## Tuesday, November 29, 2011

### Interview with an Astronomer

Today, David and I conducted an interview with Richard Ellis, an astrophysics professor at the California Institute of Technology and my adviser. Professor Ellis didn't require much in the way of guiding questions; he covered a lot on his own, so instead of writing this up in a question-answer format, I'm going to try to write up his stories based on my notes. [I may interject every now and then, but rest assured my statements will be safely walled off behind brackets. Other than that, all that follows should be a paraphrase of Prof. Ellis, as transcribed from my notes. I may write my reflection on the interview in another post]

## Sunday, November 20, 2011

### Water, Water Everywhere

While I was looking things up for my previous post about water on Europa, I ran across this article. It basically summarizes the search for liquid water, both in the solar system and outside it. I haven't looked through the article thoroughly, so I can't really vouch for the accuracy or the up-to-date-ness of its data.

It did, however, remind me of another article that I'd read this past summer:
A water cloud containing the equivalent of 140 trillion times the water held in Earth's oceans has been detected around a quasar powered by a giant black hole 12 billion light years away.
Note that this is water vapor, not liquid water or ice. Also note that this cloud is huge: it spans hundreds of lightyears around a supermassive black hole. Pretty cool.

### Europa

Jupiter has 65 satellites, according to the Carnegie Institution for Science. They vary greatly (four of them make up almost all the mass, and Ganymede makes up a third of "almost all" by itself). It is these four, known as the Main Group or the Galilean Moons, that have captured the most interest, perhaps mostly because, as the largest, the are the easiest to see. They are known as the Galilean Moons because they were discovered in 1609 and 1610 by Galileo Galilei, who also famously recorded their orbits around Jupiter, bringing more support to the heliocentric view of the solar system.

As an aside, Galileo originally wanted to name the moons after the four brothers at the head of the powerful Medici family: Cosimo, Francesco, Carlo, and Lorenzo. This idea, though, proved to be less popular outside Florence, leading to the names we know today: Io, Ganymede, Europa, and Callisto, four lovers of the god Zeus in Greek mythology.

Europa, a Galilean moon and the sixth from Jupiter, is a bit smaller than Earth's moon, has a surface made up mostly of silicate rock, and probably has an iron core. It even has a thin atmosphere of oxygen. These and other geological features of the moon have led some to speculate that Europa holds large reserves of subterranean water, possibly entire oceans underground. Because of this, Europa is one of the leading candidates in the solar system for potential habitability and the presence of extraterrestrial life (well, at least since Mars let us down).

Which leads me into this new development. Geophysicists at UT Austin have possibly discovered a large (about the volume of the Great Lakes), surface lake of liquid water on Europa (and by surface, I of course mean several kilometers below the surface). The lake, like much of Europa's surface is mostly obscured by a thick ice shell, and it's not really known what is below it. A large subsurface ocean is one possibility, but another model claims that the top layer, composed of cold and brittle ice, covers a much thicker layer of warmer, convecting ice.

As far as I can tell, this new find doesn't necessarily support either model. This simulation of the formation of the lake seems to ignore that which lies far beneath the surface, instead showing how the lake can form just beneath the top ice sheet. It's possible, though, that I'm not interpreting this correctly.

This certainly isn't enough to get into the idea of actual life on Europa (though apparently some people have even been talking about colonizing it). Liquid water is, however, a step in that direction; it is also just interesting to learn more about the features of our solar system.

## Tuesday, November 15, 2011

### Stars and the Virial Theorem

The Virial Theorem is a theorem in mechanics that relates the kinetic energy of particles in a system to the potential of the system. This can be useful when talking about stars, which balance both an immense gravitational collapsing force from their huge mass with the outward force from the internal particles heated from fusion at the star's core.

## Sunday, November 13, 2011

### Professional Astronomy II

How does one become a professional astronomer?

## Wednesday, November 9, 2011

### Happy Birthday, Carl Sagan

Today would be Carl Sagan's 77th birthday. He was an American astronomer and popular science writer, known for such works as Cosmos, The Demon Haunted World, and the novel Contact, which was later adapted into a film. But my personal favorite book of his is Pale Blue Dot, from which the above speech was taken. He is referencing this picture:

 Picture from www.realscience.us
I don't know how well you'll be able to make it out, but in the line of light on the right-hand side of the picture, there is a little blue dot. That is the Earth, as seen from the Voyager 1 spacecraft 6 billion kilometers away, at the edge of the solar system. This joins another poignant picture of the Earth from space, Earthrise:

 Picture from Wikipedia

This one was taken by Apollo 8 crewmember Bill Anders, on December 24, 1968.

### The Initial Mass Function II: The Luminosity Strikes Back

Continuing the problem from the previous post (and ignoring the lame Star Wars reference), we now have 3 mass categories of stars in the cluster, and the total masses contained in each of these categories in the cluster.

Now suppose we want to find the total luminosity of the cluster. If the luminosity scaled with mass the same way for all masses of stars, then this would be easy. We could just use the total mass and convert it to luminosity using that relation.

It's not quite that simple, though, which is why in the previous post it was necessary to break the stars into categories: different masses of stars have different mass-luminosity relations. Here they are for low, intermediate, and high mass stars, respectively:

$\bg_white L=M^5$
$\bg_white L=M^3$
$\bg_white L=64M$

So, with each of the masses calculated in the previous post, we can use each of these relations to find the total luminosity for each category. The sum of these will be the total luminosity of the cluster.

I left those masses in terms of the solar mass, so finding the luminosity in erg/s will take some calculation. Help me, WolframAlpha, you are my only hope...

Once the luminosities are found, one can use them and Wien's Displacement Law to find the maximum-intensity wavelengths of the stars. Because younger stars are generally more luminous than older ones (they have more fuel which they use more wuickly), the average output radiation will be toward the blue end of the spectrum; older, cooler stars tend to be red.