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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?

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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. –  AstroFloyd Nov 17 '13 at 13:58
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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 Nov 17 '13 at 14:04
    
@AstroFloyd you said light only took a few million years to get here how can we determine which light is from which galaxy. –  SpringLearner Nov 18 '13 at 3:22
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Galaxies outside our own can be visually distinguished quite easily, given the right optical equipment. See the deep field images from Hubble. –  Mitch Goshorn May 15 at 2:42
    
@MitchGoshorn Thanks for the link –  SpringLearner May 15 at 2:56

3 Answers 3

up vote 3 down vote accepted

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.

How to Learn a Star's True Age

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.

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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. –  Donald.McLean Nov 19 '13 at 22:30
    
@fractal21 Are you trying to describe the main sequence cut-off approach of determining the age of a population of stars? –  astromax Nov 21 '13 at 4:18

The most straightforward approach is to use a so-called Hertzsprung-Russell diagram: stars of a given mass "burn" in a fairly predictable way, so by looking at a star's spectrum and luminosity one can infer its possible age.

http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram

Wikipedia also has an overview of some additional methods:

http://en.wikipedia.org/wiki/Stellar_age_estimation

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fractal21 and user16653 are more or less right, but not completely.

What you use with a cluster of stars is an isocrone, a line theoretically calculated that gives you where in the HR diagram the stars of a given age are depending on their mass. This can be done because bigger stars start moving on the HR diagram earlier than smaller ones when they became supergiants.

So what we astrophysicists do is to compare a set of isocrones with the HR diagram of a cluster, and we assign to that cluster the age of the best fitting isocrone.

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