Thursday, November 29, 2007

Sorry to have been quiet...

It's been a quiet week here on the blog, and today will continue to be quiet. I was traveling for the Thanksgiving holiday, and was supposed to be home yesterday. But then I came down with a 24-hour bug, and had to delay my travel.

The airline was nice and only charged me $350 (a change fee plus fare difference) to come home a day later and avoid infecting an entire cabin with whatever I was carrying. I had thought they might be a little forgiving with the change fees, but I overestimated them. You would think the airlines would be happy to facilitate a change to keep sick people off their plane -- I would have been happy to go to an urgent care clinic to get a doctor's note verifying that I was ill.

But, anyway, I'll be back tomorrow. And hopefully healthy, too.

Wednesday, November 21, 2007

"Just plain weird" -- An odd star proves to be a new type of object

On the night after Thanksgiving two years ago, I was at the Keck Telescope, trying to identify some very blue objects in the sky. I was looking for white dwarfs, which are the compressed ashes left behind when a star ends its life. The technique I use, spectroscopy, involves splitting its light up into component colors. Different types of astronomical objects have vastly different spectra, depending on the objects temperature, chemical composition, distance, and other physics. So, we can learn a lot from a spectrum.

I'd been working on this project for quite a while, and I could easily identify the spectra of every object we were looking at. Most were white dwarfs, some where quasars (black holes halfway across the universe scarfing down tremendous amounts of matter), and the rest were other types of stars in our Milky Way galaxy. But there was one object that mystified me -- I'd never seen a spectrum like it before. I spent a lot of time trying to identify the spectrum, before finally giving up and writing "Just plain weird" in the comment section of my observing logs.

When I returned to Arizona, I showed the spectrum to my friend and collaborator, Jim Liebert. Jim knows far more about white dwarfs than any person I know -- it this was a white dwarf, he'd be able to tell me. He stared at it for a while, rummaged through some published papers of his, looked at other white dwarf spectra he had, and finally announced that he thought he knew what it was.

The star was a white dwarf, but a very rare kind called a "hot DQ" -- the "D" stands for white Dwarf, and the "Q" stands for carbon (I still don't know why, other than "C" is taken for another type of star). Jim had discovered about a dozen similar white dwarfs out of nearly ten thousand known white dwarfs -- very rare, indeed.

Otherwise, we knew little about this star. As far as we knew, all white dwarfs are carbon and oxygen ash covered by a thin but opaque atmosphere made of helium and (usually, but not always), hydrogen. If there is any hydrogen present in my hot DQ at all, we'd have seen it, because hydrogen has very distinct signatures in spectra of white dwarfs. But helium is exceptionally transparent, and so can be hidden if you mix in just a little bit of some other element. So, Jim and I proposed that this star, and all the other hot DQ white dwarfs, had helium atmospheres with a tiny bit of carbon pollution -- maybe 1 carbon atom for every 100 or 1000 atoms of helium. But we couldn't do much else, because nobody had ever studied what happens to carbon in the extreme atmospheres of white dwarfs -- pressure millions of times that of Earth's atmosphere, and temperatures of 50,000 degrees.

So, we published a paper on our star, and went on, hoping somebody would be able to help us figure more out some day.

Around the same time, an astronomy PhD student in Montreal named Patrick Dufour was finishing up his doctoral dissertation. His topic was the atmospheres of cool DQ white dwarfs -- also white dwarfs with traces of carbon in their atmospheres, but "only" 10,000 degrees -- much cooler than my oddball white dwarf and Jim Liebert's collection. Patrick found that, for the cool DQ white dwarfs, the idea that these are helium atmospheres with a tiny amount carbon pollution is correct.

So, after getting his PhD, Patrick came to Arizona to work with Jim Liebert. Once there, Patrick turned his white dwarf atmosphere models to the hot DQ stars. The problem was, they didn't work. According to Patrick's models, all of the hot DQs should show spectroscopic signatures of helium, if they are just hotter versions of the cool DQs (as we all thought). But we don't see helium.

One day, Patrick, on a whim, tried an atmosphere that had no helium, only carbon. This seemed silly, because we knew that all white dwarfs have helium (and maybe hydrogen) in them. But the atmospheric models looked almost exactly like the spectra of the hot DQ white dwarfs. More work by Patrick confirmed this. The result: Hot DQ white dwarfs have carbon atmospheres.

Patrick's paper on his discovery is in tomorrow's issue of Nature, the most prestigious scientific journal. He's busy writing a longer and more thorough paper to publish in our astrophysics journals showing that all of the hot DQs fit this model.

To an astronomer, this is completely weird. The universe is full of hydrogen and helium; almost all objects are composed mostly of hydrogen and helium. Jupiter and Saturn are mostly made of hydrogen and helium, as are Neptune and Uranus. The Earth's gravity is too weak to hold on to hydrogen and helium, which is the only reason that the Earth and all the other rocky planets aren't mostly made of those elements.

So, the hot DQs are an entirely new type of star. But we still don't know where they come from. It may be that stars eight or nine time the mass of the sun can make white dwarfs with cores made out of oxygen and neon ash surrounded by an atmosphere of carbon, and then perhaps some helium and hydrogen further out. If this is right, why don't we see the hydrogen and helium? Or maybe these stars are normal helium-atmosphere white dwarfs that somehow re-start nuclear fusion of their atmospheres (since helium fuses to make carbon). This seems contrived.

So, now my star comes back into the picture. My hot DQ white dwarf is in a cluster of stars (Messier 35), and is the only hot DQ known in a star cluster. This gives us some valuable information, because all of the stars in a star cluster formed at the same time out of the same clouds of gas. So, we can figure out what the (now dead) parent star was like, and for my hot DQ, its parent star had to be at least five times more massive than the sun, maybe much more. There are lots of other white dwarfs in the star cluster, too, that I am currently studying. Our hope is that, with a little more study, we may be able to learn more about where my hot DQ white dwarf came from.

So, by a little bit of sheer luck, my "weird" star may be the key to understanding an entirely new group of stars! But now the real work begins -- we need a lot more information and analysis. I'll be sure to let you know what we learn!

To read press stories on Patrick Dufour's discovery, try this Reuters story and Space.com's article. Just to make it clear, my hot DQ star is not covered in Patrick's first couple of articles; we need a little more data to understand them first.

A Happy Thanksgiving!

I would like to write a nice story today about some cool new astronomy results that I am slightly involved in, but the story involves a paper in tomorrow's issue of Nature, a prestigious journal that forces us to not speak publicly about results until the date of publication. Never mind that it's been talked about among ourselves for a few months, and that I'm not even an author of the journal article. But, I don't want to get my colleagues in trouble, so I'll just have to wait another day (if I can find time to write before having to start cooking tomorrow, that is).

So, instead I will wish you all a Happy Thanksgiving, and safe travels to all who are travelling.

Tuesday, November 20, 2007

Telescopes for Christmas

Believe it or not, this Friday marks the beginning of yet another holiday shopping season in the U.S. Perhaps someone on your shopping list is pestering you to buy them a telescope. Should you do it?

My opinion on this is unchanged from my post on this subject from last year. Unless you are absolutely positive that a telescope will get a lot of use, you may want to consider buying some nice binoculars and a tripod for those binoculars instead of a telescope. There are a lot of neat astronomical objects that can be seen with binoculars (comets, the moons of Jupiter, galaxies, star clusters, and nebulae), and binoculars can be used for other interesting things, like bird watching, hunting, spying on your strange neighbors, and so on. A telescope is hard to learn to use and not much use outside of stargazing, and a lot of telescopes are purchased, used once or twice, put away out of frustration, and forgotten about.

Along with those binoculars, consider giving the gift of a subscription to an astronomy magazine like Sky and Telescope orAstronomy. Not only will a magazine keep your budding astronomer abreast of astronomy news, but it also will offer tips on what is up in the sky and some good targets to look at.

Monday, November 19, 2007

There's a whole lot of shaking going on!

Last week, a very large (magnitude 7.7) earthquake struck northern Chile near the city of Antofagasta. The disaster is striking the region hard, as would be expected from such a large temblor.

The epicenter of the earthquake was about 300km (190 miles) north of the European Southern Observatory's Very Large Telescope (actually four large telescopes) on Cerro Pachon in Chile. Although the shaking there was still strong (equivalent to a magnitude 5.7 earthquake), thankfully there was little or no damage, and the telescopes returned to operation very quickly.

I've been to telescopes about 600 km (about 400 miles) further south in Chile, where the shaking was much less, but I've always been aware that earthquakes can happen there -- many of the strongest earthquakes in the world occur in Chile. It takes a lot of energy to raise the Andes mountains 15,000 feet into the sky!

Earthquakes are a fact of life when telescopes are built on mountains. Last year, telescopes on Mauna Kea were damaged by a strong earthquake there. Lick Observatory, outside of San Jose, California, has experienced several earthquakes, including the great San Francisco earthquake of 1906. Telescopes in earthquake-prone regions are engineered to withstand strong earthquakes, and a destructive earthquake (say, magnitude >7) has not yet made a direct hit on an observatory.

My best wishes go out to the citizens of northern Chile, and I sincerely hope that repairs can be made quickly.

Friday, November 16, 2007

A plethora of planets in the Pleiades?


Image Credit: Robert Gendler and APOD

The Pleiades (the "Seven Sisters") are one of the most widely recognized star clusters in the night sky. These days, they are rising as darkness falls in the Northern Hemisphere, and are visible most of the night. Many people mistakenly call the Pleiades the "Little Dipper," since it kind of looks like a dipper, and it is fairly small on the sky (about the size of the moon), but the Little Dipper is actually elsewhere in the sky.

The Pleiades star cluster is a fairly young star cluster, as far as astronomical ages go -- it is about 125 million years old, and hasn't even had time to orbit the Milky Way once since it formed. When the Pleiades was young, dinosaurs like this one roamed the Earth, and, if they had any astronomical inklings, they might have seen several exploding stars in the young star cluster, though that excitement has long since stopped.

Although only the seven brightest stars are visible to our eyes, the Pleiades has at least 1500 stars, and it is frequently the target of astronomical study, because even the faintest stars are relatively easy for professional telescopes to see.

From the standpoint of looking for planets, the Pleiades is a great target, Not only is it nearby, and so easy to study, but we think the sun might have been about 100 million years old when the Earth had finished forming in our Solar System 4.5 billion years ago. And some evidence suggests that the Solar System was in a star cluster when the sun formed. So, looking for evidence of forming solar systems in the Pleiades may help us understand where we came from.

Yesterday, astronomers at UCLA announced that they have found evidence of rocky (Earth-like) planets forming in the Pleiades. Their evidence, from both space-based and ground observatories, is that one star a bit bigger than our sun, called HD 23514, has a lot of dust around it. Our sun has a lot of dust around it -- most of the dust comes from comets and asteroid collisions -- but HD 23514 has a million times more dust around it than the Sun does.

Dust is a very fragile thing. Light from a star will either push dust away or cause it to spiral into the star within a few thousand years, so to see a ton of it around another star means that something has happened recently to produce it. The UCLA astronomers propose that two large planets may have collided withing the last few hundred thousand years (such a collision is what made Earth's Moon), but it could also be that there are a lot of asteroids around, many more than in our Solar System, and those could be constantly colliding and grinding each other into dust.

Whatever is going on in the Pleiades, it continues to be a great place to try and study how planets form!

And, speaking of dust, if you have insomnia this weekend and clear skies, try catching the Leonid meteors. Every 30-35 years, the Leonids put on spectacular shows with hundreds of meteors falling every minute, but this year you'll be lucky to see 10 or 20 meteors per hour. Still, the Leonids zip across the sky really quickly, and the brightest ones leave behind glowing tails that can last for several minutes! The Leonids are best seen in the Early morning hours -- you won't see very many until after midnight. Monday morning will be the best time to see Leonids, though meteor showers usually last a couple of days on either side of the peak.

The Leonid meteors are caused by dust shed by Comet Tempel-Tuttle, which orbits the sun every 33 years. So, if you see a meteor speeding across the sky early in the morning this weekend, you might be seeing a little bit of comet dust. And just imagine if you were on a planet in the Pleiades, where the meteor show would be about 1 million times more exciting!

Wednesday, November 14, 2007

It's coming straight at us -- we're doomed! Oh, wait, never mind.

Sometimes (well, most of the time) astronomers are happy not to have paparazzi documenting our every move and blaring it on national news 15 minutes later. Last week was one of those times. Thankfully, I was not involved!

Several observatories in the U.S. (and around the world) spend every clear night looking for near-Earth asteroids. These are asteroids that have the potential to hit the Earth some day (maybe not for hundreds of millions of years, but someday). Roughly 5000 of these are known; most are pretty small (about 500 yards across or less -- large enough to wipe out a city, small enough that they wouldn't destroy the Earth). Most of these asteroids are discovered either just before or just after they come close to the Earth -- by close, we mean a few million miles.

On Thursday night, three different observatories discovered a near-Earth asteroid coming our general direction, and the initial estimates had the asteroid coming only 5000 miles away from Earth's surface today, which would have been the closest asteroid approach we've ever seen! There is also some error on that measurement, so we really didn't know exactly how close the asteroid would come. But it would be too close for comfort.

Bulletins raced out for more observations to better determine the asteroid's trajectory, and press releases were prepared. Then, before public announcements were made, a Russian scientist named Denis Denisenko noticed that the asteroid's path almost exactly matched that of a European space probe, Rosetta, that is swinging by Earth today on its way to visit a comet. In fact, this was no dangerous asteroid, but a human robotic spacecraft.

So, everyone could breathe a bit easier, even though many people probably feel they have a little egg on their face. The Minor Planet Center, the international organization responsible for monitoring all asteroids and comets, issued a statement calling for a better database of spacecraft orbits.

However, I see a silver lining in this fiasco. First, the system worked -- an unknown object that was going to come very close to the Earth was spotted before it arrived -- while a few day's warning would not be enough to save the Earth from a giant asteroid, it would be enough to protect people from a smaller asteroid through evacuations. (It would be possible to get a decent idea where an asteroid would impact.) Second, although the mistake was not picked up as quickly as we would have liked, it was discovered before the public was alarmed. And, last, we learned that there are some tweaks to the system that are necessary.

Tuesday, November 13, 2007

The Pale Blue Dot


Image credit: JAXA/NHK

One of the hardest concepts in astronomy for anyone to comprehend (including professional astronomers) is the distance scales we are dealing with in space. Our brains are wired to understand scales of feet, yards, and miles (or centimeters, meters, and kilometers, if you prefer). Most of us can properly imagine relative sizes on these scales. And, if you've done a lot of travelling, even larger scales are imaginable; for example, if you know it takes six hours to drive from San Jose to LA (sans traffic), and an airplane flight of that distance takes an hour, you know roughly how distant a five-hour flight will take you.

But when we go to space, our minds cease to be capable of relating our lives to the cosmic scale. The Moon is 250 thousand miles away; by airplane, that trip would take 18 days. An airplane trip to the sun would take two years (and you thought the 7 hours to Europe was bad -- imagine if you miss your connecting flight!). And the nearest stars are hundreds of thousands of times further away than the sun!

Although it is possible to imagine relative sizes (such as how long an airplane trip might take), our brain just cannot comprehend the true physical distances involved. We've never experienced it.

The above picture was taken by a Japanese moon orbiter called Kaguya. It shows the Earth setting over the south pole of the Moon. That blue globe is our home in high-definition -- yet it is still hard to make out! The land you see in the picture is Australia (upside down) and southeast Asia. Australia is about the size of the United States. Our planet seems so big, and yet, when seen from our nearest celestial neighbor in high-definition, it's hard to make out home at all.

Apollo 8 astronauts took a similar photo in December 1968, a photo which has become one of the iconic views of our planet.

As a side note, the rising and setting of the Earth is due to the orbiting of satellites around the Moon. If you were to stand on the Moon, the Earth would stay in almost exactly the same spot relative to your horizon -- it would never rise or set, though it would go through phases like the Moon. This is because the Moon's rotational speed is identical to its orbital motion (and the same reason why the same side of the Moon is always seen from Earth).

Monday, November 12, 2007

A Happy Veterans Day

Yesterday was Veterans Day, when we in the United States celebrate those who have served in our armed forces; numerous other countries also observe the day in memory of those soldiers who were killed in battle.

So, may I issue a big thank you to all who have served their country in the armed forces.

Friday, November 09, 2007

Cosmic rays

I was reading the astronomy news this morning when I came across this article with the title proclaiming "Cosmic rays believed to start in black holes." Now, we know that once something is in a black hole, it can't escape. That's the definition of a black hole. So, I read the article to find out what was really intended by the story. As I suspected, this headline was just a little artistic license. But the story is interesting nonetheless.

Cosmic rays are bits of radiation that are always passing through us. Most of the time they do no harm, and there is no way to shield yourself from the radiation, so life has evolved to live with it. Most cosmic rays are protons or electrons, occasionally other stuff like helium or even iron. These atoms are accelerated to near the speed of light. When they hit Earth's upper atmosphere, the collision (sometimes stronger than our biggest particle accelerators) releases lots of energy, including some faint light (too faint for our eyes to see) and other subatomic particles.

Cosmic rays come in a variety of energies, from pretty wimpy to some true monsters. The largest cosmic rays ever measured are strong enough that, if all of their energy could be delivered as motion energy to a 2 pound block of lead, that block would jump a foot into the air. And all of that energy comes from a single atom!

Most cosmic rays are through to come from supernovae, the explosion of a dying star. As the remnants of the star push out into space, it is possible for protons, electrons, and other atoms to get trapped at the boundary between the explosion and the gas of outer space beyond. While trapped in this boundary, it gets bounced back and forth, slowly picking up energy until it is travelling near the speed of light and it escapes.

The physics here is quite complicated, but it seems to work, and we have seen X-ray glow coming from these particles, so most people think this mechanism works. The problem is that this mechanism can't produce the monster cosmic rays. They'd escape the supernova long before they became so ultra-energetic. So, we need something more monstrous than an exploding star to make these.

Some astronomers have suggested that the monster black holes at the center of most galaxies could make massive cosmic rays. As material is falling toward the black hole (but not yet inside!) it is possible to give a tiny bit of the incoming material some ginormous amount of energy.

But, in order to prove this, we need to figure out where cosmic rays are coming from. A world-wide collaboration of scientists has used several telescopes in Argentina to image the faint flashes of light coming from cosmic rays hitting the atmosphere, and also to trace that light back to the direction the particles that caused the flash came from. And all the monster cosmic rays come from nearby galaxies known to host giant black holes.

While this news is not Earth-shattering, nor was it unexpected, it is a big triumph of both theory and observation. A lot of people spent a lot of time to develop a very complex experiment, and it worked. That is highly satisfying. And they may have cleared up one of the longest-standing mysteries in astronomy -- cosmic rays were first discovered in 1912, and we only know are certain where all the cosmic rays come from.

As for coming from inside of black holes, that isn't right -- some editor mis-interpreted the article. If you'd like to read a more technical version (but not too bad) of this story, Science Magazine has a nice summary. But I'm not sure if you'll be able to read it or not -- many (but not all) of their articles require a subscription to the magazine.

Wednesday, November 07, 2007

Five planets and counting...

Yesterday, one of the most productive teams hunting planets outside our own Solar System announced that they had discovered a fifth planet around a star already known to hold four planets.

The star, 55 Cancri, is about 20% smaller than the sun, is about 41 light-years away, and is in the constellation Cancer. So, it is a pretty normal star.

Before this announcement, the star had four planets, including one discovered by astronomers here at the University of Texas. Three of these planets are about the sized of Neptune and Saturn, and are pretty close to the star. The fourth is a whopping four times the mass of Jupiter, but is pretty far away from the star (about as far from Jupiter is from the sun).

The newest planet is about the same mass as Neptune, but it orbits its star at a distance of 80% the Earth-sun distance, and since its parent sun is 60% as bright as our sun, this means that the newest planet receives only a little less sunlight than the Earth does. In other words, the new planet, if it has a solid surface, could have liquid water on it. Or, if the planet is a bag of gas like Neptune, its moon (if it has one) could have liquid water. Maybe those aliens I was dissing a few posts back could have evolved on a moon of this planet?

To me, the most interesting part about the new find is not that the new planet may have liquid water, or even that it is the 5th planet in another solar system. After all, we have 8 or 9 planets, so our solar system is still #1 in that regard! No, I find most interesting the large variety in solar systems we are finding. Some stars don't have planets that we can detect. Some only have one as big as Jupiter that is orbiting right next to the parent star. Our solar system has four big planets, all pretty far from the star, and four small, rocky planets close to the star. 55 Cnc has four big planets close to the star, and one monster further out.

To me, this shows that extrapolating what we know about our solar system to planets across the Universe is no longer good science. For a while, it was all we could do, because it was the only example we had! But now we have hundreds of planets, and dozens of multiple-planet systems that we can study, and they exhibit a wider range of variety than we had though possible. This also tells us that making planets is easy -- you don't need a special configuration like our solar system for planets to form and stay put. A wide variety of conditions make planets, giving rise to a wide variety of solar systems.

And maybe, just maybe, this means that we shouldn't be surprised to find that our Galaxy is teeming with life. After all, if planet systems come in many shapes and sizes that look nothing like our own solar system, maybe life can come the same way, too.

Tuesday, November 06, 2007

Monday, November 05, 2007

UFOs, Aliens, and Politics

During a Democratic presidential debate last week, the topic of UFOs came up. As an astronomer, I am very often asked about UFOs, aliens, and other such things. So, here are my answers to these questions. And they are all opinion, not proven scientific fact.

Do I believe that people see UFOs?. Yes, without a doubt, but UFO means "Unidentified Flying Object", and not "alien spacecraft." People see things in the sky all the time that they can't identify -- this is especially true at night. Our eyes did not evolve to see well in the dark, and can play tricks on us. Airplanes, satellites, meteors, even planets and bright stars are often called "UFOs" because the people looking at them did not know what they were looking at. Even trained astronomers and pilots accustomed to flying at night see weird things that, in retrospect, can be explained. The number of UFO sightings that are truly unexplained is a very tiny fraction of all total reports.

Do I believe that alien beings visit the Earth? No. The reason is that space is amazingly huge and hostile, much more so than almost anybody recognizes. At current best speeds, it takes our space probes tens of thousands of years to reach the nearest stars! And, even if there is technology that allows these distances to be crossed easily, why would aliens seek us out yet remain so secretive? With all do deference to people who think they have seen alien spacecraft, the evidence is exceedingly flimsy. Even among those handful of reliable UFO reports that cannot yet be otherwise explained, there are no high-quality photographs or videos that stand up to rigorous scientific scrutiny. And finally, if there are lots of alien civilizations out there, we are not very advanced, and could not possibly be very interesting. If aliens are just curious about us (like us watching meerkats on TV), then why take all the personal risks of space travel, when a robot ship could do all sorts of testing, filming, and so on?

And please don't accuse me of being close-minded; I'd very much like to know if alien civilizations exist, and, if they are scooting about the stars, how they do it. But there has to be very rigorous evidence, and nothing of what I've seen approaches rigorous.

Again, I do not question that people who have seen UFOs have indeed seen something. I just don't believe these people have seen alien spacecraft.

What about Roswell? Read this article. I think that alien spacecraft made a great cover story for what was really a defense experiment, and so the government was willing to let the story linger (and perhaps even encourage it).

Finally, do I think alien life exists? I suspect that it does. As we have started to find planets in the galaxy, we are finding them everywhere, in every shape and size. And on Earth, we are finding life just about anywhere it can exist, and many places where we think it shouldn't. But we still have very little idea why life arose on Earth, and how narrow of conditions a planet has to experience to develop life. And, for three and a half billion years, life on Earth was nothing more than mats of bacteria and algae -- we don't know why that suddenly changed 500 million years ago. So, as we look for life elsewhere, I think the first things we are most likely to find are bacteria. But until we do discover that, Earth remains a very special place.

Friday, November 02, 2007

Modifying Isaac Newton

This week, a handful of news articles stated how last year's "proof" of the existence of dark matter may not be proof after all.

This article features work on the "Bullet" Cluster, two colliding clusters of galaxies about 3.4 billion light years away. By combining information on where galaxies are, where hot X-ray emitting gas is, and how the total mass of the cluster is bending light from even more distant galaxies, the original team claimed that the only explanation that could simultaneously explain all the observations was that dark matter actually exists.

Dark matter is quite mysterious. When we look at other galaxies and clusters of galaxies, the stars and gas in those systems doesn't move like we think it should according to Newton's Law of Gravity. In most cases, the star's movements can be explained if there is matter we cannot see that obeys the laws of gravity, but otherwise doesn't interact with normal matter. Standard physics doesn't predict such matter, but some new physics hypothesizes that particles with these characteristics could exist. This is what we call dark matter.

Despite not knowing what dark matter actually is, including these basic properties into calculations of the formation and evolution of the Universe results in predictions that are pretty close to reality, although not exactly right. Dark matter has not yet been detected or made in physics laboratories, however. Until it is, I won't feel comfortable saying that dark matter has been "proven." I am pretty sure dark matter does exist, but I want proof.

In the 1980s, another hypothesis was put forward to explain the odd motions of stars and gas in other galaxies. This hypothesis is called "MOND," or MOdified Newtonian Dynamics. MOND says, in short, that gravity works just like Isaac Newton claims it does, getting weaker as you get further away from matter, at least until a certain point. After that point, gravity falls off more slowly than Newton would predict.

It is true that MOND can explain some parts of the observations behind dark matter as well as dark matter can. And about a decade ago, it was noticed that the Pioneer spacecraft leaving our solar system are slowing down at a different rate than Newton's Law would predict (although this signal is so small that there are lots of different tiny effects, like acceleration of the spacecraft due to the sun's light warming some parts more than others, and the extra "push" the craft gets when it sends radio waves back to Earth -- extraordinarily tiny effects that are hard to calculate exactly).

The problem most astronomers have with MOND is not (as some MOND supporters claim) that it would get rid of dark matter. The problem is that the MOND is continually being tweaked to explain new observations. And, as I've said before, no hypothesis can become a mature theory until it makes a prediction that can be tested. Explaining things in hindsight is not acceptable in that regard.

For MOND to be seriously considered by a large number of astronomers, it needs to make a prediction about something we can see that cannot possibly be due to dark matter or anything other than MOND. And then we have to go look and test that prediction. If the prediction holds true, MOND will get a big boost. But, if it is found not to hold true, then MOND supporters need to re-think their theory.

Vocal MOND supporters have pointed out how, in science history, there were times when widely accepted theories were held on to long after the evidence supported throwing them out. An example is the "ether," a mysterious substance that was hypothesized as necessary for light waves to travel. But tests showed that the ether did not exist, and new theories were developed to explain how light can travel in a vacuum.

However, historical precedence does not and should not mean that MOND is more likely than dark matter. As I said, dark matter still needs to be proven in a lab, and it may be that we find that dark matter, like the ether, doesn't exist. But, unlike the ether, dark matter has made testable predictions that are, for the most part, correct. And other "phantom" particles, like neutrinos, were hypothesized long before they were conclusively detected in the lab.

So, dark matter is not dead, nor even slightly ill. And if MOND wants to challenge the reigning theory, it needs to bulk up and fight according to the scientific rules of engagement: (1)develop a hypothesis, (2) make a testable prediction that would not be predicted by the prevailing theory, and (3) test the prediction.

Thursday, November 01, 2007

Only six more weeks 'till winter!

One positive aspect about blogging with this week's holidays is that I can recycle past blogging ideas with impunity, and then just chalk it up to zombie posts -- topics that I won't let die until they get into your brain. Or maybe it's just that I'm lazy.

But today is an astronomical holiday! The date of All Saints Day was set to coincide with one of the year's four cross-quarter days, the day exactly halfway between a solstice and an equinox. I've talked about cross-quarter days before. So, today we are approximately halfway between the autumnal equinox (the first day of fall, when the sun appears to cross Earth's equator on its path southward) and the winter solstice (the first day of winter, when the sun reaches its southernmost point for the year, and the Northern Hemisphere has its shortest day).

Actually, today is not the exact halfway point, and, astronomically speaking, there is no interesting event. But it does explain why Halloween falls when it does in the calendar, and so qualifies as a bit of trivia that may win you $100 on Jeopardy! someday.