Wednesday, February 28, 2007

Onward to Pluto

Thirteen months ago the New Horizons Spacecraft was launched from Earth. Its mission is to explore Pluto and other Kuiper Belt objects. Early this morning, the spacecraft passed close to Jupiter on its way to the outer reaches of the Solar System. The probe is returning fabulous photos of the planet as it passes by.

The main reason for passing by Jupiter is to give the probe a slingshot using the planet's gravity. Before reaching Jupiter, the probe was moving at about 12 miles every second (43,000 miles per hour); now it is moving at 14.5 miles every second (or 52,000 miles per hour). That extra speed will shave years off of the trip to Pluto.

Poor Pluto has had a rough year. It was demoted from being a planet to being a Kuiper Belt Object, which is a big blow to any celestial body's ego. Hopefully the knowledge that we are coming to visit will help its spirits.

What's next for the space probe? It won't reach Pluto until July of 2015, eight and a half years from now! Since the probe won't be passing any other planets, most of the science instruments will be powered down, and the spacecraft put in a sort of hibernation until it gets close to Pluto. Every so often, we'll check in on it, but mostly we want it to rest. That reduces the possibility that something will break in the harsh expanse of space. Bon Voyage, New Horizons!

Tuesday, February 27, 2007

Dry as a bone

NASA's mantra for searching for life on other planets is pretty simple -- follow the water. As far as we know, life needs water to exist. And while there may be forms of life in the Universe that have learned to make do without water, we wouldn't know what to look for. So, we start with what we know. In our own Solar System, water is everywhere. The Earth has deep oceans, and the clouds in our atmosphere are made from water. We know Mars has some water, the debate is how much. The moons of Jupiter and Saturn have a lot of water (in the form of ice); Europa may have liquid water just under its icy outer layers. And comets are made up of ice and dust.

When astronomers look beyond our solar system, we see water everywhere. This is not surprising -- water is a very stable molecule, hydrogen is the most abundant atom in the universe, and oxygen is the third most abundant atom. We see water around dying stars and in the freezing hearts of "molecular clouds" that will one day form new stars.

So, imagine our surprise when we were able to study the makeup of the atmospheres of two planets outside our solar system and we found --- no water. None whatsoever. What gives?

First, you are probably wondering how we can study these planets when we can't yet take pictures of them. The two planets in question, around the stars HD 209458 and HD 189733, both pass between their parent star and the Earth, blocking off part of the light from the star. But around the very edge of that planet, we will see starlight that passes through the atmosphere. If we compare the spectrum of the light from the star when the planet is off to one side and the spectrum of that same star when the planet is in front of it, any differences will be the chemical fingerprints of the planet's atmosphere.

After careful study, the researchers working on each star found some spectral signatures, but no signs of water. What can this mean?

The planet around HD 209458 showed signs of dust in its atmosphere. This is not too surprising -- the planet is close enough to its parent star that dust can form clouds (like the water in our atmosphere) because it is so hot. These dust clouds would hide anything lower in the atmosphere, just like clouds in our atmosphere hide the ground underneath.

There are two other explanations for the lack of observed water. In the first explanation, maybe the atmosphere of the planet is very stable so that there is no weather. In this case, the water would not necessarily show up in a spectrum. I don't understand atmospheres, so I can't comment on how likely this is, but it sounds a bit contrived to me. The other possibility is that the water is just not there.

What is the most likely explanation? If you forced me to guess, I would say that the planets probably have water, but it is very deep in the atmosphere, buried under dust clouds and general haze. The fact is, we don't understand planetary atmospheres very well. While most everybody has heard of Jupiter's Great Red Spot, we still don't really know what makes it red. Model atmospheres on the Earth work pretty well, but we still can't predict the weather. At least one Martian probe was lost because Mars's atmosphere was thicker than normal when the probe arrived. Saturn's moon Titan has an atmosphere that we thought requires oceans of liquid methane on the moon's surface, and yet beyond a few lakes near the moon's poles, there's not much there. And there are many ways to make an atmosphere on a planet -- if you don't know exactly what is there to begin with, your guesses as to what you will see will be very wrong.

So, in short, I am not worried that planets outside the solar system don't show signs of water. My suspicion is that the water is there, just that we don't yet know how to find it. This problem will give both the observers and the atmospheric modellers plenty to work on in the coming years!

Monday, February 26, 2007

It's all relative, but to what?

This weekend I received an email from a reader who asked:

We say that everything in this universe is relative, and there is no absolute frame in this universe with respect to which we can measure everything. But I have read on many websites that our Sun is moving with a speed of 140 miles per hour, and our Galaxy the Milky Way is also moving with a speed of 190 miles per hour. With what frame we are measuring these speeds?

This is a very good question regarding what can be a tricky subject. Before we leap out into the Universe, let's start close to home. Suppose two joggers, Mick and Mack, are jogging toward each other, and we are being lazy and sitting on the porch watching them. If we measure each jogger's speed, we get 7 miles per hour. Yet if Mick Jogger measures the speed of Mack Jogger, he measures 14 miles per hour. Likewise, Mack Jogger measures Mick Jogger moving at a speed of 14 miles per hour. And both Mick and Mack measure our speed as 7 miles per hour. What speed you measure all depends on your frame of reference. Now, which of these three people measured the "right" speeds?

Many people would say that WE did, because we were sitting still. But we aren't sitting still -- the Earth is rotating, it is orbiting the sun, the sun is orbiting the galaxy, and so on. But in our daily lives, we don't need to think about all of those things. Without thinking, we choose a "convenient" frame of reference -- in this case, the people sitting on the porch -- because it makes sense to us intuitively. But, it doesn't matter whether you are Mick or Mack Jogger, or sitting on a porch, or flying in an airplane overhead. If you are trying to do any physics (say Mick is tossing a baseball in the air as he jogs and you want to measure its motion), any of those frames give you the right answer.

In fact, one of the main ideas behind Einstein's general theory of relativity is that there is no universal "inertial" reference frame -- in other words, in the Universe, there is no way to define being at rest. (This is a greatly, greatly simplified explanation, by the way, but is what is important for this question.) So, just like when looking at joggers on the street, we come up with a "best" reference frame depending on the situation.

For example, when the space shuttle docks with the International Space Station, you sometimes hear about how you have two objects moving at 17,000 miles per hour trying to gently hook up in space. But that speed is relative to us on the ground. During the final stages of docking, the relative velocity of the space station to the space shuttle is measured in fractions of a foot per minute, not tens of thousands of miles per hour!

For another example, we know that the planets orbit the sun. So when talking about our Solar System, we make the reference frame the "Solar System Barycenter," the true center of the orbits of everything in the Solar System. (It's pretty darn close to the sun, roughly 60,000 miles outside of the sun). Planetary orbits, spacecraft motions, and so on are often quoted with respect to the barycenter.

When we expand our study to include nearby stars, astronomers often change to talk about the "Local Standard of Rest," which is just the average of the motion of a lot of nearby stars, including the Sun. This Local Standard of Rest is useful for studying how stars are moving in the galaxy with respect to one another. The sun moves at about 10 miles per second in relation to this reference frame. The most widely-used reference frame is the Cosmic Microwave Background. This is the "echo" of the Big Bang, light that has been travelling unhindered through the Universe since the first hydrogen atoms formed. When this light was created, gravity had not yet had time to start creating big structures in the Universe, so most things were pretty close to at rest relative to each other. It is also pretty easy to measure our velocity relative to the cosmic microwave background using the Doppler shift -- almost exactly like a police officer uses radar to measure your car's speed. From this, we see that Local Group of Galaxies is moving at about 350 miles per second relative to this background. We think it is due to the gravitational pull of a largesupercluster of galaxies called the "Great Attractor."

So, in short, when you see mentions of how fast things in space are moving, you should ask yourself, "In respect to what?" You can get very different answers that can tell you very different things.

Wednesday, February 21, 2007

What we learned from Supernova 1987A

20 years ago on Friday, Supernova 1987A exploded; this remains the closest supernova to be seen since the invention of the telescope. The above picture, taken by the Hubble Space Telescope 3 years ago, shows about what the supernova looks like today. The bright ring is material tossed off of the star long before it exploded; the Hubble press release site contains a video (taken over several years) where you can see the ring light up as the shock wave from the supernova plows into the material. In the middle of the ring, the tenuous object you see is the expanding nebula, the remains of the star expanding outward at almost a thousand miles each second.

So, what have we learned from this supernova? First, we were surprised to learn that the progenitor star was a blue star, not a red supergiant. Astronomers had thought that a star had to swell up into a red supergiant star before the explosion. We now know that this need not be the case. As time goes on, we have seen other supernovae in other nearby galaxies, and we can identify the star they came from. Most of these are indeed red supergiants. So why was the star that made Supernova 1987A blue? This is still a bit of a mystery. Maybe the star that exploded had a fainter, nearby companion star. That companion star could have stripped away gas every time the dying star tried to puff up, keeping the hotter, bluer inner parts of the star exposed. Or perhaps the companion star and the dying star had spiraled together and merged into a single star recently, which would also make a red giant bluer. Or perhaps the material we see as rings around the star may have been blown off of a red supergiant in a near-explosion eons ago, essentially getting rid of the outer layers of the star. It's hard, now that the star is gone, to study these possibilities in too much detail.

Second, we learned that stellar death is quite messy. Supernova 1987A is surrounded by rings and shells of gas and dust, all shed by the parent star long before it exploded. This wasn't THAT surprising -- it had been suspected -- but the supernova lit up all of that invisible gas and dust so we could study its structure. This gives us clues as to when and how the star lost that material.

Finally, we've collected a ton of information about the process of a supernova itself -- details about the elements made in the explosion, about the production of various types of light from radio through X-rays. And some mysteries remain. Supernovae caused by dying stars exploding leave behind a remnanat, either a neutron star or a black hole. So far, we haven't seen anything at the center of this supernova. Did it make a black hole? Or is the expanding cloud of dust and gas still too thick to allow the neutron star to show through? We just don't know, and until the next nearby supernova goes off -- maybe tomorrow, maybe in a few hundred years -- Supernova 1987A will continue to be intensely studied from Earth.

To end, I'd like to shout out a happy anniversary to Oscar Duhalde, one of the co-discoverers of SN 1987A. I've worked with Oscar during several observing runs in Chile, and I was there last year to see him get several pats on the back on the 19th anniversary of being the first human since Johannes Kepler to see an exploding star with his bare eyes.

Tuesday, February 20, 2007

Things that go "Bang" in the night

The night of February 23rd, 1987 started off as a very typical night for observatories around the world, and it ended up being one of the most important nights for astronomy in the modern era. At the Las Campanas Observatory in Chile, one of the telescope operators, Oscar Duhalde (pictured above), took a break and went out to look up at the sky. At this time of year, the Milky Way's brightest companion galaxy, the Large Magellenic Cloud, is high in the sky. Oscar noticed a star in the LMC that he'd never seen before, but then had ti get back to work. At a neighboring telescope, astronomer Ian Shelton was taking a four-hour long picture of part of the Large Magellenic Cloud. In these days, we used photographic plates instead of electronic cameras to take astronomical images, so at the end of the exposure, Shelton developed the plate and noticed a bright star where none should have been. After talking among themselves and looking up at the sky, the astronomers at Las Campanas realized that they had detected a supernova -- a massive star ending its life in a cataclysmic explosion. Before news of the supernova filtered out, up to half a dozen people had independently "discovered" the supernova. Supernova 1987A, as the event became known, is the closest supernova to be seen since the invention of the telescope. Because the Large Magellenic Cloud is well-studied, astronomers even knew a little bit about the star that exploded. From this event, astronomers have learned a lot, much of it surprising, about the end of stars' lives. Tomorrow I'll talk about a little of what we learned from this supernova.

Monday, February 19, 2007

A "new star" in the sky

As I mentioned last week, this Friday marks the 20th anniversary of Supernova 1987A, the explosion of a star in the Milky Way's neighbor galaxy the Large Magellanic Cloud. This is the closest visible supernova to go off since the 1600s, and so is one of the most important objects for studying these stellar explosions.

As if to honor this anniversary, a nova ("new star") has appeared in the constellation Scorpius, and has been given the completely boring name of V1280 Scorpii. This past weekend it was visible to the naked eye, but it is fading rapidly. If you want to try and find it, you'll need to wake up early in the morning. Here is a sky map from Sky & Telescope magazine. Today's Astronomy Picture of the Day has a picture of the nova taken from the desert of Iran.

What is a nova? Most people have heard of supernovae, which is the explosion of an entire star. A nova is also a thermonuclear explosion, but it only happens on the surface of a specific type of star, a white dwarf, and the star lives to tell about it.

White dwarfs are the exposed nuclear furnaces of dead stars. Most stars become white dwarfs. They run out of nuclear fuel, but the nuclear furnace is too small to explode spectacularly. The outer layers of the star float out into space as a planetary nebula, leaving the white dwarf behind to slowly cool and fade away.

Some white dwarfs, though, have a companion star. This star all but ignores the death throes of its sister star, and continues to burn its own fuel. Eventually, though, it begins to run out of fuel, too, and it starts to swell up into a red giant star. But as it swells up, the outer parts of the dying star come close enough to the white dwarf that they are pulled on to the white dwarf by gravity. An artist's rendition of what this might look like can be found here.

This gas is mostly hydrogen, the typical fuel used by stars. At first, the hydrogen just sits on top of the white dwarf star, minding its own business. But as more gas continues to pour on, the hydrogen gets compressed and starts to heat up. Eventually some part of the hydrogen gets hot enough to start a nuclear reaction, and the surface of the star ignites in a huge explosion of hydrogen. The explosion pushes most of the accumulated matter, now burned into helium, off into space.

Over time, the system calms down, and more gas from the companion star starts to settle on the white dwarf, and the cycle begins anew. For some white dwarfs, the time between novae is only a few tens of years, but for most stars (such as the white dwarf in Scorpio that just went nova), the interlude can be tens of thousands of years. We astronomers have no warning of a nova -- when an amateur astronomer sees it is often the first we know about it. How many novae happen in our own galaxy that we don't know about? That I don't know.

Thursday, February 15, 2007

Spreading the word

One of the most important things astronomers do is to spread what we learn through our research to other people: our fellow astronomers, our students, and to the general public. After all, what is the point of learning new things if that knowledge sits in a desk drawer or buried in a computer file? There are stories of physicists who would make a new discovery and go to talk to Nobel-winning physicist Richard Feynman about their discovery. Feynman would listen for a while, dig through a drawer, and find some papers where he had worked out the same problem, and then say that the answer looked right to him. Then he would put his papers away.

If true, these stories are sad. Feynman hated writing papers about his work, but if one of the brilliant minds of physics failed to share his knowledge with the rest of the world, the rest of us lost out on his talent.

On Tuesday, I had the honor of giving a colloquium (an hour-long research talk) here at the University of Texas Astronomy Department. I stood up and talked about my studies of other galaxies and of what we are learning about dark matter in and near those galaxies. The talk went well, and several people contributed new ideas and new viewpoints that will help me look anew at my studies as I go ahead. And I think I gave many people here insight into research that they don't do but that might impact their work. All in all, it was a lot of work but a lot of fun to prepare and give the talk.

Wednesday, February 14, 2007

Happy Valentine's Day

In other articles I have talked about the connection between holidays and astronomical events, such the "cross-quarter Days" halfway between the an equinox and a solstice. These include Groundhog Day (between the winter solstice and the spring equinox), May Day (halfway between the spring equinox and the summer solstice) and Halloween/All Saints Day (halfway between the autumnal equinox and the winter solstice). Christmas, not by coincidence, comes near the winter solstice, Easter and Passover near the spring equinox. So, a lot of our holidays are closely related to the seasons and, thereby, astronomy.

But, I can safely say that there seems to be no connection between Valentine's Day and astronomy. We are not near any major seasonal event, nor was St Valentine (take your pick which one we are talking about) associated with astronomy. So, if you have a special person, you can take them out for a special night and not have to bore them with astronomical lore about the holiday. And if you don't have a special someone, you can fully ignore the holiday and not be neglecting some very important event.

Elsewhere in astronomy, we are coming up on the 20th anniversary of the discovery of Supernova 1987A, the closest supernova to be observed during modern astronomy (and the closest to be seen since the early 1600s). As that date draws closer, I'll talk a little about how important that event was, and how much more important the next supernova in our galaxy will be.

Monday, February 12, 2007

Lies, d*mned lies, and statistics

There are three kinds of lies: lies, damned lies and statistics. -- attrbuted to Benjamin Disraeli

A week ago I was watching the Big Football Game Whose Name I Am Not Allowed To Say Lest I Get Sued For Trying To Make Money Off the NFL Without Giving Them Their Fair Share Even Though I Don't Make a Cent From This Blog; for short, let's call it the "Stupendous Bowl." Anyway, the Chicago Bears won the coin toss, and the anouncers remarked that this was the 10th year in a row that the NFC had won the coin toss, a 1-in-1000 occurance. This quote implies that the Bears had a 1-in-1000 chance of winning that coin toss. That implication is wrong. Unless the mob has gotten involved and "fixed" the coin toss, the coin doesn't remember from one year to the next what team won the toss. Each year, there is a 50-50 chance that the coin toss will be won by the NFC team.

The statement isn't totally wrong. If I were to ask the question, "What is the likelihood that the next 10 coin tosses will be won by the NFC?" the answer would be one in a thousand, more precisely, 1 in 1024. But the chance that next year's coin toss will be won by the NFC for the 11th year in a row? 50-50.

People often mess up playing this game of statistics. Let's look at gambling in Las Vegas. Many people who play the slots stand around watching to find a machine that hasn't paid out in a long time, and then they sit down and plop their coins in. Whether they admit it or not, the chance of winning with that machine is no greater than it was the pull after the machine's last payout. By law, those machines can't remember when they last paid out money. It's completely random. Unless the game is fixed (and illegal), take any open seat.

This same game is often played in astronomy. On average, humans see a supernova (an exploding star) in the Milky Way every hundred years. The last supernova in our galaxy was seen in 1604, 403 years ago. So you often hear astronomers say, "we are overdue for another supernova!" But many people who hear this think that we must have one any day now, that somehow the chances of seeing a supernova next year are higher than average because of the long delay. Again, this is not proper thinking about statistics. Other stars in the galaxy don't know when one another explode, and they don't time themselves to go off exactly every 100 years. So, the chance of a bright supernova in the Milky Way this year is the same as the chance of one last year and the chance of one next year: about 1 in 100. (I'll sidestep the point that many supernova in our galaxy may be nearly invisible from Earth.)

What about earthquakes? On average, the San Andreas Fault produces "The Big One" every 150 years; in San Francisco, the last Big One was 100 years ago. Based on what I've said, does this mean that the chance of "The Big One" next year is 1 in 150, no matter when the Big One occurred?

Maybe, but probably not. Unlike exploding stars or coin tosses, the San Andreas Fault "remembers" when the last earthquake was. An earthquake releases stress in the rock along the fault, and we have to wait for that stress to build up again before there can be another big earthquake. This doesn't mean that earthquakes go off like clockwork, and it doesn't mean that the San Andreas Fault can't have two big earthquakes in a row. But it does probably mean that, as time goes on, the chances of a big earthquake are increasing.

Clear as mud? Probably. Statistics plays a central role in so many areas of science and even everyday life (just ask an insurance salesman), but it is very difficult to figure out what statistics mean, even when you work with them every day. So, be a bit dubious when you hear odds and statistics thrown around. And stop and think before plunking down a large chunk of money on a bet of next year's Stupendous Bowl coin toss outcome, thinking that the AFC must finally win the coin toss. The chances are only 50-50.

Thursday, February 08, 2007

A little astronomy humor

While I continue to chip away at projects with upcoming deadlines, here's a humorous spin-off of the truly sad story of astronaut Lisa Nowak.

Meanwhile, in happier news, here is a NASA press release about the ongoing work on the James Webb Space Telescope,NASA's next large telescope. It is still far from launch, at least 5 years (and probably more than that, in my opinion).

Wednesday, February 07, 2007

The onslaught continues...

Sorry to be so quiet, but several deadlines have piled up on this week, keeping me overly busy. Things will settle down after Friday, so I'll talk to you then!

Friday, February 02, 2007

Thats right, woodchuck-chuckers, it's Groundhog Day!

"There is no way this winter is ever going to end as long as that groundhog keeps seeing his shadow. I don't see any way out of it. He's got to be stopped. And I have to stop him."  -- Bill Murray, Groundhog Day

For those who haven't heard, Punxsutawney Phil failed to see his shadow this morning, meaning winter is over! Of course, he also failed to see the strong blast of Arctic air moving toward Punxsutawney. But, as far as I know, Phil is not a member of the American Meteorlogical Society, so perhaps that can be forgiven.

Phil is also not an astronomer. 6 and a half weeks from today, on March 20, is the first day of spring. So, in the literal sense, spring can never come early; on Groundhog Day, there will always be six more weeks of winter. But, you may already be noticing the first hints of spring. The days are starting to get longer quite rapidly. I noticed yesterday that, despite having to work late, it was still light when I left my office.