“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.
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.
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.
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).
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.
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.
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!
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!
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.
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.
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!
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:
- M1, The Crab Nebula: October 22, 2012
- M8, The Lagoon Nebula: November 5, 2012
- M15, An Ancient Globular Cluster: November 12, 2012
- M45, The Pleiades: October 29, 2012
Which one will be next? Join me each Monday to come, where we’ll have a new Messier Object waiting for you to discover!