Astro Bob blog: Disco ball infernosThe curious future of Mars' moon Phobos plus a look at two different ways for stars to cluster.
By: Bob King, Duluth News Tribune
Disco ball infernos
Phobos orbits above the hazy Martian atmosphere in this image taken by the High-Resolution Stereo Camera on board Mars Express on January 22, 2007. Credit: ESA/ DLR/ FU Berlin (G. Neukum)
While waiting for the latest photos from the recent close flyby of Mars' moon Phobos by the Mars Express I dug up a photo of the moon taken a couple years earlier. It's so "out there" I thought you'd enjoy seeing it. If Phobos looks small and boulderish, it's no surprise -- the little guy's only 13 miles in diameter. That's too small for the moon to crunch itself into a sphere.
Earth's moon orbits at an average distance of 239,000 miles. In this photo taken Monday, the moon is setting into snow blowing from 13,658 ft. Mt.Tom in the Sierra Nevada of California. Unlike Phobos, our moon is slowly creeping away from the Earth at the rate of 1.5 inches per year ... but that's a story for another blog :) Credit: Andrew Kirk
Phobos orbits 3,728 miles above the planet's surface or more than 60 times closer than our moon does to Earth. Mars' gravitational dominance over Phobos causes its orbit to shrink by 66 feet per century. In about 10 million years Phobos will spiral to within 2,249 miles of the surface at which point the tidal forces exerted by Mars will disrupt and shatter the moon. Remnants of Phobos will form a temporary ring around the planet. A tidal force is the difference between the strength of Mars' gravitational attraction on the front side versus the back side of Phobos. At a point called the Roche Limit, tidal forces are strong enough to overcome the smaller body's structural integrity causing it to disintegrate into fragments.
You can keep up with Phobos news by visiting the Mars Express blog.
The brightest globular cluster in the sky is Omega Centauri. It's visible from the far southern U.S. in the constellation Centaurus and contains an estimated 10 million stars. Credit: ESO/EIS
I don't know about you but we've been socked in with clouds here in the Duluth area for the past few days. The stars clusters we looked at yesterday will be well-placed in the evening sky for at least a few more weeks so there's plenty of time for viewing them. M50 and NGC 2244 are known as galactic or open clusters because they reside in our galaxy's flat disk and they're open and "airy" compared to the dense balls of stars called globular clusters. Globular comes from globulus, the Latin word for "globe". Globulars (GLOB-you-lars) resemble disco balls dangling in a vast spherical halo about the center of the Milky Way galaxy.
This is a photo of the Milky Way galaxy made by the COBE satellite in the light of infrared. Infrared penetrates the galaxy's dust clouds to show its true shape. The yellow dots I've added represent globular clusters centered on our galaxy's core or bulge. The sun and planets reside in the disk about halfway between the core and edge. Credit: NASA
Open clusters have anywhere from a couple handfuls to 10,000 stars while globulars start at 10,000 and range up to a several million stars. Beside their obvious differences in appearance and star count, each has evolved in entirely different ways. Globulars are very ancient -- 10 billion years and older -- and their origins go back to the time of the Milky Way galaxy's formation. They first appeared during a long-ago era of vigorous star formation when the massive gas cloud that was to become our galaxy collapsed under the force of gravity to create the globulars as well as the billions of stars that reside in the core and halo. Because the globulars are distributed in a sphere centered on the galaxy's core, it's likely they formed very early on and trace the shape of the original birth cloud. As the cloud spun and contracted, it gradually collapsed into a thin disk to resemble the structure of today's galaxy: a pancake-like disk with a central hub surrounded by a halo of globular clusters.
You can really see the difference between the two cluster types in these photos. To summarize: open clusters are young, reside in the galaxy's disk, have relatively few stars and break apart over relatively short times. Globulars are ancient, reside in the galaxy's halo, have far more stars and hang onto their members for a much longer time. About 1100 open clusters and 150 globulars have been discovered in the Milky Way galaxy. Photos: Bob King (left) and NASA.
Open clusters formed later from the gas and dust processed by earlier generations of stars born within the disk. If globulars are the galaxy's immortals, open clusters like M50 are the young whippersnappers. Both types of star clusters are held together by their own gravity, but the densely-packed globulars hold their many stars more tightly compared to the loose open clusters. As open clusters revolve about the galaxy's core, members stray away and the cluster dwindles. In addition to gravitational interference from random dust clouds and passing stars, each star within an open cluster also has its own individual motion. Like a wild colt, a cluster member can bolt into space on its own leaving its cozy home behind.
Other galaxies have their own assortment of clusters. In this photo of the giant galaxy M87 in Virgo, the tiny star-like points you see are just some of its more than 10,000 globulars. The blue jet next to the galaxy's core is material shot out by a supermassive black hole in its center. Credit: Hubble Space Telescope/NASA/ESA
Without enough mass to hold themselves together against the push and pull of intergalactic gas clouds or their own individual motions, many open clusters dissipate in a few million years. It's likely that a good number of the individual stars we see in the nighttime sky were once members of star clusters. New open clusters are forming all the time in nebulas like the Rosette (see yesterday's blog) because there's still plenty of recycled gas and dust in the galaxy's disk. Globulars formed a long time ago and used up the material available in the halo back then. No new globulars are seen to form in the Milky Way today.
There are open clusters that do have what it takes to hang together for much longer. Tomorrow we'll visit with the constellation of Cancer the Crab and meet one of the few survivors.