Thursday, January 31, 2008

Visitors from afar

The past couple of days, I've had visitors from distant places descend upon me. No, not extraterrestrials. It was extended family, travelling through town while on an extended vacation.

It was super to have them visit. Not only are they great people and fun to be around, but as we sat and chatted about their lives and my life and all the news of what everyone is doing, I was reminded of things that are truly important in life. Yes, I like my work, and I like to think that what I do is important in some grand scheme of things. But, to me, family and friends are more important than understanding the deepest secrets of the Universe. The stars I study will be around, virtually unchanged, for the rest of my life, and my children's lives, and their children's lives. Sometimes, quantifying their properties can wait for a day.

Tomorrow, it's back to science. And that's soon enough.

Tuesday, January 29, 2008

NASA's rough week

Image Credit: NASA

This week marks the anniversary of the three worst accidents in NASA's history.

  • January 27, 1967 Gus Grissom, Edward White and Roger Chaffee were killed in a launch pad fire during a training mission for the Apollo spacecraft. The fire was ignited by an electrical spark that ignited the pure-oxygen atmosphere of the Apollo capsule.
  • January 28, 1986 Greg Jarvis, Christa McAuliffe, Ronald McNair, Ellison Onizuka, Judith Resnik, Michael J. Smith, and Dick Scobee perished when the space shuttle Challenger exploded 73 seconds after liftoff. The explosion was caused by a combination of bitterly cold weather and poor design of joints in the shuttle's solid rocket boosters.
  • February 1, 2003 Rick D. Husband, William McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla, Laurel B. Clark, and Ilan Ramon died when the space shuttle Columbia broke apart during re-entry. A piece of foam fell of the external fuel tank during liftoff and hit the shuttle's wing, creating a hole that allowed hot gases to enter and destroy the craft.

It is important to remember these people and accidents. These men and women willingly put their lives at risk to explore space, bravery that deserves recognition. But also, each of these accidents may have been preventable. A string of human errors and cultural issues led to each accident. These errors are much easier to see in retrospect than they were ahead of time, and so we should be careful in assigning blame to freely. Yet we can and must learn from these mistakes to protect future lives; to ignore these lessons would be an unforgiveable failure.

Finally, we should all recognize that more lives will be lost in the future. Space travel is extraordinarily dangerous. As private companies also begin to open space to civilians, we must accept that there will be accidents and lives lost, and most of these will probably be due to human error. Let's just hope that those errors are due to exploration and humankind's pushing of the envelope, and not due to our failure to learn from our history.

Monday, January 28, 2008

The (miniscule) threat of a US spy satellite

Image Credit: Chris Peat, Heavens-Above GmbH

This weekend, I opened the newspaper to see a story with the title "Disabled Spy Satellite Threatens Earth." I was curious, since I didn't know that a single satellite, especially a defunct satellite, could threaten the entire planet. So, I read the article, and I still know that I was right -- Earth is not threatened by this satellite.

First, let's figure out what the story is about beyond the over-hyped headline. The if you read the story, you'll see that the facts are that a U.S. spy satellite has lost power. That also means it has lost the use of its thrusters. And that's about it for details, with some knowledgeable sources saying the satellite will re-enter Earth's atmosphere in late February or in March.

Our satellites in low-Earth orbit (those satellites orbiting less than about 1200 miles above Earth's surface) are not in a pure vacuum of space. Sure, there isn't exactly a lot of air around, but there are tiny bits of Earth's atmosphere up there. As a satellite orbits, that tiny bit of atmosphere drags on the satellite, slowing it down and causing it to slowly inch toward the Earth. As it gets closer to the Earth, the thickness of the atmosphere increases, so the drag increases, so the rate of slowing increases, so the satellite's descent speeds up, and the atmosphere continues to get thicker, and so on until the satellite falls back to Earth. It doesn't matter whether a satellite has electricity or not, or whether it is under control or not. All Earth satellites in low-Earth orbit will fall back to Earth in a timescale of months to decades, depending on how high up the satellite is.

There is one way to keep from falling back, and that is to use a rocket engine to give a satellite a little acceleration to counter-act Earth's atmospheric drag. As long as the engine still works and has fuel, the satellite can orbit as long as it wants.

The graph at the top of this post is a plot of the height of the International Space Station over time. You can see that, most of the time, the space station is slowly sinking back toward Earth, and every once in a while it very quickly moves further away again. These boosts in the station's altitude are caused by rocket engines on visiting spacecraft (Russian Progress supply rockets and the Space Shuttle). These boosts keep the station in orbit.

The Hubble Space Telescope doesn't have a rocket engine, but it is higher than the space station and so can survive longer. Every time a shuttle visits the Hubble, they use the shuttle's engines to boost Hubble back up to a safe orbit.

But satellites without rockets, or satellites that lose power and go out of control (like the US spy satellite) will eventually fall back to Earth. Most satellites disintegrate high in the atmosphere and completely burn up; larger satellites (like NASA's Skylab or Russia's Mir space station) can have pieces that survive re-entry; pieces of Skylab were found across Australia.

For that reason, countries now try and de-orbit useless satellites by using a rocket engine to cause the satellite to fall into the ocean. Astronauts on the next Hubble repair mission will attach some equipment to the Hubble that will allow a future robotic spacecraft to attach an engine to Hubble to bring it down. But, if a satellite doesn't have an engine, or if the engine isn't working, then the spacecraft could fall anywhere.

As you might imagine, some satellites have some pretty nasty stuff. Rocket fuel can be hazardous, and some satellites have radioactive power supplies (these are built to withstand re-entry intact so as not to spread radioactive particles over the Earth). If those satellites fall in a populated area, people could get sick. But most of the Earth is water, and most of the rest is unpopulated, so the chances of a noxious rogue satellite part hitting a city are very tiny.

So, the real story of the US spy satellite is, if a piece lands in your backyard, don't touch it. The chances are it wouldn't harm you, but it might make you sick. And, more to the point, there are parts of that satellite that the government doesn't want anyone to see. So picking up a piece of the satellite for decoration is probably not a good idea, unless you'd like an inside look at Guantanamo Bay.

Thursday, January 24, 2008

Beatrice Tinsley

Yesterday, I talked about the results of some black hole studies by Dr. Ramesh Narayan, who is visiting the University of Texas Department of Astronomy as the "Beatrice Tinsley Visiting Professor." Since the story of Beatrice Tinsley is not well known outside the astronomy community, I thought I would tell bits and pieces of it here.

Several people have already written Tinsely's biography on the web, and I have nothing new to add. In summary, Tinsley was born in Englang, grew up in New Zealand, married a physicist, came with him to the University of Texas at Dallas. She earned a PhD from the University of Texas at Austin, but was only given an undistinguished research position at UT Dallas. Tinsley performed ground-breaking research (more on that below), but UT Dallas did not give her a professorship. After a long struggle, she left Texas, divorced, and went to Yale in 1975, where she quickly became one of the most distinguished astronomers in the United States. After only six years, she died from cancer. For more details on the life of Tinsley, you can read this brief article from the American Astronomical Society's Committee on the Status of Women in Astronomy, a biography from the the New Zealand Edge, another detailed biography, or you can even a published biography (if you can find it in print).

Tinsely's research was some of the most influential astronomical research of the times. At the time, little was known about the evolution of galaxies -- how did galaxies form, and how did they change over time?

Galaxies, as you may know, are collections of billions of stars bound together by gravity. The Milky Way is a galaxy; the Andromeda Galaxy is a near-twin of the Milky Way located 2 million light-years away. Both galaxies are spiral galaxies. Further away from us, we find some elliptical galaxies, which look like fuzzy blobs and lack beautiful spiral arms. Moreover, elliptical galaxies are distinctly more yellowish than spiral galaxies, which tend to be quite blue. But it used to be quite a mystery what caused these different shapes and colors.

One of Tinsley's most profound insights was that, since galaxies are made out of billions of stars, we can use what we know about the lives of stars to learn about the history of galaxies. We know that bright, blue stars live only for a few million years, while yellow stars like the sun can live ten billion years. So, a galaxy that is currently blue is forming stars now, while yellow galaxies are not.

While most of the above was known or guessed at when Tinsley was working, she put a great effort into calculating not just how galaxies look today, but how they would look over time, depending on how they formed their stars. A galaxy will look different if all of its stars formed in a big flurry of activity 15 billion years ago than if its stars were formed steadily over fifteen billion years, even if the galaxy is not presently making stars. Tinsley calculated models for dozens of different types of galaxies with many different possible histories for star formation -- a tremendous effort in the days before a powerful computer could sit on everyone's desktop.

At the time Tinsley was doing her work, our view of the Universe was rapidly changing. The Big Bang had only recently been proven, and models of stars were only just beginning to tell us how nuclear reactions in stars produced all of the elements in our Universe. The theory behind the life cycles of stars was solidifying into the basic story we believe today. Tinsley was able to take all of this different information and synthesize it in her work. As telescopes allowed us to look at more and more distant galaxies, her work and methods became the central means for understanding the changes that we saw in galaxies as we looked further and further back in time.

People can (and do) discuss at length the positives and negatives of Beatrice Tinsley's life and personality (read the links in the second paragraph), but there is no denying how, in her short career, Tinsley synthesized a tremendous amount of emerging information to become one of the giants of astronomy in the 20th century. And this is why the University of Texas at Austin and the American Astronomical Society both have awards in her name.

Wednesday, January 23, 2008

Seeing black holes

Image Credit: NASA/CXC/M. Weiss

One question about black holes that we astronomers are most often asked is, "If a black hole traps everything, even light, how do we know it is there?"

This is not a stupid question. In fact, as I'll talk about below, it's the whole issue to proving that something is a black hole! But let's start with how we detect likely black holes in the first place.

The picture above is an artist's conception of what a typical black hole we can detect from Earth might look like up close. These black holes have companion stars that are close enough to the black hole that material from the companion star falls toward the black hole.

But, before getting to the black hole, the gas tends to form a disk (like a frisbee) around the black hole. In this disk, gas jostles around and heats up to temperatures of hundreds of thousands of degrees. At this temperature, the gas glows in X-rays, which we detect using X-ray telescopes like the Chandra X-ray Observatory. The gas in the disk slowly loses energy and spirals into the black hole.

So, the light that we see does not come from inside the black hole, as that is impossible. The light comes from outside the black hole. And that is how we can "see" the black hole.

But the picture above would be the same if, instead of a black hole, we had a neutron star or even a white dwarf. So, how can we be certain that the thing at the middle of the disk is a black hole?

To date, the evidence is typically indirect. We can measure how fast the companion star is orbiting the hidden object, and if that hidden object has more than three times the mass of the sun, we think it is a black hole. But that is only because theory has trouble making neutron stars that massive. However, our theories could be wrong.

Last week and this week, we've had a visitor in our department named Ramesh Narayan, an astronomy theoretician from Harvard. Dr. Narayan spoke of a clever scheme to tell if a black hole is truly present or not. This idea relies on the fact that light cannot escape a black hole.

When something falls onto something else (like a window falling out of a building, or a meteor hitting a planet), there is a tremendous crash. That crash is a release of energy -- the falling object has a lot of energy until it hits the ground, when it has to release all that energy. The energy may go into sound (as in the window shattering), it may go into moving dirt around (like a meteor making a crater), or it can go into heat. Actually, quite a lot of energy goes into heat, but for most things we deal with, the crash is not energetic enough to really heat things up a lot.

Meteorite impacts are big enough to start releasing lots of heat. Many meteor craters have little bits of glass sprinkled around them, because the heat of the impact melted the dirt, which cools as glass. The asteroid which killed off the dinosaurs released enough heat to cause tremendous forest fires around the entire planet!

Now, if the Earth's gravity is enough to accelerate an asteroid to those very high speeds to release a lot of heat, imagine what the gravity of a neutron star (all the mass of the sun squeezed into a ball about 30 miles across) or a black hole (the mass of the sun squeezed into something smaller than three miles across) could do! The gas that we see emitting X-rays heats up even more as it begins its final plunge onto the central object.

In the gas falls onto a white dwarf, a neutron star, or anything else with a surface, the impact will release a lot of heat which we would be able to see in X-rays. In fact, in many X-ray bright objetcs, we see both the heat from gas in the disk and the heat of the gas impact on the star. But in some (those that we think might be black holes), Narayan and his collaborators see the heat of the gas disk, but they don't see heat from an impact of the gas onto a surface. It's like the gas stars to fall and then just vanishes.

And that, what we should see but don't see, is some of the best proof of the existence of black holes. So, going back to the original question -- we know black holes exist because we see places where matter is being swallowed up by something, and we can see the gas until very close to the bitter end, but then it just vanishes without a trace. In this case, not seeing is believing.

Tuesday, January 22, 2008

Crunching numbers

Image credit: Popular Science, Modern Mechanix

The trick to finding a needle in a haystack is not brute force (like Jim Moran above), but a clever sorting mechanism -- whether a metal detector, or some sifting machine, or another unique scheme.

In astronomy, some of the most interesting objects are some of the rarest. This is often because the interesting objects are those that are changing rapidly (at least in a cosmic sense). For example, stars like the sun are everywhere in the sky, because they live a middle-age life for ten billion years, nearly the entire age of the Universe. But stars ending their lives, making beautiful planetary nebulae, are very rare -- the phase only lasts ten thousand years, a blink in the cosmic eye. So, compared to stars like the sun, planetary nebulae are rare. Another rare object is a supernova, or exploding star. These are only visible for a year or so before fading from sight, and the stars that make them are rare. So, in our skies right now, there are no supernovae in the Milky Way galaxy. We have to look at distant galaxies to find supernovae.

But planetary nebulae and supernovae are pretty easy to find, relatively speaking. Planetary nebulae are big and glow in very specific colors of light, so you can design a search to take pictures of the sky in those colors, and you'll find lots of planetaries. Supernovae are very bright, and so can be seen far away. So, we just look at more and more distant galaxies until we see a supernova -- in essence, we are searching hundreds of billions of stars at once, looking for a "new", bright star in a galaxy. Both of these are clever ways of searching through the haystack of the sky for that elusive needle.

I am part of a group at the University of Texas Astronomy Department that is looking for a specific kind of white dwarf star in a specific patch of the sky. White dwarfs are faint, and bright ones are rare (because they have to be close by). And the patch of sky we are looking in is large, by astronomy standards -- about the size of both of your hands held at arms' length. A typical astronomical camera can only image part of the sky as big as a part of your pinky finger's fingernail. Out of the hundreds of thousands of stars in that patch of the sky, we expect to find about ten of these white dwarfs. So, how can we find them?

The brute force method, like searching each strand of hay for the needle, is long and complex. We would have to take pictures of the entire region of the sky (dozens of nights on telescopes available to us). Then we would analyze the images and look for stars that have just the right colors. Then we go back to the telescope and take spectra of all of those stars, splitting the light up into its component colors, and determining what each one is. We then would have to analyze each spectrum. And, lastly, we have to return to the telescope to double-check each of our best candidates. So, we're talking a few weeks (at least!) on the telescope, and hundreds of person-hours to find ten objects.

Thankfully, there is an easier way -- an automated sorting machine known as the Sloan Digital Sky Survey. The Sloan survey has taken images of one-quarter of the sky, and automated software analyzed each object. Interesting objects are targeted for follow-up work, such as the spectra, which automated routines then analyze. As of right now, the Sloan database has image analysis of 280 million stars and galaxies, and spectra of 1.2 million of those objects.

So, yesterday afternoon I sat at a coffee shop and used the Sloan database query tools to find a dozen interesting white dwarfs in our patch of sky. A grand total of less than one person-hour went into my search. Granted, Sloan has used hundreds of nights of telescope time and countless thousands of person-hours to create and maintain the database. But that enormous undertaking allows small teams such as the team I'm part of to do searches that used to be prohibitively time-intensive. Computers and clever software allow a single person to search through 280 million objects for the dozen he or she is interested in, and those searches only take minutes, not years.

So, I found the needles I was looking for in the haystack of the sky, all thanks to computers and a giant team dedicated to making this possible.

Monday, January 21, 2008

Why astronomers don't believe in alien UFOs

Image © 1996 Warner Bros.

In the past couple of weeks, there have been many newspaper stories about UFO sightings in western Texas. Larry King even had a segment about the sightings.

Do I think the people of Stephenville saw something they couldn't identify at night? Yes. Do I think they saw an alien spacecraft? Absolutely not. Does this make me part of a giant government conspiracy, or a closed-minded, smarter-than-thou so-called "expert," or an ignorant, evidence-dissing idiot? No.

First, let me say that I suspect that there is life out there in the Universe somewhere. And, like Agent Mulder of television's The X-Files, I want to believe that there is alien intelligence that deigns to contact us humans. But I've never seen any evidence that leads me to suspect that there are space aliens among us.

I can't possibly present a list of all the reasons why I don't believe that people have seen alien spacecraft UFOs. There are a lot, and the reasons vary depending on the circumstances. Let me highlight a few (and note that these are just a few, and that my explanations are not detailed for each reason -- I don't have enough space).

  1. Our eyes play tricks on us. Our eyes and the brain are incredible image processing machines. In a few seconds, we can gather more information from a brief glimpse at a scene than a robot could given hours of time. We had to develop this, or our ancestors would have been eaten by lions and tigers and bears hundreds of thousands of years ago. But our brains and eyes are tuned to exploring our immediate surroundings -- finding friends and foes, determining what we can eat and what may be trying to eat us, that sort of thing. We aren't used to looking and understanding the sky. Given enough time, we can interpret and understand what we see, but even so, we (and I include myself) often get it wrong on first impressions. And when there is something out of the ordinary (like a very bright star near the horizon, or a bright planet, or an airplane or satellite), most people (again, myself included) will get it wrong unless they are making a concerted effort to understand what they are seeing.
  2. Space travel is amazingly difficult. Since humans have traveled to the moon, and since we've sent probes to the far reaches of our Solar System, we tend to underestimate how big and how hostile the Universe really is. And, though there may be physics we haven't yet discovered that allows us to travel safely among stars, as of now we don't see any evidence that such physics exists. It is true that it is poor reasoning to say that the absence of evidence of advanced physics means that it doesn't exist, but it is equally poor reasoning to say that the absence of evidence of advanced physics means that it is likely to exist.
  3. The government can't keep a secret. Government cover-ups are a commonly-quoted excuse for the lack of proof of space aliens. But our government, as big and scary as it can be, can't hide secrets from us for very long. Even big secrets, like forged documents used to justify a war, telephone wiretapping, or Stealth aircraft technology, leak to the public long before the government would like.

    In fact, the government may have a good reason to encourage alien spacecraft stories. If several UFO sightings are really secret test aircraft, keeping people barking up the tree of alien spacecraft would help to preserve the secrecy of the aircraft programs.

So, what would it take for me to believe that a UFO is a likely alien spacecraft? It would take data. Hard, incontrovertible data. What does this mean? Well, a person reporting that an object was "about a thousand feet up in the sky" is almost certainly wrong -- people are horrible judges of vertical distance. But, if you had multiple detailed reports of the exact same object (such as one person saying, "I saw the object while sitting eight feet to the west of the maple tree in my front yard, and it was 16 degrees above the horizon, 78 degrees east of due north, moving on a heading of 162 degrees at a speed of 1.6 degrees per second, and the time was 8:16:35 pm", and another person with a similarly detailed report at the same time from a different location), it is possible to determine the true altitude, heading, and speed of an object. But these reports have to be at least as detailed as the report above, or else any such calculations are meaningless.

Here is where videotape can be handy, but the video has to be of superb quality, with time stamps accurate to a fraction of second, visible landmarks, and, preferably, stars visible in the sky. You should be saying to yourself, "but nobody has that kind of detail!" And you are right -- this is why eyewitness reports, even from very astute observers, are unable to prove many of the "miraculous" actions of supposed alien UFOs -- tremendous speed, strange accelerations, and so on. With multiple simultaneous observations, one can use trigonometry to determine exactly how high and fast an object is moving -- it's even possible to get the orbits of satellites this way!

But these observations are not impossible. A NASA campaign to research the Aurigid meteor shower used accurate video cameras on two airplanes to watch the meteor shower, allowing them to determine accurate positions, altitudes, and speeds of meteors. Accurate reports and videos of a fireball (a very bright meteor) in 1992 that hit a car near Peekskill, New York allowed the orbit of the meteor around the sun to be determined. Amateur astronomers use synchronized video cameras to image asteroids passing in front of faint stars, allowing them to determine the size and shape of asteroids. But the fact remains that almost all video taken of UFOs is not of sufficient quality to do the necessary analysis.

I'll end with a true story of some "UFO" video that I had the chance to view. It was taken by a news crew in Santa Cruz, California, who heard a report of a UFO, and took video over 15 minutes showing the "spacecraft" to slowly settle into the ocean. The cameras had accurate timestamps, and from their vantage point I could tell both exactly where the camera had been placed and what direction it was looking. I then went to a planetarium program, entered the date, time, and location of the cameras, and saw that they had a very nice video of the planet Venus setting in the western sky. The news people were disappointed, and even a bit upset that I didn't agree it was an alien spacecraft landing in the Pacific Ocean. But they should have been happy, because we were able to determine exactly what they had seen. And that is the type of evidence that alien spacecraft would need.

Carl Sagan said, "Extraordinary claims require extraordinary evidence." To be safe, scientists always approach a phenomenon from the negative side -- assume something doesn't exist, and wait for it to be proven to exist. A mountain of marginal evidence (typical UFO sightings) doesn't meet the burden of proof. The evidence that will be necessary to get scientists to take alien spacecraft claims must be exquisite and incontrovertible. I wouldn't trust my own eyes as evidence.

Happy Martin Luther King, Jr. Day!

Image Credit: The Nobel Foundation

"Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that. Hate multiplies hate, violence multiplies violence, and toughness multiplies toughness in a descending spiral of destruction....The chain reaction of evil--hate begetting hate, wars producing more wars--must be broken, or we shall be plunged into the dark abyss of annihilation."
Martin Luther King, Jr., Strength To Love, 1963

Friday, January 18, 2008

Tough times for astronomy?

Image Credit:

All week, I've been talking about new astronomy results on ever-increasing size scales. But, today, it is time to snap back to reality for, what I think, was the true news for astronomers last week.

As I blogged last week from the American Astronomical Society Meeting, the astronomy budget picture is bleak. NASA expects the astronomy budget to be severely crunched, due in part to small budget increases, and in part to budget line item directives that are requiring NASA to spend a lot of astronomy's money on one specific mission that had been delayed due to the budget crisis. The mandate from Congress on the science mission in question (which shall remain nameless, is deserving science, but is very expensive) will require cuts in many other areas of NASA astronomy, probably delaying several future missions and reducing money available as research grants.

Our big pot of research money, the National Science Foundation, is also expecting fairly flat budgets for the forseeable future; our planned astronomy investments require increasing budgets. Some very painful and unpopular cuts will almost certainly have to be made.

But we are better off than our friends in Great Britain. Due to budget concerns and some politics that I don't pretend to fully understand, British astronomers have been told to expect a 25% budget cut in the coming few years, as well as a withdrawal of the United Kingdom from the Gemini Telescope project (which will increase budget pressures on the NSF here in the U.S., too). This will be a tremendously difficult budget crunch for our British friends -- a 25% cut is extraordinarily painful for any group or business, whether publically funded or a privately owned. And the budget numbers fell from the sky with little warning.

We astronomers have been more successful than many government groups at getting funding increases in a time of tight non-defense budgets, but it looks like budget realities are catching up to us around the globe. The U.S. astronomy community is begining to prepare a report on the next decade of astronomy research in the United States (we've done this every ten years for several decades now, and it helps to guide Congress, NASA and the NSF in funding decisions). This time it looks like we will have to be much more grounded in realities, as we will have to consider tight budgets for the first time in years.

Thursday, January 17, 2008

Groping about in the dark

Image Credit:

Today, I'll finish up my discussion on some of the science presented at last week's meeting of the American Astronomical Society by looking at the biggest scale we can -- the entire Universe.

A lot of people, both astronomers and non-astronomers, are very interested in dark matter and dark energy. What are they? Where do they come from? Do they even exist?

These questions are easier to answer for dark matter than for dark energy. Certainly dark matter is fairly convincing. When we look at galaxies and clusters of galaxies, we can see that the pull of gravity is stronger than we expect based on the "normal" matter there. And emerging theories of physics predict dark matter particles that have properties that mimic what we surmise dark matter should act like. But these particles haven't been proven to exist yet, so they remain a very compelling hypothesis, not proven particles.

Dark energy is a little more subtle to explain the evidence for it. Most important is to realize that there are at least two independent lines of evidence for dark energy. The first method involves using what is known as a "Standard Candle," or something that we think we know exactly how bright it is. For dark energy, we use a certain type of supernova that always appears to be about the same brightness. We look and see how faint the supernova appears, we know how bright it actually is, and we use geometry to get a distance. Then we measure how fast the supernova is moving away from us due to the expansion of the Universe. And what we find is that the expansion of the universe is speeding up, when gravity should be causing the expansion of the universe to slow down. Since some form of energy has to be speeding the Universe up, we call that "dark energy."

The second piece of evidence for dark energy comes from a satellite called WMAP. WMAP explored the echoes of the Big Bang visible on the sky, and determined that the total amount of energy in the Universe. We add up all the energy we know about (light, visible matter, and dark matter -- remember, Einstein told us that matter is energy!), and we only get about 30% of the total energy measured by WMAP. The remaining 70% is called "dark energy."

Let me admit here that the distinct possibility exists that both dark matter and dark energy are related, and maybe they just indicate that we are missing some fundamental understanding of physics in the Universe. Dark matter and dark energy make up about 95% of the Universe, and we have very little clue what it is! So maybe our theories about gravity are incomplete. However, Einstein's General Relativity, which explains how gravity works, has worked so well in every experiment designed to test it, that most astronomers are not about to throw it out yet.

So, let's assume that our theories of gravity are correct. What could dark energy be? First, maybe it is a mistake. If the brightness of the supernovae we are looking at changes over the age of the Universe, then the supernovae are not "standard candles," and the measurements we make for them would naturally give us the wrong answer. This is where it is important that we have a second indicator or dark energy, the WMAP satellite. Those data are much harder (but not impossible) to misinterpret. Since both the satellite and supernovae give us the same answer, it seems likely that this is not a mistake.

So, many smart theorist astronomers have developed some ideas as to what dark energy could be. These theories explain all of our observations so far, but they make differing predictions about dark energy that better observations can test.

But there is one alternative theorists don't like, and that is Einstein's Cosmological Constant. The Cosmological Constant is a value that Einstein put into his General Relativity. Early on, Einstein saw that General Relativity predicted the Universe had to either be growing or shrinking. At the time, we had no evidence that it was doing either. The math of general relativity allowed Einstein to put the constant in, so he did. Later, when Einstein learned from astronomers that the Universe was indeed expanding, he set the constant equal to zero. But the cosmological constant, if it is not equal to zero, allows the universe to expand faster and faster, just like dark energy.

Astronomers don't like the cosmological constant, because it has no explanation -- it's just a number allowed by math. Let's suppose I ask you, "How long will it take a car to drive to El Paso if the car is moving at 60 miles per hour?" The answer depends on how far away from El Paso the car is. Now suppose I tell you that the car is 60 miles away. Simple! The answer is 1 hour!

But, then you can ask, why is the car 60 miles from El Paso? Did it just magically appear there? Is there an auto factory 60 miles from El Paso? Does the owner live 60 miles from El Paso? Or is the owner a vacationer from St. Louis, who just happens to be driving west at the time? For the purposes of math, these are silly questions. But if you truly want to understand all that there is to know about the car, these are important questions!

It is the same with the Universe and the Cosmological Constant. The Cosmological Constant is like a starting point. The value of the starting point affects the answers we get from doing math problems involving general relativity. But the math doesn't care where the value came from, it just wants to know what the value is.

For the astronomer and the physicist, that is not good enough. We want to know why the cosmological constant has the value that it does. Is it a fluke of nature? Is there some underlying process that we don't understand that sets the value? Does the value change over time?

So, back to the American Astronomical Society meeting. A few groups presented research trying to make detailed observations of "dark energy" and to determine if one of the existing theories was better than another, or if dark energy continues to act just like a cosmological constant. And the answer is, dark energy acts like the cosmological constant. Now, we can still make better measurements -- if dark energy is mostly like a cosmological constant, but slightly different, we can't yet measure that. But it makes a world of difference in understanding dark energy if the expanding universe acts just mostly like the cosmological constant, or exactly like a cosmological constant. And, if the answer is the latter, we have a lot of tough philosophical questions ahead.

In the meantime, though, astronomers are happy to say that we need to do better measurements first. And this means that you can all keep wondering, "What is Dark Energy? Is Dark Energy Real?" And we'll just smile, shrug our shoulders, and say, "Let me get back to you on that."

Wednesday, January 16, 2008

A collision in store for our galaxy

Image Credit: Bill Saxton, NRAO/AUI/NSF

Today we continue on an expanding view of science from last week's meeting of the American Astronomical Society. Today, we'll expand our view from the sun's corner of our galaxy to look at the Milky Way as a whole.

One of the continuing mysteries of our Galaxy involves the so-called "high-velocity clouds", which are giant clouds of hydrogen gas that visible only by the radio light they emit and are moving far too fast to be part of the Milky Way. Some astronomers have thought that these clouds are raw materials for galaxies raining down on the Milky Way for the first time. Others have suggested that these clouds are actually hundreds of thousands of light-years away and not associated with out galaxy. And still others have suggested that these clouds are bits of material thrown out of the Milky Way by supernova explosions, the death throes of massive stars, that are just now falling back on to our galaxy.

Of course, as readers of this blog may suspect, probably all three explanations are right in specific cases.

The cloud pictured above is called "Smith's Cloud" after Gail Smith, an astronomer at the University of Leiden in the Netherlands in the early 1960s. Ms. Smith has since married and left the field of astronomy, but her cloud proved to be one of the enigmatic high-velocity clouds. Previous studies have found no stars in the cloud whatsoever, but these studies have detected the cloud glowing in optical light from hydrogen gas. This faint glow was consistent with the cloud being close to the Milky Way, as light from bright stars in the Milky Way can cause nearby hydrogen to glow.

Recently, a team of astronomers led by Jay Lockman used the new 300-foot-wide Byrd Green Bank Telescope , a telescope that looks at radio waves, to study Smith's Cloud. Their picture is above. Because the radio telescope allows them to get the speed of gas (from the Doppler Effect) as well as the image of the gas. From their work, Lockman and his team were able to piece together exactly how each part of the cloud is moving.

What they found was clear evidence that the cloud is indeed starting to come into contact with the Milky Way. Some parts of the cloud are being accelerated by the Milky Way's gravity, while other parts are slowing down as they run into gas from our own Milky Way Galaxy. All of these interactions allowed Lockman's team to do a complicated analysis that not only proved the cloud is falling toward the Milky Way, but also how far away it is, how big it is, and what the future holds for the cloud.

In summary, the cloud is about 8000 light-years away from our Galaxy and has enough gas to make a million stars the size of our sun. The cloud will hit the disk of the Milky Way face on in about 20 million years, which should cause a lot of that now-quiet gas to start to collapse and turn into stars. The rest of the gas will be mixed into our galaxy's private gas reserves to make stars long into the future. The impact site will be thousands of light-years away from us, though, so our descendants will have to travel quite a ways to watch the slow-motion collision in person.

An open question is, where did this gas come from? Maybe it is new material that has never been part of a galaxy before, or maybe it is a remnant of gas from a small galaxy that the Milky Way tore apart long ago. The gas is probably not the remains of old supernovae falling back into the Milky Way -- there's just too much of it, and it is a little too far away. It may not be possible to know for sure for a long time to come.

Tuesday, January 15, 2008

Taking a census of the neighborhood

Image Credit: NASA/CXC/M. Weiss

Today, I continue to talk about science presented at the American Astronomical Society meeting, held last week here in Austin, TX. Yesterday I talked about goings-on in our own Solar System. Today, we step back a bit and look at our corner of the Milky Way Galaxy. Several different people presented results of ongoing attempts to identify our Sun's closest neighbors.

It shocks a lot of people to realize that we don't know if we've found all of the sun's closest neighbors yet. But the problem is that most of the stars in our galaxy are not bright, but very faint. Our sun's closest neighbor, Proxima Centauri, is so faint, we can't see it without a telescope. Proxima Centauri has a companion, Alpha Centauri A, that is nearly a twin of our sun; Alpha Centauri A is the fifth brightest star in our sky (after the Sun, Sirius, Canopus, and Arcturus). Sirius is relatively nearby, and is also very bright. But most of the Sun's neighbors are like Proxima Centauri -- feebly glowing red dwarf stars. So, it is not surprising that many of these may have escaped our notice even to this day.

Fainter than the faintest stars, brown dwarfs are often considered "failed stars" (an artist's rendition comparing brown dwarfs to the sun and Jupiter is above). They are too small to sustain nuclear fusion, which powers all true stars, and so although they start off quite hot, they cool off and slowly fade away, like the embers in a dying fire. Because of this, brown dwarfs close to the sun may be too cool to see at all in visible light; many brown dwarfs only glow in infrared light. We are just starting to explore the infrared sky, and so new brown dwarfs are being discovered all the time, and most are within just a few dozen light-years of the Earth. But there could be very cool brown dwarfs closer than Proxima Centauri that we just haven't found yet.

In short, the results of these surveys is that they are finding new neighbors, but not tons of them. These studies are important, because the relative numbers of different types of stars in our neighborhood, where we think we've counted most or all of the stars, is used in studying clusters of stars and other galaxies, where we have no hope of seeing the faintest stars and so have to guess how many stars are there.

Monday, January 14, 2008

Planets, planets, everywhere

After 10 days of vacation and a week of conferences, it's time to settle in to work for the first time in what seems like ages. This week, I'll work on catching you up on news from the 211th meeting of the American Astronomical Society. Let's start with our Solar System.

The big news is that Mars will almost certainly not be hit by an asteroid later this month. It took many nights of observing spread over a few months to verify this, but it seems that Mars will be missed by about 26,000 miles.

The Mars story should serve as an illustration for what would likely happen if we were to find an asteroid that was on course to graze by the Earth. Let's briefly review what happened. A small asteroid was discovered heading toward Mars, with pretty low chances of hitting the planet. As time went on, the odds of an impact seemed to go up, and then, suddenly, they dropped to almost zero. And this scenario would likely replay itself in regard to Earth -- in fact, it already has, with regard to the asteroid Apophis's close brush with Earth in 2029, when it will miss Earth by a mere 18,300 miles.

Why do the odds of an impact go up and go down? It all has to do with errors. When an asteroid is first found, its orbit is unknown. After a few days, a preliminary orbit can be estimated. But there are still big uncertainties in the orbit. Where it will be on a given time in the future can only be described as a circle or oval (more accurately, a spheroid), kind of like a target at a shooting range. The less we know about the asteroid, the larger the target area has to be to describe all the possible locations. So, when an asteroid is first discovered, that target area is quite huge. If a planet (say Earth or Mars) falls within that target area, then there is a chance that the planet will be hit. But the chance is pretty small, because the target is very large, and, in terms of outer space, planets are tiny. Since the asteroid can land anywhere within the target, the chance of a hit is tiny.

Now, suppose we get better measurements of the asteroid. The size of the target will shrink by quite a bit. But, if Earth or Mars stays within the target area, the chances of a hit go way up. The size of the planet remains constant, but the area where the asteroid may go is much smaller than it used to be. As time goes on and we get more data, the target area continues to shrink. And, most often, any planet in the target area will soon be outside the target area, and the chances of a hit go to zero.

If you watch the game show "Deal or No Deal," you can see the same phenomenon. In the game show, there are suitcases with prizes hidden inside ranging from a penny to a million dollars. At the start of the game, the contestant chooses a suitcase at random. Maybe the suitcase contains a million dollars; maybe it doesn't. Then, the contestant chooses remaining suitcases to open, showing off what is inside. Now let's forget the actual gameplay, and think about what happens as suitcases are opened.

We know that one suitcase contains a million dollars. As we start opening suitcases, chances are that we won't immediately open a suitcase with a million dollars in it. That means that the million dollars is still around, and may still be inside the contestant suitcase! The odds that the contestant's suitcase contains a million dollars goes up. But, eventually, the million dollars is likely to be found in one of the other suitcases, and the chances that the contestant will win a million dollars goes to zero. If you play enough times, eventually the contestant will win a million dollars. But, most times, the contestant won't win.

Now, back to asteroids and planets. Where the asteroid actually goes is like the dollar amount in the contestant's suitcase. At first, we don't really know where the asteroid is going, but as we open up suitcases by making more observations of the asteroid, we learn where the asteroid will not be. And, if one of those suitcases contains a planet about to be hit by an asteroid, the odds of a hit may seem to go up as more and more suitcases are opened. But, almost always, the suitcase containing the asteroid hit will be opened up by more observations, and the planet is safe. However, where "Deal or No Deal" uses only 26 suitcases, the cosmic version of the game uses millions of suitcases.

As we play enough times, eventually we will "win" by getting an asteroid impact (which some might consider losing the game). In fact, in 1994, the planet Jupiter "won" when it was hit by Comet Shoemaker-Levy 9. But, while "wins" happen, they are few and far between.

So, next time you hear about an asteroid with a chance of hitting the Earth, remember what happens on TV and what happened with Mars. The chances may go up at first, and they can climb pretty high. But, in the end, the chances of a hit almost always go to zero. Still, the wait can be frustrating and nail-biting in the meantime.

Finally, in other planet news, the planet Mercury will be visited in just two hours by the MESSENGER spacecraft, the first space probe to visit Mercury since 1975. MESSENGER will fly past Mercury three times in the next few years, using Mercury's gravity as a brake each time, before the probe will be slow enough to go into orbit around Mercury in 2011. This is a tough mission -- temperatures that close to the sun can melt lead, so a special sunshield had to be designed to protect the instruments. Still, the heat from Mercury's surface is enough to damage the cameras on the craft if too many pictures are taken at once. So, the success of this mission hinges as much on clever technicians and engineers as on the scientists behind the mission. Good luck, MESSENGER team!

Friday, January 11, 2008

Last day of the American Astronomical Society Meeting

Today is the last day of the 211th meeting of the American Astronomical Society in Austin, Texas. The last day always winds down pretty quickly -- I expect to see the crowds much lighter today, and, after lunch, the convention center will be virtually vacant.

Yesterday, the National Optical Astronomy Observatory (NOAO) released their report on the future of smaller telescopes within the NOAO structure. "Small" here refers to diameters less than about 4 meters, or about 13 feet. And the NOAO structure refers to telescopes operated primarily by a budget from the National Science Foundation.

In an era when ever-larger telescopes seem to dominate the scene, small telescopes owned by NOAO have been suffering from both declining use and declining budgets (which leads to fewer cutting-edge instruments and increasing structural problems, which leads to fewer users, which leads to budget cuts, which leads to less upkeep, which leads to more problems, and so on). And a recurring question has been whether to close these telescopes, or to devote more resources to them.

Last year, a major report on the future of National Science Foundation astronomy budgets recommended that NOAO study how to best utilize small telescopes and bring them up-to-date. Robotic telescopes or remotely-operated telescopes could allow observers to stay home (saving money that would otherwise be spent on travel, or money that may not exist for travel) and observe from home. Eliminating redundant capabilities would allow new instruments to be built for existing telescopes from money that would otherwise go to maintaining the redundant instrument. There were many other suggestions; the NSF report left it up to NOAO to decide what to do.

Yesterday, the NOAO report was released to the community. It covered this variety of topics, and appears to be a well-planned roadmap to refurbishing and revitalizing small telescopes. The main problem may come from the budgets being even smaller than the committee assumed. As I said a few days ago, the astronomy budget picture is bleak. Some telescopes may have to be closed, but, as the committee pointed out, we can't afford to keep everything open and still modernize.

I am happy that the entire astronomy community came together to help with this report. Astronomy on big telescopes relies on work done on smaller telescopes. Big telescopes should only tackle problems that require a big telescope; there are many interesting problems that can be solved on smaller telescopes. And, by having such a nice roadmap, we can now begin to prioritize and bring small telescopes out of the 20th century into the 21st century.

Thursday, January 10, 2008

Mars is safe

News flash: The asteroid that was threatening Mars won't hit Mars after all. Details at 11.

What I learned on Day 2

Yesterday was the second day of the winter meeting of the American Astronomical Society here in Austin, Texas. During the day, I went to talks on 10,000 mile-per-second winds streaming away from giant black holes (the stuff in the wind was never inside the black hole, so it can escape), stars in the outer parts of nearby galaxies, what giant stars can tell us about how elements are made, and even a debate on the future of astronomy research.

I spend lunch and parts of the afternoon discussing projects I'm working on with co-workers from across the country. Although we talk by email and telephone fairly often, it is nice to have everyone in the same room to help solve some problems, put faces to people I've only spoken to over the phone, and to start some new ideas.

You'll note I haven't talked much about new news coming out of the meeting yet; that's due to a lack of time to digest the information and blog about it. But that will give me material to talk about for the next week. :)

Wednesday, January 09, 2008

American Astronomical Society Meeting, Day 2

It's a crisp morning here in Austin, as I've arrived for day 2 of the winter meeting of the American Astronomical Society. As I had a related meting just prior to the main meeting, this is actually day 4 for me.

Yesterday, things started with the former Lieutenant Governor of Texas, William Hobby, talking to us about astronomer and its relation to Texas. This was followed by an invited talk on how to find planets around other stars, and then the 1500+ of us broke into individual sessions for talks on very specific subjects.

After a first round of talks, we came back for a lecture from Michael Griffin, the administrator of NASA. He was very frank, and painted a fairly bleak picture about NASA funding. On Monday, I heard a talk from an administrator at the National Science Foundation painting a similarly bleak picture. In short, astronomy will be lucky to see a budget that matches inflation and may see a flat budget. And while this may sound bad, our budgets are doing much better than most of the federal budget.

The fact is, out of the $2+ trillion federal budget, only about $400 billion is available to be split among all non-defense related budgets, and most of those areas are seeing cuts in funding. So, we should be happy for a flat budget in some ways. The only way to change this state is to either raise taxes or hope that the economy magically takes off again.

On a different topic, last night there was a meeting of astronomy bloggers at a local restaurant. Alas, I didn't make the meeting. It's not that I didn't want to meet my fellow bloggers, many of whom are far more accomplished than me, it's that I was exhausted. So, I hope that they will forgive me for not showing up.

Tuesday, January 08, 2008

1500 astronomers in one room...

...and the sound system is playing the Muzak version of "Tequila." I'm at the opening ceremony for the winter meeting of the American Astronomical Society. We are in a gigantic ballroom, waiting for the ceremony to begin. We'll have 10 minutes of business, followed by a science lecture, and then we break up and go to smaller meetings for much of the rest of the day. The past two days were spent in meetings of other postdoctoral researchers; if I get a few minutes I hope to summarize that for you.

Friday, January 04, 2008

Beware, Austin, here we come!

Copyright © 2007 by Sidney Harris

Austin, Texas is about to be invaded. Not by a foreign army, not by aliens, and not by kudzu, but by astronomers. Roughly 2000 astronomers, or nearly one-third of all astronomers in the nation, will be in Austin next week for our 211thmeeting of the American Astronomical Society (the AAS), the nation's largest association of professional astronomers. We have meetings twice a year, and the winter meeting tends to draw the largest crowds.

This also means that next week you can expect to see astronomy in the news almost every day, as astronomers release their newest results to the public. It also means that I have been given tasks of finding several groups of friends places to have working lunches, organizing smaller related meetings, and other such fun tasks. And it means that the people of Austin may be just as likely to overhear discussions about cataclysmic variables and axion detectors as they are to hear about guitar riffs or Longhorn basketball. Like the cartoon above, astronomers tend to live in their own world and have their own jokes and speaking style, so we'll probably stick out.

So, to the city of Austin, I apologize for the invasion. It'll be over in a week. And to the AAS staff, thank you for all of your hard work in arranging a meeting for us. It's a huge task.

I will do my best to try and keep you up to date with the goings-on at the AAS in between all of my hosting duties.

Thursday, January 03, 2008

Mars in the crosshairs

Will Mars be hit by an asteroid later this month? The answer remains a resounding "maybe."

Just a couple of posts ago, I talked about how it looked like an asteroid may hit Mars. The chances were low, just one in 350. Then, late December observations of the asteroid allowed better predictions to be made. The probability went up to nearly 1-in-25 (a 4% chance) before dropping slightly to a 1-in-30 chance. More observations in the coming weeks will help to refine the orbit. But we may not know for sure until we look for a new crater after January 30.

Mars will still probably survive unscathed, though, as I said before, we would learn a lot about asteroid collisions from a hit.

However, this is also a good learning experience for the process astronomers would go through if we saw an asteroid headed for the Earth. The odds of a collision often cannot be calculated as fast as we might like. It takes years of observations to pin down an asteroid orbit to the 8000 mile precision we need to predict an Earth collision.

An example is the asteroid 99942 Apophis. At one point, this asteroid had 1-in-300 chance of hitting the Earth in 2029 and 2036; two years of observing were needed to lower that chance to 1-in-45,000 for 2036 (and virtually zero for 2029). The Wikipedia page on this object gives a timeline of these odds.

Now, imagine if, instead of the date being 21 years away, but just a few months, how much panic there may have been, especially with the odds changing rapidly. This scenario worries us. Part of the problem is that we still don't know how to deflect an asteroid, and, if we make a mistake, we can cause an asteroid that was going to miss us (even a miss by 200 miles is still a miss!) to impact the Earth.

The best remedy for this is to keep on studying asteroids, their compositions, how they are constructed. Also, we need to keep looking for these asteroids. The further in advance we find an inbound asteroid, the more options we have, and the less panic we need to cause.

Wednesday, January 02, 2008

Presidential Primaries and Astronomy

Tomorrow, the 2008 presidential races kick into overdrive with the Iowa Caucus, followed shortly thereafter by a flurry of primaries and caucuses. It's interesting to watch, as I don't recall ever seeing this wide open of a race in both parties. And, I live in a state that doesn't vote until everything may well be decided, so I haven't been bombarded with television advertisements.

Anyway, some people have asked me who I think would be best for space and astronomy. I don't really know who might be most inclined to increase funding, but I also know that, in our political system, even a president who wants to greatly increase funding for astronomy and science has to deal with Congress and budget realities, so whatever a candidate promises doesn't really matter.

Really, the best thing for science research funding is a healthy economy. A healthy economy means higher tax revenues for the government, which means more money to give out (I'm assuming we at least try to balance our budget here, which tends to be a bad assumption in federal budgets). Second, science funding is helped by relative peace. An active war drains the budget (again, assuming it is paid for in cash and not on bonds).

Finally, the president needs to be either neutral or positive on science funding. This is usually not an issue, though there are a few candidates out there who probably would request reduced science budgets.

In the federal budget process, most astronomy is funded through the National Science Foundation, with some from NASA. But astronomy lives and dies on the National Science Foundation budget, which also supports most other non-health related science research.

So, if you want your vote to support science, vote for a candidate you think will revitalize the economy and reduce our foreign military needs. Whether this is best done by ending the war, bringing in more partners, or somehow otherwise stabilizing the Middle East is influenced by your personal opinion.

In short, astronomy prospers when you do, so choose who you think will be the best overall president!