# Largest Star and Black Hole **URL:** \ **Category:** Factual Questions\ **Created:** [September 29, 2015, 1:29am UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906 "2015-09-29T01:29:32Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Horatio\_Hellpop](https://avatars.discourse-cdn.com/v4/letter/h/e5b9ba/32.png) [@Horatio\_Hellpop](https://boards.straightdope.com/u/Horatio_Hellpop)\ **Post date:** [September 29, 2015, 1:29am UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/1 "2015-09-29T01:29:32Z")
I’m not a science guy and need this to be explained to me in pretty simple terms. From a couple click bait sites, I understand the largest known star has a diameter roughly the size of Neptune’s orbit-- but there’s a supermassive black hole that’s been described in similar terms. Is this accurate? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2015, 1:38am UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/2 "2015-09-29T01:38:30Z")
Yeah, that looks about accurate: A black hole the size of Neptune’s orbit would be about a billion times the mass of the Sun, and the largest supermassive black holes can be up in that vicinity. Note that although the sizes of these two objects would be about the same, their masses would be vastly different. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [September 29, 2015, 2:19pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/3 "2015-09-29T14:19:03Z")
> [@Chronos](#): > > Yeah, that looks about accurate: A black hole the size of Neptune’s orbit would be about a billion times the mass of the Sun, and the largest supermassive black holes can be up in that vicinity. Note that although the sizes of these two objects would be about the same, their masses would be vastly different. Wiki says even larger… up to 40 billion solar masses, if correct. :eek: > **[Supermassive black hole](https://en.wikipedia.org/wiki/Supermassive_black_hole)** > > A supermassive black hole (SMBH or sometimes SBH)\[a\] is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, including light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. > **[S5 0014+81](https://en.wikipedia.org/wiki/S5_0014%2B81)** > > S5 0014+81 is a distant, compact, hyperluminous, broad-absorption-line quasar, or blazar, located near the high declination region of the constellation Cepheus, near the North Equatorial Pole. > The object is an OVV (optically violent variable) quasar, a type of blazar. It belongs to the most energetic subclass of active galactic nuclei, which are produced by the rapid accretion of matter by a central supermassive black hole, changing gravitational energy to light energy that can be visible across... ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2015, 2:55pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/4 "2015-09-29T14:55:43Z")
Yeah, when I said “in that vicinity”, that was a very handwavy sort of “vicinity”. But the bigger they are, the rarer, and there’s no theoretical upper bound on the size of a black hole. So for any given very large black hole one finds, there is probably going to be an even larger one out there somewhere, but you’ll probably have to look very hard to find that somewhere. That said, I hadn’t heard of one quite that massive. It looks like there’s some dispute about the mass (isn’t there always?), but that’s still very impressive. ---
**Author:** ![blood63](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@blood63](https://boards.straightdope.com/u/blood63)\ **Post date:** [September 29, 2015, 3:00pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/5 "2015-09-29T15:00:15Z")
> [@Horatio\_Hellpop](#): > > I’m not a science guy and need this to be explained to me in pretty simple terms. From a couple click bait sites, I understand the largest known star has a diameter roughly the size of Neptune’s orbit-- but there’s a supermassive black hole that’s been described in similar terms. Is this accurate? I think you want to be clear that you are talking about mass not volume. There are no black holes the physical size of Neptune’s orbit. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2015, 3:06pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/6 "2015-09-29T15:06:40Z")
Yes, there are: See the previous posts. And it wouldn’t make sense to talk about something with “the mass of Neptune’s orbit”-- What would that even mean? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [September 29, 2015, 5:21pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/7 "2015-09-29T17:21:46Z")
> [@Chronos](#): > > Yeah, when I said “in that vicinity”, that was a very handwavy sort of “vicinity”. But the bigger they are, the rarer, and there’s no theoretical upper bound on the size of a black hole. So for any given very large black hole one finds, there is probably going to be an even larger one out there somewhere, but you’ll probably have to look very hard to find that somewhere. > > That said, I hadn’t heard of one quite that massive. It looks like there’s some dispute about the mass (isn’t there always?), but that’s still very impressive. I think the upper limit for the radius of a black hole may be the cosmological event horizon (about 16 billion light years, slightly larger than the Hubble Horizon, but less than half the radius of the observable Universe), though I must admit I am not 100% sure about that. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2015, 6:40pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/8 "2015-09-29T18:40:55Z")
Well, when you get to that scale, where the cosmological constant becomes relevant, it can be unclear just what a “black hole” means. There are equations for it, but it’s not entirely clear how to interpret them. ---
**Author:** ![Mollie\_Magill](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mollie_magill/32/20882_2.png) [@Mollie\_Magill](https://boards.straightdope.com/u/Mollie_Magill)\ **Post date:** [September 29, 2015, 8:11pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/9 "2015-09-29T20:11:01Z")
> [@Chronos](#): > > Yeah, when I said “in that vicinity”, that was a very handwavy sort of “vicinity”. But the bigger they are, the rarer, and there’s no theoretical upper bound on the size of a black hole. So for any given very large black hole one finds, there is probably going to be an even larger one out there somewhere, but you’ll probably have to look very hard to find that somewhere. > > That said, I hadn’t heard of one quite that massive. It looks like there’s some dispute about the mass (isn’t there always?), but that’s still very impressive. Are you talking about the event horizon, or the object itself? Can we even measure the object, or do we just make calculations based on observable gravity? ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [September 29, 2015, 8:27pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/10 "2015-09-29T20:27:52Z")
Speaking of Black Holes, astronomers have discovered two giant black holes on the verge of colliding. When they do they’ll completely destroy at least one galaxy in an explosion equiv. to 10^36 megatons of TNT. Cites: [http://thenextdigit.com/26333/distant-galaxies-black-holes-collide-sooner-expected/](http://thenextdigit.com/26333/distant-galaxies-black-holes-collide-sooner-expected/) > **[Flickering Quasar May Hold Black Holes on a Collision Course](https://www.scientificamerican.com/article/flickering-quasar-may-hold-black-holes-on-a-collision-course/)** > > Gravitational waves from the most promising black hole binary system candidate yet could help shine a light on how these monsters at the hearts of galaxies merge > **[Converging black holes in Virgo constellation: Crash expected in 100,000...](https://www.sciencedaily.com/releases/2015/09/150916162123.htm)** > > Astronomers have provided additional evidence that a pair of closely orbiting black holes deep in the Virgo constellation is causing the rhythmic flashes of light coming from quasar PG 1302-102. Based on calculations of the pair's mass -- together,... (If it’s too sad to contemplate the impending demise of whatever intelligent life forms inhabit those galaxies, just take the view that the black holes _already collided 3.5 billion years ago._) ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [September 29, 2015, 10:28pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/11 "2015-09-29T22:28:26Z")
From what I can tell, the largest known burning star, by radius, is UY Scuti (not the guy who posts here) , which is around 17 AU. That puts it a couple AU inside the orbit of Uranus. But, what is the radius of a star? When they get to be that big, I would expect that the boundary between the photosphere and the corona is more like an idea than a physical thing. One could reasonably define the size of our own sun by its atmosphere, which would make it a kind of teardrop-shaped object a couple hundred AU along its long axis. A black hole, by contrast, theoretically, has a very definite radial boundary, at its event horizon. Anything above the event horizon is not (yet) port of the black hole, anything below it _is_ part of the black hole. The Sagittarius A\* object at the center of this galaxy, when you compare its apparent radius to its apparent mass, has a very low _average_ density, lower than liquid water. But, what goes on inside a black hole’s event horizon? The laws of physics, as we know them, may not make sense. Mass and density may not have meaning. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 30, 2015, 1:48am UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/12 "2015-09-30T01:48:28Z")
> [@](#): > > Quoth **Maus Magill** : > > Are you talking about the event horizon, or the object itself? Can we even measure the object, or do we just make calculations based on observable gravity? In so far as “the object itself” has any meaning, it _is_ the region inside of the event horizon. I know, I know, you’re probably looking for some sort of “substance”. But you won’t find it anywhere, inside the horizon or out: Under the standard models for black holes, they’re completely vacuum, with no substance of any sort anywhere. Now, quantum gravity might change that picture. It’s conceivable that there might be some sort of “object” deep within the hole, with properties we can’t even guess at. But that’s pure guesswork, and not even educated guesswork, since we know almost nothing about quantum gravity. ---
**Author:** ![Mollie\_Magill](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mollie_magill/32/20882_2.png) [@Mollie\_Magill](https://boards.straightdope.com/u/Mollie_Magill)\ **Post date:** [September 30, 2015, 5:26pm UTC](https://boards.straightdope.com/t/largest-star-and-black-hole/732906/13 "2015-09-30T17:26:06Z")
That’s what I had figured. I had always learned that a black hole was a point. Then I’m reading posts by more knowledgeable people talking about black holes’ radii, and was thinking that perhaps the models had changed. Good to know that Stephen Hawking didn’t change _everything_.