Inside a globular cluster

[DRT]

A few months ago there was a very interesting thread here on SGL discussing what you would see if you were close to or inside a planetary nebula. One of the most interesting facts I found out from that discussion was the vast distances between what we consider to be relatively small and closely associated groups of stars, such as the Trapezium in the core or the Orion nebula. For whatever reason that jumped into my head a couple of nights ago whilst I was looking at M13 and it led me to wonder what it would be like to be on an Earth-like planet orbiting a main sequence star deep inside a globular cluster.

I haven't researched this in any way so it might be that someone just points me to the answer, but does anyone know what this might look like in the night sky? Would we see lots of extremely bright stars, stars that were brighter than Jupiter appears to us, stars that look like small Suns, or would we simply see a sky full of stars like we see from here, only in different patterns and constellations?

 

[nightfisher]

I can only guess but i would think we would see a night sky much like our own but different constellations 

[BGazing]

probably much more mag 0 and 1 stars in the sky and our milky way laid out bare from above...

[Peter Drew]

As far as I know, the distance between individual stars in a globular cluster is far greater than the Earth-Sun distance so presumably the sky would appear to contain more bright stars than we see but not necessarily as discs.

[DRT]

Perhaps a mathematician can help with this?

M13 has around 500,000 stars and is 145 light years in diameter with the stars more tightly packed at the centre than at the outer edge.

Using some reasonable assumptions about the difference in star density, what would be the average distance between stars near the core?

[Paul M]

Ok, I cheated on Google but anyway, typically stars are separated by between the diameter of our solar system and 1ly.

I guess the night sky will be pretty awesome. Full of bright stars, perhaps barely getting dark? Perhaps too many and too similar in appearance to allow easy navigation and identification of patterns?

[xtreemchaos]

id like to think the sky would be full of diamonds, but we would probley have cloud  . intresting thought tho. charl.

[DRT]

Ok, who had "6" in the sweep for how long it took someone to mention clouds?

 

[DRT]

7 minutes ago, Paul M said:

Ok, I cheated on Google but anyway, typically stars are separated by between the diameter of our solar system and 1ly.

I guess the night sky will be pretty awesome. Full of bright stars, perhaps barely getting dark? Perhaps too many and too similar in appearance to allow easy navigation and identification of patterns?

Paul, that is interesting info. What definition is given to "diameter of our solar system"? Does that include the stuff beyond the planets or only out as far as Neptune (or Pluto depending on your preference)?

 

[Paul M]

14 minutes ago, DRT said:

Paul, that is interesting info. What definition is given to "diameter of our solar system"? Does that include the stuff beyond the planets or only out as far as Neptune (or Pluto depending on your preference)?

 

I think it's very generalised. One reference implies 100th the distance of alpha Centauri. Another says a few hundred parsecs in the globular's core.

Some globs will be denser, some sparser. So it's very general to say the least. 

 

 

[DRT]

1 hour ago, DRT said:

Perhaps a mathematician can help with this?

M13 has around 500,000 stars and is 145 light years in diameter with the stars more tightly packed at the centre than at the outer edge.

Using some reasonable assumptions about the difference in star density, what would be the average distance between stars near the core?

I sent this question to a mathematical friend of mine (www.jdawiseman.com) who wrote...

Quote

Assume that the bottom half of the galaxy’s cross-section is is like \/, with two straight lines, and that the top half is /\, all obviously symmetrical. So we’re assuming that the thickness is zero at the edge (horizontally 72½ ly from the centre); and we assume that at the centre the top-to-bottom height is h.

Volume = Integrate[2 π x h (1-x/r), {x, 0, r}] = ⅓ π h r².

Let’s say h = 10 light years. If you want differently, rework from here.

So volume per star, in ly³, is ⅓ π 10 72½² ÷ 500,000 ≈ 0.11.

How far apart are the stars? Really, some are close in binary systems, some are not. Let avoid that distributional question by assuming stars packed such that as far apart as possible.
https://en.wikipedia.org/wiki/Sphere_packing
Average density is 0.74048.

Replace each star with a ball of radius starGapR, with volume = 4/3 π starGapR³, and that volume is 0.11 × 0.74048.

So starGapR = (0.11 × 0.74048 * ¾ / π)^(⅓) = 0.269, so the distance between stars is twice this, about 0.538 ly.

This has lots of assumptions, all wrong, but might be a start.

As an example of the importance of the packing assumption, consider the following. Assume there are 500k stars, arranged as 250k tight binaries. Then each star’s distance to its closest neighbour might be a only few light minutes — perhaps the Sun-Mercury distance. And the average of these would also be a few light minutes, the size and shape of the galaxy being totally irrelevant.

I now have a headache.

But my reading of Julian's reply is that even distribution would cause the stars to be about half a light year apart, so the reality would be that many would be closer than that.

I have no idea where to go from here

[PhotoGav]

Here is an extract from a book that I am working on at the moment:

"In 1974, the Arecibo Observatory in Puerto Rico beamed a radio message at M13, signalling that there is intelligent life on Earth.   This message will not reach its intended recipients, if indeed there are any, for twenty five thousand years. Now, while the cluster will move through space during the transit time, the proper motion is small enough that the cluster will only move 24 light years, only a fraction of the diameter of the cluster. Thus, the message will still arrive near the center of the cluster.

To an observer located near the heart of M13, the sky would appear in constant daylight as the stars are packed so tightly, with over a hundred in a three-light-year radius. For contrast, the nearest star to our Sun is four light years away. Such beings might never have considered the great expanse of the universe we know, it being always hidden from sight."

So, not much chance of deep sky astronomy in M13, I reckon!!!

[DRT]

I just found this, which says that there are 56 stellar systems within 16.3 light years of our solar system. That is a lot of empty space with virtually nothing in it. Compared to a big ball of stars with an average distance from one to the next of 0.5 light years.

What size and magnitude would a red giant be if it were 0.5 light years from Earth?

[FenlandPaul]

That's a really interesting thought.  I suspect there would be so many very bright stars (somewhere between Venus and the full moon maybe?) that they would create quite a lot of glare in our atmosphere (I'm assuming we have one, right?  You didn't specify, but let's assume we do otherwise we have bigger concerns).  So we'd end up not being able to study the faint fuzzies so easily, despite being stuck inside one.

It does make you think just how uniquely placed we are to observe a lot of stuff.  We've not recently been zapped by a supernova, there's no imminent threat of consumption by a black hole, there's not so much dust around that we can't see really really deep, and there aren't so many bright stars around that we all complain of "star pollution", or whatever term we might coin.

[DRT]

23 minutes ago, PhotoGav said:

Here is an extract from a book that I am working on at the moment:

"In 1974, the Arecibo Observatory in Puerto Rico beamed a radio message at M13, signalling that there is intelligent life on Earth.   This message will not reach its intended recipients, if indeed there are any, for twenty five thousand years. Now, while the cluster will move through space during the transit time, the proper motion is small enough that the cluster will only move 24 light years, only a fraction of the diameter of the cluster. Thus, the message will still arrive near the center of the cluster.

To an observer located near the heart of M13, the sky would appear in constant daylight as the stars are packed so tightly, with over a hundred in a three-light-year radius. For contrast, the nearest star to our Sun is four light years away. Such beings might never have considered the great expanse of the universe we know, it being always hidden from sight."

So, not much chance of deep sky astronomy in M13, I reckon!!!

That is a fantastic description, Gav. When I asked the question I had a picture in my mind of a sky full of hundreds of bright disks of different colours and sizes, not just the diamonds on black velvet that we see.

 

[PhotoGav]

13 minutes ago, FenlandPaul said:

It does make you think just how uniquely placed we are to observe a lot of stuff.  We've not recently been zapped by a supernova, there's no imminent threat of consumption by a black hole, there's not so much dust around that we can't see really really deep, and there aren't so many bright stars around that we all complain of "star pollution", or whatever term we might coin.

Not to mention the luck that we only have one small moon. That's irritating enough if you are trying to take images of deep sky stuff. Imagine trying to image from Jupiter or Saturn!

[DRT]

Mathematician friend number two replied...

Quote

If you just want the answer:  <1.47 ly
 
Detail:
 
For a sphere of 145 ly diameter, the radius is 72.5 ly, so the sphere volume = (4/3)*pi*r^3 = 1596256 (ly^3)
The volume of a cube with side length 145 ly (which could just contain the sphere) = 145^3 = 3048625 (ly^3)
 
If we assume a constant packing density for the stars then at the same packing density, if the sphere contains 500,000 stars, the cube would contain 500000*3048625/1596256 = 954929 stars; the number of stars along each side would therefore be the cube root of 954929 = 98.5, so the spacing would be 145/98.5 = 1.47 ly. However, your original question stated that the "the stars more tightly packed at the centre than at the outer edge", but without knowing how much more tightly packed (2x, 10x 100x?) all we can say is that the distance between them would be less than the above statement for even packing.
 
One more observation: if you took a transparent sphere with equally distributed points within, then these would automatically appear to be more tightly packed at the centre than at the edge from any external viewpoint, because you are in essence looking through more of the sphere when you look at the centre than when you look at the edge, so you would see more points within a small viewing angle at the centre than at the edge. I imagine the person asking the question would be aware of this effect, and perhaps knows there is higher density at the centre, but I mention it in case.

Two friends who I consider to be highly competent at crunching numbers disagree by a factor of three. They might now be up all night arguing by email about who is right and who is wrong

 

[Paul M]

1 hour ago, DRT said:

I just found this, which says that there are 56 stellar systems within 16.3 light years of our solar system. That is a lot of empty space with virtually nothing in it. Compared to a big ball of stars with an average distance from one to the next of 0.5 light years.

What size and magnitude would a red giant be if it were 0.5 light years from Earth?

Maths is my weakness but I'll jumble some numbers and see if anyone agrees!

Aldebaran is 44 x the diameter of the Sun so at the Sun's distance it's angular size would be 22 degrees!

But it's 38,000,000 mile diameter at 2,939,300,000,000 miles distance (half light year) would still only subtend 3 arc seconds!

 

I made much use of Google and this tool : http://planetcalc.com/1897/  Even so, I may well have made a silly mistake somewhere!

 

[rockystar]

Not got too much to contribute, other than my main knowledge of GCs is that they are made up of really old stars, so I'm not sure many of them would be on the main sequence, would they have dropped down to white dwarves maybe? But it is a fascinating thought and I'm going to go and do some proper research and correct all the mistakes in my previous sentence

 

[LukeSkywatcher]

Regarding living close to a nebula...........

A couple of yrs ago i heard on tv that if M42 was as close to Earth as the Sun is, that the nebula would fill every inch of sky visible from anywhere on Earth all day and all night. It would be the only thing we would see........even during the day.

[SilverAstro]

On 08/08/2016 at 23:08, rockystar said:

Not got too much to contribute, other than my main knowledge of GCs is that they are made up of really old stars, so I'm not sure many of them would be on the main sequence, would they have dropped down to white dwarves maybe? But it is a fascinating thought and I'm going to go and do some proper research and correct all the mistakes in my previous sentence

 

A bell is ringing about Population 1 and 2 stars and that the group found in GC are very old, long lived, red, poor in heavy elements having been formed when the universe was young. Another bell is ringing that recently a GC (or some ? GC) confounded  astronomers as found to have some blue young star formation in it (them) ?

Yep, methinks also, some research is called for , , did you find anything since ?

[Paul M]

Other than what can be directly observed, we really know very little about globulars.

Why, how, when and where are all open to debate.

The more I read the more complicated they become.

[Peter Drew]

Are globulars really that dense at the centre?. If you consider a ball of stars of equal separation radiating out from the centre, wouldn't perspective give the appearance of increased density?.   

[DRT]

Just now, Peter Drew said:

Are globulars really that dense at the centre?. If you consider a ball of stars of equal separation radiating out from the centre, wouldn't perspective give the appearance of increased density?.   

That would be true, Peter, but I doubt that the stars are evenly distributed. Presumably gravity would play a part in holding the ball together so matter would most likely be denser at the centre than at the edge, as it is in solar systems and galaxies?

I find it quite intriguing that so little seems to be known about them. 

[Paul M]

6 minutes ago, Peter Drew said:

Are globulars really that dense at the centre?. If you consider a ball of stars of equal separation radiating out from the centre, wouldn't perspective give the appearance of increased density?.   

They do indeed get denser towards the centre.

It's caused by the gravitational gradient. The "weight" of the outer stars compress the core of the cluster.

I can't remember the deatails but it's actually one of the characteristics that globulars are classified by. Something like the ratio of the core's radius to the whole cluster's radius and can be quite significant.