# P7.3

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What is a Light Year
the distance light travels in one year
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How far away is the closes star apart from the Sun
4 light-years
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Parallax Effect
When something seems like it has moved, but where you are seeing it has changed
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What is the Parallax angle of a star?
Half of the star that has apparently moved in 6 months
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Seconds of Arc
measurement for parallax angles
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What is the equation for working out the distance of a star?
distance=speed of reccession/Hubble's constant
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What is a Parsec?
measurement of distance to a star, whose parallax angle is a second of an arc.
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Luminosity of a star
Amount of Radiation it emits per second. Luminosity depends on the temperature and the size of the star
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How can astronomers calculate the distance of how far a star is from Earth
If they know the luminosity and the observed intensiity
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Cepheid Variable Star
A star in which the luminosity is not constant, it varies. There is a regular pattern of change when the star gets bigger and smaller.
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How do Astronomers calculate distances to a cepheid variable star
measure the period, work out the luminosity from the peroid, measure the observed brightness, compare the observed brightness with the luminosity
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What did Astronomers name "Fuzzy" patches
nebulae
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What is redshift?
Light coming from distant stars to the Earth which are moving away from the Earth will be shifted towards the the red end of the spectrum
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What does Redshift show?
The Universe is expanding
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Speed of Reccession Formula
speed of recession=Hubble's constant x distance
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Speed of Recession Definition
The speed in which the galaxy is moving away from us
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## Other cards in this set

### Card 2

#### Front

How far away is the closes star apart from the Sun

4 light-years

Parallax Effect

### Card 4

#### Front

What is the Parallax angle of a star?

Seconds of Arc