Can Luminosity Give You a Star's Age?

[chefskitten]

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?

 

[howabout1337]

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.

 

[chefskitten]

Thanks for your input howabout1337! That's what I was thinking, just wanted some confirmation

 

[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?

 

[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.

 

[Drakkith]

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.

 

[Drakkith]

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

 

[Vanadium 50]

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.

 

[Chronos]

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.