Red Giant Stars

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

Posts: 76
Joined: Tue Nov 06, 2012 1:41 am UTC

Red Giant Stars

Here's an article about one of the largest stars known, almost 4 billion km. in diameter.
Its temperature is about 3490 K.
Also...

VY Canis Majoris also illustrates the conceptual problem of defining the "surface" (and radius) of very large stars. With an average density of 0.000005 to 0.000010 kg/m3, the star is a hundred thousand times less dense than the atmosphere of the Earth (air) at sea level.

http://en.wikipedia.org/wiki/VY_Canis_Majoris

Now, the problem is, increasing density always increases temperature, right? And vice versa, right? So, what's the temperature at the outer layers? And what's the density further in? I'm trying to visualize this. For one thing, what would it be like on the surface of a planet engulfed by this star? It's supposed to burn up, but not very quickly, obviously...or does it?

Are there websites with diagrams on this sort of thing?

One of the what-ifs had to do with extreme boating, on lakes of liquid titanium, mercury, sulfur etc. How does that compare with being on a planet inside a red giant with its purportedly tenuous atmosphere?

Thanks.

PM 2Ring
Posts: 3713
Joined: Mon Jan 26, 2009 3:19 pm UTC
Location: Sydney, Australia

Re: Red Giant Stars

Under terrestrial conditions, that sort of gas density is classed as high vacuum. The temperature of most of the outer region of a red giant's atmosphere is quite uniform, so you can use a simple model to calculate the variation of atmospheric density with distance from the centre of the star.

thoughtfully
Posts: 2253
Joined: Thu Nov 01, 2007 12:25 am UTC
Location: Minneapolis, MN
Contact:

Re: Red Giant Stars

I've always wondered what would happen if the Earth did get swallowed up by the Sun when it becomes a red giant. Its atmosphere and oceans will be long gone, I believe, but the rocky bits are going to be harder: there isn't a ton of heat transfer going on, by conduction or radiatively. I imagine it will continue in a decaying orbit fairly intact for quite a long time.

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.
-- Antoine de Saint-Exupery

LaserGuy
Posts: 4582
Joined: Thu Jan 15, 2009 5:33 pm UTC

Re: Red Giant Stars

gladiolas wrote:Here's an article about one of the largest stars known, almost 4 billion km. in diameter.
Its temperature is about 3490 K.
Also...

VY Canis Majoris also illustrates the conceptual problem of defining the "surface" (and radius) of very large stars. With an average density of 0.000005 to 0.000010 kg/m3, the star is a hundred thousand times less dense than the atmosphere of the Earth (air) at sea level.

http://en.wikipedia.org/wiki/VY_Canis_Majoris

Now, the problem is, increasing density always increases temperature, right? And vice versa, right? So, what's the temperature at the outer layers? And what's the density further in? I'm trying to visualize this. For one thing, what would it be like on the surface of a planet engulfed by this star? It's supposed to burn up, but not very quickly, obviously...or does it?

Are there websites with diagrams on this sort of thing?

One of the what-ifs had to do with extreme boating, on lakes of liquid titanium, mercury, sulfur etc. How does that compare with being on a planet inside a red giant with its purportedly tenuous atmosphere?

Thanks.

Well, it may be quite a bit hotter than you'd expect. The solar corona only has a density of < 1 Pa but is very hot (10^6 K), which has caused a bit of confusion for some time and is still not particularly well-understood. The concept of temperature gets a bit strange at very low pressures, though this shouldn't really be low enough for it to be a thing, IMHO.