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

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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