Monday, March 19, 2018

M67 - An Unusually Ancient Open Cluster

Open clusters are not generally long-lived objects. They are (in cosmic timescales) relatively quickly torn apart by gravitational influences as they wander through the galaxy. So after a few scant hundreds of millions of years, their individual stellar components live out the remainder of their lives as solitary stars (just like our own sun, and nearly every other star in the Milky Way Galaxy), and their parent cluster is only a memory.

But every now and again, an open cluster will manage to hold itself together, even after multiple passes through the galactic plane (the zone of maximum disruption). Messier 67 is a prime example of this rara avis. Estimated to be somewhere between 3.2 and 5 billion years old, M67 is quite the oldster. Composed of approximately 500 stars, of which about 100 closely resemble our own Sun, the cluster contains the mass of more than 1000 Suns. The consensus amongst astronomers is that M67's initial mass, some 4 billion years ago, was more than 10 times what it has managed to retain. What a sight that would have been!

But however glorious it may have looked in the distant past, M67 remains quite the eye pleaser. At apparent magnitude 6.1, it verges on being a dark sky naked eye object. Through my 8X56 binos, it is a distinct fuzzy patch just to the right of Alpha Cancri. In my 90mm Stellarvue refractor, I can make out dozens of stars enmeshed in a faux nebulosity, which in reality is the remainder of the cluster's stars which lie beyond my scope's capability to resolve. To my eye, M67 looks like nothing so much as a giant comma lying on its side. Either that, or a cat seen from behind, with its tail prominently displayed.

M67 lies somewhere between 2600 and 2900 light years from the Earth. Practically every type of star can by found within the cluster, from white dwarfs to blue stragglers, from Sun-like main sequence stars, to red giants.

M67 is not hard to find. Start by locating the dim constellation of Cancer, between bright Gemini and dramatic Leo. The Main Attraction in Cancer is of course the Beehive, or M44. The Beehive (also known as the Manger) is to the upper right of Delta Cancri, which lies smack on the ecliptic. (Fun Fact: Delta Cancri's proper name is my favorite of all the stars in the sky, the Babylonian word  Arkushanangarushashhutu - and no, I am not going to try to pronounce that!) Below and to the left of Delta Cancri is (despite its "Alpha" designation, the somewhat fainter star) Alpha Cancri. M67 is located not far to the right of Alpha Cancri.

Not so long ago, astronomers seriously wondered whether M67 was our own Sun's parent star cluster, principally due to the large number of Sun-like stars within it. But recent computer simulations of the non-intersecting paths taken by M67 and Sol have ruled such a thing to be extremely improbable.

Friday, March 16, 2018

Why is the winter sky so beautiful?

Well. Mainly, because there are so many bright stars visible in wintertime - far more than in the summer. And this, despite the fact that we enjoy the glorious star clouds of Cygnus, Sagittarius, and Scorpius during the hotter months. But let's face it, the individual stars making up those constellations are not, pound for pound, the equal of those composing Orion, Gemini, or Auriga... not to mention glorious Sirius, Procyon, or Aldebaran.

So why are there so many more brilliant stars up there in the winter? To understand why, we need to know a bit about the geography (so to speak) of our galaxy, and our place in it.

Take a look at the above illustration. Although the matter is not "settled science"just yet, there is broad consensus that the Milky way is composed of a nucleus, four spiral arms, and an uncertain number of "spurs" (broken off pieces of spiral arms). Our sun happens to be located on the inner (that is, closer to the galactic center) edge of the Orion Spur. Between us and the galactic core are no less than three spiral arms, the closest being the Carina-Sagittarius Arm. But when we look in that direction (in the summertime), our gaze has to first traverse many thousands of light years just to get to even the closest, or Sagittarius, Arm. So no matter how intrinsically bright any stars are in that direction, they're going to look dimmer just from sheer distance.

Not so in wintertime! We are smack up against the Orion Spur, and the blue-white or red supergiants in that direction are therefore that much closer. So it's mainly a cosmic optical illusion. The winter sky does not boast a greater number of bright stars - it's just that those that happen to be in that direction are significantly closer to us.

But who cares? Get out there and enjoy the spectacle of the glorious winter sky!

Thursday, March 1, 2018

After you've looked at M42...

... what is there to see in the winter sky? Actually, tons! For those of us unfortunate enough to live in the Northern Hemisphere, the winter sky contains more objects of interest than any other season - and that includes the summer Milky Way. Now I'm not going to attempt to list them here, but rather direct your attention to one undeservedly overlooked open cluster in Canis Major, NGC 2362.

This image captures what NDC 2362 looks like through my 90mm refractor.

The first time I chanced upon this Jewel of the Sky, I wasn't looking for anything in particular. I was simply letting the turning Earth beneath my feet bring new wonders into my field of view, while I allowed my scope to drift unguided. Then a most marvelous sight riveted my attention, and my aimless drifting was definitely over for the night. I simply couldn't get enough of this spectacular cluster, with its giant central star Tau Canis Majoris (which is actually part of the cluster, and not just a chance line of sight association).

To my eye, this cluster has a distinctly triangular shape, and is full of color. NGC 2362 is a baby amongst its fellow open clusters, being only about 4 million years old. Its central (and brightest) star is Tau Canis Majoris, a class O supergiant which turns out to be a fantastic multiple star system composed of at least 6 stars (and probably more). At magnitude 4.4, Tau Canis Majoris ought to be visible naked eye when observed under dark skies.

This cluster rewards the patient viewer, as many subtle details emerge as your eye becomes accustomed to the sight, and fainter members make their presence known.

NGC 2362 is approximately 5000 light years from our solar system.