# Can Luminosity Give You a Star's Age?

by chefskitten
Tags: luminosity, star
 P: 2 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?
 P: 21 http://en.wikipedia.org/wiki/Stellar_age_estimation: "As stars grow older, their luminosity increases at an appreciable rate" So I would assume the more luminous one.
 P: 2 Thanks for your input howabout1337! That's what I was thinking, just wanted some confirmation
 PF Gold P: 6,072 Can Luminosity Give You a Star's Age? 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?
 Sci Advisor PF Gold P: 9,356 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.
Mentor
P: 11,473
 Quote by phinds 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?
The more massive one has a different spectrum. It will be hotter, even if their luminosity is the same.
Mentor
P: 11,473
 Quote by Chronos 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.
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

 Mentor P: 16,097 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.
 Sci Advisor PF Gold P: 9,356 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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