Ch. 16 - Astro1010

Report
The Nature of Stars
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Chapter 16
The Nature of Stars
Stellar Distance Scales
Light Year = the distance that light travels in
one year
Parsec = the distance to a point where 1 AU
subtends one second of arc
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Remember that nearby
stellar distances can be
measured using parallax
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Nearest star to the Sun is Proxima Centauri
which is a member of a 3-star system: Alpha
Centauri complex
Model of distances:
Sun is a marble, Earth is a grain of sand orbiting
1 m away
Nearest star is another marble 270 km away
Solar system extends about 50 m from Sun; rest
of distance to nearest star is basically empty
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The 30 closest stars to the Sun
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Brightest stars were known to, and named by, the
ancients (Procyon) In 1604, stars within a constellation
were ranked in order of brightness, and labeled with
Greek letters (Alpha Centauri)
In the early 18th century, stars were numbered
from west to east in a constellation (61 Cygni) As more
and more stars were discovered, different naming
schemes were developed (G51-15, Lacaille 8760, S
2398)
Now, new objects are simply labeled by their
celestial coordinates
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Brightness Scales
Apparent magnitude
Hippachus
1st to 6th
Spica (1st Mag), Vega (0 Mag)
Brightness (Luminocity)
measured by light meter
An Apparent Magnitude difference
of 5 represents a Brightness ratio of
100/1
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Apparent
Magnitudes of
several objects
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The Absolute Magnitude (M) of stars is
defined as the apparent magnitude that the
star would have if were at 10 parsecs
distance. Then the following ratio holds:
M
10
2

m
d
2
Note: This is not exact – there are constants
left out.
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Luminosity, or absolute brightness, is
a measure of the total power radiated
by a star.
Apparent brightness is how bright a
star appears when viewed from Earth;
it depends on the absolute brightness
but also on the distance of the star
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Two stars that
appear equally
bright might be a
dimmer, nearer
star and a brighter,
farther star
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The color of a star
is indicative of its
temperature. Red
stars are relatively
cool, while blue
ones are hotter.
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Spectral classes make up a Temperature
Sequence
O, B, A, F, G, K, M
O hottest
M coolest
Oh Be A Fine Girl Kiss Me
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These are representative spectra of each
class.
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For the vast majority of stars that cannot be imaged
directly, size must be calculated knowing the
luminosity and temperature:
Supergiant stars are more than 100 solar radii
Giant stars are between 10 and 100 solar radii
Upper main sequence stars are 8 to 100 solar radii
Average stars are 0.5 to 8 solar radii
Dwarf stars are 0.1 to 0.5 solar radii
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Stellar radii vary
widely.
In the chart, note the
great changes in scale
required to show the
different sizes.
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Ejnar Hertzsprung (8 Oct,
1873 - 21 Oct, 1967) was a
Danish chemist and
astronomer.
Henry Norris Russell (Oct 25,
1877 – Feb 18, 1957) was an
American Astronomer.
Together they invented one of most useful graphs
in Astronomy
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The Hertzsprung–
Russell Diagram
The H–R diagram
plots stellar
luminosity against
surface temperature.
This is an H–R
diagram of a few
prominent stars
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Once many stars are plotted
on an H–R diagram, a
pattern begins to form.
These are the 80 closest
stars to us; note the dashed
lines of constant radius.
The darkened band is called
the main sequence, as this is
where most stars are.
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An H–R diagram of the 100
brightest stars looks quite
different, These stars are all
more luminous than the Sun.
Two new categories appear
here – the red giants and the
blue giants.
Clearly, the brightest stars in the
sky appear bright because of
their enormous luminosities,
not their proximity.
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Major Sections of
the H-R Diagram.
They start with
the Main
Sequence , then
the two Giant
Stages then
finally the White
Dwarf Stage
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This is an H–R plot of
about 20,000 stars. The
main sequence is clear,
as is the red giant
region.
About 90% of stars lie
on the main sequence;
9% are giants and 1%
are white dwarfs.
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Spectroscopic parallax: has nothing to do
with parallax, but does use spectroscopy
to extend our ability to determine the
distance to a star
1. Measure the star’s apparent magnitude
(brightness) and spectral class
(temperature)
2. Use temperature to estimate luminosity
3. Apply inverse-square law to find distance
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Spectrographic
Parallax can be seen
on this H-R Diagram
and the definition of
Absolute Magnitude
M
10
2

m
d
2
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Spectroscopic
parallax can extend
the cosmic distance
scale to several
thousand parsecs.
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The spectroscopic parallax
calculation can be
misleading if the star is not
on the main sequence. The
width of spectral lines can
be used to define luminosity
classes
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Determination of Stellar Masses
Many stars are in binary
pairs; measurement of
their orbital motion allows
determination of the
masses of the stars.
Visual binaries can be
measured directly; this is
Kruger 60:
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Equations needed to determine
individual masses
(M1 + M2)P 2= d3
M = M1+ M2
M1
d1
d2
M2
d = d1 + d2
M1d1 = M2d2
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Study of spectral lines reveals the motion of
spectroscopic binaries and hence their
spacing. From that the masses are calculated.
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The mass of a star is also correlated with its
radius, and very strongly correlated with its
luminosity.
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Mass is also related to stellar lifetime
Using the mass–luminosity relationship
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So the most massive stars have the
shortest lifetimes – they have a lot of
fuel but burn it at a very rapid pace.
On the other hand, small red dwarfs
burn their fuel extremely slowly, and
can have lifetimes of a trillion years or
more.
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In –Class Quiz
Spectroscopic parallax: has
nothing to do with parallax, but
does use spectroscopy to
extend our ability to determine
the ______________ to a star
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