P7 - observing the universe - revision cards

• A siderial day is thetime take for a star to return to its original position, its the time it takes for the earth to spin one on its axis 

• a siderial day is 23h 56mins 

• a solar day is the time it takes for the sun to appear in thesame position in the sky, itis thetime it takes for the earth to orbit the sun once.

• a solar day is 24 hours 

• the sun and the moon appear to cross the sky from east to west this is because the earth spins from west to east.

• the moon takes 25 hours to appear in the same position

lunar eclipse

•  a lunar eclipse happens when the moon passes behind the earth meaning no sun light can reach it.

• so the moon seems to disappear.

Solar eclipse

• The moon is the right size and distance away that when it passes between the sun and the earth, it can block out the sun.

• so a solar eclipse is when the moon blocks the light from the sun.

Eclipses dont happen very often  because the moon orbits the earth at a slight angle to earth's orbit around the sun. so most of the time they dont line up to cause a eclipse

               SOLAR                                                       LUNAR

• the positions of stars are measure from angles seen from earth 

• its like latitude and longitude on earth but for the sky 

The sky appears to turn as the earth spins so astronomers picked two Fixed positions to measure from: 

• the pole star - a star directly above the north pole
• celestial equator - an imaginary plane, running across the sky, extending out from the earths atmosphere.

There are two angles used to measure positions in the sky, these are : 

• Declination - measure by degrees and is celestial latitude
• Right accention - measure by degrees and time and is celestial longitude

• all planets orbit the sun in the same direction but at different speeds. The closer to the sun the faster the planet will orbit.

• without a telescope you can see the naked eye planets, these are : mercury, venus, mars, jupiter and saturn.

a way of remembering these planets : my valuable method jumps to saturn

• the planets seem to gradually move from west to east 

• the outer planets (mars to neptune) seem to change direction for a bit making a loop or squiggle.
  

• this happens because both the planets and the earth are orbiting the sun so we are seeing their movements relative to earth. 

• the further the planet the less frequent they change direction.

• it causes rays of light to converge (come together to a focus)

• all lenses have a principle axis , a line which passes straight through the middle of the lens.

• Thicker lenses bend light more, they are therefore described as more powerful. 
  

• Powerful lenses have short focal lengths. 
  

• The power of a lens is measured in dioptres (D) and is given by the formula:

Power =          1               
               focal length (m)

simple refracting lens :

• uses two converging lenses - objective lens and an eyepiece 

• the lenses are alligned so that they have the same principle axis and focal points.

• by the time the light rays from space arrive to earth the become parrallel 

• the objective lens converges (brings the light rays together) these parallel rays to form a real image between the 2 lenses.

• the eye lens is much more powerful that the objective lens .

Magnification = Focal lenth of objective lens /

                 focal lens of eye lens.

• most astronomical telscopes use a concave mirror instead of a convex objective lens.

• concave mirrors are shiny on the inside of the curve, parallel rays of light shining on a concave mirror reflect and converge.

• parallax angle is half the angle moved against distant background stars over 6 months (half a year).

• the nearer the object the greater the angle 

• parallax angle is measured in the unit of arcseconds 

• parallax is used to calculate the distance to stars, the smaller the angle the further the star is. 

• the distance to the nearby stars is measured in parsecs (about 3 light years)  

Distance in parsec = 1 / angle in arcsec

• intrinsic brightness (how bright it would be if you went to it depends on two things:

1) size of the star        2) how hot the star is

• the bigger and hotter a star is the brighter it is.

• as you move away from a star itwill look dimmer this is because  less light from the star reaches us (it spreads out through space) 

• so the observed brightness of a star from earth depends on:

1) intrinsic brightness       2) distance to the star

• cepheid variables are a group of stars

• they pulse in brightness - they get brighter and dimmer over a period of several days 

• how quickly they pulse is linked with their intrinsic brightness. the brighter the star the longer it will pulse 

• the amount of time a star pulses is called the pulse period

• stars with the longer pirsle have a higher intrisic brightness. 

• the star with the longest pulse period must be furthest away

• the universe was one gigantic galaxy 

• size of galaxy = 100000 parsecs across (parsec is unit of distance)

• the sun and and solar system was far away from the center 

• nebualea were huge clouds of gas and dust 

• nebulae were part of the milky way

• universe was made of many galaxies 

• size of galaxy = 10 000 parsecs across

• the sun is near the centre of the galaxy 

• the nebulae were other distant galaxies

• the nebulae were separte from the mily way 

• Neither shapley or curtis were completely right in theuir arguments, also so of their points were proven to be true. 

• shapley was right that the solar system was far from the centre of our galaxy 

• curtis was right that there were many galaxies in the universe , he was also right about the spiral nebulae was really far away.

• hubble showed that their was objects outside the galaxy 

• hubble helped solve the curtis-shapley debate by discovering the andromeda nebulea

• using the largest telescope at the time (hubble telescope) he managed to see that there were many stars including cepheid variables in the nebulea.

• Hubble was able to work out the distance to the nebulae by finding the distance to one of the cepheid variables (brightness and pulse freuency... see cards 13-14)

• The andromeda neulea was 2.5 million light years away ( this solved the nebulea debate)

• he studied other nebulae and found the same results (they were really far away) 

• Mega parsecs are used instead of parsecs if the distance is enormous

• when a galaxie moves away from us its wavelength of the light differs - it becomes redder this is called red shift

• by seeing how much of the light has been red shifted you can work out the reccession velocity (how fast it is moving away from us)

• by using red shift hubble found that distant galaxies are moving away fastest

HUBBLES LAW

speed of reccession (km/s)  = hubble constant (km/s per Mpc) x distance (Mpc)

• The value of the hubble constant is

70 km/s per Mpc       or           2 x 10^-18