February 2019 – M46 and M47

In the winter sky, toward midnight, you can look in the southern sky and see the brightest star in our sky: Sirius. Often giving off brilliant flashes of blue, green, and red, this bright white star is unmistakable. About a hand width straight east of it is where we find these two star clusters: M46 and M47.

M46 and M47

M47 is the cluster on the right. It sparkles with bright blue stars, set against a crowd of stars of various brightnesses. These stars are massive and young. We know this because to be that blue and bright, they must be fusing hydrogen into helium at an astonishing rate. This means they won’t live long, ending their lives in a few million years as stupendous supernova explosions. Thus, M47 is a young (astronomically speaking) cluster of stars.

In contrast, M46, the cluster visible on the left of the image, is an older cluster. Gone are its blue supergiants, along with any star that lived its life fast and died hard. These stars are all mainstream, run-of-the-mill stars, most of which will last billions of years. In fact, it’s old enough that one of its stars has entered the final phase of life: a planetary nebula. These are so named because they’re generally round, and mimic the appearance of planets through the eyepiece of a telescope. This one does seem to be associated with M46, and adds a beautiful jewel to an already lovely scene.

M47 with its planetary nebula

That M46 is still discernible as a cluster at this age is remarkable. Most clusters would have long ago dispersed into the general stellar population of the galaxy. Only the most massive star clusters have the gravitational power to prevent this. M46 is obviously massive enough to avoid that “fate.”

There are two other clusters clearly visible in this image, though they are much closer to complete dispersion. One is above M47 at about 11 o’clock, and the other is to the lower left, about 7 o’clock. Another cluster is present, but is not discernible against the starry background. We only know a cluster remnant is present because there is a statistically unlikely group of stars of similar age in its location. This cluster is to the lower right, but you can’t find it!

Every star in this image began life in a cloud of dust and gas like last month’s image of M78. The stars formed in clouds like that created clusters similar to these, with hundreds or even thousands of stars in each. As time progressed, the hot blue stars exploded and the rest were left to join the hundreds of billions already orbiting the galactic center. Last month we saw stellar nurseries. This month we see adolescent, adult, and old age stars. In March we’ll see toddler stars!

January 2019 – M78 and Barnard’s Loop

In the evening, Orion’s Belt, a row of three brilliant stars, can be seen rising in the southeast, at least for us mid-latitude northern hemisphere dwellers. It’s an iconic sight, written about by civilizations as ancient civilization itself. What those civilizations couldn’t know, is that there’s much more to Orion than meets the eye. This image includes two perfect examples, situated just north of the eastern star of the belt: Alnitak.

M78 consists of two regions of brightly glowing “reflection nebulae,” seen here at right-center. These objects are very dusty, and the brilliant light from the newborn stars embedded within the clouds of dust illuminate their surroundings with their blue light. Just like dust on your dresser, though, it is thinner and thicker in places. Where it is thicker, even the light of these brilliant stars cannot penetrate, and we see those areas in silhouette against the brightly glowing background.

M78 is crowded with young stars, most of them too dim for their light to get through even the thinner dusty regions. The light from the brighter stars will resolve that problem. Since light actually exerts pressure on atoms, molecules, and tiny grains of dust, the light from the bright stars is in fact literally blowing the dusty shroud away. Soon, relatively speaking, there will be “naked” star clusters in the place of M78. An unrelated example of one of these can be seen near the left edge of the image.

The lovely red arc that dominates the left side of the image is thought to be a supernova remnant, the almost-spent shock wave from the explosion of an ancient massive star. It is very large, basically wrapping the entire east side of Orion. E. E. Barnard, the famous mapper of the Milky Way, discovered this glowing loop, and this it’s named after him. Perhaps it’s no coincidence that there are so many starbirth regions in Orion. There is a lot of hydrogen and dust in this part of the sky. The compression of this passing shock wave may have been enough to trigger regions of gravitational collapse in this massive cloud, the first step to forming a new star.