You Are Being Diverted

In lust under a second, you should have been forwarded to the new home of the Professor Astronomy blog, http://blog.professorastronomy.com. If not, just click here and you will be taken there. Otherwise, why not update your bookmarks?

If you have any trouble reaching the new blog, contact me. Please!

The blog is moving!

Actually, I'm not moving far, just changing my hosting from private hosting to BlogSpot, so that I can make use of some of their nifty widgets and gadgets. In a few days (once I'm sure everything is working and DNS changes have propogated), I'll set this page to automatically forward to the new blog. In the meantime, set your bookmarks to: http://blog.professorastronomy.com.

Changes afoot

While I sit in the clouds and rain over the next couple of days, I'm going to try some upgrades to this blog. Â So, if you were trying to read this and getting errors, or if there are several broken links, or if things look ugly, I apologize. Â Hopefully all will be well by the end of the weekend.

On top of Mount Locke

Yesterday I drove across Texas to McDonald Observatory. Â As always, it was a long drive. Â I think there must be a buzzard convention in the area, because many times I came around a corner to see a flock of buzzards just sitting in the road, even though there was no roadkill for them to eat. Â Or maybe they were hoping to cause me to wreck, thereby getting an easy meal. Â Either way, it was a little odd. Â I also saw a tarantula crossing the road, as well as what used to be several armadillos. Â And, these days, no trip across West Texas is complete without seeing a truck or two carrying parts for the construction of wind farms. Â Yesterday I saw two semis, each carrying a single blade for a windmill. Â It's eye-opening to see how big the windmills are, when it takes an extra-long semi to carry a single blade.

Starting tonight I have seven nights on the 82-inch telescope. Â Unfortunately, as you can see above, the weather is not very favorable. Â It rained overnight, and will probably rain again this afternoon and tomorrow and Sunday. Â Thick clouds are drifting overhead, with delicate tendrils reaching down toward the mountain tops. Â Occasional bursts of brilliant blue sky appear behind the clouds. Â Some of the mountainsides have long-dormant waterfalls cascading down into the desert floor. Â It's gorgeous, but not conducive to astronomical research.

Still, I'll prepare this afternoon as if it is going to be a clear night. Â One never knows; the clouds could magically part, and if I'm not ready to go, I could miss the only clear weather of the week. Â Most likely, though, we're going to be socked in until at least Monday.

In the coming days I'll blog about what I'm looking at, and maybe make a few changes to the website along the way. Â These cloudy observing runs are good for that sort of thing.

The students are back

Image Credit: Jorge Cham

Today is the first day of classes here at the University of Texas. In the past few days, this campus has transitioned from its sleepy summertime state to a bustling hive of educational activity. Monday, things were very quiet, with the exception of some large tents being put up. But coffee carts were closed, lines at restaurants were short, and there were plenty of seats on the bus. Yesterday, the coffee carts were still closed, but the sidewalks were a steady stream of students carrying burnt-orange bags filled with overly-expensive text books, and the smell of barbecue wafted from the large tents, welcoming students back to campus. And today, the buses are overflowing, the sidewalks are full of students trying to find their way around campus, Gideons on every street corner trying to pass out New Testaments, lines for coffee are spilling out of buildings, and new students are riding the elevators up and down, wondering why it won't stop on the second floor (despite the signs saying that the elevator doesn't stop there).

Education is our primary purpose for existence here at the university, so it is hard to complain about the inconveniences when they simply mean it is time to get back to work. But academics and astronomy are an odd mix. Success in our careers is most often defined by the impact we have on astronomy research, not on how many students we teach or how well we teach them. But teaching a course requires much more than the 4 hours a week spent in the lecture hall; trying to teach multiple courses means foregoing almost all research for a semester. So it is in the summer that most research gets accomplished. With the start of the school year, as we re-enter our primary mission, our productivity will now decline. To me, that's a bit perverse. But it's the way of the world, I guess.

When the refereed becomes the referee...

Last week I talked about how much fun I was having (NOT!) responding to a peer review of one of my papers. Well, now the tables are turned, and I am working on a review of someone else's paper.

It's a bit nerve-wracking. I am supposed to look over the science and make judgements on the quality of another person's work. But more than that, the author usually knows more about what she/he is writing about than I do. So, if I want to do a good job as a peer reviewer, I need to read not only the paper that I'm reviewing, but a lot of background literature.

There are lots of things that need to be considered in a peer review. These include:

• Is the research new and useful? It doesn't need to be the most exciting astronomy ever, but if a paper has little use to anybody else, then the big astronomy journals tend not to want it.
• Is the research convincing? Do I believe the results? Even if I don't believe the conclusions, do I at least believe that the data and the methods are sound?
• Have the authors references enough of the literature? When we write a paper, we don't need to re-derive all of 500 years of astronomical research. It is okay to use the results of other people (like Newton did in his famous quote about "standing on the shoulders of giants."), but we have to give those people credit. And, as a referee, I need to make sure that the paper is not based on old results that have been discredited.
• Did the authors do the math right? Astronomy involves a lot of math. If I were to be a super referee, I would re-calculate their numbers and re-derive their equations to make sure I get the same answers. I tend not to do that. Instead, I spot check numbers and see if the results make sense. This is not an ideal method! I'm being a bit lazy here, and I am giving the referee the benefit of the doubt, assuming that they've checked their own work for mistakes. But, generally, the only way to truly check the work would be for me to re-do their project. That would take time, and delay the publication of the paper. If I suspect a problem, it is okay for me to tell the authors that I think there may be an error, and for them to check the results.
• Does the organization of the paper make sense?
• Do tables and graphs and figures convey the information the authors claim, and do so in a clear manner?

In the end, as a reviewer I get large sway over whether or not a paper eventually gets published. If I reject a paper, the authors can ask the editor of the journal to send the paper to another reviewer, and that decision lies with the editor. If I say a paper is fine, it will almost always appear in print, but the editor would have the power to request changes or even send it to another reviewer, if he/she had reason to doubt the quality of my report. So far, I have never suggested that a paper be rejected.

I do feel a little bad for the people who get my reports. I tend to be long-winded, ask lots of questions, and make lots of suggestions. I just hope that the authors on the receiving end of my reports find my long litany of comments useful. After all, it is science that needs to be served, not my whims or the authors' own desires.

Heart of Darkness

I've noticed that, when we astronomers are attacking a problem or question, we tend to attack from many different angles. Some work, some don't, and eventually the picture becomes clearer. But we have a memory of the mess that existed during the highest frenzy of research, and we tend not to see how nicely all the pieces fit together into an overwhelmingly convincing argument. It is only when someone comes along and organizes all the evidence in a linear fashion that we can admire the result.

This week on our preprint server (the webpage where astronomers can post their research before the journal containing the paper arrives in the mail), Mark Reid of the Harvard-Smithsonian Center for Astrophysics published this article, which is a scientific review of the search for a black hole at the center of the Milky Way galaxy over the past five decades. The review starts with some of the early evidence for something big at the center of the Milky Way and other galaxies. But it quickly moves to a discussion of the strongest current evidence for a black hole in the center of the Milky Way:

1. The orbits of stars at the center of the galaxy: Over the past decade, two groups of astronomers (one led by Andrea Ghez at UCLS, and the other by Rheinhard Genzel in Garching, Germany) have used infrared cameras on large telescopes to take pictures of the center of our galaxy. Over this time, they have been able to see stars completing orbits around the same point in space. One star even comes within 100 Astronomical Units (100 times the Earth-Sun distance) of this point, and moves at 6000 miles per second at closest approach, about 3% of the speed of light. Using the laws of gravity and orbits laid out by Johannes Kepler and Isaac Newton 300-400 years ago, we know that the object at the center of these orbits must have a mass about 4 million times that of the sun! So, we have four million times the mass of the sun in an area no larger than our Solar System. That's a lot of stuff! (You can see animations of the stars' movements here (UCLA) and here (Germany).
2. The focus of these orbits is at the same point as a strong radio source, but this source has to emit less infrared light than a single star. The pictures used to track the stars' orbits were taken in infrared light, but there was no light coming from the center of their orbits. So, we need an object that needs to be 4 million times the mass of the sun, is smaller than our Solar System, can emit radio signals, yet emits almost no infrared light. (We don't know about visible light, because dust between us and the center of the Milky Way blocks visible light.)
3. The size of the radio source is less than 1 Astronomical Unit across. In other words, unless only part of the object is making radio waves (which would be a very improbably feat), all of that matter, four million times the mass of the Sun, is constrained to be in an area smaller than the Earth-Sun distance. A black hole with a mass of 4 million times the mass of the sun would be about 1/8 of an astronomical unit in size, This means that the object we are looking is smaller than eight times bigger than a black hole.
4. The object is sitting still at the exact center of the Milky Way. Those stars we see in orbit around the dark object at the center of the Milky Way are pretty big, and there are a lot of them, adding up to almost 4 million times the mass of the sun themselves . If the thing at the center of their orbits is not a single object at the exact center of the galaxy, then it should be whipping through space at speeds similar to those of the stars around it. But it is sitting almost perfectly still, moving at a speed of less than a quarter of a mile a second, or "just" 900 miles per hour. That may sound fast, but remember those nearby, massive stars are zipping through at 6000 miles per second. The sun is moving at about 150 miles per second around our galaxy. So the fact that this thing isn't moving in such a harsh environment means that it is not only pretty honking massive, but it's right at the dead center of our galaxy.
5. Ordinary matter can't explain all the data and survive for very long. It is possible to make very dense clusters of faint stars, white dwarfs, or neutron stars that would be visible only in radio waves and have this large amount of material in a very small space. But if you somehow make a tight ball of fairly ordinary stars, the stars will collide with each other, merging to make a single black hole, or slingshot each other out of the middle of the galaxy. In either case, the cluster of stars would only live for a million years. This sounds like a long time, but it is the blink of an eye in the 13-billion year lifetime of our galaxy. How could you make such a dense ball of stars, and why would it be at the exact center of our galaxy right now? And when we look at other big galaxies, they all have something big and massive at their center. How could all big galaxies have a short-lived cluster of faint stars at their middles at this instant in time? There's no reasonable answer.

When you add all these things up, there is only one object in all of currently-understood physics that can be this massive and this small, and that is a black hole. Other explanations, like the very dense cluster of faint stars or balls of weird subatomic particles, are just too contrived to seem reasonable. In the parlance of a criminal court, the presence of a black hole at the center of the Milky Way is beyond a reasonable doubt.

The best part is, there are still more tests that we can do to continue to test and probe this black hole. Every theory should be tested and re-tested as often as novel techniques arise. But, I think we can move on with confidence and stop asking "Is there a big black hole in the center of the Milky Way?" Instead, we can start asking the even harder question of "Where did the big black hole come from?" Hopefully, in another review article 50 years from now, the evidence answering that question will be just as overwhelming.