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05/19/2004
No Little Green Men
We will soon celebrate the 4th of July and fireworks will explode
in our nighttime skies. This July 4th will mark the 950th
anniversary of possibly the biggest explosion seen by humans
with their naked eyes in the past thousand years. On the 4th of
July in 1054 a star suddenly appeared in the sky that was brighter
than Venus and was even visible in the daylight. Chinese
astronomers called it a “guest” star. Indeed, the guest only hung
around for about three weeks and disappeared from sight.
Fortunately, the Chinese recorded the time and location of the
disappearing star.
Seven centuries later, in 1758, the French astronomer Charles
Messier found a fuzzy bit of light at the spot where the guest star
had been. He named this little smudge of light M1 and went on
to find many more fuzzy patches of light, getting up to M109.
Today, these objects, some of them galaxies, others nebulae or
star clusters, are still known by their Messier numbers.
Now fast forward two centuries to 1967 and graduate student
Jocelyn Bell, a native of Belfast, Ireland. According to the
NASA Web site, Jocelyn failed the exam required to pursue
higher education in the British educational system. Undaunted,
her parents sent her to a boarding school - a wise decision. After
a getting a B.S. from the University of Glasgow, Bell went for
her Ph.D. at Cambridge under the direction of Antony Hewish.
At Cambridge, she spent two years helping in the construction of
a radio telescope to study quasars. (Quasars are extremely
powerful sources of radiation, now thought to be associated with
black holes sucking in surrounding material.)
The radio telescope was essentially a bunch of over 2,000
antennas arrayed on a field covering 4.5 acres. Bell’s job was to
operate the telescope and to analyze the data. In those days,
computers hadn’t taken over the taking and crunching of data.
Instead, analyzing data required hours of pouring over hundreds
of yards, maybe even miles of recordings on chart paper. (I can
empathize with Bell, having had to extract lithium battery
cycling data from rolls and rolls of chart paper before
computerizing our test facility.) Bell was taken by an “odd piece
of scruff” that she kept finding in a tiny fraction of the recordings
on the chart paper.
The “scruff” indicated repeating pulses of radio waves being
emitted at a rate that couldn’t be from a quasar. Bell recognized
the importance of the finding. Hewish’s group spent months
coming to the conclusion that the pulses, which were amazingly
regular, one pulse every 1.33731109 seconds, were being emitted
from some compact object. Tongue in cheek, they christened the
object LGM-1, in case it derived from little green men! In
reality, they had discovered “The Strange and Twisted World of
Pulsars”, the title of a special section of articles in the April 23
2004 issue of Science. One, “The Pulsar Menagerie” by Robert
Irion, stimulated this column.
The Hewish group''s publication of their findings in Nature early
in 1968 spawned intense work by others and it was soon found
that the pulses were coming from a neutron star. In 1934, Walter
Baade and Fritz Zwicky had predicted that a supernova, an
exploding star, would blow off most of its material and that the
rest would collapse to form a neutron star. As we’ve discussed
earlier, a neutron star is some piece of work. A teaspoon would
contain many tons of neutron star material! Under certain
conditions, a spinning neutron star emits beams of radiation, like
a rotating lighthouse beacon. Jocelyn Bell’s scruff was such a
star and this kind of pulsating radio star was given the shorter
name of “pulsar”. In 1974, Hewish shared the Nobel Prize for
his work on pulsars but Jocelyn Bell, the graduate student, was
left out.
What about that smudge M1, known now as the Crab Nebula?
That guest star in 1054 had all the hallmarks of a supernova
explosion. Sure enough, in 1968 workers at Arecibo and at the
radio telescope in Green Bank, West Virginia found a pulsar at
the heart of the Crab that emitted light and radio waves in pulses
at a rate of 30 times a second. This was quite exciting but the
workers seem to have been even more excited to find that the
spinning star was slowing down by 36.5 billionths of a second a
day. This slowing down confirmed a model proposed by
Thomas Gold at Cornell. His model predicted that pulsars were
spinning neutron stars that would slow down under high
magnetic fields. In slowing down, he predicted that the energy
lost would come off into space partly as cylinders of light, thus
giving rise to the lighthouse similarity.
In 1974, another grad student, Russell Hulse came up with a
computer algorithm that made detecting pulsars ten times more
sensitive than was previously possible. He worked with
astronomer Joseph Taylor at the University of Massachusetts,
Amherst. One of the pulsars Hulse picked out spun at17 times a
second. Hulse was annoyed, however, when he found the timing
pattern kept changing. He finally decided that the pulsar was
orbiting another body. Taylor hopped down to Puerto Rico,
home of the Arecibo radio telescope and quickly confirmed that
the Hulse-Taylor binary, as it was to be called, consisted of the
pulsar orbiting a nonpulsing neutron star.
Taylor combined forces with other astronomical types and
showed that the neutron stars were spiraling in towards each
other. The really exciting thing was that they were spiraling in at
just the rate predicted by Einstein many decades earlier. He
predicted that gravitational waves carry away the energy lost in
the process. Indeed, this was the first evidence that the
gravitational waves postulated by Einstein do exist. In 1993,
both Hulse and Taylor shared the Nobel Prize.
But the grad students weren’t finished. Shrinivas Kulkarni,
working at the Arecibo telescope in 1982, found the record
setting pulsar spinning at a rate of 642 times a second. If you
were standing on the surface of that star you’d be zipping along
at somewhere in the neighborhood of 20,000 miles an hour,
about a tenth the speed of light! Kulkarni was a grad student
with Donald Backer’s group at UC Berkeley. Although a
hundred of these superfast spinning neutron stars have been
discovered since then, none is faster than the one found by
Kulkarni.
These superfast pulsars are so precise in their spin rate that they
make the best clocks in the universe. They do slow down but
only by less than a billionth of a second a year! This precision
led to another milestone discovery back in 1992 when
researchers Alexander Wolszczan and Dale Frail noticed that the
times of arrival of pulses from one pulsar were fluctuating by
very teensy amounts. They postulated that the star had two
planets orbiting it that were disturbing the arrival times. Their
claims were greeted with skepticism, but time proved that there
were actually three planets, the first detected outside our solar
system. Over a hundred planets have been discovered since then.
What’s new in pulsars? This year, the team at the Parkes Radio
Telescope in Australia announced a long awaited discovery of a
pair of pulsars locked in orbit around each other. What happens
as these two pulsars battle for dominance should provide grist for
the astronomical community for years to come.
If you’re concerned about the left-out Jocelyn, Jocelyn Bell
Burnell left Cambridge in 1968 and has received many honors
and awards for her continuing work on the heavens. She’s now
the head of the physics department at Open University in
England. Just this morning I saw a segment on one of the
morning TV shows about some colleges dropping the
requirement that prospective students present their SAT scores.
Jocelyn Bell certainly proved that exams aren’t always indicative
of ultimate achievement.
Allen F. Bortrum
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