Neutron Stars and Black Holes Astronomy: Horizons 10th edition Michael Seeds.

Neutron Stars and Black Holes

Astronomy: Horizons10th edition

Michael Seeds


• Gravity always wins.– However a star lives, it must eventually die in one of

three final states—a white dwarf, neutron star, or black hole.

• These objects—often called compact objects—are small monuments to the power of gravity. – Almost all the available energy has been squeezed

out of compact objects, and you find them in their final, high-density states.


• Modern astronomers have found objects that do seem to be neutron stars and black holes.


• A neutron star contains a little over 1 solar mass compressed to a radius of about 10 km.– Its density is so high that the matter is stable

only as a fluid of neutrons.

Neutron Stars


• Theory predicts that such an object would spin a number of times a second, be nearly as hot at its surface as the inside of the sun, and have a magnetic field a trillion times stronger than Earth’s.

Neutron Stars


• Over the following years, scientists applied the principles of quantum mechanics to understand how a neutron star could support itself.– Neutrons spin in much the way that electrons do,

which means that neutrons must obey the Pauli exclusion principle.

– If neutrons are packed together tightly enough, they can become degenerate just as electrons do.

Theoretical Prediction of Neutron Stars


• White dwarfs are supported by degenerate electrons.

• Quantum mechanics predicts that a dense enough mass of neutrons could support itself by the pressure of degenerate neutrons. – However, the inside of a neutron star would have to

be much denser than the inside of a white dwarf.



• Why would the core of a collapsing star produce a mass of neutrons?

• Atomic physics provides an explanation. – If the collapsing core is more massive than the

Chandrasekhar limit of 1.4 solar masses, then it cannot reach stability as a white dwarf.

– The weight is just too great to be supported by degenerate electrons.

– The collapse of the core continues—and the atomic nuclei are broken apart by gamma rays.



– Almost instantly, the increasing density forces the freed protons to combine with electrons and become neutrons.

– In a fraction of a second, the collapsing core becomes a contracting ball of neutrons.

– The envelope of the star is blasted away in a supernova explosion.

– Only the core of the star is left behind as a neutron star.



• A star of 8 solar masses or less could lose enough mass to die by forming a planetary nebula and leaving behind a white dwarf.

• More massive stars will lose mass rapidly.– However, they cannot shed mass fast enough to

reduce their mass below the Chandrasekhar limit.– So, it seems likely that they must die in supernova

explosions.



• Theoretical calculations suggest that stars that begin life on the main sequence with 8 to roughly 20 solar masses will leave behind neutron stars.

• Stars more massive are believed to form black holes.



• Mathematical models predict that neutron stars are only 10 or so kilometers in radius and have a density of about 1014 g/cm3.



• On Earth, a sugar-cube-sized lump of this material would weigh 100 million tons.– This is roughly the density of an atomic nucleus.

– You can think of a neutron star as matter with all the empty space squeezed out of it.



• The most widely accepted calculations suggest that a neutron star cannot be more than 2 to 3 solar masses. – If it were more massive than that, the degenerate

neutrons would not be able to support the weight, and the object would collapse—presumably into a black hole.



• Simple physics predicts that neutron stars should be hot, spin rapidly, and have strong magnetic fields.



• You have seen that contraction heats the gas in a star. – As the gas particles fall inward, they pick up speed.– When they collide, their high speeds become thermal

energy.– The sudden collapse of the core of a massive star to

a radius of 10 km should heat it to millions of degrees.



– Furthermore, neutron stars should cool slowly because the heat must be radiated away by the surface.

– They are so small that they have little surface from which to radiate.

– Thus, basic theory predicts that neutron stars should be very hot.



• The conservation of angular momentum predicts that neutron stars should spin rapidly. – All stars rotate to some extent because they form

from swirling clouds of interstellar matter. – As such a star collapses, it must rotate faster

because it conserves angular momentum.



• You have learned that this happens when ice skaters spin slowly with their arms extended and then speed up as they pull their arms closer to their bodies.



• In the same way, a collapsing star must spin faster as it pulls its matter closer to its axis of rotation.– If the sun collapsed to a radius of 10 km, its period of

rotation would increase from once every 25 days to about 1,000 times a second.

– You might expect the collapsed core of a massive star to rotate 10 or 100 times a second.



• It isn’t hard to understand why a neutron star should have a powerful magnetic field.

• Whatever magnetic field a star has is frozen into the star. – The gas of the star is ionized.– So, the field cannot move easily through the gas.

– When the star collapses, the magnetic field is squeezed into a smaller area—which could make the field a billion times stronger.



• As some stars have magnetic fields over 1,000 times stronger than the sun’s, a neutron star could have a magnetic field as much as a trillion times stronger than the sun’s. – That is about 10 million times stronger than any

magnetic field ever produced in the laboratory.



• Neutron stars are very hot.

• From your understanding of black body radiation, you can predict they will radiate most of their energy in the X-ray part of the spectrum.– This radiation could not be observed in the 1940s

and 1950s because astronomers could not put their telescopes above Earth’s atmosphere.

• Also, the small surface areas of neutron stars mean that they will be faint objects.



• In November 1967, Jocelyn Bell, a graduate student at Cambridge University in England, found a peculiar pattern in the data from a radio telescope. – Unlike other radio signals from celestial bodies, this

was a series of regular pulses.

The Discovery of Pulsars


• At first, she and the leader of the project, Anthony Hewish, thought the signal was interference.

• However, they found it day after day in the same place in the sky. – Clearly, it was celestial in origin.



• Another possibility, that it came from a distant civilization, led them to consider naming it LGM—for Little Green Men.

• Within a few weeks, however, the team found three more objects in other parts of the sky pulsing with different periods.– The objects were clearly natural.



• The team dropped the name LGM in favor of pulsar—a contraction of ‘pulsing star.’– The pulsing radio source Bell had observed

with her radio telescope was the first known pulsar.



• As more pulsars were found, astronomers argued over their nature.

• Periods ranged from 0.033 to 3.75 seconds and were nearly as exact as an atomic clock.– Months of observation showed that many periods

were slowly growing longer by a few billionths of a second per day.

– Whatever produced the regular pulses had to be highly precise—nearly as exact as an atomic clock—but it had to gradually slow down.



• It was easy to eliminate possibilities.• Pulsars could not be stars.

– A normal star, even a small white dwarf, is too big to pulse that fast.

• Nor could a star with a hot spot on its surface spin fast enough to produce the pulses. – Even a small white dwarf would fly apart if it spun 30

times a second.



• The pulses themselves gave the astronomers a clue.

• The pulses lasted only about 0.001 second. – This places an upper limit on the size of the object

producing the pulse. – If a white dwarf blinked on and then off in that interval,

an observer would not see a 0.001-second pulse.



– The near side of the white dwarf would be about 6,000 km closer to Earth.

– So, light from the near side would arrive 0.022 second before the light from the bulk of the white dwarf.

– Thus, its short blink would be smeared out into a longer pulse.



• This is an important principle in astronomy.

• An object cannot change its brightness appreciably in an interval shorter than the time light takes to cross its diameter. – If pulses from pulsars are no longer than 0.001

second, then the objects cannot be larger than 300 km (190 miles) in diameter.



• Only a neutron star is small enough to be a pulsar. – It is so small that it can’t vibrate slowly enough.

– It can, however, spin as fast as 1,000 times a second without flying apart.



• The missing link between pulsars and neutron stars was found in late 1968—when astronomers discovered a pulsar at the heart of the Crab Nebula. – The Crab Nebula is a

supernova remnant.– Theory predicts that some

supernovae leave behind neutron stars.



• The short pulses and the discovery of the pulsar in the Crab Nebula strongly suggest that pulsars are neutron stars.



• The modern model of a pulsar has been called the lighthouse model.

• There are three important points to note about the model.

A Model Pulsar


• First, a pulsar does not pulse but rather emits beams of radiation that sweep around the sky as the neutron star rotates.– If the beams do not sweep over Earth, the pulses will

not be detectable by Earth’s radio telescopes.

A Model Pulsar


• Second, the mechanism that produces the beams involves extremely high energies.– However, it is not fully understood.

A Model Pulsar


• Finally, modern space telescopes observing from above Earth’s atmosphere can image details around young neutron stars and even locate isolated neutron stars whose beams of electromagnetic radiation do not sweep over Earth.

A Model Pulsar


• When a pulsar first forms, it is spinning fast—perhaps nearly 100 times a second.

• The energy it radiates into space comes from its energy of rotation.– So, as it blasts beams of radiation outward, its

rotation slows.

The Evolution of Pulsars


• The average pulsar is, apparently, only a few million years old.– The oldest is about 10 million years old.

• Presumably, older neutron stars rotate too slowly and have too little energy to generate detectable radio beams.

• Thus, you can expect a young neutron star to emit powerful beams of radiation.



• The Crab Nebula provides an example of such a system.– Only about 950 years old, the Crab pulsar is so powerful

it emits photons of radio, infrared, visible, X-ray, and gamma-ray wavelengths.



• Careful measurements of its brightness with high-speed instruments show that it blinks twice for every rotation. – One beam sweeps almost directly over Earth—

and astronomers detect a strong pulse.– Half a rotation later, the edge of the other beam

sweeps past—and astronomers detect a weaker pulse.



• You would expect only the most energetic pulsars produce short-wavelength photons and pulse at visible wavelengths.

• The Crab Nebula pulsar is young and powerful.– It produces visible pulses.



• The energy in the beams is only a small part of the energy emitted by a pulsar.

• Roughly 99.9 percent of the energy flowing away from a pulsar is carried as a pulsar wind of high-speed atomic particles.



• This can produce small, high-energy nebulae near a young pulsar.



• Not every supernova remnant contains a pulsar.

• Not every pulsar is located inside a supernova remnant.



• Many supernova remnants probably contain pulsars whose beams never sweep over Earth.– It is difficult to detect such pulsars.



• Also, some pulsars move through space at high velocity. – This suggests that supernova explosions can occur

asymmetrically.– This could be because of violent turbulence in the

exploding core.– Such a violent,

offcenter explosion could give the neutron star a high velocity through space.



• Also, some supernovae probably occur in binary systems and fling the two stars apart at high velocity. – In any case, pulsars are known to have such high

velocities that many probably escape the disk of Milky Way.

– You should not be surprised that many neutron stars quickly leave their supernova remnant behind.



• Finally, a pulsar can remain detectable for 10 million years or so, but a supernova remnant cannot survive more than about 50,000 years before it is mixed into the interstellar medium.



• Stars with masses from about 8 up to some mass limit believed to be about 20 solar masses leave behind neutron stars.

• More massive stars, though, were thought to leave behind black holes.



• The most massive stars, however, may lose mass so rapidly that they can leave behind a neutron star and not a black hole. – If that is the case, then black holes must be produced

by stars with masses between roughly 20 and 40 solar masses.



• The explosion of Supernova 1987A in February 1987, apparently, formed a neutron star.– You can draw this conclusion because a burst of

neutrinos was detected passing through Earth.– Theory predicts that the collapse of a massive star’s

core into a neutron star would produce such a burst of neutrinos.



• At first, the neutron star would be hidden at the center of the expanding shells of gas ejected into space.– As the gas expands and thins, astronomers might be

able to see the neutron star.– Alternatively, if its beams don’t sweep over Earth,

astronomers might be able to detect it from its X-ray and gamma-ray emission.



• Years after the explosion, no neutron star has been detected.– Nevertheless, astronomers continue to watch

the site—hoping to see the youngest pulsar known.



• Over a thousand pulsars are now known, and some are located in binary systems.– These pulsars are of special interest because

astronomers can learn more about the neutron star by studying the orbital motions of the binary.

– Also, in some cases, mass can flow from the companion star onto the neutron star—and that produces high temperatures and X rays.

Binary Pulsars


• The first binary pulsar was discovered in 1974.– Astronomers Joseph Taylor and Russell Hulse noticed

that the pulse period of the pulsar PSR1913+16 was changing.

– The period first grew longer and then grew shorter in a cycle that took 7.75 hours.

– Thinking of the Doppler shifts seen in spectroscopic binaries, the radio astronomers realized that the pulsar had to be in a binary system with an orbital period of 7.75 hours.

Binary Pulsars


• When the orbital motion of the pulsar carries it away from Earth, observers see the pulse period slightly lengthened—just as the wavelength of light emitted by a receding source is lengthened.– That is, observers see a redshift.

Binary Pulsars


• Then, when the pulsar rounds its orbit and approaches Earth, they see the pulse period slightly shortened.– That is, they see a blueshift.

Binary Pulsars


• From these changing Doppler shifts, the astronomers could calculate the radial velocity of the pulsar around its orbit—just as if it were a spectroscopic binary star.

Binary Pulsars


• The resulting graph of radial velocity versus time could be analyzed to find the shape of the pulsar’s orbit.

Binary Pulsars


• When Taylor and Hulse analyzed PSR1913+16, they discovered that the binary system consisted of two neutron stars separated by a distance roughly equal to the sun’s radius.

Binary Pulsars


• Yet another surprise was hidden in the motion of PSR1913+16.– In 1916, Einstein’s general theory of relativity

described gravity as a curvature of space-time.

– Einstein realized that any rapid change in a gravitational field should spread outward at the speed of light as gravitational radiation.

Binary Pulsars


• Gravity waves have not been detected.

• However, Taylor and Hulse were able to show that the orbital period of the binary pulsar is slowly growing shorter because the stars are gradually spiraling toward each other. – They are radiating orbital energy away in the

form of gravitational radiation.

Binary Pulsars


• Taylor and Hulse won the Nobel Prize in 1993 for their work with binary pulsars.

Binary Pulsars


• Dozens of binary pulsars have been found orbiting stars of various kinds.

• By analyzing the Doppler shifts in their pulse periods, astronomers can estimate the mass of the neutron stars. – Typical masses are about 1.35 solar masses—in

good agreement with models of neutron stars.

Binary Pulsars


• In 2004, radio astronomers announced the discovery of a double pulsar. – The two pulsars orbit each other in only 2.4 hours.– Their spinning beams

sweep over Earth.– One spins 44 times

a second.– The other spins in

2.8 seconds.

Binary Pulsars


• The theory of general relativity predicts that they are emitting gravitational radiation and that their separation is decreasing by 7 mm per year. – The two neutron stars

will merge in 85 million years—presumably to trigger a supernova explosion.

Binary Pulsars


• In the meantime, the steady decrease in orbital period can be measured.– This gives astronomers a further test of general

relativity and gravitational radiation.

Binary Pulsars


• Binary pulsars can emit strong gravitational waves—because the neutron stars contain large amounts of mass in a small volume.– This also means that binary pulsars can be sites of

tremendous violence—due to the strength of gravity at the surface of a neutron star.

– An astronaut stepping onto the surface of a neutron star would be instantly smashed into a layer of matter only 1 atom thick.

Binary Pulsars


• Matter falling onto a neutron star can release titanic amounts of energy.– If you dropped a single marshmallow onto the

surface of a neutron star from a distance of 1 AU, it would hit with the impact of a 3-megaton nuclear bomb.

Binary Pulsars


• In general, a particle falling from a large distance to the surface of a neutron star will release energy equivalent to 0.2 mc2

—where m is the particle’s mass at rest. – Even a small amount of matter flowing from a

companion star to a neutron star can generate high temperatures and release X rays and gamma rays.

Binary Pulsars


• Matter flowing from the normal star into an accretion disk around the neutron star can reach temperatures of millions of degrees and emit a powerful X-ray glow.– Interactions with the

neutron star’s magnetic field can produce beams of X rays that sweep around with the rotating neutron star.

Binary Pulsars


• The X-ray source 4U 1820-30 illustrates another way neutron stars can interact with normal stars. – In this system, a neutron star orbits a white dwarf with

a period of only 11 minutes.– The separation between

the two objects is only about a third the distance between Earth and the moon.

Binary Pulsars


• To explain how such a very close pairing could originate, theorists suggest that a neutron star collided with a giant star and went into an orbit inside the star. – The neutron star would have gradually eaten

away the giant star’s envelope from the inside—leaving the white dwarf behind.

Binary Pulsars


• Matter still flows from the white dwarf into an accretion disk and then down to the surface of the neutron star.– Here, it accumulates until it ignites to produce periodic

bursts of X rays.

Binary Pulsars


• Objects called X-ray bursters are thought to be such binary systems involving mass transferred to a neutron star. – This mechanism is similar to that responsible for

novae.

Binary Pulsars


• Why are neutron stars detectable at X-ray wavelengths?

Building Scientific Arguments


• First, a neutron star is very hot because of the heat released when it contracts to a radius of 10 km.– It could easily have a surface temperature of

1,000,000 K.– Wien’s law informs you what such an object will

radiate most intensely at a very short wavelength—X rays.



• However, the total luminosity of a star depends on its surface temperature and its surface area.– A neutron star is so small it can’t radiate much energy. – X-ray telescopes have found such neutron stars—but

they are not easy to locate.



• There is, however, a second way a neutron star can radiate X rays.– If a normal star in a binary system loses mass to a

neutron star companion, the inflowing matter will form a very hot accretion disk that can radiate intense X rays easily detectable by X-ray telescopes orbiting above Earth’s atmosphere.

– Further, any matter that hits the surface of the neutron star will impact with so much energy that it will be heated to very high temperatures and will radiate X rays.



• Although the physics of black holes is difficult to discuss without sophisticated mathematics, simple logic is sufficient to predict that they should exist.

Black Holes


• What objects observed in the heavens could be real black holes? – The quest for black holes is more difficult than the

search for neutron stars.– Nevertheless, it has met with success.

Black Holes


• The escape velocity is the initial velocity an object needs to escape from a celestial body.

• Whether you are discussing a baseball leaving Earth or a photon leaving a collapsing star, escape velocity depends on two things:– Mass of the celestial body– Distance from the center of mass to the escaping

object

Escape Velocity


• If the celestial body has a large mass, its gravity is strong and you need a high velocity to escape from its surface.

• If you begin your journey farther from the center of mass, the velocity needed is less.

Escape Velocity


• For example, to escape from Earth, a spaceship would have to leave Earth’s surface at 11 km/s (25,000 mph).

• However, if you could launch spaceships from the top of a tower 1,000 miles high, the escape velocity would be only 8.8 km/s (20,000 mph).

Escape Velocity


• If you could make an object massive enough or small enough, its escape velocity could be greater than the speed of light.– Relativity says that nothing can travel faster than the

speed of light.– So, even photons, which have no mass, would be

unable to escape from such an object.– This small, massive object could never be seen—

because light could not leave it.

Escape Velocity


• If the core of a star collapses and contains more than about 3 solar masses, no force can stop it.– The object cannot stop collapsing when it reaches the

size of a white dwarf—because degenerate electrons cannot support its weight.

– It cannot stop when it reaches the size of a neutron star—because degenerate neutrons cannot support its weight either.

– No force remains to stop the object from collapsing all the way to zero radius.

Schwarzschild Black Holes


• As an object collapses, its density and the strength of its surface gravity increase.

• If it collapses to zero radius, its density becomes infinite. – Mathematicians call such a point a singularity.



• In physical terms, it is difficult to imagine an object of zero radius.– Some theorists believe that a singularity is impossible

and that the laws of quantum physics must somehow halt the collapse at some subatomic radius roughly 1020 times smaller than a proton.

• Astronomically, it seems to make little difference.



• If the contracting core of a star becomes small enough, the escape velocity in the region around it is so large that no light can escape.

• You can receive no information about the object or about the region of space near it.– Such a region is called a black hole.



• If the core of an exploding star collapsed into a black hole, the expanding outer layers of the star could produce a supernova remnant.

• The core, though, would vanish without a trace.



• In 1916, Einstein published a mathematical theory of space and time that became known as the general theory of relativity.– Einstein treated space and time as a single entity—

space-time. – His equations showed that gravity could be described

as a curvature of space-time.



• Almost immediately, the astronomer Karl Schwarzschild found a way to solve the equations to describe the gravitational field around a single, nonrotating, electrically neutral lump of matter.– That solution contained the first general relativistic

description of a black hole.– Nonrotating, electrically neutral black holes are now

known as Schwarzschild black holes.



• In recent decades, theorists such as Roy P. Kerr and Stephen W. Hawking have found ways to apply the sophisticated mathematical equations of the general theory of relativity and quantum mechanics to charged, rotating black holes.– For this discussion, the differences are minor.– You may proceed as if all black holes were

Schwarzschild black holes.



• Schwarzschild’s solution shows that, if matter is packed into a small enough volume, then space-time curves back on itself.– Objects can still follow paths that lead into the black

hole, but no path leads out.– So, nothing can escape—not even light.– Consequently, the inside of the black hole is totally

beyond the view of an outside observer.



• The event horizon is the boundary between the isolated volume of space-time and the rest of the universe.

• The radius of the event horizon is called the Schwarzschild radius, RS.– This is the radius within which

an object must shrink to become a black hole.



• A bit of arithmetic shows that a 1-solar-mass black hole will have a Schwarzschild radius of 3 km, a 10-solar-mass black hole will have a Schwarzschild radius of 30 km, and so on.

• Even a very massive black hole would not be very large.



• Every object has a Schwarzschild radius determined by its mass, but not every object is a black hole.– For example, Earth has a Schwarzschild radius of about

1 cm—meaning it could become a black hole if it were squeezed inside that radius.



• This chapter discusses black holes that might originate from the deaths of massive stars. – These would leave behind cores with masses larger

than 3 solar masses.

• In later chapters, you will encounter black holes whose masses might exceed a million solar masses.



• It is a common misconception to think of black holes as giant vacuum cleaners that will suck in everything in the universe. – A black hole is just a gravitational field.– At a reasonably large distance, its gravity is no

greater than that of a normal object of similar mass.– If the sun were replaced by a 1-solar-mass black

hole, the planets’ orbits would not change at all.



• The gravity of a black hole becomes extreme only when you approach very close to it.– The figure illustrates this by representing gravitational

fields as curvature of the fabric of space-time.



• Physicists like to graph the strength of gravity around a black hole as curvature in a flat sheet. – The graphs look like funnels in which the depth of

the funnel indicates the strength of the gravitational field.

– However, black holes themselves are not shaped like funnels.



• In the figure, you should note that the strength of the gravitational field around the black hole becomes extreme only if you venture too close.



• Do black holes really exist? – Beginning in the 1970s, astronomers searched for

observational evidence that their theories were correct.

– They tried to find one or more objects that were obviously black holes.

– That very difficult search is a good illustration of how the unwritten rules of science help scientists understand nature.

The Search for Black Holes


• A black hole alone is totally invisible—because nothing can escape from the event horizon.

• However, a black hole into which matter is flowing would be a source of X rays. – Of course, X rays can’t escape from inside the event

horizon.– However, X rays emitted by the heated matter flowing

into the black hole could escape if the X rays were emitted before the matter crossed the event horizon.



• An isolated black hole in space will not have much matter flowing into it.

• A black hole in a binary system, though, might receive a steady flow of matter transferred from the companion star.– Thus, you can search for black holes by searching

among X-ray binaries.



• If the compact object emits pulses, it is a neutron star.

• Otherwise, you must depend on the mass of the object. – If the mass of the compact object is greater than 3

solar masses, it cannot be a neutron star.– You can conclude that it must be a black hole.



• To confirm that black holes existed, astronomers needed a conclusive example—an object that couldn’t be anything else.



• Another way to confirm that black holes are real is to search for evidence of their distinguishing characteristic—event horizons.– That search too has been successful.



• In one study, astronomers selected twelve X-ray binary systems.

• Of these, six seemed to contain neutron stars and six were believed to contain black holes.– Using X-ray telescopes, they monitored the systems.– They could see flares of energy as blobs of matter fell

into the accretion disks and spiraled inward.



• In the six systems believed to contain neutron stars, the astronomers could also detect bursts of energy when the blobs of matter finally fell onto the surfaces of the neutron stars.



• In the six systems believed to contain black holes, however, the blobs of matter spiraled inward through the accretion disks and vanished without final bursts of energy.



• Evidently, those blobs of matter had vanished as they approached the event horizons.– This is dramatic evidence that

event horizons are real.


Neutron Stars and Black Holes Jets of Energy from Compact Objects

• As you can see, it is a common misconception to think that it is impossible to get any energy out of a black hole.

• You should pause here to learn how a compact object and an accretion disk can eject powerful jets.


• Whether a compact object is a black hole or a neutron star, it has a strong gravitational field. – Any matter flowing into that field is accelerated

inward.

Jets of Energy from Compact Objects


• As it must conserve angular momentum, it flows into an accretion disk made so hot by friction that the inner regions can emit X rays and gamma rays.– Somehow, the spinning

disk can emit powerful beams of gas and radiation along its axis of rotation.



• The process isn’t well understood.– It seems to involve magnetic fields that get caught in

the accretion disk, and are twisted into tightly wound tubes that squirt gas and radiation out of the disk and confine it in narrow beams.



• This process is similar to the bipolar outflows ejected by protostars.

• Only, it is much more powerful.



• Apparently, SS433 is a binary system in which a compact object—probably a black hole—pulls matter from its companion star and forms an extremely hot accretion disk.

• Jets of high-temperature gas blast away in beams aimed in opposite directions.



• As the disk precesses, it sweeps these beams around the sky once every 164 days.

• Telescopes on Earth detect light from gas carried outward in both beams. – One beam produces

a redshift and the other produces a blueshift.



• SS433 is a prototype that illustrates how the gravitational field around a compact object can produce powerful beams of radiation and matter.


Neutron Stars and Black Holes Gamma-Ray Bursts

• The Cold War had an odd connection to neutron stars and black holes. – In 1963, a nuclear test ban treaty was signed.– By 1968, the United States was able to put a series of

satellites in orbit to watch for nuclear tests that were violations of the treaty.

– A nuclear detonation emits gamma rays—so, the satellites were designed to watch for bursts of gamma rays coming from Earth.


• The experts were startled when the satellites detected about one gamma-ray burst a day coming from space. – When those data were finally declassified in 1973,

astronomers realized that the bursts might be coming from neutron stars or black holes.

– These bursts are now known as gamma-ray bursts.

Gamma-Ray Bursts


• The Compton Gamma Ray Observatory reached orbit in 1991 and immediately began reporting gamma-ray bursts at the rate of a few a day. – The intensity of the gamma rays rises to a maximum

in seconds and then fades away quickly.– A burst is usually over in seconds or minutes.

Gamma-Ray Bursts


• Furthermore, the Compton Observatory discovered that the gamma-ray bursts were coming from all over the sky and not from any particular region. – This helped astronomers sort out the different

theories.

Gamma-Ray Bursts


• Some theories proposed that the gamma-ray bursts were being produced among the stars in Milky Way.

• The Compton Observatory observation that the bursts were coming from all over the sky eliminated that possibility. – If the gamma-ray bursts were produced among stars in

Milky Way, you would expect to see them most often from along Milky Way where there are lots of stars.

– The observation of bursts occurring all over the sky meant that bursts were coming from distant galaxies.

Gamma-Ray Bursts


• Studies by later satellites show that there are two kinds of gamma-ray bursts. – Short bursts last less than 2 seconds.

– Longer bursts can go on for many seconds.

Gamma-Ray Bursts


• Gamma-ray bursts are hard to study. – They occur without warning and fade so

quickly.

• Starting in 1997, though, new satellites in orbit were able to detect gamma-ray bursts, determine their location in the sky, and immediately alert astronomers on the ground.

Gamma-Ray Bursts


• When telescopes swiveled to image the locations of the bursts, they detected fading glows that resembled supernovae.– That has led to the conclusion

that long gamma-ray bursts are produced by a certain kind of supernova explosion.

Gamma-Ray Bursts


• Theory proposes that a star more massive than some upper limit of about 20 solar masses can exhaust its nuclear fuel and collapse directly into a black hole.

Gamma-Ray Bursts


• Models show that the collapsing star would conserve angular momentum and spin very rapidly.– That would slow the collapse of the equatorial parts of

the star.

Gamma-Ray Bursts


• The poles of the star would fall in quickly.– That would focus beams of intense radiation and

ejected gas blasting out along the axis of rotation.

Gamma-Ray Bursts


• Such an eruption has been called a hypernova.– If one of those beams were

pointed at Earth, it could produce a powerful gamma ray burst.

Gamma-Ray Bursts


• The evidence seems clear that the long gamma ray bursts are produced by hypernovae.

Gamma-Ray Bursts


• Short gamma-ray bursts don’t seem to be associated with hypernovae.– Some repeat—and those are believed to be produced

by neutron stars with magnetic fields 100 times stronger than that in a normal neutron star.

Gamma-Ray Bursts


• Dubbed magnetars, these objects can produce bursts of gamma rays when shifts in the magnetic field break the crust of the neutron stars.– One of these objects produced a burst of gamma rays

that reached Earth on August 27, 1998, and temporarily ionized Earth’s upper atmosphere—disrupting radio communication worldwide.

Gamma-Ray Bursts


• Not all short gamma-ray bursts are produced by magnetars.– Some bursts have occurred in parts of distant

galaxies where you would not expect to find magnetars or young, massive stars about to explode as hypernovae.

– The afterglows don’t resemble fading supernovae.

Gamma-Ray Bursts


• These bursts are believed to be produced by the merger of two neutron stars that orbit around each other, radiate orbital energy as gravitational radiation, and spiral into each other. – The final merger would produce a violent explosion as

the two neutron stars merged to form a black hole.

Gamma-Ray Bursts


• Some short gamma-ray bursts are evidently produced by the merger of a neutron star and a black hole. – As these objects spiral into each other, the neutron

star would be ripped apart and swallowed by the black hole.

– That could produce a gamma-ray burst.

Gamma-Ray Bursts


• However, the burst itself and the fading afterglow should be different from that produced by the merger of two neutron stars. – Astronomers are now working to distinguish

between these two kinds of short gamma-ray bursts.

Gamma-Ray Bursts


• Incidentally, if a gamma-ray burst occurred only 1,600 ly from Earth, the distance to a nearby binary pulsar, the gamma rays would shower Earth with radiation equivalent to a 10,000-megaton nuclear blast.– The gamma rays could create enough nitric oxide in

the atmosphere to produce intense acid rain and would destroy the ozone layer—exposing life on Earth to deadly levels of solar ultraviolet radiation.

Gamma-Ray Bursts


• Gamma-ray bursts can occur relatively near the Earth as often as every few 100 million years and could be one of the causes of the mass extinctions that show up in the fossil record.

Gamma-Ray Bursts


• There may be 30,000 neutron star binaries in each galaxy.– So, mergers must occur now and then in any

galaxy.

Gamma-Ray Bursts


• Massive stars explode as hypernovae only rarely in any one galaxy.– However, the gamma-ray bursts they produce are so

powerful that astronomers can detect these explosions among a vast number of galaxies.

– The 1000 or so gamma-ray bursts detected each year appear to be coming from the final deaths of stars far across space.

Gamma-Ray Bursts


• How can a black hole emit X rays?– Building this argument requires careful

language.



• Once a bit of matter falling into a black hole reaches the event horizon, no light or other electromagnetic radiation the matter emits can escape from the black hole. – The matter becomes lost to view.

• However, if it emitted radiation before it reached the event horizon, that radiation could escape—and you could detect it.



• Furthermore, the powerful gravitational field near a black hole stretches and distorts infalling matter, and internal friction heats the matter to millions of degrees. – Wien’s law informs you that matter at such a high

temperature should emit X rays.



• Any X rays emitted before the matter crosses the event horizon will escape.– So, astronomers can look for black holes by

looking for X-ray sources.



• Of course, an isolated black hole will probably not have much matter falling in.

• Black holes in binary systems, however, may have large amounts of matter flowing in from the companion star. – That’s why astronomers search for black holes by

looking for X-ray binaries.


Neutron Stars and Black Holes Astronomy: Horizons 10th edition Michael Seeds.

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Transcript of Neutron Stars and Black Holes Astronomy: Horizons 10th edition Michael Seeds.