We can tell by factoring in how far away they are, relative to the speed of light (we see stars however many light years away they are from us). If we know what stage a star looks like it is in, and factor in how many lightyears away it is, then we can figure out what stage the star is in.
An easy way is to look at their color. “The ages of stars are determined by knowing their current surface temperatures, luminosities and masses.” Surface temperature alters a stars color.
“A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K.”
Astronomers use a Hertzsprung-Russell diagram to determine the stage of life of a star. They gather data on the star’s temperature (color) (x) and luminosity (brightness) (y), then plot them on a coordinate plane. The resulting pattern shows different layers dependent on what type or stage of star they are looking at. Here is an example of a Hertzsprung-Russell diagram:
As you can see, there is a Dwarf group, a Main-sequence group, a Giant, and a Supergiant group.
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