"Books serve to show a man that those original thoughts of his aren't very new at all." -Abraham Lincoln

It might be Veterans Day / Armistice Day all around the world, but it's still Messier Monday here on Starts With A Bang! We may have been fighting wars for all of human history, but nearly all of the 110 deep sky objects that make up the Messier Catalogue go back long before that.

Today, we take an in-depth look at one of the brightest and closest star clusters in the entire night sky, one that -- despite being visible to the naked eye -- went unrecorded until Messier himself catalogued it in 1764. Let's take a tour of Messier 39, starting with how to find it.

Although one of the most prominent collections of stars in the night sky is known as the Summer Triangle -- consisting of the dominant stars (in its portion of the sky) Altair, Deneb and Vega -- it's actually visible after sunset well into autumn and even early winter for the Northern Hemisphere! The galactic plane passes through the summer triangle, and most of the open star clusters contained within our galaxy are found in the galactic plane, and Messier 39 is no exception to this rule.

To find it, draw an imaginary line from Vega (the brightest one) to Deneb (on the short side with Vega), and continue to head a little farther in that same direction.

The first thing you'll run into is the easily visible naked-eye star, the orange supergiant ξ Cygni, which is nearly along the imaginary line extended along the Vega-to-Deneb path. If you think about Deneb as the bottom of a circle and move clockwise towards ξ Cygni, you'll hit three more (albeit slightly dimmer) naked eye stars as you reach the top of the circle -- ρ Cygni, π2 Cygni, and Azelfafage (also π1 Cygni), in order -- and Messier 39 lies just inside of them.

Although Messier, discovering it in 1764, gave it the very terse description:

Cluster of stars near the tail of the Swan; one can see them with an ordinary telescope...

this actually turns out to be one of the most remarkable objects in his entire catalogue! Let's take a look inside.

You might first notice the prominent blue stars, and you should: there are about 30 prominent, blue-colored stars identified in M39, the bluest of which is of spectral type A0, the brightest and bluest of all the A-class stars. But one of the things that makes this cluster particularly interesting is that all of the bright stars are blue!

There are no red, orange or yellow giants, no prominent subgiants, and no stars in general found off of the main sequence!

For those of you who need a reminder, the main sequence is the line on which -- if you plot the colors vs. magnitudes of stars -- you'll find all stars burning hydrogen in their core.

Image credit: European Southern Observatory (ESO).

This is exceedingly rare! By this point in a star cluster's life, a star running out of fuel in its core takes some tens of millions of years to go from the main sequence and through the various subgiant and giant stages until it dies in a planetary nebula and white dwarf combination. With hundreds of stars expected in a typical star cluster, finding one without a single evolved star is exceedingly rare!

And yet, here Messier 39 is, doing exactly that.

With the background stars of the galaxy shining in a dim haze behind it, Messier 39 happens to be close, at a distance of just 800 light years, only the Pleiades and Praesepe star clusters are closer objects in the Messier catalogue. And yet, if we take a long exposure to really bring out the background stars, we don't really find many more that appear to be associated with these bright, blue ones! Even though -- based on what we see -- there are around 800 solar masses worth of material in there, we've only definitively identified around 60 stars in this cluster!

Only six of Messier's star clusters appear brighter to the eye, and the brightest of the blue stars here are just barely beyond the limit of human naked-eye vision under the darkest possible skies.

With long-exposure photography, the sight is nothing short of spectacular.

Based on the stars that are in there -- and assuming that higher mass ones are missing because they've evolved and disappeared -- we can estimate that this star cluster is around 300 million years old, but there are some uncertainties there.

After all, looking among what appear to be background stars, how many evolved white dwarfs from long-dead O-and-B-class stars are hanging out, invisible because of the incredible density of stars found in the galactic plane?

Image credit: flickr user The Killer Rabbit1.

You'd be very, very smart if you thought to look in the infrared; oftentimes, evolved, cooler stars that are less prominent in the visible will appear bright in the infrared!

Let's have a look at the best IR image out there of M39, thanks to 2MASS!

Image credit: The Two Micron All Sky Survey (2MASS) at IPAC.

This infrared image is what allowed us to go from about 30 (visible) stars up to around 60, but that's still a tiny number for an open star cluster! (And it's only seven light-years across; this star cluster would appear tiny if it weren't so ridiculously close!) Did something happen when this cluster was forming to end star formation early? It wouldn't be the only cluster like this, but the more of these objects I go through, the more of an appreciation I have for the sheer diversity of populations of stars that form naturally in the Universe!

My favorite of all the images that exist of M39 comes courtesy of NOAO, the National Optical Astronomy Observatories.

Image credits: Heidi Schweiker, WIYN, AURA, NSF, NOAO.

This image -- originally -- comes in an incredibly high resolution; if we take a "slice" through the center, here's what this incomparable star cluster looks like, silhouetted against the backdrop of the galactic plane!

Image credits: Heidi Schweiker, WIYN, AURA, NSF, NOAO.

This would make a lousy target for Hubble due to its incredibly large angular size, but that makes it all the more amazing as a target for amateur skygazers! And with that magnificent tour through one of the closest star clusters to us, that will wrap up another Messier Monday! Including today's entry, we've peered at the following Messier objects:

Come back next week, where another deep-sky object -- and another unique story about the Universe -- awaits you on Messier Monday!