5.4 Astrophysics And Cosmology Flashcards
what is a planetary satellite?
Moons, artificial satellite and anything else that’s orbiting planets.
What is a galaxy?
a collection of stars, dust, and gas. Each galaxy contains around 100 billion stars
what type of orbits do planets have?
circular
How is a main sequence star formed from interstellar dust and gas?
- (Interstellar dust and gas) cloud is drawn together by
gravity, forming protostar which keeps contracting - Loss in (gravitational) PE / KE increases / temperature
increases/
and volume decreases, gas pressure increases - Fusion of hydrogen nuclei to form helium
- Energy is released in fusion reactions as EM radiation
- A stable star is formed when gravitational pressure is
equal to internal / gas / radiation pressure
Where does interstellar dust and gas come from?
previous stars blew up in supernovae
Describe Evolution of a low-mass star like our Sun into a
red giant
- core hydrogen burning happens (pressure from fusion in core counters gravity trying to collapse it)
- when hydrogen runs out, fusion stops, so core contracts and heats up due to weight of star. Outer layer cools and expand forming red giant.
- materials surrounding core has plenty of hydrogen, heat from core raises temp of this material enough for fusion. This is shell hydrogen burning..
Describe how a red giant turns into a white dwarf
Core continues to contract by gravitational collapse
So temp rises in core and fusion of helium nuclei to heavy Elements occur
Lots of energy released, increasing radiation pressure outwards
Fusion finishes and starts to collapse again, outer layer of gas ejected forming planetary nebula
Hot white dense core remains, electron degeneracy pressure stops it from collapsing
Define a planetary nebula
An expanding, glowing shell of ionised hydrogen and helium ejected from a red giant star at the end of its life
Define gravitational collapse
The inward movement of material in a star due to the gravitational force caused by its own mass.
What causes radiation pressure?
It is caused by the momentum of photons released in fusion reactions, and acts outwards in the direction of the energy flow
What is electron degeneracy pressure?
The pressure that stops the gravitational collapse of a low mass star. (Stops white dwarf star collapsing)
What is the Chandrasekhar limit?
The max possible mass for a stable white dwarf = 1.4x the mass of our sun, masses above this turn into neutron stats or black holes
What is a red super giant?
A star that has exhausted all the hydrogen in its core and has a mass much higher then the sun
Temperature is high enough for helium to fuse to heavier elements, it gets an inert iron core and has layers of increasingly heavier elements
What is a neutron star and what are its properties?
The remains of the core of a red supergiant after it has undergone a supernova explosion.
It’s extremely small and dense and can rotate super fast
They emit radio waves in two beams when they rotate and sometimes are detected by earth. They’re called pulsars
What is a black hole?
The core of a red supergiants that has collapsed almost to a point. They’re very dense and very small. With a gravitational field so strong that light cannot escape (escape velocity>speed of light) at the event horizon
What is a planet?
objects with mass sufficient for their own gravity to force them to take a spherical shape, where no nuclear fusion occurs, and the object has cleared its orbit of other objects.
What is a dwarf planet?
Planets where the orbit has not been cleared of other objects
Define an Asteroid
objects which are too small and uneven in shape to be planets, with a near circular orbit around the sun.
Define a comet
Comets – small, irregularly sized balls of rock, dust, and ice. They orbit the sun in eccentric elliptical orbits.
Define a solar system
Solar systems – the systems containing stars and orbiting objects like planets.
What is the hertz sprung Russel diagram?
Luminosity-temperature plot (temp decreases from left to right)
Where would you place main sequence stars on hertz sprung Russel diagram?
Negative gradient straight line
Where would you place white dwarfs on hertz sprung Russel diagram?
Bottom left
Where would you place red supergiants on hertz sprung Russel diagram?
Top right
Where would you place red giant?
Middle right (still Above main sequence stars)
How do electrons exist in an atom?
Within discrete energy levels.
When is an electron excited?
When an electron moves from a lower energy state to a higher energy state, it requires the input of external energy. (Like absorbing photon)
Why are all energy values negative?
Negative represents the energy required to be inputted to remove an electron from the atom
Define 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
Define continuous line spectra
all visible wavelengths of light are present. They are produced by atoms of solid heated metals.
Define 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 wavelengths as the coloured lines produced when the atoms are de-excited.
What happens when an electron is de-excited in terms of emission spectral lines?
When electron is de excited it releases energy as a photon with a specific wavelength.
Energy releases is the difference between Initial energy level of the electron and the final energy level of the electron.
Each elements has a unique set of discrete levels so spectroscopy can be used to identify elements.
What can spectral lines be used for?
Identify elements within stars
What is transmission diffraction grating used for? And what equation is used
Determining wavelength of light,
As different coloured light is diffracted at different angles
dsin(theta)=nlanda,
where d is the diffraction slit separation, theta is the angle of diffraction, and n is the order of maxima.
What can stars be modelled as?
Idealised black bodies that emit radiation across a range of wavelengths, with a peak in intensity at a specific wave lighten corresponding to colour of star.
What is wein’s law?
The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature of the object.
Landamax T = constant
State Stefans Law
For a black body, the total radiant heat energy emitted from a surface is proportional to the fourth power of its absolute temperature.
L=4pir^2T^4sigma, sigma is stefans constant
What can weins and stefans law be used to find out?
Estimate the radius of a star
What are the 3 measurements of distances in space?
AU, Lightyear, parsec
Define 1 astronomical unit
Mean distance between the earth and the sun, 150million km
When is a star exactly one parsec away?
When the Angle of parallax, as the earth moves through 1 AU, is 1 second of arc (1/3600)degrees
In the equation p=1/d what does each symbol mean?
P is the parallax in seconds of arc
d is the distance in parsec
what is The principal of parallax?
the apparent position of a star appears to change when viewed from different positions
What is the cosmological principle?
on a large scale the universe is homogeneous and isotropic, and the laws of physics are universal
What is the Doppler effect?
The effect is caused by the relative motion of an observer and a source of waves.
The observed frequency(from the observer’s viewpoint) is different from the actual frequency.
The actual frequency is the frequency emitted from the source
What happens when a light source moves towards us?
Light undergoes Blue shift, shorter wavelength, higher frequencies
State Hubble’s law
v=Ho*d for receding galaxies
where v=recessional velocity (Km/s)
Ho=Hubble’s constant (Km/sMp/c
d= distance (Mpc)
What evidence supports the fact the idea the universe is expanding, how did edwin hubble show this?
Spectra from galaxies all show red shift
calculate recessional velocity from red shift
galaxies receding from Earth
distance data for galaxies
recessional velocity α distance, v-r graph straight line
universe began at a single point
what is the SI unit for Hubble’s constant?
s^-1
What is the big bang theory?
The universe started of very hot and very dense and has been expanding ever since
What is the red shift of light caused by?
space itself is expanding (not that other galaxies are flying away from us through space)
What is the experimental evidence of the big bang theory?
Cosmic background radiation
What gave rise to the expansion of time and space?
The big bang
What equation can be used to investigate the age of the universe?
t=1/Ho
What are the properties of the cosmic background radiation and how does it link to the big bang?
The intensity of the microwaves homogeneous and isotropic
These microwaves correspond to a temperature of 2.7 K
or The temperature of the universe is 2.7 K.
The expansion of the universe following the big bang led to cooling and hence we observe microwaves rather than short wavelength. (gamma waves during the early stages of the universe have been ‘stretched out’-red shifted due to expansion)
what did the continuous spectrum from the cosmic background radiation correspond to?
A temperature of 2.7K
What does the cosmic background radiation also show?
A Doppler shift, indicating Earth is moving through space
Why are there tiny fluctuations in background temperature?
tiny energy-density variations in the early universe, and are needed for the initial seeding of a star or galaxy formation
What is guessed to have happened between the big bang and 10^-4 seconds after
Big bang to 10^-43: Unknown, general relativity stops working
10^-43 - 10^-4 seconds: One grand force splits into the 4 forces as the universe expands and cools, at 10^-34 it the universe inflated. The universe is a sea of quarks, anti quarks, leptons and photons unbound. Matter antimatter symmetry breaks and there’s more matter.
What happened 10^-4 seconds after the big bang?
Temp=10^12k Universe is cool enough to form protons and neutrons. Matter and antimatter annihilate eachother leaving small excess of matter and lots of photons
What happened 100s after the big bang?
Temp=10^9K (universe is like the interior of a star) protons are cool enough to fuse to helium
What happened 300,000 Years after the big bang?
Temp=3000K cool enough for electrons to combine with He and H nuclei to form atoms. Re combinations occurs.
What happened 300,000 Years after the big bang?
Temp=3000K cool enough for electrons to combine with He and H nuclei to form atoms. Re combination occurs.
What is happening 14 billion years after the big bang (now)?
Temp=2.7K
slight density fluctuations n the universe mean that over time, clumps of matter have been condensed by gravity into galactic clusters, galaxies and individuals stars
What is happening 14 billion years after the big bang (now)?
Temp=2.7K
slight density fluctuations in the universe mean that over time, clumps of matter have been condensed by gravity into galactic clusters, galaxies and individuals stars
Describe qualitatively the evolution of the universe immediately after the big bang to the
present day. You are not expected to state the times for the various stages of the evolution.
- (At the start it was) very hot / extremely dense /
singularity - All forces were unified
- Expansion led to cooling
- Quarks / leptons (soup)
- More matter than antimatter
- Quarks combine to form hadrons / protons / neutrons
- Imbalance of neutrons and protons / (primordial) helium produced
- Atoms formed
- Idea of gravitational force responsible for formation of stars / galaxies
- Temperature becomes 2.7 K / 3 K or (the universe is saturated with cosmic) microwave background radiation
What is the percentage composition of dark energy, dark matter and matter
dark energy 70%
dark matter 25%
matter 5%
How was dark matter discovered?
Scientist calculated the mass of a cluster of galaxies based on the velocity of its outer galaxies and compared the mass that was estimated using luminosity, mass calculated using velocity was a lot more. Suggesting there was extra mass not seen.
Stars at the edge of a galaxy we’re moving faster then they should. For Newton’s laws to hold, there must be extra matter not accounted for
What are 3 explanations for dark matter?
- It’s made up of MACHO’s (massive compact halo objects). Theses are objects made from normal matter in a very dense form, giving of no light so hard to detect
—> but unlikely that MACHO’s make up all as this would require more protons and neutrons to exist - It’s is made of WIMP’s (weakly interacting massive particles). They don’t interact with EM force, but do with gravity.
—>no evidence so just theoretical - Dark matter doesn’t exist at all and is an illusion caused by mistakes in other theories
Why do astronomers think there’s some sort of dark energy?
The universe is accelerating in expansion. But it should be slowing down due to gravity.
