24 Feb 2008
Forty years ago today, a paper came out of Cambridge University in England announcing to the world the discovery of yet another bizarre heavenly object, a mysterious "pulsing" animal found quite by surprise. We now call it a pulsar.
It was actually discovered the summer before, in July 1967, by a young postgraduate student at Cambridge, Jocelyn Bell. And as is common in the sciences it was discovered without being looked for. She and her thesis advisor had been doing radio astronomy on far more distance objects when out of nowhere popped this little throbbing goblin.
Radio astronomy uses the radio part of the electromagnetic spectrum to "see" things out there, things invisible to our eyes. While searching deep space with her radio telescope she came across a strange pulsing of radio waves coming from an unknown point in space. Like a heartbeat it was very regular, unlike a radio noise one might expect.
It was such a regular pulse that it was suggested the signals might have been from an intelligent source, perhaps an alien civilization. Tongue firmly in cheek, the strange discovery was first called LGM-1, for Little Green Men.
The humor aside, the object was a mystery. But shortly after the release of the paper, astronomers figured out what it was. It turned out that the pulsar was just another stellar object we already knew about, only seen from a different point of view. It was a neutron star. But what is that exactly, and why do some pulse?
A neutron star is the core remnant of a giant star that went boom. When a massive star dies, it blows away its outer layers in a giant explosion called a supernova. But the core implodes into a superdense ball of neutrons.
When I say superdense, I mean superdense. A cubic inch of this stuff would weigh in at 100's of millions of tons. That, my friends, is superdense!
Moreover, when one forms it spins just like the star it came from, but it spins fast. You guessed it... superfast. Like an ice skater bringing in her arms to spin faster, the neutron star, shrunk to the size of a large city, can spin fast - hundreds of times a second.
This high spun neutron star has a north pole and a south pole, the result of an intense magnetic field. It is out of these poles that radio waves spew.
This all leads us back finally to the pulsar. You might already be able to figure out what those pulses are. If the neutron star is spinning like a top but the magnetic field is slightly off axis, you can see how its radio beams could sweep out an area like the searchlights at a world premiere.
I'll use the classic lighthouse analogy as an illustration. As the beam of light from a lighthouse sweeps around and around, those on the horizon - in the line of sight - see what appears to be a regular pulse of light.
That is what a pulsar is like. It is a neutron star whose expelled radio blast is in our line of sight. We see the "pulse" each time it sweeps by. Other neutron stars could no doubt be pulsars if they would just line themselves up with us.
Once they were found, astronomers went looking for them and found them by the bucketsful.
As to Jocelyn Bell, her faculty advisor got a Nobel Prize for his work in pulsars. Jocelyn was passed over. Hmm... At any rate, her discovery of the pulsar has probably inspired many young women to take up a discipline long held captive by men.
Someone say "Amen!"
Who needs a Nobel Prize, anyway.
Until next time, clear skies!
10 Feb 2008
Next week, on Wednesday the 20th, we are all invited to a celestial phenomenon for the whole family. At about 6 in the evening, in the eastern skies, our Moon will rise full but not like it usually does. There is a twist tonight. As it rises it will be crawling into our shadow in an event we call a total lunar eclipse.
The timing of this particular eclipse is perfect for those of us in southern California, and we won't get it this good again for a couple years. We essentially get to see the whole show at a reasonable hour. Now we may miss part of the first act, but that's OK; the opening act is a snoozer. Here is a synopsis of our play.
Every month the Moon makes one trip around our planet. During that circuit it manages to get itself on the side of the earth opposite the sun. When it does this it risks going through our huge shadow. Because the Moon's orbit is slightly tilted, it doesn't always manage to hit the shadow, but it will on the 20th, and thus we have ourselves an eclipse.
The whole process takes several hours, which will give you some time not only to relax and enjoy it, but some time also to observe some other cool science things.
When the moon first peeks over the eastern horizon, it will already be fully into what is called the earth's "penumbra." The penumbra is just the partial shadow an object lays down. You have unwittingly experienced it yourself. As the sun rises in the morning from behind a distant hill or mountain, we do not see the whole sun yet. When it first shows itself, when all we see is part of it, we are in the partial shadow - the penumbra - of the earth. The sun is neither completely hidden from us, nor is it blazing in all its full glory.
This is what the Moon is experiencing when an eclipse begins. For someone on the Moon, the sun appears to be moving behind the earth. But a penumbra is still so bright, most of us would never notice it.
It isn't until the Moon moves completely into the full shadow, the umbra, that the show really gets underway. This will have just begun as we see the Moon rising Wednesday. You will see it maybe a third covered in the dark umbral shadow.
Notice at this time, too, after the sun sets, how the whole eastern horizon seems to be lifting up in a purple haze. That curved semi-darkness is the earth's shadow, the same shadow the Moon is moving into.
Over the next hour or so, until about 7, watch how the shadow crosses the face of the Moon, darkening it. Notice also how the shape of the shadow is curved. It is always curved. This fact led ancient Greeks to believe the earth was a sphere, the only shape that always has a curved shadow.
The darkness will last for about an hour, it takes that long for it to pass through. It is at this time you might notice that the Moon takes on a reddish appearance. That would be because our own atmosphere bends the light from the sun, like a lens, toward the Moon. The only wavelengths from the sun long enough to make it through all our atmosphere and get the free trip to our satellite are from the red end of the spectrum. More poetically, all the world's sunsets are bleeding towards our Moon.
Just before 8 PM, the Moon will have reached the other side of our shadow. The umbra will begin to give way to the penumbra, all the main action having been completed. By about 9 PM the umbra is gone; by 10 PM the nearly invisible penumbra will fade by and the entire show is over.
This will be the last time to see a total lunar eclipse at a reasonable hour for a couple years. You and your family, or school class, or scout troupe might want to make it a nice evening of observation of one of the heaven's free dramas.