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).

That's how I understand it, too. The red giant stage is a branch point, and then, for example, our Sun will specifically branch to white dwarf (bigger mass stars will branch differently). FriendlyRiverOtter 06:20, 8 April 2007 (UTC)

[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 red because they are cool. 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)
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?

[edit] Timeline of Earth may be incorrect

I would like someone to review this for correctness:

Earth's biosphere will be destroyed as the Sun gets brighter while its hydrogen supply becomes depleted. The extra solar energy will cause the oceans to evaporate to space, causing Earth's atmosphere to become temporarily similar to that of Venus, before the atmosphere is also lost.

I understand that the Earth will lose its oceans and biosphere before the Sun becomes a red giant. The Sun is slowly brightening while on the Main Sequence at a rate of about 10% every billion years. By the time the Sun nears the end of its life on the Main Sequence, its increased luminosity will long since have boiled the oceans and destroyed the biosphere. When the Sun becomes a red giant, it won't boil the oceans at that time. Instead, it will become hot enough to melt the rocks. --B.d.mills 04:30, 6 June 2007 (UTC)

You may well be correct. Someone has probably written a paper on this. Finding it would mean scanning the ADS

That show on top is right. You probably got the info from website still uses the old calculations.

[edit] Answers about Earth timeline

Some people may still rely on Planetary nebula and Future of our solar system. They still mention what early calculations. They thought sun will expand 100 times current size swallowing Mercury and Venus and maybe even Earth, even if not; Earth would be hot enough to melt rocks. The previous calculation shows sun would penetrate 90 percent to Earth orbit they suggest the gravitation pull might tuck in the Earth orbit causing the Earth orbit to decay perhaps due to orbiting sun's outer atmosphere.

[edit] Venus' Sun

[1] The new calculation shows all planets but Mercury will escape to a wider orbit. It would be large enough to penetrate Venus orbit but in 5 Gyrs, but it's decide that Venus will not be swallowed up. It will first reach 0.8 AU just slightly past Venus orbit. Because it's loss of mass and gravity Venus will escape to 1.0 AU by then. Instead Venus will just be a hot enough to melt rocks making the surface semi-molten, and it's greenhouse atmosphere will long been gone, and viewing from it's surface you will see sun at daytime filling the whole sky, sunrise and sunset you will see 75 percent of huge sun and nigthtime sun will fill 40 percent of sky. Mercury will completely be incorporate in the sun's inner layer structure for envelopment and it will get eliminated and engulfed, and destroyed. Earth will just be scorch cinder it won't be that hot just oceans and atmosphere will be escape off into space. Yes' it's ocean will evaporate, biosphere will certainly be destroyed, Venus will still be alive since it will escape to a wider orbit. Earth will be slightly cooler but not much. That is a good news Venus will not be swallowed up when sun dies, but the chance to support life is much more impossible. The link regarding to the new calculation I show you it the blue title above. Freewayguy 01:11, 27 July 2007 (UTC)HPShu789194

[edit] Definition of Red Giant!

The text says (11:01, 11 August 2007 (UTC)):

According to the Hertzsprung-Russell diagram, a red giant is a large non-main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler giant stars. Examples include Aldebaran, in the constellation Taurus, and Arcturus.

This is ... no good! (politely said). A red giant burns helium to carbon. Said: Rursus ☻ 11:01, 11 August 2007 (UTC)

That was wrong too! Red Giants are swelled-up stars that burns H in a shell, and increases its core pressure and temperature. When fereretrerererere (sorry) ... when the helium core burning starts the star is now a horizontal branch giant. I'll fix it as soon as I'm able to, if nobody is faster than me. Said: Rursus ☻ 13:28, 11 August 2007 (UTC)
Now fixed by enhancing this red giant:ing to also include such swelling shell H-burners. With reservations for errors and ambiguous meanings of "Red Giant". Said: Rursus ☻ 14:54, 11 August 2007 (UTC)

It is not that simple. Stars with solar mass become red giants twice, once burning hydrogen in a shell (the RGB) and again (after a period of core helium burning on the so-called horizontal branch) burning helium in a shell (the AGB). The defining features of a red giant are low effective temperature (they are red!) and a large radius (they are giants!) Timb66 22:56, 11 August 2007 (UTC)

Good note, we'll see how we can include that information. That Red Giants come in RGB, AGB and Horizontal Branch (Red (?) Giant) is something it would be nice to pinpoint, but it starts to become a little complicated there, so maybe that should be second sentence, or some such... ? Said: Rursus ☻ 08:37, 13 August 2007 (UTC)
I'll take a tour reading some before "messing" any more with the text. One problem is the "redness", below what Teff is the star "red"? Are there ambiguous definitions around? Etc. ... the usual encyclopedian linguistic questions. Said: Rursus ☻ 09:03, 13 August 2007 (UTC)
My usual mumbling, here:
RGB stars are objects with masses lower than ∼2.0 M⊙ (the precise value depends on the initial chemical composition), which develop electron degenerate He-cores after the end of central H-burning, surrounded by a thick H-burning shell (thickness of the order of 0.1 M⊙ ) and a convective envelope whose chemical composition is the initial one. The envelope temperature gradient is to a large extent adiabatic, apart from the most external layers, where it becomes superadiabatic, and must be treated according to some prescribed convection theory.
This is a concise definition, but it is for RGB stars: in essence:
  1. the mass is < 2 Msun,
  2. the core is degenerate He,
  3. a H-shell contributes energy,
  4. the envelope (atmosphere) boils like boiling water (but gas of course).

One slight correction to what you written here: stars on the horizontal branch (which is basically the main sequence for core helium burning) are not red giants. The HB phase of a star comes in between the two red giant stages. Timb66 10:26, 13 August 2007 (UTC)

That sounds reasonable, and that was one of my main troubles, the other ones being:
  1. Is red giant ever "defined", or discussed by a "prototype" consensus, the latter being the usual case in a language.
  2. If the temperature limits are 2,500 to 3,500 °C, as from David Darling, then what to do about early C-R who matches late G-stars?
  3. Only the carbon stars are actually red, the rest being orange or maybe yellow, what to do about the S-stars?
  4. Some pair of colliding wd:s are theorized to become a red giant, what to do about them?
The solution would be to go back to the discoverer, then whome of Hertsprung or Russell? Or maybe Cannon or someone else? Said: Rursus ☻ 13:27, 13 August 2007 (UTC)
It's like the definition of planet – it is straightforward until you have to actually do it. Said: Rursus ☻ 13:29, 13 August 2007 (UTC)
OK, doing a few hours survey on Hertzsprung, Russel, red giant descriptions and so on, the best we can do for now is to skip an outright definition of "Red Giant" (which might be a bad thing): instead,
A "prototypic" "red" giant is very inflated and has a low surface temperature, while at the same time a high luminosity. (Yellow orange to red in color). That's some kind of universal theme of all texts writing about "red giant stars". (There are no strict limits of definition for "red giants"). To this category of stars are generally regarded stars of K and M spectral types, the rare S class stars, and also the carbon stars, of the spectral types C-R and C-N. The inflatedness is due to the electron degenerate cores.
To the red giants belongs stars in the Red Giant Branch (RGB) that have ceased "burning" hydrogen in the core, and now instead "burns" hydrogen in a shell. Red giant stars belonging to the category Asymptotic Giant Branch (AGB) instead have ceased burning "helium" in the core, and so burns it in a similar shell. RGB and AGB stars occupy almost the same place in the HR diagram, the AGB diminutively above the RGB:s. There are theories about other astrophysical mechanisms for some red giant types, for example the R CrB stars .........
Soon... Said: Rursus ☻ 18:04, 13 August 2007 (UTC)
Done. Please criticize as fitting. Said: Rursus ☻ 18:41, 13 August 2007 (UTC)

[edit] More to do on article

More todo:

  1. some more about different red giant types, AGB, RGB, possible wd merger cases, variability, Barium stars (not!) and other unnatural critters, with links to their respective article,
  2. examples of red giants, luminosity class shall be III (so not the superdupergiant Betelgeuse!),
  3. rewrite the rest of the article – there's something odd written about Horizontal Branch vs. Red Giant, which was certainly correctly thought up, but is so garbled that it's hard to see the logic...
  4. add a HR diagram, built from Image:HR-diag-no-text-2.svg, making a ring around the area of typical red giants.
  5. planet munching, masers and suspected sub-photosphere giantplanets
  6. restructure and rename the so called overview section, (since the first 3 §s are the true overview), it's not that far from good, but it's some little unclear, and may profit from sentence polishing, so that it makes a coherent whole message,
  7. stress that horiz branch giants aren't red giants, but what about the red clump?

Said: Rursus ☻ 20:21, 13 August 2007 (UTC)

[edit] For high schools?

This article needs a complete re-write if it's to be used as a high school reference text. The writing isn't even at newspaper level. Example: "Red giants evolve from main sequence stars with masses in the range from about 0.5 solar masses to somewhere between 4 and 6 solar masses." Does anyone else find this wordy and redundant? 68Kustom (talk) 02:25, 10 February 2008 (UTC)

I agree that it is a bit wordy and redndant and that it needs to be proofread a bit to be considered a "high school reference text" 11341134a (talk) 00:35, 19 March 2008 (UTC)

[edit] Sun as a red giant

A new paper has been published: "Distant future of the Sun and Earth revisited" by K.-P. Schröder and Robert Connon Smith [4] and also available on astrop-ph: [5]. Somebody might like to include this info (sorry, I don't have time). Timb66 (talk) 01:17, 29 April 2008 (UTC)

I have went through article and fix the informations. When sun turns yellow-orange, Earth would have evolve with runaway greenhouse effect.--Freewayguy Talk Contribs 00:33, 4 June 2008 (UTC)

[edit] First sentence

"...with nuclear fusion going on inside a shell inside the core but not in the core itself." If nuclear fusion is happening inside a shell inside the core, then nuclear fusion is happening inside the core (Transitive relation). Have I misunderstood? Is this (confusing) detail even necessary in the first sentence of the article? At this point in the article, I don't know what's meant by "shell" or "core", so the point is lost anyway.--Jmjanzen (talk) 19:43, 30 May 2008 (UTC)

It was wrong. I have removed from the lead. In fact, red giants have a core of helium (formed from earlier fusion of hydrogen) that is surrounded by a shell of mostly hydrogen in which fusion is occurring. In some red giants (AGB stars), the helium core is also undergoing fusion (into carbon). Timb66 (talk) 12:45, 4 June 2008 (UTC)