AST201H1 Lecture Notes - Lecture 3: Parsec, Main Sequence, More Stars

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25 Aug 2016
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AST201 Week 3
Properties of the Stars
How bright is the Sun?
- Luminosity: the amount of energy an object emits per unit time. It is the total amount of
power (energy per second) the star radiates/emits into space
Compared to a light bulb. 100-Watts  unit of power, how much a light bulb will
heat up per second/ amount of energy per second. It puts out the same amount of
light (luminosity) but its brightness depends on how far away you are from it
Luminosity of the Sun – 4x 1026 W. One solar luminosity or 1L
Luminosity tells how bright something is naturally (by looking at it)
Main property that will determine a star’s luminosity is its mass – more mass,
more luminous. It will determine the rate of nuclear fusion reaction and how
much energy is released
To measure a star’s luminosity we need to know its apparent brightness (how
bright it appears to be) and distance
Luminosity/ solar luminosity does not change no matter how many light years it is
seen from
- To determine a star’s surface temperature we would need to measure the wavelength at
which it emits the most light  peak of the blackbody spectrum, colour, spectral lines =
temperature
- To determine a star’s chemical composition we would need to measure the wavelengths
at which its spectrum shows absorption lines (spectral lines tells the chemical
composition).
- Apparent brightness is how bright a star looks to us on earth, as it appears to our eyes,
from the sky. It is the amount of starlight reaching the Earth (energy per second per
square meter)
Different sizes  misconception, glow that makes it looks bigger is what the
telescope picks up/ makes it look like this with lens
The Inverse Square Law for Light
- Of all the light a star emits, Earth receives only a small fraction of it. This fraction
determines the star’s apparent brightness as seen from Earth
- The closer a star is to Earth, the higher the fraction of its light we receive. Closer stars are
brighter, further away the dimmer it is
- The amount of light we receive from a star falls with the square of its distance 
luminosity divided by the distance squared!
- The brightness we perceive for a star decreases as its luminosity divided by the square of
its distance increases from us (further distance = less light seen)
Measuring Distances
How to do it: we can measure the distances to nearby stars using parallax: the
small annual shifts in a star’s apparent position caused by Earth’s motion around the sun.
The smaller the angle, the further away a star is. Distance to distant stars cannot be
measured using the parallax method because the angle is so small that its too difficult to
measure
Stellar parallax is caused by Earth’s orbit around the Sun
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