APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

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APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by APOD Robot » Sun Mar 24, 2019 4:07 am

Image Zooming in on Star Cluster Terzan 5

Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are less than 200 left. Over the eons, many globular clusters were destroyed by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. The featured video shows what it might look like to go from the Earth to the globular cluster Terzan 5, ending with a picture of the cluster taken with the Hubble Space Telescope. This star cluster has been found to contain not only stars formed in the early days of our Milky Way Galaxy, but also, quite surprisingly, others that formed in a separate burst of star formation about 7 billion years later.

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Blastof

Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Blastof » Sun Mar 24, 2019 6:24 am

Thats surprising that theres only 200 in the whole Milky Way since we can see at least 30 from Earth's little corner of the galaxy

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Alex_944 » Sun Mar 24, 2019 9:20 am

This kind of animation, where they zoom from outside down to particular object in the galaxy is absolutely jaw dropping. It really gives the impression of traveling through the galaxy.

heehaw

Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by heehaw » Sun Mar 24, 2019 9:29 am

The universe is very big, and, it contains things that are very small: a billion-solar-mass black hole is smaller than a single proton: the distorted spacetime around the singularity is only about the size of our solar system. This is one heck of a universe!

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by De58te » Sun Mar 24, 2019 9:30 am

Amazing that it was even discovered because you can't even see it in the beginning where the Milky Way looks like you normally see it in the sky. It doesn't even appear as a blue star until you zoom in halfway there (whatever the power of the zoom is). And even stranger that when it gets to the Hubble image that there are only a handful of blue stars and at least 99% of the stars are white or yellow. I wonder how many other distant faint blue stars are really white globular clusters?

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Ann » Sun Mar 24, 2019 11:06 am

De58te wrote:
Sun Mar 24, 2019 9:30 am
Amazing that it was even discovered because you can't even see it in the beginning where the Milky Way looks like you normally see it in the sky. It doesn't even appear as a blue star until you zoom in halfway there (whatever the power of the zoom is). And even stranger that when it gets to the Hubble image that there are only a handful of blue stars and at least 99% of the stars are white or yellow. I wonder how many other distant faint blue stars are really white globular clusters?
According to Sky Catalogue 2000.0, volume 2, Terzan 5 is a highly reddened cluster whose (reddened) B-V index is +2.77. That makes the reddened color of Terzan 5 considerably redder than the star Betelgeuse.

Why does the cluster look blue (at a certain distance from us) in the video? You will have to ask the people responsible for the picture and the video.
Information on this page tells us that the bluest filter used for the picture was centered on 606 nm, which corresponds to an orange color (although it was likely a wideband filter). There were also three infrared filters used for the image.

What this means is that anything that looks blue in the picture might in reality be orange. On the other hand, it might also be truly blue but possibly reddened to an orange color.

What Terzan 5's intrinsic integrated color is depends on how metal-poor its stars are. All globular clusters are metal-poor, but the more metal-poor they are, the bluer they typically are. That is because the really metal-poor globulars contain (many) blue horizontal branch stars, but the less metal-poor ones contain few or no such stars.


Color-magnitude diagram of metal-poor globular M13.
Source: http://abyss.uoregon.edu/~js/ast122/lectures/lec15.html
Color-magnitude diagram of less metal-poor cluster 47 Tuc. Source:
https://www.chegg.com/homework-help/que ... -q27073193


























Take a look at the color-magnitude diagram of globular cluster M13 at left. M13 is a "blue" cluster, which contains fairly large numbers of blue horizontal stars. You can see the horizontal branch of M13 at around magnitude 15 in the diagram at left, and you can see that the B-V color of the stars here is around 0.0. That is the same color as Vega or Sirius, and it is fairly blue.

But now look at the color-magnitude diagram of a less metal-poor cluster, 47 Tuc. 47 Tuc sports a tiny little horizontal branch at about magnitude 14 in the diagram at right. You can see that the B-V color of the stars here is around 0.6-0.8. That is not at all blue.

According to Sky Catalogue 2000.0, Volume 2, the integrated B-V index of "blue" globular cluster M13 is 0.69, which is comparable to the color of the Sun. The integrated B-V index of 47 Tuc, by contrast, is 0.89, which is clearly yellower than the Sun and somewhat comparable to the star Pollux.

"Blue" globular M13. Note the blue horizontal stars.
Source: https://www.spacetelescope.org/images/potw1011a/
"Non-blue" globular 47 Tuc. Note the lack of blue horizontal stars.





















The pictures of M13 and 47 Tuc are not quite comparable, because the filters for the picture of M13 were 435 nm (blue), 625 nm (orange-red) and 814 nm (near infrared), whereas the filters for the picture of 47 Tuc were 336 nm (ultraviolet), 555 (green) and 814 nm (near infrared). That means that it was easier for bluish stars to show up through the filters used for the M13 picture than through the filters used for the 47 Tuc one. Even so, there can be no doubt that M13 is a decidedly bluer cluster than 47 Tuc.

But there is another lesson here. We can't expect any (old) globular clusters to be truly "blue". At their bluest, they will be a little bluer than the Sun, but not by much. Maybe possibly maybe they might be as blue as Procyon, Alpha Perseus. At their (unreddened) reddest, they will be about as yellow as the star Pollux.

So what color is Terzan 5 really? If it wasn't so reddened by dust?

My guess is that it is a little redder than M13 but a little bluer than 47 Tuc. I base that on the (large) picture of Terzan 5 published at the ESO page (see address below the topmost picture). In the large picture, you can spot a few (but not very many) blue horizontal stars in this globular.

By the way, the blue horizontal stars of Terzan 5 likely belong to the globular's oldest, most metal-poor stars. It seems less likely that the much younger stars created in the second starburst would be metal-poor enough to give rise to such stars.

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Sun Mar 24, 2019 11:16 am

Under Physical Properties the linked to Wikipedia artical on Terzan 5 says
The absolute magnitude of Terzan 5 is at least MV=−7.5.[1] Its bolometric luminosity is about 800,000 times that of the Sun, while its mass is about 2 million solar masses.[4] The small core of Terzan 5—about 0.5 pc in size[4]—has one of the highest star densities in the galaxy. Its volume mass density exceeds 106 M☉/pc3,[10] while its volume luminosity density exceeds 105.5 L☉/pc3, where M☉ and L☉ are the Sun's mass and luminosity, respectively. The cluster also has one of the highest metallicities among the Milky Way's globular clusters—[Fe/H]=−0.21.[5]

In 2009 it was discovered that Terzan 5 consists of at least two generations of stars with ages of 12 and 4.5 billion years and slightly different metallicities, possibly indicating that it is the core of a disrupted dwarf galaxy, not a true globular cluster.[6] There are only a few other globular clusters in the Milky Way that contain stars with different ages. Among them are M54 and Omega Centauri. The cluster also contains around 1300 core helium burning horizontal branch (HB) stars,[6] including at least one RR Lyrae variable star.[10]
IMHO those who think this isn't really a true globular cluster are correct.

I also wondered about how much of the Milky Way's disk we might be looking through on the epic zoom in to Terzan 5. The wiki article says "recent estimates generally range from 5.5 to 8.7 kpc", or about 18,000 light years, give or take. Sag A* (the supermassive BH at the MY's center) is about 26,000 LYs away, so T5 is about 70% of the way in toward galactic central station, my fellow tourists.

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Sun Mar 24, 2019 11:36 am

Ann, it looks to me that the vid uses photos that have had all or most interstellar reddening removed. I (being a lover of red) would love to see an equivalent Hubble photo before it had been Visined. (eye wash advertised to "get the red out")

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Sun Mar 24, 2019 11:51 am

BDanielMayfield wrote:
Sun Mar 24, 2019 11:36 am
Ann, it looks to me that the vid uses photos that have had all or most interstellar reddening removed. I (being a lover of red) would love to see an equivalent Hubble photo before it had been Visined. (eye wash advertised to "get the red out")

Bruce
Just to be be clear to the author(s) of the video, the above is in no way meant as a critism of your work, which is great. Removing dust obscuration is needed to further the illusion of moving deeper and deeper into space. Very well done.
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by orin stepanek » Sun Mar 24, 2019 12:48 pm

It amazes me that there are these ball shaped star clusters; kind of like Christmas tree bulbs! One named Terzan; reminded me of a certain jungle man; Tarzan! :lol2:
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Chris Peterson » Sun Mar 24, 2019 2:13 pm

Blastof wrote:
Sun Mar 24, 2019 6:24 am
Thats surprising that theres only 200 in the whole Milky Way since we can see at least 30 from Earth's little corner of the galaxy
We can see about 150 globular clusters. And this isn't surprising, since they are not embedded in the Milky Way, but orbiting around it at all different inclinations. That means that most of them are in our line of sight. It's only a few that are blocked from view by the galactic center or the galactic disc.
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by markc » Sun Mar 24, 2019 4:12 pm

What is the average distance between stars in a globular cluster? And how stable are the star positions relative to each other?

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by neufer » Sun Mar 24, 2019 5:00 pm

markc wrote:
Sun Mar 24, 2019 4:12 pm

What is the average distance between stars in a globular cluster?
And how stable are the star positions relative to each other?
  • Omega Centauri has about 6 stars per cubic light year;
    however, 1 star per cubic light year is more typical.
https://en.wikipedia.org/wiki/Omega_Centauri wrote:
<<Omega Centauri (ω Cen or NGC 5139) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 15,800 light-years, it is the largest globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars and a total mass equivalent to 4 million solar masses, making it the most massive globular cluster of the Milky Way. Omega Centauri is so distinctive from the other galactic globular clusters that it is thought to have an alternate origin as the core remnant of a disrupted dwarf galaxy.>>
https://en.wikipedia.org/wiki/Globular_cluster#Mass_segregation,_luminosity_and_core_collapse wrote:
<<In measuring the luminosity curve of a given globular cluster as a function of distance from the core, most clusters in the Milky Way increase steadily in luminosity as this distance decreases, up to a certain distance from the core, then the luminosity levels off. Typically this distance is about 1–2 parsecs from the core. However about 20% of the globular clusters have undergone a process termed "core collapse". In this type of cluster, the luminosity continues to increase steadily all the way to the core region. An example of a core-collapsed globular is M15.

Core-collapse is thought to occur when the more massive stars in a globular cluster encounter their less massive companions. Over time, dynamic processes cause individual stars to migrate from the center of the cluster to the outside. This results in a net loss of kinetic energy from the core region, leading the remaining stars grouped in the core region to occupy a more compact volume. When this gravothermal instability occurs, the central region of the cluster becomes densely crowded with stars and the surface brightness of the cluster forms a power-law cusp. (Note that a core collapse is not the only mechanism that can cause such a luminosity distribution; a massive black hole at the core can also result in a luminosity cusp.) Over a lengthy period of time this leads to a concentration of massive stars near the core, a phenomenon called mass segregation.

The dynamical heating effect of binary star systems works to prevent an initial core collapse of the cluster. When a star passes near a binary system, the orbit of the latter pair tends to contract, releasing energy. Only after the primordial supply of binaries is exhausted due to interactions can a deeper core collapse proceed.In contrast, the effect of tidal shocks as a globular cluster repeatedly passes through the plane of a spiral galaxy tends to significantly accelerate core collapse.

The different stages of core-collapse may be divided into three phases. During a globular cluster's adolescence, the process of core-collapse begins with stars near the core. However, the interactions between binary star systems prevents further collapse as the cluster approaches middle age. Finally, the central binaries are either disrupted or ejected, resulting in a tighter concentration at the core.

The interaction of stars in the collapsed core region causes tight binary systems to form. As other stars interact with these tight binaries, they increase the energy at the core, which causes the cluster to re-expand. As the mean time for a core collapse is typically less than the age of the galaxy, many of a galaxy's globular clusters may have passed through a core collapse stage, then re-expanded.

The Hubble Space Telescope has been used to provide convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The globular star cluster 47 Tucanae, which is made up of about 1 million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey, which allowed astronomers to track the motion of its stars. Precise velocities were obtained for nearly 15,000 stars in this cluster.>>
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Sun Mar 24, 2019 7:07 pm

So Terzan T is thought by many to actually be the core of a dwarf elliptical galaxy that has been canabalized by the Milky Way, as also is the case with Omega Cen as discussed above. Here's another:
Messier 54 (also known as M54 or NGC 6715) is a globular cluster in the constellation Sagittarius. It was discovered by Charles Messier in 1778 and subsequently included in his catalog of comet-like objects. M54 is easily found in the sky, being close to the star ζ Sagittarii. It is, however, not resolvable into individual stars even with larger amateur telescopes.

Previously thought to belong to the Milky Way at a distance from Earth of about 50,000 light-years, it was discovered in 1994 that M54 most likely belongs to the Sagittarius Dwarf Elliptical Galaxy (SagDEG),[7] making it the first globular cluster formerly thought to be part of our galaxy reassigned to extragalactic status, even if not recognized as such for nearly two and a quarter centuries. As it is located on SagDEG's center, some authors think it actually may be its core;[8] however others have proposed that it is a real globular cluster that fell to the center of this galaxy due to decay of its orbit caused by dynamical friction.[9]

Modern estimates now place M54 at a distance of some 87,000 light-years,[3] translating into a true radius of 150 light-years across.[5] It is one of the denser of the globulars, being of class III[1] (I being densest and XII being the least dense). It shines with the luminosity of roughly 850,000 times that of the Sun and has an absolute magnitude of −10.0. In July 2009, a team of astronomers reported that they had found evidence of an intermediate-mass black hole in the core of M54.
It seems that several of the densest globulars aren't really globulars at all, but are the captured cores of dwarfs consumed by our galaxy.
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Boomer12k » Mon Mar 25, 2019 4:05 am

I wonder if there are others on the other side of the bulge that we might not be able to see....

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Boomer12k » Mon Mar 25, 2019 4:08 am

orin stepanek wrote:
Sun Mar 24, 2019 12:48 pm
It amazes me that there are these ball shaped star clusters; kind of like Christmas tree bulbs! One named Terzan; reminded me of a certain jungle man; Tarzan! :lol2:

My comment was going to be..."I just love TERZAN movies....".... :lol2:

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Mon Mar 25, 2019 8:29 am

Boomer12k wrote:
Mon Mar 25, 2019 4:05 am
I wonder if there are others on the other side of the bulge that we might not be able to see....

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There would have to be Boomer. Otherwise, there would be a big gapping hole in their natural, random distribution about the galaxy.

The preceding reasoning has been brought to y'all by the (somewhat overused) Copernican Principle; we don't occupy a special place in the galaxy or universe.

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Last edited by BDanielMayfield on Mon Mar 25, 2019 8:59 am, edited 1 time in total.
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Nitpicker » Mon Mar 25, 2019 8:41 am

BDanielMayfield wrote:
The preceding reasoning has been brought to y'all by the (somewhat overused) Coperican Principle; we don't occupy a special place in the galaxy or universe.

Bruce
Following that logic, the parts of the universe where we aren't, might be special. 😀

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by BDanielMayfield » Mon Mar 25, 2019 9:14 am

Nitpicker wrote:
Mon Mar 25, 2019 8:41 am
BDanielMayfield wrote:
The preceding reasoning has been brought to y'all by the (somewhat overused) Copernican Principle; we don't occupy a special place in the galaxy or universe.

Bruce
Following that logic, the parts of the universe where we aren't, might be special. 😀
Did I misuse or misapply the principle? (I noticed that I misspelled Copernican, since corrected.)
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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by Nitpicker » Mon Mar 25, 2019 9:21 am

No Bruce, but I'm almost certain that I did.

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Re: APOD: Zooming in on Star Cluster Terzan 5 (2019 Mar 24)

Post by TheZuke! » Mon Mar 25, 2019 3:48 pm

Thank you Art N. for writing the interstellar distances of a Global Cluster.
I just joined Starship* mostly to find that out.
Which then leads me to the questions.
If an Earthlike planet orbited one of those stars, what would the night sky look like?
With stars approximately 1 LY apart they would be much brighter (I guess 16 times -square law and all that) than Alpha Centauri, but not bright enough to eliminate night completely?
Or, how far apart in the night sky would the closer stars appear, as in degrees separation.
And with 6 stars per cubic light year as in Omega Centauri... my mind is officially boggled!