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

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

• Massive Stars• Iron collapse• Neutron Stars and Black Holes

Low Mass Stars •Red Dwarfs•Sun-like Stars

White Dwarfs

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

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 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

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

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

•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

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

Crab B1937J0437PSR0950 VelaB0329

• 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??

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

• 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

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

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

Very old BUT spun up by accreting matter from a binary companion

Binary companion slowly beingblown apart

Read Units 66, 67, 68 NovaSearch II Homework

Due THURS.