Tuesday, March 31, 2009

Late March News Nuggets

Some collected astronomical news and detritus I've not yet mentioned:

  • The space shuttle Atlantis is on the launch pad, ready to go and repair Hubble for the last time. Hmm, this sounds familiar. Oh, that's right. I blogged about it last September, before a communications failure led NASA to delay the mission. Now NASA has a spare data unit ready to go, the astronauts have trained on new procedures, and we are good to go for launch on May 12.
  • Texas has finished revising its science education standards. It's been a long six months since my first blog post on Texas's revisions to its state science standards; revisions have now been finalized. I'm not as pessimistic as many other scientists are about the outcome. The final standards are not great and allow non-science far too much room to try and wriggle into the science classroom, but they are far better than they would have been without the united efforts of concerned citizens, scientists, and teachers. While this fight over standards was not a victory for science, it was not a victory for opponents of science. I look at the new Texas state science standard results with some grim satisfaction. The outcome could have been worse, far worse, for science.
  • Galileoscopes are coming, just slowly. The Galileoscope, a high-quality, low-cost telescope kit developed for the International Year of Astronomy, has proven to be so popular that the small volunteer team filling orders has been overwhelmed. Frankly, I'm surprised that they allowed themselves to be surprised, but that's a discussion for post-project reviews. Galileoscopes will be shipped starting in late April. If you don't need a fine telescope, please consider donating money to purchase Galileoscopes for underserved children throughout the globe.
  • Help support the AAVSO! The American Association of Variable Star Observers is a large network of primarily amateur/semi-professional astronomers that make detailed observations of variable stars (stars that change their apparent brightness). Many professional astronomers use AAVSO data in our analysis of these stars. Anyway, Sky and Telescope magazine is offering a special deal where you can subscribe or renew your subscription at substantial savings, and they will donate $5 to the AAVSO. See here for more details. (I apologize to the AAVSO that I can't post the advert here on my site due to restrictions imposed by the grant that pays for my webhosting.) Kudos to Mike Simonsen for working out what sounds like a great deal!

Monday, March 30, 2009

100 Hours of Astronomy: Starting Thursday

100 Hours of Astronomy

Starting Thursday, April 2 at 15:00 UT (11 am Eastern, 8am Pacific in the USA), and running through Sunday, April 5 is the International Year of Astronomy's next major activity, the 100 Hours of Astronomy.

Throughout the end of the week and the weekend, there will be many activities online and around the globe.

The streaming webcasts will be available on the 100 Hours of Astronomy channel of UStream. For more information, go to the 100 Hours of Astronomy website (http://www.100hoursofastronomy.org). If you Twitter, you can get updates on all the fun by following 100Hrs; while you're at it, why not follow astronomy2009 (the International Year of Astronomy 2009), 365DaysofAstro (the 365 Days of Astronomy podcast), and professor_astro (yours truly)?

Thursday, March 26, 2009

Hunting planets

Image Credit: Fergal Mullally

My current observing run at McDonald Observatory is part of a project to look for planets circling white dwarfs. This project is led by Fergal Mullally, an astronomy postdoc at Princeton; I'm just helping out by taking some data.

How do we look for planets around white dwarfs? We have to be clever. Most planets have been found by measuring the Doppler shift in light from a star; the gravity of a large planet pulls its parent star around in a small circle, which causes the light from the star to be Doppler shifted as the star moves toward and away from us. But this technique doesn't work for white dwarfs, because it isn't possible to measure their speed accurately enough to detect the small motions caused by a planet. A planet can make a star move at speeds of about 20 or 30 miles per hour, but it's hard to measure the speed of a white dwarf more accurately than a half mile per second!

White dwarfs are stars that slowly cool over time. They used to be the cores of stars like the sun, but when the star ran out of fuel, its outer layers blew away in a planetary nebula, and the hot core of the star was exposed to the cold vacuum of space. Without any heat source, the white dwarf begins to cool off, just like a hot poker removed from a fire slowly cools off.

When most white dwarfs reach a temperature of about 22,000 degrees Fahrenheit, their outer layers begin to slosh back and forth. (This is due to complex physics, but if you are familiar with Cepheid variable stars, the mechanism is similar.) The sloshing causes the white dwarf to get brighter and fainter in a very regular fashion. For a handful of white dwarfs, this sloshing is so regular, we can predict the exact second the star will be brightest several years into the future! The variations in the brightnesses of these white dwarfs are so regular, they rival our best atomic clocks in accuracy.

Now comes the really clever bit. Suppose one of these regular white dwarfs has a planet. The gravity from the planet pulls the white dwarf in a little circle, just like the planets around normal stars do. This means that sometimes the white dwarf is a little close to us, and sometimes it is a little further away. Since light takes time to travel to us, and since the light will have to travel a little farther when the white dwarf is further from us, the light will take a little longer time to get to us. And remember that the brightness of the white dwarf is varying like clockwork. So, if we see the light variations arriving a little later than we predicted, that means the white dwarf is a little further away from us than it was before. And if the light variations arrive ahead of schedule, that means the white dwarf is a little closer to us than normal. In short, if the white dwarf has a planet, we should see the light variations arriving sooner than expected as the white dwarf moves toward us, and later than expected as the white dwarf moves away. (A variation of this method was used by Danish astronomer Ole Römer in 1676 to measure the speed of light; he used the predicted and observed times of eclipses of Jupiter's moons.)

So, Fergal's project involves measuring the arrival time of light variations from the most regularly-varying white dwarfs we know. He's been watching these stars for nearly six years now. Most of the white dwarfs don't show any early- or late-arriving pulses, so they don't have any planets (or at least none large enough to detect). But one white dwarf named GD 66 does show variations. A graph of those variations is at the top of this post. The squares with error bars show how early (negative) or late (positive) the light pulses have arrived at our telescope since 2003. The solid line shows how we would expect the time to vary if a planet about twice the mass of Jupiter were orbiting around GD 66 at a distance of about 250 million miles (2.75 times the distance between Earth and the sun). At this distance, it takes the planet about 6 years to go around once.

So, is there really a planet around GD 66? I think so, but we aren't sure yet. We must see the orbit start to repeat to be sure. In other words, we need to see the white dwarf start moving toward us again. Our best guess is that we should see the orbit repeat starting sometime this year. If we do see it repeat, we can celebrate Fergal's discovery of a planet. If it doesn't repeat, then we're not seeing a planet, and there's something about this white dwarf that we don't understand. And we can't rule that out yet, not until we see the orbit repeat.

And that's why I'm here at the mountain. Tomorrow I'll show you a little of the data I took. I can't tell you if the orbit is repeating yet or not -- there's a lot of analysis that Fergal has to do to turn the data I'm taking into planet orbits, and it may be a few months yet before the orbit starts to repeat. But I'll be sure to let you know!

Wednesday, March 25, 2009

It's a carefully designed science experiment, so why are people mad?

Good science experiments are hard to do, especially when it involves human subjects. That's why, as a scientist, I get upset when I see people knocking a well-designed experiment because they are mis-interpreting the conclusions.

The experiment I'm talking about is a study on weight loss published in the New England Journal of Medicine that found that the amount of weight lost by a person on a diet is determined solely by the difference between the number of calories a person consumes and the number of calories that person uses. It doesn't matter if the person is on an Atkins diet, or a Subway diet, or a nothing-but-chocolate diet. Simple enough, and, frankly, not surprising. But people are complaining. So, let's step through this. First, I'll go through the experiment and explain why I think it is scientific. Then I'll go through the complaints I've seen. And I'll end on why these people are wrong in attacking the study, even though the arguments they raise are valid. Let me start by saying I am going off of published summaries of the diet study; I haven't read it myself. If I screw up in my interpretation, then feel free to blame me for not reading the original study. (Since I'm not a medical doctor, I feel I'm likely to not understand or mis-read the professional journal article. I barely understand a large part of the astronomy literature.)

First, the study itself. There have been many studies on the efficacy of different diets, with many conflicting results. The reason for this is, in my opinion, is that many of these studies don't limit the variables. They find a pool of volunteer subjects, give them a diet to stick to, and then follow their progress. But people make lousy experimental subjects. We are all different, so genetic differences are always present. People lie about whether they stuck to their diets, because they feel ashamed when they cheat. The same food can have vastly different numbers of calories, depending on how it is prepared. People lose interest over time. These are very difficult variables to control

But these studies also often ask questions that render these variables crucial. For example, if I ask the question, "Does the Atkins diet or the Chocolate diet lead to greater weight loss after a two year period?", I'm asking for inconclusive results. What if the Atkins group gets tired of counting carbs after 6 months, but the chocolate group is happy to continue eating their allotment of Hershey bars? Maybe the Atkins meals take an hour to prepare, while the Chocolate meals are instant, meaning that the time-pressed subjects in the chocolate group are more apt to stick to the diet on a night when the kids have soccer and Girl Scouts, the spouse has a PTA meeting, and the subject has five hours of take-home preparation?

So the experiment started with a more pointed, direct hypothesis: The amount of weight lost on a diet is only due to the difference between the number of calories consumed and the number of calories used. They decided to test this hypothesis by varying the types of food consumed in a controlled manner, including diets that were low in fat, or low in carbohydrates, or low in protein, and a few other similar diets. The scientists used a large group of people and held them to pretty rigorous standards of eating only from a select menu of foods, of requiring them to write down all of their food and exercise, of routine measurements of their metabolism (rate of calorie burning during an activity), and of requiring the subjects to be as honest and as sticktoitive as possible. And the study used a large group of people and followed them for a long time.

The results were clear: the amount of weight lost by a person depended only on how many calories they ate and how many they used, not the type of diet. This is a nice, clean, clear result.

So, of course, the arguments start pouring in. Kathy Freston complains that the typical dieter will never follow the rigorous standards of the experiment. Another commenter complains that the study didn't consider the nutritive value of the diets.

But the study was not designed to test the question of whether the typical person would stick to the diet. It was designed to test the hypothesis I stated above. The very regimented meal plans and diaries were implemented to remove the variable of people's natural cravings to cheat from the equation. Nowhere does the study claim that people should follow their regimen. The conclusion form the study is solely related to the amount of weight lost as a function of consumed and used calories. It is indeed a very good question as to how to encourage people to stick to a diet, and whether certain foods might help someone stick to their diet by reducing hunger cravings. But this study was not testing that aspect, so it is unfair to attack the conclusions for not considering that variable.

Another complaint, on the nutritive value of the diets, is also irrelevant to the tested hypothesis. We know that humans need specific nutrients to remain healthy. And it may well be true that a certain balance of those nutrients will, by keeping a person healthy and/or full of energy, help a person stick to a diet or feel energetic enough to exercise. But, again, those variables were specifically excluded from the study. It is unfair to criticize the conclusions for ignoring a variable that was specifically excluded.

In short, the weight loss study was designed to test a specific question, and they came up with an answer to that question. The amount of weight a person loses in a diet depends only on the number of calories they consume and the number of calories they use. That conclusion should not be interpreted as saying that any diet is okay. The study says that I would lose weight if I ate nothing but 1200 calories of lard every day, but it was not designed to say that such a diet was healthy. I suspect that, if I went on the all-fat diet, I would rapidly become ill, malnourished, and unmotivated.

The results of this study, that a calorie is a calorie, can now be used in the interpretation of other diet studies. Dietary scientists no longer have to worry that our bodies view fiber and fat calories differently, because they don't. They can now design an experiment to test the other variables that this study purposefully limited, like the impact of nutritive balance, or the relative feeling of fullness that different foods provide, just what the critics of this study wanted to see.

In short, we shouldn't criticize individual scientific studies for not testing every possibility. This is because the best scientific studies purposefully limit the possibilities to ask and answer a specific question. And when a study does answer a specific question, we must be very careful to not draw any inferences beyond those supported by the experiment and its conclusions.

These are the times that try astronomers' souls

This week I am at McDonald Observatory, helping my colleagues with an observing run that was scheduled at an inopportune time. They took the first half of the 8-night run, and I'm finishing it out. Unfortunately, things haven't gone well so far.

First, the weather hasn't been cooperating. Most of the time it has been cloudy (like tonight), and yesterday we had a dust storm that forced us to close. Tomorrow it is supposed to be clear, but it will be very windy. That means either too windy to operate the telescopes safely, or more dust.

Then we've had some equipment problems. The main camera failed at the start of the third night of the run (a clear, still night, of course!). So, I brought up our spare camera. We installed it two nights ago, and then a new problem cropped up -- the camera lost track of the time and started taking extra, very short exposures. For many types of data, we could just throw the short pictures away. But we are trying to measure changes in the brightness of white dwarf stars on 10-second timescales, and if the camera forgets what time it is and starts taking extra pictures, we can no longer string together several hours worth of snapshots.

I woke up early today and spent the daylight hours troubleshooting the problem (it's gone away, but we don't know why). So, now that the system seems to be working again, the clouds have come back. It's supposed to clear off tomorrow, but the wind is going to pick up again toward dangerously high speeds. (Dangerous for the telescopes, that is.)

It's pretty frustrating, and there's really nothing we can do other than wait for the weather to change for the better, or for our time at the telescope to run out.

Tuesday, March 24, 2009

Not a great night for (optical) astronomy

Thick clouds to the west of McDonald Observatory don't bode for a productive evening. Oh well, cloud happens.

Monday, March 23, 2009

Thoughts on the cataclysmic variables conference

As I mentioned in previous posts, last week I was in Tucson for a conference on cataclysmic variables, systems where a white dwarf is pulling material off of a companion star.

Cataclysmic variables (CVs for short) are not my area. If you'd like to read a great summary on the conference from somebody who works in the area, check out Mike Simonsen's posts on the conference. But, as an outsider, here are some of my personal take-away points from the conference.

  • We know where CVs come from. Early last week I wrote our current hypothesis on the origin and evolution of cataclysmic variables. This story is very well supported by the observations. Evidently this is not news to the CV people, who told me they knew all of this 10 years ago. This isn't what I learned in grad school in the late 1990s, but then again we didn't have any CV researchers locally. But so many details of the CV story are supported by observations, it shows we probably have the basic story right.
  • The big questions need to be advertised. As an outsider, I come away from this conference wondering what the big questions in the field are. I had hints of some of the questions, like the space density (how many CVs there are in a given region of space), searches for "pre-CVs" (systems that are not CVs yet, but will be in the next few billion years), and other such things. But I didn't learn what I should be working on, if I were a CV person, nor how such work impacts the science outside the CV world. Though perhaps I just missed the questions, or I didn't recognize them as the big issues.
  • Amateur astronomers are indispensable. I already knew this from my interactions with amateurs (one of whom saved my current observing run yesterday), but so many of the talks and posters used data from amateurs in their analysis. The amateurs are willing to look at stars almost every night, while professional observatories need to ration time. Plus, amateur equipment is so advanced these days that they can obtain measurements as accurate as professional scopes. If they see anything interesting going on, we professional astronomers get notification and can turn the "big glass" (large telescopes) on the systems. Amateur astronomers really deserve a title like "semi-pro."
  • CV researchers are a very amiable crowd. Even when people disagreed or are in direct competition, they remain on good interpersonal terms (or at least are very good at hiding any tensions). This isn't the case in all fields of astronomy. I really like the camaraderie. There's really little reason, other than egos, to get upset with each other in astronomy. It's not like fortunes will be made or lost based on the findings of a given research project.
  • Some things remain completely mysterious. One of the more interesting talks of the conference was by Michael Shara, who was discussing the origin of the strange outburst of the star V838 Mon (which resulted in this gorgeous series of Hubble images). Shara and another astronomer, Howard Bond, disagreed on almost every aspect of this object, including what the star is, what caused its brightening, whether the other stars in the field are related, and how to go about learning more about the system. Bond and Shara are both very intelligent and well-regarded, and they also respect each other quite a bit, so if they can't agree, then we truly have a mystery for someone to solve.

All in all, I learned a lot, had a good time, and enjoyed talking with colleagues I hadn't seen in quite a while. I'm not about to switch research areas, but "wild stars" are quite interesting!

Monday Rants

It's Monday, and I didn't get enough sleep, so it's time to blow off a little steam. One of these rants is important at its roots, the other is just an annoyance. I'll let you decide which is which.

Volcano Monitoring: A few weeks ago, Louisiana governor Bobby Jindal questioned why the federal government is spending money monitoring volcanoes when we "should be monitoring is the eruption of spending in Washington." Early this morning we got our answer, as Alaska's Redoubt volcano erupted, sending plumes of ash as high as 50,000 feet and causing many airplane flights to be re-routed or cancelled (volcanic ash causes airplane engines to stop, which tends not to be a good thing for most airplanes). Of course, never mind historical eruptions like the 1980 eruption of Mount Saint Helens. Or the fact that Mount Rainier, visible from Seattle, has 150,000 people living in the potential path of volcanic mudslides known as lahars (a lahar in Colombia in 1985 killed over 23,000 people, a tragedy that a little volcano monitoring and education could have prevented).

As researchers at the US Geological Survey said, that money is not being thrown into the volcano to watch it burn. Most of it pays salaries for American citizens, and much of the rest is used to buy equipment (usually from domestic suppliers). And the research is used to better understand the hazards that volcanoes pose to the people living with them. If you'd like to see where volcano research money is going, check out the USGS Volcano Observatories in Hawaii, Alaska, and the Cascades (among others).

NASA Mission Madness: To coincide with the NCAA college basketball playoffs ("March Madness"), NASA has released an interactive website called "Mission Madness," where members of the public can vote on who should win head-to-head matchups between NASA missions. It's cute, it's geeky, and it's a great way to learn about NASA missions you've probably never heard of.

Unfortunately, someone (or some group of people) have apparently figured out how to game the system. In the first round, one of the matches pitted the venerable Mars rovers Opportunity and Spirit against the "Superpressure Balloon" experiment, which I'd never heard of. The balloon won. Not only that, but the balloon got ten times as many votes as Apollo 11, the mission where we walked on the freakin' moon. (The Mars Rovers put up a spirited defense through Twitter appeals, but came up short.) Now, in the second round, the balloon is beating yet another Mars mission, and has four times as many votes as any other mission.

Call me cynical, but I don't believe that the public finds superpressure balloons ten times more interesting than Apollo 11, or more interesting than the Mars rovers. I suspect that the balloon team is stuffing the ballot box. Is this cheating? Not really, because NASA set essentially no rules, anyone can vote as many times as they want), but I do think the balloon team is inflating the results (ha!). And that defeats the purpose of this activity, of letting the public choose their favorite space missions.

All I can say is, if I am sitting on a NASA funding review panel and the superpressure balloon people try to use this as justification of public interest in their project, I'm going to re-open this rant in the panel discussion.

Yes, balloons do excellent science. I like to think I do so, too. But I know that if you put my white dwarf work up against Apollo 11, I will lose every time. So should the balloon, IMHO.

Friday, March 20, 2009

The Evening AND Morning Star

This morning our conference is over, and I'm trying to tie up some loose ends before heading back to Austin (and then on to McDonald Observatory). I'll write a summary blog post on all of that in the next couple of days.

But I did want to point out that the next few days offers a rare opportunity -- to see the planet Venus in both the evening sky AND the morning sky on the same night.

This is possible because of geometry. Venus is closer to the sun than the Earth, so it goes around the sun faster. Every one and a half years, Venus "laps" the Earth, passing between the Earth and the sun. But the orbits of Earth and Venus are slightly tilted with respect to one another, so Venus usually passes just above or just below the Sun.

This lap, geometry works so that Venus is passing about 8 degrees (a little less than the width of your hand held at arm's length) north of the sun. That distance is far enough that people in the northern hemisphere who look for Venus low in the west right after sunset and low in the east right before sunrise will be able to see the planet, with the Earth blocking the sun.

If you are in the Northern Hemisphere and awake at both sunrise and sunset, try looking for Venus low in the sky at both times. You'll get a rare treat!

For some more information, try reading Sky and Telescope's blog post on Venus observing this month.

Thursday, March 19, 2009

The Polar EU Cancri, Part 2

As I described yesterday, my contribution to the cataclysmic variables conference involves my observations of a polar named EU Cancri in the open star cluster Messier 67.

So, why would my findings be considered interesting?

It's precisely because the polar may be in an open star cluster. Open clusters are groups of stars that were all born at the same time out of the same cloud of material. So, every star in the open cluster is the same age, and they all have the same mix of elements. M67 is a well-studied cluster, so we know the age (4 billion years) and we know that it has the same amount and mix of elements that the sun has (this latter bit is important, because some stars only have a thousandth the heavy elements that the sun has, some have twice or three times as many metals as the sun has, and the amount of metals affect the brightness and structure of stars.

Anyway, if the polar I looked at is in the star cluster M67, we know that the two stars had only four billion years to get into their present state. Four billion years is a long time, but, even so, this is one of only a few cataclysmic variables (and the only polar) where we know this timescale.

One important question remains. Is my polar actually in the star cluster? When we look out and see star clusters, we have to look through parts of the Milky Way galaxy, and the galaxy extends for tens of thousands of light years behind the star cluster. So, many of the stars we see in a picture of a star cluster like M67 are either in front of or behind the star cluster.

We can only estimate the distance to the polar I've looked at, we can't get a firm answer (yet). We think we know, roughly, how bright polars are. We know how bright this one appears to be. And the difference between those two gives us a distance (because the further away things are, the fainter they appear to be). From these calculations, we think that the polar is about 9000 light-years away. But Messier 67 is only about 2500 light-years away. So, it seems that the polar only appears to be in the star cluster, but in reality it is not. In this case, all of the information that the star cluster gives us is of no use.

It still remains possible that the polar is in the star cluster, if we've guessed wrong as to how bright it really is. There are other ways to tell distance, and we'll have to try some of those.

So, in some ways, it is disappointing that the polar may not be in the star cluster. There are many known polars, and most of those are much brighter (and so easier to study) than the one I looked at. If my polar is not in M67, then it probably won't get studied much anymore. But that would be a good thing: my polar is very faint and needs lots of time on big telescopes to study. This time would be worthwhile if we can use what we know about the star cluster to study the white dwarf, but this time is not worthwhile if it is just a normal polar floating alone in the galaxy.

So, I get to write a paper about our findings, and I got to learn a lot about a fun type of star. I won't become famous for my accidental observations of this star, but maybe next time.

Wednesday, March 18, 2009

The polar EU Cancri, Part 1

Time-resolved spectrum of the polar EU Cancri
Image Credit: Yours truly

My contribution to the conference on cataclysmic variable stars involves an accidental study of a polar (rhymes with "coal tar" or "Moltar") with the name of EU Cancri. (Read here for an explanation of how variable stars get their names).

First, let's learn what polars are. Here's an artist's impression of what a polar might look like. Remember, a cataclysmic variable is a white dwarf star whose gravity is stealing material from a nearby companion star. In polars, the white dwarf has a very strong magnetic field, tens of millions of times the strength of Earth's magnetic field. When hot plasma (like on the surface of a star) encounters such a strong magnetic field that strong, the plasma is forced to follow the magnetic field. So, in the polar, the material that gravity pulls off the companion star follows the white dwarf's magnetic field to the white dwarf's magnetic poles, where it slams into the surface of the white dwarf, releasing lots of heat and light, but only at that specific spot.

Anyway, my tale begins in January 2007. I was at the Keck Observatory, the world's largest telescope, to look at normal, single white dwarfs in the old star cluster Messier 67. My goal was to measure how massive the white dwarfs are in Messier 67. So, I had taken pictures of the star cluster and identified potential white dwarfs (white dwarfs are very faint and blue, so I just picked all the faint, blue stars). I then pointed the telescope at my candidate white dwarfs and started taking spectra. Because taking spectra involves breaking the light of stars into its component colors, and because faint stars like white dwarfs don't have much light to begin with, it was going to take several hours with the giant telescope to get the data I needed.

When I looked at the spectra, most were either normal white dwarfs or were quasars; quasars, like white dwarfs, look like faint blue stars, so I tend to find them quite a bit. But one spectrum looked different. It showed signatures of hydrogen and helium, but these lines were in emission instead of absorption (compare the first and last spectra here to see the difference between absorption and emission). Absorption is typical for normal white dwarfs, but emission means that something weird is going on.

Also, when I looked at the spectrum, it had giant humps that came and went over just 45 minutes. Humps in spectra often mean magnetic fields; they come from a process called cyclotron emission. (Electrons in a magnetic field tend to spiral around in circles and emitting light as they do so). So, I suspected that I'd found a polar, even though I'd never seen the spectrum of one before. But I didn't know what else it would be. And it turns out that I was right.

When I studied the spectra more closely, I noticed that the emission lines were moving. Sometimes they were a little bluer, sometimes they were a little redder. This is due to the Doppler shift. The companion star is orbiting the white dwarf every 125 minutes, so over the three hours I looked at the system, it went through one and a half orbits. To put this in perspective, the two stars are about 400,000 miles apart, or about 1.6 times the moon's distance from the Earth, yet they go around each other in just over two hours (while if the moon were that far away, it would take almost two months to go around the Earth)!

The picture at the top of this post is a stack of all the spectra I took, with some almost-true color added. The picture is kinda like a movie. The very top is the beginning, the bottom is the end, and in between are snapshots like individual frames. You can see the emission lines wiggling back and forth, as the companion star first moves away from us (getting redder due to Doppler shift), then toward us (getting bluer due to Doppler shift), and then back away from us again. You can also see the "humps" due to the magnetic field appear and disappear. I repeat the "movie" twice for clarity, so the spectrum really covers about 5 hours.

Anyway, I find it neat that we can see things changing so much over just a few hours. If I were to make a similar "movie" spectrum of a normal white dwarf, the movie could go for centuries with no noticeable changes. And yet here we see drastic changes in just a few hours.

As I said at the beginning, my spectra were obtained mostly by accident, because I wasn't looking for weird objects. But yet I found one. This tends to happen in astronomy; we go to the telescope looking for one type of object, and we find something completely different.

Tomorrow I will talk a bit about why people might be interested in my star, and how my results probably disappoint everyone.

Monday, March 16, 2009

Making a wild star

As I mentioned yesterday, I'm spending this week at a conference called "Wild Stars in the Old West", where we are talking about a star system known as cataclysmic variables. These are objects where a white dwarf is orbiting so close to another star that the white dwarf's gravity can pull material off of its companion. This process produces an ever-changing amount of heat and light, which we see on Earth as a single star that appears to flicker like crazy.

But where do these exotic things come from?

It all starts with the fact that half of the stars you see in the night sky are not single stars, like our sun, but often systems of two or more stars. The stars can be far apart, hundreds of times the distance that the Earth is from the Sun, or they can be close together, only about ten million miles apart (remember that the Earth is about 93 million miles from the Sun), or anywhere in between.

Stars live by nuclear fusion, turning hydrogen into helium. When a star runs out of hydrogen, it begins to bloat up into a red giant star. Our own sun, as a red giant, will swell up from its current 900 thousand mile diameter to about 150 million miles across; that's big enough to swallow Mercury and Venus!

If a star like the sun has a companion several billion miles away, the companion star will happily go about its business, continuing in its orbit and pretending not to notice that its sibling is swelling up like a balloon. But if the dying star's companion is within 100 million miles, it will soon find itself inside its sibling. You might think that would mean a fast and messy end for the companion star, sort of like the Blob absorbing a poor innocent bystander. But no!

The dying star is not gaining weight or mass, it is simply expanding. When you take a star like the sun and spread it out over a volume the size of Earth's orbit, its density drops by a factor of a million. So, while the star looks big and imposing, it's really quite fluffy and tenuous. The companion star, meanwhile, is still pretty dense. So, the companion star can actually continue to orbit inside the outermost layers of the red giant star!

This doesn't mean the companion star is completely unscathed; it does get heated up a bit, and even though the red giant star is fairly fluffy, the companion does encounter some small but persistent wind resistance. This friction causes the companion star to slowly spiral even closer to the core of the red giant star.

There are two possible outcomes. One is that the companion star spirals all the way in to the center of the red giant, where it merges with the red giant's core and loses its own identity. Later, when the red giant sheds its outer layers as a planetary nebula, it will leave behind a single white dwarf star, leaving little, if any, evidence that the companion ever existed.

However, if the companion doesn't spiral all the way in to the center, the red giant can still go through its planetary nebula phase, leaving behind a white dwarf with a scarred, but surviving, companion in a very tight orbit (In fact, the companion star often helps to eject this material, almost as if it is trying to avoid being swallowed alive). In many cases, the companion star is only a couple of million miles (or less) away from the white dwarf, completing an orbit in just a few hours (remember, the Earth takes a whole year to go around the sun). At those small distances, the strong gravity of the white dwarf can begin to steal material from its companion, and we get a cataclysmic variable.

Anyway, most of today's discussions were on these companion stars (most are tiny red dwarf stars) and how the orbits of these companions change as their outer layers are slowly sucked away by the white dwarf (first the orbits get smaller, then they get bigger. Don't ask me why -- it's complicated stellar physics that I don't feel like explaining tonight.)

Tomorrow I present my results, so I'll tell you about my findings then.

Sunday, March 15, 2009

Wild Stars in the Old West

Today I flew to Tucson, where I'm going to spend the week at a conference entitled, "Wild Stars in the Old West II".

The conference is about a type of object known as cataclysmic variables, or CVs for short. CVs are actually binary stars, typically with a white dwarf star being orbited by a small star, often a red dwarf or a brown dwarf. The gravity from the white dwarf is strong enough to pull gas off of its companion star, and as the gas falls onto the white dwarf, it heats up and glows as bright or brighter than both stars combined. So, when we look at CVs, we often can't see either star, but we see a highly variable flickering from the gas stream.

CVs are not my primary area of research, but since I work on white dwarf stars, there is a lot of overlap. I know many of the people in CV research, so it will be great to catch up with many of them.

I'm also presenting my first foray into CV research. In January 2007, I was looking at stars I thought might be white dwarfs in the star cluster Messier 67 with the Keck telescope. One of the stars I looked at was not just a white dwarf, but was also a type of cataclysmic variable called a polar (pronounced "POLE-are"). While I didn't discover this particular CV, I did obtain one of the best data sets on it, and all quite by accident.

In the coming days I'll tell you a bit about my CV and findings, and I'll tell you about any interesting developments from this conference.

Friday, March 13, 2009

It must be Friday the 13th

\begin{rant}

I'm working on a blog entry about a fun project I've been working hard on the last couple of weeks. I was hoping to finish it up for y'all tonight. But, instead, I've had to be on the phone with police, make auto repair appointments, and worry about transportation two days before I leave town for two weeks.

I take the bus to work 90% of the time. I live about 12 miles from campus, traffic during rush hour in Austin is horrible, and campus parking is expensive. Riding the bus is free for University employees and students, it is not much slower than driving, and not having to fight the traffic relieves a lot of stress. So, I drive a couple of miles to a park and ride and take the bus. I save about 20 gallons of gas a month this way, and probably save $5 or $6 a day in fuel and parking costs.

But, today when I got off the bus at the park and ride, I found that my car had been broken in to. For the second time in six months. Right in front of a security camera. Last time, they took my car stereo (a bottom-of-the-line Best Buy replacement for my car's manufacturer's stereo, which was busted). This time, they didn't take anything, presumably because I have started taking the stereo faceplate with me. But it is still $175 to replace the window.

The first time my car was burglarized, several cars were hit at the same time. The police officer thinks it was a professional thief, because the stereos were taken with tools and wires clipped to avoid reducing the value of the product. They probably got $20 fencing my stereo, and cost me and my insurance company $300. Which the insurance company is now recouping by raising my rates. (I'm not filing a claim this time!)

After that incident, I started parking in a more heavily-trafficked part of the parking lot, right near an obvious security camera. Fat lot of deterrence that proved to be.

My car is nothing special, just a boring 2000 sedan. The stereo is not special. I don't have custom speakers, just the crappy manufacturer's speakers. I don't have anything else of value in the car. So I don't think my vehicle should be attracting any attention, other then that it is parked all day long, along with dozens of other vehicles.

An online search found that this park and ride has 60 reported vehicle burglaries in the last 18 months, most being multiple break-ins on a single day. 60. 60!! And yet the bus company doesn't feel the need to post a permanent security guard, because it costs too much money, and we all park "at your own risk." Hey, how about grabbing some of that stimulus money that our governor doesn't want, and hiring a security guard for each of the park and rides? It'd reduce unemployment and crime. Or maybe Texas is hoping that the stolen goods market will revive the economy.

Here I am, trying to do the right thing by riding the bus, reducing my carbon footprint, reducing traffic on the roads, saving money. The reasons the bus company says we should take the bus. But after two burglaries, I don't think I can take the bus anymore. Clearly there's somebody who's figured out the bus schedules, knows that there will be nobody in the parking lot at certain portions of the hour, and takes advantage of that. Yet the city and bus people expect me to just swallow the costs and smile and be happy about it.

Now the city is about to start an expensive light rail service (which I support!), but it will involve more cars being parked and unattended for hours at a time. And they'll be even easier targets for thieves, because the trains will only run during rush hour, so burglars will know that they can grab 80 car stereos, have a leisurely lunch, grab another 80 and go home before a single train rolls into the station.

Forget that. I'm fixing my window, buying a parking pass, and driving in to work from now on. I'll find some legitimate way of offsetting the carbon usage. But where's the monetary savings if I have to replace a car window every 6 months? Where's the peace of mind if I have to worry about if my car will even be there 10 hours later?

Here I was just trying to Do The Right Thing. I guess no good deed goes unpunished.

\end{rant}

Thursday, March 12, 2009

clouds

The itt Peak McMath-Pierce Solar Telescope in Fog

Image Credit:
Yours truly

Clouds are the bane of the ground-based astronomer. The picture above is of the Kitt Peak McMath-Pierce Solar Telescope during an ice fog in January 2004. I was supposed to be observing that night on the Mayall Telescope, and the next, and the night thereafter. Needless to say, we didn't get much observing done. (It did finally clear off for our last night, but it was a long wait in the cold and damp weather.)

Today, it is raining here in Austin. It's our first substantial rain since last fall, and we desperately need the rain to quench a devastating drought in the region. But it is dreary and cloudy. This morning several students were complaining about the rain, and though I found the gray skies depressing, I am very happy to see the rain.

My point is that, on the Earth, clouds can both hide what we'd like to see (the sky and sun) and yet be beneficial (by bringing water). Since we can't have our clouds and see through them, too, we astronomers have learned to deal, such as by patience, using other wavelengths of light, or going above the clouds.

As we search for signs of life on other planets, such as the Earth-like planets that the Kepler mission may find, clouds may pose a challenge. Presumably any surface-dwelling, land-based life on other planets (like alien trees and alien intelligent dinosaurs) will rely on weather similar to that on Earth. But what if the clouds are hiding the surface? Some of the primary biomarkers that astronomers hope to look for, like chlorophyll, would be hidden. Others, like ozone, might well be visible above clouds.

Clouds are hard to model. We understand why they form in atmospheres, but we are surprised by the variety of clouds that we see. On the Earth, cloudiness varies by both location and time, and where there are clouds, there are many different varieties. From many light-years away, though, this wouldn't matter. Aliens looking at the Earth would get occasional glimpses of vegetation through holes in the clouds, and should be able to detect biomarkers.

But other planets and moons in our Solar System would be tougher nuts to crack. Mars is almost (but not quite!) cloud free, and we still argue as to whether life might exist on Mars. Venus is a lot like Earth in size and composition, but it is completely shrouded in clouds. Venus is also likely devoid of life, since its clouds are made of sulfuric acid and the surface is hot enough to melt lead. Saturn's moon Titan has some clouds that come and go, but those clouds are difficult to see under a layer of thick haze and smog. Jupiter has clouds of all shapes, colors, and sizes, but we don't know what many of these clouds (like the Great Red Spot) are made out of! Given this variety in our own Solar System, we can only imagine what clouds may cover an alien planet.

Beyond planets, even some stars (well, okay, "failed" stars) have clouds. At least some brown dwarfs have clouds, and those clouds may even be changing over time. Now that we are finally able to predict weather on Earth reasonably well, we have to start worrying about weather on other planets. Gosh, what bad luck would it be if I were to use Hubble to look at planet Vulcan during a planet-wide dust storm?

Tuesday, March 10, 2009

Comet dust

Stardust spacecraft encounters Comet Wild 2
Image Credit: NASA

Today, Prof. Don Brownlee of the University of Washington visited our department to give a colloquium. He was talking about the results of NASA's Stardust mission, which sent a spacecraft to a comet, collected pieces of the comet, and brought them back to the Earth in January 2006.

Comets are thought to be pristine relics from the early solar system. 4.6 billion years ago in the outer regions of the disk that was to become our Solar System, ice and dust clumped together to form "dirty snowballs" in a wide range of sizes. Some of these were drawn together by gravity to make icy dwarf planets like Pluto and Sedna, some came too close to giant planets like Uranus and Neptune and were flung out into deep space, and some stayed orbiting the sun near where they formed, remaining in a sort of cryogenic deep freeze to the present day. On occasion, one of these icy bodies is perturbed by gravity and falls toward the inner solar system, where the Sun's rays warm it and melt the ice and gas, resulting in a comet.

Professor Brownlee and his collaborators hoped that the dust and debris in a comet might give us clues about the details with which our Solar System formed. They expected to find that the dust of comets would consist of loose agglomerations of "interstellar grains," tiny dust particles that pervade space. So, they designed the Stardust spacecraft, which was launched into space, visited a comet, collected some dust, and brought it back to Earth.

The reason to bring dust back to Earth is quite simple. In our physics labs, we have big machinery and equipment that can determine the composition of individual parts of a single grain of dust. That equipment is not cheap, it is large in size, it tends to weigh several tons or more, and it requires several people to run. Those factors make it impossible to send such detailed equipment into space. Robots may be excellent at running simple experiments in space, but detailed measurements on dust grains is best done back here on Earth. So, Brownlee and collaborators brought the comet (or at least pieces of it) back to Earth by capturing dust-sized grains in a novel substence called aerogel.

The initial results are very surprising. Instead of delicate aggregates of interstellar dust grains, the comet contains pebbles and specks that were melted at very high temperatures, over 3000 degrees Fahrenheit! That material had to have been processed near the sun, but somehow had to get to the outer regions of the solar system to get encased in ice. More than that, the material in the comet looks to be more processed than the material we find in meteors (most of which come from the asteroid belt).

It's almost like we excavated a woolly mammoth from the Siberian tundra and found a kangaroo in its stomach. Not only would we not expect it and need to explain how kangaroos got from Australia to Siberia, but we'd also have to explain how we've never found any evidence for kangaroos in Southeast Asia and China. Maybe some sort of giant slingshot built by disgruntled wombats?

And, in fact, there is one idea for solar system formation that results in a sort of giant slingshot, called an "x-wind." Basically, the region near the forming sun contained a very powerful wind, and any rocky material that happened to drift too close to the wind would get flung into the far reaches of the Solar System. It sounds simple enough, but in reality it is a very detailed and complex model that has not gained full acceptance in the astronomical community. Discoveries such as those from Stardust may give the x-wind model the observational evidence it needs to become accepted.

There's a lot more work to be done, but one big question is already looming. Stardust visited a specific comet: Comet Wild 2. Is Wild 2 a typical comet (all evidence pointed that way prior to the Stardust mission)? Or did the Stardust planners make an unlucky choice of some cosmic oddball comet? Perhaps we need a Stardust 2 mission to another comet to solve the questions raised by Stardust...

For more on the initial findings of the Stardust mission, click here. If you prefer pretty pictures, try here. Or, if you have a satirical outlook on life, try this video summary:

Monday, March 09, 2009

The Crescent of Venus

One of Galileo's most important discoveries he made through his telescope was the detection of the phases of the planet Venus. (Click here for one of Galileo's drawings from the Institute and Museum of the History of Science in Florence, Italy.) The primary importance of this discovery is that it proves that Venus does not orbit the Earth, as was believed in the Ptolemaic system, but it instead orbits the sun, as do all the planets in the Copernican system. In the Ptolemaic system, there is no way for Venus to go through a full set of phases. And yet it does. Game, set and match to Copernicus.

About 15 years ago, my first astronomy professor posed a somewhat-unfair question on an exam: How would the history of astronomy had been different if Venus were just a little closer to the Earth? (I say somewhat unfair, because grading on a hypothetical question is always highly subjective). The question arises out of the fact that the human eye can see details that are larger than about one minute of arc (1/60th of a degree), and when Venus is closest to Earth, it is just over one arcminute in size. Not only that, but when Venus is closest to Earth is also when it is going through its most extreme crescent phases.

So, if Venus were just a little closer to Earth, its crescent would be visible to the unaided eye, and even ancient astronomers would have known that it went through phases. Assuming nobody else would have figured it out, the ancient Greeks would have realized that phases meant Venus had to orbit the sun (they understood the geometry that causes the phases of the moon), and by assumption the rest of the planets would, too. Perhaps what we call the Copernican system might even have been proposed by Ptolemy, if only Ptolemy had known about the phases of Venus!

There are claims that some eagle-eyed ancient astronomers might have seen the phases of Venus, but it is difficult to prove this is the case. My gut feeling (which doesn't mean I'm right!) is that the Greeks would have seen the phases if it were possible, and there's no record that they did. It is very difficult to do a fair experiment these days, because we already know that Venus has phases, and even what phases the planet should have when we stare at it, and our brain is very good at telling us we see what we expect to see.

Over the next three weeks, the bright evening star that is Venus will grow to its largest angular size as its orbit carries Venus between the Earth and the sun. This is your chance to look for the phases of Venus. Even if you don't want to try to see it with your unaided eye, even small binoculars or a low-power telescope will easily show you the thin crescent that is Venus. Or, failing all else, check out this cool Astronomy Picture of the Day from last Friday, where you can see both the crescent moon and crescent Venus.

Saturday, March 07, 2009

Launching money into space -- NOT!

Last night, while I was watching the Kepler Mission launch feed on NASA TV, I heard an interview with Dr. Ed Weiler, the Associate Administrator of NASA's Science Mission Directorate (think of his position like VP for Research). Dr. Weiler was asked about the cost of the Kepler Mission, roughly $600 million over 5 years. First, he made what I considered the obvious comment: while $600 million sounds like a lot of money (and it is!), it works out about 40 cents per year per man, woman and child in the United States. (Heck, you can make it sound even cheaper: this is about a penny per person per week for five years).

Then Dr. Weiler made a statement that, while sounding perhaps a little gruff, is an argument I haven't heard much from NASA before: NASA doesn't burn money in space. It doesn't launch stacks of bills into orbit. That $600 million is spent here in the United States, primarily paying salaries of scientists, engineers, technicians, teachers, construction workers, truckers, and janitors, among others. Those people, who live spread around the country, use their salaries to buy groceries, houses, cars, and to pay for babysitters and repairmen. Those people then use that money for the same purchases and more. That $600 million, when all of this is added up, buys a lot more than several tons of aluminum rocketry and precision electronics. This spending and re-spending of the money will buy, in total, nearly a billion dollars worth of goods and services throughout the economy via a mechanism known as the multiplier effect. In short, the government spends $600 million dollars, impacts the economy to the tune of a billion dollars, and gets a really cool science mission to boot.

More importantly, Weiler is overtly reminding us that the cost of a NASA mission does not involve throwing money into a black hole. Your tax dollars are being sent back into the economy. They're employing hundreds highly skilled people who could otherwise be drawing unemployment right now. And they're giving us a chance to find Earth-like planets to boot. Perhaps we need to be reminded where the money is really going more often.

Friday, March 06, 2009

Looking for dark matter

Dark matter is one of those topics in astronomy that really catches the public's interest. By mass, there is at least five times more dark matter in the Universe than "normal" matter (baryons). The evidence for dark matter is strong, but we don't really know what it is. How can we not know what 80% of the matter in the Universe is?

This week, I visited with some astrophysicists who are trying to detect dark matter in a laboratory on Earth. If they succeed (and are the first team to do so), it may well mean a trip to Stockholm. If they and all other groups searching for dark matter fail, it could pose a serious challenge to our current views of the Universe. It's a high stakes game, with a lot of pressure to be first and to be right.

Dark matter was first suggested by Fritz Zwicky, who was an astronomer with a, um, unique personality. Zwicky had many outlandish ideas well outside the mainstream. Many of these ideas were wrong, but many were right. One of Zwicky's ideas grew out of his studies of clusters of galaxies. The galaxies were moving faster than the gravity of individual galaxies would have predicted, so Zwicky posed that unseen matter was responsible for these fast motions. It took decades to show that this matter could not be normal matter (atoms and protons and electrons), but many lines of evidence have now shown that dark matter exists and cannot be normal matter.

But there remains a possibility that dark matter may not exist, but rather something else is going on. Perhaps our understanding of gravity is wrong, and gravity works differently on very large scales than on scales we can test in our laboratories. Perhaps the Universe folds back on itself in multiple dimensions and gravity from normal galaxies "leaks" through these folds while normal light cannot. (Don't worry if you don't understand it, I only have a vague idea myself.)

Assuming that dark matter does exist as a previously-unknown type of matter, it should be possible to detect. There are many proposed types of dark matter particles, with names such as axions, neutralinos, WIMPS, Kaluza-Klein particles, and many others. These particles react with normal matter mainly only by gravity, but very, very rarely they can actually bounce off of normal matter.

It is these rare bounces that Earth-based laboratories hope to detect. The physicists running these experiments cool a detector down to near absolute zero so that the atoms in the detector are barely moving. If a dark matter particle hits one of these atoms, the atom will move with a lot more vigor, and this extra movement can be detected with very precise measurements.

The problem is that there are lots of other, much more common events that cause the detector atoms to get excited and move around. Cosmic rays from outer space slam into these detectors with huge energies, heating many atoms up. Placing the detectors in mines deep underground protect them from cosmic rays. But normal particles called neutrinos can also penetrate the Earth and, on rare occasions, interact with the detector (in fact, many of these detectors double as neutrino detectors). Telling neutrino hits apart from dark matter hits is hard but not impossible. Another problem is radioactive decay. Even in a very pure detector, some atoms of radioactive material will be present, and radioactive decay also causes signals in the detector.

So, the entire game of dark matter detection is to look at hundreds of thousands of events and try to explain each one as a known process. The few unexplained events that remain are then considered to be dark matter candidates. Then even more tests are needed to see if these events are true dark matter. For example, as the Earth moves around the sun, the direction of dark matter particles should appear to change. So, if the average direction of the candidate dark matter events changes over the course of a year, then these events may be even more likely to be due to dark matter.

If one of the teams looking for dark matter actually finds it, and the experiment can be confirmed by other means, then we can finally be positive that dark matter exists. If the experiments are perfected yet fail to find dark matter, then many physicists will start to question if dark matter actually exists, or if some of the other, (currently) less-popular explanations like modified gravity may actually be correct.

Tuesday, March 03, 2009

News tidbits

I'm getting ready to say goodbye to some visiting relatives before heading to the airport for my own travels, so I don't have much time this morning. But here are some short blurbs that have been floating around in my head.

  • Galileoscopes can now be ordered! A little over a month ago, I blogged about the Galileoscope, a small, inexpensive telescope based on the telescope Galileo used 400 years ago to discover the moons of Jupiter, the phases of Venus, and craters on the moon. You can now order Galileoscope kits for yourself, friends and family. These kits come not only with all the parts you need, but also classroom activities on astronomy and optics. The optical quality of the telescopes is high -- you will be able to see the rings of Saturn! The cost is $15 plus shipping; if you need to order large quantities, there are discounts available. If you don't need a telescope yourself but would like to purchase some for disadvantaged school classrooms both here and abroad, you can donate money for that purpose at the Galileoscope website as well. (Perhaps you may want to alert science teachers in your school districts about these telescopes, and even offer to donate some or to collect money for that purpose?)
  • Yesterday I blogged about the Kepler Mission to look for small planets around other stars, and I failed to mention that you can watch Friday night's launch on NASA TV (streamed free over the web, but also carried by many cable and satellite TV providers). An unofficial countdown clock can be viewed here.
  • The voting is over, and you have chosen to point the Hubble Space Telescope at two colliding galaxies during the 100 Hours of Astronomy weekend in early April.
  • Speaking of voting, NASA is allowing you to vote on a name for the newest space station segment. The name "Serenity" is currently in the lead, but if you are a troublemaker, you may want to consider the name Buddy. Obviously I'm a troublemaker.

Monday, March 02, 2009

Looking for other Earths

Image Credit: NASA

Sorry for the dearth of blogging recently. About a week ago my main work computer was repaired after nearly a month of downtime, and I've been trying to catch up on computer-intensive work that was past due. I'm mostly caught up now, just in time to go on the road for a few days.

It is rare that, in advance, we can point to an upcoming astronomy experiment and say, "The results of this experiment will change the way we think about the Universe." In most cases, like new telescopes, we know that many interesting discoveries will be made, but we can only guess at what they will be. For example, we astronomers new that a working Hubble Space Telescope would open up new windows on the Universe, and we hoped that it would be able to measure the expansion rate of the Universe. It indeed accomplished this, but many of its biggest discoveries, like the evolution of galaxies over the lifetime of the Universe and numerous lines of evidence for Dark Energy, were either vague hopes or completely unexpected results.

In other cases, we know going into an experiment that, if it works as we hope (always a big if, especially when space flight is involved!), we will learn something fundamental about our Universe. The COBE satellite, the first mission to measure the ripples in the very early Universe that became galaxies and clusters of galaxies, was one of those missions. If it discovered the ripples, it would both further support the Big Bang theory, and the strength of those ripples would be important input for our models of the Universe. If it failed to discover those ripples, then the Big Bang theory would have been greatly challenged, if not invalidated. COBE did discover those ripples, and the chief scientists got a Nobel Prize in 2006 for their work.

Assuming the launch schedule doesn't slip, this Friday night another such satellite is going to launch. NASA's Kepler Mission is a satellite that has one primary purpose in life: to stare at a single patch of sky in the constellation Cygnus for over three years and to find Earth-sized planets in Earth-like orbits around other stars. If Kepler finds a lot of these planets (and I think that most astronomers expect this result), then we would know that planets similar to the Earth (at least in size) are very common in our galaxy, which increases the likelihood that there are many habitable planets in our galaxy. And if Kepler doesn't find very many Earth-sized planets in Earth-like orbits, we will know that our home planet is a very unique place, which could mean that we are one of the few habitable planets around. Either way, the results of this experiment will have an important impact on our understanding of planets and our outlook for finding other life in the Universe.

Kepler works by constantly measuring the brightness of 100,000 stars. If a star has a planet, and if the planet's orbit is tilted just right, every time the planet orbits its parent star the planet will appear to cross the face of the star, as seen from Earth. The planet will block a tiny bit of the star's light; an Earth-like planet will block just one ten-thousandth of the light from a sun-like star. This is really tough to measure -- I'm happy when I can measure the brightness of a star to better than one percent! Bigger planets will block more light, so by measuring how much light gets blocked, we can measure the size of the planet. And, since the planet will block light each time it orbits a star, we can determine how long the planet's orbit around its parent star is by measuring how often the light is blocked.

We've already detected big planets this way from the ground; many planets around other stars with sizes between that of Neptune and Jupiter have been found this way. But we don't think that life can exist on planets this big. Further, most of these planets are really close to their parent stars and have temperatures of a few thousand degrees -- hot enough to melt rock! That's not a promising place to look for life.

So, the Kepler Mission is looking for planets the size of the Earth that orbit stars like the sun about once a year. Think about how technically challenging this is: the Kepler scientists are trying to measure a change of one-ten thousandth in the brightness of a star that happens for a few hours once a year. And then we have to see it happen twice more to verify that the transit repeats with the regularity that planetary orbits require.

The Kepler Mission will last for at least three and a half years, and it will take some time for results to be confirmed. The data taking is hard, the analysis is hard, and we just have to be patient to see multiple orbits of an Earth-like planet. As long as Kepler works as advertised, though, in a few years we will know how common Earth-sized planets are. Right now, we can only guess. And that single result will have a big impact on how we view our Solar System and our home planet, and our place in this vast Universe.

(Yes, Kepler will do other good science, too. But its ultimate success or failure will hinge on its ability to tell us how many Earth-size planets there are in the galaxy.)