# P7.3: Mapping the Universe

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• Created by: Lili
• Created on: 21-06-13 15:41
What is a Light-Year?
It is the distance light travels in one year.
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How Far Away is the Closest Star After the Sun?
After the sun the nearest stars are about 4 light-years away. This means that we see light that left those star 4 years ago.
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What is the Parallax effect?
As the Earth orbits the Sun, the closest stars appear to change their positions relative to the very distant 'fixed stars'. However in reality it is the Earth that has moved not the stars.
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What is the the Parallax Angle of a Star?
It is half the angle the star has apparently moved in 6 months (When the Earth has travelled from one side of the sun to the other).
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What are Seconds of Arc?
This is the measurement for parallax angles. (1 second of arc is 1/3600 of a degree).
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What is the Equation for Working Out the Distance to a Star?
Distance to Star (in parsecs) = 1/Parallax Angle (in seconds of an arc)
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What is a Parsec?
It is the distance to a star whose parallax angle is 1 second of an arc. Astronomers use parsecs to measure distance.
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What is the Luminosity of a Star?
It is the amount of radiation it emits every second. Luminosity depends on temperature and size- the hotter and bigger the star, he more energy it radiates per second.
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What is the Observed Identity of a Star?
It describes the radiation reaching Earth from a star. It depends on the luminosity of the star and its distance from Earth.
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How Can Astronomers Calculate the Distance of How Far a Star is away from Earth?
If they know the luminosity and observed intensity.
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What is a Cepheid Variable Star?
It is a star whose brightness varies. There is a regular pattern of change as the star gets bigger and smaller. The time between peaks of brightness is the period of a cepheid variable star.
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What is the Correlation Between the Luminosity of a Cepheid Variable Star and its Period?
The greater the luminosity the longer the period.
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How Do Astronomers Calculate Distances to a Cepheid Variable Star?
Measure the period; Use the period to work out the luminosity; Measure the observed brightness; Compare the observed brightness with the luminosity to work out the distance.
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What Did Astronomers Name 'fuzzy' Patches of Light Seen Through Telescopes in the 1920s?...
Nebulae, They have different shapes including spirals. Astronomers debated spiral nebulae. Shapely thought the Milky Way was the entire universe, he dismissed the nebulae as clouds of gas. Curtis thought that the spiral nebulae were huge...
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What Did Astronomers Name 'fuzzy' Patches of Light Seen Through Telescopes in the 1920s?
...huge distant clusters of stars, other galaxies outside the Milky Way. Neither astronomers had evidence had strong enough to settle the argument.
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What is Redshift?
Astronomers study absorption spectra from distant galaxies. Compared to spectra from nearby stars, the black absorption lines for distant galaxies are shifted towards the red end of the spectrum.
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What Does Redshift Show?
It shows galaxies are moving away from us.
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What is the Speed of Recession of a Galaxy?
It is the speed at which the galaxy is the speed at which it is moving away from us. Its value can be found from the redshift of the galaxy.
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What is the Equation for Speed of Recession?
Speed of Recession (km/s) = Hubble Constant (s to the power of -1) (km/s per Mpc) X Distance (km) (Mpc)
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## Other cards in this set

### Card 2

#### Front

How Far Away is the Closest Star After the Sun?

#### Back

After the sun the nearest stars are about 4 light-years away. This means that we see light that left those star 4 years ago.

### Card 3

#### Front

What is the Parallax effect?

### Card 4

#### Front

What is the the Parallax Angle of a Star?

### Card 5

#### Front

What are Seconds of Arc?