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Last Time

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The singularity is the point where all the mass of a black hole exists ... It is not that black holes are extremely massive, it is that they are extremely dense ... – PowerPoint PPT presentation

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Title: Last Time


1
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

2
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

3
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

4
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

5
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

6
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

7
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

8
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

9
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

10
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

11
Last Time
  • Most of the star is thrown out, forming a
    supernova remnant
  • The core is left behind, forming a neutron star

12
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

13
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

14
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

15
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

16
Last Time
  • Eventually, these pulsars run out of energy
  • They slow down and die, becoming cold neutron
    stars
  • So ends a massive star

17
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

18
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

19
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

20
Last Time
  • We also talked about black holes
  • Black holes dont suck in everything within reach
  • They simply pull with gravity

21
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

22
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

23
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

24
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

25
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

26
This Time
  • Finish up Black Holes
  • Planet formation
  • Start on multiple star systems

27
Black Holes
  • Now you might think that black holes are very
    complex, but actually they are very simple
  • Black holes have no hair.

28
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

29
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

30
Black Holes
  • Remember redshift?
  • Gravity does the same thing
  • The light coming out from near a black hole has a
    gravitational redshift

31
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

32
Black Holes
  • Accretion disks can get very hot
  • Before the matter actually crosses the event
    horizon, it can put out a lot of radiation

33
Black Holes
34
Black Holes
  • There is one more weird property of black holes
    to talk about

35
Which is more dangerous to be near a low mass
black hole or a very high mass black hole?
  • Higher mass
  • Lower mass

36
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

37
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

38
Black Holes
39
Black Holes
  • So things like spaghettification will be much
    worse around a low mass black hole than around a
    high mass black hole

40
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?

41
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!

42
Black Holes
  • This is probably more likely

43
Black Holes
  • This is why John Stossel is an idiot
  • And so is this guy

44
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

45
Black Holes
  • Plus, they give us anexcuse to listen to Pink
    Floyd
  • Like we need an excuse

46
Life After Death
  • Every new beginning comes from some other
    beginning's end

47
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?

48
A Big Deal
  • This is an important question to answer
  • Where did we come from?

49
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

50
The Beginning
  • What is our gold made of?
  • Protons
  • Neutrons
  • Electrons
  • These were made in the Big Bang (more on that
    later)?

51
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?

52
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)?

53
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?

54
Au Gold, Where Art Thou?
55
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?

56
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

57
Where does the extra energy come from?
  • Another star
  • The supernova
  • Gravitational collapse

58
Death is the road to Au...
  • Supernova don't mind if we steal a little energy
    from them!
  • And this is exactly what happens

59
Neutron Capture
60
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

61
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

62
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

63
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

64
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

65
The Aftermath
  • After a supernova, gas is spread across many
    dozens or hundreds of light years
  • Eventually, it cools, and forms a ...

66
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

67
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

68
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

69
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

70
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

71
Protoplanetary Disks
72
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73
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

74
Planetesimals
75
Planetesimals
  • Movies!
  • Movie 1
  • Movie 2
  • British Accents!

76
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

77
Evaporating Worlds
78
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

79
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

80
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

81
Star Systems
  • From One, Many

82
Are stars only formed alone, or can they come in
groups?
  • They can come in groups
  • They only form alone

83
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

84
Binary Systems
  • We call this a binary star system
  • The two stars now begin a life together

85
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!

86
If the stars have different masses, will they
evolve at the same rate?
  • Yes
  • No

87
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

88
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

89
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

90
Roche Lobe
  • When will this happen?
  • When the secondary fills its Roche lobe
  • It all has to do with gravity

91
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

92
Roche Lobe
93
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?

94
Roche Lobe
95
What do we find along a stars Roche lobe?
  • Equal elevation
  • Equal gravity
  • Equal mass

96
Roche Lobe
  • Notice that each star has its own lobe
  • But there is a point in the middle where the two
    lobes meet

97
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

98
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

99
Roche Lobe Overflow
100
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

101
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

102
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

103
Type Ia Supernova
  • Type Ia supernova can be very useful
  • It is widely believed that Type Ia supernovae
    have a fairly constant luminosity
  • Why?

104
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

105
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

106
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

107
Recycled Pulsars
  • In fact, this is exactly how we think millisecond
    pulsars usually form

108
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

109
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

110
Accretion Disks
  • These disks can get to be tens of millions of
    degrees
  • These accretion disks are great sources of X-rays

111
Accretion Disks
112
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

113
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

114
Collision Ahead
  • There could be something called gravitational
    waves that would carry the energy of the orbit
    away
  • The orbit would decay

115
Collision Ahead
  • Movies
  • Colliding neutron stars
  • Colliding black holes

116
Gravitational Waves
  • Are they real?
  • We have good reason to think so

117
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118
More Binary Systems
  • There are other possibilities for binary systems
  • I told you stars come in different shapes

119
Contact Binary
  • Sometimes, two stars get so close that they share
    their outer layers
  • This is called a contact binary

120
Contact Binary
  • This star would be peanut shaped!
  • It is not even clear what exactly we mean by star
    anymoreis there one or two?

121
Finding Planets
  • How do we find other planets?
  • After all, a planet is like a very tiny binary
    companion

122
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

123
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

124
Finding Planets
  • We can see the Doppler shift in the spectral
    lines of the star
  • We can measure very small shifts using good
    spectroscopy

125
Finding Planets
126
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

127
Finding Planets
128
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

129
Measuring Masses
  • We use something called Keplers Third Law

130
Measuring Masses
  • It can be hard to measure period and separation
    exactly, but if we can it tells us the total mass

131
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

132
Star systems can form with more than 2 stars?
  • True
  • False

133
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