Neutron Star
Neutron Star
A neutronstar is the dead remnant of a massive star. A massive star ends its life as a supernova, a catastrophic explosion that flings the star’s outer layers into space, leaving only the core behind. If the mass of the core is between 1.4 and 2.5 times the mass of the sun, it will become a neutron star, a solid mass of neutrons a hundred trillion times more dense than water. Neutron stars are tiny, about 6.2 mi (10 km) across, and they rotate very rapidly and have tremendously strong magnetic fields. Because they are massive and small, they also have intense gravitational fields. They are most easily observed in the radio and x-ray portions of the spectrum, so they were not discovered until the late 1960s, when radio and x-ray telescopes capable of detecting them began to become available.
The history of neutron stars begins with English physicist Sir James Chadwick (1891–1974) who discovered the elementary particle—the neutron—in 1932. Based on this finding, German-American astronomer Wilhelm Heinrich Walter Baade (1893–1960) and Swiss astronomer Fritz Zwicky (1898–1974) suggested in 1933 that neutron stars might exist as a way to explain the origin of supernovas. British astronomer Antony Hewish (1924–) and British astrophysicist Jocelyn Bell (1943–) discovered the first reported neutron star in 1967 when they detected radio pulses coming from what they thought was a pulsar.
The guest star
In AD 1054, astronomers in China noted the appearance of a guest star in the region of the sky that astronomers now call Taurus (the Bull). The new star was bright enough to be visible during the day, but faded within a few months. When astronomers train their telescopes today on the place where this celestial interloper appeared, they see a spectacular cloud of gas, its filaments twisting and ragged. Its shape has led it to be called the Crab Nebula. The Crab is expanding, as if it had been flung violently outward from a central point.
When scientists observe the Crab with a radio telescope, something even stranger appears: there is an object at its center that flashes on and off like a strobe light, about 30 times per second. So regular are the flashes that they hardly seem to be a natural phenomenon. Indeed, Jocelyn Bell and Antony Hewish, who discovered the first of these flashing objects, toyed with the idea of calling them LGM—meaning Little Green Men. They eventually settled on the term pulsar, and today about 500 pulsars are known.
Almost a millennium ago, humans stared in wonder at a new star. Today, there is a pulsar, seemingly beaming pulses of radio radiation to Earth at precise intervals, and a cloud of gas surrounding it. What has happened?
The origin of neutron stars
When a star that is between about three to eight times as massive as the sun dies, it goes in spectacular fashion. The star’s core temperature is in excess of half a billion degrees Kelvin, and must remain this hot for thermonuclear fusion reactions involving its last reserves of fuel to take place.
Then, the fuel runs out. No longer able to produce fusion reactions to sustain it, the star collapses. The core—a few million trillion trillion tons of it—falls in on itself, and in the ensuing cataclysm the star’s outer layers are flung outward in a supernova explosion.
Intuition might tell a person that the collapsing core will keep falling, squeezing itself together until it becomes so dense that it can fall no farther. This is like when one crumples a sheet of paper into a ball. A person can only squeeze it to a certain point, after which it is too tightly packed to reduce its size any further.
In the case of a collapsing star, a law of physics known as the Pauli exclusion principle describes this phenomenon. Atoms are composed of a nucleus surrounded by electrons. Electrons do not orbit the nuclei in the sense that planets orbit the sun; rather, electrons exist in what are called energy states, meaning that they have only certain amounts of energy. The Pauli Exclusion Principle states that two identical electrons may not share the same energy state. It is therefore possible for the energy levels of an atom to become completely filled with electrons, in the same way that an auditorium can only hold as many people as it has seats. Matter with its energy levels filled like this is called degenerate.
The Pauli exclusion principle, named after German physicist Wolfgang Pauli (1900–1958), will come into play when the sun dies and its core collapses. The carbon and oxygen atoms will become squeezed together until the atomic levels are filled and the whole core becomes a ball of degenerate matter. At this point, the resistance of the electrons to gravity, or electron degeneracy pressure, will halt the contraction. This ball of hot, degenerate, carbon and oxygen atoms is called a white dwarf, and it is the fate of the sun.
If the collapsing core is between 1.4 and about 2.5 times the mass of the sun, however, the gravity will be so strong that the electron degeneracy pressure will fail. Unable to resist the weight of their own gravity, the atoms will be crushed into a ball made mainly of neutrons about 32,810 ft (10 km) across. This object is called a neutron star.
Properties of neutron stars
With twice the mass of the sun crammed into a space no larger than a small city, a neutron star is fantastically dense. A sugar-cube-sized piece of neutron star would weigh billions of tons.
Neutron stars rotate rapidly. This is because the original stellar core was rotating, and as it collapsed its rotation rate increased, in the same way figure skaters spin increasingly rapidly by drawing their extended arms in to their sides.
Neutron stars also have intense gravitational and magnetic fields. The gravity is strong because there is so much matter packed into so small a radius. If one dropped a probe onto a neutron star from, say, 50 million miles out, the probe would impact and explode with a blast larger than that of any current nuclear warhead. The magnetic field is strong because it is an intrinsic property of the neutron star’s matter. When the original stellar core collapses, its magnetic field collapses with it and intensifies until it is a trillion times as strong as Earth’s.
Observing neutron stars
Many neutron stars look like the drawing in Figure 1. The neutron star’s rotation axis is inclined with respect to its magnetic axis. (The same situation prevails on Earth.) The neutron star’s rapid rotation and intense magnetic field cause radiation to be emitted in narrow beams from the magnetic poles. If Earth happens to be in line with one of these beams, astronomers will see a flash every time the neutron star rotates and carries the beam past Earth.
This is precisely what scientists see in the Crab Nebula. The rate of the flashes indicates the neutron star is spinning 30 times per second. (Compare that with the sun’s rotation period of about one month.) If a pulsar’s beam never sweeps past Earth’s line of sight, however, astronomers will never see it.
KEY TERMS
Binary pulsar— A pulsar in orbit around another star. The pulsar drags matter off its companion, heating it to the point that it emits x rays. These powerful beams of radiation sweep past Earth’s line of sight, producing flashes in the same way that single pulsars flash in the visible and radio.
Degeneracy pressure— The tendency of degenerate matter to resist further contraction under its own gravity. Neutron stars are prevented from being crushed into black holes by the degeneracy pressure of their tightly packed neutrons, which is sufficient to resist the neutron star’s strong gravity.
Pulsar— A rapidly spinning neutron star with its magnetic axis inclined relative to its rotation axis. Radiation streams continuously from the pulsar along its magnetic axis, so if the magnetic axis passes through Earth’s line of sight as the pulsar rotates, it is seen as a flash. The rate of the flashes reveals the rotation rate of the neutron star.
Some neutron stars are members of binary systems, in which they orbit another star. The intense gravity of the neutron star drags material off its companion. The material forms an accretion disk as it approaches the neutron star. In the process it becomes so hot that it begins to emit large amounts of x rays. Only a gravitational field as strong as one predicted to be associated with a neutron star could heat the infalling gas this much.
Even more spectacular are neutron stars that combine the above phenomena: the binary pulsars. In these systems astronomers see pulses of x rays, as the powerful beams of high-energy radiation sweep past the Earth.
Neutron stars have also provided the first direct evidence of planets outside the solar system. The pulsar PSR 1257+12 rotates 161 times per second-but its pulses are not evenly spaced, as are the ones from the Crab. Observations made with the giant Arecibo radio telescope showed that some pulses arrive slightly too soon, others just a bit too late. This means something is tugging the pulsar back and forth slightly. Careful measurements showed that a pair of planets is responsible.
See also Pulsar; Star; Stellar evolution.
Resources
Baykal, Altan, et al., eds. The Electromagnetic Spectrum of Neutron Stars. Dordrecht, Netherlands: Springer, 2005.
Blaschke, D., et al., eds. Physics of Neutron Star Interiors. Berlin, Germany, and New York: Springer, 2001.
Hansen, Carl J. Steller Interiors: Physical Principles, Structure, and Evolution. Berlin, Germany, and New York: Springer, 2004.
OTHER
Nemiroff, Robert, Michigan Technological University. “Virtual Trips to Black Holes and Neutron Stars.” <http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html> (accessed October 20, 2006).
Jeffrey C. Hall
Neutron Star
Neutron star
A neutron star is the dead remnant of a massive star. A massive star ends its life as a supernova , a catastrophic explosion that flings the star's outer layers into space, leaving only the core behind. If the mass of the core is between 1.4 and 2.5 times the mass of the Sun , it will become a neutron star, a solid mass of neutrons a hundred trillion times more dense than water . Neutron stars are tiny, about 6.2 mi (10 km) across, and they rotate very rapidly and have tremendously strong magnetic fields. Because they are massive and small, they also have intense gravitational fields. They are most easily observed in the radio and x ray portions of the spectrum , so they were not discovered until the late 1960s, when radio and x ray telescopes capable of detecting them began to become available.
The guest star
In a.d. 1054, astronomers in China noted the appearance of a "guest star" in the region of the sky we now call Taurus (the Bull). The new star was bright enough to be visible during the day, but faded within a few months. When we train our telescopes today on the place where this celestial interloper appeared, we see a spectacular cloud of gas, its filaments twisting and ragged. Its shape has led it to be called the Crab Nebula. The Crab is expanding, as if it had been flung violently outward from a central point.
When we observe the Crab with a radio telescope , something even stranger appears: there is an object at its center that flashes on and off like a strobe light , about 30 times per second. So regular are the flashes that they hardly seem to be a natural phenomenon. Indeed, Jocelyn Bell and Antony Hewish, who discovered the first of these flashing objects, toyed with the idea of calling them LGM's—meaning Little Green Men. They eventually settled on the term pulsar, and today about 500 pulsars are known.
Almost a millennium ago, humans stared in wonder at a new star. Today, there is a pulsar , seemingly beaming pulses of radio radiation to us at precise intervals, and a of gas surrounding it. What has happened?
The origin of neutron stars
When a star that is between about three to eight times as massive as the Sun dies, it goes in spectacular fashion. The star's core temperature is in excess of half a billion degrees kelvin, and must remain this hot for thermonuclear fusion reactions involving its last reserves of fuel to take place.
Then the fuel runs out. No longer able to produce fusion reactions to sustain it, the star collapses. The core—a few million trillion trillion tons of it—falls in on itself, and in the ensuing cataclysm the star's outer layers are flung outward in a supernova explosion.
Intuition might tell you that the collapsing core will keep falling, squeezing itself together until it becomes so dense that it can fall no farther. This is like when you crumple a sheet of paper into a ball. You can only squeeze it to a certain point, after which it is too tightly packed to reduce its size any further.
In the case of a collapsing star, a law of physics known as the Pauli exclusion principle describes this phenomenon. Atoms are composed of a nucleus surrounded by electrons. Electrons do not "orbit" the nuclei in the sense that planets orbit the Sun; rather, electrons exist in what are called "energy states," meaning that they have only certain amounts of energy . The Pauli Exclusion Principle states that two identical electrons may not share the same energy state. It is therefore possible for the energy levels of an atom to become completely filled with electrons, in the same way that an auditorium can only hold as many people as it has seats. Matter with its energy levels filled like this is called degenerate.
The Pauli exclusion principle will come into play when the Sun dies and its core collapses. The carbon and oxygen atoms will become squeezed together until the atomic levels are filled and the whole core becomes a ball of degenerate matter. At this point, the resistance of the electrons to gravity, or electron degeneracy pressure, will halt the contraction. This ball of hot, degenerate, carbon and oxygen atoms is called a white dwarf , and it is the fate of the Sun.
If the collapsing core is between 1.4 and about 2.5 times the mass of the Sun, however, the gravity will be so strong that the electron degeneracy pressure will fail. Unable to resist the weight of their own gravity, the atoms will be crushed into a ball made mainly of neutrons about 32,810 ft (10 km) across. This object is called a neutron star.
Properties of neutron stars
With twice the mass of the Sun crammed into a space no larger than a small city, a neutron star is fantastically dense. A sugar-cube-sized piece of neutron star would weigh billions of tons.
Neutron stars rotate rapidly. This is because the original stellar core was rotating, and as it collapsed its rotation rate increased, in the same way figure skaters spin increasingly rapidly by drawing their extended arms in to their sides.
Neutron stars also have intense gravitational and magnetic fields. The gravity is strong because there is so much matter packed into so small a radius. If a friend dropped you onto a neutron star from, say, 50 million miles out, you would impact and explode with a blast larger than that of any current nuclear warhead. The magnetic field is strong because it is an intrinsic property of the neutron star's matter. When the original stellar core collapses, its magnetic field collapses with it and intensifies until it is a trillion times as strong as Earth's.
Observing neutron stars
Many neutron stars look like the drawing in Figure 1. The neutron star's rotation axis is inclined with respect to its magnetic axis. (The same situation prevails on Earth.) The neutron star's rapid rotation and intense magnetic field cause radiation to be emitted in narrow beams from the magnetic poles. If the Earth happens to be in line with one of these beams, we will see a flash every time the neutron star rotates and carries the beam past us.
This is precisely what we see in the Crab Nebula. The rate of the flashes indicates the neutron star is spinning 30 times per second. (Compare that with the Sun's rotation period of about one month.) If a pulsar's beam never sweeps past our line of sight, however, we will never see it.
Some neutron stars are members of binary systems, in which they orbit another star. The intense gravity of the neutron star drags material off its companion. The material forms an accretion disk as it approaches the neutron star. In the process it becomes so hot that it begins to emit large amounts of x rays . Only a gravitational field as strong as one predicted to be associated with a neutron star could heat the infalling gas this much.
Even more spectacular are neutron stars that combine the above phenomena: the binary pulsars. In these systems we see pulses of x rays, as the powerful beams of high-energy radiation sweep past us.
Neutron stars have also provided the first direct evidence of planets outside the solar system . The pulsar PSR 1257+12 rotates 161 times per second-but its pulses are not evenly spaced, as are the ones from the Crab. Observations made with the giant Arecibo radio telescope showed that some pulses arrive slightly too soon, others just a bit too late. This means something is tugging the pulsar back and forth slightly. Careful measurements showed that a pair of planets is responsible.
See also Pulsar; Star; Stellar evolution.
Resources
periodicals
Bailyn, C. "Problems with Pulsars." Mercury (March-April 1991): 55.
Fienberg, R.T. "Pulsars, Planets, and Pathos." Sky & Telescope (May 1992): 493.
Hewish, A. "Pulsars After 20 Years." Mercury (January-February 1989): 12.
Jeffrey C. Hall
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Binary pulsar
—A pulsar in orbit around another star. The pulsar drags matter off its companion, heating it to the point that it emits x rays. These powerful beams of radiation sweep past our line of sight, producing flashes in the same way that single pulsars flash in the visible and radio.
- Degeneracy pressure
—The tendency of degenerate matter to resist further contraction under its own gravity. Neutron stars are prevented from being crushed into black holes by the degeneracy pressure of their tightly packed neutrons, which is sufficient to resist the neutron star's strong gravity.
- Pulsar
—A rapidly spinning neutron star with its magnetic axis inclined relative to its rotation axis. Radiation streams continuously from the pulsar along its magnetic axis, so if the magnetic axis passes through our line of sight as the pulsar rotates, we see a flash. The rate of the flashes reveals the rotation rate of the neutron star.
Neutron Star
Neutron star
A neutron star is the dead remnant of a massive star. A star reaches the end of its life when it uses up all of its nuclear fuel. Without fuel, it cannot undergo nuclear fusion, the process that pushes matter outward from the star's core and provides a balance to its immense gravitational field. The fate of a dying star, however, depends on that star's mass.
A medium-sized star, like the Sun, will shrink and end up as a white dwarf (small, extremely dense star having low brightness). The largest stars—those more than three times the mass of the Sun—explode in a supernova and then, in theory, undergo a gravitational collapse so complete they form black holes (single points of infinite mass and gravity). Those stars larger than the Sun yet not more than three times its mass will also explode in a supernova, but will then cave in on themselves to form a densely packed neutron star.
Origin of a neutron star
A neutron star is formed in two stages. First, within a second after nuclear fusion on the star's surface ceases, gravity crushes the star's atoms. This forces protons (positively charged particles) and electrons (negatively charged particles) together to form neutrons (uncharged particles) and expels high-energy subatomic particles called neutrinos. The star's core, which started out about the size of Earth, is compacted into a sphere less than 60 miles (97 kilometers) across.
In the second stage, the star undergoes a gravitational collapse and then, becoming energized by the neutrino burst, explodes in a brilliant supernova. All that remains is an extremely dense neutron core, about 12 miles (19 kilometers) in diameter with a mass nearly equal to that of the Sun. A sugar-cube-sized piece of neutron star would weigh billions of tons.
Neutron stars spin rapidly. This is because the original stellar core was spinning as it collapsed, naturally increasing its rate of spin. Neutron stars also have intense gravitational and magnetic fields. The gravity is strong because there is so much matter packed into so small an area. The spinning generates a magnetic field, and the star spews radiation out of its poles like a lighthouse beacon. Neutron stars give off radiation in a variety of wavelengths: radio waves, visible light, X rays, and gamma rays.
Pulsars
If the magnetic axis of the neutron star is tilted a certain way, the spinning star's on-and-off signal can be detected from Earth. This fact led to the discovery of the first neutron star in 1967 by English astronomer Antony Hewish and his student Jocelyn Bell Burnell.
Words to Know
Black hole: Remains of a massive star after it has exploded in a supernova and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
Neutrino: A subatomic particle resulting from certain nuclear reactions that has no charge and possibly no mass.
Nuclear fusion: Process in which the nuclei of two hydrogen atoms are fused together at extremely high temperatures to form a single helium nucleus, releasing large amounts of energy as a by-product.
Pulsar: Rapidly rotating neutron star that emits varying radio waves at precise intervals.
Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.
Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.
Supernova: Explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.
White dwarf: Small, extremely dense star having low brightness.
Hewish and Bell Burnell were conducting an experiment to track quasars (extremely bright, distant objects) when they discovered a mysterious, extremely regular, pulsing signal. They found similar signals coming from other parts of the sky, including one where a supernova was known to have occurred. With the help of astronomer Thomas Gold, they learned that the signals matched the predicted pattern of neutron stars. They named these blinking neutron stars pulsars (from pulsating stars).
Since then, more than 500 pulsars have been catalogued, including many in spots where a supernova is known to have occurred. Pulse rates of observed neutron stars range from 4 seconds to 1.5 milliseconds. Scientists believe that more than 100,000 active pulsars may exist in our galaxy.
[See also Star; Subatomic particles; Supernova ]