Messier Monday: The Smallest Messier Galaxy, M32

"Dream no small dreams, for they have no power to move the hearts of men." -Johann Wolfgang von Goethe

Welcome back to yet another Messier Monday here on Starts With A Bang! Each Monday, we highlight one of the 110 deep-sky objects that make up the Messier catalogue, showing you where and how to find it in the sky and telling you a bit about the history and the physics/astrophysics behind it. Each one tells its own unique story, and today's distant wonder is no exception.

Image credit: Rich Richins, of all 110 Messier objects (in no particular order).

On a virtually moonless night like tonight, skies are at their darkest, which makes for the best conditions for observing the faintest of all the deep-sky objects: the galaxies. A full 40 of the 110 Messier objects are galaxies: more than any other type. And while we typically think of these as great, large, extended objects, there's an entire class of galaxies that's far more numerous than the spiral and elliptical giants of your imaginations: the ultra-abundant class of dwarf galaxies.

Today, let's take a look at the smallest of all the galaxies in Messier's catalogue, Messier 32. Here's how to find it.

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

After sunset tonight, the prominent "W" of the constellation Cassiopeia towers over the northern hemisphere, with a row of four bright stars -- Mirphak, Almaak, Mirach and Alpheratz (from L-R) -- below it. Take note of where Mirach is in the skies and navigate back towards Cassiopeia, and Messier 32 will unveil itself, if you know where to look.

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

Navigating away from Mirach back towards Cassiopeia, the naked eye stars μ Andromedae and, a little farther on, the slightly dimmer ν Andromedae will point the way. In fact, if you can find ν Andromedae, you're practically there; under dark skies, the Great Nebula in Andromeda -- Messier 31 -- will be unmistakeable.

But if it's Messier 32 you're after, you'll need to continue the hunt just a little further. First, take note of another naked-eye star near μ and ν Andromedae that makes an isoceles triangle with them: 32 Andromedae.

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

Because if you can locate both the Great Andromeda Galaxy and 32 Andromedae, all you have to do is look at the outer "edge" of the Andromeda Galaxy along the imaginary line that connects its center to that star, and you'll see a small, diffuse cloud.

It'd be easy to mistake it for a giant globular cluster, but it isn't; that's a galaxy all its own! That's what Messier 32 is.

Image credit: Julian Cooper, flickr user ju_cooper.

What you're looking at is a tiny elliptical galaxy, an object that's only 6,500 light years in diameter at its widest point. There was controversy for a while about whether this galaxy was actually a satellite of Andromeda or a much more distant background galaxy, but since we've measured individual stars within it -- including the characteristic evolving red giant stars that are so good for indicating distance -- we've found that it's only 2.5 million light-years away, nearly the same exact distance as Andromeda. (And for the record, we still aren't sure which one's closer!)

Image credit: Alistair Symon of http://www.woodlandsobservatory.com/.

Messier 32, on its own, is an incredibly interesting object for its closeness, compactness, and a few other surprising properties. First off, there are only about three billion solar masses in there. Yes, that's a huge number, but consider that's only about 0.75-1.5% of what's in the Milky Way. And yet -- in such a small volume -- the star density in there is tremendous!

Image credit: © 2006 - 2012 by Siegfried Kohlert, via http://www.astroimages.de/.

The densities of stars at the very core of Messier 32 are staggering: whereas you need to go a few light years in any direction to run into a star in our neighborhood, at the very central core of M32, you only need to travel about a thousandth of that distance. Because density scales as distance-to-the-negative-three power, the stellar density at the center of M32 is some 100,000,000 times what it is in our neck-of-the-woods!

Image credit: Bayfordbury Observatory, from their flickr account.

And what's even more surprising is that, despite the tininess of this galaxy, there is a supermassive black hole at its core, and it's practically the same mass as the one at the center of our galaxy, at between 1.5 and 5 million solar masses! To Messier's eyes, it was barely noteworthy at all:

Small nebula without stars, below & at some minutes [separation] from that of the belt of Andromeda [M31]; this small nebula is round, its light fainter than that of the belt. M. le Gentil has discovered it on October 29, 1749. M. Messier saw it, for the first time, in 1757, & he has not found any change.

But to our advanced instruments, we know it's full of older stars, free of dust-and-gas, and going to merge with its much bigger sister (Andromeda) sooner rather than later! And thanks to the power of Hubble, we can easily resolve individual stars within it!

Image credit: NASA Hubble Space Telescope's - Space Telescope Imageing Spectrograph (STIS).

This image is just 175 light-years on a side, just 3% of the scale of Messier 32! And as you can see, there's an excess of ultraviolet radiation coming from the very core. Why could that be? We think that tidal effects from Andromeda have caused a burst of activity at the center, either from a very recent, last gasp of star-formation before it gets torn apart by Andromeda or, potentially, because the central region is overwhelmed with stellar mergers and the formation of blue stragglers!

Either way, when you zoom all the way in, Hubble can reveal some amazing things!

Image credit: NASA / ESA / Hubble Legacy Archive, via http://hla.stsci.edu/, data from Mateo et al.

There are star clusters (lower left), an abundance of giant stars (yellow/red), and a huge number of individual faint, dim older stars! It's only a matter of a few hundred million years before this is totally torn apart and consumed by its larger neighbor within our local group, but in all probability, not even one star from within it will collide with another in Andromeda!

And with that epiphany, we’ll wrap up another Messier Monday! Including today’s entry, 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
• M37, A Rich Open Star Cluster: December 3, 2012
• M38, A Real-Life Pi-in-the-Sky Cluster: April 29, 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
• M48, A Lost-and-Found Star Cluster: February 11, 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
• 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
• M108, A Galactic Sliver in the Big Dipper: July 22, 2013
• M109, The Farthest Messier Spiral: September 30, 2013

Come back again next Monday, where another deep-sky wonder, and another unique story from the Universe, awaits you! Only here, only on Messier Monday!