Space Science Short


National Aeronautics and
Space Administration

NASA Headquarters
Washington, D.C.                                               October 1994
______________________________________________________________________________


                             Cosmic Yardsticks

Astronomers gauge the dimensions of space by using "distance indicators"
-- celestial objects with unique properties that allow for their distances to
be deduced.  Reliable distance measurements are a crucial factor in
determining a precise value for the universe's expansion rate (called the
Hubble Constant) which is needed to estimate the size and age of the
universe.  (To calculate the Hubble Constant, astronomers also need to
know how fast a galaxy is moving away from us, measured by spectral
redshift.)

Measuring the distance to a faraway galaxy involves a complicated set of
closely-linked steps.  First, distance indicators within our galaxy are used
as a stepping stone to calibrate other distance indicators in nearby galaxies,
which in turn creates yet another stepping stone to calibrate distances to
even more faraway galaxies. 

The first rung in the "distance scale ladder" can be found in our Milky
Way neighborhood, in nearby open star clusters such as Hyades and the
Ursa Major cluster.  An open cluster is a collection of young stars with a
common motion in space.  Because the Hyades and the Ursa Major cluster
are close to us, their distances can be derived using radial velocity (motion
toward or away from us) and proper motion measurements of member
stars.  This allows astronomers to obtain the intrinsic brightness, or
luminosity, of different types of stars in these open clusters. 

Astronomers then measure the brightness of stars with similar properties in
more distant open clusters.  By assuming that these stars would have the
same intrinsic brightness as their nearby counterparts, a distance to the
remote open clusters is calculated by comparing the apparent and intrinsic
brightness of their member stars. 

To obtain distances to nearby galaxies, astronomers use "primary distance
indicators."  These are objects that can be observed within our galaxy or
have characteristics that can be theoretically modeled.  Examples include
Cepheid variable stars, novae, supernovae, and RR Lyrae stars. 

Two well-defined primary distance indicators, or "standard candles," are
the Cepheids and fainter RR Lyrae stars.  They have a regular variation in
brightness, and the period of this pulsation is closely linked to the star's
intrinsic brightness.  So, if the pulsation period of a star is known, its true
brightness can be deduced.  The distance to the star can then be calculated
by comparing its true brightness with its apparent brightness.

Cepheid variable stars are often used as distance calibrators for nearby
galaxies.  They are very luminous yellow giant or supergiant stars,
regularly varying in brightness with periods ranging from 1 to 70 days. 
This type of star is in a late evolutionary stage, pulsating due to an
imbalance between its inward gravitational pull and outward pressure.

Cepheids are found in remote open clusters whose distances are known
from comparison with nearby open clusters.  It is, therefore, possible to
calibrate these Cepheids with an independently obtained ruler ot yardstick.

In the past, the best ground-based observations have detected Cepheids in
nearby galaxies within 12 million light-years.  However, all galaxies in this
region have motions due to gravitational attraction of neighboring galaxies. 
In order to study the overall expansion of the universe, it is necessary to
reach out to Cepheids in galaxies at least 30 million light-years away. 

Until the recent Hubble Space Telescope observations of Cepheids in
M100, there were no well-calibrated standard candles observable over this
distance.  Therefore, astronomers have been using other kinds of objects,
called "secondary distance indicators," to probe even deeper into the
universe.

Secondary distance indicators, such as planetary nebulae, supernovae, and
the brightest stars are used in galaxies that are so remote that only
prominent objects can be discerned.  (These secondary indicators are
calibrated in nearer galaxies, where distances are known from resident
primary distance indicators, before being applied to more remote galaxies.) 
The galaxies themselves can also be used as secondary distance indicators. 
One widely-used strategy, the Tully-Fisher method, uses a correlation
between the internal motions within galaxies (from radio observations of
cold interstellar gas) with their luminosities.  Another method, the
Faber-Jackson relation, looks at the random motions of stars in a galaxy
obtained from spectroscopic measurements.  These relationships are based
on the fact that a more massive galaxy would be more luminous, and would
rotate faster than a less massive galaxy.