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Astronomy
Basic Properties of Stars
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Kirchhoffs Three Kinds of Spectra
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A Model of a Hydrogen Atom
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Spectral Lines
A. Electrons have a definite bindingenergy.
B. Each element has its own set ofenergy levels
C. If an electron absorbs enough
energy, it jumps to a higher energylevel.
D. When an electron falls, itreleases energy in the form of light.
E. wavelength inversely
proportional to frequency F. Dark lines are produced when a
cooler gas absorbs light.
G. An emission spectrum showsthe chemical element that produced
those lines.
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Electron Distances and Energy Levels
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Possible Absorption and
Emission Lines for the
Hydrogen Atom
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An Emission Spectrum of Hydrogen
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Stellar Spectra A. Predominantly patterns of dark lines on acontinuous band of colors.
B. Stars bright visible surface is called thephotosphere.
C. As light travels through the stars outeratmosphere, the cooler gases absorb somecolors/wavelengths.
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Chemical Composition A. Our sun was the first absorption spectrum analyzed in 1814
by Fraunhofer
1. Fraunhofer lines--strongest dark lines from the sun
B. By comparing the dark lines with spectral lines from otherelements, we find whats in the sun.
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Spectral Classes A. Absorption spectra are used to classify
stars into 7 types.
B. If hydrogen lines are stronger
1. Its not because of more hydrogenALL
stars have hydrogen.
Stars are classified in the following order:
O, B, A, F, G, K, M
oh, be a fine girl/guy, kiss me !
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Spectral Classes E. So whats the difference?
1. Stars at different temperatures display
certain lines better than others.
The temperature is the difference !
class O stars are hottest.class M stars are
coolest.
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The Spectra of Radiation Emitted
with Temperatures of 4500 K,
6000 K, and 7500 K Things will become
bluer when theyare hotter.
Stars will becomeredder when theyare cooler.
If we can find the
brightest part of thespectrum of a star,we can find itstemperature.
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Temperature
B. Every chemical element hasa characteristic temperatureand density at which its mosteffective in producing certain
lines. C. At extremely high temps.--
Helium atoms are ionized;bluer stars (class O)
D. Temps. Around 5800 K--metal atoms
E. Temps. Below 3500 K--titanium oxide molecules;
redder stars (class M)
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Spectra of the Spectral Classes
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The Relative Number of Hydrogen Atoms in
the Second Energy Level for
Various Temperatures
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The Number of Hydrogen Atoms with
Their Electrons in the Second Energy Level
Compared with the Total Amount ofHydrogen, Whether in Atomic
or Ionized Form
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The Relative
Numbers of Atoms
of Different
Elements on a
Typical Star
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Other information from Spectral
Lines A. Other info is gathered from spectral lines.
B. Collisional broadening--broader lines might show
a denser star C. Rotational broadening--broader lines can show
how fast a star rotates/spins
D. Zeeman effect--split lines show magnetic fields
E. Redshift--lines shifted toward the red show a starmoving away (blueshiftmeans star is movingtowards you)
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The Spectra of a Rapidly Rotating Star
and a Slowly Rotating Star
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The Doppler Shifts of a Rotating Star
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The Parallax of a Nearby StarA parsec is a unit of distance such that a star that exhibits a shift of 1
(1 second or 1/3600 of a degree) of arc. This is only an apparent shift of
the star in the sky (and its very small) as a result of the real motion of
the earth around the sun. We are looking at the star at different angles.
The distance of a star can be found by observing its parallax angle. The
equation is: distance (in pc) = 1 / parallax angle()
Example: Alpha Centauri has a parallax angle of 0.742. So its
distance from Earth is 1/0.742 = 1.35 parsecs. To convert this to light
years (1pc = 3.26 ly): 1.35 pc x 3.26ly = 4.4 ly
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Propagation of light Remember that
light falls off
according to theinverse square law
An object 3x farther
away will appear1/32 = 1/9 as bright
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Apparent magnitude (m) Definition: a
measure of how
bright a starappears
The general rule:
the lower thenumber, the
brighter it appears
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Apparent magnitude (m)
The modern magnitudescale is set up so that a
difference in
magnitudes goes up as
an exponential function
2.512(x)
Where x is the difference
in apparent magnitudes of
A and B
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Absolute magnitude (M) Definition: a measure of how much
light a star is putting out into space
(its luminosity)
The general rule: the lower the
number, the more luminous it is
Note: you cant just say, that star
is brighterdo you mean it
appears brighter, or do you mean
that its giving off more light?
Question: Why would it matter?Answer: a really luminous star
might appear fainter simply
because its very, very far away
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Absolute magnitude (M)