APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

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APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by APOD Robot » Fri Apr 26, 2024 4:08 am

Image Regulus and the Dwarf Galaxy

Explanation: In northern hemisphere spring, bright star Regulus is easy to spot above the eastern horizon. The alpha star of the constellation Leo, Regulus is the spiky star centered in this telescopic field of view. A mere 79 light-years distant, Regulus is a hot, rapidly spinning star that is known to be part of a multiple star system. Not quite lost in the glare, the fuzzy patch just below Regulus is diffuse starlight from small galaxy Leo I. Leo I is a dwarf spheroidal galaxy, a member of the Local Group of galaxies dominated by our Milky Way Galaxy and the Andromeda Galaxy (M31). About 800 thousand light-years away, Leo I is thought to be the most distant of the known small satellite galaxies orbiting the Milky Way. But dwarf galaxy Leo I has shown evidence of a supermassive black hole at its center, comparable in mass to the black hole at the center of the Milky Way.

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Ann » Fri Apr 26, 2024 6:49 am

Wow, that's some spectacular info about tiny dwarf galaxy Leo I! :shock:

But first, let's consider foreground star Regulus:

Regulus_Dwarf_by_Markus_Horn1024[1].png
Regulus and the Dwarf Galaxy
Image Credit & Copyright: Markus Horn

Regulus is the closest of all B-type stars at 79 light-years from the Earth. It is a four-star system, whose dominant member - Regulus A - is spectral class B8IV. Regulus A has just exhausted its core hydrogen and is about to leave the main sequence. It is still blue, at a surface temperature of about 11,700 K, almost exactly twice that of the Sun, with a mass of 3.8 solar and a luminosity of ~316 times solar. Regulus A is rotating at a breakneck pace, and according to Wikipedia, its equatorial surfaces rotates at about 320 kilometers per second (199 miles per second), which is 96.5% of its critical angular velocity for break-up!!! I mean!!! :shock:

Because of its extremely rapid spin, Regulus A is a highly flattened star. This is what the four-star system looks like (sizes and distances are not to scale):


The orange and red stars in the illustration are small cool faint main sequence stars of spectral classes K and M, whereas the closest companion to Regulus A is believed to be a white dwarf. But if that is the case, then the Regulus system would be at least a billion years old, according to Wikipedia, just to account for the formation of the white dwarf. Small objection from the Color Commentator: Have the Wikipedia people heard about Sirius and its white dwarf companion? Is Sirius also a billion years old?

Anyway. If Regulus A is a billion years old, then it can't have been born with its current mass, 3.8 solar masses, because stars that massive don't remain on the main sequence for that long. So it must have gained mass during its main sequence lifetime, probably from its now-white dwarf companion. In other words, if the closest companion of Regulus A is a white dwarf, then Regulus A must be a blue straggler and a post-Algol star.


One last note about Regulus: It lies only 0.465 degrees from the ecliptic, and it is regularly occulted by the Moon!


Okay! Now let's start talking about Leo I. It's a tiny, puny galaxy, whose mass is only about (2.0 ± 1.0) × 107 M, according to Wikipedia. Is that a lot? Heck no, gimme five Omega Centauris and you've got the mass of Leo I!

Milky Way with Omega Centauri Gaia annotated.png
The Milky Way with Omega Centauri by Gaia. Credit: ESA.

But in spite of its puny size, Leo I may have a central black hole comparable in mass to the central black hole of the Milky Way, Sgr A*! The mass of Sgr A* is some 4.3 million times the Sun, and the black hole of tiny little Leo I may be some 3 million times the Sun!! That's ridiculous! 😮 😮 😮


According to Fabio Paccuci et al., Leo I, which is one of the most distant satellites of the Milky Way, may have lost up to 90% of its mass after passing close to the Milky Way twice. We must assume that Leo I is on an elongated orbit. Of course, the mass lost by Leo I would not affect the mass of its black hole.

If Leo I has lost so much mass during its passages of the Milky Way, it must have left behind an observable stellar stream:


But according to Fabio Paccuci et al., we may be viewing the Leo I stellar stream end on, so that it is very hard to see it. The Gaia telescope may reveal the stream to us, they hope.

Well, wow!!! I'm just saying!!! I'm still flabbergasted!

Ann
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by skyrish » Fri Apr 26, 2024 12:05 pm

Well I was going to ask at what point we differentiate between a star cluster and dwarf spheroidal galaxy, but thanks to the color commentary I am guessing the black hole at the center may be a factor. :idea:

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Christian G. » Fri Apr 26, 2024 12:15 pm

The "little king" star, a "dwarf" galaxy, and one HUMONGOUS black hole! As supermassive as the Milky Way's but in a galaxy fifty times smaller!
Last edited by Christian G. on Fri Apr 26, 2024 4:28 pm, edited 1 time in total.

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Chris Peterson » Fri Apr 26, 2024 12:55 pm

skyrish wrote: Fri Apr 26, 2024 12:05 pm Well I was going to ask at what point we differentiate between a star cluster and dwarf spheroidal galaxy, but thanks to the color commentary I am guessing the black hole at the center may be a factor. :idea:
Some globular clusters have central black holes.

I think there are at least two differences. A globular cluster has a different radial density gradient than a galaxy. And a globular cluster has a very different HR diagram than a galaxy, indicating a different formation process.
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Roy

Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Roy » Fri Apr 26, 2024 1:40 pm

So many unanswered questions - flabbergast to the max.
First - what mechanism causes a star to rotate at such an enormous rate? Sucking gas off another star won’t do it.
Second - try tracking down the two assertions about LEO I. Central density comes from a model which requires a central black hole to work out right. So they posit it. The Black hole comes from a study exploring possibility of a dark matter halo, which cannot be shown, but supposing a black hole makes the model more or less work. So there is no evidence! it is publish or perish to keep the rice bowls filled.
Third - tidal stream of stars. There isn’t one. Does that mean the orbital motion isn’t happening? Nonono, we are just seeing it endwise. Flabbergasting.

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Ann » Fri Apr 26, 2024 1:46 pm

skyrish wrote: Fri Apr 26, 2024 12:05 pm Well I was going to ask at what point we differentiate between a star cluster and dwarf spheroidal galaxy, but thanks to the color commentary I am guessing the black hole at the center may be a factor. :idea:
I'll try to answer. Globular clusters are typically compact and small in size.


As you can see, the Fornax dwarf galaxy is faint and extended, whereas its globular clusters are small and bright. They are bright because the stars are packed so closely together. But not all globular clusters are equally dense:


I said in my previous post that the mass of dwarf galaxy Leo I is about five (actually perhaps six) times the mass of globular cluster Omega Centauri. So how much larger is Leo I than Omega Centauri? Well, according to Google, the diameter of Leo I is about 2,000 light-years, whereas the diameter of Omega Centauri is (also according to Google) 172 light-years. In other words, the diameter of Leo I is a bit more than 10 times the diameter of Omega Centauri. Let's take a look at the sizes of the Sun and Jupiter to understand the comparative sizes of Leo I and Omega Centauri:


Another characteristic of globular clusters is that their stars are metal-poor. That is to say, their stars are made up of almost exclusively of hydrogen and helium, whereas stars like the Sun contain a much higher concentration of heavier elements like oxygen, carbon and iron.

Admittedly, Leo I is also very metal-poor and also very deficient in elements like oxygen, carbon and iron. But Leo I is too large and too far from the Milky Way galaxy - some 800,000 light-years - to be one of our galaxy's globular clusters.

Ann
Last edited by Ann on Fri Apr 26, 2024 4:07 pm, edited 1 time in total.
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Chris Peterson » Fri Apr 26, 2024 1:58 pm

Roy wrote: Fri Apr 26, 2024 1:40 pm So many unanswered questions - flabbergast to the max.
First - what mechanism causes a star to rotate at such an enormous rate? Sucking gas off another star won’t do it.
Second - try tracking down the two assertions about LEO I. Central density comes from a model which requires a central black hole to work out right. So they posit it. The Black hole comes from a study exploring possibility of a dark matter halo, which cannot be shown, but supposing a black hole makes the model more or less work. So there is no evidence! it is publish or perish to keep the rice bowls filled.
Third - tidal stream of stars. There isn’t one. Does that mean the orbital motion isn’t happening? Nonono, we are just seeing it endwise. Flabbergasting.
In fact, there are models that can explain the rapid rotation of Regulus A as a consequence of mass transfer. That is a hypothesis backed by evidence.

The mass of the galaxy, when compared with its brightness, suggests a central black hole. That is a hypothesis backed by evidence.

The possibility of the galaxy being a tidal stream is a hypothesis. It is not an assertion. Which means it provides an incentive to look for evidence.

We observe dark matter halos around galaxies by measuring the rotational speeds of stars. Current observations neither confirm nor deny the existence of such a halo. This remains something still being investigated.

You don't seem to have a very clear understanding of how science works.
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Ann » Fri Apr 26, 2024 2:09 pm

Chris Peterson wrote: Fri Apr 26, 2024 12:55 pm
skyrish wrote: Fri Apr 26, 2024 12:05 pm Well I was going to ask at what point we differentiate between a star cluster and dwarf spheroidal galaxy, but thanks to the color commentary I am guessing the black hole at the center may be a factor. :idea:
Some globular clusters have central black holes.

I think there are at least two differences. A globular cluster has a different radial density gradient than a galaxy. And a globular cluster has a very different HR diagram than a galaxy, indicating a different formation process.


Galaxies and globular clusters contain different types of stars.

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Chris Peterson » Fri Apr 26, 2024 2:13 pm

Ann wrote: Fri Apr 26, 2024 2:09 pm
Chris Peterson wrote: Fri Apr 26, 2024 12:55 pm
skyrish wrote: Fri Apr 26, 2024 12:05 pm Well I was going to ask at what point we differentiate between a star cluster and dwarf spheroidal galaxy, but thanks to the color commentary I am guessing the black hole at the center may be a factor. :idea:
Some globular clusters have central black holes.

I think there are at least two differences. A globular cluster has a different radial density gradient than a galaxy. And a globular cluster has a very different HR diagram than a galaxy, indicating a different formation process.


Galaxies and globular clusters contain different types of stars.

Ann
Exactly. Which reasonably suggests that they have entirely different formation processes.
Chris

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by JimB » Sat Apr 27, 2024 9:14 am

Chris Peterson wrote: Fri Apr 26, 2024 2:13 pm
Ann wrote: Fri Apr 26, 2024 2:09 pm
Chris Peterson wrote: Fri Apr 26, 2024 12:55 pm

Some globular clusters have central black holes.

I think there are at least two differences. A globular cluster has a different radial density gradient than a galaxy. And a globular cluster has a very different HR diagram than a galaxy, indicating a different formation process.


Galaxies and globular clusters contain different types of stars.

Ann
Exactly. Which reasonably suggests that they have entirely different formation processes.
Is it possible that globular clusters are formed around what were originally population III stars (the initial generation of short lived super heavy stars from the early universe)?

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by Ann » Sun Apr 28, 2024 11:42 am

JimB wrote: Sat Apr 27, 2024 9:14 am
Chris Peterson wrote: Fri Apr 26, 2024 2:13 pm
Exactly. Which reasonably suggests that they have entirely different formation processes.
Is it possible that globular clusters are formed around what were originally population III stars (the initial generation of short lived super heavy stars from the early universe)?
Actually, no. That’s because the stars in globular clusters all contain some heavier elements, which means that they were formed from gas that had already been ”polluted” (or enriched) by elements produced by an earlier generation of massive stars that exploded as supernovas and emptied their innards into the surrounding space.

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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by VictorBorun » Sun Apr 28, 2024 6:31 pm

Chris Peterson wrote: Fri Apr 26, 2024 2:13 pm
Ann wrote: Fri Apr 26, 2024 2:09 pm
Chris Peterson wrote: Fri Apr 26, 2024 12:55 pm

Some globular clusters have central black holes.

I think there are at least two differences. A globular cluster has a different radial density gradient than a galaxy. And a globular cluster has a very different HR diagram than a galaxy, indicating a different formation process.


Galaxies and globular clusters contain different types of stars.

Ann
Exactly. Which reasonably suggests that they have entirely different formation processes.
The HR diagram of the Milky Way..jpg
The HR diagram of the Milky Way-.jpg
...
Click to view full size image 1 or image 2
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by johnnydeep » Sun Apr 28, 2024 6:45 pm

VictorBorun wrote: Sun Apr 28, 2024 6:31 pm
Chris Peterson wrote: Fri Apr 26, 2024 2:13 pm
Ann wrote: Fri Apr 26, 2024 2:09 pm ...

Galaxies and globular clusters contain different types of stars.

Ann
Exactly. Which reasonably suggests that they have entirely different formation processes.
The HR diagram of the Milky Way..jpgThe HR diagram of the Milky Way-.jpg
...
Click to view full size image 1 or image 2
Cool! Nice work.
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by VictorBorun » Mon Apr 29, 2024 12:12 pm

johnnydeep wrote: Sun Apr 28, 2024 6:45 pm
VictorBorun wrote: Sun Apr 28, 2024 6:31 pm
Chris Peterson wrote: Fri Apr 26, 2024 2:13 pm
Exactly. Which reasonably suggests that they have entirely different formation processes.
The HR diagram of the Milky Way..jpgThe HR diagram of the Milky Way-.jpg
...
Click to view full size image 1 or image 2
Cool! Nice work.
Well I am not sure. Maybe fitting the temperature scales is better
The HR diagram of the Milky Way-3.jpg
The HR diagram of the Milky Way-4.jpg
...
Click to view full size image 1 or image 2
What stays the same is Bright Giant branch fitting of the globular cluster's stars
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by johnnydeep » Mon Apr 29, 2024 3:08 pm

VictorBorun wrote: Mon Apr 29, 2024 12:12 pm
johnnydeep wrote: Sun Apr 28, 2024 6:45 pm
VictorBorun wrote: Sun Apr 28, 2024 6:31 pm
The HR diagram of the Milky Way..jpgThe HR diagram of the Milky Way-.jpg
...
Click to view full size image 1 or image 2
Cool! Nice work.
Well I am not sure. Maybe fitting the temperature scales is better
The HR diagram of the Milky Way-3.jpgThe HR diagram of the Milky Way-4.jpg
...
Click to view full size image 1 or image 2
What stays the same is Bright Giant branch fitting of the globular cluster's stars
Ah. It's a shame these diagrams don't all use the same standard scales.
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by AVAO » Mon Apr 29, 2024 8:25 pm

johnnydeep wrote: Mon Apr 29, 2024 3:08 pm
VictorBorun wrote: Mon Apr 29, 2024 12:12 pm
johnnydeep wrote: Sun Apr 28, 2024 6:45 pm

Cool! Nice work.
Well I am not sure. Maybe fitting the temperature scales is better
The HR diagram of the Milky Way-3.jpgThe HR diagram of the Milky Way-4.jpg
...
Click to view full size image 1 or image 2
What stays the same is Bright Giant branch fitting of the globular cluster's stars
Ah. It's a shame these diagrams don't all use the same standard scales.
The diagram of M55 is only a "schematic" representation. In the Milky Way diagram, the source was only a very small excerpt or section of the existing star material. You really have to be careful not to compare apples with pears.

Well, personally I prefer to use Gaia data. these are standardized and used in the same way for many analyzes.
What's interesting, for example, is that if you use all the available data about our galaxy, the diagram suddenly becomes much "wider".:

What we normally know as the HR diagram of the galaxy is just that of our "small" solar neighborhood around our own central star.

https://www.gaia.ac.uk/multimedia/gaia- ... r-clusters
https://arxiv.org/pdf/1804.09378


In comparison, it becomes clear that globular clusters differ greatly from the overall profile of the galaxy, but are surprisingly similar and that their course is only slightly shifted due to metallicity.

Video: https://www.youtube.com/watch?v=vTKYZIgvH3c
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by VictorBorun » Wed May 01, 2024 3:59 pm

AVAO wrote: Mon Apr 29, 2024 8:25 pm
johnnydeep wrote: Mon Apr 29, 2024 3:08 pm Ah. It's a shame these diagrams don't all use the same standard scales.
Well, personally I prefer to use Gaia data. these are standardized and used in the same way for many analyzes.
Here are
25 April 2018 Gaia — More than four million stars within five thousand light-years from the Sun
10 August 2018 Gaia Data Release 2 — Composite HRD for 14 globular clusters, coloured according to metallicity
The HR diagram of the Milky Way 11.jpg
The HR diagram of the Milky Way 10.jpg
...
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Re: APOD: Regulus and the Dwarf Galaxy (2024 Apr 26)

Post by VictorBorun » Wed May 01, 2024 4:04 pm

The HR diagram of the Milky Way 10-.jpg
The HR diagram of the Milky Way 11-.jpg
...
and a fragment
Click to view full size image 1 or image 2
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