Sirius Stargazing: Globular Cluster M15

Uploaded by SiriusStargazing on 21.12.2009

Welcome to Sirius Stargazing. I'm TK and in this episode we're going to look at the Globular
Cluster Messier 15 - M15 for short - in the constellation of Pegasus. Now, unlike open
or galactic clusters, which populate the sweeping arms of spiral galaxies like the one we live
in, globular clusters occupy the galactic halo, which is an enormous, almost spherical
cloud of material around the Milky Way's core; an outer component of the galaxy much fainter
and less readily detectable than the bright nucleus or disc, which you and I are so familiar
with. This is because on average, it's very diffuse, but the globular clusters within
it are by contrast incredibly dense; tightly bound spherical ensembles of perhaps millions
of suns. In the case of M15, the number is not quite this great, perhaps on the order
of a few hundred thousand, but it is remarkable for having an unusually dense core. We know
it has undergone a process called 'core-collapse' and today, many astronomers believe that deep
in the centre lurks a medium-sized black hole. Because of this, I always think of Messier
15 as a scale model of an elliptical galaxy, and when you see it, it really looks like
one, but there are more subtle similarities as well. For example, as with elliptical galaxies,
globular clusters like M15 are composed entirely of very ancient stars. Now, in the disc of
the Milky Way, we see nebulae where stars are forming, and we see open clusters containing
very young stars, but in globular clusters no such star formation is going on any more.
In fact, so old are most of the stars in M15, that when they formed there was actually no
disc of the Milky Way at all. The galactic thin disc as we know it is a mere 9 billion
years old or so, whereas members of M15 exceed 13 billion years, and that means they have
been around for most of the history of the known Universe. Find Pegasus on your star
atlas or planisphere and look for the star Enif, or Epsilon Pegasi, which marks the tip
of its nose. Back-track up its head, and it will bring you to the star Theta Pegasi, and
you need to construct an imaginary line between these two. Continue this line back down in
the direction Pegasus is facing and past Enif for around half the distance between the two
stars you've just found, and you'll come across M15. Knowing whether you've identified it
for the first time can be tricky, so use this apparently nearby star as an indicator. It's
brighter than the surrounding stars, and it will let you know that M15 is not far away.
In poorer conditions, use averted vision to bring out more of the cluster and with practise
you should be able to see both the bright and dense core, and the much fainter halo.
At first, you might think that M15 looks just like a fuzzy star, but as a more experienced
observer, you'll come to see it as being distinctly non-stellar, even at low magnification. Seeing
M15 with a wide field-of-view, such as with binoculars, will really give you a good sense
of scale. The cluster is about 200 light-years wide, but at best will only appear as a small,
nebulous blob, because it's an incredible 34,000 light-years away, floating free from
the galaxy's main spiral. Don't let its appearance disappoint you though; the trick to enjoying
these deep sky objects is to know what it is you're looking at, and I find it amazing
to consider that small blob for what it really is: the accumulated light of so many distant
suns. It really leaves one wondering how the night sky would look to us if we lived there.
Of course, there would be tens of thousands of bright stars above us, making our constellations
either very numerous or at least very complicated, but because so many stars are bound together
so very closely, a great number would be visible even during the day. So, Sirius Stargazers
like us would never get any sleep. From a star system in M15, our home galaxy would
be a truly staggering sight, dominating the sky, its beautiful spiral structure unmistakably
clear. Maybe in the distant future, human beings will invent some kind of exotic new
spacecraft and travel out of the galaxy to look back at our home, but for the foreseeable
future at least, we can only study our neighbouring star cities, and try to infer how our own
must appear to the rest of the Universe. For now, we'll keep looking out from the inside.
Until next time, clear skies.