Podcaster: Steve Nerlich
Title: Dear Cheap Astronomy: Ep. 121: Black Holes Yet Again!
Organization: Cheap Astronomy
Links: http://cheapastro.com
Description: Cheap Astronomy finds can never be too many podcasts about black holes.
Dear Cheap Astronomy – What are squeezars?
Squeezars are stars that orbit supermassive black holes. Essentially they are stars on a slow death spiral into the black hole and the squeezing referred to is the tidal stretch being exerted upon them as the orbit closer and closer to the black hole’s event horizon. That tidal stretching heats them up, a bit like how the moon of Io, orbiting close to Jupiter is the most volcanically active body in the Solar System. So they are unusually hot and they are also unusually fast.
Dear Cheap Astronomy – Do black holes float in water?
Well, the internet says they do so it must be true. But let’s unpack this a bit. The internet also says that if you compress the Earth down to marble size it will become a black hole. This is true in a hypothetical sense, but actually compressing the Earth down to marble size is pretty much impossible. You could use some kind of gargantuan press to start the process, but once the Earth becomes denser than the material the press surfaces are made of, the press becomes useless.
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Transcript:
Why, why, why, why, why, why, why, why, why, why, why, cheap astronomy?
Yeah, why? And this is Dear Cheap Astronomy, Episode 121, Black Holes Yet Again. Yep, we just can’t seem to get enough of these exotic, compact objects and their extreme astrophysics, which, while seeming exotic and extreme to us Earth-bound mortals, is actually fairly common and ubiquitous astrophysics in the wider universe.
But although their behaviour may be becoming more familiar to us, we still don’t want to get too close to them.
What are squeezars?
Dear Cheap Astronomy, what is a squeezar? Squeezars are stars that orbit supermassive black holes.
Essentially they are stars on a slow death spiral into the black hole, and the squeezing referred to is the tidal stretch being exerted upon them as they orbit ever closer and closer to the black holes of Vent Horizon. That tidal stretching heats them up, a bit like how the moon of Io, orbiting close to Jupiter, is the most volcanically active body in the solar system. So squeezars are unusually hot, and they’re also unusually fast.
Much as water spiralling into a plug hole speeds up as it gets caught up into an increasingly tighter radius, stars around a black hole are accelerated within the black hole’s steep gravity well. Being accelerated up to faster speeds means they can maintain an orbit around the black hole, potentially for hundreds or thousands of years, but any interactions with other orbiting bodies, or just collisions with the odd dust grain, will steadily slow them down, and as they slow down, they descend into a closer orbit with the black hole. And once tidal stretching really kicks in, that is the pull of gravity on their near side becomes substantially greater than the pull of gravity on their far side, they really then do commence a death spiral, since the extra heat generated and radiated away represents a loss of orbital momentum energy.
A squeezar known as S47-11 orbits Sagittarius A star, the 4 million solar mass black hole at the centre of our Milky Way galaxy. S47-11 orbits Sagittarius A star at a speed of 24,000 km a second, which is about 8% of the speed of light. In other words, pretty darn fast.
Its current orbit is about 13 AU from the black hole’s event horizon, where for comparison, Saturn is about 10 AU from the Sun. Of course the Sun is 1.4 million km in diameter, while Sagittarius A star’s event horizon is 24 million km in diameter, and of course it has 4 million times the mass of the Sun, meaning it generates one heck of a gravity well. Measuring the velocity of stars like S47-11 is how we’ve been able to accurately estimate the size, mass and density of Sagittarius A star, which, being a black hole, is otherwise invisible.
Trekking squeezars has also allowed us to identify and quantify the mass of central supermassive black holes in other galaxies within our local group of galaxies. In galaxies that are further out, it’s no longer possible to resolve individual squeezars, but we find that a lot of further out galaxies have active galactic nuclei, which are the result of their central supermassive black holes sucking down large amounts of gas and dust, and probably stars and planets, creating huge accretion disks of hot, compressed material which radiate high energy X and gamma rays.
The finding of all these active galactic nuclei confirm our modern understanding that most galaxies in the universe have central supermassive black holes. The fact that we see active galactic nuclei at the centre of galaxies at great distances, while we most often see relatively quiet supermassive black holes at the centre of close galaxies, may just be because the further away we look, the further back in time we look. So this pattern may just reflect that in the modern universe, galaxies are more likely to be mature and stable, while galaxies in the early universe are still growing up and have a lot more free material for their central black holes to feed upon.
The original paper which proposed the name squeezars, Alexander and Morris 2003, proposed that the stars spiralling in towards supermassive black holes should have atypical luminosities for their spectral class. It was anticipated that the tidal squeezing experienced by these stars would either make them brighter as they radiated off extra heat, or the stars would expand due to heating and hence appear bigger rather than necessarily brighter. To date we are still working at close to the limit of our observing resolution, so no one has actually confirmed whether squeezars do have atypical luminosities, but nonetheless the name squeezar has stuck.
So if anyone starts talking about a star orbiting a supermassive black hole at a very high velocity, you can just say, oh you mean a squeezar?
Do black holes float in water?
One of the reasons for black holes frequent appearance in science blogs and podcasts is their mysterious nature, with so much of their real behaviour shrouded behind event horizons which is ideal for storytelling and hypothesis building since it’s often difficult for anyone to prove you wrong. But there are still some things said about black holes that do just seem a bit daft. For example, Dear Cheap Astronomy, do black holes really float in water?
Well, the internet says they do, so it must be true. But let’s unpack this a bit. The internet also says that if you compress the earth down to marble size, it will become a black hole.
This is true in a hypothetical sense, but actually compressing the earth down to marble size is pretty much impossible. You could use some kind of gargantuan press to start the compression process, but once the earth becomes denser than the material the press is made of, the press becomes useless. You can then switch to a different press made of denser material, but that’s only going to take things so much further.
Probably the only way to turn the earth into a black hole is to pile up more mass around it, until the self-gravity of that much more massive object becomes sufficient to overcome sub-nuclear degeneracy pressures in its central core, and whatever you get at the end of all that isn’t going to be marble-sized. But anyway, the principle is reasonable. If you can compress a quantity of mass to a size smaller than its corresponding Schwarzschild radius, then you will have yourself a black hole.
We’re just making the point that that is a big IF. The usual way in which black holes are formed in the modern universe is where you have a very massive star generating fusion at its centre, which pushes out the huge mass of the star, so that when the fusion fuel suddenly runs out, the whole star collapses inwards, compressing its core into degenerate matter, whose mass is more dense than its Schwarzschild radius, and so, black hole. And once you have a black hole, you’re away.
Throw more mass at it, and anything that descends past its event horizon will stay there for good, and when that mass adds to the black hole’s total mass, then that expands the radius of the black hole’s event horizon. If we consider the event horizon as the surface of the black hole, and whatever’s within the event horizon to be the black hole’s volume, then as you keep adding more mass, the volume of the black hole increases more quickly than its mass does. So by mathematical necessity, it’s always the case that larger black holes are less dense than smaller black holes.
Indeed, most supermassive black holes are less dense than water, which means they’ll float in water, and really big supermassive black holes are less dense than air, meaning they could float around like balloons. But hang on, let’s remember which podcast you’re listening to. There are so many dimensions of implausibility here, it’s hard to know where to start.
Firstly, liquid water spontaneously evaporates outside a pressured atmosphere. Secondly, if you try and drop a black hole in a giant bath, you’ll find you’re actually dropping the bath into the black hole. Even if the bath is bigger, you’re putting it in contact with something that warps space-time into a steep cliff, so the near side of the bath would spaghettify and disappear across the event horizon at something approaching the speed of light.
So you can just forget about the whole floating notion. It’s also the case that the whole issue of density is a bit of a furphy. While we don’t know what goes on in a black hole, we can be pretty confident that the mass within its event horizon is not evenly distributed throughout its volume.
It’s almost certainly all squished into the center. Indeed, there’s a plausible hypothesis that all matter within a black hole has been mashed down into point-particle quarks which occupy no volume at all. So as you keep adding matter to it, the black hole gains mass, but the mass within it has no material existence.
It’s just a quantity. Some people will say it has infinite density, but it’s probably better to say that the whole concept of density becomes irrelevant. What matters is the effect that the black hole has on space-time.
So it doesn’t really matter whether the black hole’s core is close to or distant from the event horizon, since an event horizon is an event horizon. The only thing that does matter is that a supermassive black hole’s event horizon has a much bigger surface area than a smaller black hole’s event horizon. Which means that supermassive black holes are able to suck down much bigger baths.
And don’t even get us started on where the Saturn floats in water.
[Speaker 1]
So there you go. It’s not unreasonable to suggest that a lot of the exotic claims made about black holes are unlikely to be true, but at the same time they may just be unfalsifiable since you just can’t observe what happens behind an event horizon. Mind you, while black holes floating in water does have a certain underlying mathematical logic, we can be pretty confident it’s never going to happen.
But that’s it for another episode of Dear Cheap Astronomy. If you’ve got a space science question, or you just want to float an idea past us, why not write to CheapAstro at gmail.com and we’ll try to assess its buoyancy. Thanks for listening.
Steve Nerlick, Cheap Astronomy.
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