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Physics - Astrophysics Section 7 (Motion in the universe (Largest to…
Physics - Astrophysics Section 7
Motion in the universe
HAVE TO KNOW
The universe is a
large
collection of
billions
of galaxies. A
galaxy
is a
large
collection of
billions
of
stars
. Our solar system is in the
Milky Way
galaxy.
Largest to Smallest
Universe
Galaxy
Solar System
Sun
Planet
Why do different planets have different gravitational field strength?
Because they have different sizes and masses. More mass = more gravitational field strength it has
What does gravity cause?
Things to fall to the surface of a planet
The planets to orbit the sun
Moons to orbit planets
Artificial satellites to orbit the Earth
Comets to orbit the sun.
The differences in the orbits of comets, moons and planets
Comets orbit
stars
Planets orbit
stars
(like the sun)
Moons orbit around the
planets
Comets have
elongated
orbits
All have
Elliptical
orbits
Orbital Speed
Equation:
Orbital speed in metres per second (m/s)
Orbital radius in metres (m)
Time period in seconds (s)
Stellar Evaluation
Spectral class
Oh Be A Fine Girl Kiss Me
Life cycle of stars
Look at book for description of each stage.
Hertzsprung Russell Diagram
This diagram is effectively a graph of brightness (measured by absolute magnitude) against temperature (or spectral class) for stars.
Each star has its own unique position on this graph.
Cosmology
The main argument is the Big Bang Theory
Evidence is Red shift
Doppler effect red shift and calculating recessional speed.
When will a comet be moving fastest?
The comet will be moving fastest when it is further away from the star (right end of the ellipse)
The visible Spectrum
Edwin Hubble made a series of observations of some very distant galaxies - all of them well outside the Local group.
By splitting their light into its spectrum he found that its wavelengths had all been stretched or 'shifted' to the red (longer) end of the spectrum.
Naming this effect ‘red shift’, Hubble realised that it must be caused by the galaxies all moving away from us at great speed.
Hubble
Since all the galaxies which he observed appeared to be moving away from us, with the more distant galaxies moving fastest of all, Hubble realised that these observations meant that the universe was
expanding.
Hubble was able to use the Doppler Effect equation to calculate exactly how quickly each galaxy was moving away from us
Doppler Effect Equation
Change in wavelength/ wavelength
Expanding Universe
In one sense the galaxies themselves are not moving around the Universe
This also explains why more distant galaxies seem to be moving away from us more quickly.
The key idea is that it is the space between the galaxies which is getting bigger.
There is more space between us and a distant galaxy so more space to expand.
