ASTR 209- Ch. 22: Neutron stars and Black holes
What makes Cungus X-1 a good black hole candidate?
- visible companion is 25x the mass of the sun - Cynus x-1 is thought to have a mass around 10x that of the sun - detailed studies suggest hot gas is flowing out of he bright star towards an unseen companion - X-ray radiation emitted from immediate neighbourhood implies presence of very high temp gas - size of X-ray emitting region of Cygnus x-1 must be very small These properties suggest that the x- ray emitting companion could be a black hole.
Do you think that planet-sized objects discovered orbiting a pulsar should be called planets? Why or why not?
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Imagine that you had the ability to travel at will through the galaxy. Explain why you would discover many more neutron starts than those known to observers on Earth. Where would you be most likely to find these objects?
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A meter stick in a space ship travelling half the speed of light would appear to have a length of
0.87 meter
Gamma-ray burst models (2). Two models have been proposed to explain gamma-ray bursts:
1) Merger of two neutron stars 2) Core collapse, implosion, and stalled supernova or hypernova, of a single massive star. Both models predict a relativistic fireball, perhaps releasing energy in the form of jets.
All pulsars are neutron stars but not all neutron stars are pulsars for two reasons:
1) The 2 ingredients that make the neutron star pulse - rapid rotation and a strong magnetic field - both diminish with time, so the pulses gradually weaken and become less frequent. 2) Even a young, bright neutron star is not necessarily detectable as a pulsar from our vantage point on Earth. Only if a pulsar star happens to be oriented in just the right way do we actually see pulses.
Two important properties of a newly formed neutron star:
1) They rotate extremely rapidly with periods in fractions of a second. (Due to law of conservation of angular momentum which tells us that any rotating body must spin faster as it shrinks) 2) Newborn neutron stars have very strong magnetic fields. The original field of the progenitor star is amplified by the collapse of the core because the contracting material squeezes the magnetic field lines closer together.
Millisecond pulsars are the product of a two stage process:
1) the neutron star was formed in an ancient supernova, billions of years ago 2) there was a relatively recent interaction with a binary companion, and the star achieved the rapid spin that we observe today.
The special theory of relativity
1) the speed of light is the maximum speed in the universe and all observers measure the same value for the speed of light regardless of their motion. 2) There is no absolute frame of reference in the universe; there is no way to tell who is moving and who is not. Only relative velocities between observers matter. 3) Neither space nor time can be considered independently of one another. Rather, they are components of a single entity: spacetime.
Describe two leading models for gamma-ray bursts.
1) the true end point of a binary star system or the merger of the component stars. Such a merger would likely create a violent explosion comparable in energy to a supernova and perhaps energetic enough to explain the flashes in gamma rays that we observe. 2) Hypernova or failed supernova. A massive star undergoes core collapse much as described earlier for a type 2 supernova but instead of forming a neutron star it forms a black hole. At the same time, a blast wave racing outward through the star stalls. Instead of being blown to pieces the inner part of the star begins to implode, forming an accretion disk around the black hole crating a relativistic jet. The jet punches it's way out of the star producing a gamma ray burst as it slams surrounding shells of gas expelled from the star during the final stages of it's nuclear burning life time.
A neutron star is about the same size as
A US city
How does the way in which a neutron star forms determine some of it's basic properties?
A neutron star forms when a type 2 supernova causes a massive star to collapse. When neutrinos have left the star, the neutron core continues to contract until all particles come into contact and the central portion of the core rebounds and creates a powerful shock wave that races outward through the star, expelling matter violently into space. This explains the high mass and small size of the neutron star.
Singularity
A point in the universe where the density of matter and the gravitational field are infinite, such as at the centre of a black hole.
Gravitational red shift
A prediction of Einstein's general theory of relativity. Photons lose energy as they escape the gravitational field of a massive object. Because a photons energy is proportional to it's frequency, a photon that loses energy suffers a decrease in frequency, which corresponds to an increase, or redshift, in wavelength.
Time dilation
A prediction of the theory of relativity, closely related to the gravitational redshift. To an outside observer, a clock lowered into a strong gravitational field would appear to run slow.
Millisecond pulsars
A pulsar whose period indicates that the neutron star is rotating nearly 1000 times each second. The most likely explanation for these rapid rotators is that the neutron star has been spun up by drawing in matter from a companion star.
Black hole
A region of space where the pull of gravity is so great that nothing, not even light, can escape. A possible outcome of the evolution of a very massive star.
What are x-ray bursters?
A sudden, intense flash of x rays followed by a period of relative inactivity lasting several hours. Then another burst occurs. The bursts are thought to be caused by explosive nuclear burning on the surface of an accreting neutron star, similar to the explosions on a white dwarf that give rise to novae.
General relativity
All matter tends to warp or curve space in it's vicinity.
Gamma ray bursts are observed to occur
Approximately uniformly across the entire sky
What evidence is there for black holes much more massive than the sun?
Astronomers have found that stars and gas near the centres of many galaxies are moving extremely rapidly, orbiting some very massive unseen object. Masses inferred from Newton's laws range from millions to billions of times the mass of the sun. The intense energy emission from the centres of these galaxies and the short timescale fluctuations in that emission suggest the presence of massive, compact objects.
Why do you think astronomers were surprised to find a pulsar with a planetary system?
Because any planetary system orbiting the pulsars progenitor star would have almost certainly been destroyed in the supernova explosion that created the pulsar.
Why would you never actually witness an infalling object crossing the event horizon of a black hole?
Because the object would appear to take infinitely long to reach the event horizon, and it's light would be infinitely redshifted by the time it got there.
Why don't we see pulsars at the centres of all supernova remnants?
Because: 1) not all supernovae form neutron stars, 2) the pulses are beamed, so not all pulsing neutron stars are visible from earth and 3) pulsars spin down and become too faint to observe after a few tens of millions of years.
Super massive black hole
Black hole having a mass a million to a billion times greater than the mass of the sun; usually found in the central nucleus of a galaxy.
How do astronomers "see" black holes?
By observing their gravitational effects on other objects, and from the X rays emitted as matter plunges toward the event horizon.
If the sun were to magically turn into a black hole of the same mass
Earths orbit would remain unchanged
Why aren't all neutron stars seen as pulsars?
First, the 2 ingredients that make the neutron star pulse (rapid rotation and a strong magnetic field) both diminish with time, so the pulses gradually weaken and become less frequent. Second, even a young, bright neutron star is not necessarily detectable as a pulsar from our vantage point on Earth. Only if the neutron star happens to be oriented in just the right way do we actually see pulses.
The best place to search for black holes is in a region of space that
Has strong X-ray emission
The X-ray emission from a neutron star in a binary system comes mainly from
Heated material in an accretion disk around the neutron star
What is an event horizon?
Imaginary spherical surface surrounding a collapsing star, with a radius equal to the Schwarzschild radius, within which no event can be seen, heard, or known about by an outside observer.
Why is it so difficult to test the predictions of general relativity? Describe two tests of the theory.
It is difficult to test because it's effects on earth and in the solar system - the places where we can most easily perform tests- are very small. 1- the deflection of light by the sun 2- the effect of relativity on the orbit of mercury
According to special relativity, what is special about the speed of light?
It is the maximum possible speed in the universe and all observers measure the same value for the speed of light, regardless of their motion.
A neutron stars immense gravitational attraction is due primarily to it's small radius and
Large mass
The only three physical properties of a black hole that can be measured from the outside:
Mass, charge, and angular momentum. Thus, just three numbers are required to completely describe a black holes outward appearance and interaction with the rest of the universe.
Black holes result from stars having initial masses
More than 25x that of the shn
What is the principle of cosmic censorship? Do you think it is a sound scientific principle?
Nature always hides any singularity (place where the rules break down) such as that found at the centre of a black hole, inside an event horizon. Even though physics fails, it's breakdown cannot affect us outside so we are safely insulated from any effects the singularity might have.
How do Newton's and Einstein's theories differ in their description of gravity?
Newtons theory describes gravity as a force produced by a massive object that influences all other massive objects. Einstein's relativity describes gravity as a curvature of space-time produced by a massive object; that curvature then determines the trajectories of all other particles in the universe.
Use your knowledge of escape speed to explain why black holes are said to be "black".
No form of radiation- radio waves, visible light, x rays, photons of any wavelength, are able to escape the gravity of a black hole.
What does it mean to say that the measured speed of a light beam is independent of the motion of the observer?
No matter what our motion may be relative to the source of the radiation, we always measure precisely the same value for the speed of light (300,000km/sec).
Are all supernovae expected to lead to neutron stars?
No- only Type II Supernovae. According to theory, the rebounding central core of the original star in a type II supernova becomes a neutron star.
Pulsar
Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Charged particles, accelerate by the magnetic field of a rapidly rotating neutron star, flow along the magnetic field lines, producing radiation that beams outward as the star spins on it's axis.
Gamma-ray burst
Object that radiates tremendous amounts of energy in the form of gamma rays, possibly due to the collision and merger of two neutron stars initially in orbit spurns one another.
The best evidence for super massive black holes in the centres of galaxies is
Rapid gas motion and intense energy emission.
What is he connection between X-Ray sources and millisecond pulsars?
Some X-ray sources are binaries containing accreting neutron stars, which may be in the process of being spun up to form millisecond pulsars.
The most rapidly blinking pulsars are those that
Spin fastest
Microquasar
Stellar sized source of energetic X and gamma radiation, powered by accretion onto a neutron star or a black hole.
Neutron star
The ball of neutrons left after the violence of a supernova has subsided. This is not a star in any true sense of the word because all of it's nuclear reactions have ceased forever. Average density is 10 to the power of 17.
Schwarzchild radius
The distance from the centre of an object such that, if all mass were compressed within that region, the escape speed would equal the speed of light. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible. For earth, this radius is 1 cm. Rule of thumb: the Schwarzschild radius is 3km x that objects mass in solar masses.
Gravitational light deflection
The gravitational bending of light around the edges of a small, massive black hole makes it impossible to observe the hole as a black dot superimposed against the bright background of it's stellar companion.
Lighthouse model
The leading explanation for pulsars. A small region of the neutron star, near one of the magnetic poles, emits a steady stream of radiation that sweeps past Earth each time the star rotates. The period of the pulses is the star's rotation period.
What is a favoured explanation for the rapid spin rates of millisecond pulsars?
The neutron star has been spun up (increased it's rotation rates) by drawing in matter from a companion star in a binary system. As matter spirals down into the stars surface in an accretion disk, it provides a push that makes the neutron star spin faster.
Why do many neutron stars move at high speeds relative to their neighbours?
The neutron star may receive substantial "kicks" due to asymmetries in the supernovae in which they formed. Such asymmetries are generally not very pronounced but if the supernovas enormous energy is channelled even slightly in one direction, the newborn neutron star can recoil in the opposite speed of many tens or hundreds of kms per second.
What would happen to a person standing on the surface of a neutron star?
The severe pull of a neutron stars gravity would flatten them to a size much thinner than a piece of paper.
Event horizon
The surface of an imaginary sphere with radius equal to the Schwarzschild radius and centered on a collapsing star. Defines the region within which no event can ever be seen, heard, or known by anyone outside. The "surface" of a black hole.
Equivalence principle
There is no experimental way to distinguish between a gravitational field and an accelerated frame of reference.
What are gamma-ray bursts and why have they posed such a challenge to current theory?
They are energetic bursts of gamma rays, roughly isotopic ally distributed on the sky, occurring about once per day. They pose a challenge because they are very distant, and hence extremely luminous, but their energy originates in a region less than a few 100 kms across. There also appear to be two distinct types, with different entry generation mechanisms.
What would happen to someone falling into a black hole?
They would be vertically stretched and horizontally squeezed and accelerated to high speeds in the process. Numerous and violent collisions would happen with the person and other debris and a great deal of frictional heating would happen. Material is simultaneously torn apart and heated to high temperatures as it plunges into the hole.
Why do astronomers think that gamma-ray bursts are very distant and very energetic?
Very distant because all satellites detected very far away distances, even taking into account lack of accuracy. Very energetic because they otherwise wouldn't be detected by our equipment.
X-Ray bursters
X-Ray source that radiates thousands of times more energy than our sun in short bursts lasting only a few seconds. A neutron star in a binary system accretes matter onto it's surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy.