Astrophysics Flashcards
solar system
eight planets that orbit the sun
asteroid
small rocky body that drifts around the solar system
meteriod
asteroid on a collision course with another planet
meteorites
small meteors can be vaporized due to the friction with the atmosphere - the bits that arrive on earth
comets
mixture of rock and ice in very elliptical orbits around the Sun - their tails always point away from the sun
nebulae
interstellar clouds of dust, hydrogen, helium and other ionized gases
constellation
patterns of stars have been identified and 88 different regions of the sky have been labelled
why do the constellations seem to rotate?
result of the rotation of earth about its own axis
describe the movement of the sun during the day and during the year
sun rises in the east and sets in the west winter to summer, arc gets bigger and the sun climbs higher in the sky
light year
distance travelled by light in one year
milky way
faint band of light in the night sky
why is the milky way seen to be a band?
galaxy has a spiral shape
describe the movement of the galaxy.
- rotating
- all the stars are orbiting the centre of the galaxy as a result of their mutual gravitational attraction
stellar clusters
stars are grouped
what are the two types of stellar clusters?
open or globular
open stellar cluster
containing 10^3 stars
globular stellar cluster
containing 10^5 stars
what is the vast majority of space?
vacuum
what is the source of energy for stars?
fusion of hydrogen into helium nuclear reaction
what is the reaction for the energy of stars?
411p –> 42He + 201e+ + 2v
where does the hydrogen fusion take place?
core of the star
how is the sun stable?
there is hydrostatic equilibrium between the outward pressure and the inward gravitational force
binary stars
two or more stars orbit around their common centre of mass
what are the different categories of binary star?
- visual
- spectroscopic
- eclipsing
visual binary star
one that can be distinguished as two separate stars using a telescope
spectroscopic binary star
- identified from the analysis of the spectrum of light from the ‘star’
- over time the wavelengths show a periodic shift or splitting in frequency
- involves the Doppler effect
Doppler effect
- as a result of its orbit, the stars are sometimes moving towards the earth and sometimes moving away
- when a star is moving towards the earth, its spectrum is blue shifted
- when a star is moving away from the earth, its spectrum is red shifted
eclipsing binary star
- identified from the analysis of the brightness of the light from the ‘star’
- over time the brightness shows a periodic variation
explain the ‘dip’ in brightness for an eclipsing binary star.
- as a result of its orbit, one star gets in front of the other
- if the stars are of equal brightness, this causes the brightness to drop by 50%
parallax
an object that is near to you will appear to move away when compared with objects that are far away
explain why after a period of a year some stars can appear to move between two extremes.
- the earth has moved over this period
- means that a close star will have an apparent movement when compared with a more distant set of stars
- the closer a star is to earth, the greater the parallax shift
luminosity
- total power radiated by a star
- watts
apparent brightness
power recieved per unit area
what would happen if two stars were the same distance away from the earth but one has a greater luminosity?
the one with greater luminosity would be brighter
what is the issue with apparent brightness?
two stars can have the same apparent brightness even if they have different luminosities
what are the alternative units for luminosity?
- apparent magnitude
- absolute magnitude
are stars considered black bodies?
yes
how to analyse the absorption spectra of stars?
missing wavelengths correspond to a number of elements
spectral class
stars that emit the same type of spectrum are allocated to the same spectral class
what are the seven main spectral classes?
- O
- B
- A
- F
- G
- K
- M
red giant stars
- large in size
- red in color
- comparatively cool
- one of the later possible stages for a star
- source of energy is fusion of elements other than hydrogen
white dwarf stars
- small in size
- white in color
- comparatively hot
- one of final stages for some smaller mass stars
- fusion not taking placce
- hot remnant that is cooling down
- eventually will cease to give out light when it becomes sufficiently cold (brown dwarf)
cepheid variables
- little unstable
- regular variation in brightness and luminosity
- due to an oscillation in the size of the star
neutron stars
- post-supernova remnants of some larger mass stars
- gravitational pressure forces total collapse
- mass composed of neutrons
- enormous density
pulsar
rotating neutron star
black hole
- post-supernova remnant of larger mass stars
- no known mechanism to stop the gravitational collapse
- escape velocity greater than speed of light
nucleosynthesis
creation of nuclei of different elements as a result of fission reactions
what are the necessary conditions for nucleosynthesis to take place?
- two positively charged particles need to come close enough for interactions to take place
- will repel one another so must be at a high temperature
What occurs if a cloud of hydrogen is hot enough?
nuclear reactions take place spontaneously
how is the temperature of a star constant?
power radiated by the star is balanced by the power released in these reactions
how is the size of a star stable?
outward pressure of the radiation is balanced by the inward gravitational pull
how does a cloud of hydrogen get to be a high temperature in the first place?
as the cloud comes together the loss of gravitational potential energy must mean an increase in kinetic energy and hence temperature
Hertzsprung-Russell Diagram
- what are its axes?
- what does it show?
- vertical axis is the luminosity of the star compared to that of the sun
- horizontal axis is a scale of decreasing temperature
- each dot represents a different star
What is the essential difficulty presented by parallax shifts?
when we observe light from a very distant star we do not know the difference between a bright source that is far away and a dimmer source that is closer
standard candle
star of known luminosity
cepheid variable star
- standard candle
- outer layers undergo a periodic compression and contraction and this produces a periodic variation in its luminosity
can a star continue in its main sequence state forever?
- no
- at some point the hydrogen in the core will become rare
- fusion reaction will happen less often
- star is no longer in equilibrium
- gravitational force will cause the core to collapse
- increases temperature of the core so further hydrogen and helium fusion is possible
- star increases massively in size
- become red giant star
what does the process of fusion as a source of energy end with?
- nucleosynthesis of iron
- has one of the greatest binding energies per nucleon of all nuclei
- fusion of iron to form a higher mass nucleus would need to take in energy rather than release energy
chandrasekhar limit
equal to approximatelly 1.4 times the mass of our sun
what happens below the chandrasekhar limit?
electron degeneracy pressure
fate for a star below the chandrasekhar limit
- red giant forms a planetary nebula
- become a white dwarf
- become invisible
fate for a star above the chandresekhar limit
- experiences supernova
- becomes a neutron star or collapses to a black hole
oppenheimer-volkoff limit
- largest mass a neutron star can have
- remnants above this limit will form black holes
H-R diagram
all possible evolutionary paths for stars
evolution of a low mass star
evolution of a high mass star
pulsar
cosmic source of very weak radio wave energy that pulsate at a very rapid and precise frequency
what would a rotating neutron star be expected to emit?
- intense beam of radio waves in a specific direction
- as a result of the star’s rotation, this beam moves around and causes the pulsation that we recieve on earth
quasars
- point-like sources of light and radio waves that are very far away
- red shifts are very large
- at limit of our observations of the universe
expansion of the universe
- almost all galaxies show red shifts
- rate of expansion should decrease as result of gravitational attraction between all the masses in the universe
is earth at the centre of the universe?
- no
- any galaxy would see all the other galaxies moving away from it
big bang
- creation of the universe
- all the matter in the universe was crushed together
- since the big bang universe has been expanding
- creation of space and time
cosmic microwave background radiation
- microwave radiation is coming towards us from all directions in space
- not always linked to a sources
what are the two ways of understanding CMB radiation?
- all objects give out electromagnetic radiation
- some time after the big bang radiation became able to travel through the universe
are the galaxies distributed evenly across the universe?
no - clustered together
what is the trend regarding the speed of distant galaxies?
moving away at a greater speed as the universe expands
Hubble’s law
general trend for galaxies is that the recessional velocity is proportional to the distance away from the earth
supernova
catastrophic explosions that can occur in the development of some stars
type I vs type II supernova
- no hydrogen in type I
- hydrogen in type II
how can cepheid variables be used to determine the distance to a galaxy?
- cepheid variable is standard candle so have different way to find luminosity
- use luminosity, state brightness equation
- about same distance to whole galaxy bc variable is in that galaxy
how can the parallax angle be used to measure the distance of a star?
tanσ = (distance from Earth to Sun)/(distance from Sun to Star)
describe the characteristics of a cepheid variable
- always same intensity
- as size increases, luminosity increases and brightness increases
standard candle advantage
have different way to find luminosity tied to period
why does cosmic background radiation provide evidence for the big bang?
- CBR is the same in all directions
- consistent with cooling off from big bang
is the universe expanding?
no - space itself is expanding
