I don't think anyone knows how to tell the age of an individual star just by looking at it. At the moment, I think the best that can be done is to estimate the age of a physical group or cluster of stars.
The estimate is based on indirect evidence and a theory of stellar evolution. -- It's sort of like trying to tell how long ago an egg was laid. Just by looking at the outside of the egg, you can't tell. But, if you have (1) a theory that suggests how long it takes for an egg to hatch after being laid, and (2) you look at a clutch of eggs in a nest where one egg has hatched, then you can make a pretty good guess as to how old the clutch of eggs is. -- That's similar to how astrophysicists determine the ages of clusters of stars. (1) ==>>
The theory is this: Stars occurring in clusters are thought to have been formed at the same time. A star begins as mostly Hydrogen.
Under the extreme pressure and temperature at the center of the star, the Hydrogen nucleii interact to form Helium, releasing energy and making the star shine. More massive stars have hotter >centers with greater pressure than less massive stars, so more massive stars "burn" Hydrogen to Helium at a faster rate. As a result, more massive stars run out of Hydrogen fuel earlier than less massive stars. The time for this to happen for a star of given mass can be estimated through calculation. When the star runs out of Hydrogen, the star's interior begins to collaps and heat up, causing the outer part to expand, and the star becomes a Red Giant. There's more to this story, but that's all the theory we need at the moment to answer your question about ages of stars... (2)
==>> The observation is this: Stars in a cluster are of different masses, each observed to have a brightness and a color. More massive stars are brighter (they're burning their fuel faster, remember?), and they appear a bit bluer (that means they are hot; yellow is medium and red is cool). So if you make a plot of brightness as a function of color (temperature), placing a dot on the plot for each star based on its brightness and color, then the stars of a cluster will form a fuzzy line. The plot is called a Hertzsprung-Russell (H-R) diagram. If all the stars of the cluster are still burning Hydrogen, then the line will be more or less straight, running from upper left (massive, hot and bright) to lower right (small, cool and dim). This line is called the Main Sequence. However, some clusters have a kink in the curve of their H-R diagram. The upper part of the curve bends to the right, because some of the more massive stars have become Red Giants (bright but cool and red) and have left the Main Sequence. ==>> So the conclusion is that some of the more massive stars have "hatched" into Red Giants, because they have run out of Hydrogen fuel earlier than the other, less massive, stars in the cluster. From the position on the H-R diagram where the bend takes place, one can calulate how old the cluster must be. +++>>> So, if you find a clutch of chicken eggs, and one has hatched, how old is the clutch? Could you make an "H-R diagram" for chicken egg clutches? What part of your method is theory, and what part is observation? Try it, and let me know what happens! <<<+++

Very good question.The short answer is, it's not easy.


Basically, all we have to work with is the light that the star emits that we can pick up with a telescope. We believe (from a great many observations) that we can estimate the mass of a star based on. The total ("main sequence") lifetime of the star is related to the mass because the mass determines how much nuclear fuel it has available to burn and how quickly it burns the fuel. It turns out that the more mass a star has the shorter its lifetime is. The available nuclear fuel is burned up much faster even though there is nominally more fuel
available.


As an example, if we look at a star like the Sun (G2 spectral type) we believe (after doing some very careful calculations) that it will live for about 10 billion years. We think that the Sun is about 5 billion years old because we have some other evidence, like the age of some rocks on the Earth. For most stars though, if they are in the main sequence, we don't really know how long they have been around. We only really know that they are younger then their max. lifetime but old enough that they have had time to form and become main sequence stars.


If we look at a cluster of stars of different spectral types we might
be able to estimate the age of the cluster. One example of how we can do this occurs if some stars in the cluster have exhausted their nuclear fuel and gone on to the Red Giant phase while others that still have fuel remain main sequence stars. If we find the most massive stars that are still on the main sequence we can guess the age of the cluster by assuming that the most massive stars are getting close to the end of their main sequence lifetime.

The answer to this question goes like an intricate detective story.


Scientists can deduce the age of a star by its brightness and color.


How? Through a long process of deduction:


It turns out that if you sit outside every night and observe thousands of
stars, and make a diagram based on their color and brightness, they follow
a specific relationship: Bluish-white stars are brighter than yellow
stars, which in turn are brighter than red stars.


Now anybody who likes to play with fire knows that a flame that is
bluish-white is hotter than a flame that is yellow, which in turn is
hotter than a flame that is reddish. So you can conlude that stars which
are hotter are also brighter.


Also, a fire that starts out hot will eventually cool off and die out as
the fuel runs out. So stars which are cooler and dimmer must be older
than stars which are hotter and brighter.


But what exactly is "burning" in a star? Surprisingly, the answer comes
from rainbows... You see, if take the light from the sun, and put it
through a prism, you will get a rainbow. If you look very carefully at
the rainbow of the sun you will see that there are certain very specific
colors missing. The only reason these colors could be missing is that
some substance is absorbing it, and very element absorbs certain specific
colors. This pattern of missing colors is like a fingerprint which
identifies the element. In the sun and in stars, missing colors show that
the most abundant elements are hydrogen and helium.


So it must be hydrogen or helium that are burning. How do they burn?
Unfortunately the reason we know how hydrogen "burns" is from building
hydrogen bombs. We know that when we blow up small islands in the Pacific
with H-bombs its because hydrogens fuse together to form helium in a
process called nuclear fusion. So that's why we see both helium and
hydrogen in a star! The hydrogen that has been through nuclear fusion has
become helium.


So this is how you figure out just how old is a star:
1. We know a star's temperature by looking at its color.
2. We know stars must be burning something, because they get dimmer
as they get cooler.
3. We know that stars are made out of hydrogen and helium because of the
colors missing from their rainbows.
4. We know exactly how stars burn: by turning hydrogen into helium through
nuclear fusion. We know about nuclear fusion because we are experts at
building nuclear bombs. A star is just a giant H-bomb!
5. Therefore we can use what we know about nuclear fusion to calculate the
age of a star based on its temperature.

Actually, there is no completely direct evidence for the age of stars, but there is a lot of evidence
based on several assumed models which seem to correlate to each other:
First, the only current model for stellar activity is due to energy provided by nuclear fusion processes.
Such processes have several predictable consequences such as the mass at
which a star will become a white dwarf or create a supernova. Surprisingly, because a star is so warm, the physics of stellar
interiors is thought to be better understood that the interior of our own planet. Many of the processes that complicate the earth's interior canot occur at the supposed temperatures in a star. Confiramtion of these models has come recently in the form of solar seismology -- it is possible to see large scale seismic phenomina by viewing doppler shifts in the gasses that compose the chromosphere. i.e. we can measure the sun's vibration modes and these modes are sensitive to the density and temperature of the material which makes up the stellar interior. These models seem to validate the models worked out for stellar interiors based on fusion as an energy source. So, from such models, when a star is formed,its mass, density, temperature and energy output are predictable and by finding these numbers for a star you can work out how long ago it must have formed -- asuming that the model is correct. Other evidence comes from things like the solar neutrino number (indicating that nuclear reactions do occur in the sun), and the virtual lack of lithium in stars (because it is rapidly turned into other things). Finally, in the case of the sun, there is the fossil record of the earth -- which indicates that the solar constant had been relatively constant for at least 1.2 billion years.
There are some very well written articles on this subject by Willy Fowler and by S. Chandrasekhar, both of whom won Nobel prizes for their work in the theory of Stellar Evolution. A good basic source is the text by George Abell on astronomy. A very nice overview is presented in the book: Stars by Kahler (Scientific American Library #39). Have fun!

There are several clues, based on what we know of the way stars shine. First you have to determine its color - that gives an idea of its temperature. The bluer a star, the hotter it is, and the hotter it is, the faster it burns its fuel - therefor it is on the young side. Then you look at its spectrum; the kinds of elements that are found in the outer atmosphere of a star, which are cool enough to absorb certain wavelengths of light, are important clues to a star's temperature. So, color and chemical composition tell you how hot a star is, and that tells you how fast it is using up its stellar fuel.
Next you need to determine how far away it is, so you can tell its absolute magnitude. This gives you an idea of its size and brightness or luminosity.
Once you have its temperature or spectral class, and absolute magnitude or luminosity, you can use several well-known relationships to tell the rest of the story. From luminosity you can get mass, and since luminosity is just the rate at which a star converts mass to energy, these two pieces of information give you an idea how old the star is. Not an absolute age, but an age relative to the expected lifetime of the sun, which is 10 billion years.
As I said above, the faster a star burns its fuel, the younger it is, and the shorter its lifetime relative to that of the sun. Blue giants are big, hot, burn fast, and all are much younger than the sun. They last several hundred thousand years before becoming red giants, and then going supernova. The sun, we believe, is 5 billion years old now (roughly) and will last another 5 billion before becoming a red giant. It will never get big or hot enough to go supernova. Red dwarfs are old and cool - they are going to outlive the sun.

To really tell just how old a star is, you really need a whole cluster of stars, which are all at the same distance from us. There are many steps you have to take to really determine just how old a cluster is...

My research is not related to astronomy, so I did not know the answer to your questions. However, I was able to find the answers on the web. I want to tell your class how I did it so that they can answer questions they don't know too.


I went to the following web site:


http://www.ask.com/


I typed in the question, "How do you find out the age of a star?"


It gave me a response that looked like:
"Where can I learn about _____ stars?", where there were a couple choices to fill in the blank. I chose "Where can I learn about the life cycle of stars?",and there was a great article about stars.


I hope your class can look up the article and read it, but the answer to your question is that it's actually very hard to determine the age of a single star. You can get a rough idea of a star's age, however, by looking at it's color and intensity. Further, young stars tend to be more active and therefore of a day or a week.