I’ve heard it said that Jupiter could be a sun if it had just a little bit more mass. If it did have that little bit more mass and became a sun, how big would it look in the sky? Right now, Jupiter is a dot in the sky like any other star or planet. It’s hard to believe it’s a hugely massive object. Would it look much bigger if it were a sun?
I get a guesstimate of 8 arc-minutes for apparent size of Jupiter if it had the same diameter as our Sun. For reference the actual sun is like 30 arc-minutes.
And obviously it would be a whole crapload brighter.
It would need about 13 times the mass it has now to be a brown dwarf. But because of the degeneracy pressure that sets the size of brown dwarfs, brown dwarfs are all about the size of Jupiter. It would be brighter, but the same size as it is now (and would therefore look the same size) if it were a brown dwarf.
I did some calculations of how bright Jupiter-turned-into-a-star would look in this thread. If it were a red dwarf, a la Proxima Centauri, it would be around the brightness of a full moon. If it were a second sun, we’d have twilight when it was above the horizon, even if the sun was below the horizon. A second sun in Jupiter’s orbit would be 20-30 times fainter than the Sun, depending on how far away from us it was at the time (Jupiter’s distance from us varies with where Jupiter and Earth are in their orbits).
Anne, may I ask what you’re using for a definition of a brown dwarf? (I realize what one is: a ‘failed sun’. too small to ignite fusion; what I’m asking is, I guess, what your boundaries are for the definition.) It’s my understanding that Jupiter can in some ways be considered the least massive, coldest brown dwarf – like ‘normal’ brown dwarfs, it is self-heating from ongoing gravitational collapse, although in its case it can only heat itself from about -200 C (black body equilibrium temperature) to about -100 C (observed average ‘surface temperature’).
SO: 1. Is that relatively silly but I-believe-accurate statement true? 2. What do you regard as the bounds separating jovian from superjovian from brown dwarf? And 3. What are your reasons for those bounds?
I’m not asking to be argumentative but to learn from someone who has no doubt forgotten more than I’ve ever learned, to have my own ignorance fought. Thanks in advance for any answers you offer.
I would think that if Jupiter was a star with the same diameter as it current has in its planetary form, it would simply be a brilliant point of light in the night sky. The fact that it is a star wouldn’t change its apparent diameter from the point of view of Earth, just its brightness.
Of course, a star the size of Jupiter is probably a physical impossibility…“normal” stars would be several orders of magnitude larger in diameter, while degenerate stars like white dwarfs are generally thought to be Earth-sized and smaller.
Yes, but red dwarfs aren’t much larger than Jupiter, despite being much more massive. More mass means more gravity, which squishes all that hydrogen into a smaller space. So if Jupiter were a red dwarf it would still look like a point of light, just much brighter.
If Jupiter was a G type star of the same mass as Sol, it would appear as a disk. Jupiter is about 5 AUs from Sol, so as Earth orbited around Sol our distance to Jupiter would vary between 6 and 4 AUs. But Jupiter Optimus Maximus wouldn’t orbit around Sol in the first place, they’d be twin stars that would orbit around a common center of gravity. Earth’s orbit around Sol wouldn’t be very stable.
If Jupiter was massive enough to be a small star - let’s say 13x more massive (persuant to Anne Neville’s post) - wouldn’t that screw up the orbits of the other planets? Can you have a binary star system with stable planetary orbits around the main star?
Something that is able to fuse deuterium, but not able to fuse hydrogen. I think that’s the IAU’s definition.
Yes. We can’t calculate how it would, but it would. It would matter how you got the extra mass there. If the solar system formed with Jupiter 13 times as massive as it is, or with Jupiter the same mass as the Sun, I wouldn’t expect it to look much like our current solar system. If you brought the mass in from somewhere else now, that process would affect the other planets (at a guess, it would probably destabilize their orbits).
Yes. We’ve found extrasolar planets orbiting one star in a multiple-star system.
A star is I assume a body where fusion is taking place; so there is a minimum size and internal pressure needed to cause that.
There’s an area between two stars in a binary system where they disrupt any orbits around each others’ stars; and some planets would be close enough to either star to be sufficiently stable in their orbits; and some would be far enough away to orbit the center of mass of the pair in a stable orbit. I’m not sure where earth would be if Jupiter were 10 times the mass.
As for “how big would Jupiter look?” It would be the same size it is, just a lot brighter. If it would be twice the diameter, 8 times the volume, it still would not be a discernable disc. If it were the size of the sun - the sun is about 1AU away (depending on time of year, 92 to 94 million miles). Jupiter 10 times the distance (9 to 11 AU) so about 1/10 the diameter. The sun like the moon is 30 minutes or half a degree across, IIRC.
OK, thanks. I was going on the (apparently false) usage of “object which is heated/glows under gravitational pressure heating” rather than the one you give above. That clarifies a lot for me. Thanks!
Paul Weigert and Matt Holman did do the calculations to look for stable planetary orbits in the Alpha Centauri system. The two stars (Proxima orbits much further away) have an orbit that brings them within 11 AU of each other. That’s farther apart than the 5 AU separation we’re talking about with the Sun-Jupiter system. Orbits within about 3 AU of either star in the Alpha Centauri system can be stable.
If Jupiter has the mass of the Sun and the Sun-Jupiter system is similar to the Alpha Centauri system in terms of where orbits are stable (I don’t know if it would be or not), orbits up to 1.8 AU from the Sun would be stable. Earth’s orbit would be stable if that were the case, Mars’ might be. If Jupiter is less massive than the Sun, it’s probably less able to disrupt distant planetary orbits.
Jupiter is 5.2 AU from the Sun, and a maximum of 968 million km, or about 6.4 AU, from Earth.
That depends on what you mean by “size”. If you mean radius, then it’s not impossible to have a star the size of Jupiter. Brown dwarfs are all about the same radius as Jupiter. If you mean mass, then yes, it is physically impossible to have a star the same mass as Jupiter.
I’m under the impression it’s possible to have (relatively stable) black holes with the mass of jupiter. I’m assuming from what you’re saying that it’s not possible to compress the mass of jupiter far enough to undergo fusion and have it be stable just by gravity alone afterwards (without forming a black hole). Is that true, or is it “just” that under normal conditions a mass the size of jupiter won’t get that compressed?
For a while there, there was a dispute as to what exactly to consider the cutoff for calling something a “brown dwarf”. One standard proposed for the cutoff would be an object of maximum diameter, such that adding more mass would make it shrink slightly, rather than expand. By this standard, Jupiter would be just barely over the line, and considered a brown dwarf. In the end, though, the scientific community decided to use the deuterium-burning standard Anne Neville mentioned, instead.
Of course, none of that is really science. Jupiter is what it is; all that’s at stake here is what we choose to call it.
It’s possible to have a stable black hole of pretty much any mass, but nobody knows of any way to form one less than the Chandrasekhar limit (a bit more than the mass of the Sun) in the current Universe. Presumably, if there are any Jupiter-mass black holes, they were formed during the Big Bang, or very shortly afterwards. None such have ever been observed, which might mean they don’t exist, or might just mean that we’re not looking hard enough for them.
If I understand the wikipedia article on Chandrasekhar limits, this is the “natural process” limit - i.e. the minimal mass that’s needed for a black hole to form “spontaniously”, given the right components.
What I was asking was a little more subtle than that: given unlimited resources (like say the early big bang conditions) would it be possible to compress a mass like Jupiter’s just enough to form a star but not a black hole and have it be stable by gravity alone, or would it explode/evaporate/disintegrate in a relatively short timespan?
Oops. I was thinking of Saturn. TLTG - too lazy to google.