Transcript Star Sizes - Fort Lewis College

Charles Hakes
Fort Lewis College
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Charles Hakes
Fort Lewis College
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Outline
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Remove extra folder debris
Magnitudes and Distance
H-R diagrams
Stellar Evolution
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Fort Lewis College
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Magnitudes
• Apparent Magnitude
• how bright it looks
• depends on distance
• brightness depends on distance2
• Absolute Magnitude
• Only depends on Luminosity (how much energy
is being produced)
• Does not change with distance
• At 10pc, Apparent magnitude= Absolute
magnitude
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Fort Lewis College
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Chapter 10
Star Temperatures
(Colors)
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Fort Lewis College
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Figure 10.7
Star Colors – Orion (20°) and the Milky Way Center (2’)
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Fort Lewis College
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Which star would be the hottest?
A) A
B) B
C) G
D) M
E) O
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Fort Lewis College
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Which star would be the hottest?
A) A
B) B
C) G
D) M
E) O
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Fort Lewis College
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Star Spectral Classification
• New order is: O, B, A, F, G, K ,M.
• Remember the order...
• Oh, Be A Fine Girl/(Guy) Kiss Me
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Fort Lewis College
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Chapter 10
HR Diagrams
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Fort Lewis College
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On the H-R diagram, red
supergiants like Betelguese lie:
A) top right
B) top left
C) about the middle
D) lower left
E) on the coolest portion of the main sequence
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Fort Lewis College
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On the H-R diagram, red
supergiants like Betelguese lie:
A) top right
B) top left
C) about the middle
D) lower left
E) on the coolest portion of the main sequence
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Fort Lewis College
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Figure 10.12
H–R Diagram
of Well-Known Stars
• Plot the
luminosity vs.
temperature.
• This is called a
HertzsprungRussell (H-R)
diagram
• Need to plot
more stars!
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Fort Lewis College
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Figure 10.15
Hipparcos H–R Diagram
Charles Hakes
Fort Lewis College
• Plot many stars
and notice that
90% fall on the
“main
sequence”.
• Add radius lines,
and now have
• luminosity
• temperature
• radius
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Figure 10.14
H–R Diagram
of 100 Brightest Stars
• Most very bright
stars are also
distant
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Fort Lewis College
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Figure 10.13
H–R Diagram
of Nearby Stars
• Most close stars
are very dim
• Best estimate
now is that 80%
of stars are red
dwarfs
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Fort Lewis College
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Chapter 10
Star Sizes
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Fort Lewis College
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Star Sizes
• The luminosity of a star depends on
the stars diameter as well as its
temperature.
• When radius is combined with
Stefan’s Law:
luminosity  radius2 x T4
( means proportional to)
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Fort Lewis College
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Star Sizes
• The luminosity of a star depends on
the stars diameter as well as its
temperature.
• When surface area is combined with
Stefan’s Law:
luminosity = 4r2 T4
(= means equal)
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Fort Lewis College
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Star Sizes
• Can directly measure the radius on
very few stars. (~dozen)
• Can calculate the radius if you know
the luminosity and the temperature.
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Fort Lewis College
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Figure 10.11
Stellar Sizes
• Giants - radius
between 10x
and 100x solar
• Supergiants larger (up to
1000x)
• Dwarf - radius
comparable to
or smaller than
the sun.
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Fort Lewis College
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Figure 10.15
Hipparcos H–R Diagram
• Plot the
luminosity vs.
temperature.
• This is called a
HertzsprungRussell (H-R)
diagram
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Fort Lewis College
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Review
• What fraction of the stars on an H-R
diagram are on the main sequence.
• Enter numbers 1-9 for 10%-90%
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Fort Lewis College
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Discussion
• What fraction of the stars on an H-R
diagram are on the main sequence.
• Enter numbers 1-9 for 10%-90%
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Fort Lewis College
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Distance Scale
• If you know brightness and distance,
you can determine luminosity.
• Turn the problem around…
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Fort Lewis College
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Distance Scale
• If you know brightness and distance,
you can determine luminosity.
• Turn the problem around…
• If a star is on the main sequence,
then we know its luminosity. So
• If you know brightness and luminosity,
you can determine a star’s distance.
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Fort Lewis College
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Distance Scale
• Spectroscopic Parallax - the process
of using stellar spectra to determine
distances.
• Can use this distance scale out to
several thousand parsecs.
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Fort Lewis College
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Figure 10.16
Stellar Distances
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Fort Lewis College
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Stellar Evolution
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Fort Lewis College
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Figure 11.16
Atomic Motions
• Low density clouds are too sparse for gravity.
• A perturbation could cause one region to start
condensing.
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Fort Lewis College
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Figure 11.17
Cloud Fragmentation
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Fort Lewis College
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Figure 11.20
Interstellar Cloud Evolution
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Fort Lewis College
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
http://discovermagazine.com/2009/interact
ive/star-formation-game/
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Fort Lewis College
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H-R diagram review
• The H-R diagram shows luminosity vs.
temperature.
• It is also useful for describing how stars
change during their lifetime even though
“time” is not on either axis.
• How to do this may not be obvious.
• Exercise - Get in groups of ~four and get
out a blank piece of paper.
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Fort Lewis College
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Group Exercise
• As a group, create a diagram with
“financial income” on the vertical axis, and
“weight” on the horizontal axis.
• Use this graph to describe the past and
future of a fictitious person (or a group
member).
• Label significant events, for example
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birth
college
retirement
death
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Fort Lewis College
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Stellar Evolution
1 - interstellar cloud - vast (10s of parsecs)
2(and 3) - a cloud fragment may contain 1-2
solar masses and has contracted to
about the size of the solar system
4 - a protostar
• center ~1,000,000 K
• Too cool for fusion, but hot enough to
see. (photosphere ~3000 K)
• radius ~100x Solar
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Fort Lewis College
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How would the luminosity of a one-solar-mass
protostar compare to the sun?
A) Less than .1x as bright
B) A little lower.
C) About the same.
D) A little brighter
E) More than 10x brighter
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Fort Lewis College
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How would the luminosity of a one-solar-mass
protostar compare to the sun?
A) Less than .1x as bright
B) A little lower.
C) About the same.
D) A little brighter
E) More than 10x brighter
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Fort Lewis College
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Figure 11.19
Protostar on the H–R
Diagram
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Fort Lewis College
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Figure 11.21
Newborn Star on
the H–R Diagram
5 - Gravity still dominates
the radiation pressure, so
the star continues to
shrink.
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Fort Lewis College
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Figure 11.18
Orion Nebula, Up Close
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Fort Lewis College
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Figure 11.23
Protostars
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Fort Lewis College
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Figure 11.21
Newborn Star on
the H–R Diagram
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Fort Lewis College
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Stars A and B formed at the same time. Star B has 3
times the mass of star A. Star A has an expected lifetime
of 3 billion years. What is the expected lifetime of star B?
A) more than 9 billion years
B) about 9 billion years
C) 3 billion years
D) about 1 billion years
E) less than 1 billion years
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Fort Lewis College
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Stars A and B formed at the same time. Star B has 3
times the mass of star A. Star A has an expected lifetime
of 3 billion years. What is the expected lifetime of star B?
A) more than 9 billion years
B) about 9 billion years
C) 3 billion years
D) about 1 billion years
E) less than 1 billion years
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Fort Lewis College
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Stellar Lifetimes
• Proportional to mass
• Inversely proportional to luminosity
• Big stars are MUCH more luminous,
so they use their fuel MUCH faster.
• The distribution of star types is
representative of how long stars
spend during that portion of their life.
• Example - snapshots of people.
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Fort Lewis College
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Figure 10.21
Stellar Masses
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Fort Lewis College
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Figure 11.24
Prestellar
Evolutionary Tracks
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Fort Lewis College
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Figure 11.25
Brown Dwarfs
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Fort Lewis College
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Three Minute Paper
• Write 1-3 sentences.
• What was the most important thing
you learned today?
• What questions do you still have
about today’s topics?
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Fort Lewis College
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