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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
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
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
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