Friday, December 21, 2007

Is Mars in the crosshairs?

Image credit: NASA/JPL

Okay, I said I was done writing for the year, and I thought I was. But that was before this story broke, suggesting that Mars may be hit by a "big asteroid" at the end of January.

First, let's clear up that, as asteroids go, this rock is not "big." Yes, the asteroid is estimated to be about 160 feet (50 meters) across. That is a big rock by our standards, but tiny by asteroid standards. If it were to hit a city on Earth, that city would be severely damaged and a lot of people could be killed. But this is much, much smaller than the asteroid that wiped out the dinosaurs, and would not threaten our civilization.

Still, we'd like to know if Earth is going to be hit by anything big enough to endanger people, so there are several programs looking for these rocks. And, in November, they discovered this previously-unknown asteroid. Until we know its orbit better, it will be known by the boring catalog name of 2007 WD5. Since the asteroid can come close to Earth, it was put on lists to be closely watched.

When an asteroid is first discovered, we tend not to know its orbit very well. It takes months, or even years, of observations to be able to calculate its orbit well enough to know if or when it might hit a planet. As time goes on and more observations are made, the "error box," or uncertainty in the orbit, gets smaller. This is why you may hear from time to time that a newly-discovered asteroid has a 1-in-10,000 chance of hitting the Earth in a few decades, and a few months later, the chance drops to 1-in-10 million or even less.

The first orbits for the asteroid gave it a 1-in-350 chance of hitting Mars in late January. This was a pretty good chance, but most astronomers expected the chances to get much smaller when new data came in. So, when the new data actually increased the chances to 1-in-75, this became exciting.

Still, the odds are that the rock will miss Mars. We'll know more in the next few weeks, as more data come in and are analyzed.

If the asteroid does hit Mars, this is a chance for some useful science. We have only been able to estimate how large of a crater a rock of a given size will make; with a direct hit on Mars, we can measure it. This would let us know how much of a danger small asteroids like this are to the Earth, in the event we ever saw one coming for us.

Small asteroids hit the Earth and Mars all the time. Every meteor you see is a space rock burning up in our atmosphere. Every few years you'll hear of meteorites hitting somebody's house or car -- these are typically baseball-sized rocks. The bigger ones, like 2007WD5, probably hit every 100 to 1000 years.

In fact, an asteroid the size of 2007 WD5 is probably what caused the Tunguska Event of 1908, where the mid-air explosion of a small asteroid or comet leveled hundreds of square miles of Siberian forest.

So, in short, Mars probably won't be hit, but the chances of a hit are much higher than we've ever seen from an asteroid in modern times. And an impact would teach us a lot about small asteroid impacts, which could help us protect ourselves if we ever find an asteroid inbound toward Earth. For more details, see the press release from NASA's Jet Propulsion Laboratory

Happy Holidays!

Winter starts tonight! Believe it or not, all the awful winter weather many Americans have been suffering has been part of this year's autumn. With the arrival of winter and the holidays, I'll be shuttering up (or maybe just shutting up) and not blogging until the new year (2008, they'll call it).

In the meantime, lots of astronomy will be happening. Mars is at its brightest, visible as the brightest object in the night sky all night long. Comet Holmes is still visible, but is harder to see because it has gotten so large. The moon will be nearly full, helping to light the way for Santa Claus. So, if you receive a telecope or binoculars, there's a lot to see!

Thank you one and all for reading this year, and I'll be back on January 2 or 3rd!

Thursday, December 20, 2007

Technology: 1, Astronomer: 0

With the end of the year so close, and so much remaining on my plate to finish up before the holidays, I was looking forward to a nice productive week at work. But that plan was in violation of Murphy's Law, so I was forced to pay the penalty.

Late last week, the hard drive and motherboard on my laptop failed (no, that's not mine pictured above, but an image I found through Google). I need my laptop both for travel and for some graphics that I'm working on, so it was off to the manufacturer for repairs. And, while I have my laptop back now with a minimal loss of important data, I need to re-install most of my software, eating up some valuable time.

In the grand scheme of things, this is just a minor annoyance. But some colleagues will be a bit miffed at what didn't get done. That's life!

Tuesday, December 18, 2007

The "Death Star" Galaxy

Image Credit: Image: X-ray: NASA/CXC/CfA/D.Evans et al.; Optical/UV: NASA/STScI; Radio: NSF/VLA/CfA/D.Evans et al., STFC/JBO/MERLIN; Illustration: NASA/CXC/M. Weiss

Astronomers often come up with strange names for things we've found. We have the "Big Bang", "Dark Energy" and even the "Atoms for Peace Galaxy". Usually these names have at least a passing resemblance to the thing they refer to. But today's addition, the "Death Star Galaxy," (see the press release) mystifies me completely. The pictures (above) don't look like the Death Star, the galaxy and its black hole are not stars, and it's much larger than a small moon. ("That's no galaxy -- that's a space station" just doesn't have the same ring to it.) The best guess I can have is that one galaxy appears to be shooting something like a laser beam at another galaxy.

That being said, what's the hubbub about?

What we see in the above image is a radio galaxy with the catalog name 3C321. The galaxy is far away, about 1.2 billion light years from Earth! When we look at the galaxy with Hubble, we actually see two galaxies that are colliding. Both galaxies have gigantic black holes at their centers, and the collision is sending tons of gas and dust in toward the black holes. Much of that gas and dust is swallowed by the black hole, but some of it is accelerated to high speeds and shot off in a jet that stretches nearly a million light-years in length.

Today's news is about a detailed study of these galaxies, using radio waves, optical light, ultraviolet light, and X-rays. Combining all these images gave the astronomers an unprecedented look at what is going on in a pair of colliding galaxies.

And that look resulted in a surprise. The jet from one of the black holes just happens to be pointed at the other galaxy, and the energetic particles in the jet are blasting through the poor target galaxy! Well, that's an overstatement. The jet seems to be hitting the second galaxy, and it is being deflected, much like a jet of water from a hose can bounce off of a car, causing the water stream to change both shape and direction. So, the second galaxy seems to be in no danger of being blasted apart into a million pieces.

This study is important, because some astronomers think that jets such as these may be important in explaining why we don't see stars forming in clusters of galaxies. The thought is that these jets may be powerful enough to evaporate all of the gas that stars form out of. Maybe, just maybe, this "Death Star Galaxy" can help us test that idea.

(For information on how the real Death Star works, check out this article.)

Monday, December 17, 2007

Making pretty pictures

Early next month, astronomers from across the U.S. (and even further abroad) will converge on Austin, Texas for our annual winter meeting of the American Astronomical Society. Several thousand astronomers will be prowling the streets and crowding into the convention center, each presenting their own research.

I'm preparing a poster with some of my research to show off at the meeting. As my poster will be competing for attention with a few hundred other posters, I'm trying to whip up some flashy graphics to draw people in. It's a little sad that flashiness and not science is part of the draw, but that's the way things are.

Anyway, the picture above is a near-true color image of the center of the star cluster Messier 67 that I put together. The images were taken with the 6.5-meter MMT telescope south of Tucson, Arizona. Messier 67 itself is about 2800 light-years away in the constellation Cancer. The stars in the star cluster are about 4 billion years old, or just a little younger than our Solar System.

If you look at the large version of the image (click on the image above), you can see several types of stars. The brighter, orangish stars are red giants, stars that have exhausted their hydrogen fuel, and have swollen up from the size of our sun to a star larger than the Earth's orbit around the sun.

At the lower right, you can see a bright, bluish star. This star is known as a "blue straggler." Based on the star's color and brightness, it should be much younger than the star cluster. But we also know that it is part of the star cluster, and we know that all stars in a star cluster are the same age. So, this star is thought to be the result of two normal stars colliding and merging into a single star.

Meanwhile, most of the faint stars you see are stars like the sun, and some of these may even have planets around them (though, so far, we haven't found any). So, when I look at this cluster, it is possible that some alien astronomer is looking back at us.

Friday, December 14, 2007

Stars don't make good Christmas presents

"While astronomy is a relatively safe hobby, keep in mind that stars are hot and will burn for millions of years if left unattended." --- The Onion

As the holiday shopping season reaches its peak in the next few weeks, you may be wondering what to buy that space fanatic for Christmas. One item that is available from some retailers and that seems to remain fairly popular is the "buy a star" or "name a star" gift. Typically, these services, in exchange for a moderate sum of money, give you the "rights" to name a star and provide you with a few goodies, such as a nice certificate, a booklet with information on stars, a sky map with your star indicated, and/or a few other personalized items.

What these companies don't go out of their way to tell you is that the name you give a star is not official. These names are not recognized by any organization of professional astronomers, nor will they ever be used by any astronomer or star catalog (except, perhaps, a catalog produced by the retailer that no astronomer will ever look at). And you get no legal rights to that star or any money that may come from that star (say someone wins a government grant or Nobel prize for studying "your" star -- don't expect to see a red cent).

In fact, I once had a relative buy me a star to name. I appreciated the gesture, I named the star after myself, and I got the little gift packet in the mail. What annoyed me was that the packet was full of a lot of false information (probably not intentionally so, but still quite wrong). I was given my star's coordinates, but there is no star there. The star that came indicated on a star chart was nearly one degree away on the sky! I also received a booklet on the "science" of stars that was full of wildly inaccurate "facts" on stars and their lives.

I would have no problem with companies that want to "sell" stars, if they would clearly state up front that the gift is not official in any manner, and if they would not provide false scientific "facts" about the stars. Neither of these things is difficult, and given the price these retailers charge; it should be insulting to the consumer that these retailers do not take any apparent effort to provide a product worth even a fraction of the cost. And these products do the science of astronomy a disservice by giving people materials that are anti-educational.

If you have already purchased or named a star through such a company, though, don't feel ashamed -- one thing these retailers do accomplish is good advertising for their product. It's just a shame that the product is a sham.

But there are similar products you can buy for that space fanatic. Why not try a framed picture from the Hubble Telescope? Several companies offer them, or you can download high-resolution pictures from Hubblesite and have the picture printed at a photo shop. And, in case your developer is worried, this is legal -- NASA pictures are in the public domain (You may want to print out this letter to give to your developer in case they are worried). Hubblesite even provides a nice step-by-step guide to this -- and there are hundreds more Hubble images than the few dozen the step-by-step guide offers; just download the highest-quality images from the gallery. Put the print in a nice frame, and you have a museum-quality Hubble picture for a gift!

Of course, there are other options for space gifts -- I just wanted to point out one that can be inexpensive and yet result in a high-quality gift, without having to shell out over $50 for bogus naming rights.

Thursday, December 13, 2007

Catch a moving planet (and maybe a meteor or two!)

Image Credit: Mouser Williams/Wikipedia

The word "planet" is derived from the ancient Greek word for "wanderer." Unlike stars, which appear in the same place relative to one another night after night, year after year, the planets move about in the sky, slowly drifting from one constellation to the next.

In modern times, most people have never seen a planet actually wandering. Well, now is your chance! Our target is Mars, the Red Planet.

This month, Mars is wandering near the constellation Orion. I've put a picture of Orion above -- it's a fairly easy constellation for most people to see, you can see it even in the glare of city lights, and it is made of bright stars. This month, Orion is sideways in the East at early evening, and then high in the southern sky for most of the night. See if you can find it! Of course, if you are in the Southern Hemisphere, Orion is upside down in the northern sky.

The planet Mars is north of Orion (higher in the sky for those living north of the equator). Mars will be the brightest thing in the sky this month (other than the moon or, if you are up early in the morning, Venus), so it's hard to miss. Mars is bright enough that you can probably notice it's reddish hue (or maybe more butterscotch than red). The star Betelgeuse in Orion (his upper-left shoulder in the picture above) is also reddish in color, but Mars is far brighter.

When you see both Mars and Orion, pay attention to where Mars is relative to Orion. Is it over his left or right shoulder? Or far to the left or right? Can you see other nearby star patterns that will help you remember where Mars is? Remember this placement. Then go inside and get warm and enjoy the evening.

A few nights later (or a week later or two weeks later, whenever it is clear next), go back an look for Mars again. It will have moved quite a bit (if you are looking in December, it will have moved to the west, or toward the right). I've been able to notice this difference in just a single night!

Once you've seen a planet move relative to the stars, you are one of the lucky few humans alive who has seen and recognized this motion. And what you are seeing is a delicate dance of the Earth and Mars around the sun -- the position of Mars is changing quickly this month because Earth is passing by Mars in our orbit -- we have the inside track around the sun, and are zipping right by pokey old Mars.

While you are outside, you may also see a meteor ("shooting star") or two. Today is the peak of the Geminid meteor shower, which produces about one meteor a minute in excellent conditions (so you could expect to see a couple if you are outside for 15 minutes or so).

Tuesday, December 11, 2007

Atlantis vs. Arachnid

Image Credit: NASA/CNN/WKMG

You may have heard that the launch of the space shuttle Atlantis, scheduled for last week, has been delayed until at least early January. NASA claims, the problem involves a fuel gauge that is reading "empty" when the tank is full, only when the mechanic looks at the sensor, it is working fine. That sounds a lot like my car!

But, as shown in this video from Orlando's WKMG news station, the problem is much more sinister. Evidently Florida is being overrun with giant bugs (this comes as no surprise to them -- from what I hear, Stephen King's "The Mist" was not set in Florida, because no Floridian would have panicked). And one of those bugs apparently attacked the space shuttle.

I'm going to follow the advice of Kent Brockman, the news announcer in The Simpsons, who watched in horror as Homer spilled an ant farm on the space shuttle and ants drifted in front of the camera lens:

I, for one, welcome our new insect overlords.

On a serious note, though, the delay of the Atlantis launch may well delay next summer's planned repair of the Hubble Space Telescope (also using the shuttle Atlantis). As of now, it seems that a small delay likely will not endanger the Hubble Telescope. But Hubble is a sick telescope, and desperately needs that repair mission. So, here's hoping that NASA's engineers can fix those fuel sensors and safely launch Atlantis early next month!

Thanks to Dave Barry for informing me of this alarming event.

Update: It's come to my attention that this video was actually from a previous shuttle launch. But the same thing could have happened again to Atlantis.

Update on the Update: There seems to be some disagreement as to whether the video from which the picture was taken is from this weekend's launch (as I originally said, and as the majority of websites seem to claim), or from an earlier launch. Probably this type of thing has happened before (scouts before the mothership moves in). And, in the big scheme of things, it doesn't matter. Also, of course, the video is now on YouTube.

Monday, December 10, 2007

The psychology of astronomy

Although we scientists take pride in our independence of thought and our ability to look at data without those pesky human emotions influencing our interpretation. Sometimes, though, we are reminded that even the best scientist is human, and some very unscientific things influence our work.

Today's example comes from astronomy's most popular preprint server. A preprint is a scientific article that is sent out to other astronomers before it is officially published in an astronomical journal. Sometimes these articles have been approved for publication and are just waiting their turn to be printed, and sometimes these articles have just been submitted for approval (and so may undergo serious revisions along the way, or, in rare cases, may never be accepted for publication). This isn't too different from rock bands that release singles before their album is available, only imagine if some of the singles had not yet been edited, and some of the un-edited songs were so bad that they don't make the final album. Anyway, in ye good olde days, preprints were produced by an astronomer and mailed to astronomy departments around the world. In the last decade or so, these paper preprints have been phased out in favor of "astro-ph", a website of electronic preprints.

Astro-ph is updated daily; papers submitted by 2pm Eastern Standard Time will appear the next day; papers submitted after 2pm will appear the next day. Since the papers are listed in the order they are received, papers that show up at 2:01pm on Monday are at the top of Wednesday's list, while papers submitted at 1:59pm on Tuesday will be at the bottom of Wednesday's list. This has led several people to time their submissions -- on a given day, three to five new papers will have been submitted within 30 seconds of 2pm; the remaining 30 or so papers are spread out over the next 24 hours.

Today, one of those first few preprints is called "The Importance of Being First" by astronomer Jörg Dietrich at the European Southern Observatory in Germany. In the paper, Dietrich presents evidence that the preprints listed at the top of each day's listings are cited more often than papers at the bottom. In other words, preprints put in at 15 seconds past 2pm get used more often than those preprints posted later in the day. Since the value of a paper is often measured in how many times a paper gets cited, this finding is bound to turn some heads.

The question then arises, why is this so? It may be that people who have a paper they consider important purposefully time that preprint's submission, but will post less-important papers at other times. Or it could be that astronomers tire of reading summaries of 30-plus preprints every day, and more astronomers stop reading after the first few. It could also be that those astronomers willing to put in the extra effort to time their submissions are generally self-promoters who try hard to increase the visibility of their work. Or maybe the finding is a statistical fluke. Dietrich put forward most of these suggestions in the paper, but has little evidence for or against any of these ideas.

Just as interesting to me is the reaction of people in our department to this article. Different astronomers believe different explanations, and often the explanation they support depends on their own personal attitude toward the preprint server. One postdoc believes the hypothesis that people try to put better papers first, because he himself has done that. A professor who admits to not paying attention to when he submits papers feels it is a statistical fluke. A third person who gets tired of reading the lists of papers likes the idea that not everyone reads through all 30 papers.

So, maybe there is some truth to all of the hypotheses. Now the question becomes, can we (or even should we) eliminate this bias? Dietrich's preprint argues for a Google-esque sorting scheme where preprints that fit a person's own interests are shown first. The order of the papers could be randomized. Or, if the papers at the top of the list are truly "better" on average than later papers, maybe the list order should be kept as is.

Regardless, I'm curious to see if, in the next few days or weeks, there is a big increase in the number of papers submitted within a few minutes of the daily deadline, hoping for an increase in the number of times their work gets cited.

Thursday, December 06, 2007

Shadow worlds

Image credit: McDonald Observatory

We now know of well over 200 planets around other stars. Most of these planets are known only by the gravitational pull they exert on their parent star. And for that reason, we know almost nothing about these planets other than their mass and how far away from their parent star they are.

A small fraction of planets actually pass in front of their parent star as seen from the Earth. When they do so, we see the star's light get fainter as the planet blocks a little light from the star. What we "see" is a shadow. From the amount of light blocked, we can learn about the size of the planet. And, from some physics and knowledge of the planets in our Solar System, we can guess what the planet is made of, but this is just a guess.

To truly understand other worlds around other stars, we need to look for chemical signatures of their chemical composition. There are several ways to do this, but one way is to look for how the spectrum (the spread of colors in light) of a star changes when the planet is in front of it. This is because the light from the star has to pass through the planet's atmosphere, and any atoms in the planet's atmosphere can absorb light of specific colors. For example, when we look at sunlight that has passed through our atmosphere, we can see ozone, water, and carbon dioxide quite easily, as these molecules absorb unique colors of sunlight.

Several years ago, astronomers used a spectrograph on the Hubble Space Telescope to do this, and they detected sodium in the atmosphere of one planet. This is not surprising -- there is a lot of sodium in the Universe (that's one reason why there's a lot of salt on Earth), and sodium likes to absorb and emit light at two very distinct colors (which is why sodium street lights look orange). Then the spectrograph on Hubble quit working (it was old).

Over the past few years, Seth Redfield, a postdoc here at the University of Texas (that's Seth's picture above), decided to use our large Hobby-Eberly Telescope to look for sodium in some other planets outside our Solar System as they went in front of their parent star. His target: a planet 20% more massive than Jupiter around the star HD 189733. This star is in the little-known constellation Vulpecula (the "little fox"), near the summer constellations of Cygnus the Swan and Aquila the Eagle. The star itself is just 1/4 degree away from the famous Dumbbell Nebula -- in fact, in the linked picture, you can see the star (about halfway between the nebula and the right edge of the frame, and the brightest star in that part of the picture).

Many astronomers were sceptical that the project would work. From the ground, there are all kinds of difficulties that the Hubble Telescope didn't have. Earth's atmosphere is the biggest problem -- it has sodium, and the amount of sodium can change with time. It's possible, but very hard, to account for this precisely. Also, as the Earth moves around the Sun, the precise colors of sodium in the other planet's atmosphere will change slightly, but measurably, due to the Doppler Shift. The star itself likely has starspots (like sunspots), and these may cause changes in the sodium line. And even effects that wouldn't seem important, like exactly where across the star the planet moves (near the middle? toward one edge?) matters in the detection.

But Seth and his team did manage to work through all of these, and they detected sodium in the atmosphere of the planet. They did a lot of computer simulations, double-checking, second-guessing, but their detection stood up to all the tests. Sometimes experiments succeed because of luck; in this case, the team succeeded because of lots of sweat and toil.

Finding sodium is not surprising, but the fact that they did it means that, even without Hubble's spectrograph (which may be repaired by astronauts next summer), astronomers have a hope of detecting atmospheres of planets in other Solar Systems, allowing us to determine what the atmospheres are made of.

And, although there is a long way to go, Seth's work may eventually lead astronomers to find evidence of life on other planets -- ozone and chlorophyll are two signatures of life some astronomers think we should be able to see.

So, congratulations Seth on your discovery!

Tuesday, December 04, 2007

Are my favorite stars getting bullied?

Image sources: H. Bond, NASA, Frank Gregorio, and Yours Truly
When stars run out of nuclear fuel, there is no celestial gas station a star can go to and refuel with fresh hydrogen (this will still be true even after humans are refueling our cars with hydrogen). The star then begins to die. We astronomers think we know how the vast majority of stars die. They swell up into a red giant star until they use every last ounce of fuel, then they puff off their outer layers to make a planetary nebula. The nebula slowly floats away, leaving behind a white dwarf -- a hot, dense lump of ash that slowly cools and fades away. And this whole process seems to be gentle, compared with the explosions (supernovae) that mark the death of the biggest stars in the galaxy.

Back in the early 1990s, Professor Volker Weideman of the University of Kiel noticed that the Hyades (a nearby star cluster that makes the "V" in the head of Taurus the Bull) don't have as many white dwarf stars as they should. We think we can make a good estimate of how many stars lived and died in a star cluster, but we didn't see nearly enough white dwarfs. So, Professor Weidemann proposed that, when a star loses its mass as a planetary nebula, the material may come off a bit lopsided. If enough matter comes off in this way, the star may get a little bit of a push, and slowly drift away from the star cluster.

When I did my doctoral thesis, I noticed that white dwarfs seemed to be missing from other star clusters as well. I also estimated how many white dwarfs astronomers might miss because they are next to a bright star that hides the white dwarf, and it wasn't enough. So, I put this forward as more evidence that white dwarfs were somehow getting a push when they were formed. Michael Fellhauer, an astronomer working with some of my colleagues, did a computer simulation that showed that the white dwarfs would only need a push of about 3 to 5 kilometers per second (about 7000-10,000 miles per hour, which sounds like a lot, but by astronomy standards, that is pretty slow). But it is hard to prove what is happening when we couldn't even find the white dwarfs, and many colleagues who worked on how stars lose mass said that there was not much evidence for a lopsided loss of material.

Yesterday, a press release was announced involving research by Saul Davis at the University of British Columbia involving a paper Davis and his collaborators had published on white dwarfs in the globular cluster NGC 6397. You can read the full story here, but in short, the newest white dwarfs in the cluster are moving around about 3 to 5 kilometers per second faster than older white dwarfs. If young white dwarfs get a push when they are born, this is what we would expect -- because of gravity, over time, will slow down any faster white dwarfs to match the average speeds of stars in the star cluster.

So, Davis's work gives some new and different evidence that white dwarfs go get a push when they end their lives. And it is nice to see someone find that evidence!

But I don't think the case is sealed yet. Theory still doesn't explain how these white dwarfs can get a push when they die, and we haven't seen strong evidence that dying stars are losing mass in the lopsided manner needed to give the white dwarfs a push. I have no other reasonable ideas as to how the white dwarfs could be getting a push (maybe bigger stars are bullying the white dwarfs, shoving them around, and putting "kick me" signs on their backs?), but until we actually see stars losing mass in a lopsided manner, the mystery remains.

Monday, December 03, 2007

The bright skies of winter

Winter is finally asserting its grip across much of the United States. As I was walking to the bus on this brisk, windy morning, I was thinking about how bright the stars can appear to be in winter. There are bright stars in the winter sky, some of the brightest visible, and this winter the planet Mars will be bright and high in the sky all night long. But there are bright stars in the summer, too, and the planets are often visible in the summer sky. So, why do the stars seem a little brighter in winter?

The answer, for most of the United States, is in the weather. For most of the summer, the skies are quite hazy. I never appreciated how hazy typical summer weather was on the East Coast until I started doing a lot of air travel. In the summer on the East Coast, you often cannot see the ground, despite it being a sunny day! So, all that murk also dims the stars quite a bit -- I am sure this is a major part of the apparently bright stars of winter.

Also in the winter, we are awake for quite some time after darkness falls, so more people see the night sky. When there is a full moon, the moon is highest in the sky during the winter, and so looks brighter, and, if a little snow is around to reflect moonlight, the entire landscape can look very bright, which probably fools our minds into thinking the sky is brighter.

Surprisingly, then, winter is often not the best time for astronomy in the United States. Much of the country is constantly being hit by cold fronts and pressure systems that, even if they don't have clouds associated with them, cause the air to be quite turbulent. This turbulence blurs our images, so the pictures we get aren't quite as sharp. (If you see the stars twinkling a lot, that is the same turbulence!)

And ice and snow storms are common on mountain tops, where our telescopes are located. So, even if the night is clear, we can't work if the dome is covered in snow! Once I had four nights in January on Kitt Peak in Arizona. Two nights we were closed by an ice fog (I put a picture of the Kitt Peak Solar Telescope taken that day above). The third night was crystal clear, and the stars were not twinkling at all, meaning the air was very still and our images would have been sharp. But there was an inch of ice on the telescope dome -- if we had opened the dome, the ice would either have jammed the dome motors or it would have fallen on the telescope, damaging the mirror. So, in spite of some of the best astronomy weather I'd seen, we had to sit around and play cards and watch movies for another night.

So, enjoy the winter skies. This year Comet Holmes is still visible to the naked eye as a fuzzy spot in Perseus, Mars will be bright and high in the sky (being brightest right around Christmas), an, in February, most of the United States will be able to see its third total eclipse of the moon in a year's time!

If you bundle up, you may also see a lot of meteors -- there are several meteor showers this time of year. The Geminids peak during mid-December, and the Quadrantids peak in early January. Both meteor showers produce a lot of meteors, and even some many days before and after the peak. So, chances are good you will see some meteors this time of year.

So, go enjoy the sky this winter! Sure, you have to bundle up to stay warm, but think how much better that hot chocolate will taste after a half hour enjoying the winter sky!