Can Luminosity Give You a Star's Age?

  1. You find two yellow stars at the same distance (but not in a binary system!). They have the same mass, but one is much more luminous than the other.
    Which one is older?
     
  2. jcsd
  3. Thanks for your input howabout1337! That's what I was thinking, just wanted some confirmation
     
  4. phinds

    phinds 8,349
    Gold Member

    I don't get it. How would you tell an old modest-sized star from a large younger star, both of which have the same luminosity?
     
  5. Chronos

    Chronos 9,881
    Science Advisor
    Gold Member

    The luminosity of a star is strongly dependent on mass, which is known as the mass-luminosity relationship. While the luminosity of a star does increase somewhat with age, the difference is small until it leaves the main sequence. It would be a wildly unreliable way to determine the age of a star.
     
  6. Drakkith

    Staff: Mentor

    The more massive one has a different spectrum. It will be hotter, even if their luminosity is the same.
     
  7. Drakkith

    Staff: Mentor

    Do you know if this graph is accurate? This seems to be a pretty big change.
    From here: http://en.wikipedia.org/wiki/Sun#Main_sequence

    [​IMG]
     
  8. Vanadium 50

    Vanadium 50 17,445
    Staff Emeritus
    Science Advisor
    Gold Member

    It's at least approximately right. When I studied these things, the sun went off the main sequence a little sooner and more quickly (9.1 BY), but there's a factor of ~2 or so luminosity increase over the course of the main sequence.
     
  9. Chronos

    Chronos 9,881
    Science Advisor
    Gold Member

    When the sun first entered the main sequence, it was about 70% of its current luminosity. Over the next 4.5 billion years, it's luminosity is expected to increase by about 67%. I do not consider that enough of a luminosity difference to be useful to estimate the age of a star.
     
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