Measuring Distances to Galaxies
29 June 1992Using NASA's Hubble Space Telescope (HST), an international team of astronomers has taken a major first step in redetermining the expansion rate of the universe. This rate, known as the Hubble Constant, is one of two critical numbers needed for making a precise determination of the size and age of the universe.
These results are being reported by Drs. F. Duccio Macchetto, Nino Panagia, and Abhijit Saha of the Space Telescope Science Institute, Baltimore, Maryland; Allan Sandage of the Carnegie Institute of Washington; and Gustav Tammann of the University of Basel, Switzerland; at the international workshop, Science with the Hubble Space Telescope, being held in Sardinia, Italy.
Utilizing HST's Wide Field and Planetary Camera (WFPC) in WF mode, the team found 27 Cepheid variable stars in a faint spiral galaxy called IC 4182, located 16 million light-years away in the northern sky constellation Canes Venatici. Only once before have Cepheids been found in a more distant galaxy (M101, located 23 million light-years away). "The few Cepheids found in M101 with ground-based telescopes were unusually bright and required an enormous effort over many years," says Macchetto. Only Space Telescope can make these types of observations. Cepheids are too faint and the resolution too poor, as seen from ground-based telescopes, to separate the images in such a crowded region of a distant galaxy.
Cepheid variable stars rhythmically change in brightness over intervals of days (the prototype is the fourth brightest star in the constellation Cepheus). Early in this century astronomers found a direct link between a Cepheid's pulsation rate and its intrinsic brightness. Once a star's true brightness is known, its distance is a relatively straightforward calculation because the intensity of light drops off at a predictable rate. Though Cepheids are rare, they are very reliable "standard candles" for estimating intergalactic distances.
The galaxy IC 4182 was chosen as a target for a Cepheid search because it is the site of a type Ia supernova explosion which occurred in 1937. Type Ia supernovae are thermonuclear explosions that may occur in systems containing a pair of white dwarf stars. Models predict that all supernovae of this type should reach approximately the same peak brightness, as if all light bulbs manufactured in the world were exactly 60-watts.
Like the Cepheids, type Ia supernova can be reliable "standard candles" - but only if astronomers accurately know their true intrinsic brightness. Type Ia supernovae are more useful than Cepheids because they are much brighter and can been seen out to far greater distances. These supernova are the next "rung" in a "ladder" of techniques for estimating cosmological distances.
The problem is that astronomers have been uncertain about the absolute brightness these supernovae reach at maximum. By accurately determining the distance to IC 4182 using Cepheids, astronomers can calibrate the intrinsic brightness of the 1937 supernova. They can essentially now "link together" two rungs in the cosmological distance ladder.
Since type Ia supernovae can be seen 1000 times farther that the Cepheids, they can be used to accurately determine large cosmological distances. This is a critical step in refining the true value of the Hubble Constant, first developed by the American astronomer Edwin Hubble in 1929. Hubble found that the farther a galaxy is, the faster it is receding from us. This "uniform expansion" effect is strong evidence the universe began in an event called the Big Bang and has been expanding ever since.
The Hubble Constant is an estimate of the rate at which the universe is expanding and is expressed in kilometers per second per megaparsec (3.26 million light-years). The Hubble Constant is one of two critical numbers needed to determine the intrinsic curvature of space and the fate of the expansion. The other number needed is the mean density matter in the universe, or an independent verification of the age of the universe. Previous estimates for the Hubble Constant vary by a factor of two (50 vs. 100 kilometers per second per megaparsec).
Using the absolute calibration of this single type of supernova in IC 4182, the researchers yield a range for the Hubble Constant of between 30 and 60 km/sec/Mpc. The most probable value is in the middle of this range, yielding a value for the Hubble Constant of 45 km/sec/Mpc, which implies a minimum age for the universe of 15 billion years.
The astronomers plan to narrow this range for Hubble Constant by detecting Cepheid variable stars in other galaxies that have had recent type Ia supernovae as well. These observations will be made next year with HST.
When a second generation WFPC is installed in HST during a Space Shuttle servicing mission in 1993, the astronomers expect to detect Cepheid Variables out to the Virgo cluster of galaxies, estimated to be 60 million light-years away.
The ultimate goal is to use HST to refine the scale of the universe to within ten percent.