How do you measure the age of stars and the universe?

The ages of stars are determined by knowing their current surface temperatures, luminosities and masses. The masses allow astronomers to compute from the Theory of Stellar Evolution, how rapidly the star can evolve in temperature and luminosity as it burns various fusion 'fuels' in its core. The current luminosity and temperature then pinpoints a particular evolutionary stage in these models. The age of the star is then estimated from the theoretical evolutionary models appropriate to the star's mass. A second important parameter in this calculation is the 'metalicity' of the star which is simply how abundant the elements heavier than helium are in the star relative to hydrogen. Other affects that influence the stellar age calculations are the detailed mechanisms occurring within the star to transport energy from the core to the surface (convection and radiative transport) and the degree of rotation of the star. If this process seems a bit arbitrary to you, you need to understand that the evolution of a star is determined by two processes; gravity and thermonuclear fusion. These are the two, best known and best understood, areas of physics and so stellar evolution is based on very firm experimental foundations drawn from a variety of areas in physics. Debates over the details of a particular star's evolution arise mainly from a lack of detailed knowledge about the element abundances in the core of the star, and the precise mass of the star. You can se how these 'evolution grids' are computed by visiting the Geneva Stellar Evolution Database. There is even a Java, Stellar Evolution Program at Cornell developed by Terry Herter for his undergraduate astronomy students.

The age of the universe is estimated from its rate of expansion at the current time, and using a simple formula from Big Bang cosmology to convert this to an age since the Big Bang. Hubble's constant has the units of kilometers/second per megaparsecs, which has the units of 1/time. Big Bang cosmology says that the age of the universe is equal to 2/3 times (1/H). This means that for H = 50 kilometers/sec/mpc you get an expansion age of about 15 billion years, and for 100 kilometers/sec/mpc you get an age of about 8 billion years. Ultimately, the universe cannot be YOUNGER than its OLDEST stars, and stellar astronomers feel pretty confident that ages for globular cluster stars are between 10 - 15 billion years, with most being older than 12 billion years. THEY would estimate an age for the universe of at least 15 billion years, however, this would require a Hubble Constant near 50 kilometers/sec/mpc which observational cosmologists insist is to small.

The current, 1999-2000 determinations of Hubbles Cnstant based on an extensive 10-year investigation with the Hubble Space Telescope leads to a number closer to 70 kilometers/sec/mpc for an expansion age of about 12 billion years. However, studies of the clustering of galaxies and distant supernovae now strongly indicate that there is a substantial 'cosmological constant' effect which is causing accelerated expansion. The bottom line is that the age of the universe is now near 14 billion years give or take a few billion.

Copyright 1997 Dr. Sten Odenwald

Return to Ask the Astronomer.