<p>There are a few different features of black holes that researchers look for:</p>

<ol>
<li>An accretion disk</li>
<li>Mass transfer from companion star</li>
<li>X-ray emission</li>
<li>Absorption of gas and other matter around it</li>
<li>Gravitational lensing</li>
</ol>

<p>You mentioned #3 in your question; quite a few black holes, including Cygnus X-1, emit X-rays.</p>

<p>1 is a common feature around black holes. The material can be pulled from surrounding gases, or, in many cases, a companion star. Accretion disks generally emit electromagnetic radiation; the exact type depends on the type of material and the object it surrounds.</p>

<p>2 is related to 1 and 3, but it deserves its own explanation because of the effects on the donor star. Lets take mass transfer on a large scale. Take the Whirlpool galaxy. It is interacting with its neighbor, NGC 5195, via a bridge of dust and matter. What are the effects on NGC 5195? Distortion, as well as loss of structure in the area near the bridge. The same thing happens on the stellar level. A star losing mass can become distorted in the direction of its companion, potentially causing changes in brightness or other features.</p>

<p>4 is, I suppose related to the others, but I should note that if there were merely gas clouds surrounding the black hole, it could also swallow those up. If a black hole is not in a binary system, it may not come in contact with another star, and will instead begin swallowing other bits of matter around it.</p>

<p>5 is the best characteristic; a better way of detecting black holes. General Relativity predicts that massive objects bend light; using Einstein's equations it is possible to determine just how much light will be bent by such an object. This has been used in many cases, such as during the solar eclipse of 1919, to confirm Einstein's theory.</p>

<p>Finally, I should note two things:</p>

<ol>
<li>These are not the only methods of detecting black holes (other effects, such as frame dragging around a Kerr black hole, offer distinctive descriptions of the black hole), but are rather straightforward</li>
<li>1-4 can also be signs of other compact objects such as white dwarfs and neutron stars, and are often only found in binary systems. Only by using a multitude of different observations, and careful analysis, can an object finally be designated a black hole.</li>
</ol>

CC BY-SA 3.0
2014-08-07T19:54:02.430Z
2015-01-10T22:47:51.647Z
2015-01-10T22:47:51.647Z
HDE 226868
<p><a href="http://www.eso.org/public/usa/news/eso0846/" rel="nofollow">16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole</a></p>

<blockquote>
  <p>"Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt," says Genzel. </p>
</blockquote>

<p>Turns out <a href="http://en.wikipedia.org/wiki/S2_%28star%29" rel="nofollow">one of the stars (S2) orbiting</a> this 4 million solar mass object orbits at "<a href="http://en.wikipedia.org/wiki/S2_%28star%29" rel="nofollow">a pericenter distance of 17 light hours (18 Tm or 120 AU) — about four times the distance of Neptune from the Sun.</a>". That doesn't leave room for much <em>besides</em> a black hole to account for the mass within its orbit.</p>

CC BY-SA 3.0
2014-08-07T23:04:05.927Z
2014-08-07T23:04:05.927Z
Wayfaring Stranger
<p>What is the direct observational evidence that black holes exist? I have read that black holes have been detected through X-ray emission. What is some other observational evidence for black holes? </p>

CC BY-SA 3.0
2014-08-07T19:27:59.903Z
Observations of visible stars that indicate they are in a binary system with a massive compact companion.
zibadawa timmy
CC BY-SA 3.0
2014-09-06T21:41:08.647Z
I found a much earlier question, with answers that I think are better than my own: http://astronomy.stackexchange.com/questions/24/how-are-black-holes-found?rq=1
HDE 226868
<black-hole><general-relativity><astrophysics>
Observational evidence for the existence of black holes
Frank
# Observational evidence for the existence of black holes

(Asked by: username_0 on 2014-08-07)

What is the direct observational evidence that black holes exist? I have read that black holes have been detected through X-ray emission. What is some other observational evidence for black holes?

**Tags:** black-hole, general-relativity, astrophysics

> **Comments:**
> **username_1**: Observations of visible stars that indicate they are in a binary system with a massive compact companion.  
> **username_2**: I found a much earlier question, with answers that I think are better than my own: http://astronomy.stackexchange.com/questions/24/how-are-black-holes-found?rq=1  

---

## 2 answers

### Answer by username_2 on 2014-08-07. Score: 5

There are a few different features of black holes that researchers look for:

1. An accretion disk
2. Mass transfer from companion star
3. X-ray emission
4. Absorption of gas and other matter around it
5. Gravitational lensing

You mentioned #3 in your question; quite a few black holes, including Cygnus X-1, emit X-rays.

1 is a common feature around black holes. The material can be pulled from surrounding gases, or, in many cases, a companion star. Accretion disks generally emit electromagnetic radiation; the exact type depends on the type of material and the object it surrounds.

2 is related to 1 and 3, but it deserves its own explanation because of the effects on the donor star. Lets take mass transfer on a large scale. Take the Whirlpool galaxy. It is interacting with its neighbor, NGC 5195, via a bridge of dust and matter. What are the effects on NGC 5195? Distortion, as well as loss of structure in the area near the bridge. The same thing happens on the stellar level. A star losing mass can become distorted in the direction of its companion, potentially causing changes in brightness or other features.

4 is, I suppose related to the others, but I should note that if there were merely gas clouds surrounding the black hole, it could also swallow those up. If a black hole is not in a binary system, it may not come in contact with another star, and will instead begin swallowing other bits of matter around it.

5 is the best characteristic; a better way of detecting black holes. General Relativity predicts that massive objects bend light; using Einstein's equations it is possible to determine just how much light will be bent by such an object. This has been used in many cases, such as during the solar eclipse of 1919, to confirm Einstein's theory.

Finally, I should note two things:

1. These are not the only methods of detecting black holes (other effects, such as frame dragging around a Kerr black hole, offer distinctive descriptions of the black hole), but are rather straightforward
2. 1-4 can also be signs of other compact objects such as white dwarfs and neutron stars, and are often only found in binary systems. Only by using a multitude of different observations, and careful analysis, can an object finally be designated a black hole.


---

### Answer by username_3 on 2014-08-07. Score: 3

[16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole](http://www.eso.org/public/usa/news/eso0846/)

> "Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt," says Genzel.

Turns out [one of the stars (S2) orbiting](http://en.wikipedia.org/wiki/S2_%28star%29) this 4 million solar mass object orbits at "[a pericenter distance of 17 light hours (18 Tm or 120 AU) — about four times the distance of Neptune from the Sun.](http://en.wikipedia.org/wiki/S2_%28star%29)". That doesn't leave room for much *besides* a black hole to account for the mass within its orbit.


---

