December 28, 2007


Petrus Plancius was a Dutch astronomer and cartographer who was keenly interested in maps and navigation as one of the founders of the Dutch East India Company. In 1613, in an attempt to bring a little more order and structure to the map of the sky, he introduced several new constellations into the modern library in places on the sky previously blank. Close to the North Pole was a region previously considered by the Greeks to be a "desert in the sky," and into this region, Plancius introduced the constellation Camelopardalis, the Giraffe.

This faint constellation spans a pretty large area of the sky adjacent to the North Celestial Pole. Surrounding it in the other three directions (going clockwise around the pole) are Cassiopeia, Perseus and Auriga. The Giraffe is so faint that none of its stars have proper Arabic names, and there is no mythology surrounding it, but as with all areas of the sky, there are some interesting objects here.

Alpha Cam is a very rare blue-white O giant. Such stars are so hot that their spectra show almost no features as all of the atoms that would normally show spectral lines have been completely ionized. They also live very short lives. Beta Cam is a yellow supergiant star likely in transition from the main sequence to the point where Helium will ignite in its core, and while most stars in this particular state would tend to vary or pulsate like Cepheids, Beta Cam doesn't. It does seem to flash periodically (doubling its brightness during a time interval shorter than a second) like a few other stars, perhaps due to a magnetic-field flaring effect on a larger scale from solar prominences that cause geomagnetic storms we are familiar with on Earth, Kaler speculates.

RU Cam is without a doubt the strangest Cepheid in the sky, or at least it used to be. From the time it was discovered, it was a fairly reliable Cepheid variable, pulsating with a period of about three weeks, and it had been very slowly changing (which itself is unusual for Cepheids, normally reliable to within a few seconds). Near minimum, it had a spectrum of an R-class star, which means numerous bands associated with Carbon atoms appeared in its spectrum. In theory, Cepheids are supposed to gradually transition in and out of their pulsation phase, but it is supposed to take on the order of 1000 years, so we have no idea what happened here. Nor has it been seen in any other star! Rest assured, it is being closely monitored. If and when it does anything unusual, the result will be at least five dissertations and 100 journal articles about it.

Z Cam is another oddball. It was the first dwarf nova ever detected. It appears as a sun-like star in its "normal" state, but every 2-3 weeks it undergoes an outburst over the course of 2 days that increases its brightness by a factor of about 10. Sometimes, these outbursts will occur regularly, but then it will go into hibernation, remaining at an intermediate brightness for months at a time before starting outbursts again. The speculation is that this is a sun-like star in orbit around a white dwarf with a very short period.

It is the remnant of a classical nova outburst, the expanding shell of which can still be seen moving away from the system. When the mass transfer rate from the sun-like star is slow, then you get regular outbursts like a normal nova system, but sometimes, some unknown process causes the transfer rate to accelerate, so you don't get the build up and the eruption but instead a steady powerful wind from the dwarf getting rid of the infalling heated gas. BZ Cam is another strange repeating dwarf nova seen in this striking image.

For the deep sky objects in Camelopardalis, I'll start with IC 342. This galaxy is actually quite close to us, only three times further away than Andromeda, which is the nearest large galaxy to our own. It is a member of the Local Group of galaxies, a loose collection of 30 galaxies close to our own and gravitationally affecting one another (though not technically "bound" together due to the influence of other nearby clusters), dominated by the Milky Way and Andromeda galaxies. The only problem with seeing it is that this galaxy is very near the plane of the Milky Way, also known as the "Zone of Avoidance".

There was a time when Astronomers were first trying to understand galactic nebulae. We didn't know whether they were clouds nearby in our own galaxy or distant objects, but we knew that few were seen near the plane of the Milky Way Galaxy. Perhaps our galaxy exerted some sort of repulsive force on these nebulae, keeping them above or below the plane, some speculated. Now, however, we can see by looking at longer wavelengths (which are better able to pass unhindered through the gas and dust in the disk of our galaxy) that galaxies uniformly populate the sky.

Another spiral galaxy just outside the local group is NGC 2403, a modest spiral galaxy that was home in 2004 to the brightest supernova explosion we've seen since Supernova 1987A in the Large Magellanic Cloud. With a magnitude of 8.4, this is one of the brightest northern sky objects that Messier missed in his catalog.

Closer to home is NGC 1502, a small cluster of about 50 stars that makes nearly a perfect equilateral triangle with Alpha Cam and Beta Cam. Pointing like an arrow to this cluster is a chance nearly linear grouping of about 20 stars known as Kemble's Cascade, a nice binocular object if you have a good finder chart for it. If you draw a line from Caph to Segin in Cassiopeia, across the tips of the W, then double the length of that line, it should end at NGC 1502.

Another galaxy in this region is NGC 1569, a small starburst galaxy about the same distance from us as NGC 2403 but apparently not a member of the Local Group. This galaxy has been undergoing a chain reaction of star formation and explosions for probably the last 25 million years. This is one of the few galaxies in the sky whose spectrum is blueshifted, meaning that a component of its motion is toward us. Most galaxies are redshifted due to the expansion of the Universe, but some nearby galaxies have random motions superimposed on top of the "Hubble flow" that cause them to have blueshifts.

Posted by Observer at December 28, 2007 06:00 PM

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