Archive for August 4th, 2008

8/3/08: Launch Pad, days 3-4

Very busy. Having much fun. I keep forgetting keen stuff and encourage you to read other attendees’ blogs for the full Launch Pad experience (and photos).

Yesterday started off with a talk by Jerry Oltion about amateur astronomy. In the astronomy world, “amateur” is not a pejorative. Many pro astronomers are also amateurs, and many significant discoveries have been made by amateurs. Even if you have an 80″ scope at your day job, you might want to have a smaller scope in your garage because you can use it whenever you want and point it wherever you want.

Cheap telescopes often brag about how much they magnify, but the important thing for astronomy is not to make the image bigger but to make it brighter, so as to see objects too dim for the naked eye. For this reason, size counts, but the diameter is more important than the length (ahem). One danger of amateur astronomy is “aperture fever” — the desire for a bigger and bigger scope. It used to be that you had to grind your own mirrors, but machine-made mirrors are now good enough that hand-grinding is no longer necessary (though it’s still a rite of passage). You can now buy one-meter mirrors for a not unreasonable amount of money.

Telescope mounts include Dobsonian (tilt and swivel, like a cannon), equatorial (also tilt and swivel, but with one axis aligned with the North Star, making it easier to follow an object as the Earth rotates), and Jerry’s own “trackball” mount (a sphere mounted on rollers). You can get computerized scopes with GPS, once you’ve aligned them you can just key in a desired object and they swing right to it, but the affordable ones tend to be cheaply made and the set-up time may cost you as much time as you save in finding each target — also, you lose the learning opportunity and the fun of the hunt.

Why do bright stars appear to have four points? This is due to diffraction effects from the “spider” that supports the secondary mirror, which usually has four supports.

Mike Brotherton then gave a high-speed, high-density lecture on “everything you always wanted to know about stars.”

People have been studying stars for a long time and there are many “palimpsests” of earlier ideas. For example, information about stars used to be presented in charts in color order, from blue to red. Now that we know that blue stars are hot and red stars are cool, the X axis of these charts now represents temperature rather than color, but they’re still shown with the blue (hot) end on the left, so the temperature increases from right to left! And the reason the spectral classes are the order OBAFGKM (Only Bad Astronomers Forget Generally Known Mnemonics) is because the original spectral classes were in order of strength of the hydrogen line in their spectrum (A = strongest, O = weakest) but we now know that O stars are the hottest and M are the coolest. What happened to C, D, E, H, I, J, and L? They were duplicates and were dropped. But the letters are still used.

Stars can be graphed on the Hertzsprung-Russell diagram with spectral class (temperature) on the X axis and luminosity (total amount of light output) on the Y axis. Most stars fall in a roughly diagonal band from hot-and-bright (upper left) to cool-and-dim (lower right), which is called the Main Sequence. This is a tidy sequence of mass from about 18 solar masses at the top to 0.1 solar masses at the bottom. It is not a temporal sequence! Most stars spend most of their life moving slowly across the width of this band.

A star’s properties are uniquely determined by its mass and chemical composition. Bigger stars burn hotter and have shorter lives.

Stars are born in areas of dense gas and dust. Something, such as a shock wave from a supernova, causes an area of the gas to begin to condense. These protostars get hotter and brighter as they condense, but after a while their luminosity actually starts to go down, even as their temperature increases, because they are getting smaller. Shortly after fusion begins they blow off their surrounding coccoon of dust and gas and become visible; this transition is called the “birth line” on the H-R diagram, even though the star is really “born” when fusion begins. The star continues to condense and stabilize, throwing off jets of material which may in turn shock the interstellar medium into new protostars, until it eventually settles down on the main sequence at a point determined by its mass.

When a star reaches the end of its life, what happens depends on its mass. A typical star will move off the main sequence toward the upper right (becoming a giant star, which gives much more light than a main-sequence star of the same temperature because of its larger surface area), then blow off its outer envelope, leaving a white dwarf remnant in the lower left (hot but small, so not very luminous). A smaller star cools to a brown dwarf; a larger star explodes violently as a supernova.

After class we drove up to the Wyoming Infrared Observatory (WIRO), which despite its name is used only for optical observation these days. When we arrived the sky was overcast, but we toured the facility, which consists of a very ordinary-looking small house with a giant dome attached. Inside that dome was the telescope, a bus-sized spindly contraption with an eight-foot mirror on one end and a three-foot cubical box on the other. We gawked and took lots and lots of pictures. The moment when they cranked open the roof was just awesome.

Now we waited for the sun to set and hoped the clouds would clear. We ate our dinner, talked with the two grad students staffing the observatory, amused the cat (Nu Bootes, pronounced “new booties,” successor to the previous observatory cat Mu Bootes, pronounced “mew booties”), played cards and chess, and enjoyed the view. The view from the mountain was spectacular, looking like a Star Trek matte painting as the sun set. Just then it started to drizzle and they had to close the dome.

Oh well, I thought, at least we got to see the telescope. But right around the time we were getting ready to bail, the clouds parted. Huzzah!

We headed back into the dome to see the grad students charge up the instrument cluster with liquid nitrogen to reduce noise. Then we were shooed out, presumably to prevent being crushed as the giant machine turned in the pitch dark of the dome.

I spent the rest of the evening alternating between the control room, where I saw live pictures of the Ring Nebula on computer screens and asked lots of questions, and the gravel lot outside the dome, where the Milky Way came out and we gawked at the night sky. We had a pair of night-vision goggles, through which I saw a satellite and the Andromeda Galaxy.

Very, very cool. I got back to the dorm around 1am, which explains why I didn’t blog yesterday.

Today started out with a nice hike around Turtle Rock in Vedauwoo, which offered spectacular views), a little rock climbing, and pleasant temperatures, but took a lot longer than originally budgeted, throwing off the rest of the day’s schedule.

After lunch we met in the university’s small planetarium, which is rare in that it is still equipped with a traditional optical “starball” (AKA “planetarium projector” or “giant ant”). Modern digital projectors are more flexible, but there’s something about the smooth motion and ineffable “directness” of the old-fashioned starball that makes it a more engaging way of learning about the night sky. Unfortunately, optical starballs are difficult and expensive to maintain… many features of this one were not working. Our host Jim Verley gave a very entertaining talk about both the workings of the planetarium and about night sky basics.

Mike Brotherton then continued his talk about stars. More than half the stars in the galaxy are members of binary (or more) groups. Stellar evolution in binaries is complicated and depends on the two stars’ relative masses. For example, in a pair that consists of a big star and a small star, the big star will blow up into a giant star first, and its smaller companion will have the opportunity to pull away some of its outer atmosphere. If the smaller star pulls away enough mass, it may become the bigger member of the pair. Later, when it becomes a giant, the white dwarf remnant of its formerly-larger companion may pull away some of its mass in turn. In some cases the larger star may completely absorb the smaller, which can take as little as a couple of months.

If one member of the pair is a white dwarf, the matter coming into it from the other star is whipped into an accretion disk due to conservation of angular momentum. This infalling gas is incredibly hot, and may outshine the original star and emit large quantities of X-rays. Hydrogen may also settle on the surface of the white dwarf in sufficient mass to begin fusing. If this occurs, it ignites all over the star at once in a spectacular explosion: a nova. Because this only affects the surface of the star, it may happen again and again, even periodically.

Our sun will eventually (5 billion years) expand to a red giant about the size of Earth’s orbit. The Earth wll move out slightly, because the sun’s mass will have decreased by then, but it’s really an academic question whether it’s broiled by falling into the sun’s atmosphere or merely toasted by proximity. Either way it’ll be a mighty warm day. Eventually the outer parts of the sun’s atmosphere will be blown away (comparatively gently) and the core will settle down as a white dwarf.

Massive stars (25 solar masses or more) burn hydrogen at the core for about 7 million years, then helium for 500 thousand years, then carbon for 600 years, then oxygen for six months, then silicon for one day. At this point the star resembles an onion, with a silicon-burning core surrounded by an oxygen-burning layer surrounded by a carbon-burning layer, and so on. Silicon fuses to iron, but iron doesn’t fuse at all. When all the silicon is used up, the core collapses, beginning a reaction that destroys the star in a massive explosion: a supernova, which produces a flood of neutrinos and creates all kinds of heavy elements. Every atom in the universe that’s heavier than iron is the result of a supernova explosion. The remaining core becomes either a neutron star or a black hole, depending on its mass.

Supernovas are rare, occurring about once every hundred years per galaxy. Most of the supernovas we see are in other galaxies. This is a good thing, because a nearby supernova (within about 100 light-years) could kill us with the neutrino flux.

By the way, we are “on the verge” of building gravity telescopes, which could detect such things as binaries consisting of two black holes or two neutron stars, which don’t emit radiation but do emit powerful gravity waves. The basic principle is to very carefully measure the distance between two masses. If that distance decreases, that means a gravity wave is passing through, changing the shape of space.

After a supernova, if the remaining stellar core is less than 3 solar masses it becomes a neutron star, with all the protons and electrons smashed into each other to create an incredibly dense solid mass of nothing but neutrons. As the core collapses, angular momentum conservation makes it spin faster and faster, with a period of a few milliseconds. The same collapse amplifies the magnetic field by a factor of 1012. Plusars (objects that pulse rapidly in the optical and radio bands) are believed to be rotating neutron stars, the magnetic pole of which is not aligned with the rotational pole. Every time the magnetic pole points in our direction we see a pulse. It may be that all neutron stars are pulsars, but we can see only the ones where the beam from the pole happens to shine on Earth. Some pulsars wobble as well as pulsing, indicating the presence of planets.

If the core is greater than 3 solar masses, its gravity is greater than the forces within the atom and collapse continues past the neutron star phase. There is no known mechanism to halt the collapse of a compact object of more than 3 solar masses. It keeps collapsing down to a single point: a singularity, or black hole.

Escape velocity from the surface of an object of given mass goes up as the object gets smaller and denser. The point at which the escape velocity is equal to the speed of light is known as the Swartzchild radius. If the object is any smaller than this it doesn’t matter. The Swartzchild radius is the “event horizon” beyond which nothing can ever be detected. This radius scales linearly with the mass of the object (3km for an object the mass of the sun, 30km for an object of 10 solar masses… a galactic-core black hole of 1.5 billion solar masses has an event horizon as big as Saturn’s orbit).

Black holes, it is said, have no hair. This means that they lose almost all of the characteristics they had before they became black holes. The only characteristics left are mass, angular momentum, and maybe electrical charge. Electrical charge is a maybe because it’s thought that a charged black hole will quickly attract enough matter with the opposite charge to neutralize it. A black hole’s angular momentum is interesting because a rotating black hole will drag the fabric of space around with it, a phenomenon known as “frame dragging.” Even though black holes do not emit anything, we can detect them by their effects on objects around them, or by gravitational lensing of the light coming from behind them.

Because of general relativity, a clock falling toward a black hole will appear to an outside observer to slow down, and stop as it passes the event horizon. At that point the light from the clock is red-shifted, meaning that it gradually fades from view. I don’t completely understand this. However, from the clock’s perspective the event horizon is undetectable (it’s like driving past the point where you don’t have enough gas in your tank to return home). However, in practical terms, long before it reaches the event horizon the clock will be torn apart by tidal forces. This phenomenon is called “spaghettification” because the object is “stretched into spaghetti” but this is far too tidy… in reality the object is ripped to pieces because every piece of it is being pulled either up or down relative to every other piece.

One last bizarre astronomical phenomenon: gamma-ray bursts (GRB’s). These are short (a few seconds) intense bursts of gamma rays. They were first detected by the military, who were looking for space-based atomic explosions. They are thought to be jets of radiation from “hypernovas” (deaths of very massive stars over 25 solar masses) in galaxies billions of light-years away.

As detailed as that was, I’ve left tons out. I can barely take notes as fast as the slides go by. This really is Astronomy 101 in a week, but I’m having a ball.

And even while I’m in Laramie, assiduously not writing, my stories are still out there and working. I just sold a story (to a market whose name I am not yet at liberty to reveal), and “Titanium Mike” will be podcast at StarShipSofa. More details as they become available.