Saturday, July 03, 2010

The reference... II

Profound Lore Records recently issued the new full-length from Chicago’s progressive/ unclassifiable act Yakuza. Entitled Of Seismic Consequences, the cover features a famous HST picture of the Cat’s Eye Nebula (image details here).



The Cat’s Eye Nebula (NGC 6543) is a well-studied example of a planetary nebula. The name “planetary nebula” is a misnomer; such objects have absolutely nothing to do with planets. To the first observers of such objects, the nebulae appeared as roughly spherical objects with subtle hints of color, mainly green or, in the case of the Cat’s Eye Nebula, blue.

Roughly evocative of planets, and to the chagrin of introductory astronomy students everywhere, these objects were dubbed “planetary nebulae” by William Herschel in England just after he had discovered the planet Uranus. To Herschel, there were striking similarities between the appearance of Uranus and many planetary nebulae, the Cat’s Eye Nebula being a prime example.

A planetary nebula is formed at the end of the life of a medium mass star (from about 0.8 to 8 solar masses), such as the Sun. After such a star has exhausted its hydrogen fuel in its core, the star briefly contracts under its own gravity, causing the core to heat and begin fusing helium into carbon and oxygen. The star’s outer envelope expands due to the helium fusion within the core, cools, and takes on a reddish color. The star is now a red giant.

A wind generated by the helium fusion occurring in the core begins to blow off the star’s outer envelope, which glows as it expands outwards to form a planetary nebula. The beautiful structure of the nebula depends upon such variables as the star’s rotation, details of the magnetic field, and whether the star has a binary companion. After a few tens of thousands of years, the nebula disperses into the interstellar medium to become part of the next generation of stars. The core of the original star that is leftover, after helium fusion stops, is a white dwarf roughly the size of the Earth. This is the Sun’s eventual fate, as the Sun will swell into a red giant about 5 billion years from now.

The Cat’s Eye Nebula is visible to most Northern Hemisphere observers throughout the year, being close to the North Celestial Pole (Southern Hemisphere observers are out of luck on this one). Unfortunately, the nebula is in the rather dim, sprawling constellation of Draco, and a small telescope is really needed to observe the nebula itself. However, from a very dark sky site, eagle eyed observers with binoculars would be able to spot the nebula as a small, spherical, blue splotch.

A bit of “star hopping” is needed to find the nebula. To begin, first spot the Big Dipper. Use the Pointer Stars of the Big Dipper (see the box in the July Sky Dome below) to find Polaris, the North Star. To use the dome, print it out, hold it overhead and face North (the chart is accurate for anyone in the mid-latitudes of the Northern Hemisphere for early July after dusk).



Next, use the detailed chart below to find the Little Dipper (Ursa Minor), of which Polaris is the star at the end of the handle. Star hop to find NGC 6543, which is just slightly east of the bowl of the Little Dipper. The nebula occupies nearly the same position as the North Ecliptic Pole (east is to the left on the chart, so hold the chart upside down while facing North).



Through a medium aperture telescope (at least 12”), the central star of the nebula is visible (use “averted imagination,” as amateurs like to joke) as a bright center to the nebula, which appears blue. Wispy structure is visible in very large aperture telescopes.

The Cat’s Eye Nebula is one of my favorites, and is a detail in my starscape tattoo…



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2 comments:

Invisible Oranges said...

Really cool! I actually understood most of that (go me). And I definitely enjoyed reading it more than I liked listening to the Yakuza album.

dschalek said...

Cosmo,

We looked at this object when we went to Mt. Pinos. By the way, my next trip up to the mountain (you are, of course, invited) will be the 10th, and I'll be testing out a new telescope that was donated to me.

I've given the Yakuza album a listen, but only for a few songs. Not really my thing, but I can see how some would go nuts for it.