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(I’m an astrophysics undergrad.) Black holes aren’t composed of anything, they’re just defined by their charge, spin and mass equivalent.

Dust clouds have those mass ranges. It’s not a galaxy-scale mass by any measure.

This thread has a lot of CS people being confident about physics.

I was always surprised that when we talk about BHs mass, charge, and spin that we really mean U(1) (electromagnetic) gauge charge and not charges from global symmetries. (If BHs had global charge, you could at least say that this or that black hole was made out of N baryons, or whatever.)

But it's really so---according to GR, black holes don't have global charges. So even if you see a star made out of baryons collapse into a black hole, once the BH settles down into a steady state you can't say it's "really" got baryons inside: the baryon number gets destroyed.

(Of course, a different model of gravity that preserves unitarity might upset this understanding.)

And that a BH made from matter and one made from antimatter are mathematically identical, and merging them would not cause any explosion.

Thanks! That made me (superficially of course) understand it. Super weird stuff.

Basically, the event horizon is the event horizon is the event horizon. If two non-steady-state matter/anti black holes merge (like before everything has hit the singularities), it will cause explosions inside the BH, but energy is mass is energy is mass, so for an external observer it's indiscernable. It will look no different from two all-matter BHs merging.

Welcome to Hacker News.

I mean, I included a disclaimer... But regardless, you appear to be wrong on both counts (or at least contradicting Wikipedia):

1. "The presence of a black hole can be inferred through its interaction with OTHER MATTER and with electromagnetic radiation such as visible light." https://en.wikipedia.org/wiki/Black_hole

2. "A dwarf galaxy is a small galaxy composed of ABOUT 1000 up to several billion stars" https://en.wikipedia.org/wiki/Dwarf_galaxy

Darn astrophysics majors being confident about astronomy! ;)

1. Your argument is about the grammar of a sentence about black holes on Wikipedia? This isn’t some kind of gotcha.

2. I missed the dwarf part, but think about what you’re arguing: the mass range of a loosely defined category (the lower bound of a few thousands is not one I’ve ever heard, btw) that has nothing to do with the paper in question. Collections of stars of any kind produce light. This doesn’t. What are you saying?

What do you think physicists do all day?

which doesn't "interact" with normal ("baryonic") matter

I think you mean it doesn't interact electromagnetically with either matter or radiation. It does interact with normal matter via gravity -- that's pretty much the strongest (only?) argument for its existence.

I'm not aware of any reason this object can't end up being a giant sphere

AIUI, most theories posit that solid spheres of dark matter are very unlikely because matter accretion is governed by electromagnetism in addition to gravity, and dark matter is not supposed to obey the former. Most models assume that dark matter is organized in gaseous clouds (halos); strictly speaking that's still a giant sphere, just not in the same way that Jupiter or the Sun or even the Oort Cloud is.

100-200 solar masses is not one of the largest known. There are many that are 1000s of times more massive than the sun.

I'm unaware of any stars in the 1000 Msun range. Wikipedia puts 291 Msun of R136a1 at the largest. After that, 195 M of R136a2 is the next. A star at 100 Msun would be in the most massive stars known.

https://en.wikipedia.org/wiki/List_of_most_massive_stars#Lis...

“ A number of the "stars" listed below may actually be two or more companions orbiting too closely for our telescopes to distinguish, each star possibly being massive in itself but not necessarily "supermassive" to either be on this list, or near the top of it. “

“ More globally, statistics on stellar populations seem to indicate that the upper mass limit is in the 120-solar-mass range,[1] so any mass estimate above this range is suspect. “

There are good theoretical reasons why a star shouldn’t normally get as big as the ones on the top of the list. Long story short: they’d very quickly shed mass due to their intense luminosity. Some of them might even be boiling with bubbles of pure radiation.

https://en.wikipedia.org/wiki/Eddington_luminosity

Beyond that, there’s also the possibility of pair-instability supernova, which might cause the most massive stars to literally disintegrate.

This confused me too from all those solar object size comparisons I’ve seen. Turns out there are stars that are 1000s of times bigger than the sun, but they aren’t the same density.