Astrophysics for NERDSSS hahahahaha ur mom L+Ratio (goodluck on ur exam pookie bear) Flashcards
what is the sun? the male child of a dad
What is a black body radiator?
A perfect emitter and absorber of all possible wavelengths of radiation.
(Stars are approximated as black bodies.)
What is Stefan’s Law?
The power output of a black body radiator is directly proportional to its surface area and its (absolute temperature)^4
(on data sheet)
What is Wein’s Displacement Law?
The peak wavelength of emitted radiation is inversely proportional to the absolute temperature of the object.
The peak wavelength of light is released at maximum intensity.
(on data sheet = 0.0029 meters Kelvin)
Black Body Curves?
Following Stefan’s Law, as the intensity of the light increases, so does it’s temperature.
Following Wein’s Law, as the temperature increases, so does the peak frequency.
How does intensity relate to power?
Intensity in inversely proportional to the distance between star and observer.
I = P / 4πd^2 (NOT on data sheet)
How can stars be classified and a proof of procedure?
Into different spectral classes based on the strength of absorption lines, which depend on the temperature of the star because the energy of the particles which make up the star is dependant on its temperature.
Hydrogen Balmer lines are found in the spectra of type O, B and (strongest is) A stars. They are caused by the excitation of Hydrogen atoms from the n = 2 state to higher/ lower energy levels.
If the temperature of a star is too high, the hydrogen atoms may be excited to higher levels or the electrons may be ionised. If the temperature is too low, the hydrogen atoms may not become ionised. In both cases Hydrogen Balmer lines will not be present.
Spectral Class - Colour - Temperature range (K) - Prominent Absorption Lines
O - Blue - 25,000-50,000 - He+, He, H
B - Blue - 11,000-25,000 - He, H
A - Blue/White - 7,500-11,000 - H, ionised metals
F - White - 6,000-7,500 - Ionised metals
G - Yellow/ White - 5,000-6,000 - Ionised and neutral metals
K - Orange - 3,500-5,000 - Neutral metals
M - Red - <3,500 - Neutral atoms, titanium oxide
How are Hertzsprung-Russell Diagrams set up?
Absolute magnitude on the y-axis ((bottom)15 to (top) -15).
Temperature, logarithmically, on the x-axis.
(maybe a little diagram is needed)
How can the Sun be classified?
It is a main sequence star, in spectral class G with absolute magnitude 4.83
How will a Sun like star’s evolution trace onto a HR diagram?
- Once the Main Sequence star uses up all the hydrogen in its core, it will move up and to the right as it becomes a Red Giant. Red Giants are brighter and cooler.
- Once the Red Giant uses up all the helium in its core, it will eject its outer layers and will move down and to the left as it becomes a White Dwarf. A White Dwarf is hotter and dimmer.
(maybe a cutie pie little diagram is needed)
What are the stages of stellar evolution?
- Protostar: clouds of gas and dust (nebulae) which clump together under gravity. Under gravity and for the conservation of angular momentum, the clumps form a dense centre (a protostar).
- Main Sequence: gravity is balanced by the energy produced through fusion of Hydrogen into Helium so the star is stable and in equilibrium.
The greater the mass of the star, the shorter its main sequence period is (they sue fuel more quickly).
3.1 Red Giant (if < 3 solar masses): hydrogen runs out and the temperature of the core increases to begin fusing helium nuclei into heavier elements. The outer layers expand and cool.
4.11 White Dwarf (if <1.4 solar masses): when a Red Giant uses up all its fuel, fusion stops and the core contracts as gravity is now greater than the outwards force.
The outer layers are thrown off forming a planetary nebula around the very dense remaining core.
5.11 Black Dwarf: white dwarfs eventually cool to form black dwarfs.
3.2 Red Supergiant (if > 3 solar masses): a larger scale process occurs. They can fuse elements up to iron.
4.2 Supernova: When all fuel runs out, fusion stops and the core collapses inwards very suddenly and becomes rigid, the out layers then rebound off of the core, launching them out into space in a shockwave (can also cause gamma ray bursts).
As the shockwave passes through surrounding material, elements heavier than iron can then be fused.
Supernovae rapidly increase in absolute magnitude and may release around 10^44 J of energy (which the sun outputs in 10 billion years).
5.21 Neutron Star (if 1.4 < it < 3 solar masses): Gravity forces protons and electrons together to form neutrons.
Their density is about 10^17 kgm^-3.
Pulsars are spinning neutron stars that emit beams of radiation from the magnetic poles as they spin.
5.22 Black Holes (if > 3 solar masses): when the core of a giant star collapses, neutrons are unable to withstand gravity forcing them together.
The event horizon is the point at which the escape velocity is greater than the speed of light.
The Schwarzchild radius is the radius of the event horizon.
What is a binary system?
A system where two stars orbit a common centre of mass.
What are the type of supernova?
Type I: when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass.
Type II: the death of a high mass star after it runs out of fuel.
What are type 1a supernova and why are they used as standard candles?
Type 1a supernova are Type 1 supernova with a white dwarf (the companion star runs out of hydrogen and expands allowing the white dwarf to accumulate its mass). When the white dwarf star reaches a critical mass, fusion begins and as its mass continuous to increase, the white dwarf explodes.
All types of supernovae occur at the same critical temperature, so they have very similar peak absolute magnitudes, about -19.3, and produce very consistent light curves, maximum peak about 20 days after the steep incline in magnitude (although this is conventionally day 0).
Why do scientist believe there are supermassive black holes at the centre of all galaxies?
The stars and gas near the centre of galaxies appear to be orbiting very quickly, so there must be a supermassive object at the centre with a very strong gravitational field attracting them.
How do supermassive black holes form?
-The collapse of massive gas clouds while the galaxy was forming.
-A normal black hole accumulating huge amounts of matter.
-Several normal black holes merging together.
What does the brightness of Type 1a supernovae show?
The expansion of the universe is speeding up.
If it was slowing down, more distant objects would appear to be receding more quickly, since expansion was faster in the past.
And objects would also appear brighter than predicted as they would be closer than expected. However, Type 1a supernovae have been seen to be dimmer than they were expected to be, meaning they are more distant than Hubble’s law predicted. This suggests that the expansion of the universe is accelerating and it is actually older than Hubble’s law estimates.
What is dark energy?
Dark energy is described as having an overall repulsive effect throughout the whole universe.
Since gravity follows the inverse square law, it decreases with distance. But since dark energy remains constant all throughout the universe, it has a greater effect than gravity and is therefore thought to be causing the expansion speed to increase.
Dark energy is controversial because there is evidence for its existence but no one knows what it is or what is causing it.
How does the doppler effect provide evidence for an expanding universe?
The Doppler effect causes the line spectra of distant objects to be shifted either towards the blue end of the visible spectrum when they move towards the Earth or towards the red end of the spectrum when they move away from the Earth.
The more distant the object, the greater its red-shift, suggesting that the universe is expanding.
Red shift formulae (on data sheet) were derived without taking relativistic accounts into effect.
The z value is also known as the redshift (so is positive for red shift and negative for blue shift).
What are spectroscopic binaries?
Binary star systems in which the stars are too close to be resolved.
They can be identified by using the doppler shifts of each star.
What are eclipsing binaries?
When the plane of orbit of the stars is in the line of sight from Earth to the system, meaning the stars cross in front of each other as they orbit.
They can be identified by their light curve characteristics. (a wittle diagram pwease) (die in a ditch).
What is Hubble’s Law?
Hubble’s Law states that a galaxy’s recessional velocity is directly proportional to its distance from the Earth.
Therefore, the universe is expanding from a common starting point.
What’s an estimate of the age of the universe?
v = Hd
1 / H = d / v
1 / H = t
(H needs to be converted into SI units (should be about 15 billion years))
What does the Big Bang Theory say and what is some evidence for it?
That the universe started with a huge explosion from a singularity that was infinitely small and infinitely hot.
When this happened , high energy radiation would have been produced. As the universe expanded and cooled, the radiation would have lost energy and been red shifted. Its remains are CMBR.
During the early stages of the Big Bang, nuclear fusion would have converted hydrogen nuclei into helium nuclei, before the universe cooled and nuclear fusion would have stopped. The fact that the relative abundance of H:He is 3:1 is more evidence for the Big Bang model of the universe.
What are quasars?
Quasars are active galactic nuclei (a supermassive black hole surrounded by a disc of matter which causes jets of radiation to be emitted from the poles as it falls into the black hole.
They are characterised as having the following features:
-large optical red shifts
-extremely powerful light outputs
-not much bigger than the sun in size
Quasars are thought to be some of the most distant measurable objects in the known universe.
The inverse square law shows that quasars can have the same energy output as several galaxies.
Quasars have much greater radio emissions than stars.
