What I Learned Today

We all grew up being told that the Milky Way was a spiral galaxy, twisting beautiful starry arms through inky space like our photogenic neighbor Andromeda. So it may have come as a surprise to you, as it did to me, to learn that in 2005 the Spitzer Space Telescope confirmed that in fact we live in a barred spiral galaxy instead. This isn’t that strange; apparently up to two-thirds of all spiral galaxies contain a bar.

Even more interesting, our galaxy has four arms (in our current understanding of its structure) and they have each been named. We reside in the Orion arm, a small offshoot between the Carina-Sagittarius and Perseus arms. The main two other arms are Scutum-Crux and Norma.

As a side note, the wikipedia page on barred spiral galaxies includes this rather unfortunate statement:

“Studying the core of the Milky Way, scientists found out that the Milky Way’s bulge was peanut-shaped. This led to the conclusion that all barred spiral galaxies have a peanut shaped bulge.”

I’m guessing that more was involved than simply generalizing from a single example.

Also fun is this list of Ten Things You Don’t Know about the Milky Way Galaxy. I was pleased to discover that I knew all but #8 and #9 — and also pleased that there were two new things to learn.

Radio telescopes allow us to listen in on distant sources and learn about fascinating objects such as pulsars, quasars, and even (maybe?) extraterrestrial civilizations. Directional antennas for these telescopes have greater sensitivity than omnidirectional antennas, but then they must be pointed in the appropriate direction. However, large telescopes can be prohibitively heavy. Arecibo, which at 300 meters wide is the largest dish in the world, doesn’t even try; it sits in a depression in the ground and lets the Earth’s rotation sweep it around on a daily basis. As a consequence, there are areas of the sky that it cannot study, and everything else can only be imaged for a short time each day. Other facilities such as Green Bank and the Deep Space Network use massive motors and gears to rotate their telescopes to reach other regions in the sky or to focus on a specific target for longer periods of time.

But why move if you don’t have to? Engineers have developed a clever way to simulate a directional antenna from a collection of smaller, stationary, omnidirectional ones. Given multiple antennas in a line, if you shift the phase of each one progressively more, then combine the signals together, the result is the same as if you had rotated a single larger antenna to point to the side. If you have the ability to digitally change the phase shift for each antenna, then you can “point” your array anywhere you like without moving anything physically. This is called “digital beamforming.” (Technically, beamforming permits the manipulation of both the phase and the amplitude of each component antenna’s signal.) The Allen Telescope Array in northern California is an example of an array that uses beamforming (e.g., to listen to the New Horizons spacecraft).

Even more exciting is the recent advance in adaptive digital beamforming. Here, each of the phase (and amplitude) shifts (weights) are modified on the fly to maximize the resulting signal quality. Apparently, some radio transmitters even send “training sequences” to help an adaptive receiver quickly identify the best weights to use.

Thanks to Toby Haynes for his excellent “Primer on Digital Beamforming,” which is both exceedingly accessible (even for those of us without a formal signal processing background) and satisfyingly detailed (with field strength diagrams for different antenna types and component diagrams for beamforming).

Escape velocity, as we know, is the speed at which an object must travel to be released from another object’s gravitational field. It depends on the mass of the second object and how far apart the two are. We’re familiar with velocities calculated for escaping planets or stars, but in fact they can be calculated for any collection of objects: a solar system, a galaxy, the universe. While we may escape the Earth, or the sun, or the solar system, or even the galaxy, today I listened to Dr. Richard Wolfson posit that the escape velocity of the universe might well exceed the speed of light, rendering us forever trapped within its immense gravity well. (He did not elaborate on what might possibly be “outside” the universe for us to visit.) Even more interesting is that, if the escape velocity does exceed the speed of light, then the universe itself meets the definition of a black hole. We could all be living inside a black hole with such a large diameter that we haven’t (yet) felt any distorting forces (“tides”) that would be created by whatever and wherever its center would be. On the other hand, if the universe has infinite extent then it isn’t even meaningful to talk about “escaping” it. [Image by Don Dixon.]

I was listening to the other lecture on the sampler CD from The Teaching Company, which was lecture 15 from “Einstein’s Relativity and the Quantum Revolution.” This was an interesting contrast to the great books lecture on Gibbon’s Decline and Fall of the Roman Empire. For one thing, I knew more about the basic subject here (general relativity and black holes) than I did with Gibbon. But it was still thoroughly enjoyable and a fun educational experience.

Dr. Wolfson used the word “gravitating” in an unusual (to me) fashion: he employed it as an adjective to describe an object that exerts a gravitational field. I’m more familiar with it used as a verb, as one object may “gravitate” towards another. It seems a little odd to bother with it as an adjective, since isn’t every object in the universe “gravitating”?

He also made a nice point about the common conception that black holes sit around “sucking things up”. As he pointed out, the black hole has no more gravitational force than the object(s) it originally came from–mass is mass. What’s special about black holes is that their mass is compressed into a small enough volume that the escape velocity ramps up past that magical number, c. “Regular” objects avoid this phenomenon by filling more space; gravitational force falls off as distance squared, keeping us all safe from such extremes. But a black hole and a planet with the same total mass exert the same gravitational force at a distance. So long as you’re outside a black hole’s event horizon, you’re just as safe as you would be if it were a whole and healthy planet.

One last interesting tidbit: our term, “black hole”, focuses on the inability of light to escape from the object. But the Russians’ word for it instead captures the notion that (from the outside perspective) time inside the black hole slows down… and stops. They call them “frozen stars.”

The EPOXI mission was born out of the desire to make use of the Deep Impact spacecraft, after it successfully hit Comet Tempel 1 with a separate smaller spacecraft. Two missions were selected to make use of Deep Impact: EPOCh (Extrasolar Planet Observations and Characterization) and DIXI (Deep Impact eXtended Investigation). If you stick DIXI and EPOCh together, you get… EPOXI. The proposers got kudos from NASA HQ for this acronym.

EPOCh has just finished its main investigation, which involved observing seven stellar targets that were believed to have planetary companions. I recently attended an excellent talk summarizing the results by Dr. Drake Deming, the deputy PI for EPOCh. They used Deep Impact’s camera to watch for the characteristic dip in stellar brightness when a planet transits across it. Since the camera was not designed for observing distant stars, it had no automatic stabilization, and the star would appear to wander all over the CCD. Tracking the star in the data once it was downlinked to Earth, and applying a different correction for each pixel in the CCD, makes ground processing challenging. However, they’ve been able to analyze this data and extract some interesting findings.

• They studied a Neptune-sized planet (radius about 4 times that of Earth) orbiting the red dwarf star GJ 436. It has an eccentric orbit that is likely to be influenced by a second, smaller planet. EPOCh has searched industriously for a signal from this smaller planet, so far not yet finding it (down to 1 Earth radius, the limit of what they can see with this instrument).
• A secondary transit happens when a planet goes behind its host star, from our perspective. This also causes a (smaller) dip in total brightness because the planet no longer reflects light from the star. This dip can help provide an upper bound on the albedo (brightness) of the planet. (Neat!)
• They also observed the Earth from Deep Impact, treating it as if it were an exoplanet and trying to see if they could accurately infer its properties. These observations serve as the perfect validation set to help us do a good job of interpreting similar observations of other planets, when we get to the point of having them. More details will appear in a paper on this subject of how an alien observer would view planet Earth.

Even better, the data collected by EPOCh will be released to the public in the spring. So you can try your hand at analyzing it, too!

And of course, stay tuned for news from the Kepler mission, set to launch on March 6. It will stare at (relatively) nearby stars specifically seeking Earth-sized planets in the “habitable zone” (where liquid water is stable). It will survey so many stars that even a null result (if they don’t find any Earth-sized planets) would make an interesting statement about the distribution of planets in the galaxy. It’s far more likely, though, that they will find such planets. We live in such exciting times!

Today I attended a fascinating talk by Dr. Michelle Thaller about the Spitzer infra-red telescope and the search for exoplanets. I love hearing about the ongoing discoveries of planets orbiting around other stars. This is cutting-edge observational science! The first exoplanet was detected in 1995; before that, they were only hypothetical.

Spitzer is an IR telescope that orbits the Sun, lagging behind the Earth in its orbit. This lets it observe out away from the IR signal of the Earth and Moon. Dr. Thaller opened the talk with some fun (and fascinating even if you’ve seen an IR camera before!) demos showing how in IR, you can see through some things (black plastic bags) but not others (optically transparent glasses). She noted that the Earth’s atmosphere is opaque in IR, which helps explain both the greenhouse effect and why you need a space-based telescope to observe the universe in IR. More than that, since dust is opaque optically but often transparent in IR, Spitzer has given us our first views deep into the center of our own galaxy (dust blocked optical telescopes’ view into the Milky Way). We subsequently learned that we live in a barred spiral galaxy (previously thought to be just a spiral).

Spitzer doesn’t have the resolution to pick out individual planets orbiting other stars, but it can detect a swept-out gap in a stellar disk that can indicate where a planet has formed. That can guide more detailed investigations for exoplanets, such as astrometry and radial-velocity studies. You can browse a catalog of discovered exoplanets, sorted by their method of discovery or an even more attractive atlas of the planets and their stars. We’re currently up to 326 (from 1, only 13 years ago!). You can follow along with the latest planetary discoveries at PlanetQuest, and even download a desktop/Dashboard widget tracking the exoplanet tally.

The talk was exceedingly well timed. Just yesterday, it was announced that the first ever images of exoplanets had been recorded: Fomalhaut b by Hubble and three planets around HR 8799 by ground-based telescopes Keck and Gemini, using adaptive optics (see more pictures here). The full scientific papers are available here (AAAS subscription needed for full text PDF):

• Optical Images of an Exosolar Planet 25 Light-Years from Earth by Kalas et al.
• Direct Imaging of Multiple Planets Orbiting the Star HR 8799 by Marois et al.

Despite this pile of planetary discoveries, the hunt is still on for “Earth-like” planets: similar to our home world in terms of mass, size, temperature, and atmospheric composition. It’s bound to happen soon!