Winding Down: Pulsar and Astro-Timing

Winding Down: Pulsar and Astro-Timing

Adam Frank for McGraw-Hill

How do Astronomers tell time? How do they gage the passing ages for objects that are thousands of light years away or more? This is a pretty critical issue for anyone interested in understanding how the heavens are constructed since, on some level, most questions involving physics come down to measurements of space or time. On Earth of course all you need for a good measurement of time is an accurate watch. That is easy enough to procure for laboratory work. For distant astronomical objects however, it seems unlikely that Nature would be kind enough to hang a giant clock next to a star just for our benefit. But in the case of Pulsars it is almost true, unlikely as it may be. Let me introduce you to Pulsars, the Rolex's of the night sky.

Pulsars were first discovered using radio telescopes in the 1960s and immediately they began freaking out astronomers. A pulsar appears in the sky as a series of perfectly timed on-and-off flashes of radio energy. The accuracy with which the pulses repeat themselves stunned scientists and a few even suggested that pulsars might be radio beacons of distant space-faring civilizations. It didn't take long for an explanation to emerge that was easier to swallow. The new theory (which we still hold today) was, in some sense, physically stranger than the "E.T. The Extra Terrestrial" story.

Astronomers now have overwhelming evidence that every pulsar is a rapidly spinning neutron star, an ultra-dense cinder of a deceased massive star. Neutron stars are strange beasts in themselves. Born in titanic explosions called supernova, matter is so tightly packed in a neutron star that a mere thimble-full would weigh as much as a skyscraper. When appearing as a pulsar, these ten-kilometer across stellar gravestones spin at extreme speeds, sometimes whipping around hundreds of times a second. A neutron star becomes a pulsar if it is born rapidly rotating and has a strong magnetic field. The actual pulses form when electromagnetic waves from the neutron star are emitted into space. The magnetic field beams the waves along the poles of the star turning it into a kind of cosmic lighthouse. If the Earth is in the path of the beam we can see the pulsar, if not we miss it.

The enormous power locked up in their rotation makes pulsars great timekeepers. If you have ever tried to slow a spinning bicycle wheel with your hand you know it can be hard to accomplish. The only way to slow something which is spinning is to take away some of its rotational energy (like burning your hand via friction in the case of the bicycle wheel). Pulsars have so much mass (somewhere around 1.5 times that in the Sun) and rotate so quickly it's very hard to slow them down. That means the time between each pulse is pretty constant, exactly what you need in a good clock.

Pulsars do slow down though. The energy they lose into space with each pulse actually robs them of a little spin. You might think that makes them flawed as timekeepers but its actually a huge bonus for astronomers. The rate of the slow down can be measured with extraordinary accuracy just like length of the pulses. This extra piece of information gives astronomers an entirely new handle on the lifecycle of pulsars.

It is believed that pulsars are born spinning hundreds of times a second or more. Then they slow down continuously as they get older. Since astronomers see pulsars with a range of periods it confirms this idea. The fast pulsing pulsars are younger and the slower pulsing pulsars are older.

An example of this pulsar age determination cropped up just a few months ago when a team of astronomers in Australia measured the slowest pulsar known. Clocked with a pulse rate of 8.51 seconds, Pulsar J2144-3933 is spinning so slowly most astronomers thought it should be unable to send out radiation. It should, they thought, be a dead pulsar. Exactly why it is still pulsing is unclear but what J2144-3933 does clearly show is the link astronomers have forged between pulsar pulse rates and their ages.

So, it seems, in at least one kind of object, Nature was kind to us and did hang a giant clock in the sky.

Check Out These Pulsar Websites

The Princeton Pulsar Group

http://pulsar.princeton.edu/

Jodrell Bank Pulsar Research Group

http://www.jb.man.ac.uk/~pulsar/Education/pulsar_intro.html

Questions to Ponder

Some astronomers thought pulsars where signals from ETs. Can you imagine how astronomers would be able to determine if a signal did have an intelligent species as its source?
We can only see the pulsars whose radiation is beamed at us. Do you think we are missing the majority of pulsars? How do you think we might estimate the number of pulsars that really exist?