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How do I find the age of a star?

  1. Feb 16, 2012 #1
    I'm a yr 12 student doing a full year research assignment and i have decided to find the age of the star, Sirius. I have access to a pretty good telescope and equipment but I am not sure where to start. Suggestions anyone?
  2. jcsd
  3. Feb 16, 2012 #2


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  4. Feb 16, 2012 #3
    As Drakkith has advised this is generally not possible with individual stars, what you could do however is start your assignment on the pretence of finding the age of the star. Then outline the reasons this is not going to be particularly accurate as your main body of work - would still be a good assignment.

  5. Feb 16, 2012 #4
    Couldn't you get a good approximation by its spectra? The more heavy elements, the older the star.
  6. Feb 16, 2012 #5
    I was hoping to do just that and use the Hertzsprung-Russell diagram which requires the luminosity and colour of a star to determine the age of the star. How would I do get this information using a telescope?
  7. Feb 16, 2012 #6
    Thanks for the advice Cosmo Novice and Drakkith the link was very useful thank you
  8. Feb 16, 2012 #7


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    This isn't correct. Stars create more heavy elements through fusion as they age, but these heavy elements remain in the core of the star and don't migrate to the surface. The spectrum only sees the surface of the star, and hence just tells you what the initial composition of the star was when it was born. So stars with more heavy elements were formed more recently, and the amount of heavy elements you see in the spectrum doesn't change as the star ages.
  9. Feb 16, 2012 #8


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    Using stellar evolutionary models, we have a fairly good guess as to the age of Sirius. The presence of its white dwarf companion star was an obvious plus given an accurate mass estimate is vitally important in such modelling. See http://www.astronomy.com/en/sitecor...s/2005/07/The life and times of Sirius B.aspx
    The original paper can be found at: http://arxiv.org/pdf/astro-ph/0507523v2.pdf
    Note that the authors had access to some pretty sophisticated data, and the proximity of Sirius to earth certainly didn't hurt.
  10. Feb 16, 2012 #9


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    The Hertzspurng-Russel diagram does not let you determine the age of the star. It merely shows "the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures." -Quoted from wikipedia.

    This means that if you have a star that is 10 times the Suns luminosity and and was white in color it would be a Main Sequence star. If the star had 100 times the luminosity of the Sun and was very Red in color, it's surface temperature is very cool, meaning it's grown to a large size and is on Branch III, the "Giant" branch.
  11. Feb 16, 2012 #10
    This method [a new one presently limited to certain types of stars] is based on how fast the star is spinning.


    Last edited: Feb 16, 2012
  12. Feb 17, 2012 #11


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    Nice! Never heard of that before, thanks Radrook.
  13. Feb 17, 2012 #12
    Thanks for the suggestion Radrook. Do you think that I can measure stellar rotation with a 10" LX200 SCHMIDT CASSEGRAIN TELESCOPE. It looks like this:
  14. Feb 17, 2012 #13

    This is what they use to make these observations possible:

    So we have:

    1. Hectospec & Hectochelle
    2. Telescope on Mt. Hopkins
    3. Keppler Mission Data

    Links for the SCHMIDT CASSEGRAIN TELESCOPE that delineate its capabilities.

    Observation from the ground without using Keppler will significantly limit the range of stars that can be assessed

  15. Feb 17, 2012 #14

  16. Feb 17, 2012 #15
    Since I cannot find the age of a star individually how would I find the age of a star cluster?
  17. Feb 17, 2012 #16


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    This is something you would use a Hertzsprung-Russell Diagram for. The idea is that for stars on the main sequence, the ones that are bluer, hotter, and more luminous don't "live" for as long as the stars that are redder, cooler, and dimmer. The reason for this is that the hotter and more luminous stars have a larger mass. More mass means a higher central core temperature. A higher core temperature means a higher rate of nuclear fusion. Now, the "lifetime" of a star is defined as the amount of time it spends on the main sequence. The main sequence is defined as the portion of the stellar life cycle during which stars produce energy by nuclear fusion of hydrogen into helium in their cores. If the hotter and more luminous stars have a higher fusion rate, then they will run out of their nuclear fuel (hydrogen) in their cores sooner than the dimmer stars. Therefore, the hotter and brighter stars will spend less time on the main sequence. Once they run out of core hydrogen and fusion ceases, the stellar radii and surface temperatures change. This causes those stars to move off the main sequence on the H-R diagram. They move up and to the right onto the red giant branch (corresponding to becoming brighter and having cooler surface temperatures).

    The net effect of this is that if you assume that all the stars in the cluster were roughly born at the same time, then there will be certain point on the main sequence to the left of which stars simply don't exist (i.e. that portion of the main sequence is missing). The reason is because the lifetimes of stars bluer than this are shorter than the age of the cluster. So stars of those spectral types have already "veered" off the main sequence and onto the giant branch. We call this point on the H-R diagram at which the main sequence cuts off the "main sequence turn-off point." The older a cluster is, the farther to the right the main sequence turn-off point will be (because in an older cluster even the cooler, dimmer, and more longer-lived stars have "died" i.e. evolved off the main sequence).

    So, you can age a cluster simply by looking at where the main sequence turn-off point occurs. The age of the cluster is equal to the main sequence lifetime of the stars at the turn-off point.
  18. Feb 21, 2012 #17
    Thanks Cepheid. So you're saying all I need to do is find the luminosity and colour and apply this to the HR diagram. I only have two questions.
    1. How do I measure luminosity and colour using a telescope?
    2. How do I convert the point on the HR diagram to the age of the cluster?
    Sorry for asking basic questions but there is little information on the internet on how to do this.
  19. Feb 21, 2012 #18


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    Direct measurement of color and luminosity requires expertise and equipment generally more sophisticated than accessible to most amateurs. I would suggest using published values from databases such as Vizier. Padova might be a good source for modelling cluster age, although I'm sure there are others.
  20. Feb 21, 2012 #19


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    What you are asking about is pretty much astrophotography. Head over to cloudynights.com and hit up the forums there, as that site is dedicated to the hobby. If you can't find the information on their forum, just post a thread asking about it and I'm sure someone can give you some links. The basics of astrophotography are required for your project, so even if it seems like some of it doesn't apply to you, it probably does. But be warned, this is probably not something you can learn to do in just a few weeks or even months. Getting used to using a telescope and the associated equipment, along with the night sky, is probably going to take upwards of a year or so. (Based on my personal experience in the hobby) If you have someone who can be there to show you everything would make things go much faster, but if you don't, then it's going to take a while probably.

    As Chronos suggested, getting the values directly from a listing would probably be the quickest and easiest way.
  21. Feb 21, 2012 #20
    Thanks Drakkith. I attended a session for astrophotography last year and there is an expert at the school. So I don't think I will have a problem taking photos. I'm just not sure how to extract this information from the photo. Also can anyone answer my second question from my last post. How do you convert the turn off point on the HR diagram to the age of the cluster?
  22. Feb 21, 2012 #21


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    I'm with Chronos on this one. I think that obtaining the actual data for this project might be somewhat beyond the means of an amateur, and I don't really don't see how it is merely "astrophotography."

    If you have to ask this question, then you didn't understand my explanation of the main-sequence turn-off point. Give it another read and let me know what parts of it aren't making sense. In the mean time, I would re-iterate this point in particular:

  23. Feb 23, 2012 #22
    If I have understood this correctly. (The length from start to turnoff/ the length from start to finish) * lifespan of star class = age of cluster.
    Assuming this is right, why cant i do the same with an individual star?

    Also this is a website that showed how to determine colour;
    This requires measuring the flux which this website says can be easily done with a CCD camera;
    http://curious.astro.cornell.edu/question.php?number=37 [Broken]
    Last edited by a moderator: May 5, 2017
  24. Feb 23, 2012 #23
    see, the way you figure out how old a cluster is by just saying, for example:

    There are no stars to the left of F in temperature in the given cluster. How long do stars *right at the point where there are no more stars* remain on the main sequence? That is how old the cluster is.

    Like, let's say you look at a cluster and you see the image I attached to this post just pretend you're only looking at the main sequence stars. You can see that there are no stars past a certain point on the main sequence. What we can then say is that the age of the cluster is similar to the time that it takes for a star at that point to leave the main sequence.

    So like in this one it turns out I ended up with stars like our sun as the turn-off point. We now say that the age of the cluster is about as old as the time it takes a sun-like star to leave the main sequence.

    age of cluster = length of time that the stars at the turn-off point take to turn off.

    if you just look at any given star, all you see is the luminosity and the spectra and you can also tell how fast it *looks like* it's heading towards you.

    To figure out the age of a star you'd need to know how much fuel it started with and how much fuel it has right now and also the rate at which its using up that fuel. If you could know those things then you could figure out the age of the star. However, all the fuel of a star is in the center of a star, and the fuel that it used to have is in the past and impossible to measure. I think you can figure out the rate at which it uses up its fuel based on its temperature, but that's it.

    I THINK I got that right.

    Attached Files:

    • HR.png
      File size:
      22.8 KB
    Last edited by a moderator: May 5, 2017
  25. Feb 24, 2012 #24
    Thank you SHISHKABOB. I think i understand what you're trying to say but I don't think I will be able to do it within 6 months. On th other hand look at this HR diagram with Sirius plotted on it. If I take the length of the main sequence to be 1 billion years the life span of Sirius the length from the start of the main sequence to the point where Sirius is the age of Sirius is 340 million years. This is reasonably close to what has been listed in this website;

    I'm sure there must be something wrong with what I have done.
    http://outreach.atnf.csiro.au/education/senior/astrophysics/images/stellarevolution/hrgenericsml.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  26. Feb 25, 2012 #25
    The mass of the star puts an upper limit on its main sequence life.

    The metalicity puts an approximate age on the material it formed from (although this method leaves much to be desired in terms of accuracy).

    However, accurately measuring the spectra and inferring the metalicity of one specific star is no trivial task for an amateur astronomer. It might be more doable if you were say, comparing an entire cluster to stars in the disk.
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