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What’s up with the spiral arms of our galaxy?

The short answer:

This is awesomely weird. Imagine a bike wheel – each spoke is an “arm.” When the wheel rotates, the arms rotate, right? This is NOTHING like our galaxy. Imagine a beanie hat with a propeller on top. Hold the propeller still, and spin the hat underneath. That’s a little more like it. The hat is the stars, dust, gas, and everything our galaxy is made of. It spins around, passing THROUGH the arms (the propeller) of our galaxy. The arms aren’t made of anything. Whoa, what?

Okay, interested? Confused? Ready for more? Good, because that wasn’t a very accurate description, it was just enough to make you think.

Arms are Denser Areas

When I said “the arms aren’t made of anything” that’s because they’re just denser areas of the same stuff the whole galaxy is made out of, but as the galaxy spins, the arm is made up of different stars, different dust from one eon to the next. The stars and stuff glide right through the arms, though they may slow down while they’re there.

Try this at home:

1. Take a face-on mug shot of your favorite spiral galaxy (how’z about the Milky Way?)
2. Mark your 1000 favorite stars in one arm.
3. Wait 100 million years.
4. Take another photo.
5. Find those 1000 favorite stars, and where the arms are now, compared to before.
6. The arms may have moved a bit, but the stars will have moved a LOT. Some of them will be halfway around the galaxy again, some will only have moved a touch, and still others may have gone around a couple times. The arms will now be made up of different stars, but they’ll still be there.

Okay, obviously you can’t really do this at home for about 10 very good reasons, but you get the idea.

Stars Travel through the Arms (Traffic Jam)

Think about it another way. Spiral arms are like traffic jams of stars. You know how there’s always a traffic jam at the Renton S-Curves, or by the U District on I-5? If you take either of these routes, you’ll be in the traffic jam for a while, and then once you’re out of the bad area, you can get back up to speed. The traffic jam is still there though? (usually true … we’re talking about I-5 here), but you’re out, it’s someone else’s turn to bite their lip and grumble at traffic.

Same way with stars. The traffic jam is always there, but different people are in it at different times.

There’s a PERFECT animation over at: Traffic Waves.

Density Wave Motion

Now that we’ve established that spiral arms are density waves (technically a “spiral density wave”), and it’s the stars that move through it, not the arm spinning around like a bike wheel, I’ve got a shocker for you. The arms move too, just at a different speed. That’s called the “pattern speed.”

Watch this video – see how the traffic jam moves around the circle, but the cars move differently?

Now this one – it’s a little easier to see both motions with this spinning galaxy.

Density Wave Formation

How do the spiral arms get started? Umm… that’s an open question, but I can tell you a couple of useful things.

First – the galaxy rotates differentially. Oops, jargon. The middle rotates at a different speed than the outer edge. Ta-da! Differential rotation. The Sun and Jupiter do this too. A bicycle wheel does not.

This differential rotation is due to Kepler’s laws. objects closer in revolve around the center a lot faster than objects further out.

Second, the likelihood that the galaxy started as one big perfectly even ball of gas is pretty low. There were pockets of lots of gas, and pockets of less stuff. These pockets will spread out and spin into denser areas.

Galaxy Rotation

So that’s a pretty easy explanation. There are still some problems we need to solve, but that should give you an idea.

Lastly, a way in which the arms keep themselves dense is due to the gravity of more material in the arm itself: stars speed up as they enter and arm, and then slow down as they leave it – the opposite of a traffic jam.

The Wind-Up Problem

But how come the arms don’t twist themselves up? Well, that’s one of the problems with the formation hypothesis I just showed you. This is the question that led to the idea of density wave arms in the first place.

It doesn’t twist itself up because the arms are not made of material, they’re just a denser area, there’s no reason for the arm to change.

Want More?

Astronomy Cafe has some really useful, deep answers.

Check out Voyages to the Stars and Galaxies by Fraknoi, Morrison, and Wolff

Is hosted over at As(si)tronomi! (in Egypt!) Oops, actually, I think Assi herself is in Egypt temporarily, but if I’m reading the signs (.se) correctly, she and her blog are based in Sweden. That would explain the umlauts in her other blog entries.

Hey y’all!

See that 8-bit blond girl behind the “AstroInfo” desk that adorns much of my AstroInfo correspondence? Well, she’s cute, but she’s a little generic, and she doesn’t fit in some places (like my banner). I generated her with one of those avatar makers. She’s got glasses, she’s blond-ish, she clearly likes stars, and she’s clearly the author of AstroInfo – but … she lacks something.

Alice

So, it’s time for a competition! I need a new AstroInfo logo/avatar. I know at least six of you out there are great artists and logo-designers, and I’m sure there’s more I don’t know so well.

Guidelines:

– Must scale well to 128px by 128px
– Must be family appropriate
– Preferably it’ll scale up to ~400px by 400px for printing
– If you use fonts, convert them to outlines, so I don’t lose them (and tell me which font you used so I can perhaps use it in other titles and such)
– File formats: .jpg, .ai (CS3 or earlier), .psd, .pdf (if you must, but I’d like some flexibility), ask about others … oh, I’d prefer it submitted digitally, but paper submissions will be entertained.
– CC license it, at least to me. (I need to be able to make alterations, such as text stuff)
– Not an actual photo of me.
– If you give the name of the blog it is correctly referred to as “Alice’s Astro Info” (two spaces) or “AstroInfo” (no spaces)

Suggestions:

– I like the fact that she’s a person, and a girl who vaguely resembles me.
– She and I both LOVE STARS!
– Blue is an awesome color – but then again, all colors are pretty. (P.S. There’s blue in my hair)
– I’m open to all media – photos, drawings, comic-style, realism, clay … as long as you take a picture of it and I can use that … whatever.
– I’m usually not much of a fan of abstract, unless it’s full of triangles.

Extra Credit: A related banner!

– Aspect ratio of 770px by 200px

What You Win

– The competition
– My gratitude and excitement
– International recognition (hey, I have at least one reader in Portugal!)
– $0.00
– I’m willing to entertain reasonable suggestions for other compensation, but no guarantees.
– If you want them, I can arrange for free tickets to Pacific Science Center, but you have to tell me you want them. If you want tickets to see Lucy’s Legacy you’ll have to also tell me a compelling reason why I should give my comps to you and not to my parents … but we’ll see what we can do.

Judging

– Solely by me. I get to pick, and I don’t guarantee I’ll be fair.
– I promise I’ll look at anything that’s sent to be before November 10th, 2008.

Email me at [the name of this blog] with no spaces, apostrophes, or punctuation {at} gmail

Thank you!! :-)

So, I’m going to be teaching Astronomy 100 in the winter. I am dissatisfied with the text used. Please help me choose a better one.

I’m very open – it doesn’t really have to be a textbook, but it has to be good and thorough. If you don’t see one you like here, put in another.

[polldaddy poll=1020608]

You can also vote in the sidebar – I think the answers are tallied together.

The Main Idea:

The Phoenix Mars Lander’s weather instrument (a LIDAR – like radar, only with a laser) detected snow falling from clouds above the Phoenix Lander on 9/29/2008. The snow sublimed before it hit the ground, but we’re in Seattle, we’re used to celebrating snow that doesn’t hit the ground.

More?

I know, I’m sorry, I’m really late posting this. But this is a “Mars has Water!!” discovery that I actually think merits some kerfuffle (of the last eight I think there have been about two of discoveries that do).

So here’s the thing – we’ve known for since forever that there is ice on Mars (though we verified it in 1970), and in 2006 we saw evidence of water that had actually been flowing within the last year. All the other announcements are varying levels of confirmation of these facts. For instance just a month or two ago, Phoenix watched ice subliming. Before that, the various rovers and landers have imaged clouds (that include H2O) in Mars’s atmosphere.

This is DIFFERENT. This is the missing section of the water cycle – not water on the surface, not water evaporating or subliming, not water in clouds, but water falling back down again!

So, if that closed the water cycle what are we still waiting to find? We still want to see liquid water, and I wouldn’t mind seeing some of that falling snow actually hit the ground, but I’m not going to hold my breath for that one.

More Posts About the Snow:

Bad Astronomy: Snow Place like Mars
Universe Today: Snow is Falling From Martian Clouds
Martian Chronicles: Phoenix Sees Snow and Carbonates on Mars!

The Press Release:

NASA Mars Lander sees Falling Snow

“Wow, what a day. Great day to get out in the park and meet some new planets in this JANUS YOU STOP THAT RIGHT NOW beautiful solar system. Really, you work all week, on a Saturday you just need to enjoy the TETHYS GET OFF OF PANDORA GET OFF GET OFF TETHYS NO JANUS DON’T YOU GET INVOLVED TETHYS GET OFF GET GET little bit of relaxation. Maybe I’ll get to see that little blue-green number out by COME ON PALLANE COME ON STOP SNIFFING COME ON I HAVE PLACES TO BE Mars. Hope I don’t run into Uranus, though. He’s a nice enough guy but all those toilet jokes, jeez. Wow, I thought it was going to be cold, but the sun is IAPETUS GET OFF THAT COMET I’m so sorry, ma’am IAPETUS NO WAIT TETHYS YOU LEAVE PANDORA ALONE NO JANUS I SAID GET DOWN DIONE TELESTO NO IAPETUS Oh, no, is that Terra, KIVIUQ DO NOT GO OVER THERE NO KIVIUQ IAPETUS DON’T ENCOURAGE JANUS ALL OF YOU STOP THIS RIGHT NOW BEBHIONN SKOLL HYRROKKIN THAT’S IT ALL OF YOU ARE GOING TO THE POUND YOU HEAR ME THE POUN-

Oh, hey, Terra. Moons, huh? But you gotta love ‘em anyway. They’re a planet’s best friend.”

Buy one of your own … or buy one for me (hint hint!)

Check out this week’s carnival, election-style, at Lounge of the Lab Lemming!