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Dgennero
2004-Sep-04, 11:51 PM
I'm using the celestia program and have been comparing the sun's absolute magnitude/brightness to other comparable stars (G2V or G3V).
What amazes me is that the sun is very dim for its spectral class, even some G3 or G4 stars are brighter.
My explanation is that the sun is relatively young, compared to these stars (stars inflate and brighten slowly during their HRD lifetime).
On the other hand: Shouldn't stars that grow older also grow cooler and thus leave G2, if they started as G2?
So why is the sun dimmer than Toliman and other G2 and even G3 stars?

dgruss23
2004-Sep-04, 11:58 PM
On the other hand: Shouldn't stars that grow older also grow cooler and thus leave G2, if they started as G2?
So why is the sun dimmer than Toliman and other G2 and even G3 stars?

As a star ages its core temperature gets hotter and therefore its surface temperature gets hotter as long as it remains on the main sequence. For a star like the sun its core temperature was ~ 7 million K when fusion began and is estimated to be about 15 million K now. When it first formed its energy output was only ~70% of what it is today which leads to the faint young sun paradox - why wasn't the Earth frozen over when the Sun was that much cooler?

Dgennero
2004-Sep-05, 12:05 AM
Three things could explain that:

1.) The sun had more mass, so Earth's orbit was a little closer.
2.) The Earth had a different atmosphere with a stronger greenhouse effect.
3.) There was more internal heat and more volcanism.

dgruss23
2004-Sep-05, 12:26 AM
Three things could explain that:

1.) The sun had more mass, so Earth's orbit was a little closer.
2.) The Earth had a different atmosphere with a stronger greenhouse effect.
3.) There was more internal heat and more volcanism.

Most scientists think that choices 2 and or 3 explain it. Certainly the Earth did have a different atmosphere and a stronger greenhouse effect. There may have been a smaller contribution from #3 as well.

bigsplit
2004-Sep-05, 05:35 PM
[/quote] faint young sun paradox - why wasn't the Earth frozen over when the Sun was that much cooler?[/quote]


How do we know the Earth wasn't frozen over?

Crimson
2004-Sep-06, 05:09 PM
I'm using the celestia program and have been comparing the sun's absolute magnitude/brightness to other comparable stars (G2V or G3V).
What amazes me is that the sun is very dim for its spectral class, even some G3 or G4 stars are brighter.
My explanation is that the sun is relatively young, compared to these stars (stars inflate and brighten slowly during their HRD lifetime).
On the other hand: Shouldn't stars that grow older also grow cooler and thus leave G2, if they started as G2?
So why is the sun dimmer than Toliman and other G2 and even G3 stars?

There may be subgiant contamination of the sample. In other words, some of the stars listed as luminosity class V may really be luminosity class IV.

You do know the famous story of why Marcy and Butler never observed 51 Pegasi? Because it was misclassified as IV when it was really V?

Brady Yoon
2004-Sep-07, 05:51 AM
It might also be that the star's compositions are different.