Life and Death of Stars Flashcards
What is the first fusion reaction within a star once it has become main sequence?
Hydrogen/proton 1H burning to produce Helium 4H
Why does nuclear fusion begin in stars?
gravity makes protostars contract, and when the density and pressure becomes high enough, fusion can begin and the stars begin to produce energy
how do stars achieve hydrostatic equilibrium
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.
why is the main sequence a band not a line?
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.
since the outer layers of a star are too cool to support fusion, what happens to the hydrogen eventually?
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.
when does a star complete its life cycle on the main sequence
when there is an inert core of He surrounded by a H mantle which cannot be fused due to low temperatures
why are 90% of stars main sequence stars
because the average star spends 90% of its life fusing hydrogen on the main sequence.
what determines how long a star spends on the main sequence
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.
how do red dwarf stars die?
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
what are stellar winds
fast flowing streams of particles emitted from stars
how do giants and supergiant stars form
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.
what solar mass is required to ignite a stars helium core
0.4solar masses
what does helium burning produce
ashes of carbon and oxygen
what solar mass is required to ignite the carbon and oxygen ashes
greater than ~8 solar masses. stars between 0.4 and 8 solar masses cannot ignite.
in stars with solar masses 0.4-8, what happens once all helium is burnt? what branch of the H-R do they join?
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.
what is s process nucleosynthesis? what type of stars do this
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.
what is the s in s-process denoting
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.
what do the released neutrons from s-process nucleosynthesis do?
they react with Fe and other seed nuclei which formed from previous generations of stars, to produce even heavier elements.
why are heavier elements formed much more efficiently in low mass (0.4-8solar mass) stars compared to higher-mass stars?
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
what solar mass is required to ignite the C/O core? what products are produced?
8-10 solar masses required for C/O burning. to produce Ne and Mg.
when do stars begin to shed material?
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.
how are planetary nebula formed?
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.
what do the remains of low and medium sized stars turn into?
white dwarfs
do white dwarfs host fusion reactions?
no - they have no energy source hence the initially hot white dwarfs cool down with time
what is electron degeneracy pressure in white dwarfs? what role does it play?
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.
why do white dwarfs have such enormous masses?
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
what is the Chandrasekhar limit?
the maximum mass of a white dwarf (~1.44 solar masses)
what happens if white dwarfs continue accreting mass from its companion e.g. binary star system beyond the Chandrasekhar limit (1.44 solar masses)?
the star is no longer supported by the electron degeneracy pressure and it explodes as a type Ia supernova (SNIa).
what are supernova classified using
the differences in their light emission curves as measured by spectroscopy
what elements do Type 1a supernova produce and disperse
Fe-peak elements from Ca to Zn
What is a standard candle?
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
what is an example of a standard candle?
Type 1a supernovae
What solar mass is required to fuse Ne in the core? What are the products?
> 10 solar masses. The products aof Ne fusion is Mg and O.
Whta is the product of Oxygen burning in massive stars? what do the products later produce?
O -> Si, S and P
Si -> Ni and (primarily) Fe
What happens to the duration of successive nuclear fusion reactions in stars?
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
Why does heavy element fusion end with Fe?
nuclear reactions using Fe do not produce energy because Fe has a tightly bound nucleus.
what will the composition of a supergiant star be after fusion has finished?
A dense core of Fe with multiple concentric layers surrounding the core.
what happens in a core collapse supernova (ccSN)?
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.
in a ccSN, once the outer layers of the star are expelled in the explosion, what are the two potential remains of the cores?
progenitor stars <20solar masses = neutron star.
progenitor stars >20solar masses = black hole
what types of ccSN can you get?
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.
in what 3 ways do ccSN play an important role in producing the elemental fingerprint of the Universe (and solar system)
- they eject the previous products of stellar nucleosynthesis (H to Si burning) releasing C, O, Mg etc.
- the extreme T and P from supernova shockwave causes additional elemental production (explosive nucleosynthesis). most Fe produced from explosion rather than being from core.
- produces cosmic rays which produces lighter elements through nuclear fission, Li, Be and Boron
what is explosive nucleosynthesis?
production of lots of heavy elements e.g. Fe from extreme T and P of ccSN.
what role do cosmic rays play in nucleosynthesis
cosmic rays produce lighter elements, Li, Be and B, through nuclear fission
Rank neutron stars, black holes and white dwarfs from highest to lowest density
-> these are the 3 densest objects in the solar system
Densest
Black holes
Neutron stars
White dwarfs
Least dense
what was the first evidence for the existence of neutron stars?
their pulsars/emissions of directed beams of electromagnetic EM radiation
Pulsar = pulsating radio source (pulsa… r)
what are neutron stars important for in nucleosynthesis
building elements heavier than Fe by neutron addition via the r-process
what forces are neutron stars supported by?
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.
what forces are black holes supported by?
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.
what is the boundary of the region from which no escape is possible called?
the event horizon
how is the presence of black holes inferred
- formation of quasars as hot accretion disks around black holes
- disruption of stars that pass too close
- gravitational lensing of EM radiation.
