How do scientists know the age mass and rotation period of stars?

Answer 1

Stars (apart from the Sun and Betelgeuse) only appear as point sources of light in telescopes, no mater how "powerful" the telescope. This is because they are a long way away. However we can see or Sun rotating by observing sunspots and we can do this similarly with Betelgeuse because it is so big it presents a disk. Similarly we can observe the rotation of pulsars (collapsed stars left after supernova explosions) by the beams they produce and the pulses these beams make as they sweep round during rotation.

With Star mass and age we need to look at the star's brightness and spectrum and obtain a measure as to how far away the star is from us (by parallax). This tells us what the stars surface is made of, it color and how bright is actually is.

We can then plot the star on the Hertzsprung-Russell diagram which will tell us its size/mass and potentially its age.

This is because stars start off fusing hydrogen into helium and once there is no hydrogen left in their cores they start fusing helium into heavier elements and get hotter and larger. This all becomes apparent on the Hertzsprung-Russell diagram and with a knowledge of nuclear physics we can determine the speed at which a star of a given mass will use up its hydrogen.

Huge Stars live only a few tens of millions of years, while small read dwarf stars will survive for a very long time.