The Gaia spacecraft has revealed the Hyades star cluster is losing stars in two directions under the influence of the Milky Way's gravitational tug. The discovery confirms our theories about the way stars move within galaxies, and could help us understand the evolution of relatively nearby stars.
If you've spent much time looking at the sky, particularly late in the year, some of it was probably spent gazing at the Hyades, our closest star cluster. Although somewhat overshadowed by the Pleiades, also in Taurus and much more compact, the Hyades have been known since antiquity for the V-shape their brightest stars form with the even brighter Aldebaran. The Sun passes near them in April, and in places where this is a rainy month, the cluster has worked its way into folklore and song as the April Rainers. A century ago, it was the bending of light from the Hyades that Sir Arthur Eddington used to prove Einstein right.
However, like your favorite band, star clusters eventually break up – at least so we assume. Clusters form when gas clouds collapse to become multiple stars initially bound together by their common gravity. As time goes on, however, external influences, particularly from the galactic center, should pull clusters apart. We've seen this process underway with large clusters and swallowed dwarf galaxies, but proof the same thing happens to smaller clusters has been elusive.
Enter the Gaia Satellite. By precisely measuring stars' movements Gaia allows us to chart their journeys, sometimes revealing common origins for stars now widely separated. Dr Siegfried Röser of Heidelberg University used this data to study all the stars within 350 light-years of the Sun, and has revealed two “tails” of stars spreading out from the Hyades.
Like most stars, the Hyades are slowly orbiting the galactic center, and one tail trails behind. The other, perhaps misleadingly named, “tail” stretches in front of the cluster.
The center of the Hyades lies 150 light-years away, and the cluster formed around 625 million years ago. Although we are familiar with the four reasonably bright stars of the V, and several others visible to the naked eye, telescopes have previously identified 724 stars in the cluster, with a total mass more than 400 times that of the Sun.
In Astronomy and Astrophysics Röser reports more than 500 stars that originated within the cluster compose the two tails. Although the stars were all formed at the same time, their different sizes mean they have progressed through the stellar life cycle at very different rates – five stars identified in the tails are now white dwarfs, sometimes known as dead stars.
As time goes on, more stars will leave the cluster, wandering alone through the galaxy, until the cluster dissolves entirely.