Last Time

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Last Time

• Last time, we reviewed the lives of massive stars
• Stars up to about 20 solar masses live their main
  sequence lives much like low mass stars

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Last Time

• The differences started when the high mass stars
  started to fuse Helium
• No Helium flash
• Instead, the temperatures were already high
  enough to fuse Helium gradually

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Last Time

• After Helium was used up in the core, it was
  fused in a shell
• But this time, temperatures could get high enough
  to fuse Carbon

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Last Time

• Once the Carbon was used up, this was burned in a
  shell, and Oxygen was burned in the core
• This scenario continued from Oxygen to Neon, Neon
  to Magnesium, and Magnesium to Silicon

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Last Time

• After Silicon, we were left with a core of Iron,
  and a layered structure to our star
• Like an onion
• But Iron can not be fused
• So nothing to support the core

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Last Time

• Without a source of support, the Iron core began
  to collapse
• Electron degeneracy pressure could not stop it
• Electrons and protons became neutrons

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Last Time

• Finally, the collapse of the core stopped thanks
  to neutron degeneracy pressure
• Neutrons are anti-social too, but not as much as
  electrons
• Can squeeze them much tighter
• They can support more mass

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Last Time

• Although the core stopped collapsing, the outer
  layers of the star did not
• These are now rushing towards the core at
  thousands of kilometers per second

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Last Time

• When they hit the core, they pile up on one
  another
• Energy is transferred back through the layers
• A tremendous shock wave forms

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Last Time

• We call this a supernova
• To be specific, a Type II supernova
• These events are extremely energetic, and
  outshine the other stars in their host galaxies
• But only 0.01 of the energy comes out as light
• Most is neutrinos

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Last Time

• Most of the star is thrown out, forming a
  supernova remnant
• The core is left behind, forming a neutron star

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Last Time

• Neutron stars are extremely dense
• They are as massive as the sun but are only 10
  kilometers in diameter
• One teaspoon weighs a trillion pounds

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Last Time

• Neutron stars also have very strong magnetic
  fields
• This produces synchrotron radiation
• Radio emission is beamed from the magnetic poles
• Neutron stars also rotate very quickly

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Last Time

• When the radio emission is beamed in our
  direction, we call this a pulsar
• Pulsars are like stellar lighthouses
• The fastest pulsars spin hundreds of times a
  second

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Last Time

• These are extreme objects, and they are useful
  laboratories
• They rotate with the precision of atomic clocks
• We can use them to study many things, like
  gravity and quantum mechanics

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Last Time

• Eventually, these pulsars run out of energy
• They slow down and die, becoming cold neutron
  stars
• So ends a massive star

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Last Time

• We also talked about the supermassive stars that
  are more than 20 solar masses
• They live a dangerous life
• They are always at risk at being blown apart by
  radiation pressure

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Last Time

• The cores of these stars form black holes
• When the neutron degeneracy pressure cannot even
  slow down the collapse, the core goes straight to
  a black hole
• We call these events hypernova

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Last Time

• Hypernova are 1000 times more energetic than Type
  II supernova
• The most violent explosions in the universe
• They are still hypothetical, but may explain
  gamma-ray bursts

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Last Time

• We also talked about black holes
• Black holes dont suck in everything within reach
• They simply pull with gravity

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Last Time

• Far away from a black hole, things are pretty
  normal
• If we replaced the Sun with a black hole of less
  mass, Earths orbit would not change

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Last Time

• We only notice something is up when we get too
  close to a black hole
• Gravity just keeps getting stronger, because the
  mass is always interior to us
• If we get too close, we could suffer
  spaghettification

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Last Time

• The point of no return is called the event
  horizon
• This is just the region of space where the escape
  velocity is greater than the speed of light
• If we could avoid spaghettification, then nothing
  new would happen once we crossed the event horizon

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Last Time

• But once we cross the event horizon, we are
  trapped
• The singularity is the point where all the mass
  of a black hole exists
• Singularities are what make black holes special

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Last Time

• It is not that black holes are extremely massive,
  it is that they are extremely dense
• All the mass is squeezed into a point
  infinitely small
• But stay out of the even horizon, and you could
  escape

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This Time

• Finish up Black Holes
• Planet formation
• Start on multiple star systems

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Black Holes

• Now you might think that black holes are very
  complex, but actually they are very simple
• Black holes have no hair.

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Black Holes

• Black holes can be completely described by their
  mass, and how they rotate
• Sometimes charge, too
• All black holes are more or less the sameit is
  only mass that makes them different

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Black Holes

• So what would it be like to live around a black
  hole?
• Remember time dilation?
• Gravity does the same thing!
• Around a black hole, time moves very slowly

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Black Holes

• Remember redshift?
• Gravity does the same thing
• The light coming out from near a black hole has a
  gravitational redshift

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Black Holes

• But how can black holes emit light?
• It is not actually the black hole
• Often, as mass falls into a black hole it forms a
  disk
• This is called an accretion disk

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Black Holes

• Accretion disks can get very hot
• Before the matter actually crosses the event
  horizon, it can put out a lot of radiation

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Black Holes
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Black Holes

• There is one more weird property of black holes
  to talk about

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Which is more dangerous to be near a low mass
black hole or a very high mass black hole?

• Higher mass
• Lower mass

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Black Holes

• It seems counterintuitive, but low mass black
  holes are more dangerous to be near
• This is because the increase in gravity is more
  gradual in a high mass black hole

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Black Hole

• Think of it like a hill
• If falling into a black hole is like going down a
  hill, a super massive black hole is like a gentle
  slope
• A low mass black hole is like a steep cliff

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Black Holes
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Black Holes

• So things like spaghettification will be much
  worse around a low mass black hole than around a
  high mass black hole

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Black Holes

• More myths
• According to the ABC News special report, a
  black hole swallowing the Earth is the number 2
  threat of mass extinction
• Is this at all likely?

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Black Holes

• Was it likely stars would collide?
• NO!
• From far away, the gravity of a black hole is NO
  different than that of a star
• And there are fewer black holes out there!

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Black Holes

• This is probably more likely

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Black Holes

• This is why John Stossel is an idiot
• And so is this guy

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Black Holes

• We could spend a whole course talking about black
  holes
• I hope you now see that black holes are very
  misunderstood
• They arent all that bad, they just need a little
  tender loving care

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Black Holes

• Plus, they give us anexcuse to listen to Pink
  Floyd
• Like we need an excuse

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Life After Death

• Every new beginning comes from some other
  beginning's end

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A Simple Question...

• Recall one of our lecture questions... Could
  pristine material make planets?
• The answer was No
• So then where did the material that makes our
  world and bodies come from?

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A Big Deal

• This is an important question to answer
• Where did we come from?

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All That Glitters...

• To answer this question, lets follow the life of
  a single atom of gold...
• Because gold is pretty
• And because alchemy, the quest to turn everyday
  materials into gold, is exactly what we are going
  to do here

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The Beginning

• What is our gold made of?
• Protons
• Neutrons
• Electrons
• These were made in the Big Bang (more on that
  later)?

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The Beginning

• But the Big Bang did not actually make our gold
  atom, just the building blocks for it
• Like Lego's, or, if you will, Tinker Toys that
  have not yet been put together
• So how did the pieces get put together?

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Alchemy

• Before our gold was gold, the pieces came
  together to make Hydrogen
• Could have also been Helium, but less likely
• This was really all that was made in the Big Bang
  (elementally speaking)?

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Alchemy

• Somehow, we went from Hydrogen to gold
• This was done in the furnace of a star
• But wait, where is gold on the periodic table?

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Au Gold, Where Art Thou?
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A Problem...

• Does anybody see the problem here?
• Gold is heavier than Iron
• But I told you nothing heavier than Iron can be
  made in stars
• Did I lie?
• Where else could the gold have come from?

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A Resolution...

• To make elements heavier than Iron, we need to
  add energy into the system
• Stars have no energy to give - it all goes into
  trying to stop gravitational collapse
• But, there is something out there with loads of
  extra energy out there

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Where does the extra energy come from?

• Another star
• The supernova
• Gravitational collapse

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Death is the road to Au...

• Supernova don't mind if we steal a little energy
  from them!
• And this is exactly what happens

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Neutron Capture
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Neutron Capture

• The supernova creates extremely high energy
  neutrons, which then join up with other nuclei
• This process is called neutron capture
• We just keep adding neutrons

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Radioactive Decay

• But we make new elements by adding protons
• How do we get new elements through neutron
  capture?
• These isotopes that form from neutron capture are
  not stable

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Radioactive Decay

• Since they are unstable, they undergo radioactive
  decay
• Our gold is first turned into a heavy isotope,
  then decays down to actually become gold

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Life After Death

• This happens many times in a supernova, and many
  tons of gold is made
• The same or similar processes produce all the
  elements from Iron to Uranium
• It also produces some elements lighter than Iron
  not efficiently produced through normal Fusion

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Life After Death

• Now the question becomes, How did all that
  material end up here, on Earth?
• To answer this question, we look at how planets
  form

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The Aftermath

• After a supernova, gas is spread across many
  dozens or hundreds of light years
• Eventually, it cools, and forms a ...

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Deja Vu..

• We have come full circle
• Take a minute to appreciate what we have
  uncovered...
• From the death of a massive star, we can now
  create new stars
• It is from this act of destruction that we have
  an act of creation...think about it

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Well, Not Quite the Same

• But this time, there is an important difference
• We we first encountered molecular cloud, the
  material was pristine...it had never been in a
  star before
• Now, the material contains some heavy elements

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Protoplanetary Disks

• The proto-stars will form, as will the dirty
  disks that exist around them
• But now, there is heavy elements present in the
  disk

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Protoplanetary Disks

• Some of these heavy elements, like Iron and
  Silicon, begin to join up
• They make dust
• Then the dust particles join up
• They make grains

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Protoplanetary Disks

• Then the grains join up to make rocks, rocks into
  boulders, boulders into planetesimals
• As this happens, things heat up
• We start to get chemistry
• We can start to make compounds, like ores and
  other oxides

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Protoplanetary Disks
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Protoplanetary Disks

• These planets form in much the same way as stars
• Once a clump of mass grows big enough, it pulls
  more mass onto itself
• Eventually, you get lots of small planetesimals
  that collide to form planets

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Planetesimals
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Planetesimals

• Movies!
• Movie 1
• Movie 2
• British Accents!

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And Thus, Planets

• From these violent beginnings, planets grow
• We can even explain why we see some elements on
  certain planets but not on others

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Evaporating Worlds
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Evaporating Worlds

• Just like water, other elements and compounds are
  driven away by the active, young star
• Light elements, like Hydrogen and Helium, end up
  in the outer planetary system
• Rocky elements end up in the inner planetary
  system

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Life

• If conditions are right on these planets, then
  life may also form
• This is where we get the famous saying We are
  all stardust

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A Grand Connection

• Despite our own smallness, and the smallness of
  our world and solar system, and even galaxy,
  compared to the vast universe, we all share a
  connection with the cosmos
• We truly are the stuff of stars

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Star Systems

• From One, Many

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Are stars only formed alone, or can they come in
groups?

• They can come in groups
• They only form alone

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Back To the Beginning

• When stars form from molecular clouds, there is
  no reason that two stars cannot form close to one
  another
• They may become gravitationally bound

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Binary Systems

• We call this a binary star system
• The two stars now begin a life together

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Binary Systems

• It is important to note that normally, we expect
  the stars to have different masses
• They will orbit each other
• Specifically, they orbit the center of mass
• Demos!

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If the stars have different masses, will they
evolve at the same rate?

• Yes
• No

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Binary Systems

• The more massive star will go through its life
  more quickly than the low mass star
• Depending on the exact mass of the primary, it
  could form a white dwarf, neutron star, or black
  hole
• We call all three of these compact objects

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Binary Systems

• This is where things get interesting
• When the second star is a normal, main sequence
  star, it is unlikely to be affected by the
  compact object
• But

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Binary Systems

• When the second star becomes a giant, it may not
  have a strong enough hold on its outer layers
• The compact object will pull mass onto itself

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Roche Lobe

• When will this happen?
• When the secondary fills its Roche lobe
• It all has to do with gravity

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Roche Lobe

• How many of you are familiar with contour
  elevation maps?
• These work by drawing lines of constant elevation
• For example, pick an elevation (say 2000 feet)
  and draw a line on the map to indicate everywhere
  that has this elevation

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Roche Lobe
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Roche Lobe

• Now, instead of elevation, imagine we drew lines
  to indicate equal gravity
• Everything along the line would experience an
  equal gravitational pull
• What would it look like for our stars?

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Roche Lobe
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What do we find along a stars Roche lobe?

• Equal elevation
• Equal gravity
• Equal mass

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Roche Lobe

• Notice that each star has its own lobe
• But there is a point in the middle where the two
  lobes meet

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Roche Lobe

• Remember that, for gravity, it only matters how
  much mass you enclose
• So the radius of the star can change, but the
  Roche lobe stays the same

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Roche Lobe

• But what happens if the second star grows bigger
  than its Roche lobe?
• Mass will be transferred to the compact object
• It will flow through the point where the two
  Roche lobes meet

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Roche Lobe Overflow
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Roche Lobe Overflow

• Now some interesting things can happen, depending
  on the compact object
• If it is a white dwarf, we could get a Type Ia
  supernova

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Type Ia Supernovae

• Remember that electron degeneracy pressure can
  only support so much mass
• To be exact, it can only support 1.44 solar
  masses
• But we are adding mass

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Type Ia Supernovae

• When the white dwarf crosses the threshold, it
  collapses
• A lot of material will undergo fusion
• This explosion will blow the white dwarf apart
• Movie

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Type Ia Supernova

• Type Ia supernova can be very useful
• It is widely believed that Type Ia supernovae
  have a fairly constant luminosity
• Why?

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Type Ia Supernovae

• Because all Type Ia supernovae are thought to be
  more or less the same
• They always occur in white dwarfs of just over
  1.44 solar masses
• We think all white dwarfs are made of degenerate
  carbon

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Type Ia Supernovae

• If we know the luminosity of something, what can
  we find?
• This is an example of a standard candle
• Something of a known, standard luminosity

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Other Possibilities

• What if we have a neutron star?
• It is unlikely that the neutron star will gain
  enough mass to collapse to a black hole
• But this could be a way of bringing a dead pulsar
  back to life

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Recycled Pulsars

• In fact, this is exactly how we think millisecond
  pulsars usually form

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Black Holes

• We might also have a black hole for our compact
  object
• In this case, we can throw mass onto it all day
• In all cases, though, something cool forms around
  the compact objects

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Accretion Disks

• Before the matter falls onto the compact object,
  it forms a disk
• Like water circling the drain
• The gas in this disk will get squeezed to high
  pressures and temperatures
• Friction also heats the disk

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Accretion Disks

• These disks can get to be tens of millions of
  degrees
• These accretion disks are great sources of X-rays

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Accretion Disks
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White Dwarfs, Pulsars, Black Holes, Oh My!

• Eventually, the second star will also go through
  its life
• It could also form a compact object
• These double compact objects systems can be cool

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Collision Ahead

• If the compact objects are close enough to each
  other, they could eventually collide
• What would cause them to stop orbiting stably?
• Gravitational radiation

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Collision Ahead

• There could be something called gravitational
  waves that would carry the energy of the orbit
  away
• The orbit would decay

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Collision Ahead

• Movies
• Colliding neutron stars
• Colliding black holes

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Gravitational Waves

• Are they real?
• We have good reason to think so

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More Binary Systems

• There are other possibilities for binary systems
• I told you stars come in different shapes

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Contact Binary

• Sometimes, two stars get so close that they share
  their outer layers
• This is called a contact binary

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Contact Binary

• This star would be peanut shaped!
• It is not even clear what exactly we mean by star
  anymoreis there one or two?

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Finding Planets

• How do we find other planets?
• After all, a planet is like a very tiny binary
  companion

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Finding Planets

• There popular way is to look for the slight
  wobble in the star due to the planet
• As the planet goes around the star, it pulls on
  the star every so slightly

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Finding Planets

• This tiny wobble can be hard to detect, but we
  can do it using the Doppler shift
• When the star rotates away from us, it is red
  shifted
• When the star rotates towards us, the light is
  blue shifted

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Finding Planets

• We can see the Doppler shift in the spectral
  lines of the star
• We can measure very small shifts using good
  spectroscopy

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Finding Planets
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Finding Planets

• If the system is not oriented properly, we can
  also look for a shift in the position of the star
• Must measure position very closely
• Or we could look for a slight drop in the flux of
  a star when a planet passes in front of it
• Also not the easiest

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Finding Planets
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Measuring Masses

• Finally, we should talk about measuring masses
• If we can measure the period of the binary
  systems orbit, we can measure the total mass of
  the system

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Measuring Masses

• We use something called Keplers Third Law

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Measuring Masses

• It can be hard to measure period and separation
  exactly, but if we can it tells us the total mass

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Triple Systems and More

• I will finish by simply saying that we can have
  star systems with more than 2 stars
• These are rare, but not impossible
• Five or six is probably the upper limit, just
  because its hard to get so many stars together

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Star systems can form with more than 2 stars?

• True
• False

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