IR Photography

This weekend I was playing around with an infrared filter on my digital camera, and realized that the resulting photos could help illustrate some points in astronomy.

Different types of light are important in astronomy, as they are tracers for different processes and temperatures inside the objects we're looking at. Since we can never go to most of the objects we study, it's crucial that we get as much information as we can from light alone. For example, IR light is good for studying cold objects, and for studying objects that are hidden inside a cloud of dust.

Take a look at the three photos below.  They are (in order) normal visible light as our eyes see it, a black and white version of what our eyes normally see (visible light), and an infrared image (displayed in black and white). (Click on the images for bigger versions.)





Notice how in the last image, the water and the sky appear darker. This means that they are not sending as much IR light towards the camera as they are sending visible light. Specifically, this is the same reason as the sky is blue: light from the Sun comes in all colors. Red light (and infrared light) tends to go straight through the Earth's atmosphere. Blue light, however, gets scattered by dust in the air, so as a result the whole sky looks blue. Since the red light goes straight through, this is also why at sunset the Sun looks red. Since the IR light goes straight through as well, when we look at the sky in IR light, we do not see the IR light from the Sun scattered throughout the sky, so the sky looks dark. Lakes look bright blue because the blue light of the sky is then reflected back at our eyes, so in IR lakes and rivers would also appear dark. Note however, that the clouds on the horizon of the IR image do appear bright - the high water content in these clouds does scatter the IR light back towards the camera.

Similarly, IR light goes right through thin clouds in space (just like it goes through our atmosphere), but it does eventually get stopped by thicker clouds (just like the clouds on the horizon in the last image). Because of this, we have to use radio waves to see through the densest clouds. And also because of the clouds and moisture on Earth, we cannot actually do sensitive IR imagery from the ground, and have to instead do it from very high mountaintop observatories (above much of the Earth's atmosphere), or from space telescopes like Spitzer.

Science in DC

This week I'm in Washington DC for the annual meeting of my teacher's union - yes higher ed professionals are unionized too, especially the ones in public higher ed. I've had three interesting science sightings.

1. The DC Dental Spa has advertisements all over the Metro stations. Click through to that and look in the upper left. Look familiar?


2. Further down the same platform, I saw ads for the Spy Museum sporting photos of the Very Large Array. I wasn't able to Google a picture of their actual ad, nor snap one in the Metro (anti-terrorism security and all that), but the ad had a shot much like this one with the colors altered to yellow and red tones, and the very strong implication that the dishes were intended for communications with spy satellites. (And yes I'm sure it was the VLA - I always examine the pedestals for the three feet since when I visited Socorro, NM, that was the easiest part for us to see other than when I got to walk out on the face of one.)


3. And the last interesting sighting: in the Vendor Exhibition Hall of the NEA convention I saw a giant model T-Rex head. Delighted at the prospect of literature saying how evolution is a great tool to teach about the Scientific Method and critical thinking, I instead discovered that it was a booth by the Creation Museum. (No I'm not linking to them to drive up their traffic - if you really care, they're the first Google hit.) I am planning to tomorrow bring my camera and get a picture in front of the booth, just because I have to. It'll be funnier than my Segway shot.

Earth-like Planets commonly form, says Spitzer study

Spitzer (the "IR Hubble") scientists have recently put out an interesting paper implying that the majority of Sun-like stars may form Earth-like planets. (Hat tip to the Bad Astronomer for the head's up.) They studied a set of 300 Sun-like stars and the dust around them - dust glows in the infrared, so Spitzer easily sees it.

The color and brightness of the glowing indicate the temperature of the dust. The temperature of the dust is only (essentially) affected by the light from the parent star, so the temperature depends on the distance from the star. So by looking at each star, even though they can't actually resolve the disks they can from the *color* of the disk determine how the dust is distributed around the star.

Next thing they did is determined the age of the stars. I'm less clear on how that was done, since Never mind, I read the actual article (PDF), and they say "Ages for these stars were estimated from pre-main sequence evolutionary tracks, as well as kinematic association with groups of known age (e.g. Mama jek et al. 2002)." I'm actually a little skeptical about this, since those methods also rely on the color and brightness of the star/disk (since we cannot resolve the two separately), so it seems like circular reasoning to me. I ended up posting a question about this on the Bad Astronomy blog to see if I can get any response.

One thing I'm skeptical about is the method they used to determine the age of the stars. For the most part the stars and the disks cannot be resolved. They get information about the disk from the IR spectrum of the star/disk combination. They get the age of the stars (according to the actual paper) from evolutionary tracks on the HR diagram - which rely upon the spectrum of the star/disk combination. Isn't there some circular reasoning there? Or are the evolutionary tracks based upon the visual spectrum and we're able to assume the visual wavelengths are entirely uncontaminated by the disk?

However, let's move on. Assuming they know the disk description, and assuming they know the age of the star, and they selected stars that are all like the Sun, they can then track how the disk changes with the age of the star. They clearly find that the disk thins out at an Earth-like orbital radius as the star ages. One possible explanation of this is that an Earth-like planet formed and cleared out the dust (by accreting it). It's also possible that the dust was just blown out by the stellar wind, but I am under the impression that the study ruled this out - that dust around and Earth orbit is preferentially dissipating as the star ages, not that all dust is dissipating which would be the result of a stellar wind.

So the conclusion: nearly all Sun-like stars form Earth-like planets. Very interesting. We have yet to *see* these planets, but there's evidence they may exist.