what evidence suggests that certain stars are relatively young?

Answer 1

A star that has settled down to hydrogen burning has approximately the same color and luminosity for a long time---billions of years for a star like the Sun. Such stars are said to be "Main Sequence" stars. If you plot the luminosity vs. color for lots of stars, you will find that they lie on a (somewhat curvy) line that represents the Main Sequence. Many stars that are near interstellar gas clouds have a higher luminosity for a given color than Main Sequence stars. This can be understood theoretically---these stars are still emitting energy from their initial gravitational contraction. These are called "Pre-Main-Sequence" stars, and they all lie a bit above the Main Sequence line in a color-luminosity plot. Pre-Main-Sequence stars are only a few million years old, as opposed to a few billion years old for most Main Sequence stars. Inside interstellar star-forming clouds, there are stars that have much higher luminosity than even Pre-Main Sequence stars for a given color. These are even younger and are called Proto-stars. Lots of details about the interactions of these Proto-stars with the star-forming clouds can be studied, particularly at radio and infrared wavelengths.

Answer 2

When a star goes super nova it creates a lot of debris that then begins to form into more stars, or it spreads out too far and becomes a nebula. Very young stars are white, or blue like the hottest part of a flame. As a star ages it gets cooler until it turns yellow, and then red. Also as it ages it gets bigger and bigger from an excess of left over hydrogen. Eventually the star will either go super nova, explode, and create nebula or other stars. The other options are it implodes on itself and creates a black hole, or it just goes out and becomes a white dwarf, which is a mass of frozen helium in a ball.

Answer 3

The age of stars is approximated by measuring the star's mass and intensity, and plotting its evolutionary history. If a star is particularly massive, we know that it is young, because more massive stars burn through their fuel more quickly and may exist for as little as one million years; this is relatively young in an astrological sense. In addition, if a star is metal-rich, that is, contains elements heavier than helium, it must be fairly young as well, because those elements can only form in the fusion reactions occurring in dying, older stars, and so the metal-rich stars must have been born after the older stars died.