What I Learned Today

In a meeting today about the upcoming Square-Kilometer Array, I got to learn a little bit about pulsars and cosmology. Or at least, about what a radio telescope can do to observe pulsars and the earliest origins of the universe.

It turns out that pulsars, at least distant ones, do not manifest themselves as a blink of bright light (radiation). While presumably the signal starts out at a single frequency, over long distances it suffers dispersion that smears the pulse out over a range of frequencies. Even worse, the arrival time also gets smeared, so the higher frequencies arrive first and then “slide” down towards the lower ones (see example at right, taken from this handy explanation of pulsar dispersion). This means that if you aren’t sweeping frequencies at just the right time, you might totally miss a pulsar signal. The SKA will be hunting for these. Astronomers are particularly eager to find pulsars close to black holes, because pulsars are extremely reliable clocks and you should be able to test some interesting relativity hypotheses with a pulsar that’s within the black hole’s distortion field. (Related: see this ultra-cool animation of a recently discovered pulsar pair.)

Even more intriguingly, the SKA will be looking deep into the earliest evidence of the universe’s existence. Light takes time to travel, so the further away a source is, the farther back in time we are seeing it. If we look far enough, we can see back to when clouds of neutral hydrogen (a proton plus an electron) roamed the universe, before stars were created. At that time, we believe that clumps of dark matter formed. These aren’t directly observable, but they attracted the neutral hydrogen (which is observable and opaque). The hydrogen acts as a “tracer” for dark matter. Once stars formed, the radiation they gave off ionized the hydrogen (stripped off the electrons), and it became transparent… so dark matter once again could only be observed indirectly (e.g., through gravitational lensing). Yet due to distance and the limitation of light speed, we can still see back to the time of neutral hydrogen and record the shapes of dark matter then in existence.

On a lighter note: The History of the Universe in 200 Words or Less.

Unlike the International Space Station, the aurora remains a sky phenomenon that I have yet to witness with my own eyes. Yet I have read about and seen glorious images of the colored curtains of light, shifting suspended over arctic landscapes. While often yellow or green, some auroras are red or even purple (and some occur in the UV, not a “color” we perceive visually).

What causes the glorious color? What you’re seeing in the aurora is the result of a collision between charged particles streaming out from the sun and bits of the Earth’s upper atmosphere, where the energy resulting from that collision is given off as light. According to a recent Physics Today article titled “How do auroras form?” (by Robert J. Strangeway), the colors indicate the energy of the solar particles prior to collision:

For energies of a few electron volts or so, the aurora is red and emitted at altitudes above 200 km. Particles with energies of about 1 keV penetrate to lower altitudes and are responsible for the dominant yellow-green color of auroras. Even more energetic particles with energies above 10 keV can get to altitudes below 100 km; at such locations auroras are a deep red or purple color.

It makes sense to me that particle energy would correlate with penetration depth, and even with the energy (frequency) of the collision’s resulting radiation, which would dictate the color — although it seems a bit odd that this would not be monotonic (why would both low- and high-energy particles result in low-frequency colors (reds)?). However, wikipedia’s entry on auroras claims instead that the colors indicate what sort of atom or molecule in the atmosphere was on the unlucky receiving end of the collision:

Most aurorae are green and red emission from atomic oxygen. Molecular nitrogen and nitrogen ions produce some low level red and very high blue/violet aurorae. The light blue colors are produced by ionic nitrogen and the neutral nitrogen gives off the red and purple color with the rippled edges. Different gases interacting with the upper atmosphere will produce different colors, caused by the different compounds of oxygen and nitrogen.

So, is it the altitude (dictated by particle energy) or the composition? Probably both! Wikipedia continues:

Auroral electrons created by large geomagnetic storms often seem to have energies below 1 keV, and are stopped higher up, near 200 km. Such low energies excite mainly the red line of oxygen, so that often such auroras are red.

The energy dictates both altitude and which compositional responses you get.

The aurora has been captured from above (onboard the ISS), too (see left).

There will be a total solar eclipse this Friday, August 1, and if you aren’t fortunate enough to be in a good viewing location (Canada-Greenland-Mongolia-Russia-China), you can watch the event on NASA TV remotely. The broadcast goes from 6 to 10 a.m. EDT, and totality will occur from 7:08 to 7:10 a.m. EDT. That’s a bit early for us West Coasters, but maybe some of you further east can catch it. If you don’t have NASA TV on your TV, you can watch it on the web.

Watching for meteors is a fun pastime on warm summer nights out in the dark desert; you lie back on a blanket and wait for the sky to present its fireworks. The American Meteor Society (AMS) can help you plan your observing times, with this year’s meteor shower calendar and a weekly meteor outlook. But it turns out that you don’t need dark skies, or even night at all. You can observe meteors via radio instead of by sight.

I learned about this technique from an excellent book I am reading titled “The Sky is Your Laboratory: Advanced Astronomy Projects for Amateurs.” The first chapter is devoted to meteor observations, and I was interested to learn that you can record your observations (such as number of meteors observed per hour during a given event) and contribute them to the AMS, which uses this information in the aggregate to characterize meteor activity. Their visual observations webpage is a little out of date (last updated in 2006), unfortunately, but presumably you can still submit your logs.

At any rate, the book then proceeds to describe how you can listen for meteors with your FM radio. Effectively, you use an FM station’s signal as your probe. Signals at the frequencies used by FM stations are high enough that they generally go right through the ionosphere, but if they hit the ionized gas created by a meteor whistling by, they instead bounce back. So you tune your radio to the frequency of a station that is too far away to be received normally (300-600 miles), listen to the static, and wait for a glimpse of non-static. When the signal successfully bounces, you get a snippet of music or speech, and then back to static. You can count these observations just like you’d count meteors streaking across the sky — except that you can count them 24 hours a day, regardless of sky conditions. Awesome!

Yesterday, I attended the first day of the Astrobiology Science Conference, or AbSciCon. The day began with a great talk by Lord Martin Rees, who is the Astronomer Royal in England. He wrote a book called “Just Six Numbers” about six parameters of our solar system and Earth that have allowed for conditions conducive to the existence of life (and human beings). He was introduced by Paul Davies, who wrote “The Goldilocks Enigma”. Both books are now candidates for inclusion in my to-read list.

I next attended some talks about the galactic habitable zone. While I’ve read about the “habitable zone” inside our solar system (largely determined by the temperature range within which water exists as a liquid), this was the first time I’d encountered its galactic counterpart. In the galaxy, the constraints relevant for habitability (specifically, the creation of planets) involve the probability of a nearby, disruptive, supernova (so you don’t want to be too close to the galactic center, where stellar density is high) and the availability of metals for forming planets, which are more available where stellar density is high since they’re created by stars (so you don’t want to be too far away from the core). Our Sun is at 8.5 kiloparsecs from the galactic center, although it apparently wobbles in and out a bit, and there’s enough uncertainty in the measurement that it’s more like 7.5 to 8.8 kpc.

The discovery of exoplanets (planets outside our solar system, orbiting other stars) is, in my opinion, one of the foremost scientific discoveries of the past decade or so. It sounds like science fiction, but it’s real (we’re up to 287 exoplanets so far!). So far we’ve predominantly found only Jupiter-sized planets that are close to their host stars (and very, very hot), but most expect that this is because those are the planets that are easiest to detect. The hunt is on for Earth-sized planets that reside in their star’s habitability zone. In particular, a five-year study is beginning to collect 100,000 observations of Alpha Centauri B in hopes of detecting terrestrial planets. No planets have yet been detected in the complex (triple star!) Alpha Centauri system. However, in planet formation simulations, 42% of the Earth-sized planets that formed fell into the habitable zone around Alpha Centauri B. Therefore, if there are planets there, they might be very interesting to study (and much closer than many of the other stellar systems with planets). There are also reasons that planets in a binary or triple star system would be less likely to exist (e.g., gravitational disruptions could prevent them from accreting), but it seems like a good place to look.

But maybe our own location isn’t always so habitable, either. It’s been observed that if you plot the number of extant species as a function of time on Earth (a biodiversity curve), there is a certain cyclicity to the peaks and troughs. Fourier analysis identifies frequencies that have a strong correlation with the signal. It was previously thought that there was a 26-27 Myr periodicity, but this is now viewed as an artifact of the sampling rate (through time) of the curve. After the recent revision of the geological time scale, a stronger signal is found with a period of 62 Myr. So, what might be happening to cause biodiversity to peak and fall every 62 Myr? There are a lot of ideas, including the Nemesis theory of a companion star repeatedly passing through and disrupting the solar system, a sharp increase in the number of mantle plumes in the Earth, solar nuclear oscillations, and, intriguingly, oscillations in the position of the solar system with respect to the galactic plane. We seem to rise up (“north”) of the galactic plane and dip down (“south”) with a period of about 64 Myr, which could be a potential match. When we head north, we move in “front” of the galaxy as it travels through the intergalactic medium, exposing us to more of the incoming cosmic rays, which are known to have negative effects on life. Three of the five known mass extinctions in history coincide with a peak of this vertical oscillation (one is already quite firmly believed to have been caused by a meteor impact and therefore need not fit with the cosmic ray periodicity). A less exotic explanation for the cyclicity is that sea level changes have affected how well fossils are preserved, therefore making it appear that there are fewer of them when preservation rates are low. In fact, strontium isotope ratios, which indicate the degree of rock weathering and erosion going on, seem to have a strong 59 My periodicity. I’d say that the jury’s still out on this one.