Early quiet sun theory

[K.I.L.E.R]

My information from:
http://science.slashdot.org/comments.pl?sid=440610&cid=22286820

If stars that are young run hotter than older ones, then how is it that an early sun could be dimmer?

I assume that even though the stars are hotter when they are younger, they are also smaller and the radiation requires more distance to travel, therefore losing more energy along the way before it hits Earth. Is my assumption correct?

 

[Fred]

The sun was dimmer in the past, and has gradually brightened over the course of life formation on earth. This is known pretty accurately based on stellar formation models.

The paradox I gather (im not an expert) is that this contradicts paleontology records, which suggest that their was more radiation early in the earths life.

I gather the resolution is simply that the atmosphere was less well formed and more transparent to stellar radiation than it is today (less ozone). So even though the sun was cooler back then, the amount of radiation that hit the earth was larger.

 

[Davros]

little theory right off the top of my head (no doubt wrong)
today more of the sun is helium,
helium is more dense therefore the hydrogen is under more pressure burning hotter

 

[Frank]

Helium isn't fused yet. If that starts to happen, the Sun will explode (very slowly). But it's true, that there is a growing core of heavier helium, and that the radiation of the hydrogen fusing around it is what is pushing the Sun outwards.


If the radiation makes the Sun expand too much, the pressure needed to fuse the hydrogen reduces, which makes the Sun compress, which results in higer pressure, etc.

If there isn't enough hydrogen left, the Sun will compress as far as needed to start burning helium, which makes it expand into a red giant, and swallow the Earth. If the Sun was more massive, it would explode faster.

 

[nutball]

stars that are young run hotter than older ones

What gives you that idea?

 

[K.I.L.E.R]

What gives you that idea?

I read it in a few different books when I was 9-12 years old.

 

[ShootMyMonkey]

I read it in a few different books when I was 9-12 years old.

Perhaps you're confusing it with something about stars that die young? Or perhaps something about smaller stars? Another possibility that I wouldn't be too surprised by would be that you generalized something about some individual case about a certain development pattern to involve a star that even in its young stages is significantly hotter than the sun (which is a standard benchmark for us Earthlings).

Even otherwise, your assumption about the distance radiation has to travel would only be part of it. There's also the fact that "burning hotter" is only an indicator of energy *density*, not of *total* energy. A star that is not quite as hot but extremely large can easily be greater in apparent brightness even if they're further away than some other smaller hotter star.

 

[K.I.L.E.R]

Interesting. Thanks.

 

[Gubbi]

Even otherwise, your assumption about the distance radiation has to travel would only be part of it. There's also the fact that "burning hotter" is only an indicator of energy *density*, not of *total* energy. A star that is not quite as hot but extremely large can easily be greater in apparent brightness even if they're further away than some other smaller hotter star.

Temperature is important though as the energy in a black body spectrum scales with T^4.

For example compare Rigel to Betelgeuse. Betelgeuse is ~700 times larger than Rigel by volume. That means it has 80 times the surface area. However the surface temperature of Betelgeuse is 3500K and 11000K of Rigel and subsequently both have around the same luminosity.

Cheers

 

[Albuquerque]

Now I'm going to drag my US high school-level of thermodynamics and luminosity into this discussion with an obvservation and subsequent question:

Burning hotter does not always mean "brighter" in the visible spectrum. A blue flame is quite hotter than a yellow flame, but the hotter blue color will also not "light up" a room nearly as well as a cooler burning yellow flame.

So, when we're talking about star luminosity versus surface temperature, am I to assume we're still talking about human-visible spectrum? And if so, why doesn't this work the same way within an oxygen environment (aka, in my house with an oxyacetelene torch?)

 

[Gubbi]

Burning hotter does not always mean "brighter" in the visible spectrum. A blue flame is quite hotter than a yellow flame, but the hotter blue color will also not "light up" a room nearly as well as a cooler burning yellow flame.

I was referring to bolometric luminosity (total energy radiated).

And as a curious matter of fact, black body radiation at any given frequency monotonically increases with increases in temperature. So your blue flame *does* dump more energy in the visible spectrum (even the red part of the visible spectrum) than you yellow flame (if they are the same size).

Cheers