The birth of a starChapter 11

1. Where are the birth places of stars?

2. What are the main components of a protostar?

3. When and how a new is born?

4. What prevents a star from collapsing?

Questions to be addressed:

How does a star form?• A cloud of hydrogen gas began to gravitationally

collapse.• As more gas fell in, it’s potential energy was

converted into thermal energy.• Eventually the in-falling gas was hot enough to

ignite nuclear fusion in the core.• Gas that continued to fall in helped to establish

gravitational equilibrium with the pressure generated in the core.

How can collapse occur?

• No collapse if thermal pressure wins over gravity

• When clouds too cold, pressure insufficient to balance gravity: collapse

• During collapse (compression) temperature increases: gravitational energy converted into thermal energy

Molecular cloud

Molecular cloud

Cool molecular cloudsgravitationally collapseto form clusters of stars

Stars generatehelium, carbonand iron throughstellar nucleosynthesis

The hottest, mostmassive stars in thecluster supernova –heavier elements areformed in the explosion.

New (dirty) molecularclouds are leftbehind by thesupernova debris.

The Stellar Cycle

Proto-stellar disk crucial:It is where planets form

O

Stellar Evolution in a Nutshell

Mass controls the evolution of a star!

M < 8 MSun M > 8 MSun

Mcore < 3MSunMcore > 3MSun

A main sequence star is the one which is supported by hydrogen

fusion

From cloud to protostar: gravity is the key for the collapse

Initial cloud with some rotation

Cloud spins up as it collapse

A protostar

The structure of a protostar

Herbig-Haro objects

Dark band is theproto-stellar diskseen edge-on

From a protostar to a true star

• Gas is heated when it is compressed• The central part of a protostar is compressed the

most, and when the temperature there reaches 10 million K, hot enough to ignite hydrogen fusion, the collapse is halted by the heated generated by the nuclear reaction

• A new star is born, and its internal structure is stabilized, because the energy produced in the center matches the amount of radiation from the surface

A main-sequence star can hold its structure for a very long time. Why?

ThermalPressure

GravitationalContraction

41H --> 4He + energy ( E = mc2

)Two ways to do this fusion reaction:

In the Sun, about 500 million tons/sec are needed!

If M<1.1Mo: p-p chain

If M>1.1 Mo: CNO cycle

Energy output of p-p cycle depends mildly on T: 10% Dt 46% De50% of energy in 11% of mass

Energy output of CNO has steep dependence on T: 10% Dt 340% De50% of energy in 2% of mass

p-p cycle is a “direct way to fuse 4 H into 1 He

CNO cycle needs the help of C, N and O (catalysts)C, N and O simply assist the reaction, but do not partecipate

Final output is the same: 4 H fuse into 1 He

Balance happens thanks toflow (transport) of radiation

from center (hotter) to surface (colder)

• Conduction, radiation, convection• Opacity is key to efficiency of radiation

transport• p-p stars: radiative core, convective

envelope• CNO stars: convective core, radiative

envelope• Small stars (M<~0.4 Mo) all convective

Pressure and Temperature of a Gas

This balance between weight and pressure is called hydrostatic equilibrium.

The Sun's core, for example, has a temperature of about 16 million K.

How does a star hold itself?

Outward thermal pressure of coreis larger than inward gravitational pressure

Core expands

Expanding core cools

Nuclear fusion ratedrops dramatically

Outward thermal pressureof core drops (and becomessmaller than inward grav. pressure)

Core contracts

Contracting core heats up

Nuclear fusion raterises dramatically

The Stellar Thermostat

Why is there a Main Sequence?

• The Main Sequence is just a manifestation of the relationship between Mass and Luminosity: L ~ M3.5

• The more massive the star the larger its weight

• The larger the weight, the larger the pressure

• The larger the pressure, the higher the temperature

• The higher the temperature, the more energetic the nuclear reaction

• The more energetic the nuclear reactions, the more luminous the star

• Also, the more energetic the nuclear reactions, the faster the rate at which fusion occurs

• The faster the rate, the quicker the star burns its fuel, the shorter its life