Unit 3 - Stars, Galaxies and Cosmology Flashcards
What is apparent brightness (m)?
how bright a star appears from Earth, also known as apparent magnitude
Explain the scale for apparent brightness
the more negative the number the brighter the star / body
Why may a star appears brighter than those around it?
1) it is more luminous (emits more light energy)
2) it is closer to Earth
What is absolute magnitude?
a measure of the stars luminosity, how much light intensity it emits
proportional to surface area
What wavelength of light do colder stars emit?
longer wavelength, mostly red
What wavelength of light do hotter stars emit?
shorter wavelength, mostly blue
What is plotted on the Hertzsprung-Russell diagram?
absolute magnitude (luminosity) against temperature
What does the Hertzsprung-Russell Diagram tell us?
1) the temperature and brightness of stars is related
2) the sun is a star, due to its position in one of the main regions on the diagram
Describe the characteristics of a red giant
more luminous compared to main sequence stars of the same temperature, therefore much larger since luminosity is proportional to surface area
Describe the characteristics of super giants
more luminous than red giants or main sequence stars of the same temperature, therefore larger again
Describe the characteristics of white dwarfs
less luminous than main sequence stars of the same temperature, significantly smaller since luminosity is proportional to surface area
Explain the formation of stars
gravitational collapse of the nebula (gas cloud), as it contracts it heats at the centre forming a protostar, when the core becomes hot enough for nuclear fusion of H (opposes gravity) it is classified as a star
Why do newborn stars gather in stellar nurseries?
radiation from new born stars illuminates surrounding gas and dust which can stimulate further star formation
Why do massive, hotter and brighter stars have a shorter lifetime?
the burn rate of H is greater since inward gravitational forces are larger so outward pressure must be more to maintain equilibrium
Why is the top left part of a Hertzsprung-Russell diagram missing?
most massive stars have already burnt up all their hydrogen so are dead or dying
What happens after the star has used up all its H?
1) nuclear fusion ceases so collapses due to gravitational forces
2) begins to heat up until hot enough for H fusion to occur in the shell
3) outer layers expand therefore luminosity increases
4) surface temp drops since surface moves away from heat source
What happens after the star has become a red giant following the previous stage?
core is still contracting until it becomes hot enough for fusion of He - C, continues to expand forming a super giant
What happens after the star has become a super giant following the previous stage?
when He becomes depleted star blows off outer layer leaving only the core and becoming a white dwarf
What happens after the star has become a white dwarf?
core contracts until EDP balances pressure, for a star of similar mass to our Sun it will radiate and gradually cool to become a black dwarf
Explain degenerate matter and electron degeneracy pressure
plasma which has such a high density that pressure is dependent on density not temperature
the repulsion between electrons (fermions) which makes them more energetic at high densities
What occurs in the core of a massive star before collapse?
fusion of elements up to iron since more mass means more gravitational contraction so higher temperature
What happens after fusion of iron in the core?
core begins to collapse since there is no outward pressure to balance gravity
Why doesn’t electron degeneracy pressure stop gravitational collapse in massive stars?
there is an upper limit to the mass which Chandresekhar calculated (maximum mass of a white dwarf) so the iron core collapses rapidly
What reactions occur in the collapse of the star?
proton + electron = neutron and neutrino
As a result of this reaction what halts the complete collapse of the star?
neutron degeneracy pressure, sudden halt causes the outer core to rebound producing a supernova
What is left after a supernova?
a neutron star in which gravity is balanced by neutron degeneracy pressure
How are neutron stars detected?
they strongly emit short wavelengths of radiation (x-rays) due to their heat
What are pulsars?
neutron stars which emit rapid pulses of EM radiation, as the spin axis and the magnetic axis of a pulsar are different it produces the lighthouse effect
What happens when the mass of a neutron star exceeds the upper mass limit?
there is nothing to oppose gravity since NDP isn’t strong enough so it collapses to a singularity known as a black hole
What is the escape velocity of a black hole?
greater than the speed of light so nothing can escape its gravitational pull
How are black holes detected?
as a star rotates around a black hole, the x-ray signal measured on Earth will vary as the star blocks the accretion disk
What does the hazy band in the night sky suggest about the Milky Way?
it is not uniform in all directions since if it was brightness would be uniform in all directions across the sky
What assumptions did Herschel make when estimating our position in the Milky Way?
1) all stars have the same brightness
2) space is transparent between stars
Why was his ‘map’ of the Milky Way wrong?
the space between stars contains microscopic dust particles which scatters light
What are the 3 components of the Milky Way?
disk - contains young stars, lots of gas and dust
central bulge - contains old stars more closely spaced than in the disk
halo - contains globular clusters and oldest stars
What does the rotation curve for a rigid body look like?
straight line, distance and velocity are directly proportional
tells us the mass is distributed uniformly
What does the rotation curve look like for a planet in our solar system?
decreasing curve
tells us most mass is concentrated at the centre
What is the difference between the predicted rotation curve of the Milky Way and the observed curve?
predicted - Keplerian motion, most mass at the centre with the centre of galaxy itself rotating like a rigid body
observed - large fraction of mass are at the edge, beyond the sun’s orbit
What does the rotation of curve of the Milky Way provide evidence for?
dark matter
matter at the outskirts of the galaxy hasn’t been detected
What is dark matter?
matter with lots of mass which does not emit radiation or interact with other matter other than via gravity
Why don’t spiral galaxies form from differential erosion alone?
the rotation of stars is too fast to sustain a stable spiral structure
What does the differential model assume?
the spiral arms always consist of the same stars and rotate at the same speed as the stars
What is the density model of spiral galaxy formation?
spiral arms are ‘density waves’ or clumps of stars which don’t always contain the same stars and move more slowly than the stars
Why are infrared observations more useful for studying the core of the Milky Way and what has been discovered from this?
the core is a huge concentration of stars and is shrouded in dust and infrared scattered less than visible due to longer wavelength
supermassive blackhole at the centre
What are the characteristics of a spiral galaxy?
1) significant internal structure - central bulge, disk and halo
2) significant rotation
3) generally bright, lots of young bright stars
What are the characteristics of an elliptical galaxy?
1) little or no internal structure, variety of shapes
2) not much rotation
3) generally dim, lots of old mostly red stars
What are the 2 models for the formation of elliptical galaxies?
cloud density model - clumps of highest density, star formation is so fast an elliptical galaxy forms then low density clump star formation is slower so spiral galaxy forms
merger model - collisions or mergers between 2 oppositely rotating spiral galaxies
What are the characteristics of irregular galaxies?
1) enormous patchy appearance
2) thought to be remnants from collisions between galaxies
What evidence is there for the spiral structure of the Milky Way?
1) optical map of type O + B stars (hot, blue stars)
2) radio emission from H clouds
What is an eclipsing binary system?
a smaller star eclipsing a much larger and brighter second star
on earth an oscillation in brightness is seen
MAX - light from both stars
LESS - brightness from larger star only
LEAST - brightness from smaller star only
What are Cepheid variables?
single unstable star of large mass, starting to run out of fuel so the size, luminosity and temperature all oscillate together
How can Cepheid variables be used to measure the distance to galaxies?
the period of oscillation and absolute magnitude of the star are directly proportional so using equation for absolute magnitude distance to star can be calculated
How can the velocity of galaxies be measured?
using the Doppler Effect (redshift)
the observations of many galaxies produced redshifts meaning almost every galaxy is moving away
Explain the relationship between mass, gravity and spacetime
mass curves spacetime and gravity arises from the curvature of spacetime
light then follows the shortest path through distorted spacetime
Where can light be seen to bend due to gravity?
light passing close to the Sun passes through a ‘dent’ in spacetime due to the Sun’s large mass appearing to bend
around nearby galaxies, which distorts the image of the galaxy to form a ring, known as ring formation due to gravitational lensing
Why did Einstein produce a cosmological constant?
general relativity predicted the universe should be expanding or contracting but Einstein believed the universe should be static
What did Hubble’s results reveal about the relationship between the velocity of galaxies and their distance from us?
directly proportional meaning furthest galaxies are receding fastest which suggests the universe is expanding
then discovered the rate of expansion is accelerating
What conclusions can be made from an expanding universe?
1) suggests that at some point in the past all galaxies were together at a single point - The Big Bang
2) galaxies are not expanding through space but spacetime is expanding and ‘carrying’ galaxies along with it
What happened at t=0 during The Big Bang?
everything started in a tiny volume (less than the size of an atom)
shortly after 3/4 fundamental forces unite to form on force leaving gravity
What is the next stage in the expansion of the universe?
huge energy release makes universe inflate enormously
enormous temperatures but begins to cool as it expands
strong nuclear force ‘separates’ from the other 2 forces
What is the following stage in the expansion of the universe?
further expansion and cooling
cool enough for protons and neutrons to form light nuclei
creates a very dense plasma of nuclei and electrons, opaque plasma scatters light
What happened approx. 100,000 years after The Big Bang?
further expansion and cooling
electrons and neutrons bind to form neutral atoms
universe becomes transparent, light can travel long distances
cosmic microwave background radiation is produced
What happened approx. 100 million years after The Big Bang?
further expansion and cooling
gravity becomes the dominant force
H and He are pulled together to form stars
stellar cores form heavier elements up to Fe
elements heavier than Fe formed in supernova
Explain what is meant by cosmic microwave background radiation
when atoms first formed and the universe became transparent, it was a hot body
therefore must have been emitting radiation, for 3000K it would have been infrared
however due to the expansion the wavelength of this radiation should now be in the microwave region
increase in wavelength - cosmological redshift
What is the existence of CMBR strong evidence for?
The Big Bang Theory
shows that the universe was once hot and dense in the past
What is meant by the horizon problem?
early temperature maps of CMBR showed it was isotropic
problem - widely separated regions of space should not be ‘aware’ of each other
What explains the isotropy of CMBR?
the inflation model
theory states the universe expanded enormously at a rate >c although before this all areas of the observable universe were easily ‘within reach’ of each other
The idea of the expansion of the universe slowing down due to gravitational attraction depends on what?
1) strength of gravity (average mass density of the universe)
2) the shape / curvature of the universe which is determined by mass
What determines the geometry of the universe and what does CMBR show?
how much stuff is in the universe
the universe appears to be flat which means density of the universe = critical density
What problem arises from this observation?
measurement of the universe’s mass (including dark matter) only shows a density of 35% critical density
What makes up the 65% of missing density?
dark energy
evidence of this comes from the measurements of distances to and velocities of most distant galaxies using supernova
What does the existence of dark energy provide evidence for?
the accelerating expansion of the universe since dark energy is effectively gravitationally repulsive unlike ordinary and dark matter
