January 28, 2008

Canis Major

To close out the Winter Circle, I'll finish with the Big Dog, Canis Major, home to both the largest star in the sky and the brightest star in the night sky. Located about 35 degrees above the SSW horizon on Winter evenings, Canis Major has five bright stars visible to the naked eye from within a city, enabling you to see the trapezoid formed by the front and hind legs as well as a tail. Sirius represents the front shoulder of the dog. Canis Major is commonly represented as one of Orion's two hunting dogs, perhaps chasing nearby Lepus the Rabbit, but there are many other tales to go with this bright constellation.

Sirius is the nearest star to us visible to the naked eye (8.7 light years away) with the exception of Alpha Centauri, and it is the 5th nearest star outside of our solar system overall. It is an A-type main sequence star about 25 times more luminous than our Sun. Like many nearby stars, Sirius has a notable proper motion, moving the equivalent of a full moon diameter in the sky about every 1000 years. The name translates to "the scorching one". When Sirius rises with the Sun during late summer, those are the hottest days of the year in the Northern hemisphere.

The Greeks first called these days the "dog days" because only dogs were crazy enough to go outside during the heat. Some felt that Sirius being so bright and lined up with the Sun was what caused the intense heat. Precession by the Earth's axis since that time has changed the date of Sirius' conjunction with the Sun. In 1844, famous mathematician and astronomer Friedrich Bessel mapped the proper motion of Sirius very precisely and found that it was wobbling back and forth on the sky due to the influence of an unseen companion.

It was another 20 years or so until telescopes developed to the point where we were able to photograph this companion, known now as Sirius B. Astronomers were puzzled by the fact that it was both very hot and yet very dim and only a fraction of the Sun's mass. Astronomers eventually concluded that it is a white dwarf, a bright, hot compact stellar remnant roughly the size of the Earth, only the 2nd ever discovered and the closest to Earth.

Some historians have theorized that Sirius appeared red a couple of thousand years ago due to descriptions by some astronomers and observers of the time (but not all agreed). The theory goes that perhaps Sirius B was a red giant back then, causing the overall light from the system to be red and has since blown off its outer layers as a planetary nebula. Burnham addresses this by pointing out that such a transition takes about 100 times longer than the given time. There are much simpler explanations to resolve this, poetic license being one I favor, though Burnham speculates that our eye's response to color has changed enough over time that perhaps things appeared redder long ago.

Looking South toward Canis Major, you can imagine the dog climbing up, facing forward away from the horizon, and Sirius is the front shoulder. The front foot to the right of Sirius is Murzim (or Mirzam), Beta Canis Majoris. This translates to "the announcer" since, like Procyon, it rises just before Sirius and so heralds the arrival of this bright and important star in the sky.

Murzim is the prototypical star of a small subset of variables. It is a hot B-star, pulsating rapidly (about every 6 hours) with a very small amplitude oscillation (just a few percent in brightness). It's a Cepheid-like pattern, though not clear enough to be used as a distance estimator. Gamma CMa is Muliphein, one of three stars that make up the triangle of the dog's head above Sirius, and there are questions about why Bayer designated this one so early in the Greek alphabet when it is the 15th or 20th brightest star in Canis Major.

Perhaps it was brighter long ago and has since faded? Burnham and Kaler both point out a historical note that it seems to have vanished in 1670 only to reappear in other records later, but this could easily be an observational error. This is one of several B-class giant or supergiant stars in Canis Major, which an intrinsic luminosity nearly 3000 times that of our Sun. We've never seen stars like this fade so rapidly, and even if a passing interstellar cloud were the cause, we would see a color change (reddening).

Epsilon CMa (Adhara), Delta CMa (Wezen) and Eta Cma (Aludra) make up the hind leg and the tail of the dog. Adhara and Aludra are both bright B-class giants like Murzim, though they don't oscillate. Omicron 2 is similar in type and is along the body, about 75% of the way from Sirius to Wezen (the star marking the hip of the dog). All four of these bright giants are luminous enough that they would dominate the brightness of Sirius or even the planet Venus if they were as close to the Earth is Sirius is. As it is, all are on the order of 1000 light years away and so comfortably dim to us. Wezen is a yellow supergiant, with a contracting core about to initiate Helium fusion, after which it will become a red supergiant like Antares in Scorpius.

A couple other variable stars of note: UW Canis Majoris and VY Canis Majoris are both located in the region just above the tail of the dog. UW is a rare, superhot O-type star with a bright, hot companion. The two orbit very close together (period less than 5 days!) and frequently partially eclipse each other, leading to regular brightness variations. Both are spinning rapidly and flattened due to their close orbit, and much gas is lost in an expanding halo around the system. Burnham speculates that this may be a very young binary system, with the companion stars still contracting.

VY Canis Majoris is a red hypergiant star, now considered to be the largest known star in the galaxy, with a diameter 2100 times that of our Sun. In our solar system, its radius would extend nearly out to the orbit of Saturn. It's large size means that it has a very weak hold on its outer layers, and so as it pulsates irregularly, it throws off arcs and shells of gas into the surrounding regions. This one is definitely a supernova candidate, but it's hard to say how soon it will erupt. All of the irregular gas outflow from the system makes it hard to get good information on the central star itself.

Now for the deep sky objects in Canis Major. The only Messier Object in Canis Major is M41, located right about where you would expect the heart of the dog to be, about four degrees south of Sirius. This is an open galactic cluster, around 200 million years old, so many of its massive stars have already finished their main sequence phase and moved on to become red, luminous giant stars. It gives this cluster a very nice array of colors in deep photographs.

There is one other very nice cluster present as well: NGC 2362, also known as the Tau Canis Majoris cluster since it is an array of stars surrounding the relatively bright Tau CMa. This is a much younger cluster, less than 25 million years old (perhaps as young as 5 million years old), and Tau is a very hot O-class star at the heart of it. This is a 40-50 solar mass star with an incredible 50,000 solar luminosities, so it won't last long. Already the powerful wind from this young cluster (as well as nearby UW CMa) has cleared out a cavity in the surrounding interstellar medium, creating a glowing bubble known as Sharpless 310 visible on red prints from the Palomar Sky Survey.

A very beautiful nebula is found above the head of the dog by following a line from Sirius to Muliphein and going that far again in the same direction. This is NGC 2359, otherwise known as Thor's Helmet thanks to its distinctive shape. This is a smaller version of an interstellar bubble, blown up by a massive star nearing the end of its main sequence lifetime. What happens, basically, is that a lot of the heavier elements in the core like Carbon, Nitrogen and Oxygen, get dredged up to the surface, and they are more opaque than Hydrogen and Helium. As a result, the outgoing light from the star pushes a lot harder on these atoms. This is known as radiation pressure, and it causes some of the atmosphere to get carried away, pushing aside any surrounding interstellar gas and dust as well.

Although the Milky Way crosses through one side of Canis Major, there are still a few interesting galaxies visible in this area of the sky, probably out of reach of most amateur telescopes, though. One very nice spiral near Adhara is NGC 2280, very similar in morphology to our own galaxy or the Andromeda galaxy. A bit further east is a pretty barred spiral galaxy known as NGC 2217 with a noticeable outer ring, perhaps the result of a merger in the recent past.

The Hubble Space Telescope caught an ongoing merger of the two spiral galaxies NGC 2207 and IC 2163 in a striking photograph. You can see the tidal effects stretching out both galaxies in the image, and we'll see a lot more of this once we move to some constellations that are well outside the plane of the Milky Way.

The final galaxy I'd like to mention is the Canis Major Dwarf, a recently discovered small galaxy all but invisible to all but the largest telescopes. This small galaxy has been captured by the Milky Way and slowly torn apart over the past few billion years. It is now just a tiny fragment of its former self as tidal forces have strung out the stars in this galaxy over a weaving path nearly a million light years in length. Follow this link to see some very nice animations of how Astronomers have reconstructed this process. This is officially the closest galaxy to our own now!

Posted by Observer at January 28, 2008 01:19 PM
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