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Cats Eyes in the Skies
updated: Jun 07, 2014, 11:00 AM

By Chuck McPartlin

Astronomers, with their tendency to skulk around in the darkness, often feel an affinity for nocturnal animals like owls and cats. The eyes of these creatures have evolved to perform well at night, and often have an interesting appearance and structure, like vertical slit pupils and a reflective tapetum. Some astronomical objects have been likened to the eyes of cats, so let's take a look at them. We'll start with another mainly nocturnal animal.

High over Santa Barbara's ocean horizon in the hours around midnight in early June is the constellation Scorpius, the Scorpion. To the ancient Greeks, this represents the scorpion that killed Orion, the Hunter, after he boasted that he would kill every animal on the Earth. To keep them from feuding, Zeus placed them on opposite sides of the sky. The bright red supergiant star at the heart of Scorpius is Antares, Greek for "rival of Mars", and you can compare them in our skies this summer, since Mars is in Virgo, westward along the ecliptic from Scorpius. (Constellation image from Starry Night Pro.)

There are no scorpions in New Zealand, so the Maori tell a different tale. One day Maui was out fishing, and he caught what he thought was a large fish. When he pulled it up, he found it was the north and south islands of New Zealand, and all of the inhabitants. He was so surprised, he threw his hook into the sky - the Fish Hook of Maui.

The two bright stars that form the stinger of the scorpion are often called the Cat's Eyes. From left to right they are Shaula and Lesath, both names coming from Medieval corrupted versions of Arabic for "stinger" and "poisonous bite".

Shaula is about 570 light years away, and Lesath is a bit farther at around 580 light years. Shaula seems to be a complex system of three stars, resolvable by spectroscopy and interferometry, but not by backyard telescopes.

Let's move a little farther out, to another feline. The Cat's Eye Nebula, also known as NGC 6543, is a different kind of animal, known as a planetary nebula. Planetary nebulae are the end stages in the lives of low-mass stars like our Sun. As the star uses up the hydrogen fuel in its core, it begins to sputter, and swell and shrink. Eventually, the core collapses until it is hot and dense enough to start fusing helium, and it burps off its outer layers with a strong burst of stellar wind. The ultraviolet light from the hot core, now a white dwarf star, causes the ejected layers to fluoresce, just like a neon light. They really have nothing to do with planets, but many appear small, round, and greenish-blue in a telescope, like Uranus and Neptune, so they were called planetary clouds.

The vivid green color of the Cat's Eye Nebula is from ionized oxygen. It is about 3,300 light years away, and was first described by William Herschel in 1786. He also coined the term planetary nebula. NGC 6543 is in the constellation Draco, the Dragon, winding between the Bears in our northern sky.

My small scope and DSLR don't do justice to the intricate structure within the nebula. Here is a composite view made from X-ray, visible, and ultraviolet data collected by NASA's orbiting Chandra and Hubble Space Telescopes.

The distances and absolute dimensions of planetary nebulae are generally not very well established, since these objects vary greatly in their behavior. For some of the closer ones, like NGC 6543, the distances have been determined by comparing Hubble images over a period of several years, combined with spectroscopic measurements. The spectra reveal the radial velocity of the expanding material via the Doppler shift of the emission lines. The Hubble data show how much the material has expanded in angular extent between images. These values give you a pretty good estimate of the distance, from which you can then get the physical dimensions, too.

Even larger and farther away is yet another object - the Cat's Eye Globular Cluster, also known as M4 or Messier 4 for its catalog designation. Globular clusters are gravitationally-bound, roughly spherical assemblages of tens of thousands of mostly very old stars. Around 200 of these swarms of stars orbit our galaxy in a scraggly spherical halo reaching out tens of thousands of light years. They seem to be baby failed galaxies that were forming at the same time as the Milky Way, roughly 12 billion years ago. The Milky Way grew bigger quicker, and sucked away all their star making material.

M4 is one of the sparsest and closest globular clusters, at a distance of 7,200 light years, and an estimated 30,000 stars. It is visible in binoculars just to the right of and slightly down from Antares in the view of Scorpius shown above. In 1993, a planet was discovered orbiting a pulsar in M4. Tiny regular variations in the timing of the radio pulses revealed the 2.5 Jupiter-mass planet's gravitational tugs on the pulsar.

The Cat's Eye Galaxy, also known as M94 or Messier 94, is a spiral galaxy about 16 million light years away, in the dim northern constellation Canes Venatici, the Hunting Dogs. A ring of star formation around its bright nucleus gives us the ocular impression. The ring could have been caused by a collision with a smaller galaxy.

There may have been a mountain lion peering from the brush as I took these images from the end of West Camino Cielo the other night, but I wasn't worried about terrestrial cat's eyes. I always observe with someone who runs slower than I do; at least, I hope so.

References for a Cloudy Evening

Cat's Eye Nebula
Messier Catalog
Hubble Images

 

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