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Outline - March 25, 2010 Recap: Evolution and death of low mass
stars (pgs ) Evolution and death of high mass stars (pgs ) Stellar
Remnants (white dwarf, neutron star, pulsar, black hole) Novae and
supernovae H-R Diagram About 90% of stars in the sky are Main
Sequence stars
All main sequence stars are stable (gravity exactly balances
pressure) and energy source is fusion of HYDROGEN to form HELIUM
All of the non-main sequence objects are no longer burning H in
their cores (are evolved stars) Build-up of Inert Helium Core
Eventually, the star builds up a substantial He core, with H
burning in a shell around the core. The H burns into layers of the
star that are thinner, and thinner, making it harder to hold the
star up against gravitational collapse. The He core can provide a
little bit of help by contracting (conversion of gravitational
energy, just like a protostar). As the core contracts, the outer
envelope expands and the star leaves the main sequence. Evolution
of a Low-Mass Star
As He core contracts, the star moves up the HR diagram. As outer
envelope expands, the star becomes physically larger (increases
luminosity) and the surface temperature cools (becomes redder).
Star becomes a Red Giant. Onset of He burning in the core happens
quite suddenly (helium flash) once the temperature and density of
the core are high enough to fuse He. Helium flash doesnt disrupt
the star (localized region of 1/1000 of the star), but does cause
the core to expand a little bit (and envelope shrinks in response).
Red Giant Phase for Low Mass Stars
Core is now 100 million Kelvin (about 10x hotter than when the star
was a main sequence star) Two sources of energy: 1. H to He in a
shell 2. He to C in the core Final Stage of Evolution of Low-Mass
Star
Its only a matter of time before the star gets in trouble again
This time its CARBON ash that has sunk to the center (non-burning
carbon core, surrounded by a shell of He burning, surrounded by a
shell of H burning). Most low mass stars can repeat the core
contraction process, and ignite Carbon fusion (which produces
Oxygen). But, once a significant amount of oxygen has built up in
the core, its game over for the star!! Death of a Low Mass Star
Carbon-Oxygen core contracts in an attempt to help hold the star up
against gravitational collapse; but there isnt enough mass in the
star to make the temperature and density high enough to fuse the
oxgygen Core shrinks down to about the size of the earth, and cant
go any farther because of a quantum mechanical effect Can only
compress electrons so far - this is what stops the core contraction
Pressure in the core is provided by degenerate electron gas and the
core becomes stable (no longer contracting) Burning fronts (H, He,
C) plow out into the very light, fluffy layers of the (enormous!)
star, and the outer layers of the star lift off due to radiation
pressure Formation of White Dwarf and Planetary Nebula (end of a
low-mass star)
Outer layers of star lift off, revealing small, hot core = White
Dwarf and creating a Planetary Nebula Sirius B (white dwarf
companion to Sirius A) Evolutionary Track on the HR Diagram
(Low-Mass Star) Evolution of High-Mass Stars
Unlike low mass stars, high mass stars make a steady transition
from H fusion in the core to He fusion in the core (no helium
flash), to O fusion in the core, and they keep on going to heavier
chemical elements. High-mass stars evolve off the main sequence to
become supergiant stars. Onion Layers of Fusion in a High-Mass
Star
Star undergoes cycles of core contraction and envelope expansion,
fusing heavier and heavier chemical elements, until an iron core
forms. Once silicon starts to fuse, the star has about a week to
live. Timescales of Fusion (Mstar = 20 Msun)
H fusion in core: 10 million years He fusion in core: 1 million
years C fusion in core: 1000 years O fusion in core: 1 year Si
fusion in core: 1 week Whats so special about Iron (Fe)?
Fusion of nucleii that are lighter than iron result in a net gain
of energy (takes less energy to bring the nucleii close together
than you get from mass loss) Fusion of nucleii that are as heavy or
heavier than iron result in a net loss of energy (takes more energy
to bring the nucleii close together than you get from mass loss)
Bottom line: star cant use iron as a nuclear fuel to support itself
from gravitational collapse, because fusing iron is a losing
proposition in the energy balance! Death of a High-Mass Star
Supernova: Implosion followed by Explosion
Once substantial amount of iron has built up, star implodes on
itself Core reaches temperature of 10 billion Kelvin (=
tremendously high energy photons), the nuclei are split apart into
protons and neutrons (photodisintegration) In less than 1 second,
the star undoes most of the effects of nuclear fusion that happened
in the previous 11 million years!!!!! High-energy photons are
absorbed, giving rise to loss of thermal energy in the core, the
core becomes even more unstable, and the collapse accelerates
Protons and electrons in the core combine together
(neutronization), resulting in nothing but neutrons in the core
Collapse continues until its not possible to squeeze the neutrons
together any tighter (size of core = size of Manhattan) Collapse
starts to slow, but overshoots and outer layers of star are driven
out into space (perhaps by bounce off the neutron core) in a
massive explosion Supernovae Generate Tremendous Amounts of
Energy
At their maximum brightness, supernovae are as bright as an entire
galaxy. Peak luminosity is about 1051 ergs = the suns total output
of energy over 10 billion years! How long does a supernova
last?
Type II supernovae are exploding high-mass stars Type Ia supernovae
are something else entirely (and involve binary star systems) Why
should you care about supernovae?
Extraordinarily bright, so can use them to measure distances to
galaxies that are very far away: b = L / (4 d2) Supernovae are the
source of all heavy chemical elements! The heavy chemical elements
are produced during the explosion itself, when there is more than
enough energy to fuse nuclei heavier than iron (doesnt matter that
there is a net loss of energy - the star is already VERY far out of
equilibrium) Supernova Remnants (high-mass star guts) Cycle of Star
Formation and Supernovae
Stars form out of gas in the ISM, evolve, and blow much of
themselves back into the ISM Massive stars create heavy chemical
elements during the explosions, which enriches the ISM with heavy
chemical elements New stars form, and make yet more heavy chemical
elements It takes about 500 cycles of massive star formation to
account for all the heavy chemical elements in the universe More
than enough time for this to happen (universe is 14 billion years
old, massive stars take a few million years to evolve and explode)
Stellar Remnants Whats left behind after a star dies?
Main sequence mass < 5 Msun: white dwarf Main sequence mass
between 5 Msun and 40 Msun: neutron star Main sequence mass > 40
Msun: black hole All of these are stable (neither expanding nor
contracting), so long as they are left alone. Pressure in white
dwarf and neutron star is somewhat exotic (not normal gas pressure
or radiation pressure) due to their highly-compressed states. White
Dwarfs Pressure comes from degenerate electron pressure
Electrons packed together as tightly as quantum mechanics allows;
their speeds support the WD against gravitational collapse WD acts
a lot like a metal (same temperature and density throughout)
Maximum WD mass = 1.4 Msun (Chandrasekhar limit) WD with mass = 1
Msun is about the size of the earth, weight of 1 teaspoon of WD
material = about the weight of a small truck If all alone in space,
WD simply cool off (no internal source of energy) and eventually
become black White Dwarfs in Binary Systems
Most stars are found in binary systems May have situation where WD
orbits a giant or supergiant star at a relatively close distance
Outer layers of the giant or supergiant are very light and fluffy,
and may be pulled over onto the WD by gravity Material from
companion star builds up in an accretion disk around the WD, and
eventually winds up on the surface of the WD White Dwarfs in Binary
Systems, II
What happens to the WD when mass is dumped onto it depends on how
much mass, and how fast. Slow accretion of not much mass (not
enough to make the mass of the WD > 1.4 Msun): nova Fast
accretion of a lot of mass (enough to make the mass of the WD >
1.4 Msun rather suddenly): supernova (Type 1a) Novae Thin layer of
(mostly) H from the companion star builds up on surface of WD
Sudden flare in brightness (increases by about a factor of 10,000
or more), then fades over the course of about a month Flare is due
to hydrogen fusion on the surface of the white dwarf Novae happen
about 2 or 3 times per year in our Galaxy Can recur (i.e., same WD
can go nova, but not very predicable) Novae H fusion on the surface
of a WD
Naked eye nova; picture taken in the Varzaneh Desert in Isfahan,
Iran (February 2007) White Dwarf Supernova Supernova Type Ia
If the companion star to a WD dumps a lot of mass onto the WD very
quickly, making the mass of the WD exceed the Chandrasekhar mass
(1.4 Msun), the WD explodes as a supernova! WD is much like a hot
metal ball, same temperature and same density throughout Addition
of extra mass causes WD to contract (gravity wins over pressure
from the electrons) and instantaneously the carbon starts to fuse
throughout all parts of the WD, blowing the WD to bits Two Basic
Types of Supernovae Note: Supernovae NEVER repeat!
Remnants of two different supernovae.Left: a Type Ia supernova
(WD).Right: a Type II supernova (high mass star).This is a happy
alignment of images - the two stars werent related to each other!
Whats left behind after a massive star goes supernova?
If the mass of the core is less than about 3 Msun, a neutron star
is left behind. If the mass of the core is greater than about 3
Msun, there is no source of sufficient pressure to keep the core
from collapsing completely under gravity, and a black hole is
formed. Neutron Stars Even more compressed than WD
Typically the size of a city (about 10 km in radius) with mass
between 1.4 Msun and 3 Msun Density is such that the weight of one
teaspoon of NS material would weigh 100 million tons (vs. 1 ton for
WD material) NS supported by neutron degenerate pressure (again,
quantum mechanical phenomenon having to do with how tightly
neutrons can be packed together) Must rotate extremely fast
(conservation of angular momentum); a star that was originally
rotating once per month would now have to rotate a few times per
second! Compression of the material also compresses the magnetic
field, and amplifies its strength (making it trillions of times
larger than the earths magnetic field) Pulsars Rapidly-Rotating
Neutron Stars
First discovered by Jocelyn Bell (1967) as pulses of radio light
coming from the Crab Nebula The pulses lasted 0.01 seconds, and
repeated every 1.4 seconds In 1974, Jocelyns PhD advisor (Tony
Hewish) got the Nobel Prize for explaining what puslars are Now
know of 100s of pulsars, most with periods between 0.03 and 0.3
seconds (meaning they rotate between 3 and 30 times per seconds)
The fastest known pulsars have periods of milliseconds and are
rotating at speeds approaching 0.25c !!!! The radio light from
plusars is called synchrotron radiation, a type of light that is
emitted by electrons as they move on spiral paths around magnetic
field lines.The synchrotron radiation pulses are proof of the fast
rotation rates of neutron stars and the presence of an incredibly
strong magnetic field. Pulsar at Center of Crab Nebula
Supernova observed by Chinese astronomers in year 1054 Crab Nebula
- remnant of supernova explosion Pulsar Recordings
http://www.astrosurf.com/luxorion/audiofiles-pulsar.htm
Crab rotations per second Vela - 11 rotations per second PSR (a
millisecond pulsar) rotations per second Curve boundaries for
Midterm #2:
Midterm Exam #2 Curve boundaries for Midterm #2: A > 90.5% A %
to 90.5% B % to 87.5% B % to 81% B % to 75% C % to 69% C % to 66% C
% to 63% D % to 57% F < 50% Class letter grade average based on
the curve is between B and B- (2.9 / 4.0) Your score on this exam:
78.5 / 100 Your ranking in the class on this exam: / 47 Approximate
letter grade on this exam: B This info is on the last page of your
exam. Approximate Mid-semester Grades
A % to 92% B % to 86% B % to 82% B % to 78% C % to 70% C % to 66% C
% to 62% D % to 57% F < 50% Approximate mid-semester grades
based on average of midterm exams, best 3 of 4 home work
assignments, and average of 2 labs.