a105 stars and galaxies this week’s units: 66,67,68 news quiz today novasearch ii homework due...
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A105 Stars and Galaxies
This week’s units: 66,67,68 News Quiz Today NovaSearch II homework due Thursday 2nd Exam on Thursday, Nov. 2
Today’s APOD
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Announcements…• Solar Lab at 11 AM TODAY• Kirkwood Obs. open Weds
night 8-10 PM• Rooftop session Thurs @ 9• All WEATHER PERMITTING
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Review – Compact objects form from the death
of stars
• Massive Stars• Iron collapse• Neutron Stars and Black Holes
Low Mass Stars •Red Dwarfs•Sun-like Stars
White Dwarfs
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Helix Nebula
A Planetary Nebula• 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
3 light y
ears
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Planetary
Nebulae!
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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 tonsNeutron 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
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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!)
•no fuel to burn•residual heat inside
•mainly carbon and oxygen•thin H or He surface layer
initially hot (150,000K) >gradually cool > finally, a black dwarf
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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…!)
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White Dwarfs
•composition depends on mass of progenitor•helium•carbon•carbon-oxygen•oxygen-neon-magnesium
•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
•remnants of low mass stars•0.1-1.4 solar masses
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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
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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
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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
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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
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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
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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
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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
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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
Crab B1937J0437PSR0950 VelaB0329
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• Rotating neutron stars• An object as big as the Sun with a one-
month 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
What Are Pulsars??
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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.’
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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
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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
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Geminga
• 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
•Name: Gemini gamma ray source•In Italian, “Gh’ e minga” means “it’s not there”
•Detected 30 years ago as a gamma ray source (2nd brightest)
•Not detected in visible light until 1993
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Formation of Neutron Binaries
• 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
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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
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Black Widow Pulsar
•Rotates 625 times per second•Weak magnetic field•About 10 miles in diameter
Very old BUT spun up by accreting matter from a binary companion
Binary companion slowly beingblown apart
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Read Units 66, 67, 68 NovaSearch II Homework
Due THURS.