"We find them smaller and fainter, in constantly increasing numbers, and we know that we are reaching into space, farther and farther, until, with the faintest nebulae that can be detected with the greatest telescopes, we arrive at the frontier of the known universe." -Edwin Powell Hubble

With 110 deep-sky objects scattered throughout the heavens, the Messier Catalogue provides skywatchers across the globe with a number of spectacular targets, from nearby nebulae and clusters to spectacular, distant galaxies. Each Monday, we spotlight a new one right here.

Image credit: Rolando Ligustri, taken over many years, retrieved from http://www.itelescope.net/

But there's only one Messier object that can lay claim to literally having changed our picture of the entire Universe. If you only join me for one Messier Monday ever, make sure it's this one. Here's how to get started finding Andromeda, also known as Messier 31.

Just after sunset (at this time of year), look towards the eastern portion of the sky. High above the horizon and slightly towards the north, you'll find the classic "W" in the constellation Cassiopeia, while closer towards the horizon (below the bottom of the "W") you'll find a line of four stars that stand out: Mirphak (the brightest), Almaak, Mirach, and Alpheratz (from L-R, above).

Focus on Mirach and Alpheratz, and that will take you towards M31, the Great Nebula in Andromeda.

From Mirach, head "up" from the horizon (or back towards Cassiopeia, except slightly less north), and you'll see two naked-eye stars in a row: μ Andromedae (closer to Mirach) followed by ν Andromedae, which is slightly dimmer. If you've got a telescope, binoculars, or just dark skies and good vision, continue upwards just a single degree from ν Andromedae, and you won't be able to miss the spectacular extended object that awaits you.

This is the one astronomical object, moreso than any other, that changed our perception of what a galaxy is, and gave rise -- less than a century ago -- to our modern understanding of the entire Universe. Even though it was known since at least the 10th Century, details were pretty sparse for a very long time. Messier himself, writing in 1764, observed the following:

The beautiful nebula of the belt of Andromeda, shaped like a spindle; M. Messier has investigated it with different instruments, & he didn't recognise a star: it resembles two cones or pyramides of light, opposed at their bases, the axes of which are in direction NW-SE; the two points of light or the apices are about 40 arc minutes apart; the common base of the pyramids is about 15'.

Remember, without an insanely large telescope or photographic technology, what one can see is very limited; to Messier, it probably looked somewhat like this.

Image credit: Richard Helmich, who officially wins his earlier challenge to me.

Although telescopes became larger and larger over the 18th and 19th centuries, it wasn't until the development of astrophotography -- and the pioneering techniques of an amateur, Isaac Roberts -- that we realize that this object had a spiral structure, something that only came out due to the long-exposure techniques he developed.

What you're looking at, below, is his magnum opus, and the first-ever photograph of another galaxy, dated all the way back to 1888.

Image credit: Isaac Roberts, of the Great Nebula in Andromeda, 1888.

Around 1900 or so, there was an intense debate that raged for decades as to what these spiral nebulae actually were. Were they proto-stars, as some claimed: nebulous regions within our own Milky Way that were in the process of forming new stars and planetary systems? Or were they, as contested by others, "island Universes" in their own rights, well outside of the Milky Way, each consisting of a vast array of stellar systems not so different from the ones in our own galaxy, which would then be just one of many?

Looking at a modern picture of Andromeda -- the largest galaxy as viewed from Earth -- it's easy to forget how we came to learn what we know today. You see, Andromeda is the largest of all spirals in the night sky from our vantage point; regardless of what it was, it was reasonable to assume the reason it was so big is because it was closer to us than all the others.

In the early 1920s, a young Edwin Hubble was working at Mount Wilson observatory, where he had access to the 100-inch (2.5 meter) Hooker Telescope, which was the most powerful telescope in the world at the time.

Image credit: Mt. Wilson Observatory.

In October of 1923, Hubble, along with his assistant, Milton Humason, was searching for novae, or flare-ups that happen when white dwarfs accrue enough material on their surface to briefly ignite nuclear fusion.

While looking at the area around M31, Hubble saw one, then another, and then yet another. And then something unexpected happened: he saw a fourth, in the same location as the first.

That was no nova; novae take anywhere from a few years to thousands of years to refuel, this simply couldn't happen in a matter of hours. No, the only thing that could flare up, go away, and then flare up again was a variable star. This was such an exciting discovery that Hubble crossed out the "N" for Nova and wrote "VAR!" for variable star.

Why was this so exciting?

Image credit: NASA, ESA and Z. Levay (STScI); science by Hubble Heritage Team & AAVSO.

Because there's a relationship between the period of variability of a star and how intrinsically bright it is! So if you measure -- as Hubble easily could -- how bright this star appears, and you know how intrinsically bright it actually is, you know its distance!

And that was how we figured out that this object -- the Andromeda Nebula -- was actually the Andromeda Galaxy, located far beyond the full extent of the Milky Way galaxy.

Image credit: Aggelos Kechagias, via his flickr account.

It wasn't long before variable stars were measured in dozens of galaxies, and that a relationship was noticed between the distances to these galaxies and their recession speed relative to us; by the end of the 1920s, we had discovered the Hubble expansion of the Universe!

And as our eyes on the Universe have opened in different wavelengths, we've learned ever so much more about our nearest, giant galactic neighbor.

Image credit: Multiwavelength images of M31, via the Planck mission team; ESA / NASA.

For one, the full extent of the Andromeda Galaxy is about three degrees across, making it about six times the diameter of the full Moon as seen from Earth. It's the largest galaxy in our local group, at least 25% more massive than we are (despite this flawed study) and containing around a trillion stars, whereas we top out at 200-400 billion.

Image credit: NASA / ESA / Spitzer Space Telescope. Click for a giant version, if you dare!

We know how many stars are there from infrared studies, and we also know something a little unexpected: Andromeda has a supermassive black hole, just like we do. But unlike our black hole, which is around 4 million solar masses, Andromeda's is closer to 100 million, as shown in blue, below, thanks to Chandra's X-Ray eyes.

Image credit: S. Murray, M. Garcia, et al., (CfA) SAO, CXO, NASA.

The Ultraviolet part of the spectrum shows hot, young blue stars, as well as ionization regions that emit at very particular wavelengths when electrons and atomic nuclei come together. Depending on what part gets highlighted by an observatory can give very different results: compare the mosaic from SWIFT, below,

Image credit: NASA / Swift / Stefan Immler (GSFC) and Erin Grand (UMCP).

with that of GALEX, also in the UV.

Image credit: NASA / JPL / California Institute of Technology.

Andromeda is our big sister in the local group; we're the second largest galaxy by a wide margin over #3, the Triangulum Galaxy, but Andromeda's got us beat!

While the rest of the Universe recedes away from us, Andromeda, the Milky Way and the rest of the local group are gravitationally bound together. Based on the way Andromeda is moving towards us, we should expect it to merge with our own galaxy in just under another 4 billion years. Could we fast-forward in time, the sights we'd see would be spectacular.

Image credit: NASA; ESA; A. Feild and R. van der Marel (STScI), T. Hallas, and A. Mellinger.

While the rest of the Universe beyond our local group flits away from us as the Universe expands, Andromeda is the largest exception to the rule, as it hurtles towards us across the millions of light years separating the two of us.

I often tell you that the most spectacular images of any Messier object comes from the Hubble Space Telescope, whenever it's available. This is still true, but Andromeda (M31) is far too big to fit into a single Hubble frame. Instead, I'd like to show you a much rarer view of this galaxy that comes from Hubble.

Image credit: NASA, ESA and T.M. Brown (STScI).

This is just a small, tiny portion of the disk of the Andromeda Galaxy, at a far-reduced resolution. If we instead head on over to the full-resolution version of this image, we can see -- amazingly -- that there are all sorts of interesting objects contained within even the dense star-fields of Andromeda, including a number of distant galaxies that, so far, only Hubble's eyes can open for us.

Image credit: NASA, ESA and T.M. Brown (STScI).

And there's no topping that! So that concludes another Messier Monday! Including today’s entry, we’ve taken a look at the following Messier objects:

Come back next week for another look at one of the 110 unique objects that make up the Messier catalogue, only here, only on Messier Monday!