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.
The Hubble Constant (Ho) is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. This long-sought number indicates the rate at which the universe is expanding, from the primordial "Big Bang."
The Hubble Constant can be used to determine the intrinsic brightness and masses of stars in nearby galaxies, examine those same properties in more distant galaxies and galaxy clusters, deduce the amount of dark matter present in the universe, obtain the scale size of faraway galaxy clusters, and serve as a test for theoretical cosmological models.
In 1929, American astronomer Edwin Hubble announced his discovery that galaxies, from all directions, appeared to be moving away from us. This phenomenon was observed as a displacement of known spectral lines towards the red-end of a galaxy's spectrum (when compared to the same spectral lines from a source on Earth). This redshift appeared to have a larger displacement for faint, presumably further, galaxies. Hence, the farther a galaxy, the faster it is receding from Earth.
The Hubble Constant can be stated as a simple mathematical expression, Ho = v/d, where v is the galaxy's radial outward velocity (in other words, motion along our line-of-sight), d is the galaxy's distance from earth, and Ho is the current value of the Hubble Constant.
However, obtaining a true value for Ho is very complicated. Astronomers need two measurements. First, spectroscopic observations reveal the galaxy's redshift, indicating its radial velocity. The second measurement, the most difficult value to determine, is the galaxy's precise distance from earth. Reliable "distance indicators," such as variable stars and supernovae, must be found in galaxies. The value of Ho itself must be cautiously derived from a sample of galaxies that are far enough away that motions due to local gravitational influences are negligibly small.
The units of the Hubble Constant are "kilometers per second per megaparsec." In other words, for each megaparsec of distance, the velocity of a distant object appears to increase by some value. (A megaparsec is 3.26 million light-years.) For example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s.
The value of the Hubble Constant initially obtained by Edwin Hubble was around 500 km/s/Mpc, and has since been radically revised because initial assumptions about stars yielded underestimated distances.
For the past three decades, there have been two major lines of investigation into the Hubble Constant. One team, associated with Allan Sandage of the Carnegie Institutions, has derived a value for Ho around 50 km/s/Mpc. The other team, associated with Gerard DeVaucouleurs of the University of Texas, has obtained values that indicate Ho to be around 100 km/s/Mpc. A long-term, key program for HST is to refine the value of the Hubble Constant.