Monday, September 28, 2009

The sausage making of science


I have one last week of serious telescope proposal writing that will likely keep me from blogging very much.  In the meantime, here's an article by planetary scientist Dr. Mike Brown about some of the problems with doing science.  As I've said before, science is not devoid of emotion and egos, as much as we'd like to pretend it is.   I'm unfamiliar with the papers in Dr. Brown's article, so I don't know who's right scientifically.  All I can say is that these issues pop up much more often then I'd care to see.

Tuesday, September 22, 2009

Book Review: Backyard Guide to the Night Sky




National Geographic Backyard Guide to the Night Sky

I've been a subscriber to National Geographic magazine for many years, and I've always been impressed by the quality of that publication and other productions by the National Geographic Society.  So, when I was asked if I would write a review of their new book, Backyard Guide to the Night Sky, I was tickled pink and jumped at the opportunity.


The book I received (cover shot above) is a paperback, roughly eight inches tall and five inches wide.  It's easy to carry around, and the pages are a fairly heavy stock, similar to other high-quality field guides I have from the Audubon Society.  I wanted to test and see how the pages hold up to damp conditions like backyard astronomers might experience on a dewy night, but due to the ongoing massive drought here in Austin, I didn't have such a night for a test.  However, it did rain about a week ago, so I sat outside (under a porch) and read for about an hour with no noticeable effect on the book.  I did notice that my fingers would leave prints on some of the black pages in the book, but these prints disappeared within a few minutes.   The numerous prints, photographs, and illustrations were all sharp with vivid colors.  So, the overall impression I have is that this is both a beautiful and durable book.

Thursday, September 17, 2009

Data, Analysis, and Reality


As a scientist, I must always struggle to remember the difference between actual data and inferences based upon those data. This difference is often pretty clear. For example, if I take a picture of a star cluster, the actual data I've collected are counts of the number of photons arriving from different parts of the sky. I then infer that an excess of photons coming from a single point is a star, while an excess coming from an elongated smudge is likely a galaxy. My data are counts of photons, and my inferences are the nature of the object. Clear enough.

Sometimes the line is blurrier. For example, suppose I take a spectrum of the light from a star, where I split the light into its component colors. Typically, I see light from many colors of the rainbow, with perhaps a few specific colors missing (see some examples here). Those missing colors are due to individual elements, like hydrogen, helium, or oxygen, absorbing that light. Most often, that light is absorbed in the atmosphere of the star I'm looking at, and there are diagnostic tools that I can use that look at the lines and tell me the temperature of the star and how much of the given element exists in that star.

A few years ago, I wrote a paper on spectra of a group of white dwarf stars. I noticed that about one quarter of the white dwarf spectra showed the fingerprint of calcium. Calcium is an element that is very easy to see in stars, and it usually indicates the presence of many other elements, like iron and magnesium, that are not as easy to see. This is interesting, because the gravity of white dwarfs is so high that elements heavier than hydrogen and helium should sink out of sight below the stars' surfaces in a matter of years. Those white dwarfs with calcium and other heavier elements must therefore be swallowing this material from somewhere, and there's good reason to think that this material may be from asteroids or comets, the remains of solar systems around these dead stars. So, did 1/4 of my white dwarfs once have planets?

Monday, September 14, 2009

Looking for Planets Around White Dwarfs

An asteroid passing too close to a white dwarf gets shredded by gravity
Image Credit: NASA / JPL-Caltech
Our Solar System is such a nice, ordered place. Eight (or nine or thirteen) planets have been happily circling our sun for 4.6 billion years, and the whole system is stable enough to life to have formed on at least one planet. And the solar system looks like it will continue to be a nice, stable place to live, at least for another six billion years. At that time, the sun will run out of hydrogen fuel, swell up into a red giant star (swallowing Mercury, Venus, and maybe Earth in the process), expel about half of its mass as a planetary nebula, and then shrink into a white dwarf. When the sun loses mass, its gravitational pull will weaken a little. This will cause the remaining planets to move outwards in their orbits a little bit, but they should remain bound to the white dwarf sun, forever circling the ashes of our star.

Now that we astronomers are finding planets around all kinds of stars, we can estimate that at least ten percent of stars have planets of some sort. Many of these planets are very close to their parent star, much closer than Mercury is to our sun. But many are further away, like Jupiter. And, like Jupiter, these planets should survive the death throes of their parent star, and be circling white dwarfs.

Here at Texas, my colleagues have been looking for planets around white dwarf stars, with only one possible success so far. It's a long, hard, slow process that involves looking for the gentle tug of gravity that the planet exerts on its parent white dwarf.

Another of my colleagues, Dr. Mukremin Kilic of Harvard University, has been looking for white dwarf planets another way. He's trying to detect the infrared light given off by giant planets, a heat left over from the very creation of those planets. A newborn giant planet can be as hot as a few thousand degrees on its surface, and even Jupiter still glows at a more feeble one hundred degrees Kelvin (a modest -300 degrees Fahrenheit). With an infrared light telescope, it may be possible to detect these planets around white dwarf stars.

Today, Dr. Kilic released a study that he and his collaborators completed using the Spitzer Space Telescope to look for extra infrared light coming from 14 individual white dwarf stars. They specifically targeted white dwarfs that came from stars three to five times more massive than the sun. This is because astronomers have noticed that bigger stars tend to have bigger planets, and that bigger stars don't live very long. That means that any planets around these stars should be bigger and brighter (because planets cool over time). They found no evidence for any planets.

Does this mean that white dwarfs just don't have planets? Not necessarily. Even though Kilic looked at what should be fairly bright and young planets, he and his team were still limited to finding planets about five times the mass of Jupiter. Planet searches around living stars have found that these behemoth planets are rare; it's more common to find smaller planets than bigger ones. Also, more massive stars become larger red giant stars, and so they can swallow planets out to much larger distances. While the Earth may or may not survive a red giant sun, Jupiter might have trouble surviving around a star five times the mass of the sun.

And perhaps Kilic and his collaborators may just have gotten unlucky. Roughly 10% of stars have planets, and they looked at 14 stars, so they might have only expected 1 star to have planets. And, in the funny way that statistics work, expecting one and finding none doesn't prove very much. But, this isn't the first work on planets around white dwarfs that Kilic has published; he and his collaborators have now searched about 40 white dwarf stars for planets, so they might have expected to find four. If you expect to find four planets and find none, then you may be on to something.

There's another possibility. Many planetary systems we find tend to be chock full of planets. Our own Solar System has four giant planets. Remember how I said that when the sun loses mass, the planets will move a little further away? In that process of moving away, it is possible for the planetary orbits to become unstable, and one or more planets could be flung out of the system. So, maybe big planets won't be found around white dwarfs because of this. We don't know.

Astronomers today are finding planets everywhere, and we do expect to find them around white dwarfs, too. But studies like Kilic's are showing that these remnant solar systems, fossils of planetary systems once similar to our own, are not easy to find. Perhaps they are just harder to see than we thought. Perhaps they didn't survive the death of their parent star. Time will tell. If nothing else, we just need to wait six billion years and watch what happens to Jupiter, Saturn, Uranus and Neptune. One way or another, we'll learn what happens to planets when their parent star dies.

Wednesday, September 09, 2009

New Hubble pictures!

Hubble's new view of Stephan's Quintet
Image Credit: NASA, ESA and the Hubble SM4 ERO Team

Yes, I know I'm a whole 6 hours late to the game in bringing you these photos, during which time every other science-related blog in the world already posted them. But they are just so cool, I'm going to post them anyway.

Last May, the space shuttle Atlantis visited the Hubble Space Telescope for their fifth and final servicing mission. Hubble has certainly been one of the crowning achievements of NASA and the Space Shuttle program; the astronauts transformed a dud of a telescope into one of the most important astrophysical laboratories ever built. In May, astronauts replaced two instruments and repaired two others, plus gave Hubble new batteries, gyroscopes and some other minor repairs. Today, after nearly five months of sometimes frustrating checkout and calibration, NASA released its first images from the repaired Hubble.

You can look at all of today's images here. There are pictures of the colorful center of the Milky Way Galaxy's biggest globular cluster, omega Centauri. That picture shows the power of Hubble. From the ground, the center of omega Centauri looks like a blob of starlight, there are so many stars! Yet Hubble resolves them all. The Hubble also imaged the birth of a new star and the death of an old one.

For those of you who can speak spectra (the splitting of light into its component colors), there are some amazing spectra from the uber-massive star eta Carinae (a star nearly 100 times the mass of the sun, about as big as any single star can get!), spectra of gas near a supermassive black hole showing changes over the past 10 years, and more. While spectra are often less inspiring for the general public to look at, they carry far more information than you might guess. From a spectrum, we can determine the speed of a moving object, its atomic composition, its temperature, its atmospheric pressure, and other important quantities that a picture alone could never tell.

My favorite image (and the one at the top of this article) is of Stephan's Quintet, a tight grouping of five galaxies. Four of the galaxies (the yellowish ones) are located 290 million light years away and are so close together, gravity is ripping off pieces of the galaxies, creating streams of stars and rings of new star formation. The fifth galaxy, the whiter one in the upper left of this image, is not related to the others. It is only 40 million light years away, and just happens to be along the line of sight to the other galaxies. Just think -- when the light that Hubble saw left the more distant quartet of galaxies, the first dinosaurs were just starting to roam the Earth. When that light passed the closest galaxy, dinosaurs had already been dead for 25 million years, and the earliest humans were still 38 million years away. But the neatest part of this image is the zoomable version, so you can zoom in and around the image to see amazing details. You can see individual stars in the closest galaxy! Cool stuff.

Anyway, Hubble is now doing hard core science again (and has been for a few weeks). Thanks once again to the amazing team of astronauts and ground crew for fixing this marvelous telescope!

Note: At times I've been getting some errors when trying to access views of some images. If this happens to you, take a deep breath and try again. They must be popular!

Tuesday, September 08, 2009

News tidbits

It's telescope proposal time again, so I'm spending much of my day and energy writing short applications to use various telescopes. I'll take an easy path out here and just post some news snippets.

  • Refurbished Hubble pictures come out tomorrow! It's hard to believe that the most recent and final Hubble Space Telescope repair mission was four whole months ago. Tomorrow, NASA will officially release its first pictures from the refurbished telescope (if you don't count those pictures of Jupiter from July). I've been seeing some colleague's new Hubble pictures for several weeks now, despite threats from NASA that they'll never get Hubble time again if they show the pictures. Astronomers can't keep secrets, even when dire threats are made. Anyway, Hubble is now mostly doing science again, with some final calibration and engineering activities filling up the rest of the its time. If you want to see the release of the new images, check out NASA TV at 11am EDT (8am Pacific, 15:00 UT).
  • NASA's Review of U.S. Human Space Flight Plans Committee's Summary Report is out. This was a committee charged with reviewing NASA's future plans for manned space flight. A description of the committee and a link to the 12-page summary itself can be found on this page. The two major findings are (a) that the shuttle program will have to rush to finish its current launch program by the end of 2010, potentially risking safety, so NASA should be given some funding to allow the schedule to expand into the first half of 2011, and (b) that, at current NASA budget levels, the International Space Station (which isn't even finished being built yet!) will have to be de-orbited in 2015, NASA's newest Ares rockets will not be ready until at least 2016, and that a return to the moon could not happen before 2030. The committee also outlined several options where, for an extra $3 billion / year, the space station can be run to at least 2020, and missions to the moon or other near-Earth places could begin in the early 2020s.
  • Neil Armstrong admits the moon landings were faked. Or not. Last week, the satirical newspaper The Onion published an article claiming that Armstrong had been convinced by a conspiracy theorist that he had not, in fact, landed on the moon (warning: the Onion often contains adult language). Evidently some foreign newspapers, not realizing that the Onion is satirical (i.e., it makes everything up), picked up the story and unknowingly presented it as real. This just goes to show that you shouldn't believe everything you read. Not even on the Internet.

Tuesday, September 01, 2009

Happy New Year!

It's the start of a new school year here at the University of Texas. The building is full of freshmen wondering why the elevator doesn't stop at certain floors, our large number of weekly seminars are starting up again, and we've welcomed a dozen or so new graduate students into our program, and we're awaiting the arrival of a new faculty member and a few new postdocs.

Life in academia is centered on the school year, not on the calendar year. And, because of the incredibly slow bureaucracies common at universities, it's already time to start thinking about applying for money and jobs that would start next September.

For the past three years, my position here has been financed by a fellowship from the National Science Foundation. That money enabled me to expand my research programs, explore new and exciting areas of astronomy, and spend a lot of time writing blog posts and surfing the astronomy internet. That changed at the stroke of midnight last night, as my research fellowship came to an end.

In reality, there's not much change for me in the short term. For the next several months, I'm getting paid out of a different research grant from the National Science Foundation. It means a slight pay cut and some new research responsibilities, but otherwise few changes. I'll still be blogging as I can, I'll still be studying white dwarfs, and I'll still be looking for that ever-elusive permanent, tenure-track position.

So, sit back, relax, and enjoy the ride. With the start of a new year, the possibilities seem endless.