Star Formation

• Our Sun formed billions of years ago• We see evidence that star formation is a constant process

Star Formation

• Star formation starts with Dark Dust Clouds– Relatively Dense, but very cool– Cloud starts to contract under its own gravity– Contraction causes heating– Nuclear Reactions begin

• Many things oppose the gravitational contraction– Heat– Rotation– Magnetism

Stage 1: Interstellar Cloud

• DDC contracts– Very large (tens of parsecs)– Very massive (thousands of Sun masses)– Very cool (~10K)

• Internal cloud pressure initially balances against gravity

• Something happens for it to become unstable– Star explosion, formation

• Cloud begins to fragment

Stage 2: Collapsing Cloud

• The fragmented cloud is about 100x larger than our solar system

• It is still very cool because it radiates away its heat out into space

• The very center is the warmest part ~100 K

• Eventually the cloud will contract enough (becoming dense enough) where it will trap most of the heat generated

Stage 3: Fragmentation Ceases

• At this point the cloud is the size of our solar system

– The gas at the center is very dense

– Radiation cannot escape

– Temperature increases to ~10,000 K

– This region is called a Protostar

• Protostar is still contracting

– Internal pressure cannot yet counteract gravity

Stage 4: A Protostar

• Protostar’s temperature increases with contraction– Reaches ~1,000,000 K– (10,000,000 K needed for nuclear ignition)

• Has a luminosity 1000 times greater than our Sun– No nuclear reactions

– Very Large

– Releasing gravitational potential energy as it shrinks

– Can plot it on H-R diagram

Stage 4: A Protostar

• Protostar’s temperature increases with contraction– Reaches ~1,000,000 K– (10,000,000 K needed for nuclear ignition)

• Has a luminosity 1000 times greater than our Sun– No nuclear reactions

– Very Large

– Releasing gravitational potential energy as it shrinks

– Can plot it on H-R diagram

Stage 5: Protostar Evolution

• The Protostar is very hot – the gas is at high pressure

• But it still isn’t enough to counteract gravity

• The Protostar continues to shrink

• This is called the “T Tauri” phase

Stage 5: Protostar Evolution

• The Protostar shrinks to about 10X the size of the Sun

• Central Temp = 5,000,000 K

– Not enough for fusion

• Contraction slows

– Harder for gravity to compress hotter gas

Stages 1 – 5

Stage 6: Newborn Star

• 10 million years have passed

• The Protostar has shrunk down to almost 1 solar radius

• Its central temperature reaches 10,000,000 K

• Fusion of Hydrogen begins

• It is still not on the main sequence

Stage 7: The Main Sequence

• For 30 – 50 more million years, the new star continues to contract

• Eventually gravity is exactly balanced by the outward pressure of the Nuclear reactions

• The star also exactly balances the energy it produces by fusion and the energy it radiates out

• The core is at 15,000,000 K and the surface is at 6000 K.

• It will stay like this for 10 billion years

• This takes long? Full grown in 4 months!

Other Mass Stars

• How do stars of other masses evolve?

• The basic steps are the same, but they will settle on the main sequence at different points.

• But then why is main sequence so broad?

• Stars have slightly different compositions

Failed Stars

• Some of the gas clouds that could form stars don’t. Why not?– They contract under gravity– They get hot– Where does it go wrong?

• The gravity crushes the cloud, but not enough to ignite thermonuclear fusion

• There just isn’t enough mass to create a large enough gravitational force

Failed Stars

• These failed stars are called Brown Dwarfs

• Brown dwarfs are less than 0.08 solar masses,

– (80 times the mass of Jupiter)

– The lower limit for Brown Dwarfs is 12 Jupiter masses

– Anything less massive is just called a planet

• These objects eventually become cold balls of gas

• We estimate there are 100 billion of these in our Galaxy

Very High Mass Clouds

• So the lower limit for stars is ~ 0.08 Solar Masses

• Is there an upper limit?

• We thought it was about 100 solar masses

• There is so much gravitational force that the nuclear fusion begins quickly

• Then there is so much nuclear fusion that it overcomes the force of gravity holding the star together

• These stars blow themselves up early in their lifetime

Very High Mass Clouds

• In 2010 Astronomers announced that they found a star with a birth weight of about 320 solar masses

• Much larger than models predict could be stable

• Current mass is about 265 solar masses

Main Sequence Stars

• How do main sequence stars know how big to be?

• What properties dictate temperature?

• Will the star always remain this way?

PVT Thermostat

• Stars have P, V, T, , and values

is the rate of nuclear reactions

• They have a self-regulating feature to balance the internal and external forces

• If then Q T P V

Stellar Evolution

• Stars have only two significant regions during their evolution

• Core – where the fusion is taking place

• Envelope – an inert layer of gas

– There is no convection between core and envelope

• As Hydrogen gets turned into Helium, the He will be inert (not hot enough to fuse)

• Later in the star’s life it runs low on Hydrogen

Stellar Evolution

The Active Sun

• The Sun seems steady and predictable

– The luminosity (energy output) is nearly constant

• The surface activity of the Sun is changing and unpredictable

– Doesn’t affect the evolution of the Sun

– Greatly affects the Earth

Sunspots

• Sunspots are dark areas on the Sun’s surface

• Typically about 10,000 km across

• Umbra – dark center

• Penumbra – grey surroundings

• Due to temperature change

Sunspots• Sunspots appear randomly, in different numbers

• However, there is an 11 year cycle

Sunspots • Sunspots are caused by magnetic field lines blocking

convection

• The magnetic field is locally strong and breaks through the surface

• If this happens so we see it at the side of the Sun, it is a prominence

The Sun and Earth

• These factors affect the climate of the Earth

• Luminosity actually decreases slightly when sunspots are absent

• “Maunder minimum” was a period of solar inactivity from 1645 – 1715

• May be linked to the “Little Ice Age” in Europe

• We haven’t been studying these things long enough to conclude the effects yet!

The Sun and Earth

The Sun and Earth

The Active Sun

• Prominences eject hot solar gases

– They follow magnetic fields

– Gas cools and falls back into the Sun

• Releases massive energy

– 1025 Joules

– 1 Billion years of Earth’s energy production

The Active Sun

• Solar Flares are more energetic than prominences

• Caused by rapid release of magnetic energy

• Equivalent to millions of 100-megaton bombs

• Gas breaks free of magnetic field lines and escapes into space

The Active Sun

• Coronal Mass Ejections are a release of a “magnetic bubble” of gas

• This gas interferes with the normal solar wind and can interact with the Earth’s magnetic field

• Imparts a large amount of energy and disrupts communications

Nuclear Fusion

• The Sun (and all other stars) convert mass directly into energy according to Einstein’s equation:

E = mc2

Energy = Mass X (speed of light)2

• The mass of individual atoms is very small, and sub-atomic particles (protons, neutrons) is even less

• Usually measured in atomic mass units (u) instead of kilograms

Nuclear Fusion• 1 atomic mass unit = 1.66053886 × 10-27 kilograms

• Converting 1 amu directly to energy:

• E = mc2 = (1.66x10-27 kg)(3x108 m/s)2 = 1.5 x10-10 J

• Doesn’t seem like a lot, does it?

• One kg of mass transformation equals 9x1016 J!

• The United States uses approximately 4,000,000 Giga Watt hours per year. (1.5x1019 J)

• That is only 167 kg of mass turning into energy!

• Hiroshima bomb only turned 1 gram of mass into Energy

Nuclear Fusion

• Similar electrical charges repel• Protons are both positively charged• However, if they have enough energy they can

overcome the repulsive force

Nuclear Fusion

• Protons fuse to Deuterons, Deuterons fuse with protons to form Helium-3, Helium-3 fuses to form Helium-4

Nuclear Fusion

• Mass of a Hydrogen atom (proton): 1.008178u

• Mass of Helium-4: 4.003976

• Does the mass of Helium-4 equal the sum of its parts (4 Hydrogen atoms)?

4.032712 = 4.003976 ?

• No. The parts are more than the whole. Where did the mass go?

Nuclear Fusion

• The missing amount of mass is called the Mass Defect

– The Mass Defect is converted directly to energy

• This energy represents the binding energy of the nucleus

• For low atomic masses, a larger nucleus is more stable and therefore lower energy

– Fusion releases energy

• For large atomic masses a smaller nucleus is more stable

– Fission releases energy

Nuclear Fusion

• By looking at the luminosity of the Sun, we can calculate that it gives off 3.9x1026 W

• The mass difference between He and 4 H is 0.028736 amu

• This equals an energy of 4.3x10-12 J

• At this rate 600 million tons of Hydrogen must be converted into Helium every second

Stellar Evolution

• Because there is no convection between the core and envelope, there is no Hydrogen re-supply

• The Helium builds up in the core

• With less nuclear reactions: – The temperature goes down

– The pressure goes down

– Gravity starts to win vs. pressure

– The star contracts

Stellar Evolution

• As the star contracts, it will heat up– (Gravitational PE conversion)

– Temp well above 10 Million K

– Not hot enough for HE fusion

• 100 Million K

• Hydrogen shell of the core burns fast and furious– Increased energy causes star to

“puff” outward

Stellar Evolution

Stage 8 Star• The star’s surface

temperature drops– Luminosity increases slightly

– Radius increases by ~3x

• On its way to becoming a Red Giant– Will take ~100 million years

– Envelope expands due to increasing gas pressure while core is shrinking

Stellar Evolution

Stage 9: The Red Giant Branch• The star grows to about 100x its main sequence size

– The surface temperature stays constant

– Cooler surface is becoming opaque to interior radiation

– Luminosity is very high because of its size

• While the outer shell has grown, the core continues to shrink – It is mostly non-burning Helium “ash”– No pressure to counteract gravity– Temperature increases to above 100 Million K

Stellar Evolution

Stage 10: Helium Fusion

• Two Helium atoms collide and create Beryllium-8

– Triple Alpha Reaction

• Beryllium-8 is very unstable

– Usually breaks apart in 10-12 s

• Because of high density in core, Be-8 will fuse with a Helium nucleus to form Carbon-12

4 4 8

8 4 12

He He Be

Be He C

Stellar Evolution

• The core is developing layers like an onion• Hydrogen is fusing in outer core shell• Helium is fusing in middle layer• Inert Carbon ash is

building up in its core– Not hot enough to fuse

into another element

• Gravity is continuing to compress the core– The core is getting hotter

Stellar Evolution

• The core will shrink until it reaches “electron degeneracy”

– Ionized electrons are swimming around in the core

• At the degeneracy point gravity has contracted the core so much that individual electrons “touch”

– At this point gravity can’t contract it any more

• The core is supported by this electron “pressure”

– Temperature continues to increase

Stellar Evolution

• The core temperature is rising

• The PVT Thermostat should result in a pressure increase, increasing the volume of the star

• However the pressure is fixed

– Pressure of the electron degeneracy

– This breaks the PVT Thermostat

• Helium fusion is running rampant

– Dumps a lot of energy into the core: Helium Flash

Stellar Evolution

• Helium Flash is an explosion that expands the star’s core outwards

• How can the explosion overcome gravity?

• The Helium Flash creates a lot of photons

– Results in a large “Radiation Pressure”

– This pressure overcomes gravity

• Core becomes “normal” again, pressure – gravity equilibrium is re-established

Stellar Evolution

• Hydrogen and Helium continue fusing

• As a result of the Helium flash the core cools

• This reduces energy output

• It moves on the H-R diagram

• Where it goes depends on mass

• 20-30% escapes

Stellar Evolution

• The carbon core continues to get larger

• Hydrogen and Helium burning gets more intense

• Temperatures get hotter and hotter

Stage 11• Star expands back out to

the Giant Branch

Stellar Evolution

• Gravity cannot compress the carbon core enough to raise the temperature enough to fuse Carbon– 600 Million K required

• Pressure is so high that the electrons are degenerate again

• Density is 1010 kg/m3

– A grape would weigh 1000 kg

• The big problem is that the core no longer produces energy

Stellar Evolution

• Outer layers are expanding due to heat convection from the core; the core is contracting

• As the core runs out of fuel, it heats up and blasts out the envelope forming a Planetary Nebula

• The name has nothing to do with planets

Stages 13 and 14

• What is left at the center of the nebula is a White Dwarf

• It is super-massive, and hot only due to stored energy

– About the size of Earth

– Mass is about half that of the Sun

• Its heat will eventually dissipate away into space

• At that point it will be a black dwarf

• It is a cold, dark, massive portion of space

Death Path of a Star

Stellar Evolution

Higher Mass Stars

• The main difference between a star like our Sun and a star of higher mass (~ 2.5 times greater) is that they can produce higher temperatures

• More mass means greater gravity

• Greater gravity means greater core compression

• Greater compression means hotter temperatures

• Hotter temperature means easier fusion

Higher Mass Stars

• Above 2.5 solar masses helium fusion happens easily, because core does not become degenerate

• There is no Helium Flash

• Above 8 solar masses the temperature of a contracted core is hot enough to fuse Carbon (and heavier elements)

Stellar Explosions

• Low mass stars end in a fizzle

• Higher mass stars can explode dramatically

– Novae– Supernova Type I– Supernova Type II

Stellar Explosions

Novae:• Need a white dwarf and Evolving Red Star

– Stars revolve around their common center of mass

• White dwarf draws mass off of companion star– Accretion disk forms– Friction causes particles to

fall inwards– Disk gets hot, gives off

radiation

Stellar Explosions• As gas falls onto surface of white dwarf:

– Becomes degenerate– Hydrogen starts to fuse– Heats gas up even more– Causes more reactions– So many photons are created that there is an explosion

• Accretion disk gets blown off of white dwarf– Great increase in luminosity– Can re-occur relatively quickly

Stellar ExplosionsSupernova Type I

• These detonations are “hydrogen poor”

• The process is essentially the same as a Nova

• Chandrasekhar Limit is the difference– 1.4 Solar masses

– Carbon Fusion begins instantly, everywhere

– NOTE: Collapse of Core due to gravity

– Star detonates

– No remnants are left behind

Stellar Explosions

Supernova Type II

• Stars with mass greater than ~8 solar masses

• Star evolves as usual

• The “onion” layers inside the star build to Iron

• Due to high mass, fusion of these heavy elements is “easy”

Stellar Explosions

Fusion Energy Release

• Energy is released by fusing atoms as long as mass is converted to energy

• From hydrogen to iron, the mass per nucleon decreases as atomic number increases

• Above iron, mass per nucleon decreases

• Fusion cannot continue

Nuclear Binding Energy

Stellar Explosions

Making heavy elements

• How do the plentiful elements heavier than iron form if fusion is an endothermic process at that point?

• Neutrons are a byproduct of other nuclear reactions

• Neutrons (no electric charge) are absorbed by a heavy nucleus

• Creates an isotope

Stellar Explosions

Neutron Capture

• The isotopes created are usually unstable and decay in time (minutes to years) – Beta decay

• When this happens in evolving stars, the process is relatively slow so it is called the s-process

• This explains the synthesis of non-radioactive elements

Stellar Explosions

Back to the Type II

• As the high mass star runs out of fuel, the large (3X+ the mass of the Sun) core collapses under the force of gravity

• Core shrinks heats up becomes degenerate

• Core gets so hot that photons get enough energy to destroy an iron nucleus

• The core is now a soup of protons, neutrons, and electrons

Stellar Explosions

Photoelectron Dissociation

• Electrons in this soup are moving near the speed of light

• Fast enough to smash into a proton and result in a neutron (Reverse Beta decay)

• Neutrinos are created in this decay, and there are so many they blast off the envelope of the star

• ALL THIS TAKES 1 SECOND

Stellar Explosions

• The blast can be a billion times brighter than our Sun• It will outshine the galaxy it is in• In 1054 Chinese Astronomers witness the Supernova

explosion we know as the Crab Nebula

• It outshone Venus

• It could be seen in the day

• This lasts for ~ a month

Stellar Explosions

• What happens to the star after the supernova explosion? Is there anything left?

• In a Type II Supernova just the envelope is blown off, the core remains

• What that core becomes will depend on its mass

Stellar Explosions

• If the remaining core is between 1.4 and ~3 solar masses, the neutrons are compressed to the degeneracy

• The result is a Neutron Star

• They are approximately 10 – 20 km in diameter

• One teaspoon would weigh a billion tons!

Black Hole Sun

• A single off of Soundgarden’s ’94 album Superunknown

• If the star has a core with a mass greater than about 3 solar masses, the Supernova remnant may become a Black Hole

• Above about 3 solar masses gravity crushes the degeneracy pressure and the core collapses to infinite density

• This will make the gravity immediately near the star huge

2

GMg

d

Black Holes

• By compressing a large amount of mass into such a small space, the gravity is so large that:– Light passing by is

“gravitationally Doppler shifted”

– Space time is significantly warped

– The escape velocity is faster than the speed of light

Black Holes

• Schwarzschild Radius: how small you would have to squeeze a mass down to for it to become a black hole

• Event Horizon: the point near the black hole where you cannot escape its gravitational pull– You cannot see “events” beyond this horizon because the light

cannot make it to you

2

2s

GMr

c

Black Holes

If light can’t escape a black hole, then how do we find one?

• We have to look for the gravitational effects:– Look for a binary star system where you can only see

one visible object– Look for a binary star system where one star is

accreting mass to the other one– Measure the effect on other nearby stars– Look for binaries that are black hole-black hole or

black hole-neutron star

Chapter 16 Test

• 10 Multiple Choice– Some from text

• 5 Free Response – Answer 4

• The mass of a single Hydrogen atom is 1.008178 amu. The mass of one Helium atom is 4.003976 amu. Four Hydrogen atoms are fused together to produce one Helium atom.

– How much energy is released by the fusion of 4 H atoms into 1 He atom?

– The Sun produces 3.9 x 1026 Joules of energy every second by converting Hydrogen to Helium. If an oil tanker carries 4.6x1033 Hydrogen atoms in its oil, then how many tankers would the Sun need to consume every second?

Chapter 16 Test

• Nuclear reactions occur within the core of a star at tremendous temperatures and pressures. Explain what occurs in the core on an atomic level. What happens to an atom made up of many protons, neutrons, and electrons? What causes the particles to fuse together? How is energy released and where does it come from?

• Describe the properties of the Radiative Zone of the Sun. This zone is cooler than the core, how does this affect the atoms found there? How is energy transmitted through this zone?

Chapter 16 Test

• Describe what happens in the Convective Zone of the Sun. What is the dominant form of heat transfer (and describe its properties)? How quickly does energy transfer through this zone?

• What is the significance and main properties of the Photosphere? What is the main method of energy transport?