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APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:07 am
by APOD Robot
Star Cluster Omega Centauri in HDR
Explanation: Behold the largest ball of stars in our galaxy.
Omega Centauri is
packed with about 10 million stars, many older than
our Sun and packed within a volume of only about 150
light-years in diameter. The
star cluster is the largest and brightest of 200 or so known
globular clusters that roam the
halo of our Milky Way galaxy. Though most
star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of
different stellar populations with a spread of ages and chemical abundances. In fact,
Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. The featured image shows so many stars because it merged different exposures with
high dynamic range (HDR) techniques.
Omega Centauri, also known as NGC 5139, lies about 15,000 light-years away toward the
southern constellation of the Centaurus.
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Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:35 am
by Blastov
Beautiful. i am struck by its seemingly spherical shape and wonder how does an enormous mass of stars such as this not begin to rotate and flatten out the same way galaxies and protoplanetary accretion disks do?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:59 am
by Chris Peterson
Blastov wrote:Beautiful. i am struck by its seemingly spherical shape and wonder how does an enormous mass of stars such as this not begin to rotate and flatten out the same way galaxies and protoplanetary accretion disks do?
The flattening of a spinning body occurs when the individual particles that make it up are able to interact in a way that is fluid-like, typically through electromagnetic interactions. This doesn't happen with stars orbiting at random inclinations around a common center of mass. That includes galaxies, as well- those that are flat are presumably that way because of interactions involving much denser material very early in their formation. When galaxies are disrupted, the don't reform into flat structures. Accretion disks are the only example you've given where you have fluid dynamics occurring- a dense medium where angular momentum is continuously being transferred between particles, resulting in the ejection of a lot of those particles and the consolidation of the rest into a plane.
Some technical details on the image
Posted: Tue Jul 11, 2017 5:04 am
by MikeODay
A deep look at Omega Centauri ( NGC 5139 )
This image is an attempt to look deeply into the Omega Centauri star cluster and, by using HDR techniques, record as many of its faint members as possible whilst capturing and bringing out the subtle colours of the stars, including in the core.
Image details:
Size: 58.6 x 39 arcmins
Centre: 13h 26 min 50.4 sec, -47deg 28' 39.1''
Orientation: up is -89.9 East of North ( ie. E^ N> ).
Telescope: Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ).
Corrector: ASA 2" Coma Corrector Quattro 1.175x.
Effective Focal Length / Aperture : 1410mm f4.7.
Mount: Pier mounted Skywatcher AZ Eq6 GT ( backyard rolling-shed observatory )
Guiding: TSOAG9 Off-Axis-Guider, Starlight Xpress Lodestar X2, PHD2 .
Camera:
Nikon D5300 (unmodified) (sensor 23.5 x 15.6mm, 6016x4016 3.9um pixels).
Location:.
Blue Mountains, Australia ( approx. 700m altitude, 33.7 deg South )
Moderate light pollution ( pale green zone on darksitefinder.com map ).
Capture:
9 sets of sub-images with exposure duration for each set doubling ( 1s to 240s ) all at ISO800.
Processing:.
Calibration: master bias, master flat and no darks.
Integration in 9 sets.
HDR combination.
Pixinsight
May 2017
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:07 am
by MikeODay
Blastov wrote:Beautiful. i am struck by its seemingly spherical shape and wonder how does an enormous mass of stars such as this not begin to rotate and flatten out the same way galaxies and protoplanetary accretion disks do?
Thank you - much appreciated.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 6:02 am
by Boomer12k
Really great image, and work.
How can something be... "In fact....maybe...."?
:---[===] *
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 6:19 am
by LMMT
What would the average distance between stars be? I guess much less than 4 ly, the one to alfa centauri. Would a planet inside the cluster show a night sky similar to ours or would it be incredibly full of stars?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 10:54 am
by MikeODay
Boomer12k wrote:Really great image, and work.
How can something be... "In fact....maybe...."?
:---[===] *
Thank you, much appreciated.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 11:12 am
by MikeODay
LMMT wrote:What would the average distance between stars be? I guess much less than 4 ly, the one to alfa centauri. Would a planet inside the cluster show a night sky similar to ours or would it be incredibly full of stars?
A very bight night sky I imagine, especially towards the core!
As a first approximation perhaps one could assume a sphere with radius 75 light years and total of 10 million stars evenly spread through the sphere giving an average cubic volume per star of a little under 0.6cu light years ( if there is such a unit
) or say an average separation of 0.6 light years.
Wikipedia states
"Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster.[28] The typical distance between stars in a globular cluster is about 1 light year,[29] but at its core, the separation is comparable to the size of the Solar System (100 to 1000 times closer than stars near the Solar System)"
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 11:13 am
by neufer
http://www.atlasobscura.com/places/world-s-largest-popcorn-ball wrote:
<<Sac City, Iowa, isn’t known for much. But what it is known for - the world’s largest ball of popcorn - it is quite proud of. The ball draws in visitors from all over (and many who are directed to the city and its popcorn ball, which is stored in a shed, by billboards that run alongside the nearby highways), which helps nearby businesses with tourism dollars. The ball of popcorn is believed to weigh more than 3,400 pounds, beating the previous record holding ball of popcorn by more than 200 pounds. It measured eight feet in diameter and has a circumference of just over 24 feet. The ball of popcorn in Sac City ws remade in February of 2009 by more than 250 volunteers who spent 12 hours and 40 minutes constructing the sculpture. The reconstruction required 900 pounds of popcorn, more than 2,500 pounds of sugar, and 1,400 pounds of syrup.>>
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 12:08 pm
by Ann
Looking at today's APOD, I'm reminded of the only time I have ever looked at a globular cluster through a telescope. As a Color Commentator who gets a huge kick out of the colors of the universe, especially the tints of blue, I was very disappointed at the sight of M13. It was all white, with the faintest tinge of sickly green. I've never wanted to look at a globular cluster through a telescope again.
At first I thought that today's APOD looked much like that, but after I've looked at the full size image I have changed my mind. The stars in today's APOD do have colors, at least some of them. (And it is absolutely right that some of them
should look colorless, because many stars in Omega Centauri are similar to the Sun in their temperature, and they are therefore white to our eyes.)
I like the subtle colors of the blue stars of Omega Centauri, and I believe that this is the "true color" of them. They are "blue horizontal branch stars", and while they are blue, they are not all that bright. The blue horizontal branch stars are no brighter than RR Lyrae stars, and some of them are fainter. And the RR Lyrae stars are typically no brighter than 30-50 times that of the Sun.
Admittedly it is hard to know how blue the "typical" blue horizontal branch stars really are. The bluest stars in the diagram at right seem to have a B-V index of -0.8 or so, which is absolutely crazily blue and utterly unlike anything you can find in the more nearby and more metal-rich stellar populations. One of the stars with the very bluest B-V index in the nearby universe is Mu Columba, whose B-V index is -0.274 ± 0.013. You can't even compare that with a B-V index of -0.8. Of course, the blue horizontal branch stars in Omega Centauri are extremely metal-poor, and this just might give them a bluer B-V index, for all I know. (But I still can't believe that a star can be as blue as -0.8, and I don't even believe in a star with a B-V index of -0.5. Sorry.)
In any case, the stars of Omega Centauri must definitely be dust-reddened to some extent. So whatever their "true" B-V index might be, the light that reaches us from these stars here on Earth can't possibly be a lot bluer than, say, -0.2. There are various ways to portray a color like that, but there is a delicate subtlety in showing it as something very pale. And certainly, when I have looked at blue stars through a telescope, many of them have looked blue, but most of them have been quite pale in color, not unlike the car at left.
So what about the "red" stars? Aren't they red, or at least orange, or at least deeply yellow-orange? Not in my opinion, no. Most of them were just yellow, some of them a quite
pale shade of yellow, when I last looked at them through a telescope.
So I appreciate the pale, subtle colors of the stars of today's APOD.
Ann
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 12:44 pm
by neufer
Ann wrote:
<<I'm reminded of the only time I have ever looked at a globular cluster through a telescope. As a Color Commentator who gets a huge kick out of the colors of the universe, especially the tints of blue, I was very disappointed at the sight of M13. It was all white, with the faintest tinge of sickly green. I've never wanted to look at a globular cluster through a telescope again.
I like the subtle colors of the blue stars of Omega Centauri, and I believe that this is the "true color" of them. They are "blue horizontal branch stars", and while they are blue, they are not all that bright.
So what about the "red" stars? Aren't they red, or at least orange, or at least deeply yellow-orange? Not in my opinion, no. Most of them were just yellow,>>
Sickly green, yellow, not all that bright... Globular clusters are being cyber bullied.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:05 pm
by pinguwin
Mike Oday, I get the same math of .18 cubic l.y. but this would mean that each star is .56 l.y. away from each other. That is assuming equal distribution but of course it's not.
Does anyone have what it might look like inside such a globular cluster? My searching skills failed me. The only thing I can find is an apod that uses a bird flock (
https://apod.nasa.gov/apod/ap080906.html). Any good artistic renderings or simulations?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:39 pm
by zendae1
MikeODay wrote:LMMT wrote:What would the average distance between stars be? I guess much less than 4 ly, the one to alfa centauri. Would a planet inside the cluster show a night sky similar to ours or would it be incredibly full of stars?
A very bight night sky I imagine, especially towards the core!
As a first approximation perhaps one could assume a sphere with radius 75 light years and total of 10 million stars evenly spread through the sphere giving an average cubic volume per star of a little under 0.18cu light years ( if there is such a unit
) or say an average separation of 0.18 light years.
Wikipedia states
"Globular clusters can contain a high density of stars; on average about 0.4 stars per cubic parsec, increasing to 100 or 1000 stars per cubic parsec in the core of the cluster.[28] The typical distance between stars in a globular cluster is about 1 light year,[29] but at its core, the separation is comparable to the size of the Solar System (100 to 1000 times closer than stars near the Solar System)"
Thanks for the info. As a follow up question, how close can two of these stars be, on average, before starting to interact with each other? Are they sufficiently far apart to not interact?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:46 pm
by Maxedwell
I, too, am extremely curious as to what a night sky on a planet in the center of the cluster would look like- sans moon(s). An artist rendering would be much appreciated. Perhaps someone who is better at math than myself could extrapolate the numeric increase of brightness compared to an Earth sky and at least come up with a number such as "10 times brighter, 100 times brighter, or whatever would be the correct answer? Then perhaps we could "photo shop" an earth night sky, on a moonless night, and increase the brightness artificially?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:53 pm
by joyfaith
Isaac Asimov's short story Nightfall is a powerful piece about what life might look like in a globular cluster.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 4:57 pm
by rstevenson
pinguwin wrote:Mike Oday, I get the same math of .18 cubic l.y. but this would mean that each star is .56 l.y. away from each other. That is assuming equal distribution but of course it's not.
Does anyone have what it might look like inside such a globular cluster? ...
If I've run the numbers correctly, .5 ly is about 800 times the radius of Pluto's orbit. From Pluto, our average-size Sun is just a speck of light in the night sky. So, to imagine what the view is from a planet in a globular cluster like this, imagine the night sky full of bright specks -- kind of like our night sky, in fact. "My God, it's full of stars!"
Rob
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:00 pm
by rstevenson
joyfaith wrote:Isaac Asimov's short story Nightfall is a powerful piece about what life might look like in a globular cluster.
Not quite. From its
Wikipedia article...
The fictional planet Lagash (Kalgash in the novel adaptation) is located in a stellar system containing six suns ..., which keep the whole planet continuously illuminated; total darkness is unknown, and as a result, so are all the stars outside the planet's stellar system.
Rob
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:01 pm
by heehaw
Pity the poor astronomers on some planet of a star that is near the center! The whole sky would be sparkling with thousands upon thousands of bright stars! They would surely think that that was the entire universe! And globular clusters are so old, that those astronomers have had millions of years to ponder that: and also to develop bigger and bigger telescopes, and ... see that there is a beyond! In extreme contrast, our astronomers are in a very empty part of our own galaxy and can look out with clarity at our truly vast universe. Let us be grateful!
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:05 pm
by bystander
LMMT wrote:Would a planet inside the cluster show a night sky similar to ours or would it be incredibly full of stars?
pinguwin wrote:Does anyone have what it might look like inside such a globular cluster?
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:10 pm
by Visual_Astronomer
I am fortunate to live far enough south that Omega Centauri gets about 12 degrees above the horizon. Even at such a low angle, on nights of very stable air, it becomes the most amazing object in the sky. It is so large and so many stars are resolved... it is the very reason I am a visual astronomer!
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:31 pm
by pinguwin
rstevenson wrote:If I've run the numbers correctly, .5 ly is about 800 times the radius of Pluto's orbit.
If I've run the numbers correctly too
but .5 ly is 1/8 the distance to the nearest star, then there are a few more 6-7 ly away. But the .5 ly is how far each star would be away from each other assuming even distribution.
The cluster is 75 ly in diameter. At this distance we have 2500-ish stars (
http://teacherlink.ed.usu.edu/tlnasa/re ... 0123d.html) away from out sun (yes, this is a WAG on my part). 75 ly from the center of the cluster is 10,000,000 stars.
According to Mike Oday's wikipedia reference, the center would be crazy densely populated. And thanks for the "life inside" picture (and to all posters, a great discussion).
Going to be going to our local astronomy club's viewing night soon and a globular cluster will be one my requests.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 5:56 pm
by joyfaith
rstevenson wrote:joyfaith wrote:Isaac Asimov's short story Nightfall is a powerful piece about what life might look like in a globular cluster.
Not quite. From its
Wikipedia article...
The fictional planet Lagash (Kalgash in the novel adaptation) is located in a stellar system containing six suns ..., which keep the whole planet continuously illuminated; total darkness is unknown, and as a result, so are all the stars outside the planet's stellar system.
Rob
Yes, Lagash is in a system of 6 "suns", but that was only all they could see. At the end of the story it says
Lagash was in the center of a giant cluster. Thirty thousand mighty suns shone down...
http://www.astro.sunysb.edu/fwalter/AST ... htfall.htm
How amazing that sight would be.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 6:53 pm
by MikeODay
Ann wrote:Looking at today's APOD, I'm reminded of the only time I have ever looked at a globular cluster through a telescope. As a Color Commentator who gets a huge kick out of the colors of the universe, especially the tints of blue, I was very disappointed at the sight of M13. It was all white, with the faintest tinge of sickly green. I've never wanted to look at a globular cluster through a telescope again.
At first I thought that today's APOD looked much like that, but after I've looked at the full size image I have changed my mind. The stars in today's APOD do have colors, at least some of them. (And it is absolutely right that some of them should look colorless, because many stars in Omega Centauri are similar to the Sun in their temperature, and they are therefore white to our eyes.)
...
I like the subtle colors of the blue stars of Omega Centauri, and I believe that this is the "true color" of them. They are "blue horizontal branch stars", and while they are blue, they are not all that bright. The blue horizontal branch stars are no brighter than RR Lyrae stars, and some of them are fainter. And the RR Lyrae stars are typically no brighter than 30-50 times that of the Sun.
...
So I appreciate the pale, subtle colors of the stars of today's APOD.
Ann
Thank you Ann for your detailed reply and the links to the analysis of star colours in Omega Centauri. I always enjoy your posts because one of my main goals in processing my photos is to try to reproduce as best I can the "true colours" of the objects and stars I image. I have only been into astrophotography for 3 years and I still have a great deal to learn so the insights you provide are very valuable to me. Thanks.
Re: APOD: Star Cluster Omega Centauri in HDR (2017 Jul 11)
Posted: Tue Jul 11, 2017 7:06 pm
by MikeODay
pinguwin wrote:Mike Oday, I get the same math of .18 cubic l.y. but this would mean that each star is .56 l.y. away from each other. That is assuming equal distribution but of course it's not.
Does anyone have what it might look like inside such a globular cluster? My searching skills failed me. The only thing I can find is an apod that uses a bird flock (
https://apod.nasa.gov/apod/ap080906.html). Any good artistic renderings or simulations?
Thanks for correcting my error - way too long since I have done any maths
