I assume that by now you have all seen that diagram that shows the relative size of the Earth to the the Sun, the Sun to another, bigger star, until you get to this giant star 20,000c bigger then the sun.
What would life be like on planets around that sun?
warm and bright, i would imagine.
Depends. How far away from the star are you? How hot is the star burning? Bigger stars can be cooler than the Sun.
The biggest difficulty is that large stars live fast, die young, and leave a beautiful corpse. Whereas our Sun has lived for about 5 billion years, and is expected to have about that much more, very large stars can live for as short as a million years or less. Life does seem to have formed very quickly on our world, but we’re still talking hundreds of millions of years when we say “quickly”. So it seems likely that life would never have a chance to appear in the first place around a giant star.
I recall from one of Asimov’s essays that you couldn’t very well use a planet of Betelgeuse for life in realistic science fiction. Sure, you could have a planet so far from its center that it would still be a couple of hundred million miles from the surface (on the average, as I will explain). And, for that matter, the surface of a red supergiant star is far, far less bright per square unit than our own sun (and type G stars in general). The main problem is stability. Such a star is constantly pulsating, since it is a variable star. You can’t really have an orbital distance that would consistently compare to Earth’s orbit around our Sun. Not only that, but Betelgeuse continually blows off gas, in a way our own star can’t begin to compare with. This would add to the problems with climate.
BTW, your figure of 20,000 x Sol seems a little high. I recall the extreme being about the size of Saturn’s orbit. If so, it would be considerably smaller in diameter than TWO thousand x Sol. More than 10 times smaller than your round figure.
Unless infrared stars can get that big. I don’t think much was known about them when I was in college, studying Physics and Asronomy, and wolfing down every easy-to-read Asimov essay and article.
Life wouldn’t have time to evolve around a giant star, but let’s say some terraforming was done to make a suitable planet “briefly” (~1 million years) habitable.
If you’re talking about a mainstream star which is larger, hotter and brighter than Sol because it’s more massive, then to be habitable a planet orbiting such a star would have to further away. A hot star like Sirius or Rigel would look like a tiny, blindingly bright welder’s flame- in fact one problem would be how sighted animals could avoid burning out their retinas by an accidental glance at such a light source. Perhaps a perpetually clouded planet would be a necessity.
If you’re talking about stars that have left the mainstream and are giants or supergiants because they are in the latter stages of stellar evolution, then there are several factors to consider. One is increased variability, as mentioned upthread. If you had a star that had been white or blue-white when on the mainstream, then on the way to red giant stage it might pass through a point where it’s surface was about as hot as Sol’s- a “yellow giant” or “yellow supergiant”. Provided it’s planets were sufficiently far away, it would have the same surface brightness and angular diameter as Sol, and that wouldn’t make much difference. If the star in question had ballooned to red giant stage, then to give enough light and warmth with it’s lower surface temperature, the star would have a larger angular diameter as seen from the planet. You would see a red orb dim enough to look at comfortably taking up a much larger portion of the sky.
One interesting quesiton would be if a star’s ecosphere (the distance life could comfortably exist on a planet at that distance) could be large enough that more than one habitable planet could orbit stably in such a system. Some people think even Venus and Mars could have made the cut if factors were more favorable, and a larger star gives more room.
Bolding mine
Assuming that a terrestrial planet could form around one of those stars, would there even be enough time for the materials to coalesce, much less form a place cool enough for life? And a related question: What about a gas giant? My WAG is that they would grow faster than a rock, but would they be a recognizable planet before their star goes boom?
Iota Draconis b provides an example of a gas giant around a giant star. HD 149026 b has a relatively large metallic core, and it orbits a yellow subgiant, substantially larger than our sun.
I don’t think there are any planets you would describe as “terrestrial” orbiting giant or supergiant stars, really, but you have to throw out most of your preconceptions of what a planet is like when dealing with this sort of thing. Take Gliese 436 b, for instance, which has a water core that’s hundreds of degrees - yet frozen, because of the high pressure from gravity. Scalding ice forms the surface of the planet.
So there may be planets orbiting giant stars which aren’t necessarily terrestrial but still can let life form - the possibilities can be so fantastic, you’d never be able to predict them. And life is similarly fantastic; organisms surviving around deep-sea vents without any access to light are a good example of how life’s resiliency can shock us and go way past our preconceived notions of what’s habitable.
I was thinking more along the lines of a star like Rigel or supergiant things on up. The ‘live fast, die young’ variety. (I think I understand the Wikipedia on supergiants but please correct me if I’m wrong.) I don’t have a problem seeing how a planet could form around a star that’ll be there for a couple hundred million years or more, or planets around stars that grew giant with age. (It was my impression that the extra-solar planets we’ve found so far orbit main sequence stars, but I don’t really know.)
I think I can state my question better as: Is there enough time for either a terrestrial or gaseous planet to form around a star that only has a few million years to live? And would the mass that they throw off significantly affect the process?
That being said, Gliese 436 b sounds, well, not fun, but Dante-esque in a cool way. 
The first exosolar planet found at all was around a pulsar, which is about as far from the main sequence as you can get. The original star must have been significantly larger than the Sun, and therefore lived for a shorter time, to have formed a neutron star, and the planets must have formed before the supernova, but I don’t have the exact numbers readily handy.
I read that as 20,000 times bigger, not wider. Wiki gives Saturn’s orbital radius as somewhere between 1.3 and 1.5 million km, which by google gives a volume 8 billion times greater than the sun. Turns out, that’s not a bad estimate - Wiki also has a list of largest stars (of course), topping out at 1800-2100 times wider than the sun (that’s 5 million - 9 million times bigger, in case you’re wondering).
Yeah, I got nuthin’ about life in such a system.
I seem to recall that it is possible to have a habitable planet orbiting a red giant (an old star), but in order to be in the habitable zone the planet would be tidally locked i.e. having one face to the sun at all times, so it would be hot on one side and cold on the other.
I think it depends on what exactly is being asked here.
Life on a planet 1 A.U. from the center of a small red giant wouldn’t be. That is, the *insolation *-- yeah, that’s spelled right, and it means the total amount of energy received from a sun – would be so high that life would not have evolved, or if it evolved prior to the star going red giant, would likely have gone extinct. Temperatures would be in the triple digits Celsius.
A planet around a red giant or supergiant that received the same insolation as Earth would be a long way off from it. I don’t have the math to identify exactly how far out, but “many A.U.s” would be accurate. And what the star would look like in the sky would of course be a bit more spherical trigonometry – how far distant the planet would be to get the proper amount of insolation, and the star’s apparent diameter at that distance, which would be a factor of its actual diameter and the distance.
Another factor, of course, is the star’s location on the H-R chart. “Giant star” can mean two or three distinct things: a star at the upper left of the main sequence on the H-R chart, hence very hot and bright (“blue giant”), a distended Type III star with a very hot core having pushed the photosphere out to the point it’s relatively red and cool (“red giant”), or the inbetween state where a large, hot star has produced some distention, but not enough to cool the visible surface to red-giant levels. Notice on the H-R diagram how close the O and B stars at the upper left of the main sequence are to the hottest supergiants. A very cool red giant might have a habitable zone, receiving about terrestrial or Martian insolation, that is close enough to produce an apparent disk of several degrees (compared to the half degree of Earth’s Sun and Moon). A Type O star at the top of the main sequence, on the other hand, might have the habitable zone so far out as to visually be effectively a retina-burning point source.
(If someone with the requisite math could give more precise results on apparent dimensions relative to insolation, I’d greatly appreciate it – my statements were of course vague ballpark approximations.)
And, of course, as others have pointed out, it’s unlikely that life would have evolved and survived on a planet orbiting a giant star – their stellar lifespans would likely be too short, for one thing, and if what had been a Type A or F main sequence star had gone into post-main sequence evolution and turned red giant, after staying on the main sequence long enough for life to have evolved, any planet that had been getting the proper insolation before the star went off the main sequence would now be far too hot, while any planet now in the habitable zone would have formerly been too cold. So for our purposes here, we have to assume a SF-story background: life achieved intelligence and space flight and colonized the now-habitable world discussed above, perhaps terraforming it.
Quoth Walker in Eternity:
You’re thinking of a red dwarf: A planet in a red giant’s habitable zone would feel less tidal forces than we do from the Sun. There are still ways around that for a red dwarf, though: Asimov once posited a system with a gas giant in the habitable zone, and life on a moon of that gas giant. The moon was locked to the gas giant, not to the primary star, and so still rotated relative to the star and spread out the light and heat.
Quoth Polycarp:
The current convention is to refer to all main sequence stars as dwarfs, even the big, hot ones. “Giant” is reserved for stars that have moved off of the main sequence. There are still blue giants under this definition, though, mostly on what’s called the “helium main sequence”: A star near the end of its life can still go on for a while burning helium, after the usable supply of hydrogen is exhausted. As a rough rule of thumb, each subsequent fusion stage lasts about a tenth as long as the one before it, so a star that lives on the (hydrogen) main sequence for ten million years would stay on the helium main sequence for about one million.
All main sequence stars are dwarfs, but not all dwarfs are main sequence stars: there are also brown dwarfs, which are arguably on the main sequence but are not stars, and white dwarfs and two different kinds of black dwarfs, which were once main sequence but have evolved far off of it.
The math is actually pretty easy. The insolation goes as the solid angle subtended by the star and as the fourth power of the star’s surface temperature (in Kelvin): this is just the Stefan-Boltzmann blackbody equation. So for a red giant with a temperature half that of the sun, say, the flux density is reduced by a factor of 16, and so the solid angle must be increased by a factor of 16 to compensate. So the red giant subtends an angle of about four times that of the sun, about 2°.
If the red giant has a diameter of 100 solar diameters, say, then at 100AU it would span the same solid angle as the sun, so it has to be at 100AU/4 = 25AU to span 2° on the sky.
Not to disparage the good doctor’s math, but is it possible he fudged things for dramatic purposes? Is it even possible to have any moons at all in orbit around a planet locked in 1:1 resonance with its star? ISTR reading someplace that if tidal forces are strong enough to cause locking, then there are no stable orbits around the locked body, at least within the geologic time-frame necessary to form a habitable ecosystem.
*** Ponder
I don’t know.
Thanks to Chronos for correcting my giant/dwarf error.
And mine – of course, I grew up in the era when the counterintuitive usage of “blue giant” for Type O and B main sequence stars was in vogue. I’m actully happy to learn my posted standard is outmoded – I had always felt it’s one of those annoying perpetuated stupidities.
Thanks also to Omphaloskeptic for giving us actual data.
You beat me to the punch on this one. Red giants, and more so red supergiants (such as Betelgeuse), have extremely low densities; red dwarves have much higher densities than Sol (which has an overall density of about 4/3 x specific gravity of water.)
Once I worked out that the tidal force upon a surface satellite would be proportional to the density of the primary.
This came to me during a blitz of mathematical creativity about a half dozen years after college graduation. It started with realizing that surface gravity can be expressed as proportionally: radius times density, with the caveat that there is a problem applying that to lumpy bodies, even rather oblate spheroids.
It’s a way of seeing simply why Uranus, about 4 xs wider than Earth (a bit more equatorial, a bit less polar, as I’ve just Wiki-spotted), has a bit less surface gravity. It’s just that the density proportion is more than 4 xs smaller. Of course, the “traditional” approach to surface gravity proportionality is mass over radius squared. This seems to me to make it less obvious why Uranus has less surface gravity than Earth. You have to square the radius proportionality, and then divide the mass proportionality by that.
I usually think of the proportionality as diameter times density, but any consistent linear comparison will work. Heck, you could use circumference, or semi-circumference, as long as you are consistent.
Beyond that, surface escape velocity and surface orbital velocity would each be proportional to linear comparison times the **square root **of density. Hence Uranus would have a greater surface escape velocity than Earth. This may seem strange to some, but it must be remembered that the gravity curve falls off much more slowly with a larger radius. By the same token, Saturn would have about 3.2 times the surface escape velocity, despite a surface gravity only about 6.5% greater than ours.
One of the other things I worked out, again, was that the surface tidal factor would be proportional to (just plain) density.
We can even make an adjustment to satellites much further away. Simply project what the density of the primary would be if expanded to the center of the secondary body.
So great a difference between red dwarves and various kinds of red giants!
Extending that a bit, the tidal force is proportional to the cube of the apparent size times the density. For instance, from Earth, the Moon and Sun both have the same apparent size, so their tidal forces on Earth are proportional to their densities. The Moon is about twice as dense as the Sun, so it has twice the tidal force.
You might also be interested to note that surface orbital period (around a spherical body) depends only on density, being proportional to 1/sqrt(density). So it takes about an hour and a half to go around Earth (a basically rocky object), and it also takes about an hour and a half to go around any other basically rocky object like Mercury, Mars, or Ceres.