Chapter 19 Stars Flashcards
Definition of a planet
An object in orbit around a star
- has mass large enough for gravity to give it a round shape
- has no fusion reactions (else a star)
- has cleared its orbit of most other objects
A dwarf planet hasn’t cleared its object of most other orbits, thus not a planet
Planetary satellites
Include both moons and man made satellites, anything that orbits a planet
Comets
Small irregular bodies made of ice dust and small pieces of rocks
All comets orbit the sun in highly eccentric orbits
Solar system
Contains the sun and all the objects that orbit ti
- planets, comets etc
Galaxies
A galaxy is collection of stars , interstellar dust and gas
A lot of these stars will have their own solar system too
Universe
Everything that existed
- em rsdistiln , energy, space time, stars matter everything
What is nebullae?
A collection of dust and gas (mostly hydrogen)
They form over many years as tiny gravational attraction between particles pull them together , forming vast regions of cloud
How does a nebullae become a protostar
As they get closer to Esch other gravational collapse accelerates
Eventually a denser region of Nebullae forms, and now it begins to heat up as gravational energy is transferred to kinetic and so the dense region begins tk heat up
This forms a protostar
When does a protostar become a star
Only if the temperatures in the core and pressures are high enough for fusion of hydrogen tk occur , if this point never reaches a star is never born . This is because for fusion tk occur, electrostatic forces of repulsion between hydrogen nuclei must be overcome such that they can get close enough to experience the string nuclear force , and fuse
As soon as hydrogen fuse, it enters the main sequence of a star
What happens when the star is in its main sequence
The gravational force of attraction acts tk compress the star
But this is countered by the RADIATION and GAS pressure due to fusion
As a result the star remains in a steady state as long as it is fusing
Why do bigger stars have a much lower time as main sequence than smalller stars?
Bigger stars mean much hotter cores, due to more gravational potential energy being converted to kinetic in the core
As a result the rate of fusion of hydrogen is massively faster, as higher temps and pressures are achieved , so their sequence is less
But smaller mass has lowe temps and pressures, so rwte of hydrogen fusion is less and its sustained over lomger periods of time
Again so why are higher mass stars more hotter cores?
Because more scope for more GPE to be converted to kE and thus temp in the core due to bigger mass
Remember what is the outward pressure (2 things)
Radiation AND gas pressure due to fusion
How do low mass stars become red giants
Once fusion of hydrogen in core is run out, there is no longer any outward pressure
- thus the gravational force is greater and acts to compress the star. The core begins to collapse and shrink
- as the core shrinks, this increases the pressure and temperature inside the core.
- as a result , conditions are right for fusion of hydrogen in the SHELL around the core.
- this creates some last outward pressure which causes the outward layers of star to expand outwards and move away from the core
- eventually they cool and form a red layer
The star is now a red giant
What happens once the hydrogen in the shells of the core run out too?
What is left
- By now most of the layers have drifted away as planetary NEBULAE
- And so there is no more radiation pressure, and so the gravational force will act to compress
What’s left behind is a what dwarf, which is extremely hot and no more fusion
But doesnt collapse immedialty yet
What’s stopping the white core from collapsing?
There is the pauli exclusion principle which stated two electrons can’t exist in the same energy state at the same time
- thus a certain pressure exists due to electrons squeezing closer to each other called ELECTRON DEGENERACY pressure .
- this counters the extreme collapse of the whit dwarf core due to gravity
However this only happens to a limit
What’s the Chandrasekhar limit?
The limit it can withstand grav Collapse is if the core is less than 1.44M of the sun
More than that, and it camf withdntajd the collapse
This is known as the Chandrasekhar limit
What are freaturee of the white dwarf?
- very dense
- mass similar to the sun (remember has tk be less than 1.44M if it was to even exist)
- no fusion left
- very hot surface temp due to photons from earlier life
Finally a low mass star white dwarf ends as what?
Cools to form a black star and that’s it
HIGH MASS STAR
What happens after main sequence for a high mass star?
1) again the outward radiation and gas pressure drops, and now the gravational force exceeds the outward pressure
2) this causes the star to collapse, along with the core
3) core gets hot enough now , but because much more mass, the core gets hot enough for fusion of HEAVIER ELEMENTS to fuse in the core (as we,k as hydrogen in shells)
4) this causes immense radiation pressure and caused the outer layers to expand and cool snd become red again
5) as the most inside part of core is the hottest, this is the one that does fusion the most. It fuses until it becomes iron, as after this fusion csn’t happen anymore as iron the most stable isotope . This forms a core of shells of heavier elements, with iron in the middle, and les heavy as you come out
6) however eventually the whole core finally fused to become an iron core.
What happens when fusion stops and thr core is iron for a high mass star?
Now no fusion = no outward pressure
- the gravational force is much stronger and pulls in the layers
- these bounce of the layers as an implosion leading tk a shockwave that ejects all the layers into space
This is a SUPERNOVA
After the supernova, what’s left of the core for a super red giant
What are rhe two possibilotedfirst
It can either become a neutron star, or a black hole
How does the core after supernova of a super red giant become a neutron star
- if the mass of the core exceeds the chanreskehar limit, then thr grav force will counter the electron degeneracy pressure
- thus the collapse continues, and you form a neutron star
As it collapsed a lot, the neutron star is a very small volume but HIGH DENSITY object
Features of a neutron star
Mad elf entirely neutrons
- very small volume
- very high density
# therefore very high mass
