85 votes
Accepted

Why not take a picture of a closer black hole?

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 ...
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58 votes

If two black hole event horizons overlap (touch) can they ever separate again?

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 ...
53 votes

If two black hole event horizons overlap (touch) can they ever separate again?

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 ...
45 votes

What keeps galaxies together?

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 ...
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42 votes
Accepted

Why didn't the Event Horizon Telescope team mention Sagittarius A*?

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 ...
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42 votes

Why do things float in space, though the gravity of our star is always present?

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 ...
  • 94.2k
34 votes

Why not take a picture of a closer black hole?

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 ...
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33 votes

Why do things float in space, though the gravity of our star is always present?

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 ...
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31 votes

What keeps galaxies together?

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 ...
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30 votes
Accepted

What will happen to the shape of a galaxy when a super massive black hole lying in its center dies(evaporates out)?

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. ...
28 votes
Accepted

What is it exactly about these flares of infrared light from Sgr A* that "confirms" it is a supermassive black hole?

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 ...
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28 votes

Can an entire star pass through the event horizon of a black hole unharmed?

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 ...
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27 votes

Why do some supermassive black holes have a * in their name?

How the name came to be chosen is discussed in this paper1 (§3, pp. 4–5): Eight years after the discovery, one of us (Brown) invented the name Sgr A∗ to distinguish the compact source from the other ...
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25 votes
Accepted

Time according to the gravity of Sagittarius A*?

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, ...
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25 votes
Accepted

Why is there no color shift on the photo of the M87 black hole?

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). ...
  • 120k
24 votes
Accepted

Could there be dark matter black holes?

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 ...
  • 120k
24 votes
Accepted

How can a supermassive black hole cause so much energy to enlighten its matter when its massive gravity prevents light to escape?

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 ...
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24 votes

What will happen to the shape of a galaxy when a super massive black hole lying in its center dies(evaporates out)?

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 ...
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24 votes
Accepted

Why can't supermassive black holes merge? (or can they?)

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 ...
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23 votes
Accepted

How can a supermassive black hole be 13 billion years old?

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 ...
  • 120k
23 votes

Why do some supermassive black holes have a * in their name?

Sgr A is a radio source from near the centre of the galaxy. In early radio wavelength maps it wasn't clear if this was a single source or multiple. With higher resolution it is clear that Sgr A is ...
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21 votes

Why didn't the Event Horizon Telescope team mention Sagittarius A*?

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 ...
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20 votes

Why not take a picture of a closer black hole?

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 ...
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20 votes

Why do things float in space, though the gravity of our star is always present?

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; ...
17 votes

How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?

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 ...
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16 votes
Accepted

Can magnetism escape a black hole?

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 ...
16 votes
Accepted

Why black holes are extremely cold?

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. ...
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16 votes
Accepted

Would Hubble Space Telescope improve black hole image observed by EHT if it joined array of telesopes?

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 ...
  • 2,585
16 votes
Accepted

Black Hole Collision & Gravitational Waves

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 ...
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Only top scored, non community-wiki answers of a minimum length are eligible