by Ann » Wed Feb 16, 2022 6:26 am
VictorBorun wrote: ↑Wed Feb 16, 2022 1:53 am
So, how to tell at glance a galaxy core center including a massive black hole from a new open stellar cluster 100 thousand times less massive?
My guess is to look at colors.
The number of blue super giants in a galaxy core center matches the number of red dwarfs in a new open stellar cluster, but the color tells the difference
Now I wonder what a few most bright stars in the Hidden Galaxy's center may be. Are they stars at all? Maybe they are men of the hour Hubble telescope was taking that picture?
I don't think that the visible colors in a typical photo of the core of a galaxy taken with almost any telescope except Hubble will reveal the blue stars in the core. At least not if the camera settings, exposure times etc are not specifically chosen to reveal details in the core only. The core is typically so much brighter than the rest of the galaxy that you are going to overexpose the core if you try to get a good picture of the disk and spiral arms of the galaxy.
In any case, there is almost always a lot of yellow light in the core from huge numbers of old stars. The way I understand it, stars have formed in the core of a galaxy over billions of years, and stars may also have migrated there. Any predominantly old population of stars is going to be predominantly yellow. But the huge numbers of old stars in the core doesn't mean there can't be young blue clusters there, too.
Compare the two pictures of galaxy M100. The picture on the left was taken with ESO's Very Large Telescope, while the picture at right was taken with Hubble. As you can see, the core of M100 is overexposed in the ESO image, but we can see colors in it in the Hubble picture.
(Admittedly we can also see the colors of the core of M100 in
this SDSS/Giuseppe Donatiello image.)
The core of galaxy M94 by Hubble.
A galaxy with an interesting core is M94. The blue ring surrounding the core is quite large, and can be seen with other telescopes than Hubble. Interestingly, you can see that the color of the star field inside the blue ring is not the same everywhere. Closer to the blue ring, the star field is slightly greenish, which suggests that there is a significant amount of "intermediately aged" stars here, or in other words: This part of the center of M94 is not completely dominated by old stars. But further in, the color of the core is all "old and yellow", until we get to the nucleus itself, which is too overexposed even for Hubble.
Could there be a blue star cluster extremely close to the core of M94, a cluster that we can't see because the nucleus is too overexposed? I guess there could be.
Another very interesting galaxy is NGC 3081:
The same Hubble image processed by Steve Byrne.
As you can see, NGC 3081 has an outer blue ring, an inner blue ring and a "knotty" core. Could there be more star clusters inside that knotty core? Yes, I find it likely.
And take a look at galaxy NGC 1300:
There are definitely young blue star clusters in the core of NGC 1300. Okay, so why was it so much easier for Hubble to photograph the star clusters in the core of IC 342 than in all these other galaxies?
Uhmm... because IC 342 is so much closer to us than the other galaxies here?
Ann
[quote=VictorBorun post_id=320687 time=1644976421 user_id=145500]
So, how to tell at glance a galaxy core center including a massive black hole from a new open stellar cluster 100 thousand times less massive?
My guess is to look at colors.
The number of blue super giants in a galaxy core center matches the number of red dwarfs in a new open stellar cluster, but the color tells the difference
Now I wonder what a few most bright stars in the Hidden Galaxy's center may be. Are they stars at all? Maybe they are men of the hour Hubble telescope was taking that picture?
[/quote]
I don't think that the visible colors in a typical photo of the core of a galaxy taken with almost any telescope except Hubble will reveal the blue stars in the core. At least not if the camera settings, exposure times etc are not specifically chosen to reveal details in the core only. The core is typically so much brighter than the rest of the galaxy that you are going to overexpose the core if you try to get a good picture of the disk and spiral arms of the galaxy.
In any case, there is almost always a lot of yellow light in the core from huge numbers of old stars. The way I understand it, stars have formed in the core of a galaxy over billions of years, and stars may also have migrated there. Any predominantly old population of stars is going to be predominantly yellow. But the huge numbers of old stars in the core doesn't mean there can't be young blue clusters there, too.
[float=left][img3="Galaxy M100. Photo: ESO and the FORS instrument on ESO’s Very Large Telescope at Paranal Observatory in Chile. The core is overexposed."]https://cdn.eso.org/images/screen/potw1330a.jpg[/img3][/float][float=right][attachment=2]stsci-h-p1848e-z-1000x990[1].png[/attachment][c][size=85][color=#0040FF]M100 photo taken by Hubble. We can see the colors of the core.
https://www.nasa.gov/feature/goddard/2017/messier-100 [/color][/size][/c][/float]
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Compare the two pictures of galaxy M100. The picture on the left was taken with ESO's Very Large Telescope, while the picture at right was taken with Hubble. As you can see, the core of M100 is overexposed in the ESO image, but we can see colors in it in the Hubble picture.
(Admittedly we can also see the colors of the core of M100 in [url=https://commons.wikimedia.org/wiki/File:M100_group_(28104666950).jpg]this SDSS/Giuseppe Donatiello image[/url].)
[float=left][attachment=1]Starburst_galaxy_Messier_94[1].jpg[/attachment][c][size=85][color=#0040FF]The core of galaxy M94 by Hubble.[/color][/size][/c][/float][float=right][img3="Galaxy M94 by R. Jay GaBany. The field of view is much larger here than in the image at left, and we see much more of the galaxy. The core therefore looks much smaller. Note that there are two blue rings visible in GaBany's image, but the Hubble image only shows us the inner blue ring."]https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Messier_94.jpg/1024px-Messier_94.jpg[/img3][/float]
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A galaxy with an interesting core is M94. The blue ring surrounding the core is quite large, and can be seen with other telescopes than Hubble. Interestingly, you can see that the color of the star field inside the blue ring is not the same everywhere. Closer to the blue ring, the star field is slightly greenish, which suggests that there is a significant amount of "intermediately aged" stars here, or in other words: This part of the center of M94 is not completely dominated by old stars. But further in, the color of the core is all "old and yellow", until we get to the nucleus itself, which is too overexposed even for Hubble.
Could there be a blue star cluster extremely close to the core of M94, a cluster that we can't see because the nucleus is too overexposed? I guess there could be.
Another very interesting galaxy is NGC 3081:
[float=left][img3="This Hubble image shows us an outer blue ring and an inner blue ring of NGC 3081, as well as a 'knotty' core. https://cdn.spacetelescope.org/archives/images/large/potw1423a.jpg"]https://cdn.spacetelescope.org/archives/images/large/potw1423a.jpg[/img3][/float][float=right][attachment=0]NGC 3081 Hubble Steve Byrne.png[/attachment][c][size=85][color=#0040FF]The same Hubble image processed by Steve Byrne.[/color][/size][/c][/float]
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As you can see, NGC 3081 has an outer blue ring, an inner blue ring and a "knotty" core. Could there be more star clusters inside that knotty core? Yes, I find it likely.
And take a look at galaxy NGC 1300:
[float=left][img3="The bulge, bar and core of NGC 1300. Image Credit: Hubble Heritage Team, ESA, NASA"]https://apod.nasa.gov/apod/image/0501/ngc1300_hst_core.jpg[/img3][/float][float=right][img3="The core of NGC 1300. Note the blue star clusters. Source: Researchgate, uploaded by Richard de Grijs."]https://www.researchgate.net/profile/Richard-De-Grijs/publication/323571321/figure/fig1/AS:601024356163584@1520306781236/Examples-of-rejected-rings-Left-Red-dusty-ring-in-the-elliptical-galaxy-NGC-3258-the_Q640.jpg[/img3][/float]
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There are definitely young blue star clusters in the core of NGC 1300. Okay, so why was it so much easier for Hubble to photograph the star clusters in the core of IC 342 than in all these other galaxies?
Uhmm... because IC 342 is so much closer to us than the other galaxies here?
Ann