Download neutron star

Astr115
Tuesday, March 5, 13
Grand Canyon
http://apod.nasa.gov/apod/ap130303.html
03/05/13
1
TWO TYPES OF SUPERNOVA
Massive star supernova:
Iron core of a massive star reaches
white dwarf limit and collapses into a
neutron star, causing total explosion.
White dwarf supernova:
Carbon fusion suddenly begins as a white
dwarf in close binary system reaches the white dwarf
limit, causing a total explosion.
Tuesday, March 5, 13
2
SUPERNOVAE TYPES
• Type
II (High mass -- has hydrogen lines)
• Type
1 (No hydrogen lines)
• Type
Ia - white dwarf collapse (must be binary)
• Type
Ib - high mass (may be >20Msun)
• stellar
• Type
Ic - really high mass (>20Msun)
• stellar
• may
Tuesday, March 5, 13
wind blew off hydrogen layers
winds blew off hydrogen and helium layers (WR star)
cause gamma-ray bursts
3
One way to tell supernova types apart is with a light curve
showing how luminosity changes with time.
Tuesday, March 5, 13
4
NOVA OR SUPERNOVA?
• Supernovae are MUCH MUCH more luminous (about
10 thousand times)!!!
• Nova: H to He fusion of a layer of accreted matter,
white dwarf left intact
• Supernova: complete explosion of white dwarf, nothing
left behind
Tuesday, March 5, 13
5
SUPERNOVA TYPE:
MASSIVE STAR OR WHITE
DWARF?
• Light curves differ.
• Spectra differ (exploding white dwarfs don’t have hydrogen
absorption lines).
Tuesday, March 5, 13
6
WHAT IS A NEUTRON STAR?
Tuesday, March 5, 13
7
A neutron star is
the ball of
neutrons left
behind by a
massive-star
supernova.
Degeneracy
pressure of
neutrons supports
a neutron star
against gravity.
Tuesday, March 5, 13
8
Electron degeneracy
pressure goes away
because electrons
combine with protons,
making neutrons and
neutrinos.
Neutrons collapse to the
center, forming a neutron
star.
Tuesday, March 5, 13
9
A neutron star is about the same size as a small city.
Tuesday, March 5, 13
10
HOW WERE NEUTRON
STARS DISCOVERED?
Tuesday, March 5, 13
11
DISCOVERY OF NEUTRON STARS
• Using a radio telescope in 1967, Jocelyn Bell noticed very
regular pulses of radio emission coming from a single
part of the sky.
• The pulses were coming from a spinning neutron star—a
pulsar.
Tuesday, March 5, 13
12
Tuesday, March 5, 13
13
Crab Nebula in visible light
Tuesday, March 5, 13
14
PULSARS
• A pulsar is a neutron
star that beams
radiation along a
magnetic axis that is
not aligned with the
rotation axis.
Tuesday, March 5, 13
15
PULSARS
• The radiation beams
sweep through space
like lighthouse beams
as the neutron star
rotates.
Tuesday, March 5, 13
16
WHY PULSARS MUST BE NEUTRON STARS
Circumference of NS = 2π (radius) ~ 60 km
Spin rate of fast pulsars ~ 1000 cycles per second
Surface rotation velocity ~ 60,000 km/s
~ 20% speed of light
~ escape velocity from NS
Anything else would be torn to pieces!
Tuesday, March 5, 13
17
Pulsars spin fast
because a
stellar core’s
spin speeds up
as it collapses
into neutron
star.
Conservation
of angular
momentum
Tuesday, March 5, 13
18
THOUGHT QUESTION
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
Tuesday, March 5, 13
19
THOUGHT QUESTION
Could there be neutron stars that appear as pulsars to
other civilizations but not to us?
A. Yes
B. No
Tuesday, March 5, 13
20
WHAT CAN HAPPEN TO A NEUTRON
STAR IN A CLOSE BINARY SYSTEM?
Tuesday, March 5, 13
21
Insert TCP 6e Figure 18.3
Matter falling toward a neutron star forms an accretion
disk, just as in a white dwarf binary.
Tuesday, March 5, 13
22
Accreting matter
adds angular
momentum to a
neutron star,
increasing its spin.
Tuesday, March 5, 13
23
THOUGHT QUESTION
According to the conservation of angular momentum,
what would happen if a star orbiting in a direction
opposite the neutron’s star rotation fell onto a
neutron star?
A. The neutron star’s rotation would speed up.
B. The neutron star’s rotation would slow down.
C. Nothing. The directions would cancel each other out.
Tuesday, March 5, 13
24
THOUGHT QUESTION
According to the conservation of angular momentum,
what would happen if a star orbiting in a direction
opposite the neutron’s star rotation fell onto a
neutron star?
A. The neutron star’s rotation would speed up.
B. The neutron star’s rotation would slow down.
C. Nothing. The directions would cancel each other
out.
Tuesday, March 5, 13
25
X-RAY BURSTS
• Matter accreting onto
a neutron star can
eventually become hot
enough for helium
fusion.
• The sudden onset of
fusion produces a
burst of X rays.
Tuesday, March 5, 13
26
WHAT IS A BLACK HOLE?
Insert TCP 6e Figure 18.12c
Tuesday, March 5, 13
27
A black hole is an object whose gravity is so powerful
that not even light can escape it.
Tuesday, March 5, 13
28
THOUGHT QUESTION
What happens to the escape velocity from an object if
you shrink it?
A. It increases.
B. It decreases.
C. It stays the same.
Hint:
Tuesday, March 5, 13
29
THOUGHT QUESTION
What happens to the escape velocity from an object if
you shrink it?
A. It increases.
B. It decreases.
C. It stays the same.
Hint:
Tuesday, March 5, 13
30
ESCAPE VELOCITY
Initial kinetic
energy
=
(Escape velocity)2
=
2
Tuesday, March 5, 13
Final gravitational potential
energy
G × (mass)
(radius)
31
Light would not be able to escape
Earth’s surface if you could shrink it
to < 1 centimeter.
Tuesday, March 5, 13
32
“SURFACE” OF A BLACK HOLE
• The “surface” of a black hole is the radius at which the
escape velocity equals the speed of light.
• This spherical surface is known as the event horizon.
• The radius of the event horizon is known as the
Schwarzschild radius.
Tuesday, March 5, 13
33
The event horizon of a 3MSun black hole is also about as big
as a small city.
Tuesday, March 5, 13
34
A black hole’s mass strongly warps space and time in the
vicinity of its event horizon.
The event horizon is larger for
black holes of larger mass.
Tuesday, March 5, 13
35
NO ESCAPE
•
Nothing can escape from within the event horizon
because nothing can go faster than light.
•
No escape means there is no more contact with
something that falls in. It increases the hole mass,
changes the spin or charge, but otherwise loses its
identity.
Tuesday, March 5, 13
36
NEUTRON STAR LIMIT
•
Quantum mechanics says that neutrons in the same
place cannot be in the same state.
• Neutron degeneracy pressure can no longer support
a neutron star against gravity if its mass exceeds about
3Msun.
• Some massive star supernovae can make a black hole
if enough mass falls onto core.
Tuesday, March 5, 13
37
SINGULARITY
• Beyond the neutron star limit, no known force can resist
the crush of gravity.
• As far as we know, gravity crushes all the matter into a
single point known as a singularity.
Tuesday, March 5, 13
38
THOUGHT QUESTION
How does the radius of the event horizon change when
you add mass to a black hole?
A. It increases.
B. It decreases.
C. It stays the same.
Tuesday, March 5, 13
39
THOUGHT QUESTION
How does the radius of the event horizon change when
you add mass to a black hole?
A. It increases.
B. It decreases.
C. It stays the same.
Tuesday, March 5, 13
40
WHAT WOULD IT BE LIKE
TO VISIT A BLACK HOLE?
Insert TCP 6e Figure 18.12c
Tuesday, March 5, 13
41
If the Sun became a black hole, its gravity would be
different only near the event horizon.
Insert TCP 6e Figure 18.12
Black holes don’t suck!
(Nothing sucks, it vacuums!)
Tuesday, March 5, 13
42
Light waves take extra time to climb out of a deep hole in
spacetime, leading to a gravitational redshift.
Tuesday, March 5, 13
43
Time passes more slowly near the event horizon.
Tuesday, March 5, 13
44
THOUGHT QUESTION
Is it easy or hard to fall into a black hole?
A. easy
B. hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus.
Tuesday, March 5, 13
45
THOUGHT QUESTION
Is it easy or hard to fall into a black hole?
A. easy
B. hard
Hint: A black hole with the same mass as the Sun
wouldn’t be much bigger than a college campus
Tuesday, March 5, 13
46
Tidal forces near the
event horizon of a
3MSun black hole would
be lethal to humans.
Tidal forces would be
gentler near a
supermassive black hole
because its radius is
much bigger.
Tuesday, March 5, 13
47
DO BLACK HOLES REALLY EXIST?
Tuesday, March 5, 13
48
BLACK HOLE VERIFICATION
• We need to measure mass by:
— Using orbital properties of a companion
— Measuring the velocity and distance of orbiting gas
• It’s a black hole if it’s not a star and its mass exceeds the
neutron star limit (~3MSun)
Tuesday, March 5, 13
49
Some X-ray binaries contain compact objects of mass
exceeding 3MSun, which are likely to be black holes.
Tuesday, March 5, 13
50
One famous X-ray binary with a likely black hole is in the
constellation Cygnus.
Tuesday, March 5, 13
51
http://apod.nasa.gov/apod/ap111124.html
“In this artist's illustration, two distant galaxies formed about 2 billion years after the big bang are
caught in the afterglow of GRB090323, a gamma-ray burst seen across the Universe. Shining through
its own host galaxy and another nearby galaxy, the alignment of gamma-ray burst and galaxies was
inferred from the afterglow spectrum following the burst's initial detection by the Fermi Gamma Ray
Space Telescope in March of 2009. As seen by one of the European Southern Observatory's very large
telescope units, the spectrum of the burst's fading afterglow also offered a surprising result - the distant
galaxies are richer in heavy elements than the Sun, with the highest abundances ever seen in the early
Universe. Heavy elements that enrich mature galaxies in the local Universe were made in past
generations of stars. So these young galaxies have experienced a prodigious rate of star formation and
chemical evolution compared to our own Milky Way. In the illustration, the light from the burst site at
the left passes successively through the galaxies to the right. Spectra illustrating dark absorption lines
of the galaxies' elements imprinted on the afterglow light are shown as insets. Of course, astronomers
on planet Earth would be about 12 billion light-years off the right edge of the frame.”
Tuesday, March 5, 13
52
WHERE DO GAMMA-RAY
BURSTS COME FROM?
Insert TCP 6e Figure 18.17
Tuesday, March 5, 13
53
GAMMA-RAY BURSTS
• Brief bursts of gamma
rays coming from
space were first
detected in the 1960s.
Tuesday, March 5, 13
54
•
•
Observations in the 1990s showed that many gamma-ray
bursts were coming from very distant galaxies.
They must be among the most powerful explosions in the
universe—could be the formation of a black hole.
Tuesday, March 5, 13
55
WHAT CAUSES GAMMA-RAY
BURSTS?
Insert TCP 6e Figure 18.18
Tuesday, March 5, 13
56
SUPERNOVAE AND
GAMMA-RAY BURSTS
• Observations show that at least some gamma-ray bursts are
produced by supernova explosions. (>2sec)
• Others may come from collisions between neutron stars (or
neutron star/black hole, or black hole/black hole). (<2sec)
Tuesday, March 5, 13
57
SOFT GAMMA REPEATER
• Thought
• Occurs
to be caused by ‘starquakes’ on neutron stars.
approximately once per decade.
• Twists
in the super-strong magnetic field of magnetars exert stress on
the surface of the neutron star and produce the starquake.
• Magnetars
have stronger magnetic fields than normal neutron stars and rotate slower (only once every 10 seconds - compared to more
than one rotation a second)
Tuesday, March 5, 13
58
DISTANT OBJECTS
GRB 090423
520 million years after the
Big Bang
http://www.cosmosmagazine.com/news/4350/gamma-ray-burst-closest-yet-big-bang
Tuesday, March 5, 13
MACS0647-JD
420 million years after the
Big Bang
Image from nasa.gov
59
CHAPTER 19
OUR GALAXY
Tuesday, March 5, 13
60
WHAT DOES OUR GALAXY
LOOK LIKE?
Tuesday, March 5, 13
61
Dusty gas clouds
obscure our
view because
they absorb
visible light.
This is the
interstellar
medium that
makes new star
systems.
Tuesday, March 5, 13
62
All-Sky View
Tuesday, March 5, 13
63
We see our galaxy edge-on.
Primary features: disk, bulge, halo, globular clusters
Tuesday, March 5, 13
64
If we could view the Milky Way from above the
disk, we would see its spiral arms.
Tuesday, March 5, 13
65
SPIRAL
http://apod.nasa.gov/apod/ap120107.html
Tuesday, March 5, 13
66
SPIRAL
http://apod.nasa.gov/apod/ap120107.html
Tuesday, March 5, 13
http://apod.nasa.gov/apod/ap120325.html
67
ELLIPTICAL
http://apod.nasa.gov/apod/ap070208.html
http://apod.nasa.gov/apod/ap120914.html
Tuesday, March 5, 13
68
IRREGULAR
http://apod.nasa.gov/apod/ap050618.html
Tuesday, March 5, 13
69
HOW DO STARS ORBIT IN
OUR GALAXY?
Insert TCP 6e Figure 19.2 unannotated
Tuesday, March 5, 13
70
Stars in the disk all orbit in the same direction with a
little up-and-down motion.
Tuesday, March 5, 13
71
Orbits of stars
in the bulge and
halo have
random
orientations.
Tuesday, March 5, 13
72
Tuesday, March 5, 13
73
THOUGHT QUESTION
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium.
B. Gravity of disk stars pulls them toward the disk.
C. Halo stars knock them back into the disk.
Tuesday, March 5, 13
74
THOUGHT QUESTION
Why do orbits of bulge stars bob up and down?
A. They’re stuck to interstellar medium.
B. Gravity of disk stars pulls them toward the disk.
C. Halo stars knock them back into the disk.
Tuesday, March 5, 13
75
Warped spiral disk of ESO 510-13
http://apod.nasa.gov/apod/ap120304.html
Tuesday, March 5, 13
76
The Sun’s orbital motion (radius and velocity) tells us the
mass within Sun’s orbit:
1.0 × 1011MSun
100000000000 MSun
Tuesday, March 5, 13
77
ORBITAL VELOCITY LAW
r×
=
Mr
G
2
v
• The orbital speed (v) and radius (r) of an object on a
circular orbit around the galaxy tell us the mass (Mr) within
that orbit.
Tuesday, March 5, 13
78
HOW IS GAS RECYCLED IN
OUR GALAXY?
Tuesday, March 5, 13
79
Star–gas–star cycle
Recycles gas from old stars into new star systems.
Tuesday, March 5, 13
80
High-mass stars
have strong stellar
winds that blow
bubbles of hot gas.
Tuesday, March 5, 13
81
Lower mass stars return gas to interstellar space through
stellar winds and planetary nebulae.
Tuesday, March 5, 13
82
Insert TCP 6e Figure 19.6
X rays from hot
gas in
supernova
remnants reveal
newly made
heavy
elements.
Tuesday, March 5, 13
83
A supernova
remnant cools and
begins to emit
visible light as it
expands.
New elements
made by a
supernova mix into
the interstellar
medium.
Tuesday, March 5, 13
84
Radio emission
in supernova
remnants is
from particles
accelerated to
near light
speed.
Cosmic rays
probably come
from
supernovae.
Tuesday, March 5, 13
85
Multiple
supernovae create
huge hot bubbles
that can blow out
of the disk.
Gas clouds
cooling in the
halo can rain
back down on the
disk.
Tuesday, March 5, 13
86
Atomic hydrogen gas forms as hot gas cools, allowing
electrons to join with protons.
Molecular clouds form next, after gas cools enough to
allow atoms to combine into molecules.
Tuesday, March 5, 13
87
Molecular clouds in
Orion
•
•
•
•
Tuesday, March 5, 13
Composition:
Mostly H2
About 28% He
About 1% CO
Many other
molecules
88
Gravity forms
stars out of the
gas in molecular
clouds,
completing the
star–gas–star
cycle.
Tuesday, March 5, 13
89
Radiation from
newly formed
stars is eroding
these starforming clouds.
Tuesday, March 5, 13
90
SUMMARY OF GALACTIC RECYCLING
Gas Cools
• Stars make new elements by fusion.
• Dying stars expel gas and new elements, producing hot
bubbles (~106 K).
• Hot gas cools, allowing atomic hydrogen clouds to form
(~100–10,000 K).
• Further cooling permits molecules to form, making molecular
clouds (~30 K).
• Gravity forms new stars (and planets) in molecular clouds.
Tuesday, March 5, 13
91
THOUGHT QUESTION
Where will the gas be in 1 trillion years?
A. blown out of galaxy
B. still recycling just like now
C. locked into white dwarfs and low-mass stars
Tuesday, March 5, 13
92
THOUGHT QUESTION
Where will the gas be in 1 trillion years?
A. blown out of galaxy
B. still recycling just like now
C. locked into white dwarfs and low-mass stars
Tuesday, March 5, 13
93
We observe the star–gas–star cycle operating in Milky Way’s
disk using many different wavelengths of light.
Tuesday, March 5, 13
94
Radio (atomic hydrogen)
Visible
21-cm radio waves emitted by atomic hydrogen show
where gas has cooled and settled into disk.
Tuesday, March 5, 13
95
Radio (CO)
Visible
Radio waves from carbon monoxide (CO) show the
locations of molecular clouds.
Tuesday, March 5, 13
96
Infrared (dust)
Visible
Long-wavelength infrared emission shows where young
stars are heating dust grains.
Tuesday, March 5, 13
97
Infrared
Visible
Infrared light reveals stars whose visible light is blocked by
gas clouds.
Tuesday, March 5, 13
98
Visible
X-rays
X rays are observed from hot gas above and below the
Milky Way’s disk.
Tuesday, March 5, 13
99
Gamma rays show where cosmic rays from supernovae
collide with atomic nuclei in gas clouds.
Tuesday, March 5, 13
100
WHERE DO STARS TEND TO FORM
IN OUR GALAXY?
Tuesday, March 5, 13
101
Ionization nebulae are
found around short-lived
high-mass stars, signifying
active star formation.
Tuesday, March 5, 13
102
Reflection nebulae
scatter the light from
stars
Why do reflection
nebulae look bluer than
the nearby stars?
Tuesday, March 5, 13
103
What kinds of nebulae do you see in this photo?
Tuesday, March 5, 13
104
Halo: no ionization nebulae, no blue stars
⇒ no star formation
Disk: ionization nebulae, blue stars ⇒ star formation
Tuesday, March 5, 13
105
Much of the star
formation in the disk
happens in the spiral
arms.
Whirlpool Galaxy
Tuesday, March 5, 13
106
Much of the star
formation in the disk
happens in the spiral
arms.
Ionization nebulae
Blue stars
Gas clouds
Whirlpool Galaxy
Tuesday, March 5, 13
107