Transcript 8hrdiagram1s

The HR Diagram
Astronomy 315
Professor Lee Carkner
Lecture 8
Flux and Luminosity
The flux is the amount of energy per
second per unit area received from a
star
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Luminosity is the amount of energy per
second emitted by the star
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We measure the flux, but we want to
know the luminosity
Inverse Square Law
What determines how bright a star looks?
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A star radiates energy in all directions
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The flux is the luminosity divided by the
area of that sphere
where d is the distance from the star
Flux decreases as the inverse square of the
distance
Inverse Square Law
Distance
How do we find distance?
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Need to use indirect methods
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Parallax
Parallax is the apparent shift in position
of an object when viewed from two
different point
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There is a simple geometrical
relationship between the shift and the
distance to the object
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Parallax Explained
p
d
½B
View position of star
now and then 6 months
later

If d is the distance to the
star and B is the baseline
(distance across Earth’s
orbit) then:
tan p = ½ B / d
Using Parallax
We normally use a simplified version
of this equation:
p is in arcseconds (60 arcseconds per
arcminute, 60 arcminutes per degree)
d is in parsecs (1 pc = 3.26 light years =
3.09 X 1013 km)

Can only use parallax to get distances out
to 100 pc (1000 pc from space)
Parallax in the Classroom
p2
Measure the offset from
straight ahead for each
position
Average p1 and p2 to get p
Measure B
p1
d
p2
p1
½B
Absolute Magnitude
We can measure the apparent
magnitude of a star (m)
If we know the distance we can find the
absolute magnitude (M)

They are related by:
m-M = 5 log d - 5
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Classifying Stars
We can measure:
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We can now find the temperature and
luminosity of near-by stars
What results do we get for a large group of
stars?
The H-R Diagram
Make a plot of luminosity versus
temperature (or absolute magnitude v.
spectral type)

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What do you see?
Stars concentrated in a diagonal band that
rums from high L, high T to low L low T
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HR Diagram
Regions on the Diagram
The line that the bulk of stars fall on is
called the Main Sequence
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Below the main sequence the stars are
hot but dim
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Above the main sequence we have stars
that are bright but cool
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Size of Stars
We can relate the temperature and
luminosity to the size with the StefanBoltzmann law
L = sAT4 or L = 4pR2sT4
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What do we find?
Red giants -- large, white dwarfs - small
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Called red dwarfs
Radius of Stars
Luminosity Classes
Luminosity classes are used to specify
where a star falls on the HR Diagram
In order of increasing brightness and size:
V -IV -III -II -I --
The luminosity class is given after the
spectral type:
e.g. the sun is G2V
Luminosity Classes
Census of Stars
A quick look at an HR diagram makes it
seem as if all regions are equally populated
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If you take a certain region of space and
count all of the stars in it, you find:
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Reasonable numbers of medium main
sequence stars and white dwarfs
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Relative Numbers of Stars
Selection Effect
Most stars are faint
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From a casual look at the sky it would
seem like most stars are bright
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When you select a group of stars to
study, the criteria you use to select
them affects your answer to your study
Spectroscopic Parallax
The main sequence is very well defined
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If you know the luminosity and you
measure the flux you can find the
distance (F = L/4pd2)
Called spectroscopic parallax
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Determining Star Properties
Physics: apparent shift Physics: Spectral lines
of object from
depend on
different vantage
temperature
points
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Physics inverse square
law
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Physics: StefanBoltzmann Law
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Finding Star Properties
Next Time
OBSERVING TONIGHT
6:30pm, observatory
Bring your observing templates
Read Chapter 10
Question of the Day:
How do we know the masses of stars?
List 1 due Friday