Astrophysics And Cosmology Flashcards
Define a planet
A body with mass sufficient for its own gravity to force it to take a spherical shape, where no nuclear fusion occurs, and the body has cleared its orbit of most other objects
Define a dwarf planet
A planet where the orbit has not been cleared of other objects
Define a planetary satellite
A body in orbit around a planet
Define an asteroid
A body which is too small and uneven in shape to be a planet, with a near circular orbit around the sun
Define a comet
A small, irregularly sized ball of rock, dust, and ice. It orbits the sun in eccentric elliptical orbits
Define a solar system
A system containing stars and orbiting bodies like planets
Define a galaxy
A collection of stars and interstellar dust, and gas. Each galaxy contains around 100 billion stars
How do stars form?
- over millions of years, the gravitational attraction between dust and gas particles pulls them together to form clouds called nebulae
- as they come closer together, the gravitational collapse accelerates, and some regions become denser and pull in more dust and gas
- the gravitational potential energy of the particles is converted to kinetic energy, and as the moving particles collide the temperature increases. This results in a sphere of very hot, dense dust and gas called a protostar
- for a star to form, the temperature and pressure must be high enough for hydrogen gas nuclei in the protostar to overcome the electrostatic forces of repulsion and undergo nuclear fusion
- this nuclear fusion turns hydrogen nuclei into helium nuclei, and gives off a lot of heat, energy and light, leading to the emergence of a star
State and explain how the main phase of a star varies with its size
Larger stars are hotter, and so undergo fusion faster, using up available hydrogen nuclei more quickly. This means they have a shorter main phase than smaller stars
What happens to keep a star stable during its main phase?
During the main phase of a star the gravitational force of particles is balanced by radiation pressure from photons emitted in fusion and gas pressure from nuclei in the core
What does solar mass (M) refer to?
The mass of our sun’s core
Describe and explain the evolution of a low mass star
- once the hydrogen supplies are low, the gravitational forces overcome the radiation and gas pressures, so the star begins to collapse inwards. It evolves into a red giant
- the core of the red giant is too cool for helium to fuse, but the pressure in the outer shell is great enough for fusion to occur there
- as helium nuclei run low, the red giant evolves into a white dwarf. The outer shells begin to drift off into space as a planetary nebula, and the core remains as a very dense white dwarf
- a white dwarf star has no remaining fuel for nuclear fusion and so becomes cooler and cooler until eventually it becomes too faint to detect. It is now referred to as a black dwarf
Compare the characteristics of a low mass star and a massive star
- low mass stars have a mass between 0.5M and 10M, whereas massive stars have a mass exceeding 10M
- low mass stars have a smaller, cooler core
- low mass stars remain in the main sequence for longer
- low mass stars eventually evolve into red giants, then into white dwarfs, and finally black dwarfs
- massive stars evolve into red supergiants, then either a neutron star or a black hole depending on mass of the remaining core
Describe and explain the evolution of a massive star
- as hydrogen supplies deplete, radiation pressure from nuclear fusion reduces and the gravitational forces cause the core to collapse slightly
- as the core collapses, temperature and kinetic energy of helium nuclei increase sufficiently to overcome electrostatic forces of repulsion and undergo fusion
- this causes the star to expand, leading to the formation of a red supergiant
- the core fuses increasingly heavy elements to form layers, until an iron core is formed
- iron cannot fuse, so once this core is formed the star becomes unstable. Gravitational forces exceed the forces of radiation and gas pressure and the outer layers are forced inwards. They bounce of the iron core and explode outwards in type 2 supernova, in which elements heavier than iron are formed
- this leaves behind an inert iron core
- if the remaining core mass is greater than 1.44M, protons and electrons combine to form neutrons. This produces an extremely small, dense neutron star
- if the remaining core mass is greater than 3M, the gravitational forces are so strong that the escape velocity of the core is greater than the speed of light. This is a black hole, which even photons cannot escape
State the characteristics and explain the stability of a white dwarf
- a white dwarf has a temperature of around 30000K, and no fusion occurs
- photons which were produced earlier in the evolution leak out, dissipating heat
- as the star collapses, electron degeneracy pressure prevents the core from collapsing
- as long as the core mass is below 1.44M, then the white dwarf star is stable. This is the Chandrasekhar limit
State the characteristics of electron energy levels
- electrons bound to an atom can only exist in certain discrete energy levels
- the electrons cannot have an energy value that is between two levels
- each element has its own set of energy levels
- all energy level values are negative, with the ground state being the most negative
- an electron that is completely free from an atom has energy equal to 0
- the negative sign is used to represent the energy required to be inputted to remove the electron from the atom
Explain how electrons can move between energy levels
When an electron moves from a lower energy state to a higher energy state, it is ‘excited’. This requires the input of external energy, e.g heat, absorption of a photon. When an electron is de-excited, it moves towards the ground state. It releases energy in the form of a photon with a specific wavelength. Electrons can only be excited to the other discrete energy levels for that element
What are emission line spectra?
Each element produces a unique emission line spectrum because of the unique set of energy levels associated with its electrons. It appears as a series of coloured lines on a black background
What are continuous line spectra?
When all visible wavelengths of light are present. They are produced by atoms of solid heated metals
What are absorption line spectra?
A series of dark spectral lines against the background of the continuous spectrum, with each line corresponding to a wavelength of light used to excite atoms of that element. The dark lines are at the same wavelength as the coloured lines produced when the atoms are de-excited
Explain why emission line spectra are different for each element
The energy of the photon released when an electron is de-excited is equal to the difference between the initial energy level of the electron, and the final energy level of the photon. As each element has a unique set of discrete energy levels, the energy of photons of light produced will be different for each element. Since E = hc/λ, and both h and c are constants, we can see that the wavelengths of light must also be different for each element.
What is spectroscopy?
Spectroscopy is the technique used to identify elements based on the wavelengths of light emitted when atoms in a gas are excited. The characteristic emission line spectrum is produced - the spectral line is black, apart from emission line spectra at specific wavelengths
Give the two equations that can be used to find the energy of a photon
E = hf
E = hc/λ
What are diffraction gratings and how do they work?
Diffraction gratings are components with regularly spaced slits that can diffract light. Different colours of light have different wavelengths, and so will be diffracted at different angles.
Give the equation that can be used to determine the wavelength of light passing through a diffraction grating
dsinθ = nλ
d - diffraction slit separation
θ - angle of diffraction
n - order of maxima
Why do stars appear a certain colour?
The surface temperature of a star affects its colour. At any temperature above 0K, bodies emit electromagnetic radiation of varying wavelength and intensity. Stars can be modelled as idealised black bodies that emit radiation across a range of wavelengths, with a peak in intensity at a specific wavelength, corresponding to the colour of the star
State and explain Wien’s Law
Wien’s Law states that:
- the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.
Wien’s Law is used to relate the absolute temperature of the surface of a star with the peak wavelength of the electromagnetic radiation emitted by the star
What is the equation that comes from Wien’s Law?
λmax ∝ 1/T
So, λmaxT = Wein’s constant (2.9x10^-3mK)
λmax - the peak wavelength
T - absolute surface temperature of the body
State and explain Stefan’s Law
Stefan’s Law states that:
- for a black body, the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature
Stefan’s Law is used to relate the temperature of a star with its luminosity, L. The luminosity is the radiant power output of the star, and is also proportional to the surface area of the star
What is the equation that comes from Stefan’s Law?
L ∝ 4πr²T⁴
So, L = 4πr²T⁴σ
Where σ is Stefan’s constant (5.67x10^-8)
How could the radius of a star be determined?
If the colour, and hence peak wavelength of the star is known, then Wien’s Law can be used to calculate the absolute temperature of the star. If the luminosity is also known, then the radius of the star can be determined using Stefan’s Law.
What is 1 Astronomical Unit (AU)?
1AU = 1.5x10^11m - it is the average distance from the Earth to the sun, and is mostly used to express the distance of planets from the sun
What is 1 light year (ly)?
1 ly = speed of light x number of seconds in a year
= 3x10^8 x 3.16x10^7
= 9.5x10^15m - it is the distance light travels in one year, and is used to express the distances to stars and other galaxies
What is 1 parsec (pc)?
A parsec is defined as the distance at which a radius of 1AU subtends an angle of 1 arc second.
1pc = 3.1x10^16m
1 arc second = 1/3600th of a degree
What is stellar parallax?
Stellar parallax is the apparent shift in position of an object against a backdrop of distant objects (that don’t appear to move). It is accurate for distances up to 100pc. Beyond this point, the angles involved are so small they are hard to accurately measure
State the equation that uses parallax to calculate distance
d = 1/p
d - distance between observer and body
p - parallax angle
This relationship is only true where d is measured in parsecs and p is measured in arc seconds
State the cosmological principle
The universe is isotropic and homogenous, and the laws of physics are universal.
Isotropic means that the universe is the same in all directions to every observer, and it has no centre or edge
Homogenous means that matter is uniformly distributed - for a large volume of the universe the density is the same
What is the Doppler Effect?
The Doppler Effect is the apparent shift in wavelength occurring when the source of the waves is moving. If the source is moving towards the detector then the wavelength appears to decrease. If the source is moving away from the detector, the wavelength appears to increase
Explain how the Doppler Effect can be used to determine the relative speed of a star
In star light, the Doppler Effect shifts the position of spectral lines. We can determine the absorption spectra of hydrogen in the lab, and compare it to what is detected in the light from a star. We can then use the Doppler equation:
Δλ/λ = v/c
v - velocity of the star relative to Earth
λ - wavelength of the hydrogen spectral line on Earth
Δλ - change in wavelength of the hydrogen spectral line
State Hubble’s Law
The recessional velocity, v, of a galaxy is proportional to its distance from Earth
What is the equation that comes from Hubble’s Law?
V = H₀d
H₀ - Hubble constant (67.8kms^-1Mpc^-1)
What evidence is there to suggest the universe is expanding?
Hubble’s Law provides key evidence for the model that states that the universe is expanding. Almost all light from distant galaxies is red shifted, showing that the galaxies are moving away from Earth. This suggests that the fabric of space and time is expanding, and any point in the universe is moving away from any other point.
What does the Big Bang Theory state?
The Big Bang Theory states that all objects were initially contained in a singularity, which suddenly expanded outwards. The universe has not stopped expanding since then
What evidence is there to support the Big Bang Theory?
- Hubble’s Law shows that the universe is expanding, through red shift of light from distant galaxies
- the existence of microwave background radiation which is detected as a constant interference. Originally there were high energy gamma photons, but as the universe expanded, the wavelength of these photons was stretched into the microwave region
How can the age of the universe be estimated?
Hubble’s Law can be used to estimate the age of the universe.
t = 1/H₀, where H₀ is measured in seconds
= approximately 14 billion years
Outline the evolution of the universe
- Big Bang: time and space are created; the universe is a dense, hot singularity
- 10^-35s: universe expands rapidly. There is no matter, only high energy gamma photons and electromagnetic radiation
- 10^-6s: 1st fundamental particles gain mass
- 10^-3s: most of the mass is created through pair production. 1st hadrons come from quarks
- 1s: production of mass is halted
- 100s: protons and neutrons fuse to form deuterium, helium, lithium and beryllium nuclei, but nothing heavier. Rapid expansion continues. 25% of matter is helium nuclei
- 380 thousand years: now cool enough for the 1st atoms to form
- 30 million years: 1st stars form, fusion creates heavier elements
- 200 million years: our galaxy forms as gravitational forces pull together clouds of hydrogen and existing stars
- 9 billion years: solar system forms by a nebula from a supernova. This is followed by the formation of our sun, and then the Earth almost 1 billion years later
- 11 billion years: primitive life begins on Earth
- 13.7 billion years: 1st modern humans evolve
What is dark energy?
Dark energy is a hypothetical form which fills all of space and accelerates the expansion of the universe. It should make up 68% of the total energy in the universe, but so far experiments have been unable to find the form of the energy
What is dark matter?
Some observations suggest that mass is not concentrated in the centre of a galaxy, but instead is spread out. All of the observable mass in galaxies is concentrated in the centre, so there must be another type of matter we can’t see, called ‘dark matter’. It should make up 27% of the mass in the universe
State the order of planets in our solar system from closest to the sun to furthest away
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
What is the Schwarzschild radius?
The radius at which nothing can support a star against its own gravitation and the matter collapses towards a point of infinite density called a singularity
State the equation for escape velocity of a sphere of mass M and radius R, and rearrange it to give an equation for Schwarzschild radius
Vesc = √2GM/R
And since the escape velocity must be at least as great as the speed of light for a black hole to form
Rs = 2GM/c²
