# Tag Info

84

I was surprised too when I first heard they were trying to image M87's black hole. The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A*, which they are also attempting to image. A cursory search of wikipedia's ...

56

You have already got some good answers, but I'll just try to provide one more intuitive solution on why the event horizons will never separate again if overlapping each other: First, imagine a speck of dust that comes inside the EH of a black hole. I believe we'll agree this speck can never escape the black hole, because nothing can come back from behind the ...

53

If the event horizons ever touch and become one continuous surface, their fate is sealed - the two black holes will merge all the way in. They can never separate again, no matter what. There are several possible ways to explain it, with varying degrees of rigorousness. An intuitive explanation is that escape velocity at the event horizon equals the speed ...

45

The galaxy is kept together by the combined mass of the matter in the galaxy, of which the supermassive black hole is a negligible part. There are galaxies that don't have a central black hole (such as the Triangulum galaxy), and they are also held together by their combined mass. In particular, the dark matter of the galaxy is what provides most of the mass ...

42

It is not true that "objects float around" in the solar system. Perhaps you have seen video from the space station, and you can see things floating. This is not because there is no gravity, but because everything in the space station going at the same speed in the same direction. This makes it look as if things are floating. In fact the space station and ...

41

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make no promises, and they're not assuming they'll be successful). That said, I'm not wholly surprised that we ended up seeing M87, rather than Sgr A*, for a couple ...

34

There are a few criteria necessary to see a black hole with the Event Horizon Telescope. They are, in importance: Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, consuming about 90 Earth masses a day. Apparent size. Even though it is 53 million light-years away, M87 ...

33

To help with James K's excellent answer, a visual representation might help. Let's look at a thought experiment - Newton's Cannonball. Let's say you have a cannon, high enough that it's being held above Earth's atmosphere. You fire it, and it falls to Earth a little ways away ("D" in the below diagram). You fire another one with more power so it's moving ...

31

This was studied many years ago. Not only do galaxies have to hold together, but there also has to be enough matter to hold it tightly enough to spin at the speed it turns. (Imagine swinging an object on a string around your head, the faster it spins, the more force you have to apply.) (Note that galaxies don't actually rotate like a single solid object ...

28

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the central object is $M\approx4\times10^6M_{\odot}$; this alone is fairly good evidence for a supermassive black hole, and we can constrain the size of the object ...

27

Presumbably we rotate beacuse of the BH. No. The galaxy is being held in one piece due to its own total gravity. The black hole is only a small fraction of that. Basically, the BH doesn't matter. When the black hole dies in our galaxy The BH will probably be the last thing left of our galaxy at the end. And even then it will take some incredibly long ...

27

In order to survive, the star's self-gravitation must be larger than the tidal stretching forces provided by the black hole. If not, then the star will get spaghettified before it crosses the event horizon. The tidal acceleration on a freely-falling star at the event horizon of a (non-spinning) supermassive black hole is approximately g_{\rm tidal}\simeq 2\...

25

Not at all a dumb question. As you have heard, it is true that time is affected by gravity. The stronger the gravitational field, the slower time passes. If you're far from any gravitating matter, time passes "normally". But to answer your question, we must specify what is meant by "the black holes's time" (let's call the black hole $\mathrm{BH}_\mathrm{Sgr\... 24 The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity. This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing ... 24 The picture isn't a "colour" picture - it is monochrome. i.e. It is obtained at a single microwave wavelength of 1.3 mm, and so not at any wavelength you could see (Akiyama et al. 2019). There isn't therefore any spectral information that would reveal the expected Doppler effect. Any difference of colour in the "false-colour picture" is ... 23 It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon you are never coming back. That applies equally to light, and matter. Around the black hole there may be matter in orbit. Since the Black hole has such strong ... 23 The answer to this is unknown at the present time. The issue is that an accreting "seed black hole" can only accrete at a limited rate. The limitation is provided by radiation pressure from the material it is accreting. This provides negative feedback and defines a maximum accretion rate for spherical accretion known as the Eddington limit, which ... 22 Answer: Not much The Milky Way's central black hole (BH) masses about 5 million suns, while the galaxy masses 100 billion to a trillion suns. Consequently, the central BH is pretty much irrelevant to the dynamics of stellar orbits except very close to the center. But what do you mean by "the black hole dies"? Do you mean evaporates through Hawking ... 22 The main problem is angular momentum. In order for two gravitationally bound objects to merge (whether black holes, supermassive black holes, planets, stars, etc.), they must shed enough angular momentum for their orbital separation to become small enough. Average orbital separation (semi-major axis) is determined entirely from the angular momentum of the ... 21 I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the black hole in our Milky Way, because the material around it moves very fast: the vortex rotates around its axis in 20 minutes. Compare it to a toddler who has ... 20 As Ingolifs says, Sgr A* and M87* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87* first: But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear. — The ... 20 Ok, gotta quote XKCD on this. This is not how space works: This is: Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface. To avoid falling back into the atmosphere, you have to go sideways really, really fast. The ... 17 The idea behind the paper (Shannon et al. 2013) that article is based on is to measure the gravitational wave background (GWB) produced by mergers of supermassive black holes, and determine which models of SMBH merger histories can replicate the SMBH population and the corresponding gravitational wave background. In this paper, measuring the GWB is done ... 16 Nothing "escapes" a BH - in the sense that a signal originating inside the event horizon remains forever inside. If something is observed moving away from the BH, then it was generated outside the event horizon. If it was generated inside, it would never be observed at all, forever and ever. Gravity itself does not "escape" a BH - and neither does "not ... 16 Stellar mass black holes form from the collapse of massive stars at the end of their lives. You can then find them scattered throughout galaxies, just like you find massive stars. They typically have a mass a few times the mass of the sun. Supermassive black holes are found at the centers of galaxies. They typically have mass of millions of Suns. Recently ... 16 Under General Relativity (GR) alone, a Black Hole's (BH's) event horizon is a point of no return -- anything that passes through the event horizon is lost and gone forever, and nothing comes out. Hence, under GR alone, BHs are utterly black and don't have a temperature at all. This is why the absorption of radiation (or anything else) by a BH doesn't raise ... 16 No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the ... 16 Part of the answer is easy. The strain measured in that event was about$0.25\times 10^{-21}$. That is an object$1m$long would be squeezed by$0.25\times 10^{-21} m\$ in one direction and stretched by the same amount in the orthogonal direction. The strain drops off linearly with distance from the black hole, so to achieve a distortion of 1mm in something ...

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Another quick note - They are trying to get a photo of Sag. A*: From Space.com The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A*. This latter object, while still a supermassive black hole, is a runt ...

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