Astronomy - Chapter 11: Supernovas, Neutron Stars, and Black Holes

You are on a windowless airplane. The ride is smooth. Is it possible to figure out whether you are moving?

No. There is no experiment that you can do inside the plane to determine whether the plane is moving at constant speed or is stationary.

Why is it called Special Relativity? (what is a key aspect of it)

It only applies to specific situations where the different frames of reference aren’t accelerating (train example: person standing on train and person standing on platform - not moving)

Postulates of Special Relativity

1. The laws of physics are the same in all inertial reference frames (i.e. regardless of your perspective if not moving - standing still on platform or train)

2. The speed of light (in a vacuum) is the same for all observers (whether or not the light source is moving)

Type 1a Supernova

Carbon Detonation - White dwarf in binary system accumulates enough mass to reach Chandrasekhar limit and explodes

Type II Supernova

Core Collapse - Once 1.4 solar masses of Iron core burn up the supermassive star collapses and explodes in milliseconds because iron can’t fuse into anything else, creating either a neutron star or black hole

Explain the sequence of fusion shell formation in massive stars and thus the generation of nested shells

The nested shells of a massive star each contain different elements, and when each layer is depleted, the core contracts and becomes hotter which makes a new, hotter fusion process begin. There is an iron core at the center, and from the outside the layers go (1) Hydrogen, (2) Helium, (3) Carbon, (4) Oxygen, (5) Neon, (6) Magnesium, (7) Silicon, then the iron core

A small to medium sized star (Redgiant) will transform into what?

A white dwarf (then eventually a type 1a supernova if in a binary system)

A massive star (Blue giant) will transform into what?

A neutron star or black hole

How are neutron stars and white dwarfs similar?

Both neutron stars and white dwarfs have a very high density, and they (can) represent the final evolution of a star's life - before a possible supernova explosion. 

What are pulsars?

Pulsars are neutron stars that rapidly pulse at different periods depending on the type of pulsar they are - radio, x-ray, millisecond, binary

How can an old pulsar have a very short pulsar period, say less than 0.03 seconds?

If a pulsar is part of a binary system and is accreting matter from a stellar companion via an accretion disc, then it may also gain angular momentum. This process can 'rejuvenate' old pulsars, boosting their rotation and making their periods as short as a few milliseconds.

If neutron stars contain no nuclear fuel, why are they hot?

They remain hot because they are the result of a supernova explosion that generates an immense amount of heat

If the Sun and stars are supported by gas pressure, what supports a neutron star?

They are primarily supported by degenerate neutrons/neutron degeneracy. This is the pressure exerted by neutrons in a dense state and happens when core collapse (type II supernova) causes the pressure and density in a star to increase dramatically, which then allows electrons and protons to combine to form neutrons.

What evidence do we have that pulsars are neutron stars?

Only a neutron is small enough to be a pulsar, as pulses from pulsars cannot be longer than 0.001 seconds, and therefore cannot be larger than 0.001 light-seconds in diameter which is definitely smaller than a white dwarf

What is the difference between a planetary nebula and a supernova remnant?

Planetary nebulae have low temperatures, allowing them to emit a lot of optical light that can be broken into specific wavelengths corresponding to various elements. Whereas supernova remnants are so hot that they emit x-rays instead of optical radiation 

What is the difference between a supernova and a nova?

In a nova, a star flares up (in an explosion) and then returns to dormancy, allowing this process to happen again. Whereas in a supernova, the star is completely destroyed from its explosion. A supernova is also incredibly more luminous than a nova. 

What is a nova?

A nova starts as two stars in a binary system just as a Type 1a supernova does, where a white dwarf accretes matter from a regular star, eventually causing fusion reactions and an explosion.

What observational evidence do we have that black holes exist?

1. X-ray telescopes can observe light in binary systems where matter is being accreted from the outer layers of a companion star and see that it is creating the accretion disk of a black hole (which is what gives off x-rays)

2. If the mass of a compact object is significantly greater than 3 solar masses, it can't be a neutron star and must be a black hole. 

3. We can distinguish between a neutron star and black hole because neutron stars emit pulses and black holes don't emit regular pulses because they don't have a solid body.

Why can't a massive star generate energy through iron fusion?

Each fusion reaction of light elements in the core of a high mass star always has a mass defect. That is, the product of the reaction has less mass than the reactants. However, when you fuse iron, the product of iron fusion has more mass than the reactants. Therefore, iron fusion does not create energy; instead, iron fusion requires the input of energy.

Why do supernova remnants have limited lifetimes?

Because the remnants gradually mix with the interstellar medium surrounding it and vanishes.

Why do we not expect to find a 5-solar-mass neutron star?

This is because the mass limit for a neutron star is 3 solar masses, so if a larger star of 5 solar masses collapsed it would create a black hole

Why does our theory predict that neutron stars will have strong magnetic fields?

Quantum mechanics calculations predict that, at any moment, about 10% of the particles inside a neutron star are charging back and forth from neutrons to protons to electrons that have electrical charges, so powerful electric currents can flow in a neutron star and sustain a very strong magnetic field.

Why does our theory predict that neutron stars will spin rapidly?

The principle of conservation of angular momentum - as a star collapses it must rotate faster because it conserves angular momentum, meaning it must spin faster as its matter is pulled closer to the axis of rotation.

Why might we suspect that a black hole in a binary system could emit X-rays?

X-ray emissions from the accretion disk of a black hole are a direct result from matter of its companion star being pulled into the black hole and heating up to x-ray emitting temperatures. 

Magnetars

_________ are neutron stars that have magnetic fields 100 times stronger than the average neutron star.

Synchrotron radiation

_________ is a form of electromagnetic radiation produced by rapidly moving electrons spiraling through magnetic fields.

A gravitational redshift

_________ occurs when light travels out of a gravitational field, loses energy, and its wavelength grows longer.

Neutron star

A _________ has a radius of about 10 km and is supported by the pressure associated with degenerate neutrons.

A black hole can be thought of as…

…a massive body of infinitely small size

A pulsar requires that a neutron star…

I. rotates rapidly.
II. has a radius of at least 10 km.
III. has a strong magnetic field.
IV. rotates on an axis that is different from the axis of the magnetic field.

I, III, & IV 

A star on the main sequence has 7 solar masses. The star will evolve into a ____________ and its core will be supported by ______________.

white dwarf, electron degeneracy pressure

A supernova is almost always associated with

a white dwarf in a close binary system

A Type Ia supernova is believed to occur when…

a white dwarf’s mass exceeds the Chandrasekhar limit

A type II supernova…

occurs when the iron core of a massive star collapses and is characterized by a spectrum that shows hydrogen lines

Although neutron stars are very hot, they are faint and not easy to see at visual wavelengths because…

they have only a small surface area from which to emit and the peak of their thermal emission is at much shorter wavelengths than visual

An isolated black hole in empty intergalactic space would be difficult to detect because…

1. It could not emit light from inside its event horizon.

2. There would be no stars behind it whose light would be affected by gravitational lensing.

3. No companion stars would be affected by its gravitational field.

4. No matter would be falling into it to create an X-ray-emitting accretion disk.

An isolated black hole in space would be difficult to detect because…

very little matter would be falling into it

As material flows into a black hole as viewed from well outside the event horizon

the material will experience time dilation and the material will become hotter

At extremely high densities and temperatures, electrons can be forced to fuse with protons. This reaction produces…

neutrons and neutrinos

Cygnus X-1 was finally determined to be a black hole when the mass of the unseen companions was confirmed to be…

more than 3 solar masses

Giant and supergiant stars are rare because…

the giant or supergiant stage is very short