Transcript Stellar Remnants

A105
Stars and Galaxies
Today’s APOD
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This week’s units: 66,67,68
News Quiz Today
NovaSearch II homework due Thursday
2nd Exam on Thursday, Nov. 2
Announcements…
• Solar Lab at 11 AM TODAY
• Kirkwood Obs. open Weds
night 8-10 PM
• Rooftop session Thurs @ 9
• All WEATHER PERMITTING
Review – Compact objects
form from the death
of stars
Low Mass Stars
•Red Dwarfs
•Sun-like Stars
White Dwarfs
• Massive Stars
• Iron collapse
• Neutron Stars and Black Holes
Helix
Nebula
A Planetary Nebula
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•
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About 650 LY distant
About 3 LY across
A cylinder of gas
expelled from the
central star
• We are looking
DOWN the cylinder
• Probably looks more
like the “Retina Nebula”
from the side
Retina Nebula
Planetary
Nebulae!
Compact
Objects
White Dwarfs
•Formed from the cores of stars less
than 8 times the mass of the Sun
•up to 1.4 solar masses
•made of compressed He, C-O, or Fe
•about the size of the Earth
•One cc would weigh about 3 tons
Neutron Stars
•Formed in supernova explosions
•from 1.4 to about 3 solar masses
•Made of pure neutrons – a giant atomic nucleus
•About 10 km in radius
•One cc would weigh about a million tons
Black Holes
•Formed in Supernova explosions
•Usually a few times the mass of the Sun
•A solar mass black hole is about 3 km in
diameter
•Density is infinite
White
Dwarfs
•mass similar to the Sun’s
•diameter about that of the Earth
•high surface temperature (typically 25,000 K)
•but very dim (small size!)
•mainly carbon and oxygen
•thin H or He surface layer
•no fuel to burn
•residual heat inside
initially hot (150,000K) >gradually cool > finally, a black dwarf
Famous White
Dwarfs
• Sirius B
– about 9 LY
– orbits Sirius A every 50
years
• Procyon B
– about 11 LY
– orbits Procyon A every 42
years
• 40 Eridani B
– 16 light years
– orbits 40 Eri B (home
system of Vulcans…!)
White Dwarfs
•remnants of low
mass stars
•0.1-1.4 solar masses
•radius inversely proportional to mass
•solar mass white dwarf the size of Earth
•1.38 solar mass WD is much smaller
Density much greater than Earth
•Earth = 5.5 grams/cc
•WD = tons/cc
•composition depends on mass of progenitor
•helium
•carbon
•carbon-oxygen
•oxygen-neon-magnesium
White Dwarf
Structure
• White dwarfs are in hydrostatic equilibrium
– Gravity is balanced by the quantum electron pressure
– Quantum pressure allows the white dwarfs to shrink with
increasing mass
• A white dwarf’s high density (106 g/cm3) implies that
nuclei are separated by distances less than the
normal radius of an electron orbit
White Dwarfs Are Weird
• Quantum electron pressure and the
Chandrasekhar Limit
– when a quantum gas is compressed, it heats
up, but this temperature increase does not
increase the pressure
• quantum gases are less “springy”
– Adding mass to a white dwarf makes it shrink
• the white dwarf will collapse when enough mass is
added
• maximum mass for collapse is called the
Chandrasekhar Limit and has a value of 1.4 M
• NO white dwarfs have masses above 1.4 solar
masses
WDs Can Become Supernovae
• Type I supernova from exploding white
dwarfs
• Novae may finally become supernovae
• Mass of white dwarf continues to grow
• Eventually the mass becomes to great to
support
• Carbon white dwarf collapses
• Carbon nuclear reactions ignite, blow the
white dwarf apart
Type I Supernovae Are Important
• Type I’s produce most of the
iron in the universe
• Type I’s are very bright (can be
seen from a long distance)
• Because Type I’s arise in a
uniform way, they are
“standard candles”
• Type I’s are used to study the
large scale structure of the
universe
White dwarfs can
also become novae
Main Ideas
•Novae are white dwarfs in
binaries
•Mass is transferred to
white dwarf
•Mass builds up
•Ignites nuclear fusion
•Surface of white dwarf
explodes
Nova in Aquila, 1999
Novasearch:
Update
The Light Curve of
Recurrent Nova
T Pyxidis over 80 years
Data from AAVSO International Database
Credits: M. Shara, R. Williams, (STScI), R. Gilmozzi (ESO), NASA
Compact
Objects
White Dwarfs
•Formed from the cores of stars less
than 8 times the mass of the Sun
•up to 1.4 solar masses
•made of compressed He, C, or Fe
•about the size of the Earth
•One cc would weigh about 3 tons
Neutron Stars
•Formed in supernova explosions
•from 1.4 to about 3 solar masses
•Made of pure neutrons – a giant atomic nucleus
•About 20 km in diameter
•One cc would weigh about a million tons
Black Holes
•Formed in Supernova explosions
•Usually a few times the mass of the Sun
•A solar mass black hole is about 3 km in
diameter
•Density is infinite
Discovery
• First detected by Jocelyn Bell in 1967
– graduate student in England
• an odd radio signal with a rapid pulse
rate of one burst per 1.33 seconds
• more pulsating radio sources were
discovered and eventually were named
pulsars
• No clue what they were!
When pulsars were first discovered, it
thought they might be evidence of other
intelligent life in the Galaxy
B0329
PSR0950
Vela
Crab
J0437
B1937
What Are Pulsars??
• Rotating neutron stars
• An object as big as the Sun with a onemonth rotation period will rotate more
than 1000 times a second if squeezed
down to the size of a neutron star
– This happens when a massive star’s iron core
collapses
– magnetic field beams radiation energy in
opposite directions
– Spinning beams make the pulsar pulse
Lighthouse Model
Pulsars emit beams of radio light. As the pulsar rotates,
the beams sweep across the sky. When the beam
"sweeps" over Earth, we detect the radiation, as a
‘pulse.’
But What IS a
Neutron Star?
• Dense ball of collapsed matter
• Atoms so compressed that electrons
and protons are forced together to
become neutrons
• Basically, a giant atomic nucleus
• Neutron stars have three layers:
– a millimeter thick atmosphere,
– an iron crust of a few hundred meters
– a superfluid neutron core with (having
virtually no friction or magnetic fields)
• The core and crust spin
independently
The Crab Nebula Pulsar
•only 1000 years old!
•still inside its supernova
remnant
•emits a pulsar wind and
jets
•produces visible pulses –
only young pulsars have
enough energy to do this
•20-km in diameter and is
spinning at 33 rpm
Geminga
•Detected 30 years ago as a
gamma ray source (2nd brightest)
•Not detected in visible light
until 1993
•Name: Gemini gamma ray
source
•In Italian, “Gh’ e minga”
means “it’s not there”
• The closest known pulsar to Earth
• About 500 light years away
• About 12 miles in diameter
• “Tails” from a shock wave as Geminga plows through
the interstellar medium
Formation of
Neutron
Binaries
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More massive star evolves first to a neutron star
Less massive star grows larger as it evolves
Mass begins to transfer from the less massive star to the more massive star
Accretion disk forms
Material falling onto the neutron star emits X-rays
Neutron Binary
Stars
– Intense X-rays from neutron stars in binary
systems There are several types of X-ray
binaries
• X-ray bursters from gas falling on the neutron
star
• X-ray pulsars from hot-spots on the neutron star
• infalling gas can “spin up” an old neutron star
Black Widow Pulsar
•Rotates 625 times per second
•Weak magnetic field
•About 10 miles in diameter
Binary companion slowly being
blown apart
Very old BUT spun up by
accreting matter from a
binary companion
 Read Units 66, 67, 68
 NovaSearch II Homework Due
THURS.