Starship Asterisk*

The Colors and Magnitudes of M13

Explanation: M13 is modestly recognized as the Great Globular Star Cluster in Hercules. A ball of stars numbering in the hundreds of thousands crowded into a region 150 light years across, it lies some 25,000 light-years away. The sharp, color picture of M13 at upper left is familiar to many telescopic imagers. Still, M13's Color vs Magnitude Diagram in the panel below and right, made from the same image data, can offer a more telling view. Also known as a Hertzsprung Russell (HR) diagram it plots the apparent brightness of individual cluster stars against color index. The color index is determined for each star by subtracting its brightness (in magnitudes) measured through a red filter from its brightness measured with a blue filter (B-R). Blue stars are hot and red stars are cool so that astronomical color index ranging from bluer to redder follows the relative stellar temperature scale from left (hot) to right (cool). In M13's HR diagram, the stars clearly fall into distinct groups. The broad swath extending diagonally from the bottom right is the cluster's main sequence. A sharp turn toward the upper right hand corner follows the red giant branch while the blue giants are found grouped in the upper left. Formed at the same time, at first M13's stars were all located along the main sequence by mass, lower mass stars at the lower right. Over time higher mass stars have evolved off the main sequence into red, then blue giants and beyond. In fact, the position of the turn-off from the main sequence to the red giant branch indicates the cluster's age at about 12 billion years.

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A question, SVP:
Lovely photo, but how many of the “several hundreds of thousands” of stars can be individually analyzed and converted into data points in the H-R diagram? Surely not even one of several hundreds of thousands. Sooo.... how can they be sure they have a representative sample? Couldn’t there be all sorts of strange critters lurking in the dense, bright miasma of the central region that can’t be seen? I dunno.....it’s like that in my neighborhood.

ranging from bluer to redder -->
ranging from redder to bluer

An interesting way to look at an object and its data...

My M13 is not so...detailed...
:---[===] *

A bit confusing! It is weird that stars formed in these ball clusters in the first place! IMHO

That is an interesting question..how do stars form in a star cluster and why are there star clusters?

Hineni wrote: ↑Thu Jun 13, 2019 6:40 am A question, SVP:
Lovely photo, but how many of the “several hundreds of thousands” of stars can be individually analyzed and converted into data points in the H-R diagram? Surely not even one of several hundreds of thousands. Sooo.... how can they be sure they have a representative sample? Couldn’t there be all sorts of strange critters lurking in the dense, bright miasma of the central region that can’t be seen? I dunno.....it’s like that in my neighborhood.

A very large percentage of these stars CAN indeed be analyzed individually. In a photo like today's APOD stars (especially toward the center) appear to overlap, but that is due to the fact the the size of points of light on our display devices are much larger than the actual angular diameter of the stars themselves. Hope that helps.

Bruce

sillyworm 2 wrote: ↑Thu Jun 13, 2019 12:52 pm That is an interesting question..how do stars form in a star cluster and why are there star clusters?

The Globular Cluster Wikipedia article contains just about all we know about the subject, presented in an accessible way. (There's another article about Open Clusters, but M13 is a globular.)

Rob

Beautiful! I have a soft spot for M13. It's the first object I found in a telescope all by myself. I love globular clusters, they're lovely and such interesting objects, too. I've never seen the H-R diagram for M13 before. Nice one today.

So I get it that the disc of light from each star is a much greater diameter than the star itself. But if one were theoretically on a planet in the dense center, what would the sky look like? The relative same as it does now? What is the average distance between two stars there?

My favorite glob!!

When I first saw today's APOD, I thought, No way the stars in M13 can be that blue! Look at the color indexes of the stars, where the bluest ones are -1.5, that's ridiculous! NO stars can get that blue ever!!! And look at that main sequence, it is almost all bluish!!!

Then I realized that the color index used in today's APOD is not the customary B - V, but instead B - R.

When I looked at today's APOD and saw how "blue" the stars were, I realized how much the "normal" B-V index actually "reddens" stars, or at least reddens our perspective of them. Vega, for example, has a B-V index of 0.00, and it is used as a "zero point" in the description of star colors and as a perfect example of a purely "white" star.

Well, that's ridiculous! I have taken part in showing people Vega through a telescope and asked them what color they thought it was. All of them said that Vega is blue.

Vega's "whiteness" is a product of the workings of the B-V system, where Vega comes out neutral. (All right, it is also a product of our human night vision, because the blue-sensitive rods in our retinas are not good at responding to blue light when the blue light source is very faint and is seen against a very dark background.) Well, Vega's color wouldn't be neutral, but clearly blue, in a B-R color index system. In B-R, our own Sun would probably have a color index close to zero, which means it would be "white".

I keep insisting that our Sun really is white. Not because it possesses some magical quality of "whiteness", but because humanity has evolved under the light of the Sun, and it has been to our advantage to see the Sun as white. Oh, we don't see it as white, you say? No, but we see daylight as white, or neutral, and daylight is sunlight, pure and simple.

Ann

zendae1 wrote: ↑Thu Jun 13, 2019 3:41 pm So I get it that the disc of light from each star is a much greater diameter than the star itself. But if one were theoretically on a planet in the dense center, what would the sky look like? The relative same as it does now? What is the average distance between two stars there?

M13 is about 145 light-years in diameter, and it is composed of several hundred thousand stars, the brightest of which is a red giant, the variable star V11, with an apparent visual magnitude of 11.95. M13 is about 22,200 light-years away from Earth.

It wasn't until 1779 that the single stars in this globular cluster were resolved. Compared to the stars in the neighborhood of the Sun, the stars in M13's stellar population are more than a hundred times denser. They are so densely packed together that they sometimes collide and produce new stars. The newly-formed, young stars, so-called "blue stragglers," are particularly interesting to astronomers.[12]

Since according to that Wikipedia quote the stellar density is more than 100 times denser than around here then stars at M13's core would be about 0.04 light years apart. That would only amount to about 15 light days. That's about 2600 AU. (AU ~ radius of Earth's orbit)

Ann wrote: ↑Thu Jun 13, 2019 7:02 pm I keep insisting that our Sun really is white. Not because it possesses some magical quality of "whiteness", but because humanity has evolved under the light of the Sun, and it has been advantageous to humanity to see the Sun as white. Oh, we don't see it as white, you say? No, but we see daylight as white, or neutral, and daylight is sunlight, pure and simple.

Ann

I agree with all of that (except for the "humanity has evolved" part). The Sun should be called white, but since they call Main Sequence stars Dwarfs that would make the Sun a White Dwarf, and that name is already taken (by a class of stars that actually are dwarfs).

Bruce

BDanielMayfield wrote: ↑Thu Jun 13, 2019 1:44 pm

Hineni wrote: ↑Thu Jun 13, 2019 6:40 am A question, SVP:
Lovely photo, but how many of the “several hundreds of thousands” of stars can be individually analyzed and converted into data points in the H-R diagram? Surely not even one of several hundreds of thousands. Sooo.... how can they be sure they have a representative sample? Couldn’t there be all sorts of strange critters lurking in the dense, bright miasma of the central region that can’t be seen? I dunno.....it’s like that in my neighborhood.

A very large percentage of these stars CAN indeed be analyzed individually. In a photo like today's APOD stars (especially toward the center) appear to overlap, but that is due to the fact the the size of points of light on our display devices are much larger than the actual angular diameter of the stars themselves. Hope that helps.

Bruce

A friend mentioned that there is specialised software used for extracting the data from "crowded fields" eg http://www.star.bris.ac.uk/~mbt/daophot/ but I imagine there are limits for how much overlap in the psf's of adjacent stars can be handled. You also have to somehow reject all the chance aligned field stars that are not actual members of the cluster, as using apparent magnitude on the y axis of the plot, makes the implicit assumption that all the stars are at more or less the same distance, which is true enough for members of the cluster.