# Stars

What is the definition of radiant flux?
Radiant flux is the amount of energy released by a star, or alternatively, the amount of energy falling on a particular surface
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What is the definition of luminosity? What is it measured in?
Luminosity is the total amount of energy released by a star per unit time. The unit of measurement is the Watt (W).
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What is the difference between observed brightness and luminosity?
While very similar, luminosity refers to the total amount of energy released by a star; observed brightness refers only to the amount of radiation that is released in the 'visible light' part of the spectrum.
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Why do the sun and moon appear to be the same size from Earth?
Because they have a very similar angular diameter.
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How do you calculate angular diameter?
The diameter of a body divided by the distance to it.
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How would you define the phenomenon of parallax?
The effect whereby the position or direction of an object appears to differ when viewed from different positions
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What is the definition of a parsec?
A parsec is the distance that arises when 1 astronomical unit subtends an angle of 1 arcsecond
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What term refers to an objects propensity to change colour by virtue of its temperature alone?
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How much radiation does a black body reflect?
None. All the radiation is absorbed.
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What two laws loosely characterise the trends seen in black body radiation?
The hotter the whiter, the hotter the brighter
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What is Wien's law?
Wien's law is, very loosely, the hotter the whiter. It relates the surface temperature of a star to the wavelength of the visible light it emits. Mathematically:
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What is Stefan's law?
Stefan's law is, very loosely, the hotter the brighter. It combines the radiant flux with surface temperature. As the temperature increases, the amount of radiation also increases.
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What is an O type star?
A star with a surface temperature of 30,000K +
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Blue, White, Yellow, Orange, Red. Rate in which order the stars would be the hottest.
Hottest: Blue, white, yellow, orange, red.
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What are the scales used on a Hertzsprung - Russell diagram?
Luminosity in a logarithmic scale (Lsun) on the Y axis - Surface Temperature (Kelvin)
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What are the three main groups on a HR diagram?
Main sequence, red giants, white dwarf.
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What four reasons may one star appear brighter than another?
1) The star may be more luminous 2) The star is closer to you than the other star 3) The radiation from that star has been scattered or absorbed en route 4) The radiation emitted from the star is not in the visible part of the spectrum
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Explain what is meant by one arcsecond
Just as one hour is split into 60 minutes, and each minute divided into 60 seconds. One circle is composed of 360 degrees. 1 degree is made up of 60 arc minutes and 1 arc minute is made up of 60 arc seconds. This means 1 arcsecond is equal to 1/3600
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What conditions are necessary for an object to be a black body?
It must absorb all radiation incident on it; it must remain at the same temperature
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Why are there no green stars?
Because any star who has a peak in the green wavelength is also radiating blue. For some reason, it is the blue wavelength that our eyes pick up on. If it were a bit cooler, it would appear red.
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## Other cards in this set

### Card 2

#### Front

What is the definition of luminosity? What is it measured in?

#### Back

Luminosity is the total amount of energy released by a star per unit time. The unit of measurement is the Watt (W).

### Card 3

#### Front

What is the difference between observed brightness and luminosity?

### Card 4

#### Front

Why do the sun and moon appear to be the same size from Earth?

### Card 5

#### Front

How do you calculate angular diameter?