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Star Formation
Daniel ZajfmanDepartment of Particle PhysicsWeizmann Institute of Science
Stars
Planets
Galaxies
Black holes
Nebulae
Red giants
White dwarf
MoonsSupernovae
Pulsars
Neutron stars
Why so many different objects? Why all the stars are not alike?
Stars are not permanent objects: They are born, live and die,
just like human being
Big Bang Nucleosynthesis
Mainly Hydrogen, Deuterium and Helium
Star should “work” with these materials
Elements and IsotopesWe define an “element” by the number of protons in its nucleus.
There can be “isotopes” with different numbers of neutrons.
Big Bang
Particle Physics
Nuclear Physics
Matter-RadiationEquilibrium
Atomic & MolecularPhysics
H++e-H+hν“Recombination era”
Pre-galactic gas clouds
First generation of stars
1012 K
5x109 K
5x108 K
4x103 K
4He, D, 3He, 7Li
100 s
1000 s
106 years
106 years
Time scale Temperature
The Universe after the Big-Bang is “uniformly” filled with Hydrogen, Deuterium, and Helium
Small fluctuations (finite number of particles) create small lump of matter, which start to collapse under their own gravity
Formation of protogalaxies
Anatomy of an interstellar cloud
Collapse of molecular clouds:• Not in a single piece (clumps formation)• Clumps collapse to form stars• 10-1000 stars can be formed from one single cloud
Mostly in secondgeneration clouds
Horsehead Nebula
Barnard68
Eagle
Interstellar clouds are the nursery of stars.
Some clouds, called molecular clouds, containa minor (but important) fraction of molecularspecies.
RCW38
The beginning: The birth of a star
Cloud collapse
Method 1Build up of small clouds to giant onesClouds stick together and growGravitation takes overVery slow process (low interstellar density)
Method 2 Gravity and radiation pressure
Method 3
Compression by supernova blast waves
Not for first stars!
Two hindrances to collapse
Internal heating:Potential energy Kinetic energy (Gas particles speed up and collide)
Temperature increases Pressure build up which slows (or stops) the collapse
Energy is radiated away
Angular momentum L=mass x vel. of rotation x radius (L=mvr) Conservation of angular momentum: Constant for a closed system Thus, as the cloud shrinks due to gravity it spins fasters
Collapse occurs preferentially along path of least rotationThe cloud collapses into a central core surrounded by a disk
Gravity makes the cloud collapse!
Orion cloud~1000 ly
Proplyds
Protostarand
Proplyds
Planet formation???
Protostar
The process can be very “unstable”and often yields to the productionof “jets” for about 100,000 years
Protostars and jets
Protostar formationThe central core is called a protostar• Surface ~ 300 K, the internal temperature is steadily increasing• Undergoing continuous gravitational contraction • Self-compression heats the central core
Nuclear Fusion reaction starts A star is born
Planets are probably formed later in the remaining disk of the protostar
A more detailed look at the collapse process allows to extractthe critical mass of a cloud so that a star can be formed
Sir James Jeans: the critical mass, called today the Jeans mass
Can we estimate it easily??
RLet’s assume we compress the gas slightly.It will bounce back to its original size in a time
soundsound R/ct
At the same time, the gravity will attempt to contractThe system, and will do that in a “fall-free” time
Gρ1/tff G is the universal gravitational constantρ is the gas density
If we want Gravity to win, we need:
soundff tt
1/23/23sj ρG/cM Jeans Mass
Star formation – The movie
Gas
clo
udY
oung
sta
rs
100 million years
The size of the cloud changes from million of km down to few thousands of km. The temperature increases from -270 oC to million of degrees. At this temperature, the nuclear fuel (hydrogen) is “light up”.
Light is emitted, and the star starts its life: on one hand, gravitational force pushes inward, while on the other hand internal pressure due to the nuclear reactions pushes outward.
Thermonuclear Fusion
In order to get fusion, one must overcome the electric repulsion.You can do this by having high density (lots of particles) and high temperature (particles moving very quickly).
For Stars, size matters
A star mass determines which fusion reaction are possible in the core, and hence itsluminosity, surface temperature and lifetime.
Object with mass smaller than 8% of the solar mass (75 times Jupiter mass) neverignite fusion, and therefore fade to obscurity in about 100 million years .These are Brown Dwarf.
Sun mass: 2 x 1030 kgJupiter Mass: 2x1027 kg
First ever observed brown dwarf in October 1994
How many brown dwarfin the Universe?
The sun: A typical star
Age: ~ 4-5 billion
years old
The Power of the Star:The Proton-Proton Cycle
This is the primary source of energy for main sequence stars
Minimum temperature: 5 millions K
In this reaction cycle, 4 protons are transformed in one He nuclei, 2 positrons, gamma rays and
2 neutrinos
Another view of the proton-proton cycle
Each reaction cycle requires 4 hydrogen (protons) and yields about 25 MeV of energy
The proton-proton cycle is the most important reaction in the sun.Is there enough Hydrogen?
Let’s estimate the lifetime of the sun
In the p-p cycle, each time 4 protons react, and produce one 4He nuclei
4p 4He + 2e+mp=1.67x10-27 kgmHe=6.6326x10-27 kgme+=9.1139x10-31 kg 4mp=6.68x10-27 kg
mHe+2me+=6.6344x10-27 kg
Mass difference: ∆m=4.56x10-29 kg
Where did this mass goes?? E=∆mc2!!
How much energy is thus produced in one p-p cycle?
E=∆mc2 = 4.56x10-29 kg x (3x108)2 (m/s)2 = 4.1 x 10-12 Joule
That’s by the way 25 MeV!
We know that the total power output of the sun is: L=3.9 x1026 Joule/second (eq~ 100 billion nuclear bomb/second).
Lifetime of the sun (cont.)
Thus, the number of p-p cycle per second in the sun is:
Total power/energy per cycle=L/E=3.9x1026/4.1x10-12=9.5x1037 reactions/second
Since each p-p cycle requires 4 protons, the number of protons usedevery second in the sun is :
np=4x9.5x1037 =3.8x1038 protons/second
How many protons are in the sun?
#protons~ mass of sun/mass of protons = 2x1030 kg/1.67x10-27 kg ~ 1x1057 protons
Thus, the lifetime of the sun is approximately: 1x1057/3.8x1038=2x1018 seconds
which are about 60 milliard years. However, the sun uses only 10% of its hydrogen…so lifetime is of the order of (very roughly) 6 milliard years
The CNO Fusion CycleFor more massive stars
(higher temperature)
In this cycle, 4 protons are converted into 1 Helium, 2 positrons, gamma rays and 2 neutrinos
Why more massive stars?Because of the electrostatic repulsionof the Carbon nuclei
In the sun, this produce only 2% of thetotal energy!
The triple alpha process
Three Helium nuclei are converted into a carbon nucleus and gamma rays
For star leaving the main sequence (called Red Giants)
Nucleosynthesis!
Comparison of the p-p and CNO cycle
Usually the CNO cycle is more important for heavier stars, as it is hotter inside
The lifetime of a star depends (mainly) on its mass
Higher the mass, shorter the lifetime!
High mass: M > 8Msun
Intermediate mass 2Msun< M < 8Msun
Low mass: M<Msun
Convection between core and surface
Convection only in the core
Can we prove (experimentally) that all that is correct?
The Solar Neutrino (ex)-ProblemIf the sun is really powered by nuclear (fusion, p-p cycle) power, then it has to produce some special particles called neutrinos.
“The” solar neutrino problem:
"the sun does not produce enough neutrinos"
These particles have almost no interactions with matter, get out of the sun core, and canbe detected by terrestrial neutrino detectors.
The 37Cl neutrino detector is a tankcontaining 375,000 liters of Perchloroethylene in a cavity 1,500 m below ground
When a neutrino (with the right energy)collides with a 37Cl atom, it producesan atom of 37Ar (and an electron) whichis radioactive, and can be detectedlater.
Ray Davis, 1966
Nobel prize in 2002
First experiment in Homestake mine
Only ~ 1/3 of the expected Neutrinowere measured
The super Kamiokande detector
Detecting neutrino coming fromthe center of the sun.
Produce the first evidence (1998) thatsomething was “wrong” with the neutrino physics
The final word
It is the physics of neutrino which was “wrong”
Neutrino oscillation
The solar fusion theory is correct
Neutrino have masses
(Particle Astrophysics is a very rich and exciting field of Physics)
The Hertzsprung-Russel (HR) Diagram
Star Characterization
Reversed scale!!!
When one plot the data of a group of star (for example close to us)This is what we see on the HR diagram
What is the “Main Sequence??”
The HR diagram
Temperature, Size and Luminosity
Hotter objects are brighter Energy radiated per unit of time and unit of area is proportional to T4
Thus, larger Temperature means more energy radiated
Bigger objects are brighter Energy radiated per unit of time and unit of area is proportional to T4
Thus larger surface means more energy radiated
42σTrπ 4L
Let’s assume all stars are the size of theSun, but the hotter ones are more luminous, just because they are hotter
Then all the stars would fall on the blueline
In math-language it means:
Surface Stefan-Boltzman Law
In reality: Not really true!
But we learned something: The coolest main sequence stars are a lot smaller than the sun.
The hottest main sequence stars are a lot bigger than the sun.
The Hertzsprung-Russel (HR) Diagram
Spectral classes instead of temperature
Our sun is spectral class G
In general, the HR diagram allows to categorize the different stars using “measureable” parameters. Different type of stars are located in differentregion of this diagram.
M5 cluster with more data points and a calculated isochrone line
The line represent thecalculated “behavior” ofa star in the H-R diagramassuming all stars have thesame age (but were bornwith different initial size)
The Best Physics we know todayis in good agreement with observations
Stellar Lifetimes
Next episode:
•Stellar evolution•Nucleosynthesis•Binary systems•Final stages•Supernovae•Black Holes•Quasars•Pulsars•Interstellar medium