# Can Luminosity Give You a Star's Age?

1. Oct 25, 2013

### 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?

2. Oct 26, 2013

3. Oct 27, 2013

### chefskitten

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

4. Oct 27, 2013

### 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?

5. Oct 27, 2013

### 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.

6. Oct 28, 2013

### Staff: Mentor

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

7. Oct 28, 2013

### 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

8. Oct 28, 2013

Staff Emeritus
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. Oct 28, 2013

### 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.