Messier Monday: The Flattened Fake-out Globular, M19

Through a telescope, this swarm of star appears extremely oblate. But is it really?



Image credit: via http://vilagbiztonsag.hu/keptar/displayimage.php?album=19&pid=19138.


“If I take dust in my hand and ask you if that is all the dust there is, you will answer that dust is everywhere on earth. More specks than can ever be numbered. So I can give you a handful of truth only. Besides this there are other truths. More than can ever be numbered.” -Nadeem Aslam

And yet some of the most enduring truths are written not anyplace in this world, but in the heavens above, hundreds, thousands or even millions of light years away. One of the first comprehensive collections of these cosmic stories out there was put together by Charles Messier more than 200 years ago; his 110 deep-sky objects endure as some of the greatest sights visible through a telescope on Earth.

Image credit: Tenho Tuomi of Tuomi Observatory, via http://www.lex.sk.ca/astro/messier/index2.html.

For every galaxy out there in the Universe, there are hundreds — if not thousands — of globular clusters: collections of hundreds of thousands of stars formed billions of years before our own, densely packed into a sphere just a few dozen light-years across. Today’s object, Messier 19, is one of some 200 globulars in our galaxy alone, and one of the most unusual-looking. Here’s how to find it.

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

There are a great many Messier objects located in-and-around the constellations of Sagittarius, Ophiuchus and Scorpius, the last of which contains the brilliant orange giant star, Antares. Prominent in the southern part of the sky during the Northern Hemisphere’s summer, this part of the sky is very close to the galactic center and, as such, contains a large number of star clusters.

To find Messier 19, find Antares and look slightly towards the east.

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

Most prominent of all the stars in that portion of the sky, θ Ophiuchi is a third-magnitude, blue star, and an imaginary line connecting it to Antares will guide you to your destination. About 4.5° west-southwest of θ Ophiuchi, you’ll come to a pattern of stars easily visible in binoculars or a low-power telescope, that has a single star towards the southeast and then two stars close together northwest and two more slightly farther apart just a little farther on towards the west-northwest.

Just south of that first pair of stars, you can’t miss the distinct fuzzball that is Messier 19.

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

This was one of a great many objects in the catalogue initially discovered by Messier himself, on June 5, 1764, just over 250 years ago. He wrote:

“I have discovered a nebula, situated on the parallel of Antares, between Scorpius and the right foot of Ophiuchus: that nebula is round & doesn’t contain any star.”

Through Messier’s optics, it would have been very difficult to resolve the cluster’s individual stars, or even its non-spherical shape.

Image credit: Guy Campeau, via http://www.astro-caaq.org/4images/details.php?image_id=306&sessionid=b9d46d604ffc57d00eb683de67e38e5b.

But even just a 6" (150 mm) aperture telescope today is up to the challenge! This globular cluster is located relatively close to the galactic center — just 5,000 light-years away — which is quite close when you consider that we’re some 25,000 light-years distant from the Milky Way’s center! Is that intense gravitational pull from our galaxy’s center responsible for the odd shape of this globular?

Image credit: Jim Mazur of Skyledge, via http://www.skyledge.net/Messier19.htm.

Not a chance. Even if this globular cluster was not 5,000 but only 500 light-years from the galactic center, the gravitational force would be woefully insufficient to distort it like this. And this is a highly unusual shape: globulars might vary in how concentrated they are towards they center (on a scale of I-to-XII, with Messier 19 coming in as class VIII), but they’re normally very spherical.

Yet this cluster is very clearly oblate, which means it bulges on its long axis and is compressed along its short axis, similar to planet Earth but on a much more severe scale!

Image credit: Doug Williams, REU Program / NOAO / AURA / NSF, via http://www.noao.edu/image_gallery/html/im0567.html.

What’s causing this, then? A color image like the one above contains a hint. Do you notice how the stars on the “bulging” sides (top-and-bottom) appear slightly bluer than average? And that the ones on the “flattened” sides (left-and-right) appear slightly yellower/redder than average?

There are tiny dust grains in space, particularly in the galactic plane and especially in the direction of the galactic center. When background light passes through a dusty region, that dust absorbs a portion of the light, but because of the particular sizes of cosmic dust grains and the properties of different wavelengths of light, more blue light and less yellow-to-red light is absorbed by the dust. In fact, if we looked in the infrared portion of the spectrum, where dust has negligible absorptive effects, we ought to be able to see this globular’s “true” shape.

Image credit: The Two Micron All Sky Survey at IPAC, via http://www.ipac.caltech.edu/2mass/gallery/images_globs.html.

Lo and behold, this globular cluster isn’t flattened at all, but just appears that way because of the unusual configuration of having two dust-rich regions intervening on either side of this object!

In reality, what we’re looking at is a huge globular cluster, some 70 light-years in radius and containing about 1,100,000 times the mass of our Sun inside of it: one of the largest globular clusters in our galaxy! At 29,000 light-years distant, it’s slightly on the other side of the galactic center from us, and its stars are typically about 11.9 billion years old, or nearly three times the age of the Sun.

Image credit: Jim Misti of Misti Mountain Observatory, via http://www.mistisoftware.com/astronomy/Clusters_m19.htm.

An age like this means that all the class O, B, and A stars that existed in this cluster are long dead, having burned through their fuel billions of years ago. All that ought to still be around are the white, yellow, orange and red stars that are much lower in mass, that burn through their fuel much more slowly. But despite the paucity of blue stars, they still exist, formed by the recent (and fairly frequent) mergers of two smaller stars!

Although Jim Misti’s outstanding photo, above, can’t quite capture it, these mergers (and hence, these blue straggler stars) form most frequently in the globular cluster’s center. That’s something that the Hubble Space Telescope can show us! Have a look:

Image credit: NASA / STScI / Hubble-Wikisky.

There are quite a few, but you’ll notice that they’re outnumbered by those bright orange stars you see. What are they? For the most part, they’re red giants, or more evolved stars that are burning helium in their core and nearing the end of their lives. As large globular clusters like this continue to get older, their red giants will live for many hundreds of millions of years, giving us the opportunity to see many hundreds of them all at once. So don’t miss this spectacle of the summer skies; when you know what you’re looking at, the Universe contains all the more to marvel at!

And that will do it for another edition of Messier Monday! Take a look back at all our previous objects here:

And come back next week, for another deep-sky wonder and another story of the most fascinating, accessible objects in our Universe, only here on Messier Monday!