APOD: Globular Star Cluster NGC 6752 (2020 Jan 23)

[APOD Robot]

Globular Star Cluster NGC 6752

Explanation: Some 13,000 light-years away toward the southern constellation Pavo, the globular star cluster NGC 6752 roams the halo of our Milky Way galaxy. Over 10 billion years old, NGC 6752 follows clusters Omega Centauri and 47 Tucanae as the third brightest globular in planet Earth's night sky. It holds over 100 thousand stars in a sphere about 100 light-years in diameter. Telescopic explorations of the NGC 6752 have found that a remarkable fraction of the stars near the cluster's core, are multiple star systems. They also reveal the presence of blue straggle stars, stars which appear to be too young and massive to exist in a cluster whose stars are all expected to be at least twice as old as the Sun. The blue stragglers are thought to be formed by star mergers and collisions in the dense stellar environment at the cluster's core. This sharp color composite also features the cluster's ancient red giant stars in yellowish hues. (Note: The bright, spiky blue star at 11 o'clock from the cluster center is a foreground star along the line-of-sight to NGC 6752)

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[Ann]

I didn't know that NGC 6752 is the third brightest globular of the Milky Way. That's interesting!

Click to view full size image

Color Magnitude diagram of globular cluster M55.
B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope.

And of course, I must talk about the colors of the stars of NGC 6752. Please note the number of blue stars in this cluster!

Take a look at the color-magnitude diagram of the stars of globular cluster M5. Main sequence stars are the ones that shine by fusing hydrogen to helium in their cores. At the turnoff point, the stars have used up the hydrogen in their cores, their cores shrink and their outer layers expand, and they turn into red giants. But at a certain point, their helium cores get hot enough to start fusing helium into oxygen and carbon in their cores.

Stars that contain very few "metals" - which is astronomy-speak for elements heavier than hydrogen and helium - typically shrink rather drastically when their helium burning gets going. And the more they shrink, the hotter their photospheres get, and the bluer they look. These stars are called horizontal branch stars.

(And then, after some time, the stars on the horizontal branch use up the helium in their cores, and then they expand for the second time into cool giants and turn red, until they shed their outer layers altogether and become white dwarfs.)

Note a few points here. The brightest stars in a globular cluster are always the brightest red giants. But the second brightest stars are the blue (or bluish) horizontal branch stars, assuming there are any blue horizontal branch stars at all. Not all globulars have them.

Click to view full size image

Ground-based (David Malin/AAT) and Hubble/NASA/ESA/R. Gilliland
images of 47 Tucanae. There are no blue horizontal branch stars here.

Click to view full size image

Color magnitude diagram of 47 Tucanae with a "red clump"
(instead of a horizontal branch) and blue straggler stars (in the box).
Image: Puragra Guhathakurta from archival HST /WFPC2 data.

The second brightest globular of the Milky Way, 47 Tucanae, lacks blue horizontal branch stars. The HST color image of 47 Tuc was taken through ultraviolet (336 nm), visible (555 nm) and near infrared (814 nm) filters, and should have caught any horizontal branch stars if there were any.

Both 47 Tuch and NGC 6752 have blue straggler stars, however. Blue straggler stars are still fusing hydrogen to helium in their cores, and they brighter and bluer than other that are still on the main sequence. The blue stragglers of 47 Tuc are marked in the box in the color-magnitude diagram of 47 Tuc. But as you can see from the diagram of M55, blue straggler stars are almost always fainter and usually less blue than the horizontal stars of a globular cluster.

Ann

[starsurfer]

This has always been one of my favourite globular clusters ever since seeing the AAO image of it in a book more than 20 years ago.

[orin stepanek]

NGC6752LRGBcrop1024.jpg (58.57 KiB) Viewed 4433 times

Kind of like a bunch of moths going after a streetlight!

Very symmetrical! almost like mini galaxies!

[TheZuke!]

I didn't know that NGC 6752 is the third brightest globular of the Milky Way. That's interesting!

Ann

Another reason to add a Southern Hemisphere star gazing trip to my Bucket List!

[BillLee]

I wonder what the night (?) sky would look like to an imaginary resident of a planet circling one of those stars in that cluster.

Regards,

Bill Lee

[TheZuke!]

I wonder what the night (?) sky would look like to an imaginary resident of a planet circling one of those stars in that cluster.

Regards,

Bill Lee

I've asked a similar (the same?) question recently, and the answer was something like;
Night sky would be as bright as a night with a full Moon, without the Moon.
A sort of rule of thumb (misunderstanding?) I took from all the answers/equations is the stars on average are 4 light years apart, such as Alpha Centauri is to us, but there would be a would a whole lot more of them.

Maybe it is in the FAQ (if there is one), else do a site search for my username, it was one of the first postings I made.

[Guest]

I didn't know that NGC 6752 is the third brightest globular of the Milky Way. That's interesting!

Ann

Another reason to add a Southern Hemisphere star gazing trip to my Bucket List!

Well, actually M22 is brighter so NGC 6752 is the fourth brightest globular. Unfortunately Wikipedia contains wrong info there.

[InfinitiesLoop]

I wonder what the night (?) sky would look like to an imaginary resident of a planet circling one of those stars in that cluster.

Regards,

Bill Lee

I've asked a similar (the same?) question recently, and the answer was something like;
Night sky would be as bright as a night with a full Moon, without the Moon.
A sort of rule of thumb (misunderstanding?) I took from all the answers/equations is the stars on average are 4 light years apart, such as Alpha Centauri is to us, but there would be a would a whole lot more of them.

Maybe it is in the FAQ (if there is one), else do a site search for my username, it was one of the first postings I made.

Would clusters be good places to look for signs of life? Lots of older stars, lots of chances.. unless the conditions in those things is just too hostile.

[neufer]

Would clusters be good places to look for signs of life? Lots of older stars, lots of chances.. unless the conditions in those things is just too hostile.

We're still waiting for a reply.

---

<<The Arecibo message is a 1974 interstellar radio message carrying basic information about humanity and Earth sent to globular star cluster M13. The message was broadcast into space a single time via frequency modulated radio waves at a ceremony to mark the remodeling of the Arecibo radio telescope in Puerto Rico on 16 November 1974. The message was aimed at the current location of M13 some 25,000 light years away because M13 was a large and close collection of stars that was available in the sky at the time and place of the ceremony. The message forms the image shown here when translated into graphics, characters, and spaces.

The "Arecibo answer" is a hoax by people that created an imprint in a crop field (crop circle) in 2001 near the Chilbolton radio telescope in Hampshire, UK, portrayed as a response from an extraterrestrial civilization. The crop circle is a near replica of the Arecibo message. The feature forms the same 23 x 73 grid because these numbers are primes and most of the chemical data remains the same with the exception that in the section detailing important chemical elements, the main focus is altered from carbon to silicon, and the diagram of DNA has been rewritten. At the bottom, the pictogram of a human is replaced with a shorter figure with a large, bulbous head.>>

[Chris Peterson]

I wonder what the night (?) sky would look like to an imaginary resident of a planet circling one of those stars in that cluster.

Regards,

Bill Lee

I've asked a similar (the same?) question recently, and the answer was something like;
Night sky would be as bright as a night with a full Moon, without the Moon.
A sort of rule of thumb (misunderstanding?) I took from all the answers/equations is the stars on average are 4 light years apart, such as Alpha Centauri is to us, but there would be a would a whole lot more of them.

Maybe it is in the FAQ (if there is one), else do a site search for my username, it was one of the first postings I made.

Would clusters be good places to look for signs of life? Lots of older stars, lots of chances.. unless the conditions in those things is just too hostile.

Probably not. With stars so close together, it's unlikely that many planetary systems would be stable over many millions of years, which (using Earth as our only example) argues against planets in globulars as being likely to produce complex life, much less technological life.

[BDanielMayfield]

I've asked a similar (the same?) question recently, and the answer was something like;
Night sky would be as bright as a night with a full Moon, without the Moon.
A sort of rule of thumb (misunderstanding?) I took from all the answers/equations is the stars on average are 4 light years apart, such as Alpha Centauri is to us, but there would be a would a whole lot more of them.

Maybe it is in the FAQ (if there is one), else do a site search for my username, it was one of the first postings I made.

Would clusters be good places to look for signs of life? Lots of older stars, lots of chances.. unless the conditions in those things is just too hostile.

Probably not. With stars so close together, it's unlikely that many planetary systems would be stable over many millions of years, which (using Earth as our only example) argues against planets in globulars as being likely to produce complex life, much less technological life.

Also arguing against many planets being inside globular star clusters is their extreme age. Back when they where forming the elements needed to form rocks where much rarer than they are today, or even back when our system formed.

Bruce

[Ann]

Also arguing against many planets being inside globular star clusters is their extreme age. Back when they where forming the elements needed to form rocks where much rarer than they are today, or even back when our system formed.

Bruce

Exactly. There were few heavy elements available for planet-making in those days, which is also why these metal-poor globular clusters have a relatively large population of blue horizontal branch stars.

Ann

[sillyworm 2]

As orin mentioned...isn't a globular cluster pretty much what a galaxy grew from? What would distinguish them from being called a mini galaxy?

[Chris Peterson]

As orin mentioned...isn't a globular cluster pretty much what a galaxy grew from? What would distinguish them from being called a mini galaxy?

It is not generally believed that galaxies grow out of globular clusters. Globular clusters probably form very early in the process of galaxy formation, with the clusters condensing and separately, the galaxy condensing- the details of the latter probably depending strongly on the formation of a supermassive black hole, and on the impact of dark matter. Globular condensation is particularly poorly understood, especially given their general lack of both central black holes and dark matter halos.

[Ann]

As orin mentioned...isn't a globular cluster pretty much what a galaxy grew from? What would distinguish them from being called a mini galaxy?

Good question. I don't have a good answer, but I will say that galaxies come in a veritable zoo of sizes (and to some extent, of shapes and colors). Globular clusters, by contrast, are usually more or less the same size.

Click to view full size image

Large galaxies. Note the Milky Way in the center.

Click to view full size image

Small galaxies. Note the Milky Way in the center.

Globular clusters can be dense or loose, full of hundreds of thousands of stars or mostly "evaporated", but typically they are more or less the same size. A very few globulars can be noticeably elongated, but most of them are really quite spherical.

Click to view full size image

Four globular clusters in the Fornax dwarf spheroidal galaxy.
From left to right, top to bottom: Fornax 1, Fornax 2, Fornax 3 and Fornax 5.
Image credit: NASA / ESA / S. Larsen, Radboud University, the Netherlands.

Click to view full size image

The Fornax dwarf spheroidal galaxy with some of its globulars annotated.
NGC 1049 is also known as Fornax 3. Credit: ESO/Digitized Sky Survey 2.

Click to view full size image

Globular clusters and some backgrund galaxies of giant elliptical galaxy M87.
Photo: Juan Carlos Forte et al., Gemini Multi-Object Spectrograph (GMOS).

By definition, however, a globular cluster is in orbit around the center of a galaxy, and then it doesn't matter if the galaxy that the globulars orbit is large or small. The picture at right shows at least 500 of the many thousands of the globular clusters of giant elliptical galaxy M87. Some background galaxies are also visible. The background galaxies are typically more extended than the globulars, and are often elongated, or show disks.

Can globular clusters "grow into" galaxies of their own? Given the definition that they must orbit the center of a galaxy, I don't think so.

Ann

[sillyworm 2]

Thanks for all the links Ann! I brought up the question because some globular clusters appear to be just 1 Billion years younger then ..let's say,The Milky Way.What happened in those early years? Did most of the Galaxies form quickly..practically instantaneous(considering how long 1 Billion years is) and what were the circumstances that created Globular clusters? You would think that after 11 or so Billion years..these clusters would have merged with the Galaxies they orbit.

[Chris Peterson]

Thanks for all the links Ann! I brought up the question because some globular clusters appear to be just 1 Billion years younger then ..let's say,The Milky Way.What happened in those early years? Did most of the Galaxies form quickly..practically instantaneous(considering how long 1 Billion years is) and what were the circumstances that created Globular clusters? You would think that after 11 or so Billion years..these clusters would have merged with the Galaxies they orbit.

Galaxies did appear to form quickly in the early universe. For the most part, there's no reason for a globular cluster to merge with a galaxy. Clusters are in orbit. They are no more likely to merge with their galaxy than a planet is to merge with its star. What messes things up is encounters with other galaxies. That results in tidal forces that can disrupt orbits, and in that case, globulars might end up passing through the galaxy and eventually merging.