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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?

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  • $\begingroup$ 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. $\endgroup$
    – Steve Linton
    Commented Apr 14, 2019 at 10:43
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    $\begingroup$ Also, the observations are made in radio, where intervening gas and dust hardly absorbs anything. $\endgroup$
    – pela
    Commented Apr 14, 2019 at 10:47
  • $\begingroup$ It isn't perfectly dark. $\endgroup$
    – ProfRob
    Commented Apr 14, 2019 at 21:35

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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.

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