Taking the Pulse of the Universe
They keep time as well as the best clock ever made. One day they may act as beacons for starships speeding through the dark reaches of interstellar space. They are the strange cosmic objects known as pulsars — the collapsed remnants of massive stars which emit regular radio or X-ray pulses — and the particular interest of Dr. Vicky Kaspi, Professor of Astrophysics at McGill University.Kaspi and her research team recently discovered more than 20 pulsars in a single star cluster in the Milky Way. The discovery is impressive for several reasons. First, pulsars are notoriously hard to find. Their signals are faint and other stars produce so much "noise" that it's difficult to pick out individual signals. Powerful computers and huge amounts of data are needed.
Second, their discovery helps confirm a leading theory about the formation of star clusters. "One of our models of star formation predicts this particular cluster should have many, many pulsars in it," says Kaspi. "But for a long time, we couldn't find any. By solving the mystery of where all those pulsars are, we validated the model. That's a relief, because it means theorists don't have to go back to the drawing board and start all over again." To locate pulsars, scientists use radio and X-ray telescopes to search the sky for regular pulsations. They "download" vast quantities of data from the cosmos and then comb through it with supercomputers, looking for regular signals amid the background cosmic buzz.
Kaspi uses radio telescopes on Earth as well as orbiting X-ray telescopes to study pulsars and neutron stars. Data from these observatories are collected and analyzed by her group's Beowulf mini-supercomputer on the third floor of McGill's Rutherford Physics Building. Kaspi's overall specialty is "compact objects," a category that includes some of the odder items in the known universe: pulsars, black holes and neutron stars. These objects are formed when stars much more massive than our sun run out of fuel and collapse on themselves. When the most massive stars collapse, they form black holes, dense concentrations of mass with gravitational fields so powerful that not even light can escape. Because black holes don't emit light, they can be observed only by noting their effect on other cosmic objects.
When less massive stars collapse, they form neutron stars. "These are still visible and can be observed," Kaspi says. "By studying them we can learn about how matter and energy behave under extreme conditions - extreme density, extreme gravity and extreme magnetic fields. Understanding what happens under these extreme conditions lets us push the frontiers of physics in ways we just can't do in labs."
Some neutron stars are found in binary systems where two stars orbit each other at extreme speeds. In these circumstances, Newton's laws break down because the stars are moving so fast - sometimes at significant fractions of the speed of light. "At these speeds, Einstein's theory of relativity applies and we can actually test his theory by looking at how these stars move," Kaspi says. "We can also measure how much mass they have. Maybe one day we will be able to use these measurements to determine what mass a neutron star must reach to become a black hole. That's a question we would like to answer."
A teaspoon of star stuff would weigh billions of tons
PulsarNeutron stars are extremely dense. A teaspoon of neutron star matter, if it could be
weighed,
would tip the scales at billions of tons. Neutron stars spin, sometimes sending out a pulse
of radio, X-ray
or optical light energy with each rotation. Such stars are known as pulsars - the focus of Kaspi's recent
groundbreaking discovery.
Pulsars and other compact objects are by no means the only research focus in modern astrophysics. Another important area is cosmology, the origin of the universe in the Big Bang and its evolution from a primordial soup of matter to the galaxies and stars we see today. The origin and evolution of planets is also of great interest to astrophysicists. In the last few years, scientists have realized that many stars have planets. They're now trying to understand how these planets form and to learn if other Earth-like planets exist.
While the tools and methods of astrophysics are ultra-modern, the roots of the discipline can be traced at least as far back as ancient Egypt. "From time immemorial, people have studied the stars and planets, but the Egyptians were among the first to try to rationally understand cosmic objects," Kaspi says. "For instance, they realized the Earth was round and even calculated the relative size of the Earth, the moon and the sun.
The mystery of dark matter
"We've come a very long way since then, but major questions still remain. For instance, about 70 per cent
of the universe is made of what we call dark matter. We don't know what it is. It exerts gravity but
it doesn't shine. So in fact, we don't know what the bulk of the matter is in the universe.
That's a pretty big question."
At the moment, astrophysics is a booming field. New telescopes are being built and exciting discoveries are being made. It also remains one of the sciences that captures the imagination of non-scientists everywhere. "We've all looked up at the stars and wondered what they are," Kaspi says. "When we see some of the amazing images from the Hubble Space Telescope, we realize there's something bigger than us. We're inspired. "Our students feel that inspiration. They love astrophysics and astronomy because these sciences offer them a glimpse of something beyond the planet, something bigger than human life - and it's not just a movie, it's the real thing."