“The ruins were just a reminder that what had been was no longer. That everything we are will be gone someday. That I will be forgotten.” -Megan Miranda
Some four-and-a-half billion years ago, a ultra-massive cloud of cold gas and dust collapsed, giving rise to thousands of stars of all different types, from hot, massive, quick-burning blue stars down to low-mass, cool red dwarfs. And within a few hundred million years, that open star cluster dissociated, flinging the individual stars that once made it up throughout the galaxy. The lone remnant of that cluster known to us lights up our daytime sky: our Sun.
There are over a thousand known open star clusters in the Milky Way, with 33 of the brightest and closest making their way into the Messier catalogue. But while the vast majority of open star clusters don’t even make it to an age of even one billion years before gravity takes its ultimate toll and flings the stars off into interstellar space, there’s one star cluster in the Messier Catalogue that’s old enough that the Sun would be right at home.
Visible all night long during the winter, the open cluster Messier 67 is the oldest open star cluster in the Messier Catalogue, and here’s how you can find it tonight.
As you move to the east of Orion and Sirius, you can find the two very bright stars Procyon and, a little farther, Regulus, the 7th and 20th brightest star systems in the entire night sky. About midway between the two, just a tiny bit north of the imaginary line connecting them (and just a little south of Praesepe, M44, our Messier Monday from just a few weeks ago) is the faint open cluster M67, which you can find in a pair of binoculars by finding Acubens, the brightest star in the constellation of Cancer.
Messier 67 appears faint and nondescript, and there are no bright, blue stars in it, which is your first clue that it’s very old! Remember, when star clusters are first born, they form the following seven different classes of stars, in roughly equal mass-fractions.
However, the biggest, bluest, brightest stars burn up their fuel the fastest, and so have the shortest lifetime. The Pleiades, for example, at about 80 million years old, has no more O-stars, but plenty of B-stars. Praesepe, mentioned earlier, at about 600 million years of age, has no O-or-B-stars, but plenty of A-stars.
But Messier 67 is so old that it doesn’t even have any A-stars left, and the “bluest” star type present is of class F!
At only 2,600 light-years distant, it’s a relatively close open cluster, and there are over 500 stars in it, with more than 100 of those being Sun-like (or G-type) stars. There are also — as you can better tell in the image below — quite a few blue stars in this cluster, mostly concentrated at its core.
But these are not original, main-sequence stars; these are blue stragglers, or stars that have formed from the merger of lower-mass stars at the core of this cluster.
You see, this cluster wasn’t always so “light,” with a total mass of around 1,000-to-1,400 solar masses, depending on whom you ask. But as clusters age, the heavier mass stars move towards the center and the lighter-mass stars either migrate towards the outskirts or get kicked out, in a process known as mass segregation, which M67 has clearly undergone.
As a result, the stars that remain after some 4-billion-years (the latest estimate of the age of M67, although modern estimates range from 3.2-to-5 billion years) are much more tightly bound, gravitationally speaking, than younger star clusters are.
But the simple way to “date” the age of a cluster is to plot its stars on a color-vs.-magnitude diagram, and the curve that results allows you to figure out the age of the cluster from the stars that make it up!
Messier 67, because it “turned off” of the main sequence at the point illustrated above, can be reliably dated to be “around” 4 billion years old, making it the oldest open cluster in the Messier catalogue. (The oldest open cluster thus far discovered is Berkeley 17, at a whopping 10-to-13 billion years old!)
Because of the way its stars are now concentrated together, Messier 67 is expected to remain together for perhaps another 5 billion years before dissociating, provided it doesn’t experience a catastrophic gravitational interaction with an object capable of tearing it apart!
At only 12 light-years across, M67 is one of the more compact open clusters known, and the stars in it are not only the same rough age as our Sun, they also have a nearly identical elemental composition to our Sun, which means that these 500 known stars may have worlds and planetary systems very much like our own!
The best-resolution image we have of the core of this cluster comes from the Sloan Digital Sky Survey, which really shows off not only the colors of the stars inside, but also the very dim red M-dwarfs beyond the reach of other telescopes!
As always, you can click to enlarge the above image, but to best show-off the resolution, I’ve taken just a narrow strip from the middle of it, to showcase exactly what we can see inside.
And finally — I’m not entirely sure — but either there’s a funny, ring-shaped artifact around one of the bright, red-giant stars at the lower-right of the SDSS image, or we’re seeing the early stages of gas being blown off into a planetary nebula!
After all, the cluster is the right age for this to be happening, and is that what we’re seeing here? Or is it a problem of overexposing a relatively bright, red giant star with the camera/CCD in the telescope itself? I’m not sure: you be the judge!
Regardless, I hope you enjoyed a look inside one of the most unremarkable-looking objects in the Messier Catalogue, that just happens to have one of the most remarkable stories behind it!
So far, including today’s entry, we’ve taken a look at the following Messier objects:
- M1, The Crab Nebula: October 22, 2012
- M8, The Lagoon Nebula: November 5, 2012
- M13, The Great Globular Cluster in Hercules: December 31, 2012
- M15, An Ancient Globular Cluster: November 12, 2012
- M30, A Straggling Globular Cluster: November 26, 2012
- M37, A Rich Open Star Cluster: December 3, 2012
- M41, The Dog Star’s Secret Neighbor: January 7, 2013
- M44, The Beehive Cluster / Praesepe: December 24, 2012
- M45, The Pleiades: October 29, 2012
- M67, Messier’s Oldest Open Cluster: January 14, 2013
- M78, A Reflection Nebula: December 10, 2012
- M81, Bode’s Galaxy: November 19, 2012
- M102, A Great Galactic Controversy: December 17, 2012
Come back next Monday (and every Monday) for another Messier Monday, where we’ll take another look into the brightest, closest deep-sky objects visible from right here in our own backyards!