APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

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APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by APOD Robot » Sun Jul 31, 2022 4:05 am

Image Starburst Galaxy M94 from Hubble

Explanation: Why does this galaxy have a ring of bright blue stars? Beautiful island universe Messier 94 lies a mere 15 million light-years distant in the northern constellation of the Hunting Dogs (Canes Venatici). A popular target for Earth-based astronomers, the face-on spiral galaxy is about 30,000 light-years across, with spiral arms sweeping through the outskirts of its broad disk. But this Hubble Space Telescope field of view spans about 7,000 light-years across M94's central region. The featured close-up highlights the galaxy's compact, bright nucleus, prominent inner dust lanes, and the remarkable bluish ring of young massive stars. The ring stars are all likely less than 10 million years old, indicating that M94 is a starburst galaxy that is experiencing an epoch of rapid star formation from inspiraling gas. The circular ripple of blue stars is likely a wave propagating outward, having been triggered by the gravity and rotation of a oval matter distributions. Because M94 is relatively nearby, astronomers can better explore details of its starburst ring.

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Ann » Sun Jul 31, 2022 6:49 am


I love this galaxy, and I love this Hubble image of it! :D

I love the fact that the bright center is yellow-white, and I love the fact that the smooth bright area outside the center is ever so slightly greenish. This color contrast is extremely obvious in James D. Wray's Color Atlas of Galaxies, where galaxies are shown in UBV. The greenish area contains a larger proportion of A- and F-type stars than the bright yellow center does.

Of course, what I love best about this Hubble image is the exquisitely lovely "tiara" of brilliant blue-white sapphires, interspersed with red giant rubies, that M94 is wearing around its bright green and yellow "head"!

It is really striking how both the bright yellow center and the faintly greenish area surrounding it are "smooth", whereas the garland of blue stars is extremely grainy. Very many of the blue stars pop out at us individually, because they are so bright (and they are seen against a not-so-bright background), whereas the stars near the center of M94 are typically much fainter, and they are seen against a much "starrier" background too, where the light from individual stars tend to blend together.

This brings us to the question of stellar populations are typically like. How bright do stars of different spectral classes tend to be, and what color are they?

Let's start with a picture of the rich Wild Duck Cluster, M11, whose estimated age is around 300 million years:


I apologize to bystander for posting a picture that is too big, but it is not that big after all.

The Wild Duck Cluster is rich in bright blue-white stars, but there are also a number of bright yellow-white stars. The yellow stars are evolved red giants, whereas the blue stars belong to spectral classes A and F. At least one of them, HD 174512, is evolved and brighter than it used to be.

The bright stars in M11 clearly look brighter than the background stars. Note that some very red stars are scattered all over the background. My guess is that many or most of these are very dust-reddened background stars, although some might be extra cool M-type giants (or even, in a few cases, very nearby M-type dwarfs).

But what does a typical galactic bulge of yellow stars look like close up, when zoomed in?


As you can see, most of the stars in the bulge of the Milky Way are small stars. The ones we can see in the picture probably belong to spectral classes G and K, whereas most main sequence stars of spectral class M are probably too faint to show up here. The bright stars are probably in most cases evolved giants, though a few may be foreground stars, perhaps in some cases of spectral classes A and F.

Finally, look at this picture of Andromeda by Hubble:


But go to this page, read the caption and find the zoom tool. And then you can use the zoom tool to zoom in and zoom in to Andromeda and marvel at the enormous numbers of individual stars that can be resolved everywhere, even near the core. Because there are individual bright stars everywhere, at least red giant stars, even in the reddest and deadest galaxies or in the yellowest parts of galaxies.

Ann
Last edited by bystander on Sun Jul 31, 2022 5:13 pm, edited 2 times in total.
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by 251billyg » Sun Jul 31, 2022 8:32 am

If the stars are less than 10 million years old(all the blue ones in the star forming spiral), then how did the light reach us-being that M94 is 15 million light years distant ?

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Ann » Sun Jul 31, 2022 8:59 am

251billyg wrote: Sun Jul 31, 2022 8:32 am If the stars are less than 10 million years old(all the blue ones in the star forming spiral), then how did the light reach us-being that M94 is 15 million light years distant ?
Some blue stars are older than 10 million years. But regardless of that, the vast majority of the blue stars in the starburst ring of M94 were less than 10 million years old when the light from M94 that is now reaching us was emitted.

It will take another 15 million years before we can see what M94 looks like "now". Sorry.

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by orin stepanek » Sun Jul 31, 2022 12:35 pm

M94_Hubble_960.jpg
Truly is beautiful; There are stars in the brown-gray (yellow) center
area but I had to zoom to see them! The beautiful blue stars must
be large!
kn7wz036va161.jpg
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Chris Peterson » Sun Jul 31, 2022 1:07 pm

251billyg wrote: Sun Jul 31, 2022 8:32 am If the stars are less than 10 million years old(all the blue ones in the star forming spiral), then how did the light reach us-being that M94 is 15 million light years distant ?
In astronomy, we almost always consider age to be the age at which we see something. Because that's all that matters.
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by VictorBorun » Sun Jul 31, 2022 9:05 pm

Ann wrote: Sun Jul 31, 2022 6:49 am individual stars that can be resolved everywhere, even near the core. Because there are individual bright stars everywhere, at least red giant stars, even in the reddest and deadest galaxies or in the yellowest parts of galaxies.
Ann
I wonder if a direct link to zoomable Andromeda works.

I have never seen a stellar population from outside the galaxy disk.
Well, it seems red giants are many and blue giants are only at some clumps in far outskirts

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by johnnydeep » Sun Jul 31, 2022 9:42 pm

VictorBorun wrote: Sun Jul 31, 2022 9:05 pm
Ann wrote: Sun Jul 31, 2022 6:49 am individual stars that can be resolved everywhere, even near the core. Because there are individual bright stars everywhere, at least red giant stars, even in the reddest and deadest galaxies or in the yellowest parts of galaxies.
Ann
I wonder if a direct link to zoomable Andromeda works.

I have never seen a stellar population from outside the galaxy disk.
Well, it seems red giants are many and blue giants are only at some clumps in far outskirts
Well, the zoomable link works as long as I click on it and follow it. And, my god - it's full of stars all the way down! :)

andromeda zoomed in portion.JPG
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Chris Peterson » Sun Jul 31, 2022 10:03 pm

Ann wrote: Sun Jul 31, 2022 6:49 am

But go to this page, read the caption and find the zoom tool. And then you can use the zoom tool to zoom in and zoom in to Andromeda and marvel at the enormous numbers of individual stars that can be resolved everywhere, even near the core. Because there are individual bright stars everywhere, at least red giant stars, even in the reddest and deadest galaxies or in the yellowest parts of
"Resolved" only in a very limited sense of the word, of course. Every star in this image is much smaller than a single pixel, so none are actually resolved. It's just that if a star is bright enough it forms an image above the background stars. We don't really have a good word for that... so "resolved" will have to do, with qualifications.
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by VictorBorun » Sun Jul 31, 2022 11:37 pm

if M94 is 15/2.5 = 6 times as distant as Andromeda we need a telescope with a primary mirror of 2.4*6 = 14.4 m in diameter to resolve stars as good as HST did.
Alas JWST is just 6.4 m.
But earth-based telescopes with guiding Na troposphere "stars" to estimate and correct atmosphere fluctuations are large…

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by VictorBorun » Sun Jul 31, 2022 11:53 pm

I wonder why large dusty lanes of M94:
1) are not symmetric by 180° rotation around the centre of the core
2) seem to be in front of the ring of the blue giant stars

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Chris Peterson » Sun Jul 31, 2022 11:57 pm

VictorBorun wrote: Sun Jul 31, 2022 11:37 pm if M94 is 15/2.5 = 6 times as distant as Andromeda we need a telescope with a primary mirror of 2.4*6 = 14.4 m in diameter to resolve stars as good as HST did.
Alas JWST is just 6.4 m.
But earth-based telescopes with guiding Na troposphere "stars" to estimate and correct atmosphere fluctuations are large…
It doesn't really matter how far away the galaxy is. HST can "resolve" stars in M94 as well as in Andromeda, because it is actually resolving them in neither, merely detecting them. So the only role distance and aperture play is in how bright a star needs to be to give a signal sufficiently above the noise. You can "resolve" a supernova in a distant galaxy with your eye and a pair of binoculars.

To actually resolve a (very large) star in Andromeda would require a mirror at least 50 km in diameter.
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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by VictorBorun » Mon Aug 01, 2022 10:32 am

Chris Peterson wrote: Sun Jul 31, 2022 11:57 pm To actually resolve a (very large) star in Andromeda would require a mirror at least 50 km in diameter.
what are stars that we do resolve?

with its 2.4 m mirror, Hubble can resolve 2 stars .5 μm/2.4 m = .2 μradian apart; at Andromeda's distance of 2.5 million ly those 2 stars are .5 ly apart.

If the dense thin disk of Andromeda is 2000 ly thick, or 5000 ly thick at an angle, then one star volume is 0.5 ly × 0.5 ly × 5000 ly = (10 ly)³.
10 ly is probably too bad resolution to make Suns but maybe good enough to make Siriuses and rare Betelgeuses among Siriuses ?

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Re: APOD: Starburst Galaxy M94 from Hubble (2022 Jul 31)

Post by Chris Peterson » Mon Aug 01, 2022 2:17 pm

VictorBorun wrote: Mon Aug 01, 2022 10:32 am
Chris Peterson wrote: Sun Jul 31, 2022 11:57 pm To actually resolve a (very large) star in Andromeda would require a mirror at least 50 km in diameter.
what are stars that we do resolve?

with its 2.4 m mirror, Hubble can resolve 2 stars .5 μm/2.4 m = .2 μradian apart; at Andromeda's distance of 2.5 million ly those 2 stars are .5 ly apart.

If the dense thin disk of Andromeda is 2000 ly thick, or 5000 ly thick at an angle, then one star volume is 0.5 ly × 0.5 ly × 5000 ly = (10 ly)³.
10 ly is probably too bad resolution to make Suns but maybe good enough to make Siriuses and rare Betelgeuses among Siriuses ?
Resolving two stars as separated objects, though, is very different from resolving a star.
Chris

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