Some of the oldest stars in our Milky Way galaxy have been recently discovered by astronomers. Scientists at Georgia State University determined locations and velocities of stars to identify how many old stars are present in the galaxy.
Milky Way is 13.6 billion years old, and is estimated to contain 200–400 billion stars, and probably at least 100 billion planets. Much of its mass also appears to be dark matter and interstellar gas. Some of its oldest stars developed about six to thirteen billion years ago. The oldest known star in the Milky Way galaxy with an accurate determination of its age is about 6000 light years away from the Earth, and is nearly as old as the estimated age of the universe itself.
Stars, just like us, have a life cycle – stellar evolution. It is determined by their mass. The larger the mass, the shorter the life cycle. It starts with birth from collapsing clouds of gas and dust (nebulae). The star then gradually grows in size. Then for low-mass stars, the core collapses, and the outer layers form planetary nebula. The core remains as a dense white dwarf, and eventually cools to become a black dwarf. High-mass stars, however, have another cycle and undergo a supernova explosion. If the core survives after the explosion, it will become a neutron star. If the core collapses, it will become a black hole.
In this study, the scientists focused on cool subdwarfs. Subdwarfs are old, low-mass stars. It is believed that subdwarfs are relics of early star formation, surviving to our days thanks to their small mass: the duration of their evolution exceeds the time of existence of the universe.
The astronomers first observed the stars with the 0.9-meter telescope using an astrometry technique which allows to measure the stars’ positions. This allowed to determine the stars’ distances, motions across the sky and orbiting companions. This method helped to increase the amount of known old stars in our galaxy by 25 percent. The researchers even managed to discover two old binary stars (stars orbiting around their common barycentre) which is not typical for older stars.
“Maybe we can find some ancient civilizations around these old stars,”
The scientists used several methods to identify these old stars. The first one, Hertzsprung–Russell (H-R) diagram (developed in 1913), plots each star on a graph measuring the star’s luminosity, spectral type, colour, temperature and evolutionary stage. After mapping the subdwarf stars, the authors then took a closer look on how fast they move across the sky.
“In my research, I’ve found that if a star has a tangential velocity faster than 200 kilometers per second, it has to be old. So, I can evaluate whether a star is an old subdwarf or not. In general, the older a star is, the faster it moves.” said Dr. Wei-Chun Jao.
The researchers found additional 29 previously unidentified old stars after applying the tangential velocity cutoff and comparing stars in the subdwarf region of the H-R diagram to other existing star databases.
What are other methods to determine distance, age and size of stars? Over time, scientists have come up with several techniques, such as parallax (measuring the angle through which the stars appear to move over a period of time; was first applied in 1838 by Friedrich Bessel); luminosity increase (as stars grow older, their luminosity increases; this works only for calculating stellar age on the main sequence); standard candles (studying relatively to bright objects with a known fixed absolute luminosity); standard ruler (relatively to an object for which the actual physical size is known; light echoes can also be used); standard siren (observing waveform, amplitude of gravitational waves); studying supernovae (measuring photosphere, light curves, etc.); studying star clusters (permits assignment of rough ages to stars present within the cluster); gyrochronology (estimation of age from a star’s rotation period; developed empirically in 2007).
In 2018, results ESA’s Gaia mission which is currently measuring accurate positions and distances for millions of stars in the Milky Way, will make finding older stars much easier for astronomers.
“Maybe we can find some ancient civilizations around these old stars,” Jao said. “Maybe these stars have some planets around them that we don’t know about.”
Want to read more about our reaching out into the cosmos? Try this one where the wonderful Graham Moore looks into the paper work guarding the galaxy.