Life story of a star Microworld Macroworld Lecture

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 l. R ๏ = 696, 000 km (109 x Rearth) l. M= 2 x 1030 kg ( 3 x 105 x Mearth) • Rotation period: 25 days(equator) 30 days (poles) Composition: 70% Hydrogen 28% Helium

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

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

Solar fusion processes + 1. 4 Me. V + 5. 5 Me. V + 12. 9 Me. V

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 Red Giant (Sun)

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 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

Planetary nebula

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

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

Betelgeuse is a red supergiant star about 600 light years distant 1. This is a Hubble Space Telescope image - the first direct picture of the surface of a star other than the Sun. 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. 3. Betelgeuse is also known as Alpha Orionis, one of the brightest stars in the familiar constellation of Orion, 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 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 Ordinary matter ~few grams/cm 3 White Dwarfs ~1 ton/cm 3 Neutron star ~108 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

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

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

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? http: //en. wikipedia. org/wiki/Triple-alpha_process http: //en. wikipedia. org/wiki/Neon_burning_process http: //en. wikipedia. org/wiki/Silicon_burning_process

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

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

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” Object Mass Radius Earth 6 x 1024 kg 1 cm Jupiter 300 x Earth 3 m Sun 300, 000 x Earth 3 km

Mearth = 6 x 1024 kg Normal density R=6400 km If the Earth was the density of a white dwarf If the Earth was the density of a neutron star R≈10 km R≈2. 5 m If the Earth was Compressed into A Black Hole Rhoriz≈1 cm

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 (RSch= 2 GM/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) Mo • Mass determined by Doppler shift measurements of optical lines

Galaxy M 84 core = “Super-massive”Black Hole? Gas, stars moving toward Gas, stars us Image of M 84 moving across Gas, stars moving away from us Area STIS observes Spectrogram of gas and stars moving around the core Space Telescope Imaging Spectrograph spectrogram The core of Galaxy M 84 contains a total mass = 300 million x M๏ in R<26 cyr!

http: //www. youtube. com/watch? v=dip. FMJck. ZOM&feature=player_embedded