the stars formation of a star created september 2015 by joshua toebbe

The StarsFormation of a star

Created September 2015 by Joshua Toebbe

Learning Goals

The Stars: Formation of a star

• How are stars born?• How do stars make energy?• How do stars maintain stability?• What evidence do we have?


How are stars born?



How are stars born?


• Space is not exactly empty• It consists of a thin dispersion of gases among other things• These gases slowly clump together through gravitational forces• Creating giant molecular clouds• These are what give birth to stars



How are stars born?

StarsDiameter: 2 X 10 5 to 2.5 X 10 10 kmMass: 10 28 – 10 32 kgDensity: 1 – 100,000 g/cm3

Temp: 4,000 – 40,000 k

Giant molecular clouds• Diameter: 10,100 X 10 10 km• Mass: 10 150 kg• Density: 1 X 10-20 g/cm3

• Temp: 5 - 10 K


How are stars born?


Molecular clouds can not start forming stars spontaneously, four things stand in the way.1. Thermal Energy

Even at only 10 K a hydrogen atom can move at 800 mph.2. The interstellar magnetic field

Ions can not move freely through a magnetic field.3. Rotation

As a cloud contracts its increasing rotation can resist further contraction.4. Turbulence

Strong currents in the cloud can cause molecules to resist gravitational collapse.


How are stars born?


For a molecular cloud to begin star formation something else must occur.• The first trigger is a shockwave.

These can be produced by supernovae.


How are stars born?


• Large stars can also ionize nearby gas and blow it outwards creating a pseudo shock wave.• Two clouds could collide causing partial collapse.• Or a cloud could get compressed as it passes through one of the galaxies

spiral arms.

• In any case, the disturbance allows gravity to overcome the other barriers to collapse.


How are stars born?


• Even after the molecular cloud begins to collapse it must still increase its temperature by thousands of kelvin before it is hot enough to become a star.• As the star starts to draw material to its center you can consider the

molecules to be falling.• As they fall they gain speed.• This is known as Free fall contraction.• As the particles approach the center the rate of collision increase

converting the gravitational energy into thermal energy.


How are stars born?


• The cloud continues to collapse from the inside out developing a protostar at its center.• A protostar is a relatively loose term but generally refers to a pre-stellar

object that is hot enough to radiate infrared radiation, but not hot enough to sustain nuclear fusion.• The outer layer of the cloud that has yet to collapse remains dark and cold,

and are referred to as cocoons, which hide the protostar. (some astronomers think the Oort Cloud is the remant of the Suns cocoon)• The cocoon absorbs all of the energy radiated by the star and re-emits it as

infrared radiation.


How are stars born?


• As the contraction continues the cloud slowly flattens into a Protostellar disk. (much like the one astronomers think earth formed from)• As the particles in the disk collide they lose angular momentum and sink

into the protostar.• When the protostar becomes hot enough it generates a stellar wind.• In addition, photons can exert radiation pressure when they interact with

other particles.• The stellar wind and radiation pressure combine to blow away the

protostars cocoon making it visible.


What evidence do we have?



Evidence for star formation?


1. T Tauri stars are very young, and still in the process of contracting (as confirmed by observations). These stars can be seen all over the night sky.

2. Visual and infrared observations reveal bok globules which are small dusty clouds.

3. New born stars have just the right temperature and luminosity that would be expected according to modern theories.

4. Jets of gas and waves of ionized gas can be seen compressing molecular clouds, producing star formation pillars.


Evidence for star formation?


• The entire process of star formation can almost be seen in the Orion nebula.• The visible nebula is illuminated by very bright and hot stars but it is only a

portion of a larger molecular cloud.• The nebula is expanding ionized hydrogen which is compressing the nearby

molecular cloud.• Hidden within the molecular cloud are several protostars visible with

infrared telescopes.• Many of the stars in the nebula are still surrounded by a protostellar disk.


How do stars make energy?





• Many people mistakenly think that stars burn. However, stars cannot burn as there is nowhere near enough oxygen in space.• When something is burned energy is released by the breaking and forming

of chemical bonds. Chemical bonds are a result of electron configuration, so we can say energy is a result of the electromagnetic force.• The sun generates energy by breaking and forming bonds inside the atomic

nuclei. Therefore the energy is generated as a result of the strong and weak nuclear forces which hold the nuclei together.• Stars generate energy when their cores become hot enough to generate

and sustain nuclear fussion.




• Nuclear fusion generates energy by releasing bonding energy. • For example (4) hydrogen nuclei fuse to create (1) helium nuclei.

• Helium nuclei are 0.7 percent lighter than four hydrogen nuclei.

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• This extra mass doesn’t just disappear.




• Instead it is released as energy.• That’s right mass is destroyed and energy is created, it all follows one

simple rule that you may have heard of:

• Based on the difference in mass the energy generated is:

• The sun converts 5 million tons of matter into energy every second, just to resist its own gravitational collapse.




• This is where it gets complicated…




• This process is known as the proton-proton chain.• Energy is emitted in the form of gamma rays, neutrinos, positrons and the

kinetic motion of the particles.• The positrons re-combine with free electrons and vanish producing a

gamma ray• Some of the neutrinos combine into photons, the rest race out of the star

near the speed of light and carry off about 2% of the energy produced in the reaction. The rest is re-used to help sustain fusion.




• Because protons carry a positive charge they repeal each other with an electrostatic force, which creates the Coulomb barrier.

• The temperature must be millions of kelvin before particle collisions are violent enough to penetrate the coulomb barrier and fuse together.




• This is where it gets complicated… Again…


12C13N

13C

14N 15O

15N

12C



• This process is known as the CNO cycle.• This process is highly temperature sensitive, requiring an environment

around 16 million kelvin.• Carbon has more protons than hydrogen which means it has a higher

positive charge.• Therefore carbon has a higher Coulomb barrier and thus requires more

violent collisions to penetrate that barrier.


How do stars maintain stability?





• According to the first law of thermodynamics, energy will flow from hot to cold regions (without work input of course).• The surface of the sun transfers energy into the cold dark of space just as

the core transmits energy to the surface of the star.• There are three ways this energy can be transmitted inside a star:1. Conduction2. Radiation3. Convection




• Conduction is the transfer of thermal energy from particle to particle.• As particles vibrate faster and faster they begin to bump into each other.• With each collision momentum is conserved and transferred from one

particle to another.• As the rate of collisions increase the neighboring particles begin to vibrate.• Each collision transfers energy from one particle to another.• Conduction only takes place in dense materials which only occurs in the

most peculiar of stars.




• Radiation is the transfer of energy through wave motion.• When electrons excited by thermal energy drop higher energy levels to

lower energy levels they must emit that energy as electromagnetic radiation in the form of photons (yay for quantum mechanics).• Photons are essentially energy packets.• Energy flow depends on how difficult it is for the photons to travel through

the gas.• The denser or more opaque the gas, the less radiation penetrates the gas.




• The radiation passes through the low density areas of the stars easily.• Due to the energy production at the core the gas is extremely hot and has

relatively low density.• As the energy being radiated reaches the cooler outer layers of the star it

has trouble penetrating the gas and energy builds up.• This energy begins to heat the gas causing it to expand and rise.• The cooler more dense gas at the surface begins to sink according to

Archimedes principle (the buoyant force on an object is equal to the fluid displaced by that object).• This produces convection currents which transfer energy through the

cycling of matter.Created September 2015 by Joshua Toebbe



• When combining this with our knowledge of nuclear fusion (proton-proton chain and CNO cycle).• Large stars using the CNO cycle generate more than 50% of their energy in

less than 2% of their mass.• This massive energy generation prevents radiation from draining the

energy away fast enough causing the core to churn like a convection oven.• Farther out the energy density becomes less allowing the star to radiate

energy outward.




• Stars generating most of the energy through the proton-proton chain generate energy in a much larger area.• The sun generates about 50% of its energy in around 11% of its mass.• This allows energy to radiate away from the core.• Closer to the surface the cooler more dense gas becomes opaque causing

convection to occur.




• But regardless of what order this takes place in the majority of the energy transfer happens through radiation.• Only a small region near the surface of medium stars or the core of large

stars undoes convection.• But low mass stars do not generate enough heat to decrease the density of

the gas and reduce its opaqueness, meaning the bulk of their mass undergoes convection, since radiation cannot occur.




• The relationship between pressure and temperature is very important.• If a star generates too much energy, the outward pressure exceeds gravity

causing the star to expand.• This expansion would lower the central temperature of the star reducing

energy production.• If the star does not generate enough energy gravitational compression

exceeds its outward pressure• The compression of the star raises the internal temperature increasing

energy production.• In this way a star can maintain its stability.


Sources

• Cover image underlay: from genkkis.deviantart.com

• Seeds, M. A. [2008] Foundations of Astronomy: Tenth Edition. Thomson, Brooks/cole.



http://genkkis.deviantart.com/art/Honorverse-Starmap-372007527

the stars formation of a star created september 2015 by joshua toebbe

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