Download Lecture 13, PPT version

Outline - March 4, 2010
• How does the sun shine? (pgs. 495-497, 499-503)
• Lifetimes of stars: gas guzzlers vs. econoboxes (pgs. 533-534)
• Where are the oldest known stars? (pgs. 536-538)
• Interstellar Medium (pgs. 544-547)
• How are stars made? (pgs. 549-551)
Proton-Proton Chain
(all stars with M < 8 Msun)
Net result: 4 protons are fused, producing 1 helium nucleus
So where does the energy come from????
The mass of 4 protons is less than the mass of 1 helium nucleus.
The mass that is lost is converted into energy (in the form of light).
The sun (and all stars that are not white dwarfs or “neutron stars”) are
very slowly losing mass in order to power themselves.
Note: only a tiny amount of mass is actually lost. By the end of its
lifetime the sun will have lost about about 10% of its total mass to
energy generation.
In principle, how long could the sun last by “burning” hydrogen at its
present rate?
Mass of 4 protons = 6.690x10-27 kg
Mass of 1 helium nucleus = 6.643x10-27 kg
Mass lost (mlost) = 0.047x10-27 kg
Energy gained = mlost c2 = (0.047x10-27)(3.0x108)2 = 4.23x10-12 J
Energy produced by the sun every second = 3.8x1026 J
Sun must run this fusion reaction 8.9x1037 times every second or it would collapse
under gravity!!!!
In other words, the sun must fuse 6.0x1011 kg of hydrogen every single second.
That’s a lot of hydrogen, but the sun has a lot of mass…
In principle, how long could the sun last by “burning”
hydrogen at its present rate?
The sun must fuse 6.0x1011 kg of hydrogen every single second.
The sun’s mass is 1.99x1030 kg, and at a current age of 4.5x109 years, we know
that 70% of that mass is in hydrogen, or 1.39x1030 kg of hydrogen remains.
If the sun converted ALL of its remaining hydrogen into helium (at today’s
rate of “nuclear burning”), how much longer could the sun live?
Remaining lifetime in seconds = remaining H mass / rate of H fusion
Remaining lifetime in seconds = 1.39x1030 / 6.0x1011 = 2.32x1018 seconds
Remaining lifetime in years = 73.4 billion years!!
In principle, how long could the sun last by “burning” hydrogen at its
present rate?
So, if the sun could turn ALL of its hydrogen into helium at its present rate,
you would think the sun would live a total of (4.5 + 73.4) = 77.9 billion years.
But, sadly, the sun’s lifetime is limited to only about 10 billion years because
it can’t actually convert all of its hydrogen into helium.
HUGE structural changes will happen to the star long before it can “burn up”
all of its hydrogen.
What determines a star’s Main Sequence lifetime?
It’s all about MASS.
The more massive is a star, the hotter and denser is the star in its core.
The hotter and denser it is in a star’s core, the FASTER the conversion of
hydrogen to helium happens.
High-mass (> 8 Msun) stars are “gas guzzlers”
Low-mass (< 2 Msun) are “economy cars”
Main Sequence is a MASS Sequence
The highest mass stars live
only a few million years.
They have a lot of fuel and
they’re burning it really
fast.
The lowest mass stars live
for 100’s of billions of
years. They have very
little fuel, but they’re
burning it extremely
efficiently.
Estimating the Age of the Universe
(What are stars “good for”?)
It stands to reason that you are younger than your mother.
It therefore stands to reason that the objects within the universe
cannot be older that the universe itself.
The ages of the oldest stars puts a limit on the minimum age
of the universe!!
The Oldest Stars in the Milky Way
Globular Star Clusters
Spherical groupings of 10,000 to 1 million stars (about 158 known in our
Galaxy). All of the stars formed at roughly the same time. Globular
clusters have lots of RED stars, but no BLUE stars (because they
died long ago and were not “replenished”).
Globular Cluster H-R Diagram
Globular Cluster M55
Globular clusters have short, stubby main sequences that “turn off” to the
red giant region. The “turn off” point tells you the approximate age.
Oldest Stars in the Milky Way
Globular cluster M4 is one of the oldest known star clusters (about 13 billion
years old), and contains many white dwarfs (the dead cores of low-mass stars
that used up all their fuel).
Interstellar Medium (ISM)
• Material between the stars
• Most of space is a better vacuum than can be made in a
laboratory!
• About 1/5 as much mass in the ISM as in stars in our Galaxy
• Some regions of space contain clouds gas (some clouds are
hot: > 10,000 K, some clouds are very cold: 10K-30 K)
• Chemical composition of ISM: 70% H, 28% He, 2% other
elements (by mass)
Why should you care about the ISM?
• Stars had to come from somewhere (the Big Bang didn’t
make stars)
• When stars die, their guts have to go somewhere
• If those “somewheres” weren’t the same place, we wouldn’t
be here! (a topic for after Spring Break)
Association Between Cold Clouds and Stars
“Heir ist wahrhaftig
ein Loch im Himmel”
Wm. Herschel
Image taken in optical / visible light
Cold clouds are transparent in the infrared and radio
Milky Way: Optical
Milky Way: Radio
Milky Way: Infrared
Cold clouds obscure our view at visible wavelengths, but infrared and
radio light penetrates the clouds.
Cold (Molecular) Clouds in the Milky Way
The Boston University-Five College Radio Astronomy Observatory
Galactic Ring Survey
Molecular gas clouds, as revealed by radio light
emitted by the molecule CO (carbon monoxide).
The full moon would
appear to be this big on
the image above.
Molecules are fragile - they are easily broken apart by high energy light or
strong collisions (both of which happen in high-temperature environments)
Cold clouds contain many types of molecules
+
HCO
HCO
+
Molecules in the cold
clouds range from
simple molecules like
carbon monoxide (CO) to
more complex molecules
like alcohol (CH3CH2OH)
HNC
HNC
HCN
HCN
N2NH2H++
Some cold clouds have intriguing shapes
Some cold clouds are long and snaky
The “Nessie” Nebula
Size > ~100 pc x 0.5 pc
Molecular Clouds
Stellar Nurseries
• Very, very cold (10K to 30K)
• Typical density is 300 molecules per cubic centimeter (vastly
less than the density of air at sea level, but vastly more
than the density of the ISM on average in our Galaxy)
• Gas is primarily H2 molecules, but you can’t detect them
directly! (Note: Helium does not form molecules because it is
chemically inert.)
• Most common “tracer” molecule is CO (carbon monoxide)
• About 1% of the mass in molecular clouds is in “dust”
“Dust” in the ISM
•
Not dust bunnies, more like the
microscopic particles in smoke
•
Size of dust grains is smaller than
bacteria (typical size is 1 micron =
10-6m)
•
Dust grains made mostly of some
combination of carbon, silicon,
oxygen, and iron
•
Dust blocks wavelengths of light
that are smaller than the size of
the grains ( < 10-6 m)
•
Dust easily blocks UV and visible
light, but IR and radio light can
(usually) pass right through
Horsehead Nebula (in Orion), optical image
Cloud Structure: Gravitational Equilibrium
A stable cloud has a
balance of two forces:
INWARD: Gravity
OUTWARD: Pressure
No net force => No motion
The “Jeans Mass”
Sir James Jeans showed that gravity becomes stronger than
pressure when the mass gets large enough.
If clouds (or fragments of clouds) acquire enough mass, the cloud
will collapse.
Problem: Although gravity is the most pervasive, long-range
force in nature, it is also the weakest force in nature.
How can gravity get the upper hand and win the pressuregravity tug of war?
What do we mean by “pressure” in a cloud?
•
Why does a balloon maintain its
shape?
•
What happens to a balloon if you
blow it up at room temperature, then
put it in the freezer for a couple of
hours?
•
This is what is known as “thermal”
pressure (the common pressure for
gasses)
•
Easiest place for gravity to “win” over
pressure is in a cloud of gas that is
very cold (= low pressure)
Collapsing Clouds
This cold, dark cloud is
collapsing and forming
cores that will eventually
become stars
This is a cloud where gravity
has won the tug-of-war!
Most Stars are Born Inside Clusters
Most molecular clouds contain
MUCH more mass than would make
a single star
Most molecular clouds are very
LUMPY (not smooth)
Likely scenario is that many lumps
(which are more dense than the
average) contract to form stars at
about the same time
Pleiades Star Cluster
Single star formation is possible but
probably very rare (because you
need an unusually dense, yet lowmass cloud)