How do scientists determine the age of stars?
I was Googling about the oldest galaxies in the Universe. Everywhere is written that their age is known by the light. Referring to this line "Since light travels at a set speed, if you look at a star ten light years away, you're really seeing what it looked like ten years ago" on this site.
So my question is, as we know light is continuous, how do scientists know that a particular ray has started its journey some 1000 light years back? How is the age of stars determined?
I think you're confusing at least two things - the age of a star, and the time it took its light to arrive on Earth. The first tells how long ago a star was formed, the latter how far away it is.
Also, what do you mean with 'oldest galaxies'? We see the galaxies that are farthest away from us while they are still very young, since their light took about the age of the universe to reach us. In contrast, we see nearby galaxies while they're old, since their light only took a few million years to get here. Yet, both are about equally old (formed at roughly the same time after the Big Bang).
@AstroFloyd you said **light only took a few million years to get here** how can we determine which light is from which galaxy.
Galaxies outside our own can be visually distinguished quite easily, given the right optical equipment. See the deep field images from Hubble.
@SpringLearner .. AstroFloyd is correct. You are asking two different questions here (maybe three). Your first question is determining the age of stars and is nicely answered by Professor Jeffries. Your second one is "How do we determine the distance to stars and galaxies?", to which I would refer you to Wikipedia (Cosmic Distance Ladder)/ ie: light left a star ten light years away ten years ago. Third, if you are looking at articles (googling) that talk about oldest galaxies, they are really talking about "most recently formed" and the James Webb Space Telescope should see a of more of these!
Scientists look at groups of stars to determine the ages. As another poster already said, the HR diagram is the tool used to determine ages. The colors emitted from the stars is also used to determine ages because color is indicative of where the star is on its life cycle.
As the article says, stars are assessed by clusters. The stars formed around the same time and from there scientists determine the age. We have the following going on: brightness, color, and clusters. Size can be used as a determinant, however there are white dwarf stars which are very small, are dim as observed from Earth, yet are very old stars.
Also to clarify-- just because a star is far away and therefore can take thousands of light years for the light to reach is does not mean it's old. The speed of light is simply a measurement of time of travel.
Sorry, but color has very little to do with current age, only potential age. Most of the rest is correct, but that one mistake completely undermines the validity of your answer.