Sunday, August 22, 2010

Preparing to teach

I'm preparing to teach my first introductory astronomy course.  During my 9 hour plane ride back to the states yesterday, I worked on outlining the topics I want to cover during the course.

One of my teaching resources contains a list of topics students expect to be covered in an introductory astronomy course, many of which are not covered in most courses and texts.  I figure that I will ask my students what they expect to be covered, and if there is something both popular and appropriate, I'll be sure to work it in somehow.

But I also thought I'd throw open the question to you: What topics would you hope would to be covered in an astronomy course that often are not?  I put a poll on the side of this page.  If your favorite answer is not listed, feel free to add it to the comments below. In case you are unfamiliar with topics often covered, here's a collection of astronomy course syllabi collected by Reggie Hudson at Eckerd College

Friday, August 20, 2010

Looking for planets around white dwarfs



Image Credit: NASA / JPL-Caltech

Today is the final day of the 17th bi-annual European White Dwarf Workshop.  It has been a great meeting, but I am ready to go home.

Many of the talks yesterday and today have focused on a search for planets around white dwarfs.  Since white dwarfs come from stars like the Sun, and since at least 10% of sun-like stars have planets, we would expect that 10% of white dwarfs will have planets.  We think the Jupiter and Saturn will certainly continue to orbit the white dwarf sun several billion years from now.


The problem is that it is hard to find planets around white dwarfs.  Many different methods have been tried (my own collaborators included) but there are no confirmed planets around white dwarfs yet.  Does this mean planets, even those far from a star, cannot survive a red giant?  Or do they  fly off into space when the star makes a planetary nebula?  Or have we just not looked at the right stars?  Or perhaps we aren't looking with the right methods yet?

I would guess that the problem is with our methods.  Back when astronomers were looking for the first transiting planets around sun-like stars, they didn't find any.  Based on numbers alone, it was starting to get a little uncomfortable -- we should have found some planets, but nobody had found any.  Then the astronomers involved tweaked the methods, wrote better computer programs, and took lots more data, and the first transiting planets were found.  Now we find them everywhere, and NASA even launched the Kepler Mission to look for Earth-sized planets by finding transits.

I think the same thing will happen with white dwarfs.  Once the first white dwarf planet is confirmed, smart observers will figure out better ways to get the same result, and we'll start finding planets everywhere.  Maybe even by the next conference in two years!

It is time to pack and head home.  Next week, we'll follow my adventures as a brand-new, first-time professor.  Stay tuned!

Thursday, August 19, 2010

An easy day


Wednesday was a fairly easy day, at least mentally, at the white dwarf conference I'm attending in Germany.  We only had a short morning of talks, half of which I skipped out on to do some shopping. 
In the afternoon, 150 white dwarf astronomers boarded buses for a conference excursion to the Swiss city of Schaffhausen and the Rhein Falls.

Wednesday, August 18, 2010

White Dwarf Workshop Day 2: Extreme Physics


Yesterday was the second day of this year's white dwarf workshop.   I gave my presentation yesterday, and it went well.  (In a few weeks, video of the talks will be online here. Just not yet.)

Much of the first half of the day focused on physics.  While a lot of astronomy often may appear to involve describing objects, one of our main goals in astronomy is to understand the physics behind all of the beautiful objects in the sky. 

White dwarfs are a great physics laboratory.  Because white dwarfs can contain as much matter as the sun squeezed into a ball only the size of the Earth, the material is very dense.  It is so dense, in fact, that white dwarfs create forms of matter that do not exist on the Earth.  White dwarfs are one of the few ways to study the physics of these extreme environments.

As I mentioned briefly yesterday, white dwarfs slowly cool over time.  The matter in their cores, which starts off as a dense hot plasma, also cools off.  When it cools enough, the matter changes from a plasma to a solid, in fact a crystal.  Since white dwarfs are made out of carbon, and since on Earth crystalline carbon is also known as "diamond", we often claim that crystallized white dwarfs are true diamonds in the sky.  This is not strictly true, since a polished piece of crystalline white dwarf the size of a normal Earth diamond would weigh several hundred pounds. 

Yesterday, one of the talks focused on crystallization.  Since white dwarfs crystallize from the inside out, you might think it isn't possible to catch white dwarfs in the process of crystallizing.  But as white dwarf material crystallizes, it releases heat.  Most crystals, like ice, do the same thing.  The amount of heat released is small, but when you add up a sun's worth of material, it adds up to a lot of heat.  This release of heat temporarily slows the white dwarf's cooling.

When you look at a group of white dwarfs, you will tend to see a whole bunch of different temperatures, since the individual white dwarfs formed at different times and have therefore been cooling for different times.  This slow down in cooling from crystallizing will cause there to be more white dwarfs at one temperature (the crystallizing temperature) than at other temperatures.  It is sort of like a car hitting the breaks on a busy freeway -- cars will quickly pile up where the first car slowed down, and someone watching from an airplane or helicopter can easily spot the slow down.

When we look at groups of older white dwarfs, we indeed see a pile up at certain temperatures.  The exact temperature of the white dwarf pile up depends on what is crystallizing (carbon and oxygen) and the physics of the crystallization.  And we find that the pile up occurs at a different temperature than the physics theorists predicted!

This finding has made a lot of physicists angry.  Some have claimed that the astronomers have made the measurement incorrectly, because everyone who has calculated the crystallizing temperature agrees on the "right" answer.  But this is science - we test theories with observations.  And if the observations don't fit the theory, and if the observations are done properly, then something must be wrong with the theory.

Some physicists have done the scientifically right thing, which is to go back and look at the theory again.  And they've found that some of the assumptions made in the old calculations were wrong.  For example, the old calculations assumed that the crystallizing material was pure carbon.  But white dwarfs are a mix of carbon and oxygen.  And just like adding salt to water changes the freezing point of water, adding oxygen to carbon changes its freezing point. 

This is good science at work.  The observers test scientific hypotheses, and the theorists re-visit ideas that are proven wrong.  Now it is up to us observers to go back to the white dwarfs, measure the crystallization of even more stars, and more tightly constrain the revised theoretical calculations.

Tuesday, August 17, 2010

White dwarfs, day 1

Yesterday was the first day of the 17th bi-annual European White Dwarf Workshop, a weeklong-conference about white dwarf stars.

White dwarf stars are the slowly-fading embers left behind when most stars exhaust their fuel.  Stars shine due to nuclear reactions in their cores.  They fuse hydrogen atoms to make helium, and then they fuse helium atoms to make carbon and oxygen.  The heaviest (most massive) stars can even fuse carbon and oxygen atoms into still heavier elements, but these stars are rare.

Once a star has fused as many atoms as it can into as heavy of elements as it can, the star swells up into a red giant star (the red giant sun will swell from its current diameter of about 1 million miles into a diameter of about 200 million miles, swallowing Mercury, Venus, and maybe even the Earth in the process).  After spending some time as a red giant, a star will blow off its outer layers in a beautiful planetary nebula, exposing the white-hot nuclear reactor that was its core.  This core, which can contain up to half the mass of the original star, is only about the diameter of the  Earth.   This is what we call a white dwarf.

Because white dwarfs are formed in this way, they are one of the few glimpses we astronomers have into the central nuclear engines that power stars.   So, while you could say that this week is dedicated to studying dead stars, it is more like stellar forensics.

Yesterday one of the topics were white dwarfs with carbon and/or oxygen atmospheres.  I've worked in this field quite a bit, and I was part of the team that discovered that many of these carbon-atmosphere white dwarfs change their brightness.  Yesterday's talks made us wonder if maybe all of the carbon-atmosphere white dwarfs change their brightness.  Every single one of these stars that has been studied in enough detail do indeed vary, which is unheard of in other kinds of white dwarfs.

Today I give my talk.  I'll let you know how it goes.

Friday, August 13, 2010

Greetings from Germany


During the quiet stasis that has been my blog of late, I have packed up my things and moved to a new apartment in the town of Rockwall, Texas, just outside of Dallas.  There I will start my new job next week.  More on that in week or so.
Having been in my new digs for three full days, it was time to pack my suitcase and fly to Germany.  This coming week, the 17th bi-annual Euorpean White Dwarf Workshop will be held in Tübingen, Germany.  Virtually everyone who works on white dwarf stars will be here talking about their research during this conference. 
I missed the last workshop, held in Barcelona in 2008, as I had an observing run with the Keck telescope at the same time.  So it will be nice to reconnect with many colleagues for the first time in four years.
So, dear readers, after two months of near silence from me, you are about to be pounded by a week's worth of white dwarf research news. Thanks for hanging in there.