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Astronomy 101
Lecture 19, Apr. 2, 2003
Neutron Stars and Gamma Ray Bursters– (Chapter 22.1 – 22.4 in text)
In last lecture, we saw a Type II supernova blowing off its outer layers
in a gigantic explosion, due to a shock wave that bounced off the central
neutron core. The supernova lights up the sky and seeds the
neighborhood with heavy elements used for complex life.
What’s left of the star after the explosion?
If the mass of the sphere of neutrons is less than about 3 solar masses,
it remains as a
Neutron star.
Recall that the neutron core formed just before the supernova erupted
had very high density. The neutrons are as close as quantum mechanics
will allow them (degenerate). The density of the neutron star is about
the same as inside an nucleus of the atom – but for the neutron star,
instead of having 10 – 200 neutrons and protons, there are now about
1057 of them inside the sphere. And there are no protons left – all were
converted to neutrons by the reaction
p + e- → n + n
just preceding the final collapse
The density inside the neutron star is huge – about 1018 kg/m3, or a million
billion times the density of water. So the size of the neutron star is tiny
by star standards – about 20 kilometers across.
The large mass and small radius means
that gravity is incredibly strong at its
surface:
Fgrav = GMm/R2
A 150 pound person would weigh about
a million tons on the surface.
There is no way to burn neutrons into
anything else, so no new heat is
generated. The star cannot contract
further due to the degenerate neutron
pressure. So the neutron star just
radiates blackbody radiation and cools
off.
neutron
star
Manhattan
The neutron star rotates very rapidly, because of the conservation of
angular momentum.
For any object, the angular momentum
– the product of its mass times
rotational velocity times radius – is
conserved. The star before collapse
had a radius of about 1000 solar radii
and turned on its axis perhaps once
per month. The neutron star is only
10 km in radius, and turns on its axis
many times per second to maintain the
same angular momentum.
In addition, the neutron star has very strong magnetic fields. These too
get larger during the collapse as they are due to electric currents that
become larger as the star becomes smaller and rotates faster.
In time, the rotation rate and the magnetic fields become smaller due to
the radiation of energy from the neutron star.
Observing a neutron star
We see spinning neutron stars as PULSARS. In 1967 a graduate student
at Cambridge saw a light curve from a star (intensity vs. time) with very
short pulses every 1.34 seconds.
1.34 seconds
The pulse period is extremely regular, and initially people speculated
that they came from LGMs (little green men) as evidence of a civilized
society beaming us signals from deep space. By now we have seen
hundreds of these pulsars, each with its own distinct pattern and
period. Some are seen in visible light, others in X-rays.
How does the neutron star pulsar work?
Magnetic field lines emerge from the magnetic south pole, circle around and come
back into the neutron star at the north magnetic pole. The magnetic field is
particularly strong near the poles. The interstellar gas of ions and electrons
circle the magnetic field and fall toward the poles. When they bend in the field,
they emit ‘synchrotron radiation’ in the form of radio waves, light, ultraviolet, and
X-rays. Most of this radiation is emitted along the magnetic pole axes.
The magnetic field lines look
approximately like the pattern
of iron filings surrounding a
short bar magnet.
rotation
axis
S pole
magnetic axis
N pole
The magnetic pole axis is not typically aligned with the rotation axis. So the
beams of light along the magnetic axes turns in space like a searchlight. If
the earth is in the line of the searchlight, we see the pulses every time the
star rotates.
Magnetic axis
neutron
star
radiation
If earth is here, we
don’t see the pulses.
Axis of rotation
If earth is here, we
do see the pulses.
Geminga pulsar seen in images taken 0.05 seconds apart – the (gamma
ray) intensity blinks on and off with a 0.24 sec. Period.
Nearest neutron star to us (at 60 pc)
is not a pulsar. It was discovered by
Stony Brook astronomers. It is moving
across the sky at 110 km/s, faster
than typical for stars. The high speed
is probably due to a kick given during
the supernova explosion. This neutron
star is seen as an X-ray source.
If the neutron star is in a binary pair, matter (hydrogen) can be transferred
from the normal companion to the neutron star. When enough hydrogen
accumulates to raise the temperature, it ignites in the very strong gravitational
field and emits a burst of X-rays.
This mechanism is very similar to what happens when gas from a companion falls
on a white dwarf to form a nova, but more powerful.
The infalling gas spirals in and hits the neutron star moving parallel to its
surface. This impact can speed up the rotation, leading to pulsars with
very short (0.001 sec) periods. Such pulsars have surfaces moving at a
quarter the speed of light!
Gamma ray bursters
Military satellites looking for bomb blasts on earth found bursts of gamma
rays in the universe, lasting from a fraction to several seconds.
They occur uniformly over
the sky, so seem to be
outside our galaxy (which lies
in a single plane).
Some gamma ray bursters have been identified with optical objects
that are moving away from us with huge speeds, using the observed
Doppler shifts of known spectral lines. This indicates that they are
very very far away (we will make this connection between recessional
velocity and distance clear later – it’s called the Hubble expansion of
the universe).
From the energy we observe on earth and knowing the distance, we
can calculate the total luminosity of the gamma ray burster.
Assuming that the burster radiates equally in all directions, we find
that they are hundreds of times more luminous than a supernova – the
brightest known objects in the sky.
We know they are small, since there are substantial changes in
intensity in milliseconds – only possible for small objects.
Stony Brook scientists propose that Gamma Ray bursters arise from a
rotating black hole whose powerful magnetic field sweeps through the
accretion disk formed from a companion star. There is not yet
consensus on what makes the GRBs work!
Midterm Exam 2 – Monday April 7, in class. Bring your ID, pencil for
marking the Scantron form.
Be sure to record the exam type in the first digit labelled ‘birth date’
(exam types 0, 1, 2, 3)
Exam will be of similar form to first midterm – 40 multiple choice questions
and two essay questions or problems.
As review, lets look at AST101 Midterm 2 in Fall 2002.
In the proton-proton chain, an important intermediate step involves the
formation of 2H (deuteron). 2H is:
a)
A hydrogen atom with 2 electrons
b)
A hydrogen nucleus with 2 protons
c)
A hydrogen nucleus with 2 nucleons: 1 proton and 1 neutron
d)
A hydrogen nucleus with 2 neutrons
e)
An isotope of helium
A cubic centimeter of an interstellar molecular cloud contains about _____
molecules of hydrogen on average.
a)
3
b)
30
c)
300
d)
300,000
e)
3,000,000
A star with parallax of 0.2 arc seconds is at a distance of about
a)
5 AU
b)
500 AU
c)
0.5 parsecs
d)
5 parsecs
e)
0.2 parsecs
d (pc) = 1/parallax angle (arcsec)
If the mass of the sun was 10 times its value and Earth remained in an orbit
at 1 AU (150 million km), the length of an Earth year would be:
a)
About the same as now
b)
About 10 times shorter than now
c)
About 10 times longer than now
d)
About 3 times longer than now
e)
About 3 times shorter than now
Newton’s modification of Kepler’s 3rd
Law
P2 = a3 / (Msun + Mearth)
P2 reduced by about factor 10, so P
reduced by √10 ~ 3.
The mass of a giant molecular cloud is:
a)
About the mass of the earth
b)
About the mass of the sun
c)
About the mass of 100 suns
d)
About the mass of 100,000 suns
The effect of interstellar dust on starlight is:
a)
To make stars appear less bright than expected, by absorbing light
about equally at all wavelengths
b)
To scatter the red light from stars preferentially, making them
appear more blue than expected
c)
Almost nothing, since light does not interact with dust
d)
To dim and redden distant stars by preferentially scattering their
blue light.
Where in the universe would you look for a protostar?
a)
In giant molecular clouds
b)
In globular clusters of stars
c)
In the empty space between the galaxies
d)
Near to black holes
e)
In supernova remnants
The power source for the light from an emission nebula (H II region) is:
a)
Ionization of hydrogen by ultraviolet
light from hot stars
b)
Ionization of hydrogen by infrared light
from hot stars
c)
An electric current running from the
nebula from a hot star
d)
Reactions between hydrogen and oxygen
which supply explosive energy
e)
None of the above
Emission
light
UV
In the interstellar medium, radio waves of 21 cm wavelength originate in
which component?
a)
Ionized atomic hydrogen
b)
Carbon monoxide, CO
c)
Molecular hydrogen
d)
Neutral atomic hydrogen
Relative orientation of proton
and electron spins changes
from parallel to antiparallel
and emits 21 cm radiation.
A standard candle is a star or object for which we know the __________,
luminosity
and measure _________________
.
Apparent brightness to determine the ___________
distance
a) Size, apparent brightness, distance
b)
Luminosity, apparent brightness, distance
c)
Distance, luminosity, apparent brightness
d)
Luminosity, distance, apparent brightness
e)
Apparent brightness, luminosity, distance
L = 4pd2 Iapp
What important stellar parameter can be measured from observations of
binary stars, but not single stars?
a)
Surface temperature
b)
Age of star
c)
Distance of star from earth
d)
Stellar mass
e)
Both C and D
The key property that distinguishes stars on the main sequence from all
others is:
a)
They have the same mass
b)
They are burning hydrogen
c)
They are in gravitational equilibrium
d)
They are burning hydrogen in their centers
e)
They are the same age
Which of the following is true about the rate of stellar evolution?
a)
The more massive the initial star, the slower the evolution since more
material for nuclear burning
b)
The more massive the initial star,the faster the evolution
c)
Star mass has no bearing on evolution since all stars evolve at same rate
d)
The chemical makeup of the original nebula is the major factor in
determining the rate of evolution, whatever the mass.
How long will the sun have spent as a main sequence star when it finally
becomes a red giant?
a)
1 billion years
b)
1 million years
c)
1011
d)
1010 years
e)
1012 years
years
1010 = 10 billion
Our sun will ultimately become:
a)
Neutron star
b)
White dwarf
c)
Pulsar
d)
Black hole
e)
Supernova
Thermonuclear burning and fusion can produce such ‘heavy’ elements as
carbon, oxygen etc. up to a limit beyond which no further energy
producing reactions can occur. This limit occurs at element:
a)
Uranium
b)
Oxygen
c)
Iron
d)
Silicon
e)
None of the above
When a star has exhausted hydrogen at its
core
a)
The whole star expands, becoming bigger
and less luminous
b)
The core contracts, the surface
expands, the surface cools and the
luminosity increases.
c)
The core and surface contract, making it
hotter and brighter
d)
The core contracts, the surface
expands, the surface cools and the
luminosity decreases
e)
It contracts, becoming smaller and less
luminous.
A white dwarf generates energy from what source?
a) Gravitational potential energy as star slowly contracts
b) No longer generates energy, but cools slowly
c) Nuclear fusion of hydrogen into helium
d) Nuclear fusion of heavy elements in the central core
A star of 20 solar masses will end its life as a:
a)
A planetary nebula, leaving a neutron star
b)
As a supernova, leaving a white dwarf
c)
As a supernova,leaving a neutron star or black hole
d)
As a planetary nebula, leaving a white dwarf
e)
As a supernova, leaving no remnant
Which major astronomical event was recorded by Chinese astronomers in
1054 AD?
a)
A supernova explosion in our galaxy, visible even in daytime
b)
A supernova in another galaxy, visible even in daytime
c)
A total eclipse of the sun
d)
A formation of a planet
e)
The birth of a new star
A physical characteristic of matter in a white dwarf is
a)
Extemely high density compared to ordinary stellar matter
b)
Composed only of electrons in degenerate state
c)
Composed only of neutrons
d)
Is in the form of a hollow shell with a black hole in the center
e)
Is composed only of iron
Stars which have ejected a planetary nebula go on to become
a)
Protostars
b)
Supernovae
c)
Red giants
d)
White dwarfs
A supernova of Type II (from a massive star) is powered by:
a)
Gravitational collapse
b)
Convection
c)
Fusion
d)
Fission
e)
B and C
Actually, I’d prefer to say a Type II
supernova is powered immediately by an
expanding shock wave, triggered by
gravitational collapse of material onto the
brick wall of neutron degenerate pressure.
Stellar nucleosynthesis refers to
a)
All formation of complex chemicals in interstellar space
b)
The synthesis of stars from interstellar matter
c)
The transfer of energy by synthesis
d)
The formation of chemical elements in the periodic table by
nuclear fusion inside stars, starting with hydrogen
e)
None of the above
Energy can be transported across empty space by:
a)
Convection
b)
Radiation
c)
Conduction
d)
Convection and radiation
e)
Cannot be transported in empty space without matter
A cubic centimeter of air in this room contains about _____ molecules
of nitrogen and oxygen.
a)
1000 or less
b)
100,000
c)
108
d)
1011
e)
More than 1014
At what wavelengths have astronomers mapped and studied the distribution
of giant molecular clouds in space?
a)
UV, because molecules are efficient UV emitters and the clouds are hot
b)
Millimeter wavelengths, using radio telescopes
c)
Visible light using photography
d)
Using ultrasound waves at high frequency
Why does hydrogen ignite in a shell around the helium core after the main
sequence?
a)
Cooling due to cessation of core fusion ignites the hydrogen
b)
Higher density due to core contraction starts off the fusion
c)
The hydrogen was already burning, it just continues
d)
The higher temperature due to core contraction starts off the fusion
e)
The decrease in light from the core reduces the repulsion of the
hydrogen nuclei
What is a planetary nebula?
a)
The cloud from which a protostar forms
b)
The leftovers of the supernova explosion that formed a white dwarf in
the middle
c)
The disk surrounding a newborn star
d)
The leftovers of the planets after the star has died
e)
The expelled envelope of a low or intermediate mass star at the end of
its life.