Neutron Stars and Black Holes - Astronomy Chapter 11

Gamma-Ray Bursters

..., An object that produces a sudden burst of gamma rays; thought to be associated with neutron stars and black holes.

Black holes (escape velocity, Schwarszchild Radius, Event Horizon, Singularity, Gravitational Redshift, Time Dilation, the search and leaping into the Black Hole)

Black Holes - a mass that has collapsed to such a small volume that its gravity, being so strong, prevents the escape of all radiation. The volume of space from which radiation may not escape. (It is a misconception to think that black holes are giant vacuum cleaners that will suck in everything in the universe. A black hole is just a gravitational field) ______________________ Schwarzschild Black Holes: If the core of a star contains more than 3 solar masses when it collapses, no force can stop it; thus, form into black holes under the sole influence of their own gravity. To search for black holes, astronomers must look for binary star systems in which mass flows into a compact object and emits X-rays. The Black Hole is Einstein's Theory of Relativity in 1916 of gravity being the curvature of space and time, and astronomer Karl Schwarzschild found a way to solve Einstein's equations to describe the gravitational field around a single, non-rotating, electrically neutral lump of matter called the Schwarzschild black holes. Schwarzschild's solutions showed 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 can get out). _______________ Escape velocity - the initial velocity at which an object needs to escape from a celestial body such as a black hole. This velocity depends on 2 things: 1. the mass of the celestial body - if the mass of the celestial body is large, its gravity is strong. You would need a high velocity to escape. 2. the distance from the center of mass to the escaping object - if you begin your journey farther from the center of mass, the velocity needed is less. For example, Earth's escape velocity is 11 km/s (25,000 mph), but if you would launch a spaceship from top of a tower 1000 miles high, the escape velocity would only be 10 km/s (22,000 mph). ________________ Singularity - the object of 0 radius into which matter in the black hole is believed to fall. Density and gravity becomes infinite at the center. Event Horizon - the boundary between the isolated volume of space-time and the rest of the universe. No event inside is detectable. Once an object crosses this horizon, it is gone forever. __________________ Schwarszchild radius (Rs) - the radius of the event horizon. A collapsing core must shrink inside its Schwarzschild radius to become a black hole. This radius depends only on the mass of the object (in kilograms) in a simple equation: Rs = 2GM / c² G: gravitational constant M: mass c: speed of light Every object has a Schwarzschild radius determined by its mass, but not every object is a black hole. For example, Earth (0.000003 solar mass) has a Schwarzschild radius of 0.9 cm, but it could become a black hole only if you squeezed it inside its radius. Fortunately, Earth will not collapse spontaneously to become a black hole because the strength of the rock and metal in its interior supports its weight. _________________ Leaping into a Black Hole: Time Dilation - time slows down when in strong gravitational fields such as in the black hole. This is the slowing on the edge of the hole. Gravitational Redshift - the lengthening of the wavelength (reddening of the light). When the light has entered into the hole, it loses energy and turns red as it tries to overcome the gravitational pull inside the hole. __________________ The Search for Black Holes:

Pulsars (Pulsing Star, the Lighthouse Model, pulsar wind, binary pulsars, gravitational radiation, X-ray bursters, millisecond pulsars)

For the discovery of the pulsar, in Nov 1967, a graduate at Jocelyn Bell found a peculiar pattern in the data from a radio telescope with a series of rapidly radio pulses. The 1st known pulsar was observed with Bell's radio telescope. Pulsar - a contraction of a pulsing star. ______________ The Lighthouse Model of a Pulsar (the explanation of a pulsar as a spinning neutron star sweeping beams of electromagnetic radiation around the sky): 1. 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. 2. The mechanism that produces the beams involves extremely high energies and is not fully understood. 3. 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. _______________ The Evolution of Pulsars: When a pulsar first forms, it is spinning fast. The energy it radiates into space comes from its energy of rotation, so it rotates slow when it blasts beams of radiation outwards (A spinning neutron star slows as it radiates its energy into space). Older neutron stars (oldest being 10 million years old) rotate too slow to generate detectable radio beams. Only the most energetic pulsars produce short-wavelength photons, so it pulses at visible wavelengths. pulsar winds - the 99.9% of the energy flowing away from a pulsar (emitted) that is carried in these winds of high-speed atomic particles. The energy in the beams is only a small part of the energy emitted by a pulsar. This can produce small, high-energy nebulae near a young pulsar. __________________ Binary Pulsars: The 1st binary pulsar was discovered in 1974 when astronomers Joseph Taylor and Russell Hulse noticed that the pulse period of the pulsar PSR1913+16 was changing. The period 1st grew longer and then grew shorter that took 7.75 hours. __________________ Millisecond Pulsars - the fastest pulsars with pulse periods of rotation of 1 millisecond (0.001 s). For example, if a neutron star 10 km in radius spins 716 times a second, then its period is 0.0014 second, and its equator must be traveling at about 45,000 km/s. The fastest known pulsar is, XTEJ1739-285, spins 1122 times a second. ________________ Pulsar Planets: When astronomers checked pulsar PSR1257+12, they found variations in the period of pulsation much like those caused by the orbital motion of a binary star, but the variations were much smaller. When they were interpreted as Doppler shifts, it became apparent that the pulsar was being orbited by at least 2 objects with masses of 4.1(Planet-like mass) and 3.8 (Earth mass). So planets have been found orbiting at least 1 neutron star.

Compact objects

Gravity always wins; however, a star lives. It must eventually die by collapsing into 1 of 3 final states: 1. white dwarf 2. neutron star 3. black hole. These compact objects are small monuments to the power of gravity. Almost all of the energy available has been squeezed out of compact objects. When a supernova explodes, the core collapses to very small size. The collapsing core cannot support itself as a white dwarf if its mass is greater than 1.4 solar masses (Chandrasekhar limit).

Neutron stars (Chandrasekhar limit)

a small, highly dense star composed almost entirely of tightly packed neutrons. It contains 1.4 - 3 solar masses compressed to a radius of about 10 km (6.214 miles). It is hot, spins rapidly, and has strong magnetic fields. ________________ The neutron was discovered in the laboratory in Feb 1932. In Jan 1934, 2 Caltech astronomers Walter Baade and Fritz Zwicky published a paper that showed that some novae in historical records were much more luminous than most, and suggested that they were caused by explosive collapse of a massive star in an explosion called a supernova. The core of the star would form a small and dense sphere of neutrons where Zwicky proposed the term "neutron star." Throughout years, scientists applied quantum mechanics to see if this was possible. Neutrons spin like electrons do, which means that neutrons obey the Pauli exclusion principle. This means that if neutrons are packed together tightly enough, they can become degenerate just like electrons. An even denser mass of neutrons might support itself by the pressure of degenerate neutrons. The core of a collapsing star becomes a mass of neutrons by the core collapsing inward even though the star explodes outwards. If the collapsing core is more massive that the Chandrasekhar limit of 1.4 solar masses, it cannot reach stability as a white dwarf. The weight is too great to become supported by degenerate electrons. The collapse of the core continues and the atomic nuclei are broken apart by gamma rays. The increasing density forces the freed protons to combine with electrons and become neutrons. Then the collapsing core becomes a contracting ball of neutrons. The core is then left behind as a neutron star after the star's envelope has blasted away. The more massive stars will lose mass rapidly, but they cannot shed mass fast enough to reduce their mass below the Chandrasekhar limit, so they must die in supernova explosions. Stars that begin life on the main sequence with 8 - 20 solar masses will leave behind neutron stars, while more massive stars form black holes. A neutron star cannot be more massive than 2 - 3 solar masses (2 of the most massive stars are 1.94 & 2.74 solar masses that contributes to this theory). If a neutron star was more massive, the degenerate neutrons would not be able to support the weight and it would collapse, especially into a black hole. Neutron stars should be only 10 or so kilometers in radius. A neutron star has a powerful magnetic field because it could have a magnetic field as much as trillion times stronger than the sun's since magnetic fields of some stars start with over 1000 times stronger than the sun's.

Pauli Exclusion Principle

no two electrons in the same atom can have the same set of four quantum numbers