<p>Based on the comment from @SteveLinton</p>

<p>The radius of M87 is given to be 60,000 light years, which is a volume of 900 trillion cubic light years.  Given that it contains roughly one trillion stars, there is an average of 900 cubic light years for each star.</p>

<p>Several resources referred to the diameter of the black hole in the range of 1.5 light days (38 billion km).  That's 0.004 light years.</p>

<p>If you consider the cylinder<sup>1</sup> extending from the black hole to the edge of M87, the cylinder is 0.004 light years in diameter with a height of 60,000 light years.  The volume of that cylinder is just over 3 cubic light years.  Since there is one star for every 900 cubic light years, it follows that the cylinder encompassing our line of sight between the black hole and the edge of M87 is empty of any stars.</p>

<p>Whether or not any star would even be bright enough to be visible is another matter.</p>

<p><sup>1</sup> <sub>Technically a conic frustum but I don't think the difference in volume is significant.</sub></p>

CC BY-SA 4.0
2019-04-14T19:15:58.240Z
*"The radius of M87 is given to be 60,000 light years.*". No, more like 32 - 50 kiloparsecs radius = 100,000 - 160,000 light years radius. E.g. *"[According to Stoyan et al. (2010), the distance of M87 is 54.9 million light years and its diameter is 132,000 light years.](http://astropixels.com/galaxies/M87-01.html)"* and *"[Diameter: 120,000 ly](https://space-facts.com/m87-galaxy/)"*.
Peter Mortensen
CC BY-SA 4.0
2019-04-14T19:27:17.297Z
Doesn't this answer assume that the galaxy is of uniform density?
JBentley
CC BY-SA 4.0
2019-04-14T21:12:20.723Z
@PeterMortensen I'm not sure I understand.  Your sources cite a diameter of 120,000 LY and 132,000 LY.  I'm using a radius of 60,000 LY.  Radius = 1/2 Diameter.
Bumptious Q Bangwhistle
CC BY-SA 4.0
2019-04-14T21:15:43.260Z
@JBentley  Yes and I appreciate this is a fallacy (Olbers' Paradox).
Bumptious Q Bangwhistle
CC BY-SA 4.0
2019-04-14T11:29:58.190Z
2019-04-14T11:37:30.437Z
2019-04-14T11:37:30.437Z
Bumptious Q Bangwhistle
<p>The M87 black hole is at the centre, more or less, of an elliptical galaxy of 1 trillion or more stars. So certainly there are stars and other light emitting objects (emission nebula e.g.) in the intervening 53 million light years between earth and the black hole.</p>

<p>How is it that the centre of the black hole image is perfectly dark, as if it is being observed directly, with not one point of light between the black hole and earth? I appreciate that space is mostly empty, but do we really have an unobscured line of sight to a black hole in the centre of an enormous elliptical galaxy?</p>

<p>Does it have something to do with the type of light being observed?</p>

CC BY-SA 4.0
2019-04-14T10:43:28.790Z
I think it's mostly that the field of view is incredibly tiny. The popular published image seems to be about four times the diameter of the central dark region,  so representing about 800 billion km at the distance of M87 or about 0.1 light year. At the distance of the edge of our galaxy, it would represent about 1/1000 of that. There simply isn't anything in that very very narrow cone bright enough to show up.
Steve Linton
CC BY-SA 4.0
2019-04-14T10:47:02.347Z
Also, the observations are made in radio, where intervening gas and dust hardly absorbs anything.
pela
CC BY-SA 4.0
2019-04-14T21:35:03.453Z
It **isn't** perfectly dark.
ProfRob
<black-hole><m87>
Where is the intervening light in the M87 black hole?
Bumptious Q Bangwhistle
# Where is the intervening light in the M87 black hole?

(Asked by: username_0 on 2019-04-14)

The M87 black hole is at the centre, more or less, of an elliptical galaxy of 1 trillion or more stars. So certainly there are stars and other light emitting objects (emission nebula e.g.) in the intervening 53 million light years between earth and the black hole.

How is it that the centre of the black hole image is perfectly dark, as if it is being observed directly, with not one point of light between the black hole and earth? I appreciate that space is mostly empty, but do we really have an unobscured line of sight to a black hole in the centre of an enormous elliptical galaxy?

Does it have something to do with the type of light being observed?

**Tags:** black-hole, m87
> **Comments:**
> **username_1**: I think it's mostly that the field of view is incredibly tiny. The popular published image seems to be about four times the diameter of the central dark region,  so representing about 800 billion km at the distance of M87 or about 0.1 light year. At the distance of the edge of our galaxy, it would represent about 1/1000 of that. There simply isn't anything in that very very narrow cone bright enough to show up.  
> **username_2**: Also, the observations are made in radio, where intervening gas and dust hardly absorbs anything.  
> **username_3**: It **isn't** perfectly dark.  

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## 1 answers

### Answer by username_0 on 2019-04-14. Score: 7

Based on the comment from @SteveLinton

The radius of M87 is given to be 60,000 light years, which is a volume of 900 trillion cubic light years. Given that it contains roughly one trillion stars, there is an average of 900 cubic light years for each star.

Several resources referred to the diameter of the black hole in the range of 1.5 light days (38 billion km). That's 0.004 light years.

If you consider the cylinder1 extending from the black hole to the edge of M87, the cylinder is 0.004 light years in diameter with a height of 60,000 light years. The volume of that cylinder is just over 3 cubic light years. Since there is one star for every 900 cubic light years, it follows that the cylinder encompassing our line of sight between the black hole and the edge of M87 is empty of any stars.

Whether or not any star would even be bright enough to be visible is another matter.

1 Technically a conic frustum but I don't think the difference in volume is significant.

> **Comments:**
> **username_4**: *"The radius of M87 is given to be 60,000 light years.*". No, more like 32 - 50 kiloparsecs radius = 100,000 - 160,000 light years radius. E.g. *"[According to Stoyan et al. (2010), the distance of M87 is 54.9 million light years and its diameter is 132,000 light years.](http://astropixels.com/galaxies/M87-01.html)"* and *"[Diameter: 120,000 ly](https://space-facts.com/m87-galaxy/)"*.  
> **username_5**: Doesn't this answer assume that the galaxy is of uniform density?  
> **username_0**: @PeterMortensen I'm not sure I understand.  Your sources cite a diameter of 120,000 LY and 132,000 LY.  I'm using a radius of 60,000 LY.  Radius = 1/2 Diameter.  
> **username_0**: @JBentley  Yes and I appreciate this is a fallacy (Olbers' Paradox).  

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