Life and Death of Stars

Question 1

What is the first fusion reaction within a star once it has become main sequence?

Answer 1

Hydrogen/proton 1H burning to produce Helium 4H

Question 2

Why does nuclear fusion begin in stars?

Answer 2

gravity makes protostars contract, and when the density and pressure becomes high enough, fusion can begin and the stars begin to produce energy

Question 3

how do stars achieve hydrostatic equilibrium

Answer 3

nuclear fusion within stars is exothermic process, releasing large amounts of energy. when enough energy is released as heat, pressure builds up inside the star and this force counteracts the force of gravity.

Question 4

why is the main sequence a band not a line?

Answer 4

hydrogen fusion (1H ->4He) chemical change unbalances the gravity-pressure hydrostatic equilibrium by small systematic amounts, meaning the position of the star on the main sequence moves slightly as it ages.

Question 5

since the outer layers of a star are too cool to support fusion, what happens to the hydrogen eventually?

Answer 5

conversion of H to He causes He ashes to build up in the centre of the star as the interior is not mixed by convection. such stars eventually burn up their H supply and then has a core of inert He surrounded by a mantle of unused H which cannot be fused as the temperature outside of the core is too low.

Question 6

when does a star complete its life cycle on the main sequence

Answer 6

when there is an inert core of He surrounded by a H mantle which cannot be fused due to low temperatures

Question 7

why are 90% of stars main sequence stars

Answer 7

because the average star spends 90% of its life fusing hydrogen on the main sequence.

Question 8

what determines how long a star spends on the main sequence

Answer 8

its mass. massive stars burn their fuel quickly and have short lives whilst low mass stars conserve their fuel such that they can remain on main sequence much longer. (10^6Ma for 40solar mass massive stars) compared to 56x10^9 for smallest stars.

Question 9

how do red dwarf stars die?

Answer 9

red dwarfs are the smallest and coolest stars on main sequence. they are expected to die primarily through mass loss as stellar winds drive off material from the outer layers

Question 10

what are stellar winds

Answer 10

fast flowing streams of particles emitted from stars

Question 11

how do giants and supergiant stars form

Answer 11

when massive main-sequence stars run out of hydrogen in their cores, at which point they start to expand, just like lower-mass stars. Unlike lower-mass stars, however, they begin to fuse helium in the core smoothly and not long after exhausting their hydrogen.

Question 12

what solar mass is required to ignite a stars helium core

Answer 12

0.4solar masses

Question 13

what does helium burning produce

Answer 13

ashes of carbon and oxygen

Question 14

what solar mass is required to ignite the carbon and oxygen ashes

Answer 14

greater than ~8 solar masses. stars between 0.4 and 8 solar masses cannot ignite.

Question 15

in stars with solar masses 0.4-8, what happens once all helium is burnt? what branch of the H-R do they join?

Answer 15

a core of oxygen and carbon is left at their centre, and they contract heat up and expands further. they join the asymptotic giant branch (AGB) phase of stellar evolution.

Question 16

what is s process nucleosynthesis? what type of stars do this

Answer 16

AGB stars. s process nucleosynthesis is the process where elements heavier than Fe are produced by the capture of free neutrons. the free neutrons are produced where H is mixed into the He burning shell. Combo of H, He and freshly produced carbon leads to reactions converting 12C to 16O releasing free neutrons.

Question 17

what is the s in s-process denoting

Answer 17

slow. neutron densities in AGB stars are sufficiently low to make neutron captures slow compared to the Beta decay of unstable nuclei, which are formed by neutron capture.

Question 18

what do the released neutrons from s-process nucleosynthesis do?

Answer 18

they react with Fe and other seed nuclei which formed from previous generations of stars, to produce even heavier elements.

Question 19

why are heavier elements formed much more efficiently in low mass (0.4-8solar mass) stars compared to higher-mass stars?

Answer 19

because the slower evolution of low mass stars means that substantially more neutrons are available per seed nucleus i.e. more time to form nuclei

Question 20

what solar mass is required to ignite the C/O core? what products are produced?

Answer 20

8-10 solar masses required for C/O burning. to produce Ne and Mg.

Question 21

when do stars begin to shed material?

Answer 21

stars up to 10solar masses run out of heat to fuse heavier than C/O. they become cool and expand to large diameters, their material is shed by the stars radiation and some material condenses around the star into dust particles.

Question 22

how are planetary nebula formed?

Answer 22

when a star 8-10 solar masses cools and sheds its material, a last gradual expulsion event generates planetary nebula, enriching space in freshly synthesised elements from the star.

Question 23

what do the remains of low and medium sized stars turn into?

Answer 23

white dwarfs

Question 24

do white dwarfs host fusion reactions?

Answer 24

no - they have no energy source hence the initially hot white dwarfs cool down with time

Question 25

what is electron degeneracy pressure in white dwarfs? what role does it play?

Answer 25

without fusion, white dwarfs are not protected against gravitational collapse (no internal pressure counteracting gravity). electron degeneracy is a quantum effect resulting from electrons being forced into their lowest possible quantum state. electron degeneracy pressure prevents collapse from gravity.

Question 26

why do white dwarfs have such enormous masses?

Answer 26

they are supported only by electron degeneracy pressure, where electrons are forced into their lowest quantum state. this gives white dwarfs one of the densest form of matter

Question 27

what is the Chandrasekhar limit?

Answer 27

the maximum mass of a white dwarf (~1.44 solar masses)

Question 28

what happens if white dwarfs continue accreting mass from its companion e.g. binary star system beyond the Chandrasekhar limit (1.44 solar masses)?

Answer 28

the star is no longer supported by the electron degeneracy pressure and it explodes as a type Ia supernova (SNIa).

Question 29

what are supernova classified using

Answer 29

the differences in their light emission curves as measured by spectroscopy

Question 30

what elements do Type 1a supernova produce and disperse

Answer 30

Fe-peak elements from Ca to Zn

Question 31

What is a standard candle?

Answer 31

a source that has a known consistent luminosity. since the luminosity. For distances which are too large to measure using parallax, astronomers use ‘standard candles’. Light sources which are further away appear fainter because the light is spread out over a greater area. If we know how luminous a source really is, then we can estimate its distance from how bright it
appears from Earth

Question 32

what is an example of a standard candle?

Answer 32

Type 1a supernovae

Question 33

What solar mass is required to fuse Ne in the core? What are the products?

Answer 33

> 10 solar masses. The products aof Ne fusion is Mg and O.

Question 34

Whta is the product of Oxygen burning in massive stars? what do the products later produce?

Answer 34

O -> Si, S and P
Si -> Ni and (primarily) Fe

Question 35

What happens to the duration of successive nuclear fusion reactions in stars?

Answer 35

The duration of each nuclear reaction decreases dramatically, to where Si burning -> Ni and Fe only lasts half a day in a 25 solar mass star

Question 36

Why does heavy element fusion end with Fe?

Answer 36

nuclear reactions using Fe do not produce energy because Fe has a tightly bound nucleus.

Question 37

what will the composition of a supergiant star be after fusion has finished?

Answer 37

A dense core of Fe with multiple concentric layers surrounding the core.

Question 38

what happens in a core collapse supernova (ccSN)?

Answer 38

a massive star with Fe/Ni core no longer supports fusion, supported mainly by electron degeneracy pressure. once reaching Chandrasekhar mass ~500km, the core compresses into neutrons and releases a neutron burst. The neutron degenracy pressure causes infalling material to bounce and form an outward moving shock front, blasting the material away in a massive ccSN, leaving only a degenerate remnant.

Question 39

in a ccSN, once the outer layers of the star are expelled in the explosion, what are the two potential remains of the cores?

Answer 39

progenitor stars <20solar masses = neutron star.
progenitor stars >20solar masses = black hole

Question 40

what types of ccSN can you get?

Answer 40

most ccSN are type II, which form directly from massive stars but you can also get type IIb and type IIc, where the progenitor stars of SNIb and SNIc have lost most of their H layer or both the H and He shells respectively from stellar winds or mass loss to a companion star.

Question 41

in what 3 ways do ccSN play an important role in producing the elemental fingerprint of the Universe (and solar system)

Answer 41

1. they eject the previous products of stellar nucleosynthesis (H to Si burning) releasing C, O, Mg etc.
2. the extreme T and P from supernova shockwave causes additional elemental production (explosive nucleosynthesis). most Fe produced from explosion rather than being from core.
3. produces cosmic rays which produces lighter elements through nuclear fission, Li, Be and Boron

Question 42

what is explosive nucleosynthesis?

Answer 42

production of lots of heavy elements e.g. Fe from extreme T and P of ccSN.

Question 43

what role do cosmic rays play in nucleosynthesis

Answer 43

cosmic rays produce lighter elements, Li, Be and B, through nuclear fission

Question 44

Rank neutron stars, black holes and white dwarfs from highest to lowest density
-> these are the 3 densest objects in the solar system

Answer 44

Densest
Black holes
Neutron stars
White dwarfs
Least dense

Question 45

what was the first evidence for the existence of neutron stars?

Answer 45

their pulsars/emissions of directed beams of electromagnetic EM radiation

Pulsar = pulsating radio source (pulsa… r)

Question 46

what are neutron stars important for in nucleosynthesis

Answer 46

building elements heavier than Fe by neutron addition via the r-process

Question 47

what forces are neutron stars supported by?

Answer 47

a combination of neutron degeneracy pressure and repulsion by the strong nuclear force. electrons and protons combine to form neutrons which run out of room to move and prevent further collapse. the neutrons typically glue together but at these very short distances they repel to create some outward force.

Question 48

what forces are black holes supported by?

Answer 48

none - if the remnant core of a ccSN is heavier than 2.2 solar masses, neutron degeneracy pressure and strong nuclear repulsion no longer suffice to prevent gravitational collapse. the neutron star further collapses into a black hole.

Question 49

what is the boundary of the region from which no escape is possible called?

Answer 49

the event horizon

Question 50

how is the presence of black holes inferred

Answer 50

1. formation of quasars as hot accretion disks around black holes
2. disruption of stars that pass too close
3. gravitational lensing of EM radiation.