Chap 23 and 24 Flashcards
How does a white dwarf differ from a neutron star? How does each form? What keeps each from collapsing under its own weight?
White dwarf: less than or equal to 1.4 solar mass, electron degeneracy, forms from lower mass stars
Neutron star: 1.4 to 3 solar mass, neutron degeneracy, from higher mass stars after a supernova type II
Describe the evolution of a star with a mass like that of the Sun, from the main-sequence phase of its evolution until it becomes a white dwarf.
1.) Hydrogen fusion in a shell starts following core shrinking - Red Giant
2.) Helium flash signals the start of helium fusion in the core
3. The core collapses, hydrogen and helium fusion occurs in shell - Red Giant again
4. A planetary nebula forms, leaving behind a white dwarf
Describe the evolution of a massive star (say, 20 times the mass of the Sun) up to the point at which it becomes a supernova. How does the evolution of a massive star differ from that of the Sun? Why?
The evolution of a massive star happens when it starts to run out of energy and atoms to fuse (in iron core), so it then collapses and “bounces” to create a supernova.
The difference between them is that a massive star has a core temperature a lot hotter, meaning it can fuse heavier elements in the shell
How do the two types of supernovae discussed in this chapter differ? What kind of star gives rise to each type?
Type IA: White dwarf is near a large star, such as a red giant, and it pulls material off the other star, causing the white dwarf to go over 1.4 solar mass, and creating a supernova
Type II: a giant mass star runs out of atoms to fuse in core, so it and collapses and “bounces,” creating a giant explosion, leaving a neutron star or black hole
If the formation of a neutron star leads to a supernova explosion, explain why only three of the hundreds of known pulsars are found in supernova remnants.
The supernova remnants eventually dissipate.
How can the Crab Nebula shine with the energy of something like 100,000 Suns when the star that formed the nebula exploded almost 1000 years ago? Who “pays the bills” for much of the radiation we see coming from the nebula?
The rotation of the pulsar in the center of the nebula.
Describe the evolution of a pulsar over time, in particular how the rotation and pulse signal changes over time.
The frequency and signal get weaker with rotation and time.
Arrange the following stars in order of their evolution:
1. A star with no nuclear reactions going on in the core, which is made primarily of carbon and oxygen.
2. A star of uniform composition from center to surface; it contains hydrogen but has no nuclear reactions going on in the core.
3. A star that is fusing hydrogen to form helium in its core.
4. A star that is fusing helium to carbon in the core and hydrogen to helium in a shell around the core.
5. A star that has no nuclear reactions going on in the core but is fusing hydrogen to form helium in a shell around the core.
- White dwarf, last
- Proto star, first
- Main sequence, second
- Big red giant, fourth
- Red giant, third
Astronomers believe there are something like 100 million neutron stars in the Galaxy, yet we have only found about 2000 pulsars in the Milky Way. Give several reasons these numbers are so different. Explain each reason.
The pulsar of a neutron star has to be directly at us, and they also only last usually at most 10 million years, so many aren’t found.
Say that a particular white dwarf has the mass of the Sun but the radius of Earth. What is the acceleration of gravity at the surface of the white dwarf? How much greater is this than g at the surface of Earth? What would you weigh at the surface of the white dwarf (again granting us the dubious notion that you could survive there)?
Use surface gravity formula,
g= GM/ r^2
Use G = 6.67 × 10^11
Times mass of sun: 2 x 10^30
Divide by the radius of earth,
r= 6.378 × 10^6
And then divide g by 9.8
Now take a neutron star that has twice the mass of the Sun but a radius of 10 km. What is the acceleration of gravity at the surface of the neutron star? How much greater is this than g at the surface of Earth? What would you weigh at the surface of the neutron star (provided you could somehow not become a puddle of protoplasm)?
10 km —> 10,000m
A neutron star is 2 solar mass, so 2 times the mass of the Sun
Use surface gravity formula again
How does the equivalence principle lead us to suspect that spacetime might be curved?
Light is bent by spacetime since light has no mass.
If general relativity offers the best description of what happens in the presence of gravity, why do physicists still make use of Newton’s equations in describing gravitational forces on Earth (when building a bridge, for example)?
General reality is really only needed for huge objects since smaller objects are barely impacted
The Newton formulas work fine and are also more simple to use.
If a black hole itself emits no radiation, what evidence do astronomers and physicists today have that the theory of black holes is correct?
They look at the effect of the stars orbit and x-rays coming from the accretion disk
What is an event horizon? Does our Sun have an event horizon around it?
An event horizon is a boundary with an escape velocity at the speed of light. The Sun will never have an event horizon because it’s not big enough.
