life story of a star
DESCRIPTION
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 (109 x R earth ) M= 2x10 30 kg - PowerPoint PPT PresentationTRANSCRIPT
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
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
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”
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…
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
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
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
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)
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
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
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!