Reading
Unit 55, 56, 58, 59
Sun’s magnetism is due to
• A. iron core of the Sun
• B. heating of Corona by energetic particles generated
during Solar Flares
• C. generation of magnetic fields via fluid+magnetic
field motions
• D. neutrino flows coming from the Sun’s core
Why was adaptive optics developed?
• a. To compensate for chromatic aberration
• b. To prevent distortion of mirrors by the vacuum of
space
• c. To compensate for the image distortion caused by
the Earth atmosphere
• d. To prevent fractures of the main mirror.
The PRIMARY reason for spreading many radio
telescopes across a large area and combining the
signals at a central station (i.e. combining radio
telescopes to form an interferometer) is
• a. to produce a much sharper images of radio sources
• b. to avoid interference between signals from separate
telescopes
• c. to be able to send a more powerful signal to space
• d. ensure that cloudy weather only affects a few of
telescopes, leaving the others to continue observing
The main absorber in the atmosphere for infrared
radiation, which impedes observations of
astronomical infrared objects, is
• a. electrons in the Earth's atmosphere
• b. dust in the Earth atmosphere
• c. oxygen and nitrogen, the major constituents of the
atmosphere
• d. water vapor
Pieces of metal are heated by varying amount in a
flame. The hottest of these will be the one that shows
which color most prominently?
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•
•
•
a. blue
b. yellow
c. red
d. black
To a physicist a blackbody is defined as an object
which
• a. absorbs all radiation which falls upon it
• b. always appears to be black, whatever its
temperature
• c. always emits the same spectrum of light, whatever
its temperature
• d. reflects all radiation which falls upon it, never
heating up and always appearing black.
The specific colors of light emitted by an atom in a
hot, thin gas are caused by
• a. protons jumping from level to level
• b. an electron dropping into the nucleus, producing
small nuclear changes
• c. electrons jumping to lower energy levels, losing
energy as they do so
• d. the vibrations of the nucleus
When electromagnetic radiation is Doppler-shifted by
motion of the source away from the detector
• a. the measured wavelength is longer than the emitted
wavelength
• b. the measured frequency of the radiation remains the
same, but its wavelength is shortened, compared to
the emitted radiation
• c. the speed of the radiation is less than the emitted
speed
• d. the measured frequency is higher than the emitted
frequency.
You see this every day!
• More distant streetlights appear
dimmer than ones closer to us.
• It works the same with stars!
• If we know the total energy output of a
star (luminosity), and we can count the
number of photons we receive from
that star (brightness), we can calculate
its distance
L
d=
4pB
• Some types of stars have a known
luminosity, and we can use this
standard candle to calculate the
distance to the neighborhoods these
stars live in.
Photons in Stellar Atmospheres
• Photons have a difficult time moving through a star’s atmosphere
• If the photon has the right energy, it will be absorbed by an atom and raise an
electron to a higher energy level
• Creates absorption spectra, a unique “fingerprint” for the star’s composition.
The strength of this spectra is determined by the star’s temperature.
Stellar Surface Temperatures
• Remember from Unit 23 that the peak
wavelength emitted by stars shifts with the
star’s surface temperatures
– Hotter stars look blue
– Cooler stars look red
• We can use the star’s color to estimate its
surface temperature
– If a star emits most strongly in a wavelength 
(in nm), then its surface temperature (T) is:
T=
2.9 ´106 K × nm
• This is Wien’s Law
l
Measuring Temperature using
Wein’s Law
T=
2.9 ´106 K × nm
l
Spectral Classification
• Around 1901, Annie
Jump Cannon
developed the spectral
classification system
– Arranges star
classifications by
temperature
• Hotter stars are O type
• Cooler stars are M
type
• New Types: L and T
– Cooler than M
• From hottest to coldest, they are
B-A-F-G-K-M
O-
– Mnemonics: “Oh, Be A Fine Girl/Guy,
Kiss Me
– Or: Only Bad Astronomers Forget
Generally Known Mnemonics
Interferometry
• Stars are simply too far away to easily
measure their diameters!
– Atmospheric blurring and telescope
effects smear out the light
• Can combine the light from two or
more telescopes to pick out more
detail – this is called interferometry
– Two telescopes separated by a
distance of 300 meters have almost the
same resolution as a single telescope
300 m across!
• Speckle interferometry uses multiple
images form the same telescope to
increase resolution
The Stefan-Boltzmann Law
• The Stefan-Boltzmann Law links a
star’s temperature to the amount of
light the star emits
– Hotter stars emit more!
– Larger stars emit more!
• A star’s luminosity is then
related to both a star’s size
and a star’s temperature
A convenient tool for organizing stars
• In the previous unit, we saw that
stars have different temperatures,
and that a star’s luminosity
depends on its temperature and
diameter
• The Hertzsprung-Russell diagram
lets us look for trends in this
relationship.
The H-R Diagram
•
•
A star’s location on the HR diagram is
given by its temperature (x-axis) and
luminosity (y-axis)
We see that many stars are located on a
diagonal line running from cool, dim
stars to hot bright stars
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•
Other stars are cooler and more
luminous than main sequence stars
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–
•
The Main Sequence
Must have large diameters
(Red and Blue) Giant stars
Some stars are hotter, yet less luminous
than main sequence stars
–
–
Must have small diameters
White Dwarf stars
The Family of Stars
Stars come in all sizes…
The Mass-Luminosity Relation
• If we look for trends in
stellar masses, we notice
something interesting
– Low mass main
sequence stars tend to be
cooler and dimmer
– High mass main
sequence stars tend to be
hotter and brighter
• The Mass-Luminosity
Relation:
L  M 3.5
Massive stars burn brighter!
Massive stars burn brighter
L~M3.5
Luminosity Classes
Stellar Evolution –
Models and Observation
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•
•
•
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Stars change very little over a human lifespan, so it is impossible to
follow a single star from birth to death.
We observe stars at various stages of evolution, and can piece together
a description of the evolution of stars in general
Computer models provide a “fast-forward” look at the evolution of
stars.
Stars begin as clouds of gas and dust, which collapse to form a stellar
disk. This disk eventually becomes a star.
The star eventually runs out of nuclear fuel and dies. The manner of
its death depends on its mass.
Evolution of low-mass stars
Evolution of high-mass stars
Tracking changes with the
HR Diagram
• As a star evolves, its
temperature and luminosity
change.
• We can follow a stars
evolution on the HR
diagram.
• Lower mass stars move on
to the main sequence, stay
for a while, and eventually
move through giant stages
before becoming white
dwarfs
• Higher mass stars move
rapidly off the main
sequence and into the giant
stages, eventually exploding
in a supernova
Our Sun will eventually
A. Become white dwarf
B. Explode as a supernova
C. Become a protostar
D. Become a black hole
The spectral type of a star is most directly related to
its
a. Absolute magnitude
b. Surface temperature
c. Size or radius
d. Luminocity
Which two vital parameters are used to describe the
systematics of a group of stars in the HR diagram?
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•
•
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a. Mass and weight
b. Luminocity and radius
c. Surface temperature and mass
d. Luminocity and surface temperature