Transcript Surveys of Stars, The interstellar medium

The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
And surface area on stars’
Luminosity and sorts the
Stars according to their size
The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
And surface area on stars’
Luminosity and sorts the
Stars according to their size
The Hertzsprung-Russell Diagram
The Main Sequence
- all main sequence
stars fuse H into He
in their cores
- this is the defining
characteristic of a
main sequence star.
The Hertzsprung-Russell Diagram
Red Giants
- Red Giant stars
are very large, cool
and quite bright.
Ex. Betelgeuse is
100,000 times more
luminous than the Sun
but is only 3,500K on
the surface. It’s radius
is 1,000 times that of the
Sun.
The Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram
White Dwarfs
- White Dwarfs
are hot but since
they are so small,
they are not very
luminous.
The Hertzsprung-Russell Diagram
The HR diagram separates
The effects of temperature
Mass of
Star
And surface area on stars’
Luminosity and sorts the
Stars according to their
size
Size of Star
Mass-Luminosity relation
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Most stars appear on the Main Sequence, where stars
appear to obey a Mass-Luminosity relation:
L  M3.5
For example, if the mass of a star is doubled, its
luminosity increases by a factor 23.5 ~ 11.
Thus, stars like Sirius that are about twice as massive as
the Sun are about 11 times as luminous.
The more massive a Main Sequence star is, the hotter
(bluer), and more luminous.
The Main Sequence is a mass sequence!
Review Questions
1.
2.
3.
What is the Hertzsprung-Russell Diagram?
Why are most stars seen along the so-called
main sequence?
What makes more massive stars hotter and
brighter?
L=4πR2 σT4
To calculate a star's radius, you must
know its
1) temperature and luminosity.
2) chemical composition and temperature.
3) color and chemical composition.
4) luminosity and surface gravity.
L=4πR2 σT4
If a star is half as hot as our Sun, but has
the same luminosity, how large is its radius
compared to the Sun?
1) ½ times as large
2) ¼ times as large
3) 4 times larger
4) the same
What is burning in stars?
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Gasoline
Nuclear fission
Nuclear fusion
Natural gas
Stars More in Depth:
To fully characterize stars we need to
know their Four Basic Parameters
Luminosity
Size
Mass
Surface
Temperature
Measurements of Star Properties
Apparent brightness Direct measurent
Parallax
Distance
Distance + apparent brightness
Luminosity
( L=4D2 f)
Spectral type (or color)
Temperature
Luminosity + temperature
Radius
(L=4R2 T4)
Luminosity and temperature are the two
independent intrinsic parameters of stars.
Mass: how do you weigh a star?
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Mass is the single most important property in how a
star’s life and death will proceed.
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We can “weigh” stars that are in binary systems (two
stars orbiting each other). Fortunately, most stars
fall into this category.
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Most stars in binary systems have a mass that is very
similar to its companion … we’ll see why this is
soon!
Binary Stars
Center of mass
(or baricenter)
Star A
ra
rb
Star B
Ma/Mb = rb/ra
-Each star in a binary system
moves in its own orbit around
the system's center of mass.
-Kepler’s Third Law: the orbital
period depends on the relative
separation and the masses of the
two stars:
2
4π
3
2
a
p = G(M +M )
1
2
Big p = Small Masses
Small p = Big Masses
I. Visual Binaries
1
4
2
5
1.
3
2.
The total spread (size) of the
Doppler shift gives velocities
about center of mass (gives orbit
sizes, rA+rB )
The time to complete one repeating
pattern gives period, P
Spectroscopic binaries: Doppler Shift tells if it is moving toward or away
Eclipsing Binaries:
Best binaries to measure mass
Classification of Stars: the H-R diagram
1) Collect information on
a large sample of stars.
2) Measure their
luminosities
(need the distance!)
3) Measure their surface
temperatures
(need their spectra)
The Interstellar Medium
Assigned Reading
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Chapter 10
A World of Dust and Gas
The space between the stars is not completely
empty, but filled with very dilute gas and dust,
producing some of the most beautiful objects
in the sky.
We are interested in the interstellar
medium because
a) dense interstellar clouds are the
birth place of stars
b) Dark clouds alter and absorb the light
from stars behind them
Bare-Eye Nebula: Orion
One example of an
interstellar gas cloud
(nebula) is visible to the
bare eye: the Orion nebula
The ISM
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Space between stars not empty
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Physical status of the gas characterized by:
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Gas, dust; Gas is mostly Hydrogen (80%) and Helium (20%)
Temperature
Density
Chemical composition
ISM and stars are the components of the “machine” that
makes the universe evolve: the cycle of star formation and
death, and the chemical enrichment of the cosmos.
ISM also “disturbs” observations, since it absorbs light and
modifies (reddens) colors
Interstellar Reddening
Blue light is strongly scattered and
absorbed by interstellar clouds.
Red light can more
easily penetrate the
cloud, but is still
absorbed to some
extent.
Barnard 68
Visible
Infrared radiation
is hardly
absorbed at all.
Interstellar
clouds make
background
stars appear
Infrared redder.
Interstellar Reddening (2)
The Interstellar Medium absorbs light
more strongly at shorter wavelengths.
Interstellar Reddening (3)
Nebulae that appear as dark nebulae in the
optical, can shine brightly in the infrared due to
blackbody radiation from the warm dust.
The ISM Main Components (Phases)
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Phase
Dust
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T (K)
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20-100
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50-500
103-104
105-106
20-50
Density a/cm3
size: a few mm
Present in all phases
“Metals”
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Everything that is not hydrogen
or Helium is a metal
HI Clouds
Inter-cloud Medium
Coronal Gas
Molecular clouds
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This what forms stars
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1-1000
0.01
10-4-10-3
103-105
How did a star form?
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A cloud of hydrogen gas began to gravitationally collapse.
To start the collapse, the gas needs to loose pressure, i.e.
needs to become cold
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It also needs to become dense, i.e. to have more gravity
As more gas fell in, it’s potential energy was converted
into thermal energy.
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If the gas does not cool, it cannot collapse
As it collapses, the gas gets hotter and hotter
Eventually the in-falling gas was hot enough to ignite
nuclear fusion in the core.
Gas that continued to fall in helped to establish
gravitational equilibrium with the pressure generated in the
core.
Conservation of Angular Momentum (L)
L is conserved during the collapse: if R decreases, T has to decrease, too, to keep L constant
Only, the rate of change of T is faster to keep pace with the rate of change of R 2 (because of the
second power)
The collapsing object spins up rapidly during the collapse
O
The Stellar Cycle
Cool molecular clouds
gravitationally collapse
to form clusters of stars
New (dirty) molecular
clouds are left
behind by the
supernova debris.
Molecular
cloud
Stars generate
helium, carbon
and iron through
stellar nucleosynthesis
The hottest, most
massive stars in the
cluster supernova –
heavier elements are
formed in the explosion.
The Four Components of the
Interstellar Medium
Component
Temperature
[K]
Density
[atoms/cm3]
Main
Constituents
HI Clouds
50 – 150
1 – 1000
Neutral
hydrogen; other
atoms ionized
Intercloud Medium
(HII)
103 - 104
0.01
Partially ionized
H; other atoms
fully ionized
Coronal Gas
105 - 106
10-4 – 10-3
All atoms highly
ionized H
Molecular Clouds
20 - 50
103 - 105
Neutral gas;
dust and
molecules
Interstellar Absorption Lines
These can be
distinguished from
stellar absorption
lines through:
The interstellar medium
produces absorption lines in
the spectra of stars.
a) Absorption from
wrong ionization
states
Narrow absorption lines from Ca II: Too low
b) Small line width
ionization state and too narrow for the O
(too low
star in the background; multiple components
temperature; too
low density)
c) Multiple
components
(several clouds of
ISM with different
radial velocities)
Observing Neutral Hydrogen:
The 21-cm (radio) line
Electrons in the ground state of neutral hydrogen have
slightly different energies, depending on their spin
orientation.
Opposite magnetic
fields attract =>
Lower energy
Magnetic field
due to proton spin
21 cm line
Magnetic field
due to electron
spin
Equal magnetic
fields repel =>
Higher energy
The 21-cm Line of Neutral Hydrogen
Transitions from the higher-energy to the lowerenergy spin state produce a characteristic 21-cm
radio emission line.
=> Neutral
hydrogen
(HI) can be
traced by
observing
this radio
emission.
Observations of the 21-cm Line
G a l a c t i c
p l a n e
All-sky map of emission in the 21-cm line
Observations of the 21-cm Line
HI clouds moving towards Earth
HI clouds moving
away from Earth
Individual HI clouds
with different radial
velocities resolved
(from redshift/blueshift of line)
Interstellar Dust
Formed in the atmospheres of cool stars
Mostly observable through infrared emission
Spitzer Space
Telescope (infrared)
image of interstellar
dust near the center
of our Milky Way
(Right:) Infrared
Emission from
interstellar dust and
gas molecules in the
“Whirlpool Galaxy”
M51.
Molecules in Space
In addition to atoms and ions, the interstellar
medium also contains molecules.
Molecules also store specific energies in their
a) rotation
b) vibration
Transitions between different rotational /
vibrational energy levels lead to emission
– typically at radio wavelengths.
The Most Easily Observed
Molecules in Space
• CO = Carbon Monoxide  Radio emission
• OH = Hydroxyl  Radio emission
The Most Common Molecule in
Space:
• H2 = Molecular Hydrogen  Ultraviolet
absorption and emission:
Difficult to observe!
But: Where there’s H2, there’s also CO.
Use CO as a tracer for H2 in the ISM!
Molecular Clouds
• Molecules are easily destroyed (“dissociated”) by
ultraviolet photons from hot stars.
 They
can only survive within dense, dusty clouds,
where UV radiation is completely absorbed.
“Molecular
Clouds”:
UV emission from
Molecules
nearby stars destroys
survive
molecules in the outer
parts of the cloud; is Cold, dense
molecular
absorbed there.
cloud core
Diameter ≈ 15 – 60 pc
HI Cloud
Temperature ≈ 10 K
Largest molecular
clouds are called
“Giant Molecular
Clouds”:
Total mass ≈ 100 – 1 million solar masses
Molecular Clouds (2)
The dense
cores of
Giant
Molecular
Clouds are
the birth
places of
stars.
The Coronal Gas
Additional component
of very hot, low-density
gas in the ISM:
T ~ 1 million K
n ~ 0.001 particles/cm3
Observable in X-rays
Called “Coronal gas”
because of its
properties similar to
the solar corona (but
completely different
origin!)
Our sun is located within
Probably originates in supernova explosions (near the edge of) a
coronal gas bubble.
and winds from hot stars
The Gas-Star-Gas Cycle
All stars are constantly blowing gas out
into space (recall: Solar wind!)
The more luminous the star is, the
stronger is its stellar wind.
These winds
are particularly
strong in aging
red giant stars.
The Gas-Star-Gas Cycle
Stars, gas, and dust are in constant interaction with each other.