Messier Monday: A Big, Blue, Bright Baby Cluster, M47

"Do not look at stars as bright spots only. Try to take in the vastness of the universe." -Maria Mitchell

It's time again for Messier Monday, where we highlight the various wonderous deep-sky objects of the night, and show you how to find them against the expansive backdrop of stars. The (almost) full Moon is out tonight, polluting your night sky with as much light as a large-sized urban area, but that doesn't mean all of the 110 deep-sky objects that make up the Messier catalogue are off-limits!

Image credit: Mike Keith’s delightful (a)periodic table of Messier objects!

While extended objects -- like planetary nebulae and galaxies -- may prove more of a challenge, collections of stars within our own galaxy and galactic neighborhood (like open star clusters and globular clusters) are perfect targets on a night like tonight, even with the severe light pollution brought on by the full Moon! While the Messier catalogue boasts 110 total objects, five of them were declared missing for a long time because of various minor errors.

In the case of today's object, Messier 47, there was a sign error in one of the coordinates he recorded that went unresolved until T.F. Morris found the correct object (and position) in 1959! Let there be no more confusion; here's how you can find this deep-sky wonder for yourself.

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

Even with the full Moon out tonight, the great winter constellation of Orion is easy to spot, rising in the east at dusk and passive over the southern part of the horizon as the night goes on. Behind it, the brightest star in the night sky -- Sirius -- rises a little later, followed by other stars familiar to winter skywatchers. Well, if you can find Sirius and the somewhat dimmer Mirzam near it, they'll point your way to Messier 47.

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

Even in a light-polluted sky, the imaginary line from Mirzam to Sirius will have you running into another naked-eye star in short order: the fourth-magnitude Muliphein. If you continue on in a straight line the same distance again from Mirzam to Muliphein, you'll come to a place with a large number of stars at the limit of naked-eye vision; the imaginary line connecting the slightly more southerly bright pair Adhara and Wezen would meet there as well.

If you train your equipment on this region (at low magnification, for sure), this is what you're likely to see.

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

The naked eye stars HIP 36773 (in line with the Mirzam-Sirius-Muliphein triplet) and HIP 37379 (a little below it) bracket today's object -- Messier 47 -- very nicely. But this object stands out on its own, and was likely discovered independently back in 1654 before Messier rediscovered it in 1771, saying:

Cluster of stars, little distant from [Messier 46]; the stars are [brighter]; the middle of the cluster was compared with the same star, 2 Navis. The cluster contains no nebulosity.

While I could be convinced to tell you about the "sister" object of this one (Messier 46) next week, let's take a look inside this week's object first!

Image credit: Boris Štromar via the hungarian wikipedia; M46 on the left, M47 on the right.

While the brightest star in this cluster is just barely visible to the naked eye at magnitude +5.7, you're much more likely to notice that there appear to be a ton of bright, blue stars through binoculars or a telescope. That's because of a combination of two factors: the youth and the closeness of this cluster. Based on the stars that are still around, this cluster is dated at about 78 million years of age, and is one of the closest Messier objects at just 1,600 light-years distant!

Image credit: Jim Mazur's Astrophotography, via http://www.skyledge.net/.

Perhaps surprisingly, this is one of the most diffuse -- or low-density -- open clusters that we know of! Although it's only about 12 light-years in diameter, pretty typical for an open star cluster (and about the same angular size as the full Moon on the sky), there are only about 50 stars in there, somewhat surprising considering that these objects typically contain many hundreds or even thousands of stars!

Image credit: Donald P. Waid of http://www.waid-observatory.com/.

But not M47, which has maybe seven prominent blue stars, a couple of red giants, a larger number of somewhat less bright, less blue stars, and then a much larger number of dimmer stars that begins to blend in to the galactic background.

What could be happening here?

Quite simply, the dimmer stars are probably actually there, but incredibly difficult to tease out against the remainder of the galactic plane. Take a look at this deeper exposure to see what I mean.

Image credit: Maheder Hailesillasie of http://asn-astaro.blogspot.com/.

There's no evidence that this cluster is in the process of dissociating, nor that there's an absence of lower-mass stars. What happens -- and this is true for a great many open clusters in the galactic plane, which is where they tend to form -- is that it becomes very difficult to identify how stars are a part of the cluster, proper.

It's too bad, because these bright blue stars are quite the sight!

Image credit: (C) Copyright 2006 Paul Mayo, via http://campingunderstars.com/.

Despite Messier's assertion that there isn't any nebulosity, some images appear to have a little bit, which would be quite consistent with a star cluster of such a young age. You might legitimately be wondering if we could somehow see through that dust -- if it exists -- and whether there'd be a greater number of lower-mass, redder stars than we'd seen otherwise.

Well, that's what looking in the infrared can get you, and guess what we found?!

Image credit: 2MASS (Two Micron All-Sky Survey) / IPAC.

Although it's not yet proven that these stars are associated with the cluster and not the galactic background, there are, in fact, hundreds (and teetering on a thousand) stars in the vicinity of this cluster. Not bad for such a nearby object! That wraps up another Messier Monday, and including today, we’ve looked at the following objects:

• M1, The Crab Nebula: October 22, 2012
• M2, Messier’s First Globular Cluster: June 17, 2013
• M5, A Hyper-Smooth Globular Cluster: May 20, 2013
• M7, The Most Southerly Messier Object: July 8, 2013
• M8, The Lagoon Nebula: November 5, 2012
• M11, The Wild Duck Cluster: September 9, 2013
• M12, The Top-Heavy Gumball Globular: August 26, 2013
• M13, The Great Globular Cluster in Hercules: December 31, 2012
• M15, An Ancient Globular Cluster: November 12, 2012
• M18, A Well-Hidden, Young Star Cluster: August 5, 2013
• M20, The Youngest Star-Forming Region, The Trifid Nebula: May 6, 2013
• M21, A Baby Open Cluster in the Galactic Plane: June 24, 2013
• M25, A Dusty Open Cluster for Everyone: April 8, 2013
• M29, A Young Open Cluster in the Summer Triangle: June 3, 2013
• M30, A Straggling Globular Cluster: November 26, 2012
• M31, Andromeda, the Object that Opened Up the Universe: September 2, 2013
• M32, The Smallest Messier Galaxy: November 4, 2013
• M33, The Triangulum Galaxy: February 25, 2013
• M34, A Bright, Close Delight of the Winter Skies: October 14, 2013
• M36, A High-Flying Cluster in the Winter Skies: November 18, 2013
• M37, A Rich Open Star Cluster: December 3, 2012
• M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 2013
• M39, The Closest Messier Original: November 11, 2013
• M40, Messier’s Greatest Mistake: April 1, 2013
• M41, The Dog Star’s Secret Neighbor: January 7, 2013
• M44, The Beehive Cluster / Praesepe: December 24, 2012
• M45, The Pleiades: October 29, 2012
• M47, A Big, Blue, Bright Baby Cluster: December 16, 2013
• M48, A Lost-and-Found Star Cluster: February 11, 2013
• M50, Brilliant Stars for a Winter’s Night: December 2, 2013
• M51, The Whirlpool Galaxy: April 15th, 2013
• M52, A Star Cluster on the Bubble: March 4, 2013
• M53, The Most Northern Galactic Globular: February 18, 2013
• M56, The Methuselah of Messier Objects: August 12, 2013
• M57, The Ring Nebula: July 1, 2013
• M60, The Gateway Galaxy to Virgo: February 4, 2013
• M65, The First Messier Supernova of 2013: March 25, 2013
• M67, Messier’s Oldest Open Cluster: January 14, 2013
• M71, A Very Unusual Globular Cluster: July 15, 2013
• M72, A Diffuse, Distant Globular at the End-of-the-Marathon: March 18, 2013
• M73, A Four-Star Controversy Resolved: October 21, 2013
• M74, The Phantom Galaxy at the Beginning-of-the-Marathon: March 11, 2013
• M75, The Most Concentrated Messier Globular: September 23, 2013
• M77, A Secretly Active Spiral Galaxy: October 7, 2013
• M78, A Reflection Nebula: December 10, 2012
• M79, A Cluster Beyond Our Galaxy: November 25, 2013
• M81, Bode’s Galaxy: November 19, 2012
• M82, The Cigar Galaxy: May 13, 2013
• M83, The Southern Pinwheel Galaxy, January 21, 2013
• M86, The Most Blueshifted Messier Object, June 10, 2013
• M92, The Second Greatest Globular in Hercules, April 22, 2013
• M94, A double-ringed mystery galaxy, August 19, 2013
• M97, The Owl Nebula, January 28, 2013
• M99, The Great Pinwheel of Virgo, July 29, 2013
• M101, The Pinwheel Galaxy, October 28, 2013
• M102, A Great Galactic Controversy: December 17, 2012
• M103, The Last ‘Original’ Object: September 16, 2013
• M104, The Sombrero Galaxy: May 27, 2013
• M106, A Spiral with an Active Black Hole: December 9, 2013
• M108, A Galactic Sliver in the Big Dipper: July 22, 2013
• M109, The Farthest Messier Spiral: September 30, 2013

Come back next week, where just maybe we'll look at the sister object to this one, and uncover a little more in our investigations of the deep-sky objects visible from Earth. Only here, only on Messier Monday!