Stars and Cosmology Flashcards
Define planet.
An object in orbit around a star with:
- A large enough mass to have its only gravity (round).
- No fusion reactions.
- Cleared orbit of most objects.
What is the difference between a planet and a dwarf planet?
Dwarf planets have not cleared their orbit of other objects.
Define planetary satellites.
A body in orbit around a planet (eg moon, satellite).
Define comet.
- Small, irregular bodies of dust, ice and rock.
- Elliptical orbit around Sun.
Define solar system.
Planetary system with a star and at least one planet in orbit around it.
Define galaxy.
A collection of stars and interstellar dust and gas bound together by gravitational force.
What are the stages when forming a main-sequence star?
- Nebula
- Protostar
- Main sequence
What are the stages when forming a neutron star?
- Red giant
- White dwarf
- Neutron star
What are the stages when forming a black hole?
- Red supergiant
- Supernova
- Black hole
How is a nebula formed?
Tiny gravitational attraction between dust and gas pull particles together, forming vast clouds.
How is a protostar formed?
- Gravitational collapse accelerates as dust and gas get closer together.
- Denser regions form and pull in dust and gas -> gains mass and is denser.
- Hotter because GPE is transferred to thermal energy.
How is a star formed?
- Extremely high pressures and temperature in the core to overcome electrostatic repulsion between H nuclei.
- Nuclear fusion takes place (H nuclei forced together to make He nuclei)
Explain why nuclear fusion in the core of a star prevents further gravitational collapse.
Gas and radiation pressure released pushes against gravitation collapse.
Explain why nuclear fusion in the core of a star prevents further gravitational collapse.
- Fusion produces gas and radiation pressure
- Pressure pushes outwards against gravitation collapse.
Describe the similarities and differences between planets and comets.
Similarities:
- In orbit of a star
- Elliptical orbit
Differences:
- P: regular and C: irregular
- Comets cannot clear their orbit of most objects
How does a main-sequence star remain in stable equilibrium?
- Fusion produces gas and radiation pressure
- Pressure pushes outwards against gravitation collapse.
- Forces balance, maintaining equilibrium.
Explain why larger stars spend less time in their main-sequence phase.
- Greater rate of fusion
- Depletes Hydrogen in a shorter time period
Name two factors that affect the stability of a star.
- Size
- Mass
What is the astronomical unit (AU)?
Average distance from the Earth to the Sun.
1.50 X 10^11 m
What is a light-year (ly)?
Distance travelled by light in a vacuum in one year
How many arcminutes and arcseconds are there in 1°?
60 arcminutes
3600 arcseconds
Define parsec (pc).
The distance at which a radius of 1AU subtends an angle of 1 arcsecond.
How can you determine the value of 1pc with tan?
tan (1 arcsecond) = 1 AU / 1 pc
What is stellar parallax?
- Technique used to determine the distance to stars less than 100pc from the Earth.
- Relies on the apparent shift in position of nearby stars against the fixed background of distant stars.
Give the equation that relates parallax angle (p) and parsecs (d).
d (parsecs) = 1 / p (arcseconds)
Equation for intensity?
I = P / A
What is the Doppler effect?
The change in frequency and wavelength of waves from an object moving relative to the observer.
Why does a driver not experience any Doppler shifts of sound while driving?
- No relative motion between driver and wave source.
- Freq and wavelength received are the same.
What is happening when a galaxy is blue-shifted?
- Galaxy is moving towards the Earth.
- Smaller wavelength
What is happening when a galaxy is red-shifted?
- Galaxy is moving away.
- Stretched wavelength
What is the Doppler equation?
∆ λ / λ = ∆ f / f = v / c
Only use when galaxy is moving a lot slower than speed of light
Suggest how measuring the redshift of different sides of a rotating galaxy can find the speed of rotation.
- Each side of the galaxy will have a different velocity so different redshift.
- Side rotating away will have a greater velocity.
- Use Doppler equation to find relative velocities.
- Difference in velocities = speed of rotation
What is Hubble’s law?
The recessional speed of a galaxy is directly proportional to its distance from Earth.
v ∝ d
How does the velocity of a galaxy, Hubble’s constant and distance from galaxy link?
v = H₀ d
Give two units for Hubble’s constant.
s^-1
km s^-1 MPc^-1
Give two observations Hubble found that prove that the universe is expanding.
- Light from galaxies is mainly red-shifted.
- Galaxies have a relative velocity away from Earth.
What is the Cosmological principle?
The assumption that when viewed on a large enough scale, the Universe is homogeneous, isotropic and the laws of physics are universal.
What does homogeneous mean?
Matter is uniformly distributed- density of the universe is the same
What does isotropic mean?
The universe is the same in all directions to every observer, and it has no centre or edge.
What is the Big Bang theory?
The universe started from a single point which then expanded.
Give two pieces of evidence for the Big Bang theory.
- Hubble’s law
- Cosmic microwave background radiation
Does Hubble’s law support the Big Bang theory? Argue for and against.
For: Hubble’s law shows that the universe is expanding through the red-shift of light and it must have originated from a single point.
Against: the expanding universe can be explained through other theories so is a weak argument (more evidence needed).
How does microwave background radiation support the Big Bang theory?
- young, hot universe was saturated with high energy gamma photons -> as universe expanded, space stretched over time -> stretched wavelength of photons to microwave radiation
- young, dense and hot universe expanded -> temperature of space dropped to 2.7K -> universe is a black body radiator -> peak wavelength at 2.7K corresponds to microwave radiation.
How do you work out the age of the universe?
t = 1 / H₀
What proof is there that the expansion of the universe is accelerating?
Light from supernova was less intense than expected
What evidence is there for dark energy?
Light from supernova was less intense than expected -> expansion of universe is accelerating -> must be gaining energy from dark energy in order to accelerate
Explain why dark matter has been predicted.
- usually the velocity of an object decreases as distance increases from the centre.
- stars do NOT slow down
- mass of a galaxy is not concentrated at the centre
- there must be dark matter
What are the axis of the HR diagram?
x axis- average surface temperature (3000K to 40,000K)
y axis- luminosity
What is the luminosity of the Sun?
3.85 x 10^26 W
What are the equations linking energy and Planck’s constant?
E = hf E = hc / λ
What is solar luminosity?
1 L☉ = 3.85 x 10^26 W
What is an emission line spectrum?
Each element produces a unique emission line spectrum due to the unique set of energy shells.
(Black screen with coloured lines to represent wavelengths)
What is a continuous spectrum?
All visible frequencies or wavelengths present.
Rainbow
What is an absorption line spectrum?
Dark lines appear where the wavelengths of atoms are present due to the unique set of energy shells.
(Rainbow screen with dark lines)
How is an emission line spectrum produced?
- Gas atoms are heated.
- Electrons are excited and move to a higher energy level.
- Electrons drop down to lower energy levels, emitting photons with discrete frequencies that correspond to the element.
- Each line on the emission line spectrum corresponds to a specific wavelength.
How is an absorption line spectrum produced?
- Light from a source that produces a continuous spectrum passes through a cooler gas.
- Photons with the exact energy equal to the different energy levels are absorbed by the gas.
- Electrons move to a higher energy level and excite the atom.
- When specific wavelengths (photons) are absorbed, dark lines are created in the spectrum.
How can you detect elements within a star?
Compare the line spectra of elements and the absorption line spectra of the stars.
Why are some wavelengths on the absorption line spectrum of a star not detected?
Some photons are absorbed by cooler atoms on the surface so not detected.
Describe the difference between continuous spectra and emission line spectra.
Emission only has certain frequencies and wavelengths present, depending on the unique set of energy levels.
Continuous shows all frequencies and wavelengths.
Why do the wavelengths of the emission lines have the same wavelengths as dark absorption lines?
- Each element has a unique set of energy levels.
- Photons released when the atoms are excited have a specific wavelength.
What is the grating equation?
dsinθ = n λ
What is a black body?
Object that absorbs all the electromagnetic radiation that shines onto it AND emits specific wavelengths at a specific temperature.
What is Wein’s displacement law?
λ max ∝ 1 / T
What is the equation that links Wein’s constant, absolute temperature and λ max?
Wein’s constant = 2.90 x 10^-3 m K
λ max T = Wein’s
What is Stefan’s law?
The total power radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature.
L = 4 π r^2 σ T^4
What does Stefan’s law show?
Luminosity is directly prop to:
- radius
- surface area
- surface absolute temp
What is Stefan’s constant?
5.67 x 10^-8 W m^-2 K^-4
Describe the formation of the Sun and its probable evolution.
- Dust and gas drawn together by gravitational forces.
- GPE -> KE and temperature increases.
- Hydrogen nuclei fuse together
- A stable star is formed when gravitational pressure = radiation pressure
- When the star runs out of hydrogen nuclei, the outer layers of the star expand.
- Red giant is formed.
- Core will become a white dwarf.
Describe the evolution of the universe from the Big Bang to the present day.
- At the start, the universe was an infinitely hot, dense singularity.
- Universe expands and cools
- There’s a sea of quarks and leptons (antimatter)
- Universe cools so quarks combine to form particles like protons and neutrons.
- Atoms then form
- Gravitational force forms stars and galaxies.
- Temperature of the universe is 2.7K and the universe is saturated with CMBR.
Explain the process of nuclear fusion in the core of the Sun, refer to the conditions necessary.
- There is electrostatic repulsion between protons
- High temperatures of 10^7K needed
- Protons travel at fast speeds and come close enough for the strong nuclear force to take effect.
- Protons produce He NUCELI
Name the two forms of energy produced in thermonuclear reactions.
- Kinetic
- Electromagnetic
