APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

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APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by APOD Robot » Tue Oct 19, 2021 4:05 am

Image Palomar 6: Globular Star Cluster

Explanation: Where did this big ball of stars come from? Palomar 6 is one of about 200 globular clusters of stars that survive in our Milky Way Galaxy. These spherical star-balls are older than our Sun as well as older than most stars that orbit in our galaxy's disk. Palomar 6 itself is estimated to be about 12.5 billion years old, so old that it is close to -- and so constrains -- the age of the entire universe. Containing about 500,000 stars, Palomar 6 lies about 25,000 light years away, but not very far from our galaxy's center. At that distance, this sharp image from the Hubble Space Telescope spans about 15 light-years. After much study including images from Hubble, a leading origin hypothesis is that Palomar 6 was created -- and survives today -- in the central bulge of stars that surround the Milky Way's center, not in the distant galactic halo where most other globular clusters are now found.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by SpaceCadet » Tue Oct 19, 2021 4:55 am

Are these clusters visible to us with a telescope from earth?

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by Ann » Tue Oct 19, 2021 6:36 am


Can't help comparing Palomar 6 with M15, because there are a few interesting differences, but also one fascinating similarity!
Wikipedia wrote about Palomar 6:

Palomar 6 is a loose globular cluster in the constellation Ophiuchus that belongs to the halo of the Milky Way galaxy.
That's not what today's caption said.
APOD Robot wrote:

After much study including images from Hubble, a leading origin hypothesis is that Palomar 6 was created -- and survives today -- in the central bulge of stars that surround the Milky Way's center, not in the distant galactic halo where most other globular clusters are now found.
Wikipedia wrote about M15:

Messier 15 is one of the most densely packed globulars known in the Milky Way galaxy. Its core has undergone a contraction known as "core collapse" and it has a central density cusp with an enormous number of stars surrounding what may be a central black hole.
But guess what? According to Wikipedia M15 is home to 100,000 stars, whereas, according to today's caption, there are 500,000 stars in Palomar 6! So there would be five times as many stars in Palomar 6 as there are in M15! Is that possible?

The core of M15 is brilliantly bright. That is certainly not the case at all in Palomar 6, but there is, nevertheless, a "whitening" in the center of this globular.

However, there is a fascinating similarity between Palomar 6 and M15:

Wikipedia wrote about Palomar 6:

First discovered on the National Geographic Society – Palomar Observatory Sky Survey plates by Robert G. Harrington and Fritz Zwicky,[3] it was catalogued as a globular cluster, and is one of four globulars known to contain a planetary nebula.
Wikipedia wrote about M15:

It also contains Pease 1, the first planetary nebula discovered within a globular cluster in 1928. Just three others have been found in globular clusters since then.
But we need to look at the colors of the stars in Palomar 6, and find out what the colors and the luminosities of the stars mean. Take a look at the diagram below:


Let's start by looking at the B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope picture at left, where the colors are well chosen (although saturated). To understand the diagram, you also need to understand that the stars are brighter the higher up they are in the diagram, and fainter they lower down they are.

As you can see, most of the stars in a globular cluster are the really faint red ones, the ones that are located in the lower right corner of the diagram. But they are so faint that they are only resolved in very highly resolved images of globular clusters, and I don't think we can see them in today's APOD.

The brightest stars in a globular cluster are also very red. That is because the reddest of the brightest stars are the most distended, swollen red giants in a globular cluster, and they are teetering on the brink of change.

Take a look at Lithopsian's diagram at right. Can you see that there are two branches of stars reaching for the top right corner? The lower of these branches is the red giant branch. Here we find stars that have exhausted the hydrogen in their cores, and now they have inert cores that shrink under their own gravity. As the cores shrink, the temperatures of the cores rise, hydrogen fusion is taking place in a shell around the core, and the outer layers get more and more puffed up. The star gets ever larger, redder and brighter.

But then the star undergoes a "helium flash", when the core gets hot enough to start fusing helium into carbon and oxygen. As this happens, the stars shrinks prodigiously. At the same time, the outer layers of the now-compact star heats up by thousands of degrees, and the star changes in color from red to blue. The star is still bright, but it is fainter than it was when it was a huge red giant, because it is so much smaller. Note: Red giant stars turn blue after the helium flash only among very metal-poor stars, like in globular clusters.

Take a look at the color-magnitude diagram of globular cluster M55 again. You can see where the blue stars are, in the upper left part of the diagram. This part of the diagram is called the horizontal branch.

(On the horizontal branch, to the right of the blue stars, is where you find the pulsating RR Lyrae stars, which are crucial for determining the distance to globular clusters. A globular that has few or no RR Lyrae stars, because it has an underdeveloped horizontal branch, is harder to determine the distance to.)

Anyway. The blue stars on the horizontal branch fuse helium to carbon and oxygen in their cores. But when they have exhausted the helium in their cores, the core again becomes inert. Once again the core shrinks and heats up. The stars swells up and becomes red again, now rising on the asymtotic giant branch. Can you see it in Lithopsian's diagram at right, where it is colored blue? (Horror!)

But for a star on the asymtotic giant branch there are two shells surrounding the "dead" core, an inner helium shell fusing helium to carbon and oxygen, and an outer hydrogen shell fusing hydrogen to helium.

The two shells turn on and off alternatively, and the star becomes ever more unstable. Also the shrinking core grows ever and ever hotter. Eventually the star begins blowing a stronger and stronger wind and the star begins shrugging off its tenuous outer layers. Eventually the star turns into a planetary nebula and a white dwarf.

Remember? There is a planetary nebula in Palomar 6. And one in M15, too.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by De58te » Tue Oct 19, 2021 9:42 am

SpaceCadet wrote:
Tue Oct 19, 2021 4:55 am
Are these clusters visible to us with a telescope from earth?
Yes. Even though the Hubble is a space telescope, Palomar 6 was first discovered on Earth by the Palomar Observatory. They might have used the Hale telescope back in the 1950s or another of their telescopes. Wiki wasn't clear which scope they used.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by De58te » Tue Oct 19, 2021 9:57 am

Amazing to look at all those stars and to think there could be untold thousands of intelligent civilizations amongst them. All space-faring in those billions of years and fighting in "Star Wars." Am I right that since those stars are almost as old as the universe itself all those stars formed when the universe was much smaller and contracted so had plenty of hydrogen fuel to form giant blue stars that went supernova and formed those second generation yellow stars with untold rocky planets and raw material for intelligent civilizations to grow over billions of years.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by tomatoherd » Tue Oct 19, 2021 12:40 pm

De58te wrote:
Tue Oct 19, 2021 9:57 am
Amazing to look at all those stars and to think there could be untold thousands of intelligent civilizations amongst them. All space-faring in those billions of years and fighting in "Star Wars." Am I right that since those stars are almost as old as the universe itself all those stars formed when the universe was much smaller and contracted so had plenty of hydrogen fuel to form giant blue stars that went supernova and formed those second generation yellow stars with untold rocky planets and raw material for intelligent civilizations to grow over billions of years.
I don't think so. Clusters are not miniature galaxies. My understanding is they are metal-poor.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by tetrodehead » Tue Oct 19, 2021 1:09 pm

"and so constrains -- the age of the entire universe." surely the 'observable' universe?

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by Ann » Tue Oct 19, 2021 7:17 pm

De58te wrote:
Tue Oct 19, 2021 9:57 am
Amazing to look at all those stars and to think there could be untold thousands of intelligent civilizations amongst them. All space-faring in those billions of years and fighting in "Star Wars." Am I right that since those stars are almost as old as the universe itself all those stars formed when the universe was much smaller and contracted so had plenty of hydrogen fuel to form giant blue stars that went supernova and formed those second generation yellow stars with untold rocky planets and raw material for intelligent civilizations to grow over billions of years.
Globular clusters are very old, and they formed when the Universe had created far fewer heavy elements (i.e., anything more massive than hydrogen and helium) than it has now. So the ancient stars that make up the globular clusters contain far fewer "building blocks" than our Sun (and Solar system) does. It's good for us that the Earth (and we ourselves) contain good amounts of carbon, calcium, phosphorus, iron and so on. After all, it wouldn't be good for us humans if the Earth was mostly made of hydrogen and helium - and imagine us trying to be "hydrogen and helium people"! Talk about being full of gas!

Also, and perhaps just as importantly (because it is possible, after all, that there might be rocky planets with some water in a globular cluster, too) is the fact that in a crowded globular cluster, the tidal forces as the stars jostle around might easily tear planets from their suns, or force the planets into very elongated orbits. None of this would be the least bit good for life, let alone complex life.

Click to play embedded YouTube video.

You wouldn't want to be in the thick of this.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by Ann » Tue Oct 19, 2021 8:17 pm

What do you know, I just found this video about what it would be like to live inside a globular cluster! :D

Click to play embedded YouTube video.

I think the video contains several mistakes. For example, the person who made this video clearly doesn't understand the word "metals" the way that term is used in astro-speak, where it means any elements more massive than hydrogen and helium (i.e., basically any element that wasn't forged in the Big Bang but had to be created inside stars). He clearly believes that "metals" in the Universe means just what it means in "good old Earth-speak", stuff like iron, copper, silver, aluminum etcetera, so he tries to figure out what life on Earth would be like if we didn't have those metals. But he doesn't worry at all about what it would mean if we didn't enough carbon and oxygen, for example!

He also seems to think that it would be particularly hard for planets to form inside globular clusters, and I don't see why that would be the case at all. The hard thing would be for the stars to hold on to the planets that had formed near them in the crowded environment of a globular!

So don't believe everything the guy says in the video. Still, I thought it was fun watching it.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by neufer » Tue Oct 19, 2021 10:36 pm

Ann wrote:
Tue Oct 19, 2021 7:17 pm

Globular clusters are very old, and they formed when the Universe had created far fewer heavy elements (i.e., anything more massive than hydrogen and helium) than it has now. So the ancient stars that make up the globular clusters contain far fewer "building blocks" than our Sun (and Solar system) does. Also, and perhaps just as importantly (because it is possible, after all, that there might be rocky planets with some water in a globular cluster, too) is the fact that in a crowded globular cluster, the tidal forces as the stars jostle around might easily tear planets from their suns, or force the planets into very elongated orbits. None of this would be the least bit good for life, let alone complex life.
https://en.wikipedia.org/wiki/Globular_cluster#Planets wrote:
<<Globular clusters are thought to be unfavorable locations for planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters because of the gravitational perturbations of passing stars. A planet orbiting at 1 astronomical unit around a star that is within the core of a dense cluster, such as 47 Tucanae (stellar densities of 100 or 1000 stars/pc3 versus ~0.1 stars/pc3 around the sun), would only survive on the order of 100 million years. A search in 2000 for giant planets in the globular cluster 47 Tucanae came up negative, suggesting that the abundance of heavier elements—low in globular clusters—necessary to build these planets may need to be at least 40% of the Sun's abundance. Because terrestrial planets are built from heavier elements such as silicon, iron and magnesium, member stars have a far lower likelihood of hosting Earth-mass planets than stars in the solar neighborhood. Globular clusters are thus unlikely to host habitable terrestrial planets.>>
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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by XgeoX » Wed Oct 20, 2021 12:21 pm

SpaceCadet wrote:
Tue Oct 19, 2021 4:55 am
Are these clusters visible to us with a telescope from earth?
It’s 11th magnitude so a very small telescope, even a 2 inch lens, could in theory pick it up.

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by neufer » Wed Oct 20, 2021 6:06 pm

XgeoX wrote:
Wed Oct 20, 2021 12:21 pm
SpaceCadet wrote:
Tue Oct 19, 2021 4:55 am

Are these clusters visible to us with a telescope from earth?
It’s 11th magnitude so a very small telescope, even a 2 inch lens, could in theory pick it up.
https://www.astronomy-mall.com/Adventures.In.Deep.Space/palglob.htm wrote:
<<Like the Abell planetary nebulae, the Palomar globular clusters were discovered in the 1950s on the survey plates of the first Palomar Observatory Sky Survey (POSS). The list of astronomers who first identified the objects as globular clusters includes some famous names, among them Edwin Hubble, Walter Baade, Fritz Zwicky, Halton Arp and George Abell himself. Several of the Palomar globulars--including Palomar 6, Palomar 7, Palomar 9, Palomar 10 and Palomar 11--are nearby clusters of average size that just happen to be heavily obscured by dust in our line of sight. Others--including Palomar 3, Palomar 4, and Palomar 14 --are giant globulars that are very far away in the extreme outer halo of the Milky Way. Although the objects vary greatly in degree of difficulty--from easy to nearly impossible--observing the whole list is a very challenging observing project for owners of big scopes.>>
Barbara Wilson wrote:
<<I have observed all 15 Palomar globulars with a 20" telescope. Dark skies and high power are a prerequisite for definite confirmation of most of the Palomars. In order of difficulty I would place Pal 15 as most difficult of the 15.>>
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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by SpaceCadet » Tue Oct 26, 2021 4:45 am

De58te wrote:
Tue Oct 19, 2021 9:42 am
SpaceCadet wrote:
Tue Oct 19, 2021 4:55 am
Are these clusters visible to us with a telescope from earth?
Yes. Even though the Hubble is a space telescope, Palomar 6 was first discovered on Earth by the Palomar Observatory. They might have used the Hale telescope back in the 1950s or another of their telescopes. Wiki wasn't clear which scope they used.

Thank you :)

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Re: APOD: Palomar 6: Globular Star Cluster (2021 Oct 19)

Post by SpaceCadet » Tue Oct 26, 2021 4:49 am

neufer wrote:
Wed Oct 20, 2021 6:06 pm
XgeoX wrote:
Wed Oct 20, 2021 12:21 pm
SpaceCadet wrote:
Tue Oct 19, 2021 4:55 am

Are these clusters visible to us with a telescope from earth?
It’s 11th magnitude so a very small telescope, even a 2 inch lens, could in theory pick it up.
https://www.astronomy-mall.com/Adventures.In.Deep.Space/palglob.htm wrote:
<<Like the Abell planetary nebulae, the Palomar globular clusters were discovered in the 1950s on the survey plates of the first Palomar Observatory Sky Survey (POSS). The list of astronomers who first identified the objects as globular clusters includes some famous names, among them Edwin Hubble, Walter Baade, Fritz Zwicky, Halton Arp and George Abell himself. Several of the Palomar globulars--including Palomar 6, Palomar 7, Palomar 9, Palomar 10 and Palomar 11--are nearby clusters of average size that just happen to be heavily obscured by dust in our line of sight. Others--including Palomar 3, Palomar 4, and Palomar 14 --are giant globulars that are very far away in the extreme outer halo of the Milky Way. Although the objects vary greatly in degree of difficulty--from easy to nearly impossible--observing the whole list is a very challenging observing project for owners of big scopes.>>
Barbara Wilson wrote:
<<I have observed all 15 Palomar globulars with a 20" telescope. Dark skies and high power are a prerequisite for definite confirmation of most of the Palomars. In order of difficulty I would place Pal 15 as most difficult of the 15.>>
Thank you for the info. I need to get myself a telescope, that is for sure.