Life story of a star Micro-world Macro-world Lecture 20

Life Cycle of Stars Recycling Supernovae produce - heavy elements - neutron stars - black holes Martin Rees - Our Cosmic Habitat

Our favorite star: The Sun R ๏ = 696,000 km R ๏ = 696,000 km (109 x R earth ) (109 x R earth ) M= 2x10 30 kg M= 2x10 30 kg ( 3x10 5 x M earth ) ( 3x10 5 x M earth ) Rotation period:Rotation period: 25 days(equator) 25 days(equator) 30 days (poles) 30 days (poles)Composition: 70% Hydrogen 70% Hydrogen 28% Helium 28% Helium

Stars have different colors B: blue – hottest A: green – warm C: red - cool Stars have different colors Infer temperature of a star from the peak wavelength of its black body radiation

May 2006April 2004Belinda Wilkes Color, Brightness + Count them Sun

Solar fusion processes MeV MeV MeV

Neutrinos come directly from solar core

Superkamiokande

Sun as seen by a neutrino detector

What happens when the Sun’s Hydrogen is all used up?

Evolution of a Star (Sun) Red Giant

Main Sequence Evolution Core starts with same fraction of hydrogen as whole star Fusion changes H  He Core gradually shrinks and Sun gets hotter and more luminous

Evolution of the Sun Fusion changes H  He Core depletes of H Eventually there is not enough H to maintain energy generation in the core Core starts to collapse

The Sun will become a Red Giant

The Sun 5 Billion years from now Earth

The Sun Engulfs the Inner Planets

Red Giant Phase He core –No nuclear fusion –Gravitational contraction produces energy H layer –Nuclear fusion Envelope –Expands because of increased energy production –Cools because of increased surface area

Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon Helium fusion

Helium Flash He core –Eventually the core gets hot enough to fuse Helium into Carbon. –This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. –We don’t see this flash because it’s buried inside the Sun. H layer Envelope

Red Giant after Helium Ignition He burning core –Fusion burns He into C, O He rich core –No fusion H burning shell –Fusion burns H into He Envelope –Expands because of increased energy production

What happens when the star’s core runs out of helium? – The star explodes – Carbon fusion begins – The core starts cooling off – Helium fuses in a shell around the core

Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate

Sun looses mass via winds Creates a “planetary nebula” Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen  a “white dwarf star”

Planetary nebula

Hourglass nebula

White dwarf Star burns up rest of hydrogen Nothing remains but degenerate core of Oxygen and Carbon “White dwarf” cools No energy from fusion, no energy from gravitational contraction White dwarf slowly fades away…

Time line for Sun’s evolution

Sirius Comet Hale- Bop Orion Constellation ( Nebula) Brightest Star – Sirius A – (Sirius B is a white dwarf) Sirius B Betelgeuse (Red Giant)

1.This is a Hubble Space Telescope image - the first direct picture of the surface of a star other than the Sun. Hubble Space Telescope imagepicture 2. While Betelgeuse is cooler than the Sun, it is more massive and over 1000 times larger. If placed at the center of our Solar System, it would extend past the orbit of Jupiter.Betelgeusecooler than the Sunour Solar System 3.Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familiar constellation of Orion, the Hunter.Betelgeusebrightest starsOrion, the Hunter 4.The name Betelgeuse is Arabic in origin. As a massive red supergiant, it is nearing the end of its life and will soon become a supernova. In this historic image, a bright hotspot is revealed on the star's surface.The namesupernovaa bright hotspot Betelgeuse is a red supergiant star about 600 light years distant

The Sun Engulfs the Inner Planets

The Sun becomes a White Dwarf Composition: Carbon & Oxygen

What about M>1.4 M ๏ stars?

Nuclear burning continues past Helium 1. Hydrogen burning: 10 Myr 2. Helium burning: 1 Myr 3. Carbon burning: 1000 years 4. Neon burning: ~10 years 5. Oxygen burning: ~1 year 6. Silicon burning: ~1 day Finally builds up an inert Iron core

Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells

Fusion stops at Iron

Fusion versus Fission

Advanced reactions in stars make elements like Si, S, Ca, Fe

Atomic collapse  Supernova Explosion Core pressure goes away because atoms collapse: electrons combine with protons, making neutrons and neutrinos Neutrons collapse to the center, forming a neutron star

Atomic Collapse White Dwarfs ~1 ton/cm 3 Ordinary matter ~few grams/cm 3 Neutron star ~10 8 ton/cm 3

Core collapse Iron core grows until it is too heavy to support itself Atoms in the core collapse, density increases, normal iron nuclei are converted into neutrons with the emission of neutrinos Core collapse stops, neutron star is formed Rest of the star collapses in on the core, but bounces off the new neutron star (also pushed outwards by the neutrinos)

Supernova explosion

SN1987A Tarantula Nebula in LMC Neutrinos are detected Feb 22, 1987 Feb 23, 1987

Previously observed Supernova “Kepler’s Supernova” Oct 8, 1604 Kepler’s Supernova today Chosun Silok

Light curve from Kepler’s Supernova

Where do the elements in your body come from? Solar mass star produce elements up to Carbon and Oxygen – these are ejected into planetary nebula and then recycled into new stars and planets Supernova produce all of the heavier elements – Elements up to Iron can be produced by fusion – Elements heavier than Iron are produced by the neutrons and neutrinos interacting with nuclei during the supernova explosion

How do high-mass stars make the elements necessary for life?

Advanced Nuclear Burning Core temperatures in stars with >8M Sun allow fusion of elements as heavy as iron

May 2006April 2004Belinda Wilkes We are made of stardust! We are made of stardust

What about M>8 M ๏ stars?

Gravity deforms space-time Light follows curved paths

Gravity bends the path of light

Curved Space Einstein related gravity forces to space curvature. Black holes deeply warp space. Everything falls in, nothing can climb out. How does this work?

The Event Horizon Event Horizon = black hole “surface” ObjectMassRadius Earth6 x kg1 cm Jupiter300 x Earth3 m Sun300,000 x Earth 3 km

M earth = 6x10 24 kg R=6400km Normal density If the Earth was the density of a white dwarf R≈10km If the Earth was the density of a neutron star R≈2.5m If the Earth was Compressed into A Black Hole R horiz ≈1cm

A nonrotating black hole has only a “center” and a “surface” The black hole is surrounded by an event horizon which is the sphere from which light cannot escape The distance between the black hole and its event horizon is the Schwarzschild radius (R Sch = 2GM/c 2 ) The center of the black hole is a point of infinite density and zero volume, called a singularity

Black Holes Light is bent by the gravity of a black hole. The event horizon is the boundary inside which light is bent into the black hole. Approaching the event horizon time slows down relative to distant observers. Time stops at the event horizon.

Binaries Gravitational tides pull matter off big low density objects towards small high density objects. Cygnus X-1

“Seeing” Black Holes

The First “First” Black Hole Cygnus X-1 binary system Most likely mass is 16 (+/- 5) M o Mass determined by Doppler shift measurements of optical lines

Galaxy M84 core = “Super-massive”Black Hole? Gas, stars moving toward us Gas, stars moving away from us Space Telescope Imaging Spectrograph spectrogram Image of M84 Area STIS observes Gas, stars moving across Spectrogram of gas and stars moving around the core The core of Galaxy M84 contains a total mass = 300 million x M ๏ in R<26 cyr!