Jeff Hecht, contributor
What's the most important object in the history of cosmology? It may be a variable star in a nearby galaxy that has played a crucial role in opening our eyes to the true size of the universe.
On Sunday, David Soderblom of the Space Telescope Science Institute in Baltimore, Maryland, will make a case for the star's importance at a meeting of the American Astronomical Society in Boston.
The star appears on a photographic plate exposed by Edwin Hubble on the night of 6 October 1923 with the 2.5-metre telescope at Mount Wilson, better known as the 100-inch telescope. It helped him to establish the cosmic distance scale that brought about an astronomical revolution.
Earlier astronomers had recognised hazy objects they called nebulae - Latin for "clouds" - and found that many of them contained what looked like stars, some of which varied in brightness. They also had discovered that one class of variable stars, called cepheids, cycled between bright and dim over a fixed period that depended on their overall brightness. In the autumn of 1923, Hubble began systematic observations of the Andromeda nebula looking for novae - stars that grew dramatically brighter during outbursts - which had already been spotted there.
"The first good plate in the program, made with the 100-inch reflector, led to the discovery of two ordinary novae and a faint, eighteenth magnitude object which was at first presumed to be another nova," he wrote in his landmark book, The Realm of the Nebulae. Looking at other plates, he found the third star was "a typical Cepheid with a period of about a month". He calculated that its faint magnitude on the plate meant the object was a star and that it - and the rest of the nebula - must be about 900,000 light years away.
Hubble's estimate was almost a factor of three too small, but it was good enough to reveal the scale of the cosmos for the first time, showing that Andromeda was a separate galaxy, beyond the Milky Way. It was a stepping stone to fainter galaxies that are even more distant. Modern instruments have recorded objects billions of light years away, including mind-boggling arrays of galaxies at the edge of the universe. That one cepheid variable star in Andromeda got it all started, giving it an important place in the history of cosmology.
Of course, other astronomical objects have played important roles in cosmology, too. Should the cosmic microwave background radiation, the afterglow of the big bang, qualify as an "object"? It has helped pin down the universe's age and make-up, after all.
There are also objects here on Earth that have played pivotal roles in the history of cosmology. Hubble's photographic plate of Andromeda and the cepheid variable is one. Another is the venerable 100-inch Mount Wilson telescope itself, which opened the universe to us for a generation, from 1917 to 1948. Today, the Hubble Space Telescope in low Earth orbit is our main eye on the cosmos, taking us back almost to the big bang. What else should go on the list? Tell us your ideas.
What's the most important object in cosmology? STScI's Dave Soderblom will tell you at the AAS!