Black Holes! And other collapsed stars

The Hertzsprung-Russell Diagram What will happen to the Sun, once its fuel (hydrogen) ends? Size of Star Mass of Star

Evolution phases of a star like the Sun Sun today: age ~4.5 Gyrs; will last another ~5 Gyr (hydrogen burning) Yellow Giant (helium burning) Red giant Second red giant Planetary nebula

Example of a Planetary Nebula The Helix Nebula

White Dwarfs This is the fate of our Sun! A White Dwarf is a dying star, which has terminated its nuclear fuel, and has contracted to roughly the size of the Earth. WDs are prevented to collapse further by electron degeneracy (electrons, because of quantum properties, cannot be crowded more then certain limits) This fate is shared by all stars with masses below 8 MSun, and they end up with masses below 1.4 MSun [the Chandrasekar limit]. Most WDs have masses around 0.6 MSun The core of a WD is commonly a mixture of Carbon and Oxygen, and is releasing as light the contraction heat. When cold (~6,000-8,000 K) they may crystallize into `giant diamonds’ (first confirmed observationally from WD oscillations in 2004). As the heat is releases, the WD cools down and will end up a Brown, and then Black, Dwarf This is the fate of our Sun!

White Dwarfs can Flare back to life If they have a `younger’ companion

Novae are nuclear explosions on the surface of white dwarf and neutron stars Brightness changes by a factor of 4000!

White Dwarf Supernova (SN-Ia) If a White Dwarf accretes enough matter from a companion star, it will eventually nova. If, after the nova, it does not shed all the mass it gained, it will continue to accrete mass until it novas again. If this process continues (accretion, nova, accretion, nova, etc.) such that the WD continues to gain mass, once it has a mass of 1.4Msun, the core will collapse, carbon fusion will occur simultaneously throughout the core, and the WD will supernova.

Another distance indicator: White Dwarf (Type Ia) Supernovae in distant galaxies. L=4D2 l

Evolution Phases of a Star Much More Massive than the Sun (>8 Msun) Massive star, main sequence (H burning) Massive star, He burning Neutron star or black hole Red giant/supergiant Supernova explosion (SN Type II)

Example of a Type II Supernova The Crab Nebula: The supernova explosion that created the Crab was seen on about July 4, 1054 AD.

Lifetimes of Stars On the Main Sequence, stars appear to obey a Mass-Luminosity relation: L  M3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11. Thus, stars like Sirius that are about twice as massive as the Sun are about 11 times as luminous. The more massive a Main Sequence star is, the hotter (bluer), and more luminous, the star, and the shorter its life. For instance, Sirius is `only’ twice the mass of the Sun, but is 11 times more luminous, implying its life will be about 5.5 times shorter than that of the Sun: T(Sirius) ~ 2/11 T(Sun))

Neutron star A neutron star --- a giant nucleus --- is formed from the collapse of a massive star, with Mcore > 1.44 Msun. Supported by neutron degeneracy pressure. Only about 10 km in radius. A teaspoon full would contain 108 tons! Typically with very strong magnetic field

SNR N157B in the LMC 16ms period The fastest young pulsar known pulsar

Pulsar A fast rotating, magnetized neutron star. Emits both strong radiation and high-energy particles.

The Limit of Neutron Degeneracy The upper limit on the mass of stars supported by neutron degeneracy pressure is about 3.0-3.2 MSun. If the remaining core contains more mass, neutron degeneracy pressure is insufficient to stop the collapse. In fact, nothing can stop the collapse, and the star becomes a black hole.

What is a black hole? Well … mostly nothing. When the ball of neutrons collapses, it forms a singularity – a point in space with infinitely small volume and the mass of the parent material. A singularity has infinite density! The most interesting aspects of a black hole are not what it’s made of, but what effect is has on the space and time around it.

A collapsed star can be sufficiently dense to trap light in its gravity.

The Size of a Black Hole The extent of a black hole is called its event horizon. Nothing escapes the event horizon! The radius of the event horizon is the Schwarzschild radius given by: Rs = 2GM/c2

Some Examples of Black Hole Sizes A 3MSun black hole would have a Schwarzschild radius of ~10km. It would fit in Amherst. A 3 billion MSun black hole would have a radius of 60 AU – just twice the radius of our solar system. Some primordial black holes may have been created with a mass equal to that of Mount Everest. They would have a radius of just 1.5x10-15 m – smaller than a hydrogen atom!

What would a Black Hole look like?

Gravitational lensing

Some Odd Properties of Space Around a Black Hole Light emitted near the surface of a black hole is redshifted as it leaves the intense gravitational field. For someone far away, time seems to runs more slowly near the surface of a black hole. An astronaut falling into a black hole would seem to take forever to fall in.

Gravitational Redshifts A photon will give up energy while climbing away from a mass. It is trading its own energy for gravitational potential energy.

Black Holes Don’t Suck You In! Many people are under the impression that the gravity of black holes is so strong that they suck in everything around them. Imagine what would happen if the Sun were to instantly turn into a black hole. What would happen to the Earth?

Black Holes Don’t Suck You In! Since the mass of the Sun and Earth don’t change, and the Earth is no further from the Sun than it was before, the force on the Earth would remain exactly the same. The Earth would continue to orbit the black hole at a distance of 1 AU!

Black Holes Don’t Suck You In! So why are black holes so infamous? The reason is that the mass is so compact that you can get within a few kilometers of a full solar mass of material. Today, if you stood on the surface of the Sun, much of the material is hundreds of thousands of kilometers away. With a black hole, the mass is so concentrated that you can get very close to the full mass.

The tidal forces near a moderate sized black hole are lethal!

How Do We See A Black Hole? Short answer … we don’t. But we can see radiation from the material falling into one.

Evidence for Black Holes If black holes are black, how do we know that they exist? The star HD 226868 is an excellent example. It is a B supergiant. The spectral lines in the star clearly show that it is in a binary system with a period of 5.6 days, however, we see no companion star.

The companion is one of the brightest X-ray sources in the sky and is called Cygnus X-1 HD 226868 Cygnus X-1 The blue supergiant is so large, that its outer atmosphere can be drawn into the black hole. As the material spirals into the black hole, it heats up to millions of degrees and emits X-ray radiation.

Stellar Evolution on the Hertzsprung-Russel Diagram

Stellar Evolution in a Nutshell Mass controls the evolution of a star! M < 8 MSun M > 8 MSun Mcore < 3MSun Mcore > 3MSun