astrophysics + cosmology Flashcards
what is a planet?
an object with a mass sufficient for their own gravity to force them to take a spherical shape, where no nuclear fusion occurs
what is a dwarf planet?
planet where orbit has not been cleared of other objects
what is a planetary satellite?
a body that orbits a planet
how is a nebula formed?
tiny gravitational attraction pulls particles of dust and gas towards each other to form nebula clouds
how is a protostar formed?
due to tiny variations, denser regions begin to form, which pull in more dust + gas, gaining mass, getting denser + hotter and gravitational energy is transferred to thermal.
the result= hot dense protostar
what happens in the protostar stage of a star’s life cycle?
temperature and pressure must be high enough for hydrogen nuclei to overcome electrostatic forces of repulsion + undergo nuclear fusion and produce helium nuclei.
temperature increases, so thermal expansion + pressure increases
what occurs in the main sequence stage?
the star is in equilibrium
-gravitational forces act to compress star but radiation pressure from photons emitted from fusion counteract it, keeping size of star almost constant
what affects the length of time a star spends in the main sequence stage?
larger stars are hotter, so faster nuclear fusion happens and use up hydrogen quicker, therefore the star spends less time in the main sequence stage
what occurs to stars with low mass after the main sequence stage?
they become a red giant
explain what occurs in the red giant phase of a star
star glows at cool red colour
- at the start, the gravitational force is greater than the reduced force from radiation (due to reduction in energy released by fusion)
- core of star begins to collapse
- as core shrinks, pressure is high enough to start fusion of hydrogen to helium around core
- swells to radius 100x larger than the sun
what happens after a red giant swells?
it becomes a white dwarf and ejects a planetary nebula
describe what happens in the white dwarf phase?
white dwarf is dense and hot
-it emits energy as it leaks photons
-maintains similar mass as an m.s star but shrinks to smaller than Earth
-made predominantly of oxygen and carbon
helium fuses with carbon and carbon fuses with helium to make oxygen
define Pauli exclusion principle
two electrons cannot exist in the same energy state
what is the electron degeneracy pressure?
when the core of a star begins to collapse under gravity, electrons are squeezed together.
this creates pressure that prevents further gravitational collapse
the pressure caused by repulsion of electrons is electron degeneracy pressure- due to the Pauli exclusion principle
define Chandrasekhar limit
only stars with mass higher than 1.44M will keep collapsing due to gravity as electron degeneracy pressure would not be enough
what happens to stars with mass greater than the Chandrasekhar limit?
they become super red giants
what happens in the super red giant phase?
core begins to collapse under gravitational force as hydrogen runs out
they core is got so hydrogen fusion into helium nuclei occurs fast enough to overcome electrostatic repulsion and fuse helium into heavier elements, such as iron.
what are the two things that can occur after a red super giant explodes in a supernova?
it can become a neutron star or a black hole
how does star development lead to supernova?
changes in core causes star to expand and form red supergiant
- inside temperatures + pressures are high enough to fuse massive nuclei together, forming shells inside the star
- process continues until an iron core develops
- since iron nuclei can’t fuse, which makes the star unstable + leads to supernova
what is a supernova?
all core material of a star (all elements formed, etc) is ejected into space
when is a neutron star formed?
if the mass of the core is greater than 1.44M, gravitational collapse continues, forming a neutron star
what is a neutron star?
-made entirely of neutrons
-can be very small
-typical mass of 2M
electrons are pulled into the nuclei and interact with protons to make neutrons
when is a black hole formed?
if the core has mass greater than 3.0M, gravitational collapse continues to compress core. as a result, gravitational field is so strong, the escape velocity is greater than the speed of light is needed.
what is a black hole?
a region in space-time with such a strong gravitational field, not even EM radiation can escape
- varies in mass
- nothing can escape
what is the Hertzsprung-Russel diagram?
graph of stars in our galaxy showing the relationship between their luminosity on the y-axis and their average surface temperature on x-axis
what type of stars are in the top left corner of an H-R graph?
hottest, most luminous stars
what type of stars are in the bottom right corner of an H-R graph?
coldest, least luminous stars
how can you find the wavelength of light?
-shine monochromatic light through a diffraction grating
-a pattern of bright lines on a dark background is formed (maxima)
use dsinθ= nλ to find the wavelength
describe the spectrum of white light
different wavelengths within the white light are diffracted by different amounts. zero order maximum stays white. (red on outside, violet on inside)
what do hot objects emit?
a continuous spectrum in visible + infrared regions
why are energy levels negative?
external energy is required to move an electron from atom. negative values indicate that electrons are bound to positive nuclei
what does it mean if an electron has zero energy?
it is free from the atom
what is an energy level with the most negative value?
ground state
what happens when an electron moves from a lower to higher energy level?
- atom is excited
- requires external energy
what happens when an electron is moved from a higher to lower energy level?
- energy is lost
- energy is conserved
what happens as an electron transitions between levels?
a photon is emitted- de-excitation
how can you work out the energy of a photon emitted from electrons moving energy levels?
E=hf or E=hc/λ
what does a line emission spectrum show?
- the particular wavelength of light emitted
- different atoms have different energy levels so a different emission spectrum is formed.
- gases can be identified from emission spectra
what does a line absorption spectrum show?
- black lines that correspond to absorb λ
- created by shining white light through a cool gas
- photons of correct λ are absorbed by electrons to excite them to higher energy levels
how can you identify elements within a star?
by comparing absorption spectra to emission spectra
define luminosity
total energy emitted per second
give two factors affecting luminosity
temperature
surface area
give the equation for luminosity of a star
L= 4πR^2σT^4
what is σ (Stefan’s constant)?
5.67x10^-8 N/m^2/K^4
what does the peak wavelength of a star depend on?
temperature of the star
what is the relationship between intensity and wavelength?
maximum wavelength is inversely proportional to time
wavelength becomes shorter as the surface temperature of a star increases
what is one astronomical unit in kilometres?
1.5x10^6 km
what is the speed of all EM waves in a vacuum?
3 x 10^8 m/s
what is a light year?
the distance that EM waves travel through a vacuum in one year
what is a light year in metres?
9.5 x 10^15 m
what is parallax?
seeing objects in the foreground seeming to be moving faster than objects in the distance
how can you work out the distance between a star and the sun?
radius of the earth’s orbit around the sun/ angle of parallax
what is a parsec?
the distance at which the mean radius of the earth’s orbit subtends an angle of one second of arc
(a star is exactly one arc-second away from earth if angle of parallax = 1 arc-second)
what is one parsec in metres?
3.1x10^16m
define isotropic
object produces the same value no matter which direction you measure it in
define homogeneous
if you observe an object at a given location, it will look the same everywhere else
give the cosmological principle
on a large scale, the universe is homogeneous and isotropic, and laws of physics are universal, including Newton’s laws.
what is the Doppler effect?
apparent change in frequency and wavelength of an observed wave due to the relative motion between the source of the wave and the observer
in the Doppler effect, why does the frequency and wavelength change?
waves bunch together in front or behind the source
how can you determine the amount of red or blue shift?
Δλ/λ = Δf/f = v/c so c= f x λ
what does red shift show about the universe?
universe is expanding
what is red-shift?
- light source moves away
- wavelengths become longer
- frequency becomes lower
- light shifts towards red end of the spectrum
what is blue-shift?
- light sources move closer
- wavelengths become shorter
- frequency becomes higher
- light shifts towards blue end of spectrum
why would some galaxies show blue shift?
some galaxies are moving towards us due to gravitational attraction so light shows blue shift
what does the amount of red shift tell us?
the recessional velocity- how fast the galaxy is moving away
what equation links recessional velocity, distance and Hubble’s constant?
v = Hod
distance in Mpc
what are the units for Hubble’s constant?
km/s M/pc
what is the S.I unit for Hubble’s constant?
s^ -1
what is 1 Mpc in metres?
3.1 x 10^22 m
how can you convert from Ho to s^-1?
x 3.09x10^19
what is the big bang theory?
universe started off very hot and very dense and has been expanding ever since
theory suggests that red shift of light from other galaxies is caused by space expansion
what does the age and observable size of the universe depend on?
Hubble’s constant
how can you work out age of universe from Hbbble’s constant?
reciprocal
what is cosmic microwave background radiation and where does it come from?
- in early universe, there was lots of gamma radiation
- due to expansion of universe, wavelengths of CMBR have been stretched and are now in the microwave region
- CMBR was picked up and corresponded to a temperature of around 2.7K
what principle is radiation in line with?
Cosmological principle- radiation is largely the same everywhere and in all directions (homogenous and isotriopic)
describe the universe from big bang to 10^-43 seconds
infinitely small, infinitely dense, infinitely hot
describe the universe from 10^-43 to 10^-4 seconds
- one unified force
- universe expands and cools
- unified force splits into gravity, electromagnetic force, strong and weak nuclear forces
- universe went through inflation period
describe the universe from 10^-4 seconds
- temperature was around 10^12 K
- universe cools enough for quarks to join and form protons and neutrons
- matter and anti-matter annihilate, leaving photons as CMBR
describe the universe from about 100 seconds
- temperature cools to 10^9 K
- universe is similar to interior of a star
- protons are cool enough to fuse with helium nuclei
describe the universe from about 300 000 years
- temperature cooled to about 3000K
- universe is cool enough for electrons to combine with helium and hydrogen
- universe become transparent- no free charges for photons to recombine with
describe the universe from about 14 billion years - now
- temperature cooled to about 2.7K
- slight density fluctuations in universe- matter condensed by gravity into stars, galaxies etc
what is dark matter?
extra mass in the universe, giving mass to galaxies that we cannot observe
estimated that dark matter makes up about 25% of universe
what is dark energy?
energy causing the universe to expand more quickly
expansion should be slowing down but it appears to be accelerating
makes up 70% of universe
how much of the universe is made from ordinary matter?
5%
