Determine the Color or

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Determine the Color or Measure the Spectrum
and get the spectral type With consideration
given to temperature, determine which lines are
present in the spectrum Measure the apparent
brightness and compensate for distance by the
inverse square law Measure the Doppler Shift in
the spectrum Measure the width of spectral
lines Measure the period and periodic radial
velocity curves of spectroscopic
binaries Measure light curves and Doppler shifts
for eclipsing binaries
Surface Temperature ? Chemical
Composition ? Luminosity ? Radial
Velocity ? Rotation ? Mass ? Diameter ?
Stellar Quantities and Methods of Determination
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The H-R Diagram
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The H-R Diagram
One of the most important graphical trends in
astronomy is found by plotting the luminosity
versus temperature of various stars. Independently
done by Ejnar Hertzsprung and Henry Russell.
The graph is now known as the H-R Diagram.
Ejnar Hertzsprung (1873-1967) Henry
Norris Russell (1877-1957)
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Trends of the H-R Diagram
One of the most striking features of the graph
is that the stars are not randomly located.
They appear to fall into distinct groups on the
graph. What could this mean?
For example A grouping of stars seems to occupy
a diagonal region of the graph. The majority of
stars seem to fall within this group. Another
smaller group occupies the lower left region (and
it is relatively smaller than the diagonal
group). Two other groups occupy the upper right
region of the graph.
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The H-R Diagram
Star A is a rather dim and relatively cool
star. Star B is a very luminous and very hot
star. Star C is an unusual star in that it is
very dim yet extremely hot. Star D is another
odd star in that it is extremely luminous yet
relatively cool.
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The Main Sequence
The central diagonal region is referred to as the
Main Sequence. This name takes on meaning if you
consider that MOST (90) stars that are plotted
on a HR diagram fall within this region. Hence
stars mainly occupy the main sequence.
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The Giants Supergiants
The region in the upper right (representative of
extremely luminous yet relatively cool stars) is
occupied by fewer stars than the main
sequence. How can a star be extremely luminous
(energy per second) yet still be cool (low energy
star)?
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The Giants Supergiants
The answer is It must have a huge diameter.
Thus the stars in this region are known as
Supergiants and Giants. Less than 1 of all stars
are Giants or Supergiants. (though this graph is
a bit misleading in that respect)
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Size Comparison
The Sun and Siruis are Main Sequence Pollux,
Arcturus, Rigel, Aldebaran are Giants Antares
Betelgeuse are Super Giants
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The White Dwarfs
The region in the lower left is representative of
very dim yet extremely hot stars. How can such
a hot (high energy) star give off such a small
amount of light?
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The White Dwarfs
The answer is The object must be very small.
(surface area of the star must be small). The
stars in this region are known as white dwarfs.
White indicating the extreme heat associated
with the star. About 10 of stars are white
dwarfs. It is shown in later chapters
that white dwarfs are actually dead stars there
are no longer any fusion reactions within their
cores
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You may be ask, if the star is placed
lower on the HR diagram than another star, could
that just mean that it is further away rather
than stating that it is physically
smaller?. Keep in mind that we are plotting
the LUMINOSITY vs. temperature, NOT apparent
brightness. Remember that the luminosity is the
energy per time given off by the star (an
intrinsic property not dependent on an
observer)
The H-R Diagram is based on Luminosity, not
Apparent Brightness
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Gathering Information from an H-R Diagram
What types of information might be derived from
consideration of the distribution of stars on an
HR diagram?
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Assumption of Finite Stellar Lifetimes
On the following slides there is an obvious
assumption being made Stars have a finite
lifetime. Basically, stars are born, live,
and die. The nature of this evolution is given
in subsequent chapters. What is the logical
explanation for this assumption? Stars release
large amounts of energy. The energy must come
from somewhere and we have theorized that the
source of this energy is nuclear fusion reactions
within the core of the star. These reactions
convert mass into energy. Therefore if energy
is lost then mass is lost. There is NOT an
infinite amount of mass within a
star. Therefore, the star spends a finite time
as a source of radiant energy.
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Stellar Life Cycles
About 90 of all stars fall on the main sequence.
Therefore we can say that as stars evolve,
they spend about 90 of their life within this
region.
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Stellar Life Cycles
Based on estimates of chemical composition of
stars, there is enough hydrogen in the stars
core for the star to spend about 90 of its life
turning hydrogen into helium. Therefore, the
groupings on the HR diagram are representative of
stars in various stages of their life cycles.
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Distribution of Main Sequence Stars (Mass)

• More massive stars can compress their cores to
  higher temperatures based on gravitational
  contraction, therefore
• the stars capacity for fusion increases
• thus the stars luminosity increases
• thus for stars on the main sequence, there is a
  relationship between mass and luminosity with the
  more massive stars located in the upper left
  corner (the positions of high luminosity of the
  main sequence) and the less massive stars located
  in the lower right corner (the positions of low
  luminosity).

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Since the rate of fusion is higher in high mass
stars, they live much shorter lives than low
mass stars.
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Where are the Largest Stars Located?
smaller radius stars are in the lower left
corner and the larger radius stars are
in the upper right. The radius values shown
are in multiples of our Suns radius. For
example, Betelgeuse is about 1000 times larger
than our Sun. If our Sun was replaced by
Betelgeuse, Jupiter would be enveloped by the
giants outer layers!
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Our Sun (just a run-of-the-mill Main Sequence
Star)
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Building an HR Diagram
Click me
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Multiple Values of Luminosity for any Given
Temperature
We cannot determine just from the spectral class
what the luminosity of the star is. For
instance A K type star could be on the main
sequence or it could be a supergiant. These two
stars differ considerably in their
luminosities. However, if astronomers also
consider the nature of the absorption lines, they
can estimate the density of the outer layers of
the star and thus the size of the star.
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Spectral Lines Yield Information About Stellar
Density

• High Density Star of Temperature T
• The chances of a given frequency of light emitted
  by the star coming into contact with an atom
  within the stars outer layers is high
• Thus the chance that a given frequency of light
  will be absorbed in the outer layers of the star
  is high
• Thus the spectral lines are prominent
• Low Density Star of Same Temperature T
• The chances of a this absorption process are
  reduced (There simply are not as many atoms in
  the stars outer atmosphere to absorb them)
• Thus the spectral lines are less prominent than
  the high density star

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Density Yields Information About Stellar Size
The differences in the nature of the absorption
lines of a stars with similar temperatures is due
to the density of the outer layers And density
of the outer layers describes either a densely
packed small diameter star or an inflated large
diameter low density star
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Luminosity Classes of the HR Diagram
With spectral class AND density taken together,
the location of the star on the HR diagram can be
established, thus luminosity can be estimated.
The HR diagram can thus be further subdivided
into groups that astronomers call Stellar
Luminosity Class. Luminosity Class is thus
related to the diameter of the star Note that
all main sequence stars are Class V. For example
our Sun is a G2V star