Download Universe 8e Lecture Chapter 17 Nature of Stars

Roger A. Freedman • William J. Kaufmann III
Universe
Eighth Edition
CHAPTER 17
The Nature of Stars
M 39 is an Open or Galactic Cluster
Review of Previously Covered
Concepts
The distance (d) to a star can be determined from a
measurement of the star’s parallax (p).
Stellar Parallax
As Earth moves from
one side of the Sun to
the other, a nearby star
will seem to change its
position relative to the
distant background stars.
d=1/p
d = distance to nearby star in
parsecs
p = parallax angle of that star
in arcseconds
Some Nearby Stars
Proxima Centauri: p = 0.772 arcsec, d = 1/p = 1.3 pc
Barnard’s Star:
p = 0.545 arcsec, d = 1/p = 1.83 pc
Sirius A/B :
p = 0.379 arcsec, d = 1/p = 2.64 pc
1 pc = 206,265 AU = 3.26 LY
Review of Previously Covered
Concepts
The distance (d) to a star can be determined from a
measurement of the star’s parallax (p).
The “intrinsic brightness” or luminosity (L) of a star can
be determined from a measurement of the star’s
apparent brightness (b) and a knowledge of the
star’s distance.
If a star’s distance is known, its luminosity
can be determined from its brightness.
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As you get farther and
farther away from a star,
it appears to get dimmer.
Luminosity, L, doesn’t
change
Apparent brightness, b,
does change following
the inverse square law for
distance.
b = L / (4pd2)
If a star’s distance is known, its luminosity
can be determined from its brightness.
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A star’s luminosity can be determined from
its apparent brightness if its distance is
known:
L = 4p d 2 b
L = 4p d2 b
L/L = (d/d)2  (b/b)
Where L = the Sun’s luminosity
Example: The Sun
d = 1 AU = 1.51011 m
b = 1370 W/m2 (Solar Constant)
L = 4p d2 b = 1.256 101 2.251022 m2 1.37103 W/m2
L = 3.871026 W
Example: e Eridani
d = 3.22 pc = 3.22206,265 AU
= 6.65105 AU
b = 6.7310-13 b
L/L = (6.65105)2 6.7310-13
= 0.3
e Eri has a luminosity equal to
30% of the solar luminosity.
Review of Previously Covered
Concepts
The distance (d) to a star can be determined from a
measurement of the star’s parallax (p).
The “intrinsic brightness” or luminosity (L) of a star can
be determined from a measurement of the star’s
apparent brightness (b) and a knowledge of the
star’s distance.
The surface temperature (T) of a star can be
determined from a measurement of the star’s color
(or spectral type).
Today we will learn
17-7 How H-R diagrams summarize our knowledge of
the stars
17-6 How stars come in a wide variety of sizes
17-8 How we can deduce a star’s size from its spectrum
Let’s pause to examine the
spread of “L” and “T” values
among the stars that are
nearest to us (Appendix 4).
Plot “L vs. T” for 27 Nearest Stars
Data drawn from Appendix 4 of the textbook.
L and T appear to be Correlated
Nearest Stars
L and T appear to be Correlated
A few of the brightest
stars in the night sky
Hertzsprung-Russell (H-R) Diagram
Hertzsprung-Russell (H-R) Diagram
More complete mapping of stars
onto the H-R Diagram
Stars come in a wide variety of sizes
Stefan-Boltzmann law relates a star’s energy
output, called LUMINOSITY, to its temperature
and size.
LUMINOSITY = 4pR2sT4
LUMINOSITY is measured in joules per square meter of a
surface per second and s = 5.67 X 10-8 W m-2 K-4
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Small stars will have low luminosities unless
they are very hot.
Stars with low surface temperatures must be
very large in order to have large luminosities.
Determining the Sizes of Stars from an H-R
Diagram
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Main sequence stars are
found in a band from the
upper left to the lower
right.
Giant and supergiant
stars are found in the
upper right corner.
Tiny white dwarf stars are
found in the lower left
corner of the HR diagram.
Hertzsprung-Russell (H-R) diagrams
reveal the different kinds of stars.
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Main sequence stars
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Red giant stars
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Stars in hydrostatic equilibrium
found on a line from the upper left
to the lower right.
Hotter is brighter
Cooler is dimmer
Red Dwarfs (on MS) & Brown
Dwarfs (not on MS): lower right
corner (small, dim, and cool)
Upper right hand corner (big,
bright, and cool)
White dwarf stars
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Lower left hand corner (small,
dim, and hot)
Details of a star’s spectrum reveal whether it is a
giant, a white dwarf, or a main-sequence star.
Both of these stars are spectral class B8. However, star a is a
luminous super giant and star b is a typical main-sequence star.
Notice how the hydrogen absorption lines for the more luminous
stars are narrower.
LUMINOSITY CLASS
Based on the width of spectral
lines, it is possible to tell
whether the star is a
supergiant, a giant, a main
sequence star or a white dwarf.
These define the luminosity
classes shown on the left
occupying distinct regions on
the HR diagram.
The complete spectral type of
the Sun is G2 V. The “G2” part
tells us Teff, the “V” part tells us
to which sequence or luminosity
class the star belongs.
Example: M5 III is a red giant
with Teff ~ 3500K, M=0 (or
L=100 Lsun).
HR Diagram
This template
will be used in
the upcoming
test. Please
become familiar
with it. We will
do a few
examples in
class of how to
read off the
temperature,
luminosity and
size of a star
given a full
spectral type.
HR Diagram
I expect you to
know which of the
gray sequences is
which luminosity
class. From top to
bottom:
Ia, luminous
supergiants
Ib, supergiants
III, giants
V, main sequence
Examples:
G2V The Sun
M5III
B4Ib
M5Ia