Download Linking Asteroids and Meteorites through Reflectance

Astronomy 101
The Solar System
Tuesday, Thursday
Tom Burbine
[email protected]
Course
• Course Website:
– http://blogs.umass.edu/astron101-tburbine/
• Textbook:
– Pathways to Astronomy (2nd Edition) by Stephen Schneider
and Thomas Arny.
• You also will need a calculator.
• There is an Astronomy Help Desk that is open
Monday-Thursday evenings from 7-9 pm in Hasbrouck
205.
• There is an open house at the Observatory every
Thursday when it’s clear. Students should check the
observatory website before going since the times may
change as the semester progresses and the telescope
may be down for repairs at times. The website is
http://www.astro.umass.edu/~orchardhill/index.html.
• February
25th
Exam #2
– Covers from last exam up to Feb. 18
• Formulas:
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T (K) = T (oC) + 273.15
c = f*
E = h*f
KE = 1/2mv2
E = mc2
Power emitted per unit surface area = σT4
λmax (nm) = (2,900,000 nm*K)/T
Apparent brightness = Luminosity
4 (distance)2
Review Session
• 6 pm in Hasbrouck 20 on Wednesday (Feb. 24)
HWs #6, #7, #8, and #9
Milky Way Galaxy
• Interstellar Medium – matter between stars
• Made up of gas and dust
Life of a Star
• A star-forming cloud is called a molecular cloud
because low temperatures allow Hydrogen to
form Hydrogen molecules (H2)
• Temperatures like 10-50 K
Region is approximately 50 light years across
Condensing
• Molecular clouds tends to be lumpy
• These lumps tend to condense into stars
• That is why stars tend to be found in clusters
Protostar
• The dense cloud fragment gets hotter as it
contracts
• The cloud becomes denser and radiation cannot
escape
• The thermal pressure and gas temperature start to
rise and rise
• The dense cloud fragment becomes a protostar
When does a protostar become a star
• When the core temperatures reaches 10 million K,
hydrogen fusion can start occurring
• Because nuclear reactions have not yet begun in the
protostar’s core, this luminosity is due entirely to the
release of gravitational energy as the protostar continues
to shrink and material from the surrounding fragment
• http://www.youtube.com/watch?v=W13ZYepDB
vo
• http://www.youtube.com/watch?v=QFklLMB_Z
OI&feature=related
Brown Dwarfs
• Failed stars
• Not enough mass for fusion
• Minimum mass of gas need for fusion is 0.08
solar masses (80 times the mass of Jupiter)
Main Sequence
• Is not an evolutionary track
– Stars do not evolve on it
• Stars stop on the main sequence and spend most
of their lives on it
Sun ends it time on the main sequence
• When the core hydrogen is depleted, nuclear
fusion stops
• The core pressure can no longer resist the crush of
gravity
• Core shrinks
Why does the star expand?
• The core is made of helium
• The surrounding layers are made of hydrogen
And ..
• Gravity shrinks the inert helium core and
surrounding shell of hydrogen
• The shell of hydrogen becomes hot for fusion
• This is called hydrogen-shell burning
And …
• The shell becomes so hot that its fusion rate is
higher than the original core
• This energy can not be transported fast enough to
surface
• Thermal pressure builds up and the star expands
And ..
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More helium is being created
Mass of core increases
Increases its gravitational pull
Increasing the density and pressure of this region
When
• When helium core reaches 100 million Kelvin,
• Helium can fuse into a Carbon nucleus
Helium Flash
• The rising temperature in the core causes the
helium fusion rate to rocket upward
• Creates a lot of new energy
However
• The core expands
• Which pushes the hydrogen-burning shell
outwards
• Lowering the hydrogen-burning shell’s
temperature
And
• Less energy is produced
• Star starts to contract
Now
• In the core, Helium can fuse to become Carbon
(and some Oxygen)
• Star contracts
• Helium fusion occurs in a shell surrounding the
carbon core
• Hydrogen shell can fuse above the Helium shell
• Inner regions become hotter
• Star expands
– Triple Alpha Process
– 4He + 4He ↔ 8Be
– 8Be + 4He ↔ 12C + gamma ray + 7.367 MeV
http://upload.wikimedia.org/wikipedia/commons/8/8d/Triple-Alpha_Process.png
• Some carbon fuses with He to form Oxygen
• 12C + 4He → 16O + gamma ray
• Harder to fuse Oxygen with Helium to produce
Neon
Planetary Nebulae
• There is a carbon core and outer layers are ejected
into space
• The core is still hot and that ionizes the expanding
gas
Planetary Nebulae
White Dwarf
• The remaining core becomes a white dwarf
• White dwarfs are usually composed of carbon and
oxygen (can not fuse carbon)
• Oxygen-neon-magnesium white dwarfs can also
form (hot enough to fuse carbon but not neon)
• Helium white dwarfs can form
High-Mass Stars
• The importance of high-mass stars is that they
make elements heavier than carbon
• You need really hot temperatures which only
occur with the weight of a very high-mass star
Stages of High-Mass Star’s Life
• Similar to low-mass star’s
• Except a high-mass star can continue to fuse
elements
• When the fusion ceases, the star becomes a
supernova
• Supernova is a huge explosion
Fusion in High-Mass stars
• Besides fusion of Hydrogen into Helium
• The high temperatures allow Carbon, Nitrogen,
and Oxygen to be catalysts for converting
Hydrogen into Helium
CNO cycle
Fusion
• The interior temperatures of high-mass stars in its
late-stage of life can reach temperatures above
600 million Kelvin
• Can fuse Carbon and heavier elements
• Helium Capture can also occur where Helium can
be fused into heavy elements
“Deaths” of Stars
• White Dwarfs
• Neutron Stars
• Black Holes
White Dwarfs
• White Dwarfs is the core left over when a star can
no longer undergo fusion
• Most white dwarfs are composed of carbon and
oxygen
• Very dense
– Some have densities of 3 million grams per cubic
centimeter
– A teaspoon of a white dwarf would weigh as much as
an elephant
White Dwarfs
• Some white dwarfs have the same mass as the
Sun but slightly bigger than the Earth
• 200,000 times as dense as the earth
White Dwarfs
• Collapsing due to gravity
• The collapse is stopped by electron degeneracy
pressure
Electron Degeneracy Pressure
• No two electrons can occupy the same quantum
state
Electron Degeneracy Pressure
• As electrons are moved closer together
• Their momentum (velocity) increases
• Due to Heisenberg Uncertainty Principle
So What Does This Mean
• Electron Degeneracy Pressure balances the
gravitational force due to gravity in white dwarfs
One Interesting Thing
• More massive white dwarfs are smaller
White Dwarf Limit
• The mass of a White Dwarf can not exceed
approximately 1.4 Solar Masses
• Called the Chandrasekhar Limit
The Sun
• Will end up as a White Dwarf
Black Dwarf
• Black dwarf – Theoretical cooled down white dwarf
• Not hot enough to emit significant amounts of light
• Since the time required for a white dwarf to reach this
state is calculated to be longer than the current age of
the universe of 13.7 billion years, no black dwarfs are
expected to exist in the universe yet
Neutron Star
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Neutron stars are usually 10 kilometers acroos
But more massive than the Sun
Made almost entirely of neutrons
Electrons and protons have fused together
How do you make a neutron star?
• Remnant of a Supernova
Supernova
• A supernova is a stellar explosion.
• Supernovae are extremely luminous and cause a
burst of radiation that often briefly outshines an
entire galaxy, before fading from view over
several weeks or months.
• The last person to see and chronicle a supernova
outburst in our galaxy was Johannes Kepler.
• That was in 1604 rivaled Venus in brightness.
Type Ia Supernova
Type II Supernova
This stops with Iron
• Fusion of Iron with another element does not
release energy
• Fission of Iron with another element does not
release energy
• So you keep on making Iron
Initially
• Gravity keeps on pulling the core together
• The core keeps on shrinking
• Electron degeneracy keeps the core together for
awhile
Then
• The iron core becomes too massive and collapses
• The iron core becomes neutrons when protons and
electrons fuse together
Density
• You could take everybody on Earth and cram
them into a volume the size of sugar cube
Explosion
• The collapse of the core releases a huge amount
of energy since the rest of the star collapses and
then bounces off the neutron core
• 1044-46 Joules
• Annual energy generation of Sun is 1034 Joules
How do we know there are neutron stars?
• The identification of Pulsars
• Pulsars give out pulses of radio waves at precise
intervals
Pulsars
• Pulsars were found at the center of supernovae
remnants
Pulsars
• Pulsars were interpreted as rotating neutron stars
• Only neutron stars could rotate that fast
• Strong magnetic fields can beam radiation out
Black Holes
• If a collapsing stellar core has a mass greater than
3 solar masses,
• It becomes a black hole
Black Hole
• After a supernova if all the outer mass of the star
is not blown off
• The mass falls back on the neutron star
• The gravity causes the neutron star to keep
contracting
Black Hole
• A black hole is a region where nothing can
escape, even light.
Event Horizon
• Event Horizon is the boundary between the inside
and outside of the Black Hole
• Within the Event Horizon, the escape velocity is
greater than the speed of light
• Nothing can escape once it enters the Event
Horizon
How do calculate the
radius of the Event Horizon?
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It is called the Schwarzschild Radius
Radius = 2GM/c2
This is a variation of the escape velocity formula
Escape velocity = square root (2GMplanet/Rplanet)
Black Hole Sizes
• A Black Hole with the mass of the Earth would
have a radius of 0.009 meters
• A Black Hole with the mass of the Sun would
have a radius of 3 kilometers
http://www.astronomynotes.com/evolutn/remnants.gif
Can you see a Black Hole?
No
• Black Holes do not emit any light
• So you must see them indirectly
• You need to see the effects of their gravity
Evidence
• The white area is
the core of a Galaxy
• Inside the core there
is a brown spiralshaped disk.
• It weighs a hundred
thousand times as
much as our Sun.
http://helios.augustana.edu/~dr/img/ngc4261.jpg
Evidence
• Because it is rotating we can measure its
radii and speed, and hence determine its
mass.
• This object is about as large as our solar
system, but weighs 1,200,000,000 times as
much as our sun.
• Gravity is about one million times as strong
as on the sun.
• Almost certainly this object is a black hole.
Any Questions?