Transcript Oct5x

Astro 201: Oct. 5, 2010
• Today:
– The Evolution of the Sun
– The Evolution of Massive Stars
– Origin of the Elements
– White Dwarfs, Neutron Stars, Black Holes
Illustrations from Prof. Terry Herter's web site and Prof. Richard Pogge's web site
1. Protostar Phase (50 million years)
*Gas cloud undergoes Gravitational Contraction until fusion starts
* The Sun took about 50 million years to reach the main sequence
* During the collapse phase it was brighter and cooler than it is on
the Main Sequence
* The paths of protostars in the H-R diagram are called the Hayashi Tracks
2. MAIN SEQUENCE (10-11 billion years)
The Sun reached the Main Sequence about 4.5 billion years ago
At that time, it was fainter-- 0.70 x the luminosity of today's Sun
it was a little smaller -- 0.897 x the radius of today's Sun
it was a little cooler -- 5586 K
On the Main Sequence, the Sun burns hydrogen to helium in the
proton-proton process, and gets progressively hotter and brighter.
Why?
Because the pressure
P=nkT
where n= the number of atoms/volume,
k=Boltzman's constant
T=temperature.
As 4 protons -> 1 helium, n decreases
so T increases in order to keep the pressure P high enough to
counteract gravitational collapse
1.1 billion years from now, the Sun will be 10% brighter,
and there will be a significant greenhouse effect on the Earth
Independent of “Global Warming”
We can measure the CO2 concentration in the atmosphere as a function of time,
using bubbles trapped in ice layers in Antartica. (Last 50 years, direct measure).
A steady increase in CO2 began in the mid-1800's, the result of increased
burning of fossil fuels, associated with the growth of industry and urban
populations.
The increased CO2 causes an enhanced "greenhouse effect" and hence
warming of the average temperature on Earth.
Greenhouse Effect: Global warming from increased "greenhouse gas"
production, particularly in the U.S.
Greenhouse Gas:
CO2, methane (from burning of coal, natural gas, and oil; livestock);
nitrous oxide; hydroflurocarbons.
3.5 billion years from now, the Sun will be 40% brighter
there will be a runaway greenhouse effect on the Earth:
it will be like Venus.
On the surface of Venus, atmospheric pressure = 90x Earth’s
Average temperature = 737K or 900 F
hot enough to melt lead
3. End of the Main Sequence:
11 billion years after the Sun first reached the Main Sequence,
the Sun turns into a RED GIANT STAR
Hydrogen is all converted into helium in the core
Helium core begins to collapse, since energy is no
longer being produced to counteract the collapse by
gravity
Hydrogen fusion to helium still occurs in a shell
around the core
Star rearranges itself, eventually becoming a RED
GIANT
Outer layers of the Sun expand, star becomes Larger,
surface temperature is cool (red) , very luminous
For the Sun, this process will take about 1 billion
years
At the top of the Red Giant Branch in the HR diagram, the Sun will be
T=3107 K (M0 III)
Luminosity=2350 x the luminosity of the current Sun
Radius = 166 x Sun's current radius, engulfing Mercury
During this time, the outer layers of the Sun escape in a stellar wind.
The Sun will lose 28% of its mass.
4. The Helium Flash
* When the core of the star gets hot enough, a new fusion process occurs:
the TRIPLE ALPHA REACTION
Alpha = alpha particle = helium nucleus
* Triple Alpha: 3 helium --> 1 Carbon + energy
* The fusion of helium into carbon causes an enormous production of
energy in a few seconds.
* Again, the star rearranges itself to be hotter and smaller –
it becomes a so-called Horizontal branch star
5. The Horizontal Branch -- 100 Million yrs
* The Sun burns Helium into carbon and oxygen in its core as a
horizontal branch star for about 100 million years
* It still is burning Hydrogen to helium in a shell around the core
* At this point the Sun will be about R=18 x solar radius today,
T=4450 K, L=110 x luminosity of the Sun today
6. The ASYMPTOTIC GIANT BRANCH
* Eventually, the helium in the core is used up.
The core is now Carbon and Oxygen and it begins to collapse.
* There are shells of Helium burning and hydrogen burning still
* Again, the Sun rearranges itself and becomes a RED GIANT Again
* During this phase, what's left of the outer layers of the star are blowing
off in a wind, until the Sun's mass is about 0.6 x what it is today
* At the top of the Asymptotic Giant Branch, the star starts to pulsate,
and is very unstable
7. Planetary Nebula Phase
* Finally, the outer parts of the star are ejected.
The core is extremely hot and dense, and lights up the ejected
material in a "Planetary Nebula”
* Short-lived phase
The ejection of the PNE takes 100,000 years,
but then the planetary shines only 10,000 years
* Despite the short lifetime of Planetary Nebulae,
the stars which end up as PNE are common:
in the Milky Way today, there are about 10,000 planetaries
* "Planetary" nebulae have nothing to do with planets:
This is a misnomer from the days of small telescopes when the
images were small, fuzzy blobs
Images of Planetary Nebulae
The Hourglass Nebula
8. White Dwarf Phase
* The
core collapses until ELECTRON DEGENERACY PRESSURE
stops the gravitational collapse
When the pressures are very high,
electrons are squished together
and resist further collapse
Result of the Pauli Exclusion Principle
* At this point, the star is a "White Dwarf" and slowly
cools for the rest of time
* Mass is about 0.5x solar mass today,
but 200,000x more dense than Earth
* The Sun will then be about the same size as the Earth
* A teaspoon of electron degenerate material would weigh 5 tons
White Dwarfs in Globular Cluster M4 (= 100 watt light bulb at distance of the Moon)
SUMMARY:
The LIFE STORY of the Sun
1. Collapsing Protostar: 50 million years
2. 1 Msun Main-Sequence Star: 11 billion years
3. Red Giant Branch Ascent: 1 billion years
4. Helium Flash: a few seconds
5. Horizontal Branch: 100 million years
6. Asymptotic Giant Branch Ascent: 20 million years
7. Thermal Pulse Phase: 400,000 yr
8. Envelope Ejection: < 100,000 yr
9. Planetary Nebula: 10,000 years
10. 0.5 Msun White Dwarf: …
PROTOSTAR, MAIN SEQUENCE
Phases of massive stars are similar to the Sun,
just massive stars evolve faster and are much brighter
A star with 20 solar masses spends 8 million years on the
Main sequence,
and 1 million years as a red giant,
before blowing up as a supernova
GIANT/SUPERGIANT phase
stars with mass > 4 solar masses become so hot in their cores that
HELIUM CAPTURE and the CNO cycle occur.
CNO cycle:
Final Result: Onion Skin Layers of heavy elements in CORE
These stages are fast.
For example, for a 25 Msun star:
* Hydrogen fusion lasts 7 million years
* Helium fusion lasts 500,000 years
* Carbon fusion lasts 600 years
* Neon fusion lasts 1 year
* Oxygen fusion lasts 6 months
* Silicon fusion lasts 1 day
The star's core is now pure iron.
.
SUPERNOVA
* The star hits the IRON wall:
Iron is a very stable element,
and cannot fuse to form heavier elements.
* So when the core becomes IRON,
fusion no longer produces enough energy
to stop gravitational collapse
* The core collapses, until neutron degeneracy pressure
stops the collapse of the core.
* The outer parts of the star hit the core and
bounce off --> a supernova!
* What's left is a NEUTRON STAR
(if the mass is less than about 8 solar masses)
or a BLACK HOLE
Within a massive, evolved star
(a) the onion-layered shells of elements undergo fusion, forming an iron core
(b) And starts to collapse. The inner part of the core is compressed into neutrons
(c), causing infalling material to bounce
(d) and form an outward-propagating shock front (red). The shock starts to stall
(e), but it is re-invigorated by a process that may include neutrino interaction.
The surrounding material is blasted away
(f), leaving only a degenerate remnant.
Historic Supernovae:
* Supernovae become extremely bright.
* Supernovae in our Milky Way can become bright
enough to see during the day.
* Supernovae in distant galaxies are of intense interest
now for cosmology
* Famous Historic Supernova:
1054, recorded by Chinese and Native Americans,
today is the Crab Supernova remnant
1006: Southern hemisphere supernova
1572: Tycho Brahe's supernova
1604: Kepler's supernova
Since 1604, there have been no supernova explosions in the
Milky Way -- we're overdue!
In 1987, a supernova in the Large Magellanic Cloud, SN1987A
Two neutrino
experiments
operating at that
time detected
neutrinos from the
explosion
Before and after
picture
IMAGES OF SUPERNOVA REMNANTS
Crab Supernova Remnant, optical
Crab Supernova Remnant in X-rays (Hot, million degree gas)
Tycho’s Supernova Remnant
Kepler’s Supernova Remnant
Origin of the Elements
* All the carbon, oxygen, etc on the Earth, (and in humans) was
produced in the centers of stars.
* Most carbon, oxygen comes from low-mass red giant winds
* Most of the heavy elements come from supernovae
* New stars form out of interstellar gas which has been enriched
with elements by red giant winds, planetary nebulae and
supernovae.
* Older stars on the main sequence have relatively fewer
atoms of iron than younger stars, since they were formed out
of gas which had not been polluted by as many generations of stars
* We've searched pretty hard, but have never found,
pure hydrogen and helium stars.
Radioactive Dating: How we know
The age of the Earth & Solar System
or: “Clocks in Rocks”
• Some isotopes of atoms are unstable and undergo
radioactive decay, splitting into 2 or more “daughter”
atoms
• Element: determined by # of protons
• Isotope: determined by # of protons and # of
neutrons
• e.g. 87Sr, 90Sr and 86Sr are isotopes of Sr, or
strontium,
• all have 38 protons, but different number of
neutrons
• Unstable radioactive isotopes of elements, such as
Uranium-235, decay at constant, known rates over
time (its half-life, which is over 700 million years).
• When a molten rock cools, radioactive isotopes and
their daughters get frozen in the rock.
• For example, when lava cools, it has no lead content
but it does contain some radioactive Uranium (U-235).
Over time, the unstable radioactive Uranium decays
into its daughter, Lead-207, at a constant, known rate
(its half-life). By comparing the relative proportion of
Uranium-235 and Lead-207, the age of the igneous
rock can be determined.
HALF-LIFE
Oldest Rocks on Earth
Meteorites (found in Antartica)
Oldest Moon rocks
4.5 billion years old
White Dwarfs
Novae, Type 1a Supernovae
Main Sequence Stars with M < 4 solar masses
end up as WHITE DWARFs
The collapse by gravity is halted by electron
degeneracy pressure
The degenerate core which becomes a white
dwarf is mostly carbon
More massive white dwarfs are SMALLER than
less massive white dwarfs
CHANDRASEKHAR limit: the mass of a white
dwarf cannot exceed 1.4 solar masses
Subrahmanyan Chandrasekhar
(1910 – 1995)
If the core is more massive
electron degeneracy cannot
withstand gravity
Collapses to a neutron star or
black hole
1983 Nobel prize in Physics
White Dwarfs are often in binary star systems,
and the companion star may dump mass onto
the white dwarf resulting in a
nova
 cataclysmic variable or
 a Type Ia supernova

Nova
Nova radiates 100,000 x
the luminosity of the
Sun, for a few weeks
Cataclysmic Variable:
accretion disk has bright hot spot in X-rays
Type Ia Supernova
• White dwarf gets so much material dumped
on it by a companion that it explodes
• Luminosity = 10 billion times the luminosity
of the Sun, for a few weeks
• The luminosity depends on how rapidly the
Supernova fades --> measure light curve and
get the distance to distant galaxies
Light Curve of Supernova:
Brightness as a function of time since explosion
NEUTRON STARS and PULSARS
• Main sequence stars more massive than 4 solar masses explode as
supernovae, leaving behind a neutron star or black hole.
• Neutron stars are held up by neutron degeneracy pressure,
after e + p --> n
• The core of neutron stars is made up of a superfluid, which flows with no
resistance
• The surface of a neutron star forms a crust of heavy nuclei which aren't
neutronated, e.g. iron nuclei
• A paper clip made of neutron degenerate material would weigh more than
Mt. Everest
• Most neutron stars are 10 km across, but weigh as much as the Sun
(300,000 Earth masses)
PULSARS
In 1967, Jocelyn Bell found a radio source which
was pulsing very regularly.
The lighthouse model for PULSARS