APOD: Messier 2 (2019 Apr 04)

[APOD Robot]

Messier 2

Explanation: After the Crab Nebula, M1, this giant star cluster is the second entry in 18th century astronomer Charles Messier's famous list of things with are not comets. M2 is one of the largest globular star clusters now known to roam the halo of our Milky Way galaxy. Though Messier originally described it a nebula without stars, this stunning Hubble image resolves stars across the central 40 light-years of M2. Its population of stars numbers close to 150,000, concentrated within a total diameter of around 175 light-years. About 55,000 light-years distant toward the constellation Aquarius, this ancient denizen of the Milky Way, also known as NGC 7089, is 13 billion years old.

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[E Fish]

As simple as they are in appearance, I never get tired of seeing globular clusters. They're gorgeous. M13 is my favorite. It's the first one I found in a telescope on my own.

[orin stepanek]

13 billion years! these babies have been around for a while!

[Fred the Cat]

Recent findings have been presenting evidence that some galaxies with globular clusters seem to contain less dark matter. Also the rotation and motion of the stars within clusters involve complexities difficult to understand.

Attempting to resolve these issues may bring contentions among scientists trying to unravel the mysteries of galaxies lacking this elusive substance and their clusters.

[Ctrl-Z]

The estimation of 150K stars within an area of 175 light-years... how would that compare to planet Earth's view of the night sky? I realize the spacing is greater on the outskirts, so how about more towards the middle of M2? Would there be more close-looking stars, less, the same amount? Trying to get an idea of the density compared to our little section of our galaxy.

[TheZuke!]

Ctrl-Z

A couple weeks ago, Art Neufer pretty much answered your question in another daily discuss.

viewtopic.php?f=9&t=39293#p290811

Now considering that Alpha Proxima is ~4 light years away I'm guessing that would put it and it's two companions within 16 cubic light year space to us.

Anyone who knows better than this noob (me) is welcome to correct.

Is it in the FAQ? (I didn't look)

[zendae1]

Ctrl-Z

A couple weeks ago, Art Neufer pretty much answered your question in another daily discuss.

viewtopic.php?f=9&t=39293#p290811

Now considering that Alpha Proxima is ~4 light years away I'm guessing that would put it and it's two companions within 16 cubic light year space to us.

Anyone who knows better than this noob (me) is welcome to correct.

Is it in the FAQ? (I didn't look)

I would like to ask the same question in a different way:

If planet Earth was in the direct center of the cluster, and we were standing on Earth, how many stars would we see in the sky about the size of the Sun? How many would we see about 1 quarter the size of the Sun? I understand there are many parameters not being addressed here...
Let's say all the stars are about the size of the Sun, not Betelgeuse...

[Fred the Cat]

Ctrl-Z

A couple weeks ago, Art Neufer pretty much answered your question in another daily discuss.

viewtopic.php?f=9&t=39293#p290811

Now considering that Alpha Proxima is ~4 light years away I'm guessing that would put it and it's two companions within 16 cubic light year space to us.

Anyone who knows better than this noob (me) is welcome to correct.

Is it in the FAQ? (I didn't look)

I would like to ask the same question in a different way:

If planet Earth was in the direct center of the cluster, and we were standing on Earth, how many stars would we see in the sky about the size of the Sun? How many would we see about 1 quarter the size of the Sun? I understand there are many parameters not being addressed here...
Let's say all the stars are about the size of the Sun, not Betelgeuse...

I remember asking a similar question and searching for illustrations of a night sky from the center of a globular cluster.

[TheZuke!]

Okay, Freddy, Zendae, Ctrl-Z.
I don't have exactly what you are asking for, but consider Square Law.
Alpha Centauri at 4 ly is a bright start in the sky.
The same star at 1 ly would be 16 times brighter. what would that look like? I'm not sure... Venus? Moon? (although not the apparent size of the Moon)

as Neufer mentioned in the link above:
"Omega Centauri has about 6 stars per cubic light year;
however, 1 star per cubic light year is more typical."

So, I think you'd see a lot of bright stars in the sky, and they'd wash out a lot of dimmer stars, just like a full Moon washes out a lot of stars.

Where are the "pro's" of this forum, are they tired of answering this question?
My speculation doesn't take into account any stellar giants in the neighbourhood. (Are there stellar giants in globular clusters?)

[orin stepanek]

in the middle of a star cluster; I wonder if you would see any stars because it may be daylight all the time! But who knows!

[Fred the Cat]

Where are the "pro's" of this forum, are they tired of answering this question?

Let me guess? The pay was better in other forums so you might check there

[neufer]

in the middle of a star cluster; I wonder if you would see any stars because it may be daylight all the time!

But who knows!

Typically, it might be bright enough to read by (a.k.a, full moonlight bright).

[Ann]

To me this APOD is truly stunning. M2 looks amazingly rich and amazingly concentrated in the center. Farther from the center we can admire not only the strongly colored stars, but also the profusion of intermediate white-looking stars, which are probably Solar analogues.

Of course, me being me, I love the strongly colored blue horizontal stars, which appear to be very numerous in this globular.

I must add that I have contributed a little to an updated caption on the ESA page, where this picture was originally posted. Originally a part of the caption read the way it was quoted by bystander:

Spacetelescope.com wrote:
This Hubble image of Messier 2’s core was created using visible and infrared light.

But if you check the page that I just linked to, you can see that the filters used for the image were two ultraviolet ones and one blue one.

I wrote to ESA and pointed out their mistake. And lo and behold, the ESA caption has changed. Now they don't talk about infrared light any more.

Anyway, this is a gorgeous image!

Ann

[Ann]

Okay, Freddy, Zendae, Ctrl-Z.
I don't have exactly what you are asking for, but consider Square Law.
Alpha Centauri at 4 ly is a bright start in the sky.
The same star at 1 ly would be 16 times brighter. what would that look like? I'm not sure... Venus? Moon? (although not the apparent size of the Moon)

as Neufer mentioned in the link above:
"Omega Centauri has about 6 stars per cubic light year;
however, 1 star per cubic light year is more typical."

So, I think you'd see a lot of bright stars in the sky, and they'd wash out a lot of dimmer stars, just like a full Moon washes out a lot of stars.

Where are the "pro's" of this forum, are they tired of answering this question?
My speculation doesn't take into account any stellar giants in the neighbourhood. (Are there stellar giants in globular clusters?)

Click to view full size image

Globular cluster NGC 6362. Photo: ESA/Hubble and NASA.

Answer to question number one, would a lot of bright stars in a globular wash out a lot of dimmer stars in the sky to anyone who was inside the globular cluster and was surrounded by bright lights?

Answer: Yes. They would.

Answer to question number two, are there stellar giants in globular clusters?

Answer: Yes. There are.

The stellar giants in globular clusters are no true supergiants. According to Jim Kaler, the initial mass of red supergiant Betelgeuse may have been 18 or 19 solar masses. There are no such stars in ancient globular clusters, because such stars have exploded as supernovas long ago. The age-old giants that we see today in globular clusters may have started out with perhaps two solar masses or maybe even less. But these red giants are still bright, perhaps a hundred or two hundred times as bright as the Sun or even more.

Take a look at the picture of globular cluster NGC 6362. The red giants stand out very clearly here, and it is obvious that they are the brightest stars in the cluster.

Ann

[Ann]

My speculation doesn't take into account any stellar giants in the neighbourhood. (Are there stellar giants in globular clusters?)

Your question about stellar giants in globular clusters is far more reasonable than I first assumed. Because it isn't obvious from the Hubble picture of M2 that there really are stellar giants there.

Because the red stars really don't look all that much brighter than the blue stars in the APOD, even though the red giants are always the brightest stars in a globular cluster by far.

Why do the red giants look so comparatively faint in M2? Is it a consequence of the filters used for the image, two ultraviolet one and a blue one?

I complained about the recent APOD of the Horsehead region that the filters used (as well as the color mapping) made the blue stars in the region, such as HD 37776 in reflection/emission nebula IC 342, unnecessarily faint.

Here I might complain that the filters used for the image of M2 have made the red stars too faint.

Am I right here, Chris, Nitpicker and others? Isn't it true that if I photograph a nebula and its hot blue stars through Hα, OIII and SII filters, then the shortwave light from the blue stars will be poorly detected, and the blue stars will look faint?

But itsn't it similarly true that if I photograph cool red stars through two ultraviolet filters and a blue one, the longwave light from the red stars will be poorly detected, and the red stars will look faint?

I would appreciate a comment from you.

Ann

[Nitpicker]

The "wide spectrum" filters used were, I think, the F275W (B), F336W (G) and F438W (R) shown in this graph:

Click to view full size image

http://www.stsci.edu/hst/wfc3/ins_perfo ... _Wide1.jpg

I read this as the visible blue light is showing in red, and the invisible UV light is showing in green and blue. Nothing above a wavelength of 470nm was recorded. That's all I know.

[Chris Peterson]

Am I right here, Chris, Nitpicker and others? Isn't it true that if I photograph a nebula and its hot blue stars through Hα, OIII and SII filters, then the shortwave light from the blue stars will be poorly detected, and the blue stars will look faint?

All stars look faint when imaged through very narrowband filters. It doesn't really matter that much what the passband actually is.

[Ann]

Am I right here, Chris, Nitpicker and others? Isn't it true that if I photograph a nebula and its hot blue stars through Hα, OIII and SII filters, then the shortwave light from the blue stars will be poorly detected, and the blue stars will look faint?

All stars look faint when imaged through very narrowband filters. It doesn't really matter that much what the passband actually is.

Point taken, but I didn't think that the 275 nm, 336 nm and 438 nm filters are narrowband ones?

Ann

[Chris Peterson]

Am I right here, Chris, Nitpicker and others? Isn't it true that if I photograph a nebula and its hot blue stars through Hα, OIII and SII filters, then the shortwave light from the blue stars will be poorly detected, and the blue stars will look faint?

All stars look faint when imaged through very narrowband filters. It doesn't really matter that much what the passband actually is.

Point taken, but I didn't think that the 275 nm, 336 nm and 438 nm filters are narrowband ones?

No... but I was responding to your question about Hα, OIII and SII filters.

[Ann]

Click to view full size image

The Double Cluster in infrared light.
Photo: 2MASS.

Click to view full size image

Teh Double Cluster in optical light.
Photo: Fred Espenak.

I'm probably boring you out of your skulls by now, but I want to point out, again, how different objects look when photographed through different filters. The "pillow" picture is the Double Cluster photographed through the infrared J, H and K filters, centered at 1220, 1630 and 2190 nm. You can see that a few individual stars are much brighter than the rest. They are the red supergiants of the cluster. They look so bright because their light output peaks in the infrared part of the spectrum, unlike the blue stars, which are brightest in ultraviolet light.

The picture at right shows the Double Cluster in optical light, where the red giants are not obviously brighter than the blue-white stars.

Filters, filters, filters. In astronomy, you don't know what you are looking at until you have figured out the filters.

Ann

[Nitpicker]

I am guessing that the colours we see in the APOD are vaguely similar to those of a "true colour" visible light image, as the progression from blue to red still indicates decreasing temperatures. But the relative magnitudes might be quite different from a visible light image. Correct?

[Ann]

I am guessing that the colours we see in the APOD are vaguely similar to those of a "true colour" visible light image, as the progression from blue to red still indicates decreasing temperatures. But the relative magnitudes might be quite different from a visible light image. Correct?

The "problem" with the Wisps around the Horsehead Nebula APOD was that there wasn't a "natural progression from blue to red indicating ever decreasing temperatures". The Hubble palette is like that. But in the Horsehead APOD, red SII (at 658 nm?) was shown as blue, while blue-green OIII was shown as green.

Reserving the shortwave blue color for the reddest (and longest) wavelength of SII is strange, in my opinion.

Ann

[Nitpicker]

But, Ann, do the narrow bands of ionisation wavelengths relate to temperature in the same way as starlight? I think they rather relate to the elements being ionised.

[Ann]

But, Ann, do the narrow bands of ionisation wavelengths relate to temperature in the same way as starlight? I think they rather relate to the elements being ionised.

They relate to the degree of ionization of the nebula, which in turn is dependent on the temperature of (or distance from) the ionizing source.

Ann

[Nitpicker]

But, Ann, do the narrow bands of ionisation wavelengths relate to temperature in the same way as starlight? I think they rather relate to the elements being ionised.

They relate to the degree of ionization of the nebula, which in turn is dependent on the temperature of (or distance from) the ionizing source.

Ann

No, the narrow bands relate to the specific elements in the nebula, that are being ionised by photons from nearby stars. Each different ionised element emits a different, narrow band of wavelengths. The strength of the signal of each narrow band, relates to the degree of ionisation, which would depend on the photon flux and the density of the corresponding element in the nebula.