Chapter 5.5 - Astrophysics and Cosmology Flashcards
Nuclear Fusion
The process of two nuclei joining together and releasing energy from a change in binding energy
Planet
Large bodies that move in circular or elliptical orbits around a star
Planetary Satellite
A smaller body than a planet that orbits the planet (e.g the Moon)
Comets and where they come from
Rocky ice balls that travel in highly elliptical orbits around the Sun. Come from the Oort cloud
Solar System
A star (or a binary star pair) orbited by one or more planets
Galaxy
A cluster of many millions of stars
The Universe
The space in which everything exists
Gravitational Collapse
The inward movement of material in a star due to the gravitational force caused by its own mass
When gravitational collapse happens
- When a star is formed the cloud of gas undergoes gravitational collapse
- In a mature star when the internal gas and radiation pressure can no longer support the stars own mass
Radiation Pressure
An outwards pressure caused by the momentum of photons released in fusion reactions
Gas Pressure
An outwards pressure caused by the movement of the high energy gas particles inside a star
Main Sequence Star
A star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core. The star is stable since the gas pressure and radiation pressure counteract the gravitational force
Red Giant
A star in the later stages of its life that has nearly exhausted its hydrogen supply and is now fusing helium. It is larger than a main sequence star and the outer layers are cooler, giving it its red colour
White Dwarf
The end product of the life cycle of a low mass star. It is very dense but does not undergo any fusion and will slowly cool down. It does emit light as photons from past fusion reactions leak away
Planetary Nebula
An expanding shell of ionised hydrogen and helium ejected from a red giant star at the end of its life
Electron Degeneracy Pressure
An outwards acting pressure that prevents stars of mass beneath the Chandrasekhar limit from collapsing further. Since two electrons cannot occupy the same states in an energy level of an atom, when electrons are being pulled into the star due to gravity they will reach a point where they cannot be added to the volume of the star. This has the effect of exerting an outwards force.
Chandrasekhar limit
1.4 times the mass of our Sun.
The mass at which a Star will collapse further than a white dwarf and become either a neutron star or a black hole
Red Super Giant
A Red Giant that has a mass much higher than that of our Sun
Supernova
An explosion produced when the core of a red super giant collapses
Neutron Star
The remnants of the core of a red super giant after it has undergone a supernova explosion. It is very dense and composed mainly of neutrons
Black Hole
The core of a massive star that has collapsed almost to a point. They are incredibly dense and their gravitational field is so strong that, past the event horizon, not even light can escape
Hertzsprung-Russel diagram
A luminosity-temperature graph
Luminosity
The total energy that a star emits per second
How stars form
Dust and gas come together through gravitational attraction. The work done on moving these particles increases their kinetic energy resulting in an increase in temperature. This large core is called a protostar. The gravitational field of the protostar will attract more matter until the temperature and the pressure in the core is enough for the hydrogen to fuse and create helium. The gravitational pressure will become balanced with the gas pressure and radiation pressure from the fusion. It is now a main sequence star.
Overall fusion reaction
4 proton goes to
- helium-4 (2 proton + 2 neutron)
- 2 positrons
- 2 neutrinos
- gamma rays
Lifecycle of a star of mass beneath the Chandrasekhar limit
Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand and cool as the star becomes a red giant. Core collapses further until helium fuses into carbon and oxygen preventing the core from collapsing further. Once all the helium is fused the core collapses further and ejects its outer layers which form a planetary nebula. The remaining core is a white dwarf which is stable as gravitational forces are counteracted by the electron degeneracy pressure
Lifecycle of a star of mass greater than the Chandrasekhar limit (up to supernova)
Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand as the star becomes a red super giant. As the core collapses, heavier elements are created by fusion. At each stable fusion stage, the further collapse of the core is prevented by the electron degeneracy pressure and the radiation pressure. Once an iron core has built up the fusion will stop and the core will undergo further gravitational collapse. The immense gravitational forces force protons and electrons to combine to form neutrons, which releases an incredible amount of energy causing a supernova as the outer shell is blown off. During a supernova heavier elements the iron can be formed when the nuclei fuse with neutrons.
What happens after a supernova
Depending on the mass remaining in the core a neutron star may be formed. These are very small and have very high density and are composed mostly of neutrons. The magnetic field of the neutron star can cause the star to emit vast amounts of high energy radiation from its poles. This is called a pulsar.
If the neutron star is massive enough the pressure can become so large that the neutron star would collapse to a point and become a black hole
Continuous spectrum
A spectrum that contains all wavelengths over a wide range
Energy Level
Discrete energies that electrons can have when occupying specific orbits.
Emission line spectrum
The spectrum of frequencies emitted due to electron transitions from a higher energy level to a lower one
Absorption line spectrum
The pattern of dark lines that would appear on a continuous spectrum when the spectrum is shone through a medium that would absorb some frequencies of light
Energy of energy levels
0 at infinity]
-13.6eV at the lowest energy level
varying between these but the energy level is always negative
Ground State
The lowest energy level
Emission spectrum source and how they appear
They come from hot gasses when an excited electron moves to a lower energy level and emits a photon.
How the frequency of an emitted photon is calculated
delta E = hf
where delta E is the change in energy resulting from an electron changing energy level
How are emission spectra and absorption spectra related
The same wavelengths are absorbed as are emitted
How many photons does it take to change the energy level of an electron
Always just one
How elements in stars can be identified
By looking at the spectrum of light from the star. It is assumed that the star emits a continuous spectrum and thus any dark lines in the spectrum will be from that wavelength being absorbed by the elements in the stars atmosphere. Therefore by matching the dark lines to known samples of all the elements we can determine the makeup of the stars atmosphere
How come the absorption lines coming from a star aren’t filled in by those same elements emitting light of that wavelength
The gasses in the atmosphere are not hot enough to produce emission spectra
How to determine the wavelength of light from a star
Using a transmission diffraction grating to diffract the light and then using nλ = dsinθ
How to determine the number of maxima that will be produced
nλ = dsinθ
sinθ <= 1
nλ = d
n = d / λ
Wiens displacement law
λmax ∝ 1/T
where λmax is the most common wavelength emitted by a star and T is the peak surface temperature
How a graph of intensity against wavelength looks like for stars of different temperatures
Peak at λmax. Much higher area under the stars of higher temperature
Luminosity
The total energy emitted per second
Stefans law
L=4σπr^2T^4 where L is luminosity σ is stefans constant T is temperature r i s radius
Why does a star get brighter when it becomes a red giant even though it becomes cooler
Because its radius and thus its surface area increases. And luminosity is proportional to surface area
λmax = kT. what is the value of k
2.89*10^-3
How the measured intensity on Earth can be used to discern information about a star
I = L/A where I is intensity L is luminosity A is area the area in this case would be the area of a sphere with a radius equal to the distance from the star to earth since the energy has spread out over that distance
AU
Astronomical Unit
The mean distance between the Earth and the Sun
Light Year
The distance travelled by light in a vacuum in one year
Parsec
A unit of distance that gives a parallax angle of 1 second of arc using the astronomical unit as the base of the right angled triangle
Stellar parallax
The shifting in position of a star viewed against a background of distant stars when viewed from different positions, for instance when viewed from Earth at different points of Earths orbit
Arc second
1/3600 of a degree
How to calculate a distance using parallax
Measure the difference in angle between the two extremes of earths orbit. The parallax angle will be half of this. Construct a right angled triangle and do some trig ey
Simple relation between parallax angle and distance
p=1/d
where p is the angle in arc seconds and d is the distance in parsecs
This works because tan(x) = x at very small x and because parsecs are defined in terms of AU and arc seconds (this only works when a distance of 1AU has been used)
Doppler effect
The change in wavelength caused by the relative motion between the wave source and the observer
Red Shift
The apparent increased in wavelength of electromagnetic radiation observed when the source is moving away from the observer
Hubbles law
The recessional speed of a galaxy is directly proportional to its distance from us
Doppler equation
Relative change in frequency = Relative change in wavelength = v/c
How the change in wavelength from a star can be calculated
By looking at the absorption spectrum of the star and seeing how much it has shifted
Hubbles law equation
v=Hd
Inaccuracies with hubbles equation
Galaxies may be subject to local gravitational effects
Units of hubbles constant
s^-1 or kms^-1*Mpc^-1
Value of hubbles constant
75km/sMpc
How to estimate the age of the universe
1/H
Cosmic microwave background radiation
Microwave radiation received from all over the sky originating from after the Big Bang. As the universe has expanded the wavelength of the radiation has increased to just a faint microwave radiation with a peak wavelength corresponding to 2.7K
The Big Bang Theory
The universe was created from a single point where all the mass and energy was situated. Since then the universe has expanded from a small dense point to a large and comparatively cool universe. Time and space were both created at the instance of the Big Bang
Cosmological Principle
On a large scale the universe is isotropic and homogeneous and the laws of physics are universal
Isotropic
The same in all directions
Homogeneous
Of uniform density when considering a large enough volume
Experimental evidence for the Big Bang
The microwave background at a temperature of 2.7K
Experimental evidence for the expanding universe
The red shift and that galaxies further away are more redshifted than those that are closer
Temperature of the universe at the big bang
Order of 10^22K
Timeline of the universe
Universe is created and is incredibly small and dense and it begins to rapidly expand. Matter and Antimatter are formed in the form of quarks, leptons and photons. There is slightly more matter than antimatter and as they annihilate they leave a universe dominated by particles not antiparticles. Soon the universe cools enough so that quarks can come together and form protons and neutrons. Many high energy photons are released from matter-antimatter annihilation The temperature cools enough that helium and lithium nuclei can form and then electrons gradually become attached to the present protons. This is called decoupling as the universe becomes transparent and photons can move freely. This is where the photons that are now the CMB originate. Over billions of years the small irregularities that existed at the beginning of the universe become stars, planets and galaxies.
What the big bang started
space-time (a 4 dimensional property that combines the three dimensions of space and the fourth dimension of time)
Dark Matter
Matter which cannot be seen and does not emit or absorb electromagnetic radiation. It is detected by its gravitational effects
Dark Energy
An energy that opposes the attractive force of gravity and exerts negative pressure, causing the expansion of the universe to accelerate
Closed universe
The density of the universe is high enough that the gravitational force pulls the universe back in ending in a ‘Big Crunch’ where all matter and is pulled into a single point
Flat Universe
The density of the universe is enough to stop the expansion from accelerating but not enough to cause the universe to ‘deflate’. The size of the universe tends towards a certain value and effectively stops growing
Open Universe
The density of the universe is not enough for gravitational forces to prevent the universe from expanding. The galaxies will move further and further apart until gravity between them is negligible and they will then have settled at a constant speed
How we can observe dark matter
The centripetal acceleration of spinning galaxies is much higher than would be expected from the mass that we can see. There must be more mass that we cannot see that is causing this
Potential Dark Matter Candidates (4)
- WIMP
- MACHO
- Axion
- Neutrino
WIMP
Weakly Interacting Massive Particle
A hypothetical particle that may have been created near the beginning of the universe that has a high mass but does not interact very much with other particles
MACHO
Massive Astrophysical Compact Halo Objecs
Ordinary matter found in concentrated areas in the dark halos that surround galaxies. These could be brown dwarfs, neutron stars or black holes
Axion
Theoretical particle that are to do with the strong force. They have the right mass and would decay into photons.
Neutrino (Dark Matter Candidate)
A fourth neutrino that is heavier and less interacting than the other ones or simply an absolute ton of regular neutrinos.
Compostion of the universe
Dark Energy - 68%
Dark Matter - 27%
Ordinary Matter - 5%
