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Space Science Short
National Aeronautics and Space Administration
NASA Headquarters Washington, D.C.,
October 1994
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 red shift).
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 or 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.
For more information and pictures, link to
Cepheid Variables in M100
(at the Web site of the STScI).
For more information, link to
HST findings shed new light on the fate of the Cosmos
(in the Web site of Science@NASA).
Recent investigations have found that the expansion rate of the cosmos
began speeding up about five to six billion years ago.
See ARVAL - Hubble Finds Evidence for Dark Energy in the Young Universe.
This means that the "Hubble Constant" is not really constant, but varies over time as the expansion rate accellerates.
Updated: November 19 '06
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