24 May 2011

 

 

V1, the star that enlarged the Universe

 

A collaboration between the Space Telescope Science Institute and the American Association of Variable Star Observers (AAVSO) has produced a series of observations of the variable star "V1", that Dave Soderblom (STScI), who thought up the scheme, defines as "the most important star in the history of cosmology".
This star is a Cepheid variable, discovered in 1923 by Edwin Hubble within what was referred to at the time as the Andromeda nebula (M31). From the already noted period-luminosity relation that characterises this type of variable, Hubble managed to estimate the distance to V1 and realised that it was well beyond the confines of our galaxy, that some at the time thought was the whole Universe. The discovery of a star beyond the limits of our "island universe" and the demonstration of the existence of other such systems (the galaxies), was the greatest revolution that the field of cosmology has ever seen.
STScI and AAVSO wanted to commemorate the discovery of that star, and highlight the importance of Cepheids in general, by making a series of observations with the Hubble Space Telescope to demonstrate the variability of V1 (at the centre of each square above). To decide when to observe the star with the space telescope named after Edwin Hubble, the light curves constructed from 214 observations of the star, made by members of the AAVSO between July and December 2010, were used. This period spans 4 stellar pulsation cycles, each of which lasts about 31 days.
Today it seems rather easy to calculate the period of V1, because we know where to find it, but almost 90 years ago Hubble's job wasn't so easy. The astronomer spent several months in 1923 exposing many tens of photographic plates with the largest telescope of the time, the Hooker 100 inch at Mount Wilson Observatory in California. By the end of October he had identified 3 candidate novae, one of which, by comparison with images taken in the the previous months, turned out to vary periodically in luminosity with a period of 31.4 days.
From this period it was possible to derive the absolute luminosity of the star from the period-luminosity relation. By comparing this with the apparent, observed, luminosity it was then possible to calculate the distance to the star. His estimate was of 1 million light years. By the end of the following year Hubble had found 12 Cepheids in M31, allowing him to modify (even if in the wrong direction) his estimate of the distance to the galaxy. By now it could no longer be referred to as a nebula, with its new distance estimated to be 900,000 light years.

 

by Michele Ferrara & Marcel Clemens

credit: AAVSO, NASA, ESA, and the Hubble Heritage Team (STScI/AURA)