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. 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. This approach was made possible by Leavitt’s observation in 1911 that the period of a particularclass 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). 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, 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 dis- tances to increasingly distant galaxies. In 1929, Hubble made a discovery that opened up the universe even further and thus began the age of modem cosmology. He found that the further a galaxy was from Earth, the faster it was moving away.
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 parsecs). So even in our description of the far reaches of the universe, the distance scale is tied to the distance from Earth to the Sun. This distance scale is an appropriate homage to the process of spatial thinking that led from measuring the diameter of Earth to an understanding of some of the most fundamental properties of the universe.
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