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Lecture 7

Lecture 7-Chapter 15-Surveying the Stars

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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Chapter 15 ± Surveying the Stars
Measuring Stellar Luminosities
x The brightness of a star as it appears to our eyes is its apparent brightness. It is the
amount of power reaching us per unit area.
x Luminosity is the total amount of power that a star emits into space. It tells us how
bright stars are in an absolute sense.
o A 100-watt light-bulb has always the same luminosity, since it always put out
the same amount of light. However, it will look brighter if u stand closer and
dimmer if farther, thus its apparent brightness changes.
The Inverse Square Law for Light
x Apparent brightness = luminosity / (4 pie X distance^2)
x The units of apparent brightness are watts per square meter.
x $&&'FDQEHXVHGWRPHDVXUHDVWDU¶VDSSDUHQWEULJKWQess as it records how much
energy strikes its light-sensitive surface each second.
the Sun.
x The distance to an object with a parallax angle of 1 arcsecond is 1 parsec (pc).
x d (in parsecs) = 1 / p (in arcseconds)
x d (in light-years) = 3.26 X [1 / p (in arcseconds)]
The Luminosity Range of Stars
x Stars have a wide range of luminosities. The dimmest stars have luminosities
1/10,000 times that of the Sun, while the brightest stars are about 1 million times
as luminous as the Sun.
The Magnitude System
x The magnitude system is an ancient way to describe apparent brightness and
luminosity quantities by classifying stars according to their brightness ± first
magnitude (brightest) to magnitude 6 (faintest).
x Absolute magnitudes are a way of describing stellar luminosities. It is the apparent
magnitude it would have if it were at a distance of 10 parsecs.
Measuring Stellar Temperature
Colour and Temperature
x Stars come in different colours because they emit thermal radiation.
x An ideal thermal radiation spectrum depends only on the surface temperature of
the object that emits it.
x Cooler stars look red because they emit a little more blue light than red light.
x Hotter stars emit more blue light than red light and therefore have a bluer colour to
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