APOD: The Fornax Cluster of Galaxies (2023 Nov 02)

Comments and questions about the APOD on the main view screen.
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Ann
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Re: APOD: The Fornax Cluster of Galaxies (2023 Nov 02)

Post by Ann » Sun Nov 05, 2023 7:57 pm

johnnydeep wrote: Sun Nov 05, 2023 3:11 pm Oh, one more question. How far apart do you suppose the stars are in the halo, at, say, the location of the two Xs below?


And would it be similar to the separation of the stars near the Sun?


Interesting questions and hard to answer. Let me begin with the number and type of stars within 20 light-years of your red crosses in NGC 1380. I don't think you would find any A-type stars at all in a random sphere with a diameter of 20 light-years in NGC 1380.

That's not to say that there aren't any A-type stars at all in NGC 1380. There might be some, and indeed, I really think you will find some in the dust lane near the core. But because there is no ongoing large scale star formation at all in NGC 1380, and there clearly hasn't been any for at least a billion years, any A-type stars in NGC 1380 will be extremely few and far between.


Here's my point. I believe that the stellar populations are more or less the same everywhere in NGC 1380, except in the dust lane and possibly in the core.


The overall color of NGC 1380 is somewhere between Capella (far left in the second row) and Pollux (second from left in the fourth row). If you ask me, most of the light in NGC 1380 comes from stars like Capella, Pollux and Arcturus. However, most of the stars in NGC 1380 will be extremely faint M-type dwarfs (= red dwarfs), faint but a bit brighter K-type dwarfs (= orange dwarfs) and still brighter G-type stars (= yellow dwarfs).

In fact, in a random sphere with a diameter of 20 light-years at the general position of your crosses in NGC 1380, chances are high that there are no giant stars at all, no Capellas, Polluxes or Arcturuses. Chances are that the stellar population would be similar to that within 20 light-years of the Sun, except that NGC 1380 will boast no A-type stars and probably no F-type stars either.

So we could make an educated guess at what kind of stars we would find at the position of your red crosses in NGC 1380. By measuring the surface brightness of NGC 1380 at these points, and bearing in mind what type of stars we are likely to encounter here, we can indeed make an educated guess at the stellar density at these points.

Maybe, as you said, it would be similar to stellar density in our own neck of the Milky Way woods. But I don't know enough to make a good guess here.

Ann
Last edited by Ann on Sun Nov 05, 2023 8:11 pm, edited 1 time in total.
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johnnydeep
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Re: APOD: The Fornax Cluster of Galaxies (2023 Nov 02)

Post by johnnydeep » Sun Nov 05, 2023 8:06 pm

Thank you Ann.
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Ann
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Re: APOD: The Fornax Cluster of Galaxies (2023 Nov 02)

Post by Ann » Sun Nov 05, 2023 9:02 pm

johnnydeep wrote: Sun Nov 05, 2023 8:06 pm Thank you Ann.
We should bear in mind that the red dwarf stars are so faint that they affect the overall surface brightness of a galaxy hardly at all. So if there is an excess of red dwarf stars in NGC 1380, then there might be a surprisingly high density of stars in the positions of your red crosses in NGC 1380.

Normally a great excess of red dwarf stars is only found in the really large elliptical galaxies, like M87. But it seems certain that the dominant elliptical galaxy of the Fornax Cluster, NGC 1399, also has more than its fair share of them.

Whether the density of red dwarf stars near your red crosses in NGC 1380 in higher than in the solar neighborhood in the Milky Way, I frankly have no idea.

Ann
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