Download Protostar formation

Star Formation
Daniel Zajfman
Department of Particle Physics
Weizmann Institute of Science
Why so many different objects?
Why all the stars are not alike?
Pulsars
Stars
Planets
Red giants
White dwarf
Galaxies
Black holes
Nebulae
Supernovae
Moons
Neutron stars
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 Isotopes
We define an “element” by the number of protons in its nucleus.
There can be “isotopes” with different numbers of neutrons.
Time scale
Particle
Physics
100 s
Nuclear Physics
1000 s
106 years
106 years
Big Bang
Matter-Radiation
Equilibrium
Atomic & Molecular
Physics
H++e-H+hν
“Recombination era”
Pre-galactic gas clouds
First generation of stars
Temperature
1012 K
5x109 K
4He,
D, 3He, 7Li
5x108 K
4x103 K
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
Mostly in second
generation clouds
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
Interstellar clouds are the nursery of stars.
Some clouds, called molecular clouds, contain
a minor (but important) fraction of molecular
species.
Barnard68
Eagle
RCW38
Horsehead Nebula
The beginning: The birth of a star
Cloud collapse
Method 1
Method 2
Build up of small clouds to giant ones
Clouds stick together and grow
Gravitation takes over
Very slow process (low interstellar density)
Gravity and radiation pressure
Compression by supernova blast waves
Method 3
Not for
first stars!
Gravity makes the cloud collapse!
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 rotation
The cloud collapses into a central core surrounded by a disk
Orion cloud
~1000 ly
Protostar
and
Proplyds
Proplyds
Protostar
Planet formation???
The process can be very “unstable”
and often yields to the production
of “jets” for about 100,000 years
Protostars and jets
Protostar formation
The 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 extract
the 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??
R
Let’s assume we compress the gas slightly.
It will bounce back to its original size in a time
tsound  R/csound
At the same time, the gravity will attempt to contract
The system, and will do that in a “fall-free” time
tff  1/ Gρ
G is the universal gravitational constant
ρ is the gas density
If we want Gravity to win, we need:
tff  tsound
Jeans Mass

Mj  cs3/ G3/2ρ1/2

Star formation – The movie
Gas cloud
Young stars
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 its
luminosity, surface temperature and lifetime.
Object with mass smaller than 8% of the solar mass (75 times Jupiter mass) never
ignite fusion, and therefore fade to obscurity in about 100 million years.
These are Brown Dwarf.
Sun mass: 2 x 1030 kg
Jupiter Mass: 2x1027 kg
First ever observed brown dwarf
in October 1994
How many brown dwarf
in the Universe?
Age: ~ 4-5 billion
years old
The sun: A typical star
The Power of the Star:
The Proton-Proton Cycle
This is the primary source of energy for main sequence stars
In this reaction cycle, 4 protons are transformed
in one He nuclei, 2 positrons, gamma rays and
2 neutrinos
Minimum temperature: 5 millions K
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
mp=1.67x10-27 kg
mHe=6.6326x10-27 kg
me+=9.1139x10-31 kg
4p  4He + 2e+
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!
Lifetime of the sun (cont.)
We know that the total power output of the sun is: L=3.9 x1026 Joule/second
(eq~ 100 billion nuclear bomb/second).
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 used
every 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
For more massive stars
(higher temperature)
The CNO Fusion Cycle
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 repulsion
of the Carbon nuclei
In the sun, this produce only 2% of the
total energy!
For star leaving the main sequence (called Red Giants)
The triple alpha process
Three Helium nuclei are converted into a carbon nucleus
and gamma rays
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
High mass: M > 8Msun
Intermediate mass 2Msun< M < 8Msun
Low mass: M<Msun
Convection only in the core
Convection between
core and surface
Higher the mass, shorter the lifetime!
Can we prove (experimentally) that all that is correct?
The Solar Neutrino (ex)-Problem
If the sun is really powered by nuclear (fusion, p-p cycle) power, then it
has to produce some special particles called neutrinos.
These particles have almost no
interactions with matter, get out
of the sun core, and can
be detected by terrestrial neutrino
detectors.
“The” solar neutrino problem:
"the sun does not produce enough neutrinos"
First experiment in Homestake mine
Ray Davis, 1966
Nobel prize in 2002
The 37Cl neutrino detector is a tank
containing 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 produces
an atom of 37Ar (and an electron) which
is radioactive, and can be detected
later.
Only ~ 1/3 of the expected Neutrino
were measured
The super Kamiokande detector
Detecting neutrino coming from
the center of the sun.
Produce the first evidence (1998) that
something was “wrong” with the
neutrino physics
The final word
The solar fusion theory is correct
It is the physics of neutrino which was “wrong”
Neutrino have masses
Neutrino oscillation
(Particle Astrophysics is a very rich and exciting field of Physics)
Star Characterization
The Hertzsprung-Russel (HR) Diagram
Reversed scale!!!
The HR diagram
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”??
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
In math-language it means:
L  4 π r2σT 4
Surface
Stefan-Boltzman Law
Let’s assume all stars are the size of the
Sun, but the hotter ones are more
luminous, just because they are hotter
Then all the stars would fall on the blue
line
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 different
region of this diagram.
M5 cluster with more data points and a calculated isochrone line
The line represent the
calculated “behavior” of
a star in the H-R diagram
assuming all stars have the
same age (but were born
with different initial size)
The Best Physics we know today
is in good agreement with
observations
Stellar Lifetimes
Next episode:
•Stellar evolution
•Nucleosynthesis
•Binary systems
•Final stages
•Supernovae
•Black Holes
•Quasars
•Pulsars
•Interstellar medium