Star Formation:

Collapse of interstellar material under its own weight causes star formation.

•  Gravitational attraction of the dust and gas causes
   the cloud to begin to condense
  
  What two conditions must be just right for the collapse to occur?

1. The cloud must be dense enough:
• lots of mass in a given volume
• gravity is great enough to cause collapse
2. The cloud must be cold enough:
• hotter gas, higher pressure
• if gas is to hot, outward pressure can balance
  gravity to stop the collapse.

7 stages of Star Formation

1. Interstellar Cloud

2. Collapsing Cloud Fragment

3. Fragmentation ceases

4. Proto-star

5. Proto-star evolution

6. New Born Star

7. The Star (Main sequence)

Interstellar cloud:

• tens of parsecs across
• billions particles/ m³
• cloud fragments during collapse
• each fragment becomes a star
• dozens of stars from one cloud

• about 100 times the size of the Solar System
• initially doesn't heat much because radiation escapes
• eventually becomes dense enough to trap radiation and begins to heat up

• roughly the same size of the solar system
• inner part of cloud is opague and heats up ALOT
• center has temperature = 10,000K
• outer part still cool and thin
• center has 10¹⁸ particles
• inner part becomes a (proto-star)
• Kelvin-Helmboltz Contraction Phase

• *Proto-star: prestellar object hot enough to emit IR, but not hot enough for fusion.
• will continue to be proto-star until fusion begins.
• core temp 1 million K
• contraction slows, but does not stop
• from stage 4 to 6 is the Hayashi Track called a T Tauri Star
• during stage 4 the star begons to appear on the H-R Diagram

• Surface Temperature: 5,000K
• Core Temperature: 5 Million K
• Size: 10 Solar radi
• Contraction: still slowing
• Still a T Tauri Star, on the Hayashi Track

• Core Temperature reaches 10 million K, hot enough to start H fusion
• size is a bit larger than the sun
• surface a bit cooler than the sun
• luminosity a bit less than the sun

The Star (The Main Sequence)

• central density about 10³² particles
• central temperature 15 million K
• surface temperature 6,000 K
• star has arrive along the zero age main sequence (ZAMS)
• left edge of main sequence is where all stars begin thie stage of life

Stage 4-6: the Hayashi Track, called T Tauri Star
Stage 6: Hydrogen Fusion in core (then quickly to 7)

Stage 7: the star has reached the main sequence (ZAMS)

*The evolutionary path varies depending on the stars size/mass.

** A Stars mass is the #1 Factor that determines how a star evolves.

Lifetime of a Proto-Star

*depends on its mass

• 15 Solar Mass =   160,000 years
•  5 Solar Mass =    700,000 years
•  2 Solar Mass =       8 millions years
•  1 Solar Mass =     30 million years (like our Sun)
• 1/2 Solar Mass =  100 million years (red dwarfs)

**Know that: The more massive the star, the factor evolves

Notes:

If the collapsed cloud has a mass of <0.1 (0.8) Solar Mass; fusion will not begin. Core of object will not get hot enough for fusion. No fusion = No Star.

Brown Dwarfs:

About the size of Jupiter

Emit in the IR

Reddish/Brownish in Color

Difficult to detect (could be lots of them)

Evidence of Star Formation

Proto-stars could exhibit Bipolar Flow:

Bipolar Flow: two collimated jets of gas flowing out from the star in opposite directions.

• This is solar wind from the proto-star
• As cloud contracts it forms a disk around the proto-star (pizza dough)
• Solar Wind mostly perpendicular to the disk

Evidence of Star Formation cont..

As the solar wind blows away the disk, the bipolar jet fans out. Eventually the disk is dispersed and the solar wind is emitted equally in all directions.

No longer any bipolar flow!

The disks formed around new stars are important.

Clumps of matter in disks could condense to form planets (proto-planets).

Stellar Evolution

Not all stars use the PP Cycle for energy.

PP Cycle is the major energy source for the SUn but the Sun also produces some energy by the CNO Cycle

Stars in upper main sequence use the CNO Cycle

CNO Cycle

CNO Cycle requeires the presence of Carbon, Nitrogen, and Oxygen to occur (act as catallyst)

• Requires higher temperatures than the PP Cycle.
• Carbon has 6 protons in its nucleus: so to make carbon fuse with Hydrogen it takes a lot more heat - greater repulsive force between it and the proton than between 2 protons.
  ex: High mass stars use CNO cycle - they are hotter stars!

-Stellar evoultion from computer models: the star is divided into many shells; like an onion

4 Principles Used:

1. Hydrostatic Equilibrum
2. Energy Transport (how energy moves around the star: conduction, convection, radiation)
3. Continuity of Mass: total mass of star must equal sum of its shells
4. Continuity of Energy: amount of energy leaving a shell equals energy entering shell plus energy created inside the shell.

Notes

a star begins on the Zero Age Main Sequence (ZAMS)

H-R Diagram top left main sequence >>> moves down left side main sequence...

As H is burned and converted to He .....

• outward pressure decreases
• core contracts due to gravity
• heats up the core
• H is fuesd more rapidly making even more heat
• more heat causes outer parts of star to expand
• star becomes larger
• surface cools, luminosity increases

In the case of our Sun, process occurs in 10 billion yr period

Stars moves to bottom left Main sequence then begins to travel upward roght side of Main sequence:

When the Sun reached the right edge of the main sequence:

• it will be twice its present luminosity (book says 30%)
• it will have fused nearly All its H, in its H core.

The Sun will run out of H fuel about 5 billion years from now. It is approx 5 billion yr old now; which means stars stay on the main sequence approx 10 billion yrs.

• Time on the Main Sequence depends on its total mass.
• High Mass stars are hotter and burn fuel faster
• 25 Solar Mass about 7 million years
• 1 solar mass about 10 million years
• red dwarfs about 200-300 billion years

after H is Gone:

• Star enters its old age
• All H in core has been fused into He
• He Core becomes very hot, due to contraction
• He core has less energy generation
• weight of outer layers compresses corem heating it up.

Outer parts of star expand due to extra heating from H fusion shell.

Stars surface becomes cooler:

• star moves to right on the H-R Diagram
• Luminosity increases due to larger size of star
• star becomes a sub-giant.

Stage 7-8: Sub-giant stage (leaves main sequence)

Stage 8-9: Red Giant Stage

From 7-9: No fusion in the He Core, but there is fusion (H fusion) in the shell around the core.

If star has enough mass (at least 0.4 solar masses)

• He fusion begins in core
• He is fused into C via the Triple Alpha Process
• He nuclei (also called ALPHA PARTICLES)
• Requires 3 He nuclei (why called Triple Alpha Process)

Three He in = 1 C nucleus out

Stars like the Sun undergo a "Helium Flash"

• Core pressure is so great atoms are compressed together as close as possible
• This limit is called the "Pauli Exclusion Principle."
• outward pressure due to electron degeneracy pressure, not gas pressure.

When He fusion starts in the degenerate gas there is:

• no increase in pressure
• no increase in volume
• no drop intemperature

Temperature increases rapidly, rate of fusion increases rapidly.

Explosive beginning to He fusion is the Helium Flash!

Notes

Eventually, core becomes hot enough to return to normal gas!

Outer parts of star undisturbed.

Drop in luminosity after He flash.

• after He flash at stage 9, star reaches equilibrum state at stage 10 and settles down at some point on the horizontal branch.
• During this time, the star is fusing He into C.

• Stars more massive than the sun will not have He Flash, they will ignite He gradually.
• Stars less than 0.4 solar mass, will NEVER has Fe fusion.
  
  

Our SUN:

Eventually all the He will fuse into C.

He fusion slows, core begins to contract

If star is massive enough, the C core gets hot enough to start He fusion in shell around core.

• This He comes from the H fusion in shell.
• Outer parts of the shell expand (again)
• surface cool
• luminosity increases

Stage 11 is the Asyptotic giant branch.

• For stars like our Sun, the core does not get hot enough for C fusion to take place.
• But shells of H fusion and He Fusion are still working there way outwards.
• Star undergoes great expansion, while surface cools.
• Eventually outer parts of star escape into interstellar space, leaving behind a naked carbon core.

• The escaped gas can be visible, and is called a planetary nebula.
• nothing to do with planets
• historical reasons for the name
• The naked C core is a White Dwarf Star.
• Our sun will end its life as a white dwarf star after forming planetary nebula.
• The sun will not blow up!

The white dwarf is just the naked C core of the original star.

• No fuison will take place
• will slowly cool off (cold dark ball of carbon)
• It is degenerate matter
• Pauli Principle prevents further compression, not  a diamond.

• The White Dwarf must have a mass less then 1.4 solar mass.
• This is called the Chandrasekhar Limit.
• White Dwarf is usually about the size of Earth
• The planetary nebula can form beautiful, complex, colorful patterns

Stellar Evolution 2

Stars 10X the mass of the Sun

• same as sun-like stars until C core collapses
• In high mass stars, the C core does get hot enough for fusion to occur.
• As C fuses, O is mostly created along with some Ne,Mg.
• Of course the star runs out of C, leaving behind a core of O.
• fusion in core ceases, the core collapses
• the core gets hotter (core gets so hot it initiates C fusion in a shell around it)
• C left behind by the He fusion in shell, He left over by the H fusion in shell.

Stellar Evoultion 2 cont...

Oxygen core gets so hot that O fusion begins

• leaves behind mostly Ne
• Star eventually runs out of O
• Oxygen fusion in the core ceases
• core collapses, core gets real, real hot
• The core gets so hot that it initiates a shell of O fusion around it (O fm C, C fm He, He fm H)

Then Ne core gets hot enough to start Ne fusion into Si and heavier elements.

• eventuall star runs out of Ne and the core is mostly Si
• The Si core collapes, Si core gets real,real,real hot, Si core starts a shell of Ne fusion around it (Ne fm O, O fm C, C fm He, He fm H) - All working their way out to surface os star,

The Si core gets hot enough to start fusion

• leaves behind mostly Fe and a little Ni and other stuff
• Eventually the Si is used leaving Fe core
• Fe core collapses (When Fe core collapses - the star dies)......
• Supernova - spectacular explosion!

Evidence of Stellar Evolution

Theory of Stellar Evolution from computer models.

2 pieces of Evidence that computer theory correct

#1 Varible Stars: Single stars which vary in brightness

• Mira (oCeti): RA 2h 17m, dec -3.3, 9th mag to 3rd mag, 330 day period - varies alot!
• d Cepheii RA 22h 27m, dec 58.2, 3.6 to 4.3 mag, 5.4 day period, Spectral class G
• d Cepheii defines a class if stars that vary in luminosity called Cepheid Variables.
• generally giant star
• Spectral class F, G, or K
• period between 2 and 60 days
• North Star is a Cepheid Varible: 4 day period, 0.1 mag, spectral class F7

Period Luminosity Relationship: Longer period; higher average luminosity.
**Very important because Cepheid Varibles become a tool for finding distances.

- measure apparent magnitude

- determine avg absolute magnitude from period

- calculate distance

RR Lyrae varible stars: all have about the same luminosity 0.7, period about one day, and can also be used as distance indicators.

cont..

These stars pulsate:

larger - luninosity increases

smaller - luminosity decreases

Models predict region on H-R Diagram where stars will pulsate.

The Instability Strip!

evidence cont...

#2. Star Clusters: densely packed group of stars

2 types: Globular & Open

Globular: - spherical shape

              - generally very old

              - occur outside of plane of galaxy

              - 100,000 stars very crowded

Open Clusters (AKA Galactic Clusters):

- irregular shape

-  of any age

- occur in plane of galaxy

- 100's - 1000's of stars

Stars in cluster formed from the same gas cloud

Stars in cluster are:

- about same age

- about the same chemical composition

- about the same distance away

Recall that more massive stars:

- are more luminous stars (main sequence)

- evelove faster than low mass stars

H-R Diagram of clusters will show a turn-off point where stars leave the main sequence

Turn-off point position determines clusters age

***Appearance of clusters on H-R Diagram duplicated by models of stellar evidence.

Stellar Explosions

3 Kinds of Exploding Stars

1. Novae
2. Type I Supernovae
3. Type II Supernovae

Nova

Nova- exploding star that suddenly increases in brightness by a factor of 10,000.

- result of explosion on surface of white dwarf

- caused by matter falling onto the white dwarf from a
  companion star.
As H builds up on surface of White Dwarf:
- H gets real Hot, H gets very dense
- Fusion begins in surface H & quickly covers the star

- rest of star unharmed & process can start over

Supernovae Type II

-exploding high mass star

-when Fe core collapses (collision triggers explosion)

As core collapses, gravity overcomes the electron degeneracy pressure

Collapse continues, density becomes greater than that of a white dwarf

Pressure is so great that the electrons are squeezed into the nucleus of atoms

the electrons neutralize the protons: p + e > n + v

Supernovae Type II cont..

- The neutinos fly into space

- The neutrons are squeezed together until they are touching

- exclusion principle

- neutron degeneracy gas

Core has become one big neutron

neutron degeneracy pressure causes the core to rebound (like a ball bouncing off a wall)

outer parts of star also collapsing

outer parts collide w/rebounding core and are blown into interstellar space.

Supernovae Type I

- from a white dwarfin a binary system

- White dwarf accretes matter from its companion (similar to process of forming a nova) EXCEPT that:

- white dwarf gets so massive that gravity overcomes electron degeneracy pressure (>1.4 solar mass)

- white dwarf collapses.

Companion star may also become white dwarf:

-the two white dwarfs merge

-merged object exceeds 1.4 solar mass

-merged object collapses.

Supernovae Type I cont...

- C fusion begins in collapsing white dwarf

- C fusion occurs simultaneously throughout entire star

- One humongous fusion bomb!

- Supernovae brighter than all the stars in the galaxy put together!

Notes:

Type II Supernovae exhibits H lines in its spectrum

Type I Supernovae has no H spectral lines.

Beware of Supernovae:

- Supernovae w/in 50 ly would destroy life on Earth

- We would have to move underground

- Surface life would be wiped out

- climate change due to atmospheric changes

- not a pleasant place......

Famous Supernovae:

- "Guest Star" of 1054 AD Chinese discovery

- Tycho's Supernova 1572: Tycho showed Supernovae were not an atmospheric phenominum

- Kepler wrote about one in 1604
Supernovae 1987A in Large Magellanic Cloud (LAM)

**When Universe formed, only H and He:

- other elements from stars: C, N, O in our bodies from some star that ceased to exist eons ago.

- stars cannot fuse Fe or heavier and get energy.

- heavier elements created in star when it supernovaes (Gold/Silver made in supernovae many eons ago)!

Temperature

(is: mass, angular motion, electric charge)

two of the following choices are correct

(large magnetic field/rotates rapidly)

all of the above

(it will never do it again, it may leave behind a black hole, it may leave behind a neutron star)

For a nova to occur, the system must have already been a

H-R Diagram

A

Hayashi track - The object is a protostar, no fusion taking place. It is still condensing under the influence of gravity.

H-R Diagram

B

Main Sequence Star - In the core of the star hydrogen fusion takes place to form helium. The star spends alamost all of its lifetime on the main sequence.

H- R Diagram

C

Subgiant- All the hydrogen in the core has turned into helium and core fusion has ceased. The helium core collapses under the influence of gravity and becomes very hot. Hydrogen fusion takes place in shell around core. The H fusion shell causes the star to expand and its surface to cool.

H-R Diagram

D

Helium Flash - The helium core is compressed to become degenerate gas (Pauli Principle stops collapse of core) and an explosive beginning to helium fusion occurs here.

H-R Diagram

E

Red Giant Phase - Helium Fusion taking place in core to form carbon via the triple alpha process.

H-R Diagram

F

Carbon Core - All helium in core has been turned into carbon and core fusion ceases. Helium fusion shell is formed around the hot core causing the star to undergo another expansion and become a supergiant.

H-R Diagram

G

Formation of a planetary nebula - The outer parts of star escape into interstellar space forming the planetary nebula and eventually leaving behind the carbon core.

H-R Diagram

H

White Dwarf Stage  The outer part of the star has dispersed into interstellar space (planetary nebula is gone). The remaining stellar core is now a white dwarf, and is almost all carbon. It is about the size of Earth, very dense, and is initially very hot. There is no fusion taking place, so the star will slowly cool off.