Messier Monday: An Ancient Globular Cluster, M15

"When your friends begin to flatter you on how young you look, it's a sure sign you're getting old." -Mark Twain

Welcome to yet another installment of Messier Monday, where each week, I'll pick one of the 110 Messier Objects -- deep-sky objects catalogued to avoid confusion for comet hunters -- to highlight for you.

Image(s) credit: SEDS --

So far, we've taken a look at a supernova remnant, a young open star cluster, and an active star-forming nebula, a testament to the great diversity of these faint, fuzzy objects that might be easily confused with a comet. Today, I've got still another type to share with you, and unlike all the others we've seen so far, this one isn't thousands, hundreds-of-thousands or even many millions-of-years old.

Image credit: Manuel Jung, from

Today's Messier object is twelve billion years old, nearly as old as the entire Universe (at 13.7 billion) itself!

Say hello to the ancient globular cluster, Messier 15. Here's how to find it for yourself.

Image credit: Me, using Stellarium, available for free at

One of the easiest asterisms to find in the night sky is the Summer Triangle: the three very bright stars Deneb, Vega and Altair, which runs through the galactic plane. Even though it's called the Summer Triangle -- and yes, it was named in the Northern Hemisphere -- it's still visible even now in the early part of the night. If you move in the direction opposite to Vega (the brightest member of the triangle), you'll head on into the constellation of Pegasus, whose brightest star is the magnitude-2 Enif, also known as the "nose" of the Pegasus.

If you can find Enif in either binoculars or a small telescope, you can find the globular cluster Messier 15 (or M15).

Image credit: Me, using Stellarium, available for free at

Just a few degrees back towards Vega in the sky, Messier 15 is a collection of well over 100,000 stars, all contained within a sphere less than 100 light years in radius! Believe it or not, these objects are ubiquitous in the Universe, and for every big galaxy like the Milky Way, there are at least hundreds, and up to tens of thousands of these dense collections of stars: globular clusters.

Image credit: Portable Astronomy, using a 10" telescope;

These structures are the smallest independent structures in the Universe, as smaller structures will be too far below the Jeans' scale to collapse and form a gravitationally bound structure on their own.

Even though they aren't necessarily tied to galaxies, because of the way structure forms in the Universe -- more massive objects attract the mass around them -- our galaxy has picked up an estimated 10,000 of them or so over the course of our lifetime. If we look within 50,000 light years of our galactic center, though, we only find just over 100.

Image credit: William E. Harris, McMaster U., and Larry McNish.

Where did the other 98% of them go? Incredibly, they get destroyed by periods of rapid star formation, which can provide enough photon pressure to blow them apart!

What we're witnessing, when we see a globular cluster today, are the survivors. These are the most stable, the most massive ones, and the ones that have managed to avoid a starburst of sufficient strength to destroy them over their history. In the case of Messier 15, we're talking about 12 billion years, or practically the entire age of our galaxy!

We can tell this because of how stars evolve: stars above a certain mass run out of fuel after a certain amount of time, so if we construct a color (temperature) vs. magnitude (brightness) diagram for this cluster, we can figure out its age!

Image credit: Jim Napolitano of RPI from his fall 1996 astronomy course.

That's how we know that Messier 15 -- even though it's some 33,000 light-years away -- is that ancient!

But the best view of Messier 15, as is the case with most space-based objects, comes courtesy of the Hubble Space Telescope.

Image credit: ESA / Hubble & NASA.

You may notice there's a slight blue glow towards the lower left of the cluster, and you may protest that, "Hey! There weren't supposed to be any blue stars in there!"

Well, you're right, and that's not a blue star. The Hubble image is of very high resolution, so let's take a closer look at that blue glow.

Image credit: ESA / Hubble & NASA, cropped by me.

This is a planetary nebula, or the same type of stellar death that awaits our Sun. After running out of fuel -- which our Sun will do, by the way, after around 12 billion years -- Sun-like stars will blow off their outer layers and return them to the interstellar medium, resulting in a visually spectacular planetary nebula that will span a couple of light-years in size, all while the central core of the star contracts down to form a white dwarf, that will glow and cool slowly for over a quadrillion years.

That's what we're looking at when we take a look at Messier 15, one of only four known globular clusters to contain a planetary nebula and one of the oldest globular clusters in the galaxy. And believe me, if you viewed it through a small telescope or binoculars, you, too might think it's a comet! Want proof? Here's what it looks like when Comet Garradd passed by Messier 15, and this image was taken with a 17" telescope and an expensive CCD camera!

Image credit: Comet C/2009 P1 (Garradd) and M15 (2011) by The Virtual Telescope Project.

Telling them apart in even a slightly inferior scope is hard! Don't you dare underestimate the usefulness of Messier's Catalogue, even to astronomers today, more than 200 years into the future. So, including today's entry, we've covered four of the 110 so far:

Which one will be next? Join me each Monday to come, where we'll have a new Messier Object waiting for you to discover!

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I'd never underestimate the Messier catalogue, but that's because it's my primary Stuff to Look At list, not my Don't Waste My Time With Not-Comets list. If there actually was a comet within half a degree of one of the fuzzier Messiers, I'd be in trouble!

Well, you can get away with underestimating M40...

Hey Ethan! I've been on the hunt for a decent telescope, any suggestions on where to start?

Very nice explanation. I did not know.

When you say, "Where did the other 98% of them go? Incredibly, they get destroyed by periods of rapid star formation, which can provide enough photon pressure to blow them apart!"

I assume that the photon pressure of rapid star formation (?? in the globular cluster itself or in a nearby galaxy) causes the stars to move apart and hence there is no longer an organization of stars called a globular cluster. And then the independent stars eventually are attracted to a passing globular cluster or galaxy. Is this roughly correct so far??

And then what about intermediate black holes in the center of globular clusters.
I assume if such an intermediate black hole is a part of a globular cluster that some new globular cluster would begin to form around the seed of an intermediate black hole.

Hmm, does an intermediate black hole give a globular cluster enough mass to begin forming an independent structure in the first place. I read somewhere that there was a chicken and an egg problem of which came first the supermassive black hole or the galaxy; similarly is there any chicken and an egg thinking about globular clusters and intermediate mass black holes.

Just wondering. Thanks for the education

Let's see, 100-ly radius spherical cluster implies a cluster volume of about 4 million cubic ly. Then 100,000 stars gives a mean volume of about 40 cubic ly per star, or on the order of a 3-ly spacing. How does this compare to the interstellar spacing in our local neighborhood? How does it compare to spacing in denser parts of the Milky Way?

Well, the speed you can give a star by photon pressure is a bit higher than you can give a star and you can move .00001 of a solar mass by gas, but you can't do that to a star because they tend to be a much larger unit. So a sieving occurs

They do separate somewhat and, later in life, a globular cluster can undergo collapse where the stars nearer the centre fall more toward the centre and you get a bit of a two-tone globular.

You can see the effect in open clusters too.

The trapezium in the Orion Nebula is blowing away the gas but the stars aren't being equally blown apart, despite the gas cloud being several hundred solar masses in total for that area (and that visibly glowing bit is a very small part of the gas cloud that the nebula is part of.

A much simpler explanation:

The photon pressure goes up as the square of the radius of an object in that field, whereas the mass goes up as the cube of the radius. Acceleration goes as linear mass, force goes as area. Acceleration therefore is inversely proportional to the radius of the entity being pushed.

Star radius >>>>>>>> atom/molecule radius

(the normal ">>" for "very much greater than" didn't seem to be greater than enough..!)

I only sort of understand both of your explanation. they are both helpful. But I think I need an even easier explanation. Thanks; but I'm still not getting something. I'll think upon your explanation.

Every time I see a picture of one of those clusters I think about what the night sky would look like from within. With all that time to cook up organic soup, I wonder how many civilizations it has spawned and consumed during it's 12 billion year journey?

Nathan: Great stuff as usual, your blog is pretty much the only reason I still visit SB, you don't spout your opinion on the trendy topic of the week. As such it makes you stand out like a gamma ray burst against the backdrop of academic gossip mongers that litter the front page of SB.

When the gas is a gas there is very little overlap so every atom can get its own little push by the photons from the cluster.

Gas on the edge of the cluster will see 100% of the photons coming from one direction, with that fraction changing as you get closer and closer to the middle.

But at the edge, there will be every hydrogen atom will get its own little kick.

As a condensed stellar object, there is no way for the vast mass on the outer side of the star to get ANY photons from the rest of the globular cluster if it is positioned similarly.

And for those atoms more than 1 optical depth below the surface of the star (only a few hundred km of the least dense part of the star's 10^8m girth) won't get any of those photons from the rest of the cluster.

So gas gets an individually delivered push.

The star has to share what the few atoms being illuminated from outside gets throughout the entire mass of the star.

"I wonder how many civilizations it has spawned and consumed during it’s 12 billion year journey?"

Almost certainly none.

Very "metal" poor (in astronomy, anything heavier than Hydrogen is a "metal").

So no rocky body because there's nothing heavier than hydrogen and a few % helium.


To give you an idea, the actual radius (I rounded) of Messier 15 is 88 light years. 100,000 stars is a decent number estimate, so let's go with that. That means, on average, if you picked a star and drew a circle of radius 10-light-years around it, you'd pick up 147 other star systems. This does not separate out binary and trinary star system, because our resolution is not good enough for that.

Within our Sun's neighborhood, there are seven star systems within a radius of 10-light-years.

Estimating the stellar density over the entire galaxy gives you similar numbers; I get that stars are about 35 times more dense in that globular cluster than they are in the galaxy as a whole. But you must remember that the central region of a globular cluster is far more concentrated than even that, and the stellar density there is many hundreds -- if not thousands -- of times as great as a standard region in our galaxy.

Perhaps I'll touch on this in the next Messier Monday that involves a globular cluster. There is no shortage. :-)

Thanks for that explanation to Bernard. I did not know. Nice.

Thanks. As usual, an enlightening answer. Night time on a planet within the cluster would yield a mind-blowing sky to an Earthling, especially near the center.

I've also spent much time and energy attempting to figure out the "chicken" or "egg" paradox and thus declare it Mission Impossible...this message will now self-destruct ;)

Also, Ethan, Hunt no more...a Meade is perfect for beginners. I have a Meade autostar and do not use it nearly enough as I've been all caught up in the greatest mystery ever! - The chicken or the egg -

Why did the chicken cross the road? To get to the other side, of course. I'm sure Hawking would tell a different story should that road be at the even horizon.

By Jules Vern (not verified) on 16 Nov 2012 #permalink