Messier Monday: The Teapot-Dome Cluster, M28

Ethan Siegel
Sep 8, 2014 · 10 min read

At the top of the Teapot, a fantastic cluster dotted with Red Giants awaits.

Image credit: © 2005–2009 by Rainer Sparenberg, via http://www.airglow.de/html/starclusters/m28.html.

“The most difficult thing is the decision to act, the rest is merely tenacity. The fears are paper tigers. You can do anything you decide to do. You can act to change and control your life; and the procedure, the process is its own reward.” -Amelia Earhart

As the northern hemisphere’s summer draws to a close, one of the last lingering remnants of this time of year are the stars, constellations and deep-sky wonders visible from Earth. Even as the days shorten and the Earth continues its orbit around the Sun, the nights arrive earlier, providing skywatchers with an opportunity to spy many summer sky delights that you might not have been able to wait up for in June and July.

Image credit: The Messier Objects by Alistair Symon, from 2005–2009.

While the more extended objects — star-forming nebulae, stellar remnants and distant galaxies — are difficult to make out during a night with a full Moon like tonight’s Supermoon, the open star cluster and the ancient globular clusters contained within our galaxy still make for spectacular sights through a telescope. If you head outside and look towards the southern horizon just after the sky darkens, some spectacular sights await, including Messier 28, the lucky subject of our attention tonight.

For today’s Messier Monday, look towards the teapot in Sagittarius, home to the highest density of star clusters in our Milky Way, and containing the galactic center.

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

A collection of eight stars stands out in this region of the sky, not because they’re the brightest stars around, but because they have a distinctive pattern to them that we recognize as very similar to a common object here on Earth. We call such a collection of stars an asterism, and the teapot is one of the most recognizable ones. And the star at the very top of the teapot’s lid — Kaus Borealis — has a secret less than a single degree away from it.

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

Just slightly to the west of this star lies the globular cluster Messier 28, an authentic discovery of Messier himself back in 1764, or 250 years ago. It’s quite small, and although its core is concentrated, there’s a rich star field that it’s up against. (No surprise, considering how close we are to the galactic center here.) Nevertheless, there’s a star just a little farther along to the west that’s just barely visible to the naked eye: HIP 89980. If you can find that — in addition to Kaus Borealis — Messier 28 will simply pop out in your field-of-view.

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

As Messier himself described it:

Nebula discovered in the upper part of the bow of Sagittarius at about one degree from the star Lambda & little distant from the beautiful nebula which is between the head and the bow. It contains no star; it is round, it can only be seen difficultly with an ordinary telescope…

But, of course, an “ordinary telescope” in the hands of an amateur is far superior to what Messier had at his disposal all those centuries ago.

Image credit: Ron Abbott of http://www.astrolandofoz.com/GlobularClusters.html.

What appeared to Messier as a round, starless nebula is clearly a dense collection of stars that gets even more concentrated towards the center. What might not be obvious from just a glimpse is that, while the stars littered across the field in the image above range from hundreds to a few thousand light-years away, the stars of this cluster itself are 18,000 light-years distant!

Image credit: John Mirtle of http://www.astrofoto.ca/john/m028.htm.

The dust lanes of the galaxy are clearly visible in a wide-field, long-exposure image, but what makes Messier 28 so remarkable is that contained within a sphere just 30 light-years in radius — and remember, there are only about 400 stars within about 30 light-years of the Sun — is that there are at least fifty thousand stars in there, with a combined mass of around 550,000 Suns!

In addition, globular clusters like this one are old, with the stars in there containing just 5 percent the heavy metals of our Sun and having an age of around 12 billion years, or more than double the age of our Solar System.

Image credit: Kuuke’s Sterrenbeelden, via http://www.kuuke.nl/wp/alle-sterrenbeelden/sagittarius-boogschutter/.

While our Sun orbits the Milky Way’s center in a giant ellipse, the other globular-and-star clusters move about along their own paths, with the globular clusters’ motion often having very little to do with the motion of the galaxy in general. That’s why it’s such an odd coincidence that, with respect to us, Messier 28 is nearly perfectly stationary, with a redshift away from us that corresponds to a motion of just one kilometer-per-second.

What you might find interesting is that, when we look at the individual stars inside — and the brightest are magnitude +15, so break out your large-aperture telescopes — they’re exclusively red giant stars!

Image credit: NOAO/AURA/NSF, via http://www.noao.edu/image_gallery/html/im0771.html.

But where there are no blue stars (save the stragglers that form recently from low-mass stars merging), there are bound to be remnants of those initial high-mass stars that burned through their fuel ages ago. In particular, that should mean that neutron stars and black holes remain, and Messier 28 is not only home to the very first millisecond pulsar ever discovered in a globular cluster (back in 1986), but of all the globular clusters ever scanned for pulsars, Messier 28 contains the third most, and most of all among Messier objects.

In fact, here’s that very first one — PSR 1821–24 — as it emits X-rays in “pulses”, as imaged by Chandra!

https://www.youtube.com/watch?v=RE2UlFTG8Po

This globular is a little more concentrated than most towards the center, rating a concentration class of IV (on a scale of I to XII), and as evidenced by the fact that Messier marked this cluster as having a radius just 20% of what it actually is!

There’s a great image from Hubble’s old WFPC2 camera that captures this, along with the brilliant red giants found inside.

Image credit: NASA / ESA / Hubble / WikiSky, via Wikimedia Commons user Friendlystar.

But there was an even better Hubble image taken later that I think really highlights just how spectacular this cluster is at its core. Have a look, courtesy of the Hubble Legacy Archive, at a slice through this cluster in unparalleled full-resolution!

Image credit: Hubble / ESA / NASA, via the Hubble Legacy Archive at http://hla.stsci.edu/hlaview.html, from HST and Wikimedia Commons’ user Fabian RRRR. Color correction corrected again by me.

At this age, only Sun-like stars (and redder) are left, save the blue stragglers and the giant, evolving stars. There’s no better view of this cluster than from Hubble, and so with that, we’ll come to the end of another Messier Monday! Take a look back at all the previous objects we’ve covered, with today’s bringing us up to ninety-nine:

Come back next week, when we’ll pass the triple-digits mark, and then the week after, when we’ll begin counting down the final ten objects, all visible in your night sky!


Starts With A Bang!

The Universe is out there, waiting for you to discover it.

    Ethan Siegel

    Written by

    The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

    Starts With A Bang!

    The Universe is out there, waiting for you to discover it.