nitrogen and greens and blues to Oxygen: The full extent of ring development has been revealed in an HST ACS (Advanced Camera for Surveys) image, shown below. A number of individual spherical ring fronts are evident. Based on their distance from the Cats-Eye center and estimates of their speeds, these rings appear to be generated repeatedly at intervals averaging 1500 years, by a process still uncertain. The central Cats-Eye configuration is now believed to be a later stage in the history of this nebula.
The HST has caught a Red Giant in the act of finishing its existence in this state as it transposes into the Twin Jet nebula. As shown in this next image, some of its outer, less dense mass is being ejected from the main body of the star: The Red Spider nebula (NGC6537), below, shows a common feature noted in many explosive stars, namely, lobes (usually in a pair) of gas being driven outward at high speeds (1 million km/hr) by stellar winds (in this case moving at even higher velocity). The lobes result from shock that compresses the gas expelled as the nebula develops. Note the ripples in the lobes. These lobes consist of the now cast-off gases that once formed the bulk of the star whose core has survived after it has passed into the White Dwarf phase. Similar to this is what is whimsically called the "Garden Sprinkler" nebula. Its curved lobes of ejected gases are traveling well beyond the dying parent star. Still another example shows details of the wispy strands of gases and particles in a supernova found in the Vela group in the Milky Way, as imaged by the Schmidt telescope at the Anglo-Australian Observatory in New South Wales: Nebulae are probably the most photogenic of the common astronomical entities. Consider these two:
As a planetary nebula expands over time and debris organizes into these wispy strands, the entity can take on a filamentous structure as shown here in the Veil Nebula, some 2500 l.y away in the Milky Way:
This next image shows a violent explosion around NGC6302, a very hot (surface temperature of 250000°C or 450000°F) star 4000 l.y. away. Now almost gone, the destroyed star is invisible in the Hubble image below. The nebular material has much dust but also water ice crystals around particles as the expelled vapor condenses in outer space:
One of the most peculiar nebulae is the Red Rectangle, whose shape is defined by two pairs of jets emanating from a center that is a binary star pair, HD44179. The resulting X-shapes seem to have "ladder bars" joining them. Much of the red color is due to excitation of dust.
The Hubble Space Telescope has now gathered hundreds of images showing "dying"
stars, i.e., those in their last stages of fuel burning that are shedding matter
explosively. The variety and complexity of a star's final activities has proved
to be much more diverse than known from the era of conventional telescopic observations.
A recent NASA press release documents work done by astronomers at the University
of Washington and elsewhere that illustrates different observed end stages,
shown in a panel of six images typifying this diversity of gaseous envelopes
(these are typical planetary nebulae):
These brief descriptions define each observation: Top Left: A round planetary
nebula with a bright inner shell and fainter outer envelope; this uniform expansion
is the mode predicted for the final phase of the Sun's demise; Top Center: A
hot remnant star surrounded by a green (color assigned) oval in which older
gas is pushed ahead to form a bright interior rim; more gas further out shows
hot spots (red); Top Right: A spherical outer envelope and an elongated inner
"balloon" shell, both inflated by a fast wind from the interior star;
Bottom Left: A "butterfly" or bipolar (two-lobed) nebula; Bottom Center:
A bright central star at the center of a dark cavity bounded by a football-shaped
rim of dense, blue and red gas; the star's former outer layers is shown in green;
note long greenish jets; Bottom Right: A planetary nebula with a pinwheel or
spiral structure with blobs of gas ejected from the central star.
An interesting rendition of a dying star's gaseous envelope is seen in this Chandra X-ray image of NGC40, some 3000 l.y. from Earth. The Hydrogen gas has been heated by radiation from the star to a temperature around 1,000,000 degrees Centigrade, high enough o generate a strong X-ray signal. The lobed Ant Nebula (or Menzel 3), has been the subject of a recent explanation for some of the unusual shapes associated with this class of stellar objects. According to Dr. Adam Frank and colleagues at the University of Rochester, as stars age and begin to shed materials, they appear to slow their rotation. But as that material leaves the parent star, the star's core begins to rotate more rapidly. With increased rotation, the associated magnetic field becomes stronger and influences the patterns or shapes of the escaping material. The left image below shows NGC6751, in the constellation Aquila, but actually located some 6500 l.y. from Earth. It exploded less than 5000 years ago and the outer shell of the star has now moved out from the white hot central core (light yellow in center) to produce a near spherical shell about 0.8 l.y. in diameter. The outer gases (mostly Hydrogen, in orange) are cooler than the (bluish) inner gases. Notice the radial streaks of gas marking the trajectories of these streamers. The colors given to the various gas components are computer modifications of colors perceived by HST owing to excitation by UV radiation. On the right is the Eskimo nebula.
The HST has now obtained a good image and temperature data for what is called the Boomerang nebula. Despite the obvious incandescence that makes visible the two extensions, the temperatures measured in these lobes were as low as -272° K, just above absolute zero and possibly lower than the general cosmic background radiation, making this feature the coldest region of visual mass around a central star yet found in the Universe: One of the most "delicate" appearing nebulae is Barnard's Merope Nebula, just 380 light years from Earth, and found in the celestial sphere within the space defined by the Pleides. In this long-exposure ground telescope image, the material is mainly dust reflecting light from a nearby star (just beyond the upper right corner): More massive stars than the Red Giants just discussed above proceed through their final stages of fuel consumption by different processes. These result in events called Novae and Supernovae, of sufficient importance and complexity to warrant extended treatment on page 20-6. The end result can be a White Dwarf star, a Neutron Star, or a Black Hole, depending on the star's mass at maturity. A unique way in which a Red Giant is dissipating was reported in August of 2007. Using the GALEX telescope (see page 20-4) to view a well known star, Mira, in ultraviolet light, the resulting image showed the fast-moving star to be trailing great quantities of hydrogen gas in a cometlike tail that actuall extends out about 13.7 light years, as seen here:
Another way in which a star can be destroyed is by being torn apart as it approaches a Black Hole. Although extremely luminous conditions called quasars (page 20-6) are thought to be light given off as stars fall into a Black Hole, individual star destruction near a B.H. is seldom observed. However, one such event has been recorded by X-ray telescopes around a galaxy that is 700 million light years away. What takes place is shown in these next two figures, identified parts of which are actual observations but the rest are artist's rendidtions of the detected changes and a reconstruction of the sequence of events in the past and the future - read the captions for details: Having now talked about star types and their histories, let's summarize what is known or extrapolated about the estimates of the percentages of each star type in the Universe. This is difficult for all but the nearest galaxies because most stars cannot be resolved into individuals. A better inventory is available for the Milky Way: Red/Brown Dwarfs = 65% (but most of these, being small and less luminous, have not actually been seen, so this high percentage is an estimate based on theory as to mass distribution); Main Sequence (F,G,K) = 25%; White Dwarfs = 8%; Blue White M.S. stars (O, B, A) = 2%. Of note is the relative rarity of Sun-like G stars (perhaps as infrequent as 2%). Almost all types of the above stars can exist simultaneously in a galaxy. The stars usually are present as single or binary entities light years apart but we have seen that they may also be grouped as open clusters containing tens of thousands of individuals. Some clusters however can have such close-spaced stars that this grouping warrants being called a Super Star Cluster. One such cluster, about 6 l.y. across is Westerlund 1, located in the Milky Way about 10000 light years from our Solar System. It is heavily cloaked by dust but recent IR images have resolved individual stars packed so tightly that their distances are comparable to the size of the radius of the Solar System. Wolf-Rayet, OB Supergiant, Yellow Hypergiants, Red Giant, and Luminous Blue Giant types. In size, some may have diameters 2000 times that of our Sun and shine with a brilliance 100000 greater. The population of Westerlund 1 is estimated to be about a half million stars (most are smaller than the types listed above). The Super Clusters are the densest known array of stars within galaxies. The image below was made in the IR by ESO telescopes; only a few large, bright stars are visible, with the majority small ones below resolution: Because this particular cluster is so close to Earth, and contains such massive stars, whose lifespans are astronomically brief, the assemblage is probably comparatively young in terms of Milky Way history. Such large stars are destined to explode in Supernovae ( page 20-6). Their proximity to Earth may be a cause for concern as a lethal source of particulates and radiation. The numbers, varieties, and history of stars have varied over time back to the Universe's origin from the Big Bang. Stars of all sizes are still forming now but the population of very large stars was much greater in the first few billion years; since these burn up rapidly, their numbers have diminished. Various estimates of the numbers of stars at different past times have been made. Consider these two diagrams; both show relative numbers as a function of age as expressed by redshift:
The upper diagram indicates that the maximum rate of star formation occurred early - in the first two billion years - after the Big Bang. The lower diagram plots two other time estimates for the maximum, one at about 10 and the other about 5 billion years ago. The disparities in age indicate that determining star population history is still wrought with uncertainties.
As the final topic, we consider next star formation in the early Universe - the saga of the first stars. Evidence is building that stars began to form about 100 to 250 million years after the Big Bang - perhaps even earlier. Hydrogen gas that had been concentrating in protogalactic clumps or clouds was at that time hotter than in later gas clouds as the Universe matured. One reason for these is that there was little more than Hydrogen and helium as the heavier elements had not yet been synthesized and dispersed (by supernovae; see next page); such elements lower gas cloud temperatures. Thus, in the first nebulae the stars that form were mainly massive - tens to several hundred times a solar mass. In the early Universe, galaxies were closer together (the expansion had "grown" the Universe only to a few billion light years at most), so that collisions between galaxies were much more common and the galaxies themselves, being smaller, favored more frequent star-to-star collisions.
Early galaxies thus contain many more huge blue stars (O and B) relative to F, G and M stars that make up the bulk of the star populations seen in the developed galaxies we observe today. These Giants burned rapidly, typically after 3 to 5 million years following their compaction into Hydrogen-burners, and were thus short-lived. They were quite luminous and had surface temperatures in the 100000 °K range. Those with less than 250 solar masses destroyed themselves as supernovae; greater than 250 solar mass stars ended up as black holes. Of course, these stars (which fall in the category of Population III, the group consisting of just Hydrogen and some primordial helium as fuel at their start) were short-lived. At present none of these have been observed as individuals but are being looked for in the early Universe. Because of their size, they burn out and explode very rapidly. Thus, they would have existed only in the first billion (or significantly less) years. The HST cannot see star objects that far back in time but the James Webb Space Telescope scheduled to launch in 2013 may be able to detect evidence of their inferred existence. But, enough is known, or seems probable, about the astrophysics of star formation under conditions that likely prevailed in the first millions of years after the Big Bang to be able to model their inception and subsequent history during early times.
Some aspects of star formation were treated on the previous page. A quick overview of this subject is found on this Wikepedia web site. Various mechanisms have been proposed to initiate star formation within dust gas (hydrogen) giant molecular clouds. At the heart of the process is variations in the densities of dark and ordinary matter. Slightly more dense regions draw in gas from the surroundings. The gas nucleates to form the incipient star. This further increases density, drawing in more matter (dominantly hydrogen). As the protostar builds up, gravitational attraction pulls its matter into ever denser shells or zones through which tempeture increases inward. At about a million degrees Centigrade fusion starts to convert the hydrogen into helium. More massive stars heat up to higher temperatures which convert the helium to heavier elements. Such stars eventually blow up. Once large stars form and explode, the shock waves from supernovae (page 20-6), along with explosion ejecta, can exert pressures that cause parts of the remaining star-forming material to be concentrated in clots that grow and evolve into new stars. Stars consist mainly of hydrogen and some helium. Only those two elements were produced directly from the Big Bang (trace amounts of other light elements were also created. The heavier elements up to iron (Atomic Number 26) are created primarily by nuclear fusion; still heavier elements form by several neutron capture processes (see page 20-7 for details). The cores of stars (still gas) attain a range of maximum temperatures depending on their masses and densities. With time the mass consumed by fusion will change so that in larger stars there is a progression of element species creation which depends on the maximum temperatures reached. Hydrogen fusion begins at around 10 million degrees Kelvin (fusion temperatures can be replaced by the energy term "electron volts"; typical temperatures are in the range of millions of eV's). The temperature ranges involved also control the rate of element production. Here are some generalizations of element production as a function of temperature: Oxygen fusion: Silicon, Sulfur; 1 billion K or so; 8Msun
Silicon fusion: Iron; 3 billion K; 10Msun One computer-driven model (using ENZO, a cosmological hydrodynamics code) for star formation has been developed by Tom Abel (Pennsylvania State University ) and colleagues (Gregory Brant, Oxford U. and Michael Norman, UC-San Diego) over the last 7 years. The model divides regions of an opaque, dark matter-rich Universe into cells of varying dimensions. When the program is run, the primordial hydrogen filling this still dark space begins to clot as it seeks to condense. The events leading to the First Stars can be examined by the model over time and at successively higher magnifications (cell sizes cover smaller volumes). Here is a series of computer-generated images (each panel involving the growth of the Hydrogen cloud [first row] as it proceeds into a star; the second and third rows representing later, more detailed looks at the stages involved) that give rise to the initial star, which formed near the center of each Hydrogen gas cloud. See this figure's caption (click on lower right) for description of the information presented. The clouds of cold dark matter (CDM), consisting of the Hydrogen that separates to make these first stars, contained enough star material to produce 100000 sunlike stars. However, the actual star population coming out of a cloud forms fewer, more massive stars. These in a few million years reached an end-stage where they exploded as supernovae (see next page), at a rate (frequency) much greater than later stars in evolved galaxies. In so doing, driven by supernova winds, they dispersed small amounts of heavier elements into space to mix with the pervasive Hydrogen/helium. Thereafter, protogalaxies began to form along lines described at the bottom of page 20-3. This process may have been aided by the Black Holes, which themselves might coalesce, left behind after the supernovae had cleared out much of the star population. These first stars may have been numerous enough to provide radiation that helped to dissociate Hydrogen into a proton and an electron, which is the mechanism that produces re-ionization, after which the early Universe becomes transparent to electromagnetic radiation (including visible light photons), and the Universe lit up with the first stellar bodies that may have existed as star clusters. The Abel-Barnes-Norman First Star model is neatly summarized in the article "The Real Big Bang", in the December 2002 issue of Discover Magazine.
As might be expected, other models for early stars take a different position. One espoused by Kenneth Lanzetta of Princeton University also believes that first stars, almost devoid of heavier elements, formed rapidly and early in cosmic time. But these stars, he proposes, did organize into larger numbers sufficient to exist in actual protogalaxies.
In related models, these first stars that began to "precipitate" out of the hot Hydrogen gases were created within filamentous stringers (especially at crossing nodes), as shown below as an artist's depiction, which were destined to break up into protogalaxies. These, in turn, were then the gravitational attractors for more gas that helped the protogalaxies to develop into spiral, elliptical, globular, and irregular galaxies that began to proliferate after about a billion years, and then to dominate, after 2 billion years or so, the expanding Universe. Star formation was rapid in the first two billion years, when it peaked, and has been steadily declining ever since.
A group at the University of Chicago, led by Jason Tumlinson, has called attention to the likelihood that the second generation of stars may have contained a higher percentage of numerous small (low mass) stars than previously thought that contained significant amounts of the first generation of heavy elements, as shown in this diagram: Note the use of the term "Dark Ages" for the time during recombination into reionization during which we have yet to be able to see the stars and galaxies by sampling EM radiation. The second generation occurred near the end of reionization, at a redshift (see page 20-9) around 7 (at present the oldest stars detected display redshifts just greater than 6). In the Tumlinson model, over the first billion years, nearly all early stars formed as groups within clouds that evolved into the first galaxies. The process of new galaxy formation has slowed with time, so that today fewer new ones are being organized "from scratch". As time went on the temperature reduction in gas clouds that happens as heavy "metals" are dispersed from supernovae has caused increasing proportions of smaller stars so that the population of galaxies has experienced overall increases in numbers of individuals. In this model, the maximum numbers of stars within the Universe has occurred about 1-2 billion years after the Big Bang. As this happened the number of Giants decreased proportinately, as the early ones ended their lives and fewer massive stars were produced. Since this peak, the total number of stars has decreased relatively since the available Hydrogen in the galaxies (including their halos) has been dropping in quantity (no new Hydrogen is created in large amounts). In the future, the majority of remaining stars will be small ones that have long lives. This population history is summarized in the next diagram, with the dashed white line indicating the above model. There is, however, a recently reported competing model which is based on arguments favoring an intense period in early cosmic time (< 1 billion years) of stars of all sizes, with these numbers then decreasing as bigger ones are destroyed and few newer ones are created: In time the bulk of the galaxies that evolved from the clouds of original stars were enriched in Hydrogen HII, but surviving molecular Hydrogen was still available for further star formation. Protogalaxies in the early Universe were more close-spaced and tended to collide to start the growth of the galaxies extant today. As time progressed, the early massive stars exploded in large numbers, much of the debris, containing the heavier elements, were expelled into intergalactic space to mix with Hydrogen and here and there clump into new clouds that evolved into more galaxies. (Gradual enrichment of elements with atomic numbers higher than helium is the norm, since supernovae continue to occur beyond the early days of the Universe.) As we have seen above, the tendency since then has been to gather groups of galaxies into clusters that comprise present cosmic structure. When this chart was made, the actual post-Big Bang time when these first stars began to form was believed to be less than 0.5 billion years. Reliable results from the Wilkenson Microwave Anisotropy Probe (WMAP; discussed on p. 20-9) have moved the inception of star formation back in time to about 200 million years post-B.B. In the beginning there were probably a larger number of stars with masses >25 solar mass than in later times (the Type III stars mentioned above While most stars formed early in the Universe's history, many of the oldest and some of the largest have vanished by exploding or burning out. Some galaxies now have few new stars being formed. When the whole sky is examined and galactic distribution is analyzed, there seems to be a strong tendency for new star formation to be concentrated more in the filaments of galaxies than in the galactic clusters that have grown over time (see page 3a in the Galaxy subsection treated next). As galaxies have aged, the Universe-wide number of lower mass (longer living) stars has increased. Stars are responsible for nearly all the visible light in the Universe. In early eons, this visible light averaged, from all kinds of star types, wavelengths that fell in the blue region of the spectrum. Today, that average visible light radiation spread has shifted to longer wavelengths, producing a blue-green (similar to turquois) light. Of course, individual stars of a range of colors are not of that shade (aren't blue-green), but taken together their numerically weighted sums of all visible wavelengths would be represented by this blue-green value. In time, the average color will continue to shift towards the red and, tens of billions of years from now this will, in fact, be associated with the then dominant star type, the Red Dwarf. The bottom line to the gist of this page is that stars appear to be the most obvious and dominant type of any large single body making up the Universe. But as we shall see on pages 20-9 and 20-10, stars actually comprise less than one percent of the mass of the Universe. But, because of their luminosity, they give the impression that they are the "top dogs" of the Cosmos. They do have importance beyond their seeming low ranking in the mass inventory because they are necessary partners in planet formation - and as far as we are concerned, one of their kind has been the controlling "parent" of our (insignificant?) planet.
