# Two stars with the same mass equals the same luminosity?

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1. Jan 20, 2016

### JoAstro

As the Stephan-Boltzmann's Law says, the minimal change in a star's mass would have a massive effect on its luminosity, but can necessarily two stars with the same mass have the same luminosity?

2. Jan 20, 2016

### newjerseyrunner

Yes, it's entirely possible that two stars of the same mass will have the same luminosity, but mass would be just one of those factors. All type IIb supernovas have essentially the same mass and luminosity.

3. Jan 20, 2016

### sevenperforce

Although it should be stressed that two stars of equal mass will certainly have different luminosities if they are at different places on (or off) the main sequence.

4. Jan 20, 2016

Staff Emeritus
Mass isn't the only factor. Composition and age also are important.

5. Jan 20, 2016

### Chronos

Composition differences certainly complicates luminosity as in pop I vs pop III stars, Age too is a factor . Stellar composition is altered as light elements fuse into heavier elements altering its density profile. Stars that leave the main sequence can drastically change in luminosity [e.g., red giants vs white dwarves] at similar masses. For stars on the main sequence with a similar density profile it would be fair to categorize mass as the dominant force driving luminosity.

6. Jan 21, 2016

### JoAstro

I have two statements and they say as follows:

1. A 3sun star is seen to have a luminosity of 40 Lsun, which is too bright to be from the main sequence.
2. Two stars of the same mass must have the same luminosity.

I know that the first one is partly true but according to the solar-luminosity calculation: L ∝ Msun3.5
L ∝ 33.5 = 46.76 Lsun

But number two?

7. Jan 21, 2016

### sevenperforce

Statement number 2 is woefully wrong.

8. Jan 22, 2016

### Ken G

Why is that too bright to be on the main sequence? It sounds about right for the main sequence, actually, as you get below.
That is approximately true on the main sequence, though deviations also occur for all the reasons mentioned above.
That's why the first one is not very true at all, the star has a brightness that sounds a lot like it is on the main sequence, perhaps modified slightly by one of the abovementioned considerations.