Friday, 17 April 2009

A whole train of pinwheels!

One of the ways in which astronomy (and, in my humble opinion, galaxy astronomy especially) is revoltingly lucky compared to a lot of science is the outrageous beauty of our subject - though a lot of biology could make use of that too. Most weeks I could begin a blog post with "It's obviously pretty pictures week on the forum right now!" This week is certainly one such week, and I just feel like giving a tour.

Earlier today, Geoff put up a collection of pictures of the aurora, which are well worth a read. A few days ago, one of our newbies, Terry, started a thread called "space candy", many of which are SDSS treats we have tasted and loved. Incidentally, Terry's special web page about his experiences as a technician at the Mauna Kea Observing Site was fascinating. We know the big names like Newton and Herschel. Just like we know the names of kings and gallant generals. But history doesn't record all the technicians, the teachers, the mothers, the helpers on the forum. Without these quiet souls, the big names wouldn't have done so well. So I was glad when Terry got a round of applause!

Never mind; whether history records us or not, we'll have a great time. Take Tuesday's Astronomy Picture of the Day: the familiar Pinwheel Galaxy, also known as M101.

Credit: APOD.

It's often seen on the Galaxy Zoo Forum, looking like this:

Credit: SDSS.

The two pictures both show it as blue, because of its high star formation rate (mentioned in APOD). It's also an emission line galaxy - no surprise! Countless billions of atoms are so hot that their electrons are falling, radiating light energy of a very specific wavelength, as unique as a bar code: our way of telling what substances are there.

APOD also mentions that the galaxy is large enough to distort its neighbours. But look at M101 itself. Its own arms are hardly symmetrical - the one starting at six o'clock appears to be flying off into space altogether, the one trailing along behind at nine o'clock seems to fancy doing the same thing, and the one starting at twelve o'clock has a great gap and then a bunch of star formation on the far side of that gap. (Isn't it a good thing the galaxy is anticlockwise so the "timing" I'm using appears logical?)

Anyway, Waveney had the sense to zoom out on the Pinwheel Galaxy and showed us what might have happened:

M101 is very near to us, which means it's easily studied. You can have a look at it in gorgeous detail, though without the blue, on Hubble. Here is a huge file - and I mean seriously huge! This is a smaller copy:

Why do spirals so often turn up in nature? Whirlpools, ammonites, hurricanes . . . We argued endlessly about anticlockwise galaxies (which the bias study proved no more common than clockwise in the end), we worry about viewpoint of galaxies and from opposite ends of the Earth . . . and if you search for the Pinwheel Galaxy on APOD, this is what you get.

(Credit: APOD.)

Now looking at M101 in a new light: the Chandra X-ray telescope. (The good folks at which, incidentally, I wrote to recently with a question - I hope to hear from them soon!)

X-rays in space are just the same sort we use in hospitals: a kind of light whose wavelength is far too short for our eyes to see. That means its frequency and its energy are very high, and they're not something we'd like to get in the way of too often! (Hence, doctors and nurses leave the room when they give you X-rays - repeated exposure would definitely harm them.) X-rays are given off by high energy nuclear fusion - we get a few from the Sun, but most photons created in there have spent too many thousand or million years banging into one particle after another to still be that energetic when they reach the Sun's surface.

(Credit: Chandra. That page is well worth a look in detail - for example, it has a nice little video.)

The visual and the infra-red seem pretty similar, spanning pretty much the whole galaxy. That's to be expected; they're weaker forms of light radiated by stars. The X-rays mostly clump together in the middle, in definite dense and less dense areas - with just a few bright X-ray spots in the very places where the arms look very blue and bright. Chandra has another composite image helped along by Spitzer, which was excited enough about a discovery of its own concerning our familiar Pinwheel last year.

One last picture, specially from Spitzer. This is an infra-red image, the pink areas showing dust clouds and the starlight in blue. That one shows what happened in the least disrupted arm with all the star formation occurring outside a "gap" - it looks like a lot of dust got flung out there . . . The more disrupted arms appear to have little gas left. I wonder if that means they'll disappear in a few million years?

(Credit: APOD.)

Finally, I'm delighted to notice that in the Spitzer image, and in Chandra's infra-red, we can see the tiniest little bar. Just as Johan Knapen said at Astrofest, infra-red light is the best way to find bars. Now after all those pictures you will probably want to head to a real bar. Bring some space candy with you!

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