5.5 Astrophysics and Cosmology Flashcards
What is a planet?
An object in orbit around a star with mass large enough for its own gravity to give it a round shape and a cleared orbit. No fusion reactions occur
What is a planetary satellite?
A body in orbit around a planet
What is a comet? (2)
A small body made of ice, dust and rock
Highly eccentric, elliptical orbits around the Sun
What is a galaxy?
A collection of stars, interstellar dust and gas
What size of star only becomes a red giant?
0.5-5 solar masses
Small stars - stellar nebula (2)
A dense cloud of gas
Gravity pulls the gas together
Small stars - protostar (3)
No fusion
Gas under high temperature and pressure
Emits heat and light energy
Small stars - main sequence (2)
Core temperature and pressure is high enough for fusion to occur
Gravity pulls inward, balanced by outward gas pressure from fusion in core
Small stars - red giant (5)
Core hydrogen runs out
Gravity > pressure so core contracts, causing an increase in temperature
New band of hydrogen fusion forms around new core which fuses helium
Star expands until outwards pressure reduces to match gravity, surface temperature reduces
Fuses larger elements, limited by mass of star
Small stars - Planetary nebula
Outer layers of star drift away, leaving behind the core
Small stars - White dwarf (4)
No fusion
Very high temperature core
Does not collapse completely due to electron degeneracy pressure
Collapse continues if mass of core is greater than the Chandrasekhar limit
Small stars - Black dwarf
Core cools down over long time by emitting energy into space, leaving behind a cold core
What size of star forms a red supergiant?
> 10 solar masses
Larger stars - main sequence (2)
More mass so higher temperature in core
Fusion occurs faster so star has shorter lifespan
Larger stars - red supergiant (2)
Core hydrogen is depleted
Star fuses up to iron in core
Larger stars - supernova (2)
Star collapses
All heavier elements are fused
Larger stars - neutron star (4)
1.4 solar masses < core mass < 3 solar masses
Formed due to core collapsing under gravity
5-10 km radius
Made almost entirely of neutrons
Larger stars - black hole (3)
Core mass > 3 solar masses
Core collapses entirely, forming a singularity, surrounded by an event horizon
To escape the gravitational pull beyond the event horizon, a body must be moving faster than the speed of light
What is the Chandrasekhar limit? (2)
The maximum mass of a core beyond which the star will continue to collapse to a neutron star or black hole as opposed to remaining as a dwarf star
Core mass > 1.4 solar masses
What is electron degeneracy pressure? (2)
Pauli exclusion principle - electrons require an input of energy to occupy the same energy level
Star is unable to provide this energy so the force between the electrons manifests as pressure
Describe the Hertzsprung-Russell diagram (5)
Luminosity against temperature
White dwarfs - high temperature, low luminosity
Red giants - low temperature, high luminosity
Red supergiants - low temperature, highest luminosity
Main sequence - diagonal line, luminosity increases as temperature increases
How can spectral lines be used to identify elements within stars?
Different atoms have different spectral lines
What is a continuous spectrum?
All visible frequencies of light are present
What is an emission line spectrum?
Lines produced due to the unique set of energy levels in different elements
What is an absorption line spectrum?
Dark lines indicate frequencies that have been absorbed due to photons having energy exactly equal to the difference between energy levels in the atom
What is a black body?
Opaque and non-reflective objects
What is Wien’s Displacement Law?
Peak wavelength (maximum intensity) is inversely proportional to the absolute temperature of a black body
What is Stefan’s Law?
Total power radiated per unit surface area of a black body (luminosity) is directly proportional to the fourth power of the absolute temperature of the black body
Define 1 Astronomical Unit
The average distance from the Earth to the Sun
Define 1 Light Year
The distance travelled by light in a vacuum in one year
Define 1 Parsec
The distance at which a radius of one AU subtends an angle of one arcsecond
What is stellar parallax? (3)
Technique used to find distances to stars close to Earth (<100 pc)
Parallax - the apparent shift in the position of a star against the backdrop of distant stars as the Earth orbits the Sun
Distance = 1 / parallax angle (arcseconds)
Why is parallax limited to measuring small distances?
As the distance increases, the parallax angle decreases until it is too small to be measured accurately
What is the Cosmological Principle? (3)
The Universe is homogeneous - for a large enough volume, the density of the Universe is uniform, meaning that the same structures are seen everywhere
The Universe is isotropic - looks the same in all directions (no centre or edge)
The laws of physics are universal
What is the Doppler Effect?
When a wave source moves relative to an observer, the frequency and wavelength of the wave received changes
What is blue shift?
If a galaxy is moving towards Earth, the absorption lines move towards the blue end of the spectrum because the wavelengths appear shorter
What is red shift?
If a galaxy is moving away from Earth, the absorption lines move towards the red end of the spectrum because the wavelengths appear longer
How is the relative velocity of a receding galaxy calculated? (2)
Compare the absorption spectrum of a specific element in a laboratory with the light from a distant galaxy
Use the difference in observed wavelength and the Doppler equation to calculate the velocity of the galaxy
What is Hubble’s Law?
The recessional speed of a galaxy is directly proportional to its distance from Earth
Why does Hubble’s Law support the Big Bang theory?
Light from nearly all galaxies is red-shifted, showing that space is expanding in all directions
What is the Big Bang theory? (2)
All matter was once contained in a single point (singularity)
After a period of rapid expansion, the Universe became cooler and less dense, continuing to expand in all directions
What is microwave background radiation? (3)
A uniform microwave signal throughout the Universe
When the Universe was young, the wavelength of high energy gamma photons became stretched out as space expanded and cooled
Now, the temperature of the Universe is around 2.7K and the wavelengths of the photons lies in the microwave region of the spectrum
How is the age of the Universe estimated? (3)
Assumes space has expanded uniformly over time
If a galaxy is a distance d away and moving at a constant speed v then the time that has elapsed is equal to d/v which is equal to 1 / Hubble’s constant
The age of the Universe = 1 / Hubble’s constant
How did the Universe evolve? (5)
Singularity - infinitely dense and hot
Inflation - rapid, accelerating expansion
Matter created in the first second of expansion as temperature decreases
All forces are unified and then separate (gravity, strong, electromagnetic, weak)
3.8x10^5 years - atoms form, photons are free to travel, making microwave background radiation detectable
What is dark energy? (2)
The theoretical energy driving the acceleration of the expansion of space as work has to be done in order to produce acceleration
Makes up 68% of the Universe
What is dark matter? (2)
There is not enough mass to keep solar systems in orbit in galaxies, suggesting that matter that is not visible to us must exist
Makes up 27% of matter
