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Class events: week 11
Goals
Learn about stars—a primer from basic astronomy
Luminosity (and brightness)
• Distance
• Temperature
• Hertzprung-Russell diagram
• Radii
• Mass
• Life expectancies and life histories
•
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Stars—basic parameters
Luminosity
The rate at which electromagnetic energy is emitted - the total
amount of power emitted by a star over all wavelengths.
Brightness
How much energy we receive from a star. It is modified by a star’s
distance.
Distance
Key in understanding stars. Parallax is the most direct measure, if it
can be done. (100,000 stars to 1000 ly)
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Stars—basic parameters
Typical stellar temperatures
3000 K — 50,000 K
Spectra
Recall that the peak in a star’s continuum spectrum is determined by its
temperature (Wein’s Law).
Spectral lines
Furthermore, recall that the presence of absorption lines and emission
lines reveal the chemical composition of the luminous gas.
Classification
Based upon the strength of absorption lines, spectra of stars can be
classified: OBAFGKM
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The Hertzprung-Russell diagram
The many parameters of stars are confusing.
Astronomers discovered that order could be revealed by plotting two
aspects of stars on a single graph.
1) Temperature—expressed directly in Kelvins, or by color, or by
spectral type.
2) Luminosity—expressed directly in solar units (L), or in a
system called absolute magnitude.
The Hertzprung-Russell diagram is one of the most powerful tools
stellar astronomers have in understanding stars.
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The Hertzprung-Russell diagram
An example of the HR diagram in use
Recall that we know the temperatures of stars on the HR diagram.
We also know the luminosities of stars—they range from 0.001—
106 L.
Note the Stefan-Boltzmann law:
L = 4p R2sT 4
We can therefore determine the radii (sizes) of stars, and plot that on
the HR diagram.
Some stars are huge!
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Binary stars
Recap: we now know the following about stars:
Luminosities
Temperatures
Radii
Population distributions
Most stars are not single stars such as our own—most occur in binary or multiple
star systems. Some are just near each other in space, others orbit around each other.
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Binary stars
We can observe binary stars different ways. For examples…
…Astrometry charts the orbits of stars directly, over time.
…Spectroscopy is used to observe how the motions of the stars affect
their spectra.
The power of binary stars is that we can learn about the masses of stars.
Once we know a star’s mass, and we know its luminosity, we can learn
how long it will live (since a star is burning itself up, like a campfire).
t=1010(M/L) yrs
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Stars—a life of gravity vs. pressure support
Star formation begins from interstellar material, which
collapsed into dark nebulae.
The lowest mass proto-stars never quite initiate nuclear
fusion. These objects are called brown dwarfs. More
massive objects settle onto the main sequence, where
they burned hydrogen into helium.
After burning helium into carbon,
stars run out of fuel and collapse into
white dwarf stars, producing beautiful
planetary nebulae in the process.
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Stars—a life of gravity vs. pressure support
Stars more massive than our Sun (M=2—40 M) have cores that are so
hot they can burn further elements, extending their lives a few percent.
The most massive stars even attempt to burn the
element iron. This results in a catastrophic core
implosion—a supernova.
If the star does not completely blow itself apart, it may
remain as an extremely dense, compact object. This
object may be a neutron star (such as a pulsar) or a
black hole.
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Stars—a life of gravity vs. pressure support
Binary stars can have even more complicated lives.
When the giant interstellar clouds
fragment into stars, they tend to form
many low-mass stars, a medium number
of moderate-mass stars, and extremely
few high-mass stars.
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Studying stars from an astrobiological perspective
Knowing what we do of stars, we can predict their rapid formation,
long main sequence lifetimes, and speedy death processes. What are
the astrobiological ramifications?
O-B stars: ~0.1% of all stars
Timescales
Life spans of 0.5—50 million years, too short for the development of life
(although possibly enough time for planetary formation with B stars).
Radiation
OB stars produce enormous amounts of sterilizing, ultraviolet radiation.
Habitable zone
Liquid water would be stable over an enormous range of distances.
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Studying stars from an astrobiological perspective
A-F stars: ~3% of all stars
Timescales
Life spans of 1—2 billion years, enough time for at least primitive
life to form.
Radiation
Significant amounts of sterilizing ultraviolet radiation. Life would need to seek
shelter under ice, rocks, or perhaps under a thick ozone layer that might form in
response to the heavy ultraviolet irradiation.
Habitable zone
Very large compared to our Sun’s habitable zone.
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Studying stars from an astrobiological perspective
G stars: ~7% of all stars
Timescales
Life spans of 10 billion years, enough time for multicellular
life to form?
Radiation
Moderate amounts of sterilizing ultraviolet radiation. Life must seek shelter under
ice, rocks, or under a moderate ozone layer that might form in response to the
moderate ultraviolet irradiation.
Habitable zone
Inner solar system.
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Studying stars from an astrobiological perspective
K-M stars: ~90% of all stars
Timescales
Life span of 20-600 billion years; 2-60 times the Sun’s lifespan!
Radiation
Sterilizing ultraviolet radiation produced in dangerous flares that may be blocked
by a resultant ozone. Most of the radiation is produced at low energies (red,
infrared) that does not readily power biological activity, at least as it occurs on
Earth.
Habitable zone
Very small zone near the star solar system. Planets within this zone would be
tidally locked with the star; a thick circulating atmosphere might be required to
avoid the freeze-out of the atmosphere on the night side. This might be somewhat
challenging to develop with a rotation period of 70 days (a=0.5a.u., M*=1/5 M).
Summary
K-M star planets, if habitable, might represent a huge reservoir of life that has
had 10+ billions of years to develop, compared to 5 billion years for our Sun.
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Studying stars from an astrobiological perspective
L-T brown dwarf stars: presumably common?
Timescales
Life span not defined by the same standards, but they will stay warm
for very long timescales!
Radiation
Very long wavelength optical and infrared radiation.
Habitable zone
No conventional habitable zone.
Summary
Habitable only in Europa-type conditions (i.e., sub-surface tidal
heating).
2M1207b is an example of a 12 Jupiter-mass planet orbiting brown
dwarf 2M1207.
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Are multiple star systems habitable?
60% of O-K star systems are multiple. Are orbits in such systems stable?
– If the planet has a small orbit around one star, where the stars are widely spaced—
yes!
– If the stars are closely spaced, and the planet has a large orbit around both stars—
yes! (Such planets, like Kepler 16b and Kepler 47(AB)b, have been discovered.)
– If the planet’s orbit is on the same size scale as the star’s orbits—no!
Even if they are not, only about 25% of M stars are in multiple star
systems.
Overall, including M stars, about 30% of the galaxy’s stars are in multiple
systems.
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