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Fuzzy is Good
updated: Apr 26, 2014, 11:00 AM

By Chuck McPartlin

A lot of the astronomical images that we see are photographs of faint fuzzies - objects like comets, nebulae, and galaxies. You might think visual fuzziness is all the Cat of the Week and astronomy have in common, but like most things in Nature, astronomical objects have fuzzy boundaries in their definitions and categories, too. Just as a furry feline can be simultaneously pet, shorthair, mammal, piscivore, and inscrutable, as can a dog, things in the sky may often be looked at in diverse ways, depending on what specific feature we're interested in.

I was out recently taking shots of two large asteroids, Ceres and Vesta, since they are appearing quite close to each other this year in the spring and summer sky. A mosaic of images taken over a couple of months would show their rendezvous. Comet C/2012 K1 PANSTARRS was in the same general area, so I added it to my list. It wasn't a good night for imaging faint fuzzes or fine detail, with high humidity, a wiggly atmosphere, and lots of light pollution, but relatively nearby things were still worth looking at.

Within the sky visible to me, there was a whole zoo of solar system bodies, large and small, in many categories. Let's take a survey of what was seen and unseen, and what slots they fit into.

The brightest object was the biggest planet in our solar system, Jupiter, which is also a gas giant planet, and a planet with rings, though they are invisible to backyard telescopes. Also visible were Jupiter's four largest moons (the Galilean moons), Europa, Ganymede, Io, and Callisto on the other side. Ganymede is the biggest moon in our solar system, bigger than the planet Mercury, but it's classed as a moon because it orbits Jupiter. Io is a rocky moon, and the other three are icy moons. Jupiter is an outer solar system object. Here you can see the moons all lined up, as their orbital planes are pretty much edge-on to the Earth.

The next brightest target was also a planet, the red planet Mars. Mars is a small terrestrial planet, rocky like the Earth and Io. Mars is an inner solar system object, with two small moons that look like captured asteroids.

Not above my horizon were Uranus and Neptune, sometimes classed as ice giants, because they contain so much methane ice, also outer solar system objects. They are intermediates between the terrestrial planets and the gas giants. They also have tenuous rings.

The next brightest object I looked at was Vesta, a large asteroid. It was bright enough that it would have been visible to the unaided eye from a dark location. Vesta is also classed as a planetoid, because it was large enough to have undergone many geological processes, such as differentiation and volcanism, that most asteroids don't. The Dawn spacecraft recently finished orbiting and mapping Vesta, and is now on its way to Ceres.

Next was Ceres, the largest asteroid, with a diameter of 600 miles. Ceres, by itself, is almost 32% of the mass of the entire asteroid belt. It is large enough that its gravitational force is sufficient to squish its rocky constituents into a spherical shape - hydrostatic equilibrium. When Pluto was reclassified as a dwarf planet, Ceres was promoted to be one. This is a comeback of sorts for Ceres.

The first asteroid discovered, in 1801, Ceres was for decades considered a planet, as were some of the larger asteroids found later; Pallas, Juno, and Vesta. The term asteroid was coined by Sir William Herschel, the discoverer of the planet Uranus, to emphasize their stellar appearance through a telescope, and to convey his opinion that they should not be considered major planets. With the advent of photography, and the subsequent rapid discovery of dozens more asteroids, they were indeed reclassified as asteroids, or minor planets. Shades of Pluto in 2006!

The Dawn spacecraft is scheduled to enter orbit around Ceres in about a year. And in July of 2015, the New Horizons spacecraft will encounter Pluto and its five known moons. Stay tuned for some great images of dwarf planets.

Ceres was recently observed to be outgassing water vapor, a comet-like characteristic.

The dimmest thing I looked at was Comet C/2012 K1 PANSTARRS. Comets are considered small solar system bodies, like asteroids, but they may appear quite large. They've got a lot of ice, and as the Sun warms them, the ices sublimate directly to vapor, spitting off bits of dust and rock. The vapors and particles together form a large tail, with the gasses fluorescing in the ultraviolet light of the Sun. A comet's tail can span millions of miles, and yet the actual solid body of the comet may be only a few hundred meters across. C/2012 K1 will be at its brightest in late summer, and won't return to the inner solar system for almost a million years.

Comets seem to originate in the icy outer solar system, either from the Kuiper Belt, a flattened disk of icy bodies beyond Neptune, or the Oort Cloud, a spherical shell of icy debris that is thought to have a radius of up to two light years. That's the ultimate edge of our solar system, not the heliopause a mere 17 light hours away that was just breached by the Voyager I spacecraft.

There are objects in our solar system that are intermediate between comets and asteroids. These are primarily rocky bodies that may outgas like comets when they get close to the Sun, and they are called Centaurs, as appropriate for a hybrid creature. One that recently made the news is Chariklo, which was found to have a ring system when it was observed occulting a star in March.

Clouds of cometary debris in orbits that intersect the Earth's orbit give us our annual meteor showers, like the Perseids in August. But, just to fuzzy things up, there is a rocky asteroid, 3200 Phaethon, that passes so close to the Sun that it spalls off rocky fragments as it heats up, giving us the Geminid shower in December.

And, although they aren't solar system objects, and have long been considered hairless, there's some recent work on black holes that indicates they, too, may be fuzzy. So, just when you think you've got a nice clean definition of something, all you have to do is look at it from a different angle, or perhaps squint, to see that it's really a furry fuzzy dust bunny of a universe, after all.

References for a Cloudy Evening




New Horizons


Hairy Black Holes


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