Evolution of Sun-like Stars Flashcards
What is the first step in a stars life? (how it is formed)
- Cloud of dust and gas clump together due to gravity , until they are dense enough to form regions called protostars
- Eventually the temperature at the centre of the protostar reaches a few million degrees, and hydrogen nuclei start to fuse to form helium
- Releases a lot of energy and the outward pressure equals the gravity to star is stable (main sequence)
What happens at the end of the main sequence of a star?
- When all hydrogen has fused into helium, the helium core contracts (due to gravity) and heats up under the weight of the star and out layers cool, creating a red giant
- Material surrounding core still has hydrogen, so fusion continues in shell
- Helium core continues to contract until eventually it gets hot and dense enough to fuse into carbon and oxygen and outer layers expand further outwards
- When helium runs out, carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region
What is a white dwarf?
- In low-mass stars like the sun, carbon-oxygen core doesn’t get hot enough for any further fusion so it continues to contract until electrons exert enough pressure (electron degeneracy pressure) to stop it collapsing any more
- Star ejects its outer layers into space as a planetary nebula
- Star now a very hot, dense solid called a white dwarf which simply cools and fades away
What is a supernova?
For higher-mass stars, electron degeneracy pressure does not stop the core contracting, and the outer layers fall in and rebound off the core, creating huge shockwaves, causing the star to explode in a supernova.
What is a type 1 supernova? What is a type 1a supernova?
A type 1 supernova which has no hydrogen lines in its spectrum.
A type 1a supernova is formed when a white dwarf core absorbs matter from a nearby binary partner, so they all have the same mass.
What does the fact that type 1a supernovae have the same mass mean?
That they all have the same light curves with the same peak absolute magnitude, so they can be used as standard candles.
What does the light curve of a type 1a supernova look like?
Starts at an absolute magnitude of about -18, raises to about -19.5 and then slowly decreases to about -16.5 over 40 days.
What is required for a neutron star to form?
IF the star’s core is between 1.4 and 3 solar masses, it will form a neutron star.
How is a neutron star formed?
Electrons in core get squashed onto atomic nuclei and combine with protons forming neutrons and neutrinos. The core then collapses to become a neutron star made mostly of neutrons.
About how dense are neutron stars, how large are they and how fast do they spin?
Density about 4 x 10^17 kgM^-3
Typically 20km across
Can rotate at about 600 times a second
What do neutron stars emit and what is this called?
They emit radio waves in two beams as they rotate, and these beams sometime sweep past the Earth and can be observed as radio pulses. These pulsing neutron stars are called pulsars.
What is required for a black hole to form?
If the core of the star is more than 3 solar masses, a black hole will form.
How does a black hole form?
Neutrons can’t withstand the gravitational force, so the stars core collapses to infinite density.
What is the boundary at which the escape velocity is greater than the speed of light?
The event horizon.
What is the radius of the event horizon called?
The Schwarzschild Radius.
