<p>The Bondi radius is <span class="math-container">$2MG/s^2$</span> where M is the mass, G is the gravitational constant and <span class="math-container">$s$</span> is the speed of <em>sound</em> in the medium of the accretion disk.  It represents the boundary between sub-sonic and super-sonic infall of matter into the black hole.</p>

<p>The Schwartzchild radius is <span class="math-container">$2MG/c^2$</span> where <span class="math-container">$c$</span> is the speed of light and is the radius of the event horizon.</p>

CC BY-SA 4.0
2019-01-02T00:39:44.027Z
The OP's error possibly derives from the [Wikipedia page](https://en.wikipedia.org/wiki/Bondi_accretion), which gives the denominator in the expression as $c_{s}^{2}$ where $c_{s}$ is defined as the sound speed in the surrounding medium. A good lesson in paying attention to subscripts ;-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-01-01T18:38:21.597Z
2019-01-01T18:38:21.597Z
James K
<p><span class="math-container">$$2MG/c^2$$</span> </p>

<p>This is the expression for both, but some authors use one name and it has confused me. Is there any distinction?</p>

<black-hole>
Is the Bondi Radius the Same as the Schwarzschild radius?
lmm_5000
# Is the Bondi Radius the Same as the Schwarzschild radius?

(Asked by: username_0 on 2019-01-01)

$$2MG/c^2$$

This is the expression for both, but some authors use one name and it has confused me. Is there any distinction?

**Tags:** black-hole

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## 1 answers

### Answer by username_1 on 2019-01-01. Score: 7 | Accepted answer

The Bondi radius is $2MG/s^2$ where M is the mass, G is the gravitational constant and $s$ is the speed of *sound* in the medium of the accretion disk. It represents the boundary between sub-sonic and super-sonic infall of matter into the black hole.

The Schwartzchild radius is $2MG/c^2$ where $c$ is the speed of light and is the radius of the event horizon.

> **Comments:**
> **username_2**: The OP's error possibly derives from the [Wikipedia page](https://en.wikipedia.org/wiki/Bondi_accretion), which gives the denominator in the expression as $c_{s}^{2}$ where $c_{s}$ is defined as the sound speed in the surrounding medium. A good lesson in paying attention to subscripts ;-)  

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