Talk:Red giant
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[edit] Question
Is it that all stars go through the 'red giant' stage, and then branch off into white dwarfs, black holes, and neutron stars due to their masses? Or is it that the red giant stage is specific to white dwarfs?
Ahh... I see... after some research, I understand that the red giant stage is indeed a common state for all stars, and the branch off is after this, as to which final state they end in (depending on their masses).
[edit] I question this part...
"As Earth's Sun is of one solar mass, it is expected to become a red giant in about six billion years. It will become sufficiently large to engulf the current orbits of the solar system's inner planets, including Earth. However, because the gravitional pull of the sun will have weakened, it is unlikely that the planets themselves will be engulfed."
Why would the gravitational pull weaken? The same amount of mass will exist as there was before, so as long as the planet isn't actually enveloped, it feels the same strength of gravity... is there something I'm not accounting for here?
- The Sun will have lost a bit of mass during the next few billion years; if nothing else, due to its energy expenditure, and the solar wind. Ben Standeven 20:28, 12 Jun 2005 (UTC)
- When the Sun is about 12 billion years old stronger solar winds will begin to blow, and over a period of a few million years the Sun will shed about 28% of its total mass by losing the outer parts of its envelope to this wind. The lower mass of the Sun will result in less central gravity in the Solar System, and the planets will move slowly outwards in their orbit: the Earth will move out to about 1.4 AU. See The Once and Future Sun Eroica 10:07, 7 November 2005 (UTC)
Why does the sun expand to beome a red giant why does it expand especially when the core contracts.
[edit] Why the Sun expands
I am not an astrophysicist, but I believe the sequence goes as follows:
When you see the visible surface of the sun, you're not seeing the actual place where fusion is happening -- the light you see actually originated in the core. Think of the Sun as like a common frosted light bulb -- although the round glass surface of the light bulb appears to glow, the light really originates from the filament deep within. The filament is very hot; the glass surface (while hot to the touch) is nowhere near the same temperature.
The outer layers of the Sun contain hydrogen, just like the core does, but it's too cool to undergo fusion. Yet.
The core of the Sun is a place of two opposing forces: gravity causes the mass of the outer layers to push inward, and the explosive fusion reaction is constantly pushing outward. But when the available hydrogen in the core begins to run out, there's less of a reaction happening to counteract the Sun's own gravity. The Sun begins to contract a little, squeezing the core tighter.
But that squeezing increases the pressure at the core, pushing the atoms closer together and driving the temperature up -- that's what happens when you squeeze gas into a smaller volume. At some point, the temperature gets hot enough to fuse the waste helium that was produced from fusing hydrogen. So now the core is burning very hot: not only hot enough to push back against the weight of the outer layers, but also hot enough to cause those layers to ignite.
These outer layers are not under the same immense pressure as the core (since they don't have the weight of the entire Sun on top of them), so they are more free to expand. It's like setting off a bomb on the surface of the Earth as opposed to setting one off which is encased in solid rock a mile down: the first bomb will displace a lot of the surrounding atmosphere, while the second one will displace only a small amount of rock.
So the Sun expands. A lot. I think that the ultimate radius is under dispute -- some people seem to think it will swallow the Earth, some think it won't -- but regardless it'll kill off anything on the surface.
Eventually the hydrogen in the outer layers will burn up and, lacking the explosive outward pressure, they'll gradually succumb to the Sun's gravity again. The Sun will contract, only this time there's simply not enough mass to make enough pressure in the core to drive the temperature up high enough to start another cycle of fusing heavier elements. The Sun keeps contracting, growing redder and fainter, like a dying ember.
[edit] Color doesn't appear red
As I understand it, Red giant stars don't look red. They are called red giants because the main electomagnetic output is infrared radiation. To the eye, they still look white. My reference is from the magazine Analog dated 2-5 months ago, but I don't have it here and can't look it up. --Djfeldman 14:10, 26 July 2006 (UTC)
- A red giant would indeed look white, if viewed close enough because its intense light of saturates cone cells. However, viewed from a distance red giants look orangish. For example Betelgeuse looks clearly orange compared to other stars in Orion. But you're right in that red giants (or other class M stars) never look red. There's exception, the carbon stars, which are deep red.--JyriL talk 14:49, 26 July 2006 (UTC)
[edit] How Fast
Once this Red Giant phase starts, how much time does it take to complete? I read years ago, that it bloats to Red Giant in a matter of hours. Now that astro-science is more developed is that still the theory?
[edit] Fiction
*Within the Dune Universe, The Sun is in its Red Giant phase during the events that take place in The Butlerian Jihad. The book being the first of three prequel Novels. Written by Brian Herbert and Kevin J. Anderson. Current estimates require 4.5 billion years for the sun to reach its red giant phase.
The author makes a confusion between the star of the stronghold of the machines, the planet Corrin (indeed a Red Gigant), and Earth. Butlerian Jihad takes places more or less 3000 AD.
--57.68.24.50 09:06, 16 November 2006 (UTC)
- I re-added the bit about the Dune universe because, as you mentioned, a Red Giant is a part of it. Could you fix it up to reflect the concerns you stated above? Thanks! --benwildeboer(talk - contribs) 12:11, 16 November 2006 (UTC)
I fail to see how it is beneficial to the understanding of Red Giants to list every single mention of red giants in fiction in this article. Such trivia sections are little better then internal linkspam promoting other articles which are barely related, if at all. Removing. -00:31, 25 February 2007 (UTC)
- I'm a strong proponent of removing trivia from articles, but I don't think this fits. It's hardly every single mention of Red giants in fiction, but rather three very influential examples of how this stellar phenomena has influenced culture. Superman wouldn't be Superman without a red star. CovenantD 00:45, 25 February 2007 (UTC)
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- The section as it exists hardly qualifies as an explaination of the cultural influnce of red giants. Currently all that section contains is a bunch of disjointed facts that just happen to involve a red giant in some sense. There is no explaination for why the red giant is significant. Does it signify some cultural theme? Strength? Age? Death? (See FBI portrayal in the media for an example of a good "in popular culture" article). If all that the trivia section is going to contain is a random list of trivia then we're better off without it since all it's going to do is encourage every fanboy/fangirl out there to start injecting their own favored novel/anime/game in there. -Loren 00:54, 25 February 2007 (UTC)
[edit] Suspiciously precise
"They are stars of 0.4 - 10,515,478 times the mass of the Sun"
Where does the second number come from that it is justified in being given to 8 significant digits?
