HST Captures a View of Four of Saturn’s Many Moons
So, last entry I had you examining the details of an image of a galaxy pair some 70 or so million light-years away. Today, we’re going to look at at the details of an HST image of a scene that played out some 70 or so light-minutes away at the planet Saturn.
Four of Saturn's moons transiting the planet as seen by HST. (Click to embiggen -- caution-huge image!)
If you click on the smaller image, you’ll get a much bigger one that shows the details of a rare transit of four of Saturn’s moons: Titan (the large one at the top of the limb of Saturn), Mimas (below Titan and casting a shadow near the rings) and bright Dione and fainter Enceladus off to the left. These transits only happen from our point of view when Saturn’s ring plane is nearly edge-on as seen from Earth.
Later this year, on August 10 and September 4, 2009, the ring plane will appear perfectly edge-on; however, we won’t be able to see that rarity because Saturn will be too close to the Sun for good viewing. These happen periodically though — in another 14-15 years we’ll get another chance to see the rings edge-on again.
Gaze at this image (particularly the large one) for a while — note the faint banding in Saturn’s atmosphere and the sharp shadow of Saturn’s rings darkening the cloud tops. For more information on the image and how the HST folks got it, check out the web site news release. It’s got details about the exposures used, the observation times and much more.
There’s even a nifty video sequence of four “eclipses” as the moons transit the planet. You can see it here.
And, thanks to Andy Chaikin for pointing out that there’s an even COOLER pic of the transits on the Hubble Heritage site. Those moons are lined up quite nicely!
A Hubble Heritage view of the Saturn transit. (Click to embiggen.)
No doubt about it, there are strange and wonderful things to be seen in the cosmos. If you point a telescope at some area of the sky, you’re going to find weirdness. Oddities… and they stay odd until we figure out a way to explain them. At that point, they become part of the cosmic zoo — interesting, odd, but at least we kinda sorta understand them. And, the longer you point your telescope, the deeper you look and the more you find. Sometimes you even get more than you bargained for.
A color composite image of Arp 261 created from images obtained using the FORS2 instrument on the ESO Very Large Telescope (VLT), at the Paranal Observatory in Chile. (Click to embiggen.)
That’s true of the main object in today’s image released by the European Southern Observatory. At first glance, it looks like strange lump of stuff — star-studded, clumpy clouds that have a kind of barrel-like appearance. You might think this is a planetary nebula or a starbirth cloud in our own galaxy until you look at it closely. What we are seeing here is a pair of galaxies that have collided in a slow-motion dance with each other. The action of the dance is disrupting their shapes, mingling groups of stars and clouds of gas and dust.
This slo-mo tango is taking place about 70 million light-years away from us. We can’t watch it in real time because these things take a long time to unfold. So, our views are more like cosmic snapshots of an encounter that will go on for millions of years.
Although individual stars probably aren’t going to collide, the huge clouds of gas and dust certainly do crash into each other at high speed. Those collisions lead to formation of bright, hot new stars that you can see in the blue-ish regions.
The paths of that the existing stars take as they move through their galaxies get disrupted and maneuvered into new paths. These show up as swirls of light extending to the upper left and lower right of the image.
The two galaxies involved in this cosmic dance were not spirals but more like misshapen dwarfs, similar to the Magellanic Clouds that orbit the Milky Way. They weren’t what astronomers went to look for first in this part of the sky. That would have been the ordinary-looking object just to the right of Arp 261 (indicated by the lines in the second image).
A core-collapse supernova called SN1995N (indicated by lines) was the actual subject of the images astronomers took of Arp 261. They got more than they bargained for. (Click to embiggen.)
This object turns out to be an unusual exploding star, called SN 1995N, that is thought to be the result of the final collapse of a massive star at the end of its life. Astronomers call these objects “core collapse supernovae” (appropriately enough). SN 1995N is fading very slowly, more slowly than most supernovae. It’s bright enough that it still shows clearly on this image more than seven years after the explosion took place — from the distance of Arp 261. It is also one of the few supernovae to have been observed to emit X-rays. That probably happens because the exploding star was in embedded in a dense thicket of material, and as its blast waves moved out, they plowed into the material at high speed — creating conditions hot enough to emit x-rays.
This image has more than just a distant galaxy pair and an exploding supernova in it. If you look closely at an enlarged version, you can find two small asteroids that orbit in our solar system between the orbits of Mars and Jupiter. They happened to be crossing the field of view during the observations. They show up as the red-green-blue trails at the left and top of the first picture I’ve posted here. The trails happen because asteroids are moving during the exposures and also between the exposures through different colored filters. The asteroid at the top is number 14670 and the one to the left is number 9735. Each of them is probably less than 5 km across. Their reflected sunlight takes about fifteen minutes to get to the Earth.
The next closest object in the scene is a star that looks bright in the image but would be impossible to see with the naked eye. It’s most likely a sun-like star and it lies about 500 light-years from us. (For reference, Arp 261 itself, and the supernova, are about 140,000 times farther away than this star). If you look carefully, there ARE objects much more distant than Arp 261, which is in our cosmic neighborhood by comparison. These are galaxies that make up a cluster that is visible on the right of the picture. And beyond that? There’s something even more distant — we just have to look longer and deeper to see it!
