Messier Monday: A Lost-and-Found Star Cluster, M48 [Starts With A Bang]
“Always try to keep a patch of sky above your life.” -Marcel Proust
Welcome to another Messier Monday, where each week we take an in-depth look at one of the 110 deep sky objects that make up the Messier Catalogue! Messier was not the first person to try and make an accurate, large catalog of deep sky objects, but he was the first to successfully do so: most of his objects both actually exist, are deep sky objects, and had their positions recorded correctly.
Most, that is, but not all. Today, we’ll be looking at the open star cluster Messier 48, one of the objects that does exist, is an interesting deep sky object, but that he incorrectly recorded the position of. Here on Earth, we describe a position by longitude and latitude; in astronomy, their counterparts are right ascension and declination. And while Messier successfully recorded the right ascension, his declination was off by five degrees.
Here’s how you can find it for yourself.
Shortly after sunset tonight, you’ll be able to find Orion high in the sky, with Sirius and Procyon trailing behind towards the east. A little farther away, to the left of the box above, is what appears to be the naked-eye star 30-Monocerotis (also known as C Hydrae), in the tail of the unicorn. But if you look closer, you’ll find it’s actually three stars in a line, close together. And if you take Messier’s 1771 description of this object:
Cluster of very small [faint] stars, without nebulosity; this cluster is at a short distance from the three stars that form the beginning of the Unicorn’s tail.
You can’t miss it, even through a pair of binoculars or — on a very clear, dark night — with your naked eye.
There’s a collection of stars clustered tightly together: that’s Messier 48. This cluster was independently rediscovered 12 years later by Caroline Herschel, and has been well-known ever since, although Messier 48 has never acquired a nickname. But it provides us with an important example of a still young, but aging, star cluster.
When stars first form in a region of space, they typically contain anywhere from a few hundred to a few thousand new stars, and they appear roughly evenly distributed (by mass) across the seven major spectral types.
The O-stars burn through their fuel the fastest, dying the most quickly. After that, the bright B-stars run out of fuel, and that’s the end of stars that will die in a type II supernova. Over time, the biggest, brightest stars continue to burn out, becoming red giants and dying in planetary nebulae.
Messier 48′s brightest main-sequence star is of spectral class A2, which means this cluster is about 300 million years old. As you can tell by looking at it, it’s already burned all of its nebulous dust off.
What you might not be able to tell — not from the images I’ve shown you so far — is the great diversity of color in this star cluster. As you’d expect, the brightest, most massive, bluest stars dominate the light coming from this cluster that’s located about 1500 light years away.
There are at least 50 stars visible through a very good pair of binoculars on a moonless night, and at least 80 stars (and probably many hundreds more, if you include the ones too dim to be seen at this distance) are visible through a better telescope. But what might not be obvious here is that there are quite a number of big, bright red stars as well.
Red giants (and three yellow giants) are stars that were just slightly more massive than the bright blue ones still shining, and these have entered a later, helium-burning phase of their life cycle. Someday soon (astronomically speaking), these stars will blow off their outermost layers in a planetary nebula, while the inner core contracts down to a white dwarf.
All the remaining stars visible at this distance will eventually do this, but only the bright red ones are getting close.
The brightest one (here at lower-left) might be getting quite close, but don’t hold your breath: the processes that create a planetary nebula are not catastrophic, and take upwards of hundreds of thousands of years to complete.
If you take an image of this cluster that’s both wide-field (considerably over a degree, or the equivalent of 22 light-years across at this distance) and deep, you’ll find that there are some 165 identifiable stars in this cluster.
Unfortunately, none of NASA’s great observatories or giant telescopes have imaged this relatively close-to-home deep sky object. But I was able to track down a fantastic image of the core of this cluster, courtesy of the Sloan Digital Sky Survey.
The reds and blues here are so striking up against one another, I was tempted to call this the election map cluster! But I’m not the arbiter of names; so plain old Messier 48 it is. Messier wasn’t perfect, but his catalogue is the first very good one of our sky, and continues to be both useful and gloriously gorgeous to this very day!
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
M48, A Lost-and-Found Star Cluster: February 11, 2013
M60, The Gateway Galaxy to Virgo: February 4, 2013
M67, Messier’s Oldest Open Cluster: January 14, 2013
M78, A Reflection Nebula: December 10, 2012
M81, Bode’s Galaxy: November 19, 2012
M83, The Southern Pinwheel Galaxy, January 21, 2013
M97, The Owl Nebula, January 28, 2013
M102, A Great Galactic Controversy: December 17, 2012
Come back next week for another Messier Monday, where we’ll look deep into the night sky, and admire the wonders that fill our galaxy, and beyond! With seventeen down, we’ve got plenty more to go; let me know which one you’d like to see featured next!