Detecting ancient stars
Stars are found in groups known as galaxies, among which is our Milky Way. They have their own life-cycles – they are born, they evolve and they die. WDs are the typical end-point in the evolution of smaller stars, albeit up to eight times as massive as our Sun. They are among the oldest objects in the known universe. Studying these ‘fossil stars’ could provide vital information about the formation and evolution of our galaxy. However, WDs are characterised by their low luminosities. They are so faint that there are currently few identified WDs. European researchers set out to exploit the incredible technological advancements in stellar observation and experimentation to detect and classify WDs. EU funding of the ‘Unveiling the history of the Galaxy with its fossil stars’ (Galfos) project made this possible. When seen from the edge, a spiral galaxy resembles a stereotypical flying saucer. Stars in spiral galaxies such as the Milky Way are found in three different regions, namely the bulge, disk and halo. The bulge is the central concentration of stars. Surrounding it is a flat rotating disk that contains stars and interstellar matter. Further out is a faint halo of stars. The location of a star in the galaxy structure is loosely related to the star’s age. The disk consists of two different populations: younger, more rapidly rotating stars in the thin disk, and mainly older and more slowly rotating stars in the thick disk. Based on stacked images acquired on a month-by-month basis, scientists measured the relative movements (proper motion) of all stars identified with extremely high accuracy. Proper motion criteria enabled clear definition between thin disk WDs on the one hand and thick disk and halo WDs on the other. Investigators thus developed a list of high proper-motion cool WD candidates and characterised them. The Galfos project identified numerous WDs and analysed them, yielding critical information about the history of our galaxy.
