physics - astronomy (7.1 - 7.19)

7.1 weight is... acting on you

weight is force of gravity acting on you

7.1 what does gravitational field strength on surface of body (e.g. planet/moon) depend on?

mass of body

its radius (distance from its centre to its surface)

(greater mass & smaller radius = greater surface gravity)

7.2 what does our solar system consist of?

sun (our star)

8 planets & their natural satellites

dwarf planets

asteroids & comets

7.3 names & order (distance from sun) of the 8 planets

Mercury (closest to sun)

Venus

Earth

Mars

Jupiter

Saturn

Uranus

Neptune (furthest from sun)

mnemonic for remembering order of planets

My

Very

Excellent

Mother

Just

Serves

Us

Noodles

7.4 how have ideas about structure of solar system changed over time?

Ptolemy: geocentric model - earth at centre

Copernicus: heliocentric model - sun at centre

Galileo Galilei: discovered 4 of Jupiter’s moons - not everything orbits earth

more discoveries: Uranus; Neptune; Pluto; asteroids; comets

current model: 8 planets, 5 dwarf planets, thousands of comets, millions of asteroids - all move in elliptical orbits around sun; many planets have natural satellites (moons) orbiting them

7.5 orbits of moons, planets, comets & artificial satellites

elliptical orbits

7.5 orbits of artificial satellites

type of orbit depends on use

7.6 circular orbits - force of gravity leads to changing velocity of planet but unchanged speed

satellite/planet/moon in circular orbit:

• constant speed as travels

• constantly changing direction

velocity is vector quantity - orbiting body has constantly changing velocity (due to constantly changing direction)

moving object continues moving in straight line unless force acting on it to make it change speed/direction

gravitational force between earth & satellite in orbit at right angles to direction of movement - force changes its direction, not its speed

7.7 stable orbit - radius changes if orbital speed changes

satellite in lower orbit = gravitational force on satellite higher

satellite in low orbit moving much faster to stay in its orbit

satellite slows down → falls to earth → gains speed as falls until moving fast enough to stay in new, lower orbit

satellite goes low enough to encounter top of atmosphere → contact with air slows it down → eventually falls to earth

7.8 steady state theory

universe always existed

universe expanding

new matter continuously created as it expands

7.8 big bang theory

whole universe & all matter in it started as tiny point of concentrated energy

universe expanded from this point

universe still expanding

as universe expanded, gravity made matter clump together to form stars

7.9 evidence supporting big bang theory - red-shift

measure red-shift of galaxies & other distant objects - shows more distance objects moving away faster than closer ones

explained if universe expanding (BBT)

7.9 evidence supporting big bang theory - cosmic microwave background (CMB) radiation

BBT: huge amounts of radiation released at beginning of universe

universe expanding → wavelength of this radiation increased → now only detectable as microwave radiation

called cosmic microwave background (CMB) radiation

7.10 which model for origin of universe accepted & why?

more evidence supporting BBT than SST

BBT currently accepted model for origin of universe

7.11 wave source is moving relative to an observer - what happens to frequency & wavelength?

there will be change in observed frequency & wavelength

wave source moving away from observer: frequency decreases; wavelength increases

wave source moving toward observer: frequency increases; wavelength decreases

7.12 red-shift in light received from galaxies at diff. distances from earth

visible light spectrum from stars contains patterns of dark lines

star moving away from us → wavelength increases → pattern of lines red-shifted (move towards red end of spectrum)

greater red-shift = star moving away from us faster

red-shift is measure of how far along spectrum lines moved

7.13 how does red-shift of galaxies provide evidence for universe expanding?

greater red-shift = star moving away from us faster

red shift shows space between galaxies is expanding - wavelengths increased due to space expanding

7.14 how do BBT & SST of origin of universe account for red-shift of galaxies?

both theories: universe is expanding

observations of red-shift in light from other galaxies supports both theories

7.15 how did discovery of CMB radiation lead to BBT becoming currently accepted model?

CMB radiation is from radiation released at beginning of universe

SST: universe always existed - cannot explain CMB radiation

protostar

stage before main sequence

7.16 evolution of stars of similar mass to sun - nebula

cloud of dust & gas (mainly hydrogen)

materials pulled together by their own gravity

collapses due to gravitational attraction

cloud contracts - becomes denser

hydrogen becomes hotter

more mass attracted - cloud’s gravitational pull gets stronger & heats material even more

eventually temps. & pressures in centre of protostar high enough - hydrogen nuclei fuse together, form helium

fusion reactions release lots of energy as EM radiation

outward pressure from hot gases balances compression due to gravity

7.16 evolution of stars of similar mass to sun - star (main sequence)

remain stable for about 10 bil. years

fused most of hydrogen into helium - core not hot enough to withstand gravity & collapses

outer layers expand - form red giant (much larger than o.g. star)

7.16 evolution of stars of similar mass to sun - red giant

other fusion reactions happen inside red giants - e.g. combining helium nuclei to form heavier elements

star remains as red giant for about 1 bil. years

star then throws off shell of gas

rest of star pulled together by gravity

collapses to form white dwarf

7.16 evolution of stars of similar mass to sun - white dwarf

no fusion reactions inside

gradually cools over about 1 bil. years - becomes black dwarf

7.16 order of life cycle of stars of similar mass to sun

1. nebula

2. protostar

3. main sequence star

4. red giant

5. shell of gas

6. white dwarf

7.17 balance between thermal expansion & gravity in life cycle of stars

in main sequence star

outward pressure from nuclear fusion balanced by inward pressure of gravity

7.18 evolution of stars with mass larger than sun

1. fuse hydrogen into helium faster

2. become red supergiants

3. star rapidly collapses then explodes in supernova, outer layers of supergiant cast off & expand outwards

4. very big: gravity pulls remains together - forms black hole

5. less big: gravity pulls remains together - forms neutron star

7.18 order of life cycle of stars with mass larger than sun

1. nebula

2. protostar

3. massive main sequence star

4. red supergiant

5. supernova

6. neutron star/black hole

7.19 how have methods of observing universe changed over time - photography

more detailed observations & measurements than by drawings

7.19 how have methods of observing universe changed over time - computers

increased speed & detail for analysis of info. from telescopes

7.19 how have methods of observing universe changed over time - telescopes

telescopes that orbit earth - much clearer images than ground-based telescopes

detect diff. types of EM waves

7.19 why are some telescopes located outside earth’s atmosphere?

clouds & dust don’t interfere with image

atmosphere absorbs some of radiation they detect

7.19 how have methods of observing universe changed over time - space probes

investigate solar system