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July 15, 1999: Black holes are To get around this problem, astrophysicists study other high-energy
phenomena that are associated with black holes. A NATO-sponsored
Advanced Studies Institute entitled "The Neutron Star -
Black Hole Connection" was held on the Greek Island of Crete
from June 7-18. Among the Institute organizers were Marshall
Space Flight Center researchers Dr. Chryssa Kouveliotou of the
Universities Space Research Association and Prof. Jan van Paradijis
of the University of Alabama, Huntsville. Several other Marshall
scientists also attended the Institute, presenting topics on
everything from magnetars to gamma-ray bursts. The close examination
of such high-energy phenomena may someday lead to a better understanding
of black holes. |
deep
wells in the fabric of space and time. They have such immense
gravity that nothing, not even light, can escape them. This makes
studying black holes difficult - how can you see something when
it does not emit or reflect any form of energy? A black hole
seems to be the study of the invisible. |
How Black Holes are Formed A neutron star actually starts out life as a normal star, with protons, electrons and neutrons. Nuclear fusion in the core generates electromagnetic radiation. The star maintains a balance between outward radiation pressure and the inward pull of gravity, but when the star exhausts its store of nuclear fuel the star's gravitation takes over, causing the star to collapse. At this point, stars follow two paths of evolution. Smaller stars - up to eight times as large as our Sun - become "white dwarfs." A white dwarf is the cooled core of the star, gravitationally contracted to about the size of Earth. A white dwarf doesn't compress further because the pressure of its electrons resists the inward pull of gravity. Larger stars form larger cores, and when they use up their
nuclear fuel they collapse quickly, shedding material in a massive
explosion called a supernova. The remaining core forms into a
neutron star. The neutrons resist the inward pull of gravity
much as a white Right: Supernova remnant: the Crab Nebula. Chinese astronomers
witnessed the supernova that created the Crab Nebula in 1054
AD. |
the most massive stars
in the universe, but they are second only to black holes in gravitational
intensity. This gravity compresses a neutron star into a sphere
of extreme density - comparable to cramming the world's population
into a volume the size of a sugar cube. One teaspoon of neutron
star would weigh 100 million tons on Earth.
dwarf's electrons
do. If, however, the core exceeds 2 solar masses (twice as massive
as our Sun), the neutrons cannot resist the force of gravity.
The star becomes a "singularity" of infinite density
and collapses into a black hole.
Neutron stars emit their strongest energy from the poles, so a rotating neutron star appears to "pulse" energy. Like the searchlight of a lighthouse, as one of the poles swings toward the direction of Earth, we receive a short blast of energy that fades as the pole swings away again. Pulsar spin rates cover a wide range from 0.002 seconds to several minutes. Peter Woods, a University of Alabama/Huntsville graduate student working at Marshall, presented a poster about magnetars at the NATO conference. The strong magnetic fields of magnetars should cause them to spin slower and slower at a nearly constant rate. But last summer, Woods found that one magnetar spun down faster than expected around the same moment the magnetar released an intense burst of gamma energy. Because the two events occurred so closely in time, they may somehow be associated. "For 3 years, the spin-down of this magnetar was constant,
with only small deviations," said Woods. "Then we detected
this large flare of gamma energy, and around the same time the
magnetar spun down very quickly. Because magnetars are so close
to home, the Earth got hit with a huge flare of gamma energy
- greater than the gamma energy from any other cosmic source.
It actually caused a disturbance in our ionosphere, but no permanent
effects were felt. The flare only temporarily disrupted radio
communications in the South Pacific, which happened to be facing
the source when the gamma rays reached Earth." "The Chandra Observatory will be important in nailing down magnetar theory," said Woods. "We'll be able to look at these objects in much greater detail than ever before." |
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If an object is hot enough to emit X-rays, it usually also
emits light. Therefore we can see our Sun, other stars and supernovae
- they are all visible X-ray emitters. The emission of X-rays
from an unseen object could indicate the existence of a black
hole. "One of the goals is to unscramble the X-ray signatures from what's coming from the accretion disks and separate it from other sources of emission," said Weisskopf. "Someday we hope to improve X-ray telescope resolution a million times better than it is today. That sounds like science fiction, but it is theoretically possible." |
other phenomena like supernovae,
or even stars?
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The study of gamma rays is just as important as X-rays in improving our understanding of neutron stars and black holes. Magnetars, in fact, were originally called soft-gamma repeaters (SGRs) because they were first seen as sporadic bursts of gamma energy that repeated over time. Gamma-ray bursts (GRBs) were a topic of extreme interest at the NATO conference. These brief bursts of gamma ray energy are different from SGRs because they are extremely far away, have much more energy, and only flash once - never to be seen again.
The Burst and Transient Source Experiment (BATSE), which was designed to detect gamma energy throughout the Universe, has detected most of the known gamma-ray bursts. BATSE has been recording about one gamma-ray burst a day since its launch in 1991. Dr. Gerald Fishman, chief scientist at Marshall for gamma-ray astronomy and principal investigator on BATSE, gave an overview at the NATO conference of the history and observations of gamma-ray bursts observed by BATSE. Gamma ray bursts have recently been measured to be at the very limit of the universe - what scientists call "cosmological" distances. Their distance and brief life span - only a few seconds - make them difficult to detect. It is thought that the energy from a GRB is greater than any other energy source in the Universe. "I was surprised by how much gamma-ray burst discussion
there was at the conference," said Woods. "The two
main models for what cr "I've been working so hard on Chandra, I haven't been in touch with X-ray results as much as I would have liked," said Weisskopf. "I was surprised by how much and how little we've progressed! There's clearly been some progress - before, we used mathematical models to describe the data, and now, there are physical models. So there's been real progress in the physics. But some of the fundamental questions of 25 years ago are still questions today. For instance, we now know how far away gamma-ray bursts are - once a big question - but we still don't know what causes them!" Still, both Weisskopf and Woods say the NATO Institute was helpful in inspiring new thoughts and in suggesting new projects to pursue. Although many questions about high-energy objects like black holes and gamma-ray bursts remain, such conferences provide fuel to scientists in their quest to better understand the Universe. |
eates the bursts are the hypernova and
the compact merger model - where a neutron star colliding with
a black hole creates a gamma-ray burst. But many other people
had their own unique theories of what creates gamma-ray bursts.
Because it is still such a mystery, everyone wants to throw their
two cents in."|
This NATO Advanced Studies Institute
was the fifth of a series focused on neutron stars. The aim of
the Institute is to provide a systematic introduction and overview
of neutron stars and black hole systems. Topics covered |
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