Archive for August 2nd, 2008

8/1/08: Launch Pad, day 2

Launch Pad is Astronomy 101 in a week. Some of us are already getting a little crispy around the edges.

Steve Gould has a cute tiny Asus “eee” laptop. I have discovered that you can get Mac OS X to run on it. ::wants::

Mike Brotherton started off the day with some introductory remarks. Observational astronomers, he says, are night owls; theorists are the ones who schedule 8am classes. He revealed that in academia it’s standard to pay for publication of your accepted papers ($150 a page or so); this helps keep the academic journals afloat. He also shared some useful URLs, then gave us a lecture on the electromagnetic spectrum.

Almost everything we know about the universe outside of the Earth (except for some moon rocks and space dust) comes to us in the form of light and other electromagnetic radiation. He explained the relationship between frequency, wavelength, and the speed of light, and how light refracts and is broken into the spectrum by a prism because the speed of light in glass is lower than it is in air and varies according to the wavelength (the frequency stays the same, but the wavelength changes as the velocity of the wave goes down). Light is also a particle, of course, and the energy of each photon is determined by its frequency. This isn’t the same as the intensity of the light, which explains why you get sunburn from high-energy UV photons but no harmful effect from even a very intense green light.

“Black bodies” are objects that absorb light equally at all frequencies. These objects also emit light at all frequencies when they are hot. The term “black body radiation” refers to the characteristic spectrum of such a body, which peaks at different frequencies depending on its temperature. The total amount of energy emitted also depends on the temperature — if you double the temperature (measured in degrees Kelvin, i.e. degrees above absolute zero) you increase the energy by a factor of 16!

Telescopes come in two basic flavors: refracting (lens) and reflecting (mirror). Reflecting telescopes are lighter and don’t have chromatic aberration (the red and blue fringes you can see on bright objects when you look through the edges of thick glasses like mine), but the focal plane where the image appears is on the same side of the mirror as the object being observed — this is not much of a problem in real life, you can put the sensor there, or a mirror to redirect the image somewhere else, without interfering with the telescope too badly. Reflecting telescopes are also much easier to make big, and the bigger (in diameter) the better.

Modern professional telescopes use adaptive optics (tiny rapid changes in the mirror to compensate for atmospheric disturbances) and long-baseline interferometry (using several small telescopes to simulate a much larger single telescope) to achieve results nearly equivalent to space-based telescopes. However, space-based telescopes can see frequencies no ground-based telescope can see through the atmosphere, including infrared and X-rays.

Danny Dale then gave us a lecture on dust in space (say it with me: “Duust… iiiinnn… SPAAAAAACE!”) which was reasonably interesting, but as much of the presentation was seemingly meant for other astrophysicists (lots of charts) I didn’t get as much out of it as I would have liked.

Jim Verley led us through a hands-on exercise in which we got to look at glowing tubes of several different gases through diffraction gratings, trying to identify the gas by comparing the spectral lines we saw with charts of several common elements. The exercise was very cool and a lot of fun (I have never seen a band of pure teal light before), and clearly showed us that the difference between theory and practice is always smaller in theory than it is in practice. I was reminded of the classic Electron Band Structure In Germanium, My Ass.

Next up was Jerry Oltion with a couple of exercises in back-of-the-envelope calculation. He started off with an easy one: how much does a cow weigh? The answer, no shit, started off with “posit a spherical cow of uniform density…”. The next question was “if we want to build an accurate scale model of the solar system, including Pluto, inside this 30′ long classroom, how big is the sun, how large are the planets, and how far are the planets from each other?” Jerry brought an assortment of spherical objects to help visualize this (“I have the minor planets here in a bag…”).

We started off with a beachball-sized sun, which makes the Earth a 1/10″ diameter BB 100 feet away; Pluto would be an insignificant speck 4000 feet away (nearly a mile!). From there we made the sun smaller and smaller (softball, tennis ball, ping-pong ball, marble…) until we finally got down to a 0.9″ mustard seed. At this scale the solar system (well, not the diameter of the solar system, but all the planets strung out in a line to scale) just fits in the classroom. Earth is a tiny speck 9″ away, Jupiter is smaller than a grain of salt at 45″ away, and Pluto is an even tinier speck 30′ away. There’s a whole lot of empty space in the solar system. Furthermore, at this scale Alpha Centauri A and B would be a pair of mustard seeds 20-30′ from each other… 31 miles away!

The width of your finger held at arms’ length is about 1 degree of arc, by the way.

The final exercise was to view the space station docking scene in 2001 and determine its gravity, using the equation v2r = g. The station rotates once per minute and, based on the heights of the people visible in some windows, is about 150 meters in radius. This means the circumference is about 1000 meters, so v is 1000 meters per minute, which yields a simulated gravity about 1/6 of Earth’s — the same as the moon (though the people inside move as though the gravity is Earth-normal). The very tidy numbers suggest that Arthur C. Clarke told the special effects guys exactly what to do.

I had a lot of fun with the back-of-the-envelope calculations. My father did this sort of thing with me all the time when I was a kid. Some other members of the workshop were left behind, though. I imagine they must feel the way I feel when I see tanned and fit people on sailboats who just hop into the water and swim to shore for lunch.

The day ended with a party at Mike Brotherton’s house, where we chatted with members of the UWyo astronomy faculty and saw the Milky Way (faintly) and a couple of meteors. Tomorrow night we go to the big WIRO telescope up on the mountain.