Over the next several years, there were two approaches to building a picture of the structure of the universe, as revealed by the distribution of the stars. Both approaches involved the concept of a “standard candle,” thus establishing the absolute brightness (magnitude) of a star by nonparallax means. Using the absolute and the apparent brightness, it was possible to infer the distance of the star. Both methods were tied to stars whose parallax had been measured, but they allowed the extension of the distance scale beyond the solar neighborhood and the reach of the measurement of parallax.
One approach built on the H-R diagram and developed what became known as “spectroscopic parallaxes.” In this approach, the spectrum of a star was examined and then classified. By the time of the development of this method, it had been shown that spectra gave evidence not only of the temperature but also of the luminosity of a star. For example, it was possible from an examination of a star’s spectrum to distinguish between a main sequence star of spectral K0 (Figure 3.7) and a brighter (giant) star of the same temperature. So, by observing a star’s spectrum, we know about how bright it is if seen from a given distance. By comparing its known absolute brightness with its apparent brightness in the sky, we can infer how far away it is. In fact, the classification approach was sufficiently precise that the magnitude of a star’s absolute brightness could be established within approximately 10–25 percent (a few tenths of a magnitude) and the distance inferred to comparable accuracy.
In 1953, Morgan, Whitford, and Code used this technique of spectroscopic parallax to map the spatial distribution of young clusters of stars around the Sun (Figure 3.8). This analysis shows the distribution of these objects forming three distinct arms, and it was a key step in creating a sense of the spatial structure of our galaxy.
The second set of standard candles emerged through yet another astrophysical spatialization, called the period-luminosity diagram (Figure 3.9). This approach was made possible by Leavitt’s observation in 1911 that the period of a particular class of pulsating stars is an excellent predictor of its average absolute magnitude. This class of stars, called Cepheids after the prototype delta Delta Cephei, is also easy to identify from its distinctive light curve (the change in brightness over the period of pulsation) (Figure 3.10). Leavitt made her discovery by observing Cepheid variables in a nearby galaxy, the Small Magellanic Cloud. Because the distance of these variable stars relative to each other was small in comparison to their absolute distance from Earth, the scatter in their distance did not hide the relationship.
In 1924, Edwin Hubble used this new standard candle and the 100-inch telescope on Mount Wilson to identify Cepheids in the Andromeda Nebula (Figure 3.11), establishing once and for all that it was a galaxy like our own.
Hubble went on to extend his system of standard candles and measures to establish the distances to increasingly distant galaxies. In 1929, Hubble made a discovery that opened up the universe even further and thus began the age of modern cosmology. He found that the further a galaxy was from Earth, the faster it was moving away (Figure 3.12).
This led to the idea of an expanding universe, which in turn led to the “Big Bang” theory—that the universe was once all together, in a single place. With the discovery from this picture of an expanding universe, astronomy moved on to concepts such as the Big Bang, the discovery of quasars and other objects moving at very high velocities relative to Earth. The Hubble relationship became not only a map of space, but also a map of time. More distant objects, because of the finite velocity of light, are being observed at an earlier time in the history of the universe, opening up a window into the evolution of galaxies and clusters of galaxies and the “three degree” background radiation.
However, it is interesting to note that the “Hubble Constant,” the quantity that describes the expansion velocity of the universe, has the units of kilometers per second per megaparsec (a million