Transcript 22 September: Starlight

Starlight continued
How study of stellar spectra reveals the nature of the stars
Spectra (plural of spectrum)
The solar spectrum
• A fundamental measurement to extract
more information from starlight
• Spread out light according to
wavelength
• See Section 16.5
Kirchoff’s Laws of Radiation
Demo of Kirchoff’s 3rd Law
Kirchoff’s First Law + Wien’s Law
• Hot, opaque objects produce
continuous spectrum
• The hotter the object, the bluer it is
• Wien’s Law wmax = 2.9E-03/T
• The hotter an object, the brighter it is
• demo
Why does Wien’s Law look like that?
A physicist is bothered when he or she sees an
equation like:
The form which emerges from fundamental
equations of physics is:
What is the little “h” that appears in the
correct form of Wien’s Law?
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The speed of light
The scale height of the Sun’s atmosphere
The Universal Gas constant
Planck’s constant
The electrical charge of the electron
Kirchoff’s First Law + Wien’s
Law
Important point: a hotter blackbody is
brighter at all wavelengths than a
cooler blackbody of the same size
Kirchoff’s Third Law:
Absorption Spectra
See Figure 16.6
Starlight…application of
spectroscopy to stars
• Continuous spectrum gives surface
temperature (Wien’s Law)
• Spectral lines give chemical
composition, temperature (also), speed
of rotation (How?) and other properties
• Examples of stellar spectra…what can
we say?
Spectral classes of stars:
O,B,A,F,G,K,M
What can you say about the temperatures of these stars?
Examples of stellar
spectra
Question: apply Wien’s
Law to the O5 star. What
Can you say about its
Temperature (relative to the
Sun?)
With information provided by
spectroscopy, we can search for
correlations between stellar
properties
What the data
show: the
HertzsprungRussell Diagram
Highest quality
data from the
Hipparchus
spacecraft
The Sun is here
The Sun is
generically
related to most
other stars.
How? Why?
Filling out the
Hertzsprung-Russell
diagram
The Hertzsprung-Russell Diagram and
the Types of Stars
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See Figure 16.20
Types of stars, important terms
Main Sequence (luminosity class V)
Giants (luminosity class III)
Supergiants (luminosity class I)
White dwarfs
What does it all mean?
The Hertzsprung-Russell Diagram
The scientific classification scheme for
the Sun
The Sun is a class G2V star…the Galaxy probably has a
billion of them
The Hertzsprung-Russell diagram is a plotting
board for the nature and evolution of stars