Dr. Aaron Titus | Department of Physics, High Point University PHY1050      Astronomy of Stars, Galaxies, and the Cosmos home | WebAssign | textbook | course calendar

## mass and age

According the mass-luminosity relation, shown below, the luminosity of a main-sequence star depends on its mass to the 4th power.

Suppose that stars A and B are main sequence stars, and Star A has twice the mass as Star B. That means that Star A will have twice as much hydrogen to fuse into helium, compared to Star B. Which star would you expect to last longer on the main sequence? Based on the fact that it has more fuel to burn, you would expect Star A to last longer.

But it doesn't! You see, Star A will have 24 times the luminosity as Star B. That means that Star A will radiate its energy 16 times faster than Star B! As a result, it will not last as long as Star B on the main sequence.

Therefore, the more luminous (and also more massive) stars will "fuse" their hydrogen much faster and will leave the main sequence before less luminous stars.

In the simulation below, you can measure low long it takes for a star to evolve off of the main sequence. Click on a particular star and watch how it evolves. Use the buttons to stop the animation, clear it, or step through one step at a time. You can click on stars or use the slider to pick a particular mass star.

Specifically, try stars of mass 120 solar masses, 5 solar masses, 1 solar mass (our Sun), and 0.63 solar masses. Note how quickly each one leaves the main sequence.

A star of 0.63 solar masses takes a longer time than the age of our universe to leave the main sequence. That's why all stars of this mass are still main sequence stars. (We notice this in globular clusters.) No stars of this mass have yet evolved off the main sequence.

Here is an excellent simulation showing evolution of stars on a H-R diagram.

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