The Odd Couple Neutron Stars and Black Holes

The Odd Couple: Neutron Stars and Black Holes Chapter 14

Review • When massive stars evolve, many have enough mass (even after much mass loss) that gravity compresses them past electron degeneracy (a white dwarf). • Protons and electrons squeezed together to form neutrons (and a neutrino is released). • These compact objects resist further collapse due to neutron degeneracy (2 neutrons of the same state cannot occupy the same “place”)

Basic Properties • 1 -3 suns compressed into a radius of 10 km! • Core density (1014 g/cm 3) ~ same as an atomic nucleus (1 sugar cube = 100 M tons!) • Rapidly spinning ball due to conservation of angular momentum • Magnetic field is trillions x stronger than Earth • Temperature: millions of degrees (X-rays) and would remain hot due to small surface area

Neutron Star in Puppis

Discovery! • Jocelyn Bell discovered series of very regular radio pulses (unusual!). • Nearly as exact as an atomic clock • Initially named it LGM (little green men), changed to pulsar (pulsating star). • Not a white dwarf (30 rot/sec is too fast) • Must be very small due to short interval of pulses, smaller than a white dwarf! <300 km • Crab Nebula, the missing link: this neutron star is actually a pulsar (a spinning n 0 star)!

Bell’s Discovery Notes Note regular valleys.

Pulsar Model—Not Anorexic! • Misnomer: neutrons stars never actually pulse! They actually emit beams of radiation. • Stars contain much ionized gas, which “freezes” the magnetic poles in place, even when they collapse. However, as the star collapses, the magnetic field gets concentrated into a smaller region. Stronger! • As the star spins, these beams sweep through space. We see them if they line up with the Earth. Called the Lighthouse Model. • See pages 286 -87.

Lighthouse Model of Pulsars

Pulsar Properties • First, we don’t see many pulsars because their beams often don’t line up with Earth. • Many pulsars are slowing down as some of its rotational energy is converted into various kinds of EM energy (such as the beam) and powerful outflows of high-speed particles (called a pulsar wind, 99. 9% of the lost E).

Pulsar Evolution • In general, pulsars initially rotate very quickly and then slow down. Therefore, they do not tend to live long ~ 10 million years. • Young pulsars typically rotate ~ 100 times/s. • Young pulsars are highly energetic, even giving off visible light! • Some neutron stars get ejected from the remnant during the supernova or outlive the remnant remains.

Crab Nebula Pulsar Pulse period = 0. 0333 sec

The Vela Pulsar The pulsar

2 For the Price of 1! • Some pulsars have binary companions that supply them with matter and can speed up the rotation rate. One of the fastest rotates 642 times each second! Flattened shape! • Taylor and Hulse’s Pulsar (1974): • Binary (separated by ~ sun’s radius) • Shift in period due to Doppler Effect • Both objects gradually spiraling toward each other, generating gravitational waves.

Gravity Power! • An astronaut stepping onto a neutron star would be smushed into a 1 atom thick layer! • Matter falling onto a neutron star releases tremendous energy! A single marshmallow “dropped” from 1 AU would hit with an impact of a 3 -megaton warhead (~0. 2 mc 2). • Hot! X-rays and gamma rays released! • Hercules X-1: companion star is hotter on neutron side than other side! Fig. 14 -9, p. 291

Pulsar Planets? • Because pulsar rates are so precise, even tiny variations indicate another object nearby. • PSR 1257+12: tiny variations in its period indicated 3 or 4 small masses (planets) in nearby orbit. • Likely the remains of a star it devoured. • Appears “spun up” by the former star. • Would have to be lifeless due to tidal forces and radiation.

Can You Escape the BEAST? • The escape velocity for Earth is ~25, 000 mph • In 1783, Rev. John Mitchell realized (from Newton’s equations) that a big enough star would have an escape velocity greater than the speed of light! (a black hole!) • We now know that a core of more than 3 suns cannot stop collapsing to zero radius! • Called a singularity! It has infinite density and gravity. The laws of Physics are unknown in this region. A black hole!

Tycho’s supernova of 1572. No neutron star suggests that there might be a black hole instead.

A Great Event? • The boundary of a black hole is called the event horizon, because anything that takes place inside is invisible to an outside observer (the light cannot escape). • The distance from the singularity to the event horizon (for the simplest case) is called the Schwarzschild radius (RS). • Named after Karl Schwarzschild who solved for it from Einstein’s General Theory of Relativity. Gravity warps the “fabric” of spacetime. A black hole is infinite curvature, a “hole!”

The Schwarzschild Radius

Space-Time and Black Holes Gravity warps spacetime. Black holes create “holes” in the fabric, where even light cannot escape.

Radial Calculations! • Simplest case: single, nonrotating, electrically neutral object: RS = 2 GM/c 2 • G - gravitational constant. C - speed of light. M is mass. • Every object has a Schwarzschild radius, though not every object is a black hole. • Examples: Earth ~ 0. 9 cm • Sun ~ 3 km • 10 Msun star ~ 30 km • Note that even a massive star will have little effect on planets orbiting outside this radius!

Bald Black Holes • Once matter enters the event horizon, it only retains 3 things: mass, angular momentum, and electrical charge. • Black holes tend to be neutral because any net charge would attract the opposite charge and bring it back to balance. • Black holes with mass “only” (nonrotating, neutral) are called Schwarzschild black holes.

Rotating Beasts • Roy P. Kerr solved the solution for rotating black holes, called Kerr black holes. • Rotating black holes drag space-time along with them! This creates a region outside the event horizon (called the ergosphere) where nothing can resist being dragged along. • Also, energy can be extracted from rotating black holes, causing them to slow down. • Also called frame dragging, Mercury, proof

Kerr Black Hole Diagrams

Approaching Too Close • 3 things happen when things get too close: 1. Time Dilation: time slows down for outside observers watching us, stops at the E horizon. 2. Gravitational Red Shift: light leaving us would lose energy and lengthen its wavelength (gets more red). 3. Spaghettification: gravitational tidal forces pull one side of an object more than another and stretch it thin, eventually shredding it. See Fig. 14 -14, p. 297

Looking in All the Right Places • Solitary black holes would be hard to see. • So astronomers look for orbiting companions (orbital speed) and mass flow/energy release. • Cygnus X-1 is a possible candidate. A huge O star orbits a 10 -15 sun object. X-rays are produced. T = 5. 6 days • http: //chandra. harvard. edu/photo/2003/bhspin/

Other Oddities • X-ray Bursters: due to binaries containing neutron stars. When enough matter from the NS companion collects, it explosively fuses into carbon. • Jets: twin blasts of high-speed particles often explode away from compact objects from the poles along magnetic field lines. See X-1. • Gamma Ray Bursters: possibly due to merging neutron stars or hypernovae (when massive stars collapse directly into a black hole). A cause of some Earth extinctions?

Hypernovae in M 101