Discussion Explain why a star expands when its core collapses once it runs out of hydrogen in its core.

Discussion What stops the inert core of a red giant star from collapsing?

Discussion What is a white dwarf star?

Discussion Why do less massive white dwarf stars have a higher luminosity at any given temperature?

Discussion What is the CNO cycle and why doesn’t it occur in the Sun?

Discussion What is the instability strip and why are the stars in it important to astronomers?

Discussion Why does fusion in a massive star end at iron?

Discussion What causes a star to explode as a supernova?

Discussion Will the Sun ever go supernova?

Discussion What is a neutron star?

Cassiopeia A composite Red, Spitzer Orange HST Blue Chandra

HST

Cassiopeia A Chandra 300 years ago, not observed

Cassiopeia A radio

1054

11,000 years

15,000 years

Cygnus loop

Pulsar

Discussion When radio pulsars were discovered they called them LGM for little green men. Can you think of a natural way to get accurate time keeping from an astronomical object?

Discussion The fastest pulsar has a period of 0.00156 seconds, which means it must spin 641 times per second. From this astronomers deduce that these objects must be very small. Explain why.

Magnetic fields If a star with a magnetic field is smashed down to a small radius, the magnetic field will also be smashed and concentrated and therefore will be much stronger at the star’s surface.

Discussion How do you think you can get a picture of the a pulsar in its “on” and “off” state given that the pulsar is so faint that a long time exposure is required?

Discussion The Crab nebula has a luminosity of 75,000 times that of the Sun, much greater than when the star was on the main sequence. What provides all this power?

Discussion What happens to the period of a pulsar over time?

Discussion How can we get stars on the main sequence that are more massive than the turn-off, which astronomers call blue stragglers?

Discussion Stars that have radii bigger than their Roche lobe cannot form. How then can binary stars transfer matter between them?

Discussion What do you think might happen if Sirius A evolves into a red giant and fills its Roche lobe?

Nova

August 21 3013 4.3 mag

Nova Persei 1901 HST image

Nova Cygni 1992

Discussion What do you think would happen if the mass transfer happened so fast that the mass of the carbon white dwarf star exceeded the 1.4 solar mass limit?

Supernova type Ia If the mass transferred to the white dwarf is greater than 1.4 M☉, the electron degeneracy pressure will not be able to support the weight of the star and it will collapse.

The sudden collapse of the white dwarf heats the core which causes core fusion to start again. This re-ignition of the core blows the star apart.

SN 1572

SN 1604

Supernovae type Ia and II Type Ia – caused by white dwarf exceeding 1.4 M☉. Spectrum has weak hydrogen lines. Type II – caused by massive star forming a neutron star. Spectrum has strong H lines.

Discussion Why is it that a supernova type II can form a neutron star and doesn’t blow itself to bits like a type Ia supernova?

Gravity wins Neutron degeneracy pressure has a limit to how much mass it can support. If the mass of a neutron star exceeds about 3 M☉, the star will collapse. There is no known force that can stop the collapse and the matter is smashed down to infinitely small size, a singularity

Discussion If the matter is smashed down to a single point, what happens to the surface gravity? What happens to the escape velocity of such an object?

Black Holes Any object that has an escape velocity greater than the speed of light is called a black hole.

Quark Stars? Maybe the neutrons can turn into something that can stop the collapse into a black hole. Possible, but not much room to do it in. A 1.4 solar mass neutron star radius is 10.4 – 12.9 km while the escape velocity of a 3 solar mass object equals the speed of light at a radius of 9 km.

Albert Einstein

Einstein and Black Holes The existence of black holes is predicted by Einstein’s General Theory of Relativity. But black holes are so strange that Einstein refused to accept that nature would allow such a thing to exist.

Was Einstein wrong? Today most theorists believe that black holes must exist, though most of the observational evidence is still indirect. Note: the gravitational waves detected earlier last year could not have been made by two neutron stars