<p>Let's have a look at the <a href="https://en.wikipedia.org/wiki/Black_hole#Physical_properties" rel="nofollow noreferrer">Physical_properties</a> subsection of the Wikipedia article on Black holes:</p>

<p>The <a href="https://en.wikipedia.org/wiki/Schwarzschild_radius" rel="nofollow noreferrer">Schwarzschild radius</a> which is defined only for a non-rotating black hole is given there as </p>

<p><span class="math-container">$$r_S=\frac{2GM}{c^2}\approx 2.95 \frac{M}{M_{Sun}} \text{km}.$$</span></p>

<blockquote>
  <p>The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.</p>
</blockquote>

<p>So the Schwarzschild radius increases linearly with mass, by 2.95 kilometers for every solar mass you add, provided you add it carefully and don't spill any.</p>

<p>Trying to express the "size" of the event horizon for an arbitrary black hole is going to be more complicated, so I'm not going to adventure into that territory. However, @Chappo's <a href="https://astronomy.stackexchange.com/questions/28755/need-help-with-blackhole-question-asap#comment51238_28755">comment</a> suggests the dependence on mass will also be approximately linear as well.</p>

CC BY-SA 4.0
2019-01-31T06:34:33.563Z
"provided you add it carefully and don't spill any" - lol. Actually, come to think of it, that might be a lot harder than it seems, what with the various mechanisms that tend to oppose reaching 100% efficiency for accretion. Once you have a large, superheated accretion disk, things are going to get pretty difficult.
Florin Andrei
CC BY-SA 4.0
2019-01-31T06:48:50.907Z
@FlorinAndrei exactly, thank you! I didn't have my "science words" with me that day, so it's the only way I could think of to say it. ;-)
uhoh
CC BY-SA 4.0
2019-01-31T06:50:02.453Z
@FlorinAndrei I think the question shouldn't have been so heavily down-voted though.
uhoh
CC BY-SA 4.0
2018-12-17T08:54:14.387Z
2018-12-17T08:54:14.387Z
uhoh
<p>Suppose the amount of mass in a black hole doubles. Does the event horizon change? If so, how much does it change?</p>

<black-hole><mass><event-horizon>
Need help with a simple Blackhole question
user25192
# Need help with a simple Blackhole question

(Asked by: username_0 on 2018-12-17)

Suppose the amount of mass in a black hole doubles. Does the event horizon change? If so, how much does it change?

**Tags:** black-hole, mass, event-horizon


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## 1 answers

### Answer by username_1 on 2018-12-17. Score: 1

Let's have a look at the [Physical\_properties](https://en.wikipedia.org/wiki/Black_hole#Physical_properties) subsection of the Wikipedia article on Black holes:

The [Schwarzschild radius](https://en.wikipedia.org/wiki/Schwarzschild_radius) which is defined only for a non-rotating black hole is given there as

$$r\_S=\frac{2GM}{c^2}\approx 2.95 \frac{M}{M\_{Sun}} \text{km}.$$

> The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.

So the Schwarzschild radius increases linearly with mass, by 2.95 kilometers for every solar mass you add, provided you add it carefully and don't spill any.

Trying to express the "size" of the event horizon for an arbitrary black hole is going to be more complicated, so I'm not going to adventure into that territory. However, @Chappo's [comment](https://astronomy.stackexchange.com/questions/28755/need-help-with-blackhole-question-asap#comment51238_28755) suggests the dependence on mass will also be approximately linear as well.

> **Comments:**
> **username_2**: "provided you add it carefully and don't spill any" - lol. Actually, come to think of it, that might be a lot harder than it seems, what with the various mechanisms that tend to oppose reaching 100% efficiency for accretion. Once you have a large, superheated accretion disk, things are going to get pretty difficult.  
> **username_1**: @username_2 exactly, thank you! I didn't have my "science words" with me that day, so it's the only way I could think of to say it. ;-)  
> **username_1**: @username_2 I think the question shouldn't have been so heavily down-voted though.  

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