Messier Monday: The Galaxy’s First Globular with a Black Hole, M62

Nearly 250 years since its discovery, this dense, brilliant core was found to have our Milky Way’s first globular-based black hole!




Image credit: Paulino Maldonado, via http://observatorioelojodeowi.blogspot.com/2011/05/subo-algunas-fotografias-de-espacio.html.


“And then you wake up, only to see that the darkness has gone, the light now truly makes you feel vulnerable and you wonder why this darkness did not wish you well and why did it leave you so sudden, without revealing the answers you were looking for.” -Chirag Tulsiani

With 110 deep-sky objects in the Messier catalogue, and each one holding it’s own story of how it came to be in this Universe, it’s no surprise that some of them will be quite unusual. But the summer months provide a rare chance for northern hemisphere skywatchers to catch some objects well below the celestial equator, including today’s object, Messier 62.

Image credit: © Ole Nielsen 1999–2007, viahttp://www.ngc7000.org/ccd/messier.html.

There are a great many globular clusters in the Messier catalogue: 29 of them, outnumbered only by the galaxies. In addition, there are over 100 others that are located in our galaxy’s halo, and they’ve all been around for far longer than our star and planet. In fact, the stars found in globular clusters are among the oldest in the Universe, and are often home to some of the most remarkable, evolved objects known to exist in space.

Messier 62, although it’s just one particular instance of a globular cluster, has a story all its own to tell. Here’s how to find it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Shortly after sunset, just after the sky darkens, have a look due south. If you’re at a relatively high northern latitude (like I am), you’ll have to look fairly close to the horizon to find it, but the bright orange giant Antares just slightly to the west should help you out, along with the collection of stars that looks like a teapot towards the east.

Just southeast of Antares is another prominent (but not quite as bright) star, ε Scorpii, which will help guide you in the right direction.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

If you imagine that the line segment connecting Antares to ε Scorpii is the hypotenuse of a 30–60–90 right triangle, and that the right-angle-vertex is just north and slightly towards the east of ε Scorpii, that’ll put you almost exactly where Messier 62 is located!

Alternately, the naked eye star HIP 83336 is northeast of ε Scorpii, and navigating just about 2.5° north (and slightly west) of that star will plant you right at this globular, where the surrounding star field is as shown below.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

This was discovered by Messier himself back in 1771, but he didn’t accurately catalogue its position until 8 years later, writing:

Very beautiful nebula, discovered in Scorpio, it resembles a little Comet, the center is brilliant & surrounded by a faint glow.

That description — of a brilliant center with a faint glow around it — is what most globular clusters look like through a small telescope.

Image credit: Fred Espenak of AstroPixels, via http://astropixels.com/globularclusters/M62-01.html.

When star clusters form, they typically are somewhat denser towards the center, but not necessarily significantly so. For the most massive ones — and this one weighs in at about 1.2 million solar masses — the more massive stars tend to migrate towards the center. There, they merge and form blue stragglers, they run out of fuel (because more massive stars run out of fuel more quickly) and become white dwarfs, neutron stars or even black holes!

When we look at this globular, we see that it is, in fact, much denser towards the core than on the outskirts, something visible even against the backdrop of our galactic plane.

Image credit: Rick Beno of Conferring With The Sky Observatory, via http://www.conferringwiththesky.org/displayimage.php?pid=101.

There are a huge number of variable stars in this cluster — 89 at last count, compared with a mere four for its globular neighbor, M19 — as well as a large number of X-ray sources, which are thought to be a mix of binary stars and millisecond pulsars. These clearly come in two different varieties, as the X-ray image of this cluster, courtesy of Chandra, shows us.

Image credit: NASA / Chandra X-ray Observatory.

But it’s the incredibly dense central core that should interest you the most.

Image credit: ©2007–2012 Twin City Amateur Astronomers, via http://tcaa.us/Astronomy/Messier/Messier.aspx?id=M62.

Because this is where the most massive, densest, and in many ways most interesting objects can be found. In a cluster that’s maybe 10 times as massive as the smaller globulars, there are bound to be some remnants of long-dead, massive stars to look at.

One of the classes of object that should be left behind from the earliest generations of stars to form in here is a stellar-mass black hole, but for centuries, not a single one was found in a globular cluster anywhere.

Image credit: Peter Vasey of http://www.madpc.co.uk/~peterv/astroplover/LaPalma08.htm.

Traditionally, black holes are detected by using radio waves, but globular clusters are simply too dense and too noisy to have that be a reliable method. Instead, it’s X-rays from gas being accelerated and heated up by the black hole that’s the first signature, which can then be followed-up with high-resolution radio observations to provide confirmation.

The first globular cluster to house a black hole was only discovered in 2007, but that was outside our galaxy. Using that same technique, however, proved incredibly fruitful for Messier 62!

Image credit: Laura Chomiuk et al. / Chandra X-ray Telescope, NASA.

The circle, above, marks the spot where X-rays were observed (by Chandra), indicating the presence of a black hole. This was confirmed with radio observations courtesy of the Very Large Array!

Image credit: Very Large Array / National Radio Astronomy Observatory.

In fact, it’s real and it’s confirmed! We now have the very first stellar-mass black hole discovered in a globular cluster in our Milky Way, and — along with the more massive ones that have been found in others — this is a strong hint that there are likely many more to come!

The best image of this cluster’s core comes, as it so often does, courtesy of the Hubble Space Telescope, which had the occasion to image the central portion of this remarkable object.

Image credit: NASA / STScI / WikiSky snapshot tool.

And with that, we’ll come to the remarkable end of another Messier Monday. Out of the 110 objects, we’ve now taken a detailed look at 95 of them, leaving just 15 to go. Take a look back at all our previous objects:

And come back tomorrow for more wonders of the Universe, and don’t forget to look up and catch the Perseids all this week if you have the skies for it!