Slides in this set
Topic 1 Observatories and telescopes Topic 1 Observatories and telescopes
What is a telescope? - A telescope cannot make images bigger, because they are too far
-Makes visible the things that are not visible with the naked away makes them look brighter.
Telescopes can gather more eyes than our eyes because the
eye (*gasp*, they said `naked' oops!)
aperture is much greater then our pupil.
- The bigger the telescope, the more light it can gather as it -Waves spread as they pass through a aperture, narrower aperture =
can take in bigger wavelengths. more effect. Smaller wavelength = less diffraction.
-Through time, telescopes were made that can gather every - Light lends to spread out when it passes through an aperture:
type of electromagnetic radiation. diffraction. This means you see blurred blob of light rather then a
Lenses perfect spot. Large aperture causes less diffraction so it has a higher
- Simplest/earliest telescopes used lens. resolving power.
-Converging lens: it changes the direction of the light bringing - To see faint, distance stars, the collecting area of its objective
needs to be very large.
it together at a point.
-Parabolic mirrors (curved) can focus light
- A simple converging lens focuses different frequencies of light at
-Principal axis diff. points.
-Mirrors reflect rays of all frequencies/colours the same way.
-Larger diameters meant gathering more light. You cannot make
- Compare lenses by looking at: large lenses but can do with mirrors. Large lenses would change
-Lens with a long focal length is thin, not strongly curved. shape under its weight making it useless for focusing light. It is
difficult to make sure that large lenses have the uniform composition
- Short focal length fatter and more curved
throughout. Mirror's weight can be supported from the back and
-Power (D) = 1 / focal length (m) sides.
- Converging lens (fatter in middle) = magnification Bigger and better telescopes pge. 194
- 2 converging lens = telescope - High magnification telescope needs support (mountings). They
- Refractor uses lens to gather and focus light. must be moveable (diff. parts of sky)
- Refractor eyepiece lens next to eye (obviously) fatter and -Temp telescope structure expands and temp roses, vice versa.
more curved, objective lens (closest to object you are Windows in the atmosphere
observing) thinner lens. -Some built on mountain tops others are carried on spacecraft in
orbit around the earth.
- You can only see an image is the distance between the
- Visible, microwaves + radio waves pass thought atmos. X-rays,
lenses is correct (focusing). gamma + infrared are absorbed.
- Diverging (fatter at edges) eyepiece lens image is the right - Night sky looking though the atmos. `Twinkles' when starlight
way up. passes though the atmos. Atmos. Not uniform, some are denser then
Magnification pg. 190 others. As light passes though diff. densities, it changes direction =
- A telescope does not make a distant start look bugger it scintillation (twinkle).
remains a point of light. However, it spreads out groups of -Problems?: Light pollution, difficult to see from urban areas.
stars by magnifying the angle between them.…read more
Topic 2- Mapping the heavens Topic 2- Mapping the heavens
The spinning observatory -A smaller angle means a bigger distance
The Earth is spinning on its axis so the Sun rises from the East - Parsec = 3.1 X 10 ^ 13 km
to the West . - Light year= 9.5 X 10 ^ 12
The Moon is slowly orbiting the Earth from West to East (28 Luminosity:
days= complete orbit) -Temp: Hotter radiates more energy
Phases of the moon: Moon opp side of Earth away from Sun = - Size: Bigger, more surface area radiates energy
full moon, Moon in direction of Sun= new moon
Sidereal Day: -Star Temperatures
Stars appear to travel across the sky one every 23hrs 56m -Colours go from a dull red to a bluish white. Red is the cool
After 23hrs 56m Earth has rotated 360 but Sun has not end of spectrum, violet hot end
reached same position coz Earth has moved a bit in its orbit so - refer to graph image pg 217
has to turn extra (4m) to get to same position (refer to pg 205)
Constellations : -Galaxies
In winter diff to summer coz Earth travels half its orbit -Cepheid variable stars vary in brightness over regular
A star that which is rising at dusk in the summer will be setting intervals
at dusk in the winter - To determine the distance to a star Leavitt:
Planets sometimes travel in retrograde motion because the - Look for Cepheid variable, measure observed brightness and
Earth overtakes Mars in its orbit around Sun period of variation, determine luminosity, calculate dista
Shapley vs. Curtis debate:
Eclipses - Shapley: He found that stars formed in clusters aka globular
Solar eclipse: Moon blocks the Sun clusters and orbit centre of Milky Way
Lunar eclipse: Moon moves into Earth's shadow - Curtis: Studied spiral nebulae, felt they were very distant, on
Umbra: region of total darkness in an eclipse (refer to pg 209) different galaxies
Penumbra: region of partial darkness in an eclipse (refer to pg - Edwin Hubble's findings (pg 221) found Andromeda star was
209) one million light years away, bigger than our galaxy
Moon's umbra touches the surface of the Earth= solar eclipse - Distance between galaxies measured in parsecs
Moon passes into the Earth's umbra= lunar eclipse
Eclipses are rare because the Moon's tilted 5 to the orbit of -The changing Universe
Earth (not in line) -Redshift: Receding stars shifted towards the red end of the
Star distances - More distant the galaxy, greater its speed of recession
1 second of arc is 1/3600 of a degree - Speed of recession:
An object whose parallax angle is 1 second of arc, it has a = H C (72 ± 8km/s per Mpc) X distance expanding universe
distance of 1 parsec -Universe may have started by exploding from small point- big
Topic 3 Inside Stars Topic 3- continued....
The composition of stars Nuclear fusion
If light is passed through prism, gets diffracted into spectrum Balance of forces
from red to violet Strong nuclear force between protons and neutrons, but only
Emission spectrum: electromagnetic frequencies emitted by has a short range. Particles are separated enough so there is
excited atom as electron energy levels fall balance of strong nuclear force and electrostatic force
Absorption spectrum: dark lines on continuous spectrum. Light Fusion: two small nuclei join together, releasing energy and
from star passes through, cooler gas absorbs photons of forming one bigger nucleus .
particular energies. Nuclei repel each other coz of electrostatic force, not fuse.
Emitting light Close enough, attractive force takes over, energy released
-Electrons in an atom have certain values of energy , occupying Fission: large nucleus splits, releasing energy and few
ladder of energy levels neutrons forming two medi nucleus
Electron drops level, loses energy, emits single photon of light.
Photon energy = difference between two levels How gases behave
Absorbing light Sun mostly helium and hydrogen
Dark lines come about when electrons absorb energy from Pressure of air increasing as volume decreasing
white light Pressure: Particles move freely, at room temp at 450m/s
Electron in low energy only absorbs photon that can bring to As they move around, collide with each other and wall
higher level. Each collision causes tiny force, billions of collisions produce
White light now missing photons which correspond to dark gas pressure
lines in emission spectrum Heating up, cooling down
Wavelengths of absorptions reveal elements doing the Fixed volume of gas: as gas is cooled , pressure decreases
absorbing and from that we work out the elements in the Sun steadily
Fixed pressure of gas: as gas cools, volume decreases
Atoms and nuclei Absolute zero= -273C (Kelvin scale)
Geiger and Marsden observed: Temp in K = temp in C + 273
most of the alpha particles passed straight through the gold Temp in C = temp in K 273
foil, deflected by no more than a few degrees Kinetic explanation
A small fraction of the alpha particles were actually reflected Particles lose energy when cooled:
back towards the direction from which they had come If volume of gas is fixed, longer to reach wall, particles strike
Rutherford realised something positive repelling + alpha less frequently with less force = pressure decreases
particles, with big mass or alpha particle would push it out of way Pressure constant: volume of gas decreases to compensate for
And nucleus must be tiny coz some particles flew straight past weaker less frequent collisions
Eventually lose all KE, no collisions = no pressure= Absolute
Topic 4 The lives of stars Topic 4 The lives of stars
Stars have diff. brightness and colours, why? Death of a star
Stars look dim because they were a long way off. Knowing the -Fusion reactions convert H to He so the Sun's mass decreases
distance you can find out the intrinsic (true) brightness. slowly. As fusion slows down, its core cools down so there is
Diff, colours diff temps. Red is cool, blue is hot H-R diagram on less pressure and it collapses. The outer layer of the Sun
The sun is roughly at the middle of the chart. 90% of stars are contains H, falls inwards becoming hot. Causes new fusion
main sequence (most of lifetime). 10% white dwarfs (small and reactions making outer shell expand. Same time, the surface
hot). 1% red giants or supergiant (bright, not hot) temp, falls so colour changes from yellow to red red giant.
Main sequence -While the outer layers of a red giant is expanding, the core
Main sequence: surface temp of sun too cold for nuclear fusion. contacts and heats up to allow new fusion reactions to start.
Inside the sun nuclear fusion requires high temps. Energy He nuclei have a bigger positive charge than H nuclei so there
leaves the Sun from its surface photosphere. A star like the is greater electrical repulsion between them. If they are to
Sun can burn for billions of years so it must radiate energy at the fuse, they need greater momentum to overcome this
same rate that it generates it from fusion reactions. repulsion. When He nuclei fuse, they for heavier elements
The core hottest nuclear fusions occur. Hydrogen nuclei are such as C, N, O2. After a few million years, outer layer cools
fused together to form Helium nuclei, releasing energy. Photons and drift off into space. The collapsed inner core remains as a
travel outwards though the radiative zone: surrounds the core. white dwarf. Bigger starts red supergiants
Close to surface, temp. falls. Matter can flow quite readily, and Supernova
convection currents are set up, carrying heat energy to the -By fusing lighter elements together the supergiant's core has
photosphere. EM radiation is emitted by the photosphere and become largely composed of iron. . Iron nuclei absorb energy
radiates outwards through the atmos. when they fuse so the process slows down. The outer layers
Protostar are no longer held up by the pressure of the core so they
-Raw materials of stars is H. A cloud of dust and gas in space collapse inwards. The core is very dense and the outer
start to contract, pulled together by gravity. Each particle material collides with its core and bounces off, flying outwards
attracts every other particle so the cloud collapses towards the supernova. Temp rise enough to cause the fusion of
centre. Temp. of gas increases when it is compressed. It gets so medium- weight elements so the heaviest elements form up
hot that it starts to glow. When temp. is hot enough for nuclear to uranium. As a supernova becomes distributed through
fusion a star is born (like me). Material further out the disc space, it may become part of another contracting cloud of
form planets. dust and gas.
Proto starts get hot enough for fusion to start - The core of an exploding supernova remains . If the mass is
The gas idea: the star starts from a cloud of gas. When gas is less than 2.5 solar masses neutron star. The core spins
compressed (pulled together by gravity), temp. rises. faster and faster so its magnetic field becomes concentrated
The particle idea: Every particle attracts every other particle. As and this results in a beam of radio waves coming out of its
the fall inwards, they move faster (GPE is converted to KE). The magnetic poles. A massive remnant collapses even further
particles collide with each other, sharing their energy. Fast under the pull of its own gravity to become a black hole
moving particles means a high temp. strong gravity pull.…read more