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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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The Nature of Stars
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 Nature of Stars
The 30 closest stars to the Sun
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The Nature of Stars
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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Chapter 16
The Nature of Stars
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 Nature of Stars
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
m

2
2
10
d
Note: This is not exact – there are constants
left out.
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The Nature of Stars
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 Nature of Stars
The color of a star
is indicative of its
temperature. Red
stars are relatively
cool, while blue
ones are hotter.
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The Nature of Stars
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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The Nature of Stars
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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The Nature of Stars
Stellar radii vary
widely.
In the chart, note the
great changes in scale
required to show the
different sizes.
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The Nature of Stars
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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Chapter 16
The Nature of Stars
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
m

2
2
10
d
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The Nature of Stars
Spectroscopic
parallax can extend
the cosmic distance
scale to several
thousand parsecs.
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The Nature of Stars
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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Chapter 16
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Chapter 16
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