Sunday, August 30, 2015

Two Giant Black Holes Whirl at the Core of an Active Galaxy

Using the Hubble Space Telescope, a team of astronomers has found that a superbright (active) galaxy is powered at its core by two supermassive black holes whirling around each other in only a bit more than one year. This remarkable galaxy is called Markarian 231, after the Armenian astronomer who made a catalog of such active galaxies (with unusually bright centers.)
The galaxy is almost 600 million lightyears away, so we cannot see the tiny area in the center which contains the black holes directly. But a detailed study of the ultraviolet light from the core of the galaxy strongly implies that a black hole containing enough material to make 4 million Suns is whirling around a much larger black hole (with mass inside for 150 million Suns.)
Think about those numbers! You would not want to live near such overweight black holes, but luckily they are confined to the central regions of galaxies (including our own Milky Way) and are not a feature of the galactic outskirts where our solar system resides in comparative peace.
The idea that enormous black holes like this can share the same environment is not news to astronomers, but it's nice to have such a clear example. We now know that the giant islands of stars called galaxies probably all started much smaller and have been growing through "mergers" (if you'll pardon our appropriating a word from the world of business.)
Smaller galaxies are "eaten" by larger ones, or two galaxies of equal size are attracted together by their mutual gravity. If the smaller galaxies each contain a big black hole, both black holes will wind up near the center of the merged object. If the original galaxies had orbiting motion around each other, their inner black holes will have some of that motion, and can circle each other until -- later -- their gravity also pulls them together.
The fact that the two black holes in Markarian 231 take only about one year to go around means they will collide in a few hundred thousand years (a long time compared to the presidential nomination season, but short for galaxies.)
When two black holes collide you get -- surprise, surprise -- a bigger black hole. But we have caught Markarian 231 in the act of a small galaxy having been swallowed, but before the two black holes had time to merge. There are stars and huge clouds of gas and dust being pulled in by the pair of black holes and as they are torn apart and whirled around, they give off a lot of energy.
It's that energy of doomed material (before it falls into one or the other black hole) that makes Markarian 231's center so unusually bright.
The method used to find the waltzing, whirling black holes in this galaxy holds promise for finding other pairs of giant black holes in other distant galaxies. And the existence of such pairs of hungry black holes is good evidence that our merger theory of how galaxies "bulk up" is correct.

(By the way, our image, above, is a painting, based on the Hubble data.  As we said, we can't take a picture of the inner part of the galaxy.  But below is a Hubble image of the entire disturbed galaxy with its bright center.)

Sunday, August 9, 2015

Good Meteor Shower This Wednesday Night

This Wednesday evening and Thursday morning there will be one of the best meteor showers you and your family can see.

This is a great year for the Perseid meteor shower, because it is happening during the phase of the Moon called New Moon (which is when the moon is absent from the sky).  So there will be no moonlight to interfere with seeing the faint “shooting stars” of the meteor shower.

The best night is the evening of Wed, Aug. 12 and morning of Thur., Aug. 13, although there could be significantly more meteors on the night before and the night after too.  Meteors (which have nothing to do with stars) are pieces of cosmic dust and dirt hitting the Earth’s atmosphere at high speed and making a flash of light.

The peak of the shower is predicted to happen around 1 am PDT Aug. 13th, which means that after it gets dark on Aug. 12th, there should already a good number of meteors to watch for.

Here are a few tips for best viewing:


1. Get away from city lights and find a location that’s relatively dark
2. If it’s significantly foggy or cloudy, you’re out of luck
3. Your location should allow you to see as much of the dome of the sky as possible
4. Allow time for your eyes to get adapted to the dark (at least 10 -15 minutes)
5. Don’t use a telescope or binoculars – they restrict your view (so you don’t have to be part of the 1% with fancy equipment to see it; this is a show for the 99%!)
6. Dress warm – it can get cooler at night even in August
7. Be patient (it’s not fireworks): keep looking up & around & you’ll see flashes of light
8. Take someone with you with whom you like to spend time in the dark!

The Perseid meteors are cosmic “garbage” (dust and dirt clumps) left over from a regularly returning comet, called Swift-Tuttle (after the two astronomers who first discovered it).  The comet itself returns to the inner solar system every 130 years; it was last here in 1992.  During each pass it leaves dirt and dust behind and it is this long dirt and dust stream that we encounter every August.  Some experts are predicting we might briefly encounter an especially crowded part of the debris stream this time.

Each flash you see is a bit of material from the comet hitting the Earth’s atmosphere and getting heated up (and heating up the air around it) as it speeds through our thick atmosphere.  Both the superheated dust and dirt and the heated air contribute to the visible light we observe.  Since comets are left-overs from the early days of our solar system, you can tell yourself (or your kids) that each flash of light is the “last gasp” of cosmic material that formed some 5 billion years ago.

[Note: image from Mike Hanley, American Meteor Society]