Life story of a starMicro-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 SunR๏ = 696,000 km (109 x Rearth) M= 2x1030kg ( 3x105 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

Stars have different colors

Infer temperature of a starfrom the peak wavelengthof its black body radiation

May 2006April 2004 Belinda Wilkes

Color, Brightness + Count them

Sun

Solar fusion

processes

+ 5.5 MeV

+ 1.4 MeV

+ 12.9 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

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

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.

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.

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 Myr2. Helium burning: 1 Myr3. Carbon burning: 1000 years4. Neon burning: ~10 years5. Oxygen burning: ~1 year6. 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/cm3

Ordinary matter ~few grams/cm3

Neutron star~108 ton/cm3

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

SN1987ATarantula 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?

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 >8MSun allow fusion of elements as heavy as iron

May 2006April 2004 Belinda 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”Object Mass Radius

Earth 6 x 1024 kg

1 cm

Jupiter 300 x Earth

3 m

Sun 300,000 x Earth

3 km

Mearth = 6x1024 kg

R=6400km

Normal density

If the Earth wasthe density of a

white dwarf

R≈10km

If the Earth wasthe density of a

neutron star

R≈2.5m

If the Earth wasCompressed into

A Black Hole

Rhoriz≈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 (RSch= 2GM/c2)

• 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 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!

http://www.youtube.com/watch?v=dipFMJckZOM