Larger stars, with greater mass, burn their fuel more quickly.
White dwarfs, red giants, yellow giants and protostars all represent stars at various stages of development. From within the hydrogen and helium soup of interstellar gas and cosmic dust, gravity uses its force to form stars over millions and billions of years. Indeed, most stars have been around far longer than human life on Earth. Calculating the age of a star requires an understanding of a star's life cycle and the point at which it ends its main stage of evolution and moves on to the next.
Instructions
1. Identify the cluster in which the star you wish to date is located. Stars do not form individually, but in groups or clusters; within these clusters, they generally form at the same time. The age of an entire cluster, then, is established, not the age of a single star. Star clusters begin as clumps of gas within interstellar gas clouds. These clumps increase in density due to gravity, collisions and neighboring stellar explosions. Gravity continues to collapse and heat these clumps over millions of years. Eventually, these clumps form hot, dense cores and, almost simultaneously, become protostars.
2. Locate a star within that cluster that is at the main sequence turnoff point of its life cycle. The main sequence is a stage of star evolution that occurs after the protostar stage, once a star is fully formed and able to burn enough fuel in its core to offset further collapse. Stars of different sizes, or masses, burn at different rates; larger stars burn their fuel much faster. Thus, although a cluster of stars began its life at the same time, individual stars will end their life cycles at different times. Calculating the main sequence lifespan of a star at the end of that lifespan determines the age of the entire star cluster.
3. Determine the mass and luminosity of a main sequence turnoff star within that cluster. Astronomers have already cataloged the properties of many stars within the known universe. For instance, the sun's mass is 2 x 10 to the 30th power kilograms, equating to one unit of solar mass, M. The sun's luminosity is 4 x 10 to the 26th power watts, equating to one unit of solar luminosity, L. The mass and luminosity of all cataloged stars are a factor of these solar units. For instance, the star Sirius has a mass of 2M and a luminosity of 20L. Vega has a mass of of 2.9 M and a luminosity of 54L.
4. Calculate the main sequence lifespan of a star within the cluster at the turnoff point by using the formula, 10 to the 10th power x M/L, where M is the star's mass in solar units, and L is the star's luminosity in solar units. As an example, because the sun has a mass of 1M in solar units, and a luminosity of 1L in solar units, inserting those factors into the formula, we know the sun's main sequence lifespan is 10 to the 10th power or 10 billion years. Because main sequence turnoff point stars within the sun's cluster have had a main sequence lifespan of 4.5 billion years, we know the sun, too, is 4.5 billion years old.
Tags: main sequence, mass luminosity, solar units, main sequence lifespan, sequence lifespan