Astrophysics Flashcards
what is the universe?
a large collection of billions of galaxies
what is a galaxy?
a large collection of billions of stars?
what is the solar system?
a collection of planets orbiting a common star - our solar system is the Milky Way galaxy
what is gravitational field strength?
the force per unit mass on a body in a gravitational field
what is gravitational field strength measured in?
Newtons per kilogram (N/kg)
what does gravitational field strength vary with?
the mass and size of the body and is therefore different on other planets and the moon compared to Earth
gravitational force enables the various bodies to…
orbit around others
how does the moon orbit planets?
the orbits are slightly elliptical with near constant orbital speed
how do planets and comets orbit the Sun?
- the orbits of planets are slightly elliptical with near constant speed
- the orbits of comets of highly elliptical
what are examples of how gravitational force allows various bodies to orbit the Earth?
- the moon orbits planets
- planets and comets orbit the Sun
- artificial satellites orbit the Earth
the greater the orbital radius or the smaller the time period…
the greater the orbital speed
why do comets have a greater speed nearer to the star?
because the ice inside them melts as they get closer (and warmer), causing their mass to decrease
how do starts begin?
as a cloud of dust and gas called a nebula
how does nuclear fusion occur in stars?
- the particles experience a weak attraction towards each other due to gravity and begin to clump together
- they continue to clump together until the pressure and temperature is great enough for nuclear fusion to occur
- hydrogen nuclei fuse together to form helium nuclei which releases a large amount of energy and causes a great outwards pressure
- this outwards pressure balances with the inwards pressure due to gravity and the star is now stable and called a main sequence star
- eventually the hydrogen in the star is used up - there is no longer enough outward pressure from nuclear fusion and it collapses under its own gravitational attraction, becoming unstable
if a star has similar mass to the Sun…
it expands massively and becomes a red giant, then it becomes a white dwarf, and finally cools into a black dwarf
if the star has a mass larger than the Sun…
- it expands and becomes a red super giant, before exploding in a supernova
- what remains is either a neutron star, or if it was exceptionally massive, a black hole
how can stars be classified?
according to their colour - the colour of a star is related to its surface temperature, with hotter stars being bluer and cooler stars being redder
what does the brightness of a star depend on?
where it is measured - at a standard distance it can be represented using absolute or apparent magnitude
what is a stars absolute magnitude?
equal to the apparent magnitude that the star would have if it were viewed from exactly 10 parsecs (32.6) light years away
what is a stars apparent magnitude?
how bright it appears at a particular point in space
what does a HR diagram show?
the relationship between a stars surface temperature and brightness
where are the main components on a HR diagram?
supergiants = top right
red giants = below supergians
the main sequence = a diagonal strip from top left to bottom right
white dwarfs = bottom left
note - other stages don’t show on the HR diagram because a star is only in them for a short period of time
what does the Big Bang theory state?
that the universe expanded outwards from a single point
what are two examples of evidence for the Big Bang theory?
- red shift
- cosmic microwave background radiation (CMBR)
what is red shift?
- the red shift of light from galaxies shows that they are all moving away from us and that those which are furthest away are moving the fastest, suggesting that the universe was formed from an explosion at a single point, evidence for the Big Bang
- this is the principle of the Doppler effect
what is cosmic microwave background radiation?
- just after the Big Bang, lots of short wavelength radiation should have been released
- this radiation, as the universe expanded over time, would have been stretched to become microwaves
- the fact that there is CMBR present wherever you point a telescope in the sky provides evidence for the Big Bang
why is the Big Bang theory the most accepted model?
it is the only theory currently which accounts for all the experimental evidence
what is the Doppler effect?
if a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength - for example, when the siren of an ambulance is high-pitched as it approaches you and low-pitched when it goes away
what is the Doppler shift responsible for?
the red-shift of light from galaxies which are moving away from Earth, which is when the wavelength of the light coming from them increases - the faster it is moving, the more its light is red-shifted
