HE 1523-0901

HE 1523-0901 is the designation given to a red giant star in the Milky Way galaxy approximately 7500 light years from Earth. It is thought to be a second generation Population II, or metal-poor, star ( = −2.95). The star was found in the sample of bright metal-poor halo stars from the Hamburg/ESO Survey by Anna Frebel and collaborators. The group's research was published in the May 10 2007 issue of The Astrophysical Journal.

The star's age, as measured by ESO's Very Large Telescope, is 13.2 billion years. This makes it the second oldest object yet discovered in the galaxy, and nearly as old as the estimated age of the universe itself (13.7 billion years as measured by WMAP). HE 1523-0901 is the first star whose age was determined using the decay of the radioactive elements uranium and thorium in tandem with measurements of several neutron capture elements. It is believed to have formed directly from the remnants of the first-generation stars that reached the end of their longevity and exploded as supernovae early in the history of known matter.