March 21, 2008


The constellation Monoceros is the faintest large constellation in the sky, sandwiched between Canis Major, Canis Minor and Orion. The brightest stars in this constellation are about 4th magnitude, making this constellation invisible from any decent-sized city, but as the band of the Milky Way passes right through the middle, you can be sure there are plenty of neat deep sky objects in this region, including two of the most famous and photgraphed nebulae. Like Camelopardalis, this is a constellation that was squeezed into the map of the celestial sphere to help fill out all regions of the sky by Dutch astronomer Petrus Plancius.

Perhaps the most visually interesting of the three "brightest" stars in this constellation is Beta Mon, which is actually a close triple of three nearly identical stars. This triple system was discovered by William Herschel in 1781, and he praised its beauty very highly. While it doesn't really hold a candle to all of the beautiful, highly processed deep images we can get today, you have to remember that back then with poor quality telescopes that couldn't really make out much nebulosity in the sky, systems with multiple bright stars were the best thing going. Much of the emphasis in Astronomy at that time was discovering and recording the properties of binary and multiple star systems. The three stars in Beta Mon are all blue-white A stars, with two stars in a close orbit and the third looping around in a much more distant orbit. All three orbit very quickly and so throw off gaseous disks of hot material that is pervasive in this system.

Another star of note in this constellation is HR 2422, also known as Plaskett's Star, named after the astronomer who discovered it and studied its properties in 1922. Few stars are actually named after Astronomers, but those that are tend to be very faint and peculiar in some way. Astronomers wouldn't stand for a star like Rigel to be named Sagan's Star are anything like that. Names are reserved for peculiar stars that only dedicated amateurs or professionals would ever really look at. As you might imagine, Astronomers take a dim view of the whole business of selling star names to the public for a price.

Anyway, Plaskett's star is a close binary system consisting of two extremely massive and hot O stars in orbit around each other every two weeks or so. The separation is fairly close to the same separation as the Sun and Mercury, nearly 50 million miles apart. We don't know the masses precisely because the system is tilted far enough from our line of sight that we see no eclipses, so we have to guess at just how fast it is moving compared to its measureable Doppler shift. Our best guess is a combined mass in the neighborhood of 100 solar masses (!). This is easy to find, being just Northeast of the famous Rosette Nebula, and it is very likely associated with the open cluster of stars (NGC 2244) at the center of the Rosette, perhaps ejected from that region due to a gravitational interaction thousands of years ago. The distance is around 4500 light years.

For the deep sky objects in Monoceros, I'll start on the Eastern end and work west. Roughly at the location of the Unicorn's horn, about halfway between Procyon and Betelgeuse (and a few degrees North of this midpoint), we find a huge complex of gas, dust and forming stars in the region of the Cone Nebula and the Snowflake Cluster. The cone nebula shows the effect of an enormously powerful stellar wind clearing out a bubble of gas and dust except in one direction, where the wind is blocked by the smaller bubble of another forming star. So in the wake of that forming star, there is a cone-shaped dust cloud protected from the stellar winds, outlined on either side by a bright ionization front from less powerful surrounding starlight.

Here is a somewhat larger view of the region, showing the star S Mon also in the field of view, and this red image is a similar view but mainly emphasizing the Hydrogen emission from the region, which makes the details of the nearby Fox Fur nebula very visible. For a much closer look at the Fox Fur Nebula, see this image. Similar to the Cone Nebula, the Fox Fur nebula is a thick cloud of gas and dust being pushed around by a nearby bright star, and the cloud's edges are illuminated by surrounding stars and reflected starlight, making for a distinct and interesting shape.

For an even larger view of this region, see this image. Here, the Cone Nebula, S Mon and the Fox Fur Nebula are all over on the left edge, and you can also see some other nearby objects of interest in Monoceros. Among them is the old open cluster Trumpler 5, a cluster slowly breaking up due to gravitational interactions with surrounding objects in the galactic disk. Over on the left of the image is the bright reflection nebula (seen in a much larger image here) known as IC 2169, a cloud of gas and dust in the background reflecting much of the blue light from the bright cluster of newly formed stars in the foreground.

Also barely visible in the image is the ghostly comet-like object known as Hubble's Variable Nebula, also known as NGC 2661. This is a reflection nebula associated with the variable star R Mon. What's unusual about this nebula is that it changes its appearance on very short timescales, sometimes a matter of days or weeks. This is thought to be due to thick knots of dust that pass close to the illuminating star and cast shadows on the reflection nebula. I should mention a bit more about the two stars here while I'm thinking about it.

The name S Mon gives away the fact that this is a (slightly) variable star. Actually, two massive stars in relatively close orbit around each other at a distance of about 2500 light years, and they are the centerpiece of the enormous OB association of hot, young stars at the core of the huge cloud of gas and dust. Meanwhile, R Mon (which lights up Hubble's Variable Nebula) is a T Tauri star that also varies by a couple of magnitudes (which is a factor of about 10 in linear units). T Tauri stars are stars that are still in the process of forming and huge very powerful stellar winds. Like many T Tauri stars, R Mon has a dusty disk of gas and dust, perhaps a precursor to eventual planetary formation.

Next, the Rosette Nebula region in the constellation Monoceros. First, a nice image of the Rosette Nebula, a beautiful cloud of glowing Hydrogen gas lit up by a central cluster of massive, hot, young (if you consider four million years old "young", which astronomers do) stars. If only our eyes were sensitive enough, we would see that the glowing gas clouds here span an area of the night sky equivalent to five full moons. And the central bright region of the cloud, though spectacular, isn't where all of the star forming activity is found.

For that, you would need to slew your view a few degrees to the South, into the thickness of the background dust cloud. Deep within that cloud, little knots of gas and dust are swirling and collapsing and forming new stars. We can't see this optically, but the emission from such regions can be seen using radio telescopes. Radio waves are long enough that they don't get scattered by the cloud as they leave the star forming region, and so we look for very low energy emissions in the radio band from molecules such as Carbon Monoxide, which are extremely rare in interstellar space but are found very commonly in the densest parts of molecular clouds. Carbon Monoxide is a great tracer of ongoing star formation because it is a great tracer of the densest, coldest parts of the clouds which are close to collapsing.

This nebula measures about 100 light years across and is located at a distance of about 5000 light years. It is on the front edge of a giant molecular cloud, and you can see some of that cloud due to the light reflecting off of it from the bright cluster of stars at the center of the Rosette. Another view of this nebula emphasizes emission lines from Oxygen and Sulfur ions, which are found in the hottest parts of the clouds. Another closer image in a similar style reveals the details of thick dust clouds, many of which have had their rarefied outer layers stripped away, leaving behind only the thick, dense globule of a core that will eventually form a new star.

Going from the Cone Nebula region to the Rosette region and then continuing on by the same amount (about 5-6 degrees South, halfway to Beta Mon) leads you to the very nice reflection nebula NGC 2170. This is another set of very bright, newly formed stars reflecting their light off of a background molecular cloud, this one about half the distance compared to the Rosette. A very pretty picture.

Further south at the edge of this constellation, at about the center of a line connecting Sirius in Canis Major with Saiph in Orion, we find the Red Rectangle nebula. This odd pattern is formed by an old red giant star in the process of blowing off its outer layers to become a planetary nebula. The dusty disk surrounding the star has redirected this outflow into two broad conical jets, which when paired resemble a rectangle. The ladder-like rungs (seen here in this large closeup) in these cones are probably from periods of stronger outflow that happen occasionally in the unstable central star.

Now for the rest of the deep sky objects in Monoceros. Actually, this is only a smattering of the brighter and more interesting ones. There are a few dozen clusters I'm omitting just because we've seen so many similar ones already, and there aren't any that are all that spectacular here. The one cluster that does stand out is Messier 50, an open cluster found about 1/3 of the way along a line connecting Sirius in Canis Major with Procyon in Canis Minor.

This is a sparse open cluster, seen well in this image, about 3200 light years distant and 20 or so light years in diameter. Nearly 100 million years old and right in the thick of the galactic disk, this cluster is well on its way to breaking apart completely and is barely recognizable to the eye. About two degrees south of M50 is the spectacular Seagull Nebula (IC 2177), a wing-and-body shaped region of bright, excited Hydrogen gas topped by a slightly bluer head nebula surrounding a bright young star. A couple of dust lanes help define the body of the bird in deep images against a thick background of stars in the Milky Way's disk.

About 7-8 degrees due north from M 50, just a full moon's diameter to the SE of Delta Mon, we find a very nice planetary nebula known as NGC 2346 or the Butterfly Nebula (one of a few so named in the sky). At the center of this double-lobed nebula is a binary star system. One of the pair apparently engulfed the other, resulting in two stars with a common envelope. This caused the stars to emit rings of material followed by much more uniform outflow that was redirected a bit by the rings into the Butterfly shape we see. This nebula is about 2000 light years distant.

The final famous object in Monoceros is the variable star V838 Mon. In January 2002, this star emitted an incredibly bright flash and expanded greatly, becoming the brightest star in the entire galaxy for about a month before gently fading. At peak brightness, if your eyes could detect infrared light, you would've been able to see this star in the daytime thanks to all of the emitting dust particles! The light from the flash has been travelling outward from the star ever since, lighting up successive dusty shells that had been emitted by the star travelling outward for likely thousands of years much more slowly than the light wave. You can see the resulting effect in a time lapse movie here. The ring of light is now roughly double the angular diameter of Jupiter on the night sky.

It looks like the dusty shells around the star are rapidly expanding, but that's just an optical illusion. The dust is staying put. It's just the light wave expanding and lighting up successive dusty shells. So what caused this outburst, and are we likely to see it repeated in some other star? We still don't know, and we haven't really been able to find any dependable analogues in other galaxies. One theory is that this was a main sequence star, just a bit more massive and hot than the Sun, but when its core fuel ran out, it very quickly evolved to become a red supergiant, a process that normally takes thousands of years rather than a few months. This theory has fallen out of favor as better distance determinations indicate it is much further away (nearly 20,000 light years) than originally thought.

Or perhaps this was a strange nova, in which matter was dumped on to the surface of a white dwarf by a red giant companion star, and once the Hydrogen on the surface of the white dwarf got hot and dense enough, fusion ignited, causing the shell to puff out. Normally, such nova explosions are very predictable, but not this one. Why didn't the shell escape the system? Instead, it seems to have grown and cooled but then started shrinking again. Perhaps the surrounding dust (the result of a strong wind from the red giant companion) slowed the outflow? There are several other theories discussed here, and we're still not really sure which is right.

Posted by Observer at March 21, 2008 09:16 AM

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