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OCR GCSE Physics P7
Topic 3 ­ Mapping the Universe…read more

Slide 2

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Parallax Angle
· Parallax can be used to measure the distance of a star
· To find the parallax angle you need to compare the position of the star
between a 6 month interval
· You then half the total angle to get the parallax angle
· The closer the star is the greater the parallax angle will be
· Parallax angles are measure in fractions of a second arc:
­ 360 degrees in a full circle
­ 60' (minutes) of an arc in 1 degree
­ 60'' (seconds) of an arc in 1'
· An object with a parallax angle of 1 second of an arc is a distance of 1
· Distance (parsec) = 1
parallax angle (sec)…read more

Slide 3

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· The luminosity of a star depends on its temperature and
· The intensity therefore varies between stars because
they are not all the same
· You can use luminosity to measure distances between
· When measuring the distance you need to stars the you
know have the same luminosity
· Some stars that are further away appear to be quite
close because they are very luminous (don't be fooled ­
you can't assume that just because they are bright that
they are near)…read more

Slide 4

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Cepheid Variable Stars
· Cepheids or cepheid variables have a regular cycle of
getting brighter and darker
· The brightest cepheids take the longest time to get to
their peak brightness and darkness
· Cepheids can be used to measure distances:
­ Part 1:
­ Find some nearby cepheids who have known distances
­ Measure their brightness and luminosity
­ Plot your measurements on a graph of luminosity against
­ Part 2:
­ Look for a cepheid in a star cluster or galaxy of interest
­ Measure its observed brightness and period of variation
­ From the period, read the luminosity off the graph
­ Use the luminosity and observed brightness to measure
the distance…read more

Slide 5

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The Curtis and Shapley Debate
· Shapley believed that the spiral nebulae were in the Milky Way galaxy
· Curtis believed that the nebulae were outside the galaxy
· Curtis won the debate overall and it turned out that his was the correct
theory (I'm pretty sure that that's correct anyway ­ sorry if it is wrong!)
· Edwin Hubble proved the theory to be correct…read more

Slide 6

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Nebulae and Distant Galaxies
· Some nebulae are irregular `blobs' of light
· While others are roughly spiral or circular shaped
· Nebulae are sometimes just gas clouds and others they are actually distant
galaxies or clusters of stars…read more

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