Astrophysics Part 2

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Kepler’s Third Law
Square of the period of any planet is proportional to the cube of the semi major axis of its orbit
Stellar Parallax
Created by orbital position. Max shift is when the earth is at opposite sides of the sun. Once angle found, distance is calculated via trigonometry.
Coolest part of the sun. Visible surface that emits all visible light.
Low mass star
< 8 solar masses
High mass star
> 8 solar masses
Low Mass Main sequence
Fusion of H begun, low mass spends 99% of lifetime here
Red giant
Inert He core shrinks down and then the star puffs up.
Helium Flash
He begins to fuse in red giant due to a temperature rise.
Asymptotic red giant
Occurs after helium flash. Carbon core.
Planetary Nebula
Material of star thrown off by winds, leaving a carbon core.
White dwarf
Dense and degenerate. No more fusion.
1.4 Solar Masses
White dwarf mass limit
Degeneracy pressure
When protons, neutrons and electrons are packed in as tightly as possible, exerting a quantum force
Pre-star formation. Interstella medium collapses down.
High Mass main sequence
Massive and blue
Post main sequence high mass star
After H core of a high mass star exhausted, core shrinks and heats
Red supergiant
Hydrogen shell burning causing high mass star to expand
Beginning of He fusion in high mass star
T ~ 10^8K
Blue supergiant
Core has expanded after He fusion begins, slowing the fusion rate in a high mass star
Second Red giant phase
After He core exhausted, core shrinks are C fusion begins in core.
Iron core
Point at which star is doomed. Reactions from here onwards are endothermic
Core collapse supernova
Neutrons are packed so tight that they implode releasing heaps of energy.
Fusion of heavy elements
Type 2 supernovae powers…
Type 2 supernova
Core collapse supernova
Neutron core
Left by type 2 supernova
Neutron star
M < 3 Solar masses. Detected via pulse radiation or binary system.
Black hole
M > 3 Solar Masses
Pulse radiation
Use to detect neutron stars using their fast rotation due to conservation of angular momentum.
Neutron star spinning and releasing radio beams at a constant period
Neutron star accretion disk.
Sends of x-rays after getting hot due to friction of matter from a binary star being pulled into a neutron star.
Neutron degeneracy pressure limit
3 solar masses
Single point of mass as neutron star collapses into a black hole
Schwarzchild Radius
where escape velocity is speed of light
Bright x-rays
What we look for when detecting black holes.
Binary not transferring mass
Binary with one roche lobe filled
Contact binary
Both roche lobes filled
Cataclysmic variable
White dwarf accreting off donor star via roche lobe overflow
Dwarf nova
Sudden large increase in brightness, then returns white dwarf to original state.
Direct imaging
Used in exceptional circumstances. Needs a large planet with wide separation from parent star.
Only useful for nearby stars as position shifts are very small and accuracy is important. Measures position of star and looks for wobbles
Wobbles small
Downside of astrometry
Radial velocity
Most sensitive to high mass planets close to the star
Inclination needed
Downside of radial velocity
Sensitive to smaller mass planets close to star. Dimming of light tells us about the planet.
area of star blocked / area of star
Fractional change in flux during transit given by:
Gravitational lensing
Sensitive to all masses, only large distances. Rare events.
Hot jupiter
Mostly H and He, close to parent star, larger, short orbital period and low density.
Rocky planets
More numerous than gas giants.
Temperature of planet
Flux at surface x cross-sectional area
Earth’s biosignatures
Methane, liquid water and ozone
Spectrum of planet’s light
Where to search for signs of life
Small planet, with low mass, rocky and close to star
Large, high mass, icy/gaseous and far out
Rocky bodies found in asteroid belt
Ice and rock, found in Kuiper belt
100,000 ly
Length of galaxy
Galactic bulge
Centre of the galaxy
Large region of low density around galaxy
Thin with spiral arms embedded within
Detection of atomic H
Atoms have different spins. When they have slightly more energy, they drop an energy state and release a photon
Molecular hydrogen
Doesn’t have strong radiative transitions, hard to detect so we use other molecules to trace it.
H2 regions
Produced by young stars, looks like red glow. Young stars produce UV light that excites atoms.
Interstellar dust
Heated to high temperatures via starlight which is absorbed a reemitted in the IR
Blue light
Dust scatters this more easily
Scatters are absorbs
What does dust do to light?
Interstellar extinction
Light gets reddened and dimmed as light passes through a cloud.
The difference in colour we observed to that expected for object, called colour excess
observed – intrinsic
colour excess given by…
Synchrotron Radiation
Non-thermal. High energy electrons are accelerated and spiral in magnetic fields causing this.
Jets from blackholes and supernova remnants
Most common source of synchrotron radiation due to very strong magnetic fields
Thermal radia emission
Also known as Free-free emission. From free electrons with free protons in a hot ionised gas, where the proton gets accelerated by electron and releases radiation.
Signals can’t travel faster than the speed of light
What tells us that the size of Sgr A can’t be more than a light hour in size.
Keplerin rotation curve
Mass concentrated in centre, with the rotation curve declining as distance increases
Observed rotation curve of the milky way
Speed stays roughly flat as distance increases after a big jump from zero.
Dark matter
Why the observed rotation curve for the solar system is mostly flat due to not uniform density.
Baryonic dark matter
Massive compact halo objects are a type of dark matter. Dead stars and brown dwarfs which are hard to see.
Non-baryonic dark matter
WIMPS – have mass and do not react in a way giving off radiation.
Star ags star cycle
Recycles gas from old stars into new stars. Each generation has heavier elements that previous stars. ISM steadily enriched
Hot bubbles
Ionsied gas. Caused by supernovae. Atomic hydrogen forms as gas cools and then molecular clouds.
star formation
after the molecular cloud stage in the star-gas-star cycle
nuclear fusion/heavy element formation
After star formation in star-gas-star cycle
Returning gas
Supernova and stellar winds do this after the nuclear fusion stage of the star-gas-star cycle
spiral arms
Where do new stars form in our galaxy?
Categories: Astrophysics