This installment is the last of a three-part discussion on measuring the vast distances throughout the cosmos. This month the discussion turns to a tool commonly referred to as "redshift." With it, astronomers have been able to estimate distances to far away galaxies and beyond, in realms dominated by galaxy clusters and even "superclusters" — clusters of galaxy clusters.
First a quick recap. Direct distance measurement using "parallax" resulting from the shifting position of the earth cause by its motion around the sun is only good for the closest stars. For other types of stars the relationship between the luminosity (brightness) and temperature (color) help determine distance. Blue stars are hot and intrinsically bright while red stars are cool and intrinsically faint. Thus a faint blue star is probably far away. Variable stars whose brightness changes in a predictable manner can be used to measure distances to star clusters within our galaxy, and to the nearest galaxies in which individual stars can be resolved.
For more distant galaxies (i.e. the rest of the universe) something else is needed.
Spectroscopy is the study of light. It was long known that a prism breaks up sunlight into a rainbow of colors. In the 19th century features in the sun's spectrum were associated with different chemical elements found in stars. But galaxies are faint and their light is spread out compared to bright pinpoint images of nearby stars. However, the combination of spectroscopy with the developing science of photography allowed exposures to be made that were long enough to gather sufficient light on film to take images of galaxy spectrums.
Over a century ago, in 1912, photographic spectra were being made at the Lowell Observatory in Flagstaff, Arizona, by Vesto M. Slipher (1875-1969). He succeeded in detecting stellar chemical signatures in galaxy spectra. It was known that velocity toward or away from an observer causes spectral signatures to shift. A shift toward the blue end of the spectrum indicates motion toward an observer while a red shift indicates motion moving away.
By the mid-1920s Slipher had analyzed enough galactic spectra to conclude that the majority — 40 out of 41 — had redshifts, meaning that the sample of galaxies are for some reason dominated by those that are retreating away from us at various velocities. It was concluded that a relationship exists between a galaxy's recessional velocity and its distance: The faster a galaxy is retreating, the farther away it is from us.
At Mt. Wilson Observatory near Pasadena, California, Edwin Powell Hubble (1889-1953) continued the investigation of galaxies and made significant contributions involving the classification of galaxies as well as their distribution throughout the observable universe. He noticed that galaxies tend to exist in clusters and that the clusters themselves form larger structures termed "super clusters" between which are large essentially empty voids.
A galaxy's redshift value is simply the ratio between its actual spectra to the spectra of a hypothetical galaxy at rest. The puzzle is to convert this raw ratio to an actual velocity from which can be derived an actual distance. Hubble realized that a constant was needed. His "Hubble Constant" could theoretically be used to convert redshift to distance. Hypothetically, if a galaxy at the edge of the universe could be measured, then the Hubble constant could give its distance and velocity and thus provide an estimate for the age of the universe itself.
While a galaxy's redshift can be directly measured, the Hubble Constant has been debated for decades. The distance to a galaxy having a given redshift depends on the value of the Hubble Constant. And because the value of the Hubble Constant is disputed, the distance was usually expressed as a range — for example, "somewhere between X and Y light-years."
One of the Hubble Space Telescope's primary missions is to observe galaxies near the edge of the observable universe in order to better refine the Hubble Constant. That was why the telescope was named after Hubble.
At the time of his death, Hubble was preparing an atlas of galaxies. "The Hubble Atlas of Galaxies" was completed by his assistant, Allan Sandage (1926-2010). I met Sandage in his later years when he was a resident scientist at the Space Telescope Science Institute in Baltimore. He signed my personal copy of the Hubble Atlas and, when he learned that I was simply an amateur astronomer, encouraged me to seriously consider using the atlas for conducting a methodical supernova search.
In upcoming events, the Westminster Astronomical Society (WASI) is having its annual Telescope Buyers Workshop starting at 7:30 on Wednesday, November 9, at the Bear Branch Nature Center (BBNC). Mr. Gary Hand of Hands-On-Optics will present information for consumers considering the purchase of a telescope. The program is free and open to the public. BBNC is located at 300 John Owings Road in Westminster.