Questions on the Red Giant Stage of Sol

[Apollo7]

Ok so as most of us are aware the sun will become a red giant at some future point in its life cycle. At this stage the sun will expand and reden and very likely engulf mercury, venus and destroy the Earth in the process. Now, I calculated its 215 Solar Radii (or thereabouts) to a distance of 1 AU (which I assign the value 149,597,870.691Km. Anyway, if one gives an estimate the sun expanding out to 200 Radii that would give it a new Radius of 139,000,000 or about .93 AU. Considering the sun is expected to destroy the Earth this would not be far off IMHO. In my simulation in Celestia I assigned the star a spectral type of M2 and a Luminosity type of Ib, since the absolute magnitude came out to be about -3.65.

At this point the Sun would now have a luminosity of about 5904 times Solar. Anyway, am I on the right track here? I'm planning on modeling a post-expansion solarsystem, and I want to make sure the sun is relatively realistic. I calculated the "new" habitable zone of the sun at this point to be around 76 AU. Any comments?

[Don. Edwards]

Apollo7,
76 AU’s sounds too far out to me. You have to keep in mind that yes Sol is going to grow very large but it’s going to be much cooler in temperature. It will put out most of its energy in the inferred versus the white light spectrum. The surface temperature of Red giants is only several thousands of degrees not tens of thousands so the habitable zone should be much closer in. I would take a guess that it would be near Jupiter to maybe out to Saturn.
I think you got your data from a general super giant star and not a red giant. They are very different kinds of stars. Super Giants are in there middle age while red giants are at the end of there life cycle. Look at Betelgeuse, it is absolutely huge but it’s a very cool star compared to our Sun and other super giants. Remember the color of the star always dictates its temperature. Red the coolest as in red dwarfs and red giants and followed by orange, yellow, white and up to blue, the hottest of stars. I could be wrong about the habitable zone but I think if you do a little web searching you find the data you are looking for.
It was my understanding from several astronomy programs on TV that the habitable zone would be near Saturn as they often referred to Titan thawing out and it becoming a possible cradle of new life at this time. You definitely have your work cut out for you.

[ElPelado]

I have a question: Can we put Beguining and Ending dates for stars in their *.stc files??? That way we can make an animation of the sun growing up from its actual size to a red gigant(obviusly it will not be a real-time animation).

[granthutchison]

Apollo7:
Over on another thread I've been assuming a temperature of around 2500-3000K for Frank's red giant, but it looks like you're going for about 3500K? I think that's more plausible, really, and it fits with an M2 giant (luminosity class III, not I, as Don points out). My figures come out to be luminosity 6200, absolute bolometric magnitude -4.72, and a habitable zone out around 79AU.

Don:
The fact that the star's output is in IR makes no difference - infrared is heat radiation, after all, and it is just as effective at raising planetary temperatures.
Supergiants are old stars, too - they're just the giant stage of massive main sequence stars.
Although giants and supergiants are cool, they're also huge, which drives their luminosity high and pushes the habitable zone very far out.

Grant

[ElPelado]

I tryed the beguining-ending star and it doesnt work...

[Apollo7]

Well my surface temp was 3220K Class M2. The Luminosity classing was a guess, so being wrong there is no problem for me.

So we got similar figures, fact is even at 76 AU from a star of this size, the stars angular width is huge, its not as bright as the sun, and most of its energy is in the form of IR radiation. It's apparent mag is about -25.6. Celestia is flakey in a sense here it gives the radius as being 255 Solar Radii and lowballs the luminosity at 2470 Sol. Thats ok though.

It might be interesting to simluate Jupiter and Saturn's orbit, I imagine they would be VERY different places at that point heh.

[granthutchison]

It might be interesting to simluate Jupiter and Saturn's orbit, I imagine they would be VERY different places at that point heh.

Remember they'll have moved outwards to wider orbits because of the mass loss by the Sun.

The luminosity given by Celestia is visual, so it's probably quite good.

Grant

[Apollo7]

Well I realize the mass loss bit, both during the red giant stage, and as the star is sheding its outer layers. Will planets (the remaining ones anway) still be able to orbit the sun once it has reached the white dwarf sage and lost a consierable ammount of mass?

[granthutchison]

Will planets (the remaining ones anway) still be able to orbit the sun once it has reached the white dwarf sage and lost a consierable ammount of mass?

Apparently, yes. See http://www.star.le.ac.uk/~mbu/papers/espconf.pdf.

Grant

[granthutchison]

If you have access to the full text of Icarus online or in your library, Duncan JM, Lissauer JJ. The Effects of Post-Main-Sequence Solar Mass Loss on the Stability of Our Planetary System. Icarus 1998; 134: 303-10 should just about cover everything you want.

Grant

[Evil Dr Ganymede]

Apollo7:
Over on another thread I've been assuming a temperature of around 2500-3000K for Frank's red giant, but it looks like you're going for about 3500K? I think that's more plausible, really, and it fits with an M2 giant (luminosity class III, not I, as Don points out). My figures come out to be luminosity 6200, absolute bolometric magnitude -4.72, and a habitable zone out around 79AU.

Sorry I'm replying so late... been rather busy lately

The spreadsheets I made from the Geneva Stellar Structure Grids, a 1 solar mass star with solar metallicity should untimately end its life as an AGB giant (Asymptotic Giant Branch) with an effective temperature of 3258K, luminosity of 2546.83 Sols, an a radius of 158.61 Sols (0.7362 AU). I make that as a M1.5 II Bright Giant, but don't quote me on the spectral type, it seems there are several schemes that are used for that! I put the 'habitable zone' at 50.47 AU, but that's just from taking the square root of the luminosity.

Definitely not a supergiant though.

Supergiants are old stars, too - they're just the giant stage of massive main sequence stars.

Supergiants are evolved stars, but they're not old - remember that massive stars go through their main sequence very quickly (a few million years at most), so they'd have turned into supergiants probably before lower mass stars have even turned themselves on!

[granthutchison]

Supergiants are evolved stars, but they're not old

Fair enough - we can use "old" in two different ways, so what I wrote was potentially misleading.
My intention was to point out to Don that a supergiant has reached pretty much the same stage in its life-cycle as a giant - if one is "old", the other is, too - it just has a shorter life-cycle, as you've pointed out.

My parents have a fourteen-year-old cat that I'd also call "old" - even though it's turning itself off at an age I wasn't properly turned on.

Grant

[granthutchison]

OK, some useful orbital data from the Icarus reference above, in case you can't access it.
From the other online source I referenced, the rule of thumb is that orbits expand by approximately the same ratio as [initial mass of star]/[white dwarf mass of star] - about a factor of 2 for the Sun.
From the Icarus reference, the solar system remains largely stable so long as the white dwarf mass isn't less than about 0.34 Suns - below that, eccentricities increase dramatically and orbits intersect.
For a red giant that has blown off half its mass, the paper predicted pericentres / apocentres for the giant planets (in AU) as follows:
Jupiter: 9 / 10
Saturn: 17 / 19
Uranus: 33 / 38
Neptune: 54 / 57
So there are considerable, but stable, associated increases in eccentricity.

Unfortunately, they don't give data for the terrestrials under the same conditions - instead they plot the unstable orbits that result when the stellar mass goes down to an (unlikely) 0.32 solar masses.
Where Venus and Earth end up is very dependent on gas drag and tidal torque, but I guess Mars might safely have its orbital radius increased in proportion to the gas giants above.

Pluto is unstable over anything from 0.2 Gyr (high stellar mass-loss scenario) to 10 Gyr (low stellar mass-loss scenario) - it loses resonance with Neptune and is perturbed thereafter.

Grant

[Evil Dr Ganymede]

OK, some useful orbital data from the Icarus reference above, in case you can't access it.

Thanks for that, Grant

Where Venus and Earth end up is very dependent on gas drag and tidal torque, but I guess Mars might safely have its orbital radius increased in proportion to the gas giants above.

I bet the orbits of bodies in the asteroid belt must get pretty jumbled up too... I imagine it'd be quite chaotic there

[granthutchison]

I bet the orbits of bodies in the asteroid belt must get pretty jumbled up too... I imagine it'd be quite chaotic there

Yes - the authors ran some billion-year simulations involving asteroids Ceres, Pallas, Vesta, Hygeia and Martinduncan (Martin J Duncan is one of the authors ). Martinduncan was always unstable. Ceres, Vesta and Hygeia were always stable, and Pallas became unstable when the white dwarf mass was reduced to 0.4 solar masses.

Grant