Astrophysics and Cosmology

Define a planet

object with mass sufficient for their own gravity to have a spherical shape, no nuclear fusion and cleared orbit of other objects

Define planetary satellites

bodies that orbit a planet

Define comets

small irregularly sized balls of rock, dust and ice, (orbiting in eccentric elliptical orbits)

Define a solar system

the systems containing stars and orbiting objects such as planets

Define galaxies

a collection of stars, dust and gas

Explain the formation of stars

Nebulae are gigantic clouds of gas and dust, formed by gravitational attraction between particles. As they move closer, gravitational collapse accelerates, becoming more dense - gravitational energy is converted to thermal energy. The result is a protostar. If there is high enough temperature and pressure, electrostatic forces are overcome and nuclear fusion of hydrogen begins, forming helium nuclei, forming a star. The star remains in stable equilibrium, where radiation pressure counteracts gravitational forces between particles (main phase).

What is the mass of a low mass star?

between 0.5 and 10 solar masses

Explain the evolution of a low mass star

As they have a smaller, cooler core, they remain in main sequence longer. Once hydrogen supply has run out, gravitational forces overcome radiation pressure and the star begins to collapse inwards, evolving into a red giant. As helium runs low, it evolves into a white dwarf, and the outer shells drift off into space as a planetary nebula, left with very dense core with no fusion. Electron degeneracy pressure prevents the core from collapsing. If mass is below 1.44M_o, it is stable

Explain the evolution of a massive star

As hydrogen depletes, the temperature is high enough for helium fusion, making heavier elements and evolving into a red supergiant, which has layers of increasingly heavy elements around an inert iron core (iron fusion produces no energy). Once an iron core is produced, the star becomes unstable, and a supernova occurs, ejecting material into space. If the remaining core mass is greater than the Chandrasekar limit, it becomes a neutron star. If the mass is greater than 3M_o, then the gravitational forces are so strong the escape velocity is faster than the speed of light, causing a black hole

What is the Hertzsprung-Russell diagram?

shows the stellar luminosity against the stars decreasing temperature

What happens when an electron is excited?

it moves from a low to a high energy state by absorption of a photon

What happens when an electron is de-excited?

it moves from a high to low energy state by releasing a photon

What do energy levels represent?

the energy required to remove the electron from the atom

What causes lines on an emission spectra?

a de-excited electron releases energy as a photon of a specific wavelength

State the equation for the energy of a photon?

E = hc/λ, where the energy released is the difference between the initial energy level of the electron and final energy level of the photon

What is a diffraction grating?

components of regularly spaced slits which diffract light

What is the formula for diffraction gratings?

dsinθ = nλ (n is order of maxima, d is slit separation, θ is angle of diffraction)

What can stars be modelled as?

black bodies that emit radiation across a range of wavelengths, with a peak of intensity corresponding to the colour of the star

What is Wein’s displacement law?

Tλ_max = wein’s constant

(the black body radiation curve peaks at a wavelength inversely proportional to the surface temperature of the object)

What is Stefan’s law?

Luminosity (radiant power output of the star) is proportional to the surface area of the star, and to the fourth power of temperature.

L = 4πr²T⁴σ

What is one astronomical unit? AU

the average distance from the earth to the sun (1.5×10¹¹)

What is one lightyear? ly

the distance light travels in a year

What is an arcminute?

1/60th of a degree

What is an arcsecond?

1/3600th of a degree

What is a parsec?

the distance at which a radius of 1 AU subtends an angle of 1 arcsecond (1AU/arcsecond)

What is stellar parallax?

the apparent shift of position of an object against a backdrop of distance objects

What is the formula for stellar parallax?

d = 1/p (d - distance between observer and object in pcs, p - parallax angle in arcseconds)

What is the cosmological principle?

States the universe is isotropic - the same in all directions - and homogenous - uniformly distributed, density is the same

What is the doppler effect?

the apparent shift in wavelength when the source of the waves is moving (if the source is moving away, the wavelength increases)

State the doppler equation for emission spectrums

Δλ/λ = v/c (λ - original wavelength, v - velocity relative to earth)

Define Hubble’s Law

the recessional velocity of a galaxy is proportional to its distance from earth

State the formula for Hubble’s Law

V = H₀d

How do you estimate the age of the universe?

1/H₀ (in seconds)

What is the big bang theory?

the universe originated at a singularity of infinite density and temperature

What is the two pieces of evidence for the big bang theory?

• cosmic microwave background radiation - high energy photons wavelengths stretched into the microwave region, causing constant interference

• Hubble’s law, which shows the universe is expanding, through the red shift of light from distant galaxies

List the evolution of the universe

• 10^-35s - only gamma photons and EM

• 10^-6 - first fundamental particles gain mass

• 10^-3 - first hadrons created from quarks

• 1s - production of mass halted

• 100s - protons and neutrons fuse to create light nuclei

• 380k years - cool enough for atoms to form

• 30M years - first stars, heavier elements

• 200M years - galaxies form

• 9B years - solar systems form

• 11B years - life on earth

• 13.7B years - humans

What is dark energy?

hypothetical form which fills space and accelerates expansion. makes up 68%

What is dark matter?

hypothetical form of matter that does not emit, absorb or reflect EM. makes up 27%