Transcript Introduction to Astronomy

Introduction to Astronomy
• Announcements
White Dwarfs
&
Neutron Stars
White Dwarfs
Structure
Observations
• Background: the discovery of Sirius B
– Companion star to large Sirius A
– First time White Dwarf observed as part of
binary system
Sirius A: A-type main
sequence star
Sirius B: white dwarf
Structure
• Generally,
– Hot, compact stars w/ mass comparable to
the Sun & size comparable to the Earth
– Shines from residual (left-over) heat produced
in core during normal lifetime
– 25,000 K < T < 4,500 K
– Tavg = 10,000 K
• A white dwarf is nothing more than the leftover core of a low-mass star (few x MSun)
– During red giant phase, outer layers are
blown off
• Radiation pressure
– These layers are mostly H & He
• Fusion products of early star’s life
• Blown off by Helium Flash
– Not enough mass to ignite C/O fusion by
contraction
• Low mass
• So the core just cools off, radiating heat
into space
– Over ~ 10 million years, cools down to 20,000
K
• So still emits some observable light
– Would take much longer than the age of the
Universe to cool off to the point where it
doesn’t emit any visible light
• “BLACK DWARF” (still just theoretical)
Interior of a White Dwarf
• In hydrostatic equilibrium
– But no fusion pressure, so how is this
possible????
– Electron Degeneracy Pressure
• Atoms squeezed incredibly close together
• Electron orbits overlap
– Electrons easily move around, from atom to atom
– Like a “sea” of electrons flowing over atomic nuclei
• The Pauli Exclusion Principle
– A fundamental limit to the number of electrons
that can be squeezed into a given volume
– When this limit is reached, there appears a
“pressure” that keeps any more electrons
from entering the volume
– This “electron pressure” supports the white
dwarf against its own gravity
• This leads to behavior that seems to defy
common sense…
– The more mass you pile onto a white dwarf,
the smaller it gets!
• In a normal gas, pressure depends on
temperature and density
• This “sea” of electrons is what is called a
degenerate gas
– In a degenerate gas, pressure does not
depend on temperature, only density (of
electrons)
• A small chunk of “white dwarf material”
would weigh ~ couple dozen tons on
Earth!
• Gravitational
Redshift
– Like Doppler shift
due to star’s motion
– As light escapes from
the star’s surface,
gravity pulls on it,
“stretching” the light
wave
– So light from a highmass object appears
redder than it actually
is
But photons have no mass, so how does
Gravity work on it?! General Relativity!
• White Dwarfs in binary systems
– White dwarf – red giant system is potentially
very dangerous
– Outer layers of the red giant are not very
bound to the star itself
• White dwarf can “strip away” the outer layers
(feeding)
• Usually H-rich, so white dwarf gets a new fusion
fuel source
• Two options
– A Nova
– A “Type I” Supernova
• Nova (or Nova Stella)
– The white dwarf’s compression of H pulled
from the red giant heats it to fusion
temperatures
– H explodes off the surface of the white dwarf
• “Nova”
– White dwarf may begin to strip Hydrogen from
the red giant again
• Repeated novae
HST observation of a Nova
Time, t
Time, t + 7.5 months
• Supernovae
– Different from novae
– When fusion in a high-mass star stops, core
continues contracting, but no pressure to stop
it
– Temperature increases so much the Iron
nuclei start to break apart
– Collapse pushes electrons into protons,
creating bulk of neutrons at nuclear density
– Rapid implosion causes material to “bounce”
off high-density core, creates massive shock
wave that rips the star apart
“Type II Supernova”
• Type I Supernova
– Only occurs if white dwarf strips off too much
material
– White dwarf passes the Chandrasekhar Limit
• If mass of white dwarf grows beyond ~ 1.44MSun, it
compresses violently
• Squeezes Carbon & Oxygen together hard enough
to fuse into 28Si
• 2 28Si smashed together into an isotope of Nickel
• Fusion releases tremendous amount of
energy from white dwarf’s core, blows the
dwarf apart (completely)
• Leaves behind no remnant
– Just an expanding cloud of heavy elements
(C, O, Si, Ni, Co, Fe, etc)
• Type II Supernova
– Collapse of massive star (not white dwarf)
White Dwarf Observations
A team at the University of Arizona
located a star cluster made almost
completely of white dwarfs.
This cluster had to be extremely old
since every member had used up
it’s nuclear fuel.
From this, they figured the age of
this particular cluster would give
a lower bound to the age of the
Universe.
They found
τuniverse > 13 billion years
Sun-like stars
White dwarfs
M4 Globular Cluster (7,000 ly distant)
Red dwarfs
(very cool
MS stars)
• Old White Dwarfs
– Recall interiors are C, O, Si
– When cool enough, the Carbon atoms can
lock in to a crystalline structure
– What is crystalline carbon?
• A diamond!
– Old white dwarfs may be giant diamonds
floating around in space!!!
Neutron Stars
Structure
Observations
• One step beyond white dwarfs
– Requires a star of higher initial mass
• More intense collapse converts white
dwarf’s “sea of electrons” to something
else entirely…
History of Neutron Stars
• 1934, Astronomers Walter Baade & Fritz
Zwicky
– Proposed that gravity could crush the core of a
star so much that the electrons are pushed
into the nucleus
– Positive protons combine with negative
electrons to form neutrons
– This would occur when core compressed to
diameter of ~ 6 miles
• Smaller than many asteroids!
– Maximum possible mass 2 – 3MSun
Electrons pushed into nucleus
electron
Ultradense ball
of neutrons
neutron
proton
This ultrahigh density means
a single cm3 of neutron
star material would weigh
billions of tons here on Earth!
Size of a Neutron Star…
Neutron Star Observations
1997, first visible image (HST)
of a lone Neutron star.
Mysterious X-ray source found
here in 1992, but astronomers
couldn’t see anything.
HST determined temperature
~ 1.2 million F
Diameter < 17 miles
Therefore, must be Neutron star
because nothing else can be
this hot, small, and dim.
X-Ray image of wandering Neutron Star
Pulsars
• The hunt for neutron stars was on!
• But none found…
• Until a student discovered an
extraterrestrial radio signal that pulsed
precisely once every 1.33 seconds
– Initially thought to be a sign of civilization,
dubbed LGM-1 (“little green men #1”)
• Other similar discoveries soon followed
– Thought maybe they were pulsating stars
• Radius expands & contracts periodically, making
the star alternately brighter & dimmer
– But periods were too short (impossibly small
for the size-change required to change the
brightness that much)
• Not pulsating, but spinning !
– Like a cosmic lighthouse, see a pulse when
some “beam” of radiation points toward Earth
• But why so fast?!
– Pulsar in Crab Nebula rotates 30 times a
second!
OFF
ON
• Conservation of Angular Momentum
– A.k.a. the Ice-Skater Effect
– Bringing mass closer to the axis of rotation
makes the rotation speed increase
– Same principle during formation of solar
system from slowly rotating IS cloud
– If Sun shrank down to 10 km:
• Current rotational period = 30 days
• New rotational period = 0.5 millisecond !
Pulsar Emission: The Lighthouse
Effect
• Intense electric and magnetic fields strip
charged particles off star, accelerate them along
magnetic poles
• Fact: accelerating charges emit EM radiation
– Like a radio transmitter/antenna
• Charges travel along field lines, which form a
tight core at the poles
– Creates a tight cone of emission coming from each
pole = the lighthouse beam!
Protons & Electrons accelerated
quickly here
So a lot of emission near poles
Protons &
Electrons
accelerated
slowly here
Synchrotron Radiation: created by accelerating particles
• Non-thermal radiation
– Depends on the strength of the charge, the
speed of the particle, and the strength of the
magnetic field
– DOES NOT depend on the temperature
• Most “pulsar lighthouses” emit radio
waves, but some emit more broadband…
– Crab Nebula pulsar emits flashes of visible
light 30x a second
Pulsar Spin-Down
• Pulsar is constantly losing energy
– Magnetic field exerts force on charged
particles, so particles exert equal & opposite
force on magnetic field (“back-reaction”)
– Magnetic “friction” or “drag”
• Slows rotation very slightly
• Very lengthy measurements of pulsar
periods show that time between pulses is
slowly increasing
• If it slows down enough, strength of EM
radiation decreases until the “lighthouse”
beams become invisible
Hand-crank generator
• Structure (kind of like a balloon)
– Thin, gaseous atmosphere ( ~ 1 mm thick )
• Source of particles accelerated by magnetic field
– Solid iron crust ( ~ few hundred meters thick )
– Liquid sea of neutrons (bulk)
Why don’t the
surface layers
become neutrons?
Pulsar Curiosities
• X-Ray Bursters
– Infalling gas violently heated as it flows down
magnetic field to surface
– Creates thermonuclear explosion like a nova
• X-Ray Pulsars
– Infalling gas doesn’t explode, but still heated enough
to create a “hot-spot” on the surface of the neutron
star
– Rotates in and out of view, creating pulses of X-rays
X-Ray Pulsar
What We See…
• Millisecond Pulsars
– Rotate 1000x per second
– Most have companion stars
– Gravity attracts companion material into an
accretion disk around the neutron star
• This rotating disk transfers its angular momentum
(rotation) to the neutron star, speeding it up
• But some millisecond pulsars have been
observed without companions…where did
they go?
– Another star passed by, gravitationally pulled
companion away?
– Companion evaporated by intense heat from
pulsar
• The “black widow” pulsar theory
• Neutron star cannibalizes the companion for it’s
mass, which generates heat that “boils away” the
remains of the companion
NEXT TIME
• Black Holes!
– The most exotic/fascinating/misunderstood
objects in the universe