Messier Monday: An All-Season Cluster, M35

Ethan Siegel
Nov 17, 2014 · 10 min read

With an unforgettable “blue fog” caused by the galaxy itself that makes it stand out among all the others.

“Derive happiness in oneself from a good day’s work, from illuminating the fog that surrounds us.” -Henri Matisse

We might not realize it — I’m pretty sure that Messier never did — but we do live in a great cosmic fog. Not because there’s something nebulous inherent to space that prevents us from seeing what’s truly there, but because we live in the plane of our galaxy, which itself is filled with not only stars and nebulae, but light-blocking gas and dust as well. Yet if we look in just the right places on the sky, we can find not only stars, but windows into some of the more exotic deep-sky objects in the Universe!

Image credit: Jim Cornmell under a Wikimedia Commons Attribution-Share Alike 3.0.

110 of those objects are represented in the Messier Catalogue, the first large, accurate compilation of what we now know are stellar remnants, star clusters and galaxies. While the galaxies are clustered irrespective of our own location, one of the types of star clusters — the open clusters — are almost exclusively found in the plane of our Milky Way. One of them happens to be today’s object: Messier 35, a remarkable star cluster (and a spectacular site) visible practically year-round.

Here’s how to find it tonight.

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

A few hours after sunset, the famed winter constellation of Orion will rise in the east, with bright, orange Betelgeuse pointing towards the north. Other prominent stars are nearby, including brilliant Aldebaran, the incredibly bright Capella, and the “twins” Castor and Pollux, all among the 25 brightest stars in the entire sky.

If you drew a ring connecting these five stars, Messier 35 would lie somewhere very close to the middle, but that’s not a very helpful aide to find it. Instead, connect Betelgeuse to Pollux and find the blue star midway between them: Alhena (still the 41st brightest in the sky), and then “hop” in the direction of Capella.

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

There will be three stars you encounter as you star-hop that require absolutely no visual aides — like a telescope or binoculars — at all: ν Geminorum, Tejat (μ Geminorum), and then Propus (η Geminorum), which is somewhat out-of-line with the others.

If you navigate the same distance-and-direction that you jumped from ν Geminorum to Tejat, and applied that from your jump to Propus to where there are no very bright stars, you would wind up practically atop Messier 35!

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

The closest thing to a “guide star” is 5 Geminorum, but even that is at the limit of unaided vision. But if you can find it, then right next door is a visually large cluster of stars: about the size of the full Moon. And its appearance is unmistakeable, even for first time stargazers.

Image credit: Oliver Stein, via http://commons.wikimedia.org/wiki/File:Messier-35-and-NGC-2158.jpeg, of Messier 35 next to 5 Geminorum (just to its right).

Although Messier catalogued this object in 1764, it was independently discovered at least twice before him: by Philippe Loys de Chéseaux in 1745 and by John Bevis, who published it in his early catalogue in 1750. Messier himself found it and described it so:

Cluster of very small stars, near the left foot of Castor, at a little distance from the stars Mu & Eta of that constellation.

What’s striking at first, particularly through a better telescope, is that this star cluster is not alone!

Image credit: Rob Hawley of Almaden Observatory, via http://www.almadenobservatory.net/M35/index.html.

What looks like a “little sister” is actually a far more distant and much older star cluster that’s also in the plane of our galaxy. You see, it takes huge amounts and dense concentrations of gas and dust — molecular clouds — with around a million solar masses or so to collapse and trigger the formation of new stars. With very few exceptions, these molecular clouds exist only in the planes of galaxies, and the open star clusters we find typically dissipate gravitationally after a few hundred million years, with a few surviving into the billions.

But the easiest ones to find are the closest and youngest, since their stars appear the brightest to us!

Image credit: Messier 35 by Jorge Garcia, via http://www.pbase.com/image/91887467.

This is why the stars in Messier 35 appear so blue: not only because many of the brightest ones are blue (with many B-class stars still around, all the way up to B3, two full classes bluer than the bluest stars in the Pleiades), but because the stars not in Messier 35 are so much less blue.

And there are plenty of stars in the background of this cluster, including background star clusters galore (including the faint but even bluer IC 2157, off to the west), because the plane of our galaxy is simply where the richest concentrations of stars live.

Image credit: Fred Espenak of AstroPixels, via http://astropixels.com/openclusters/M35-01.html. IC 2157 is the very faint collection of blue stars off to the right, just below center.

But it’s also where the richest concentrations of gas live, and one of the things that gas can do is reflect starlight: and that’s preferentially reflecting blue starlight over all the other frequencies. The galactic “fog” we live in is illuminated by the intense blue light emanating from Messier 35, and while there isn’t enough of it to create a true reflection nebula, there is enough that a telescope like the Canada-France-Hawaii Telescope (CFHT) can pick it up.

Image credit: © 2003 Canada-France-Hawaii Telescope Corporation, using the MegaPrime cam, via http://www.cfht.hawaii.edu/News/MegaPrime/MegaPrime-PR-AstroImage-M35NGC2158.html.

This cluster is located some 2,800 light years away, and is somewhere between 60 million and 110 million years old, depending on a variety of factors. The more distant companion cluster, NGC 2158, is completely unrelated, at nearly a billion years old and at a distance of some 16,000 light years from us.

But what’s ironic is that the brightest stars found in this cluster aren’t the blue ones at all, but rather a smattering of red-to-yellow ones!

Image credit: Kfir Simon / Tango33 of PBase, via http://www.pbase.com/tango33/image/148456367&exif=Y.

That’s because these are giant stars that have evolved from the most massive, blue stars to still be alive; they simply ran out of hydrogen fuel in their core and are now fusing helium! This is the future fate of all the bright blue, white, yellow and orange stars in this cluster, as well as our own Sun. While this cluster may have formed more than four billion years after our own Sun, the most massive stars within it are giving us a preview of what awaits us!

Regardless of what your equipment is, it’s a beautiful sight, one that delights skywatchers of all levels over and over, throughout the year!

This is our next-to-last Messier object before we complete the entire catalogue, so make sure you enjoy this one any time during the year, as it makes for outstanding viewing regardless of light pollution or the presence (or absence) of a Moon. Take a look back at all our previous Messier Mondays below:

And come back next week, when we’ll finish the catalogue at last!


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.