February 11, 2009 jeb

Outline

• Examples of Star-Forming Regions
• Gravitational Competition
• Formation of Stars Like the Sun
• Stars of Different Masses
• Observations of Cloud Fragments and Protostars
• Shock Waves and Star Formation
• Star Clusters

Examples of Star-Forming Regions

• Within the Milky Way
  • Infrared image of the so-called Elephant Trunk Nebula
    • 2500 light-years distant
    • scores of young stars and protostars embedded in nebula
• Extragalactic Star Formation
  • in nearby galaxy M33
  • roughly 500 pc across

Gravitational Competition

• How do stars from?
  • region of interstellar medium collapses under its own weight
    • pulled by gravity
  • cloud heats as it shrinks
  • eventually center becomes hot enough for nuclear burning (thermonuclear fusion)
  • gravity only weakly influences interacting particles
    • it takes an enormous collection of particles to create the combined gravitational attraction required to hold them together
    • for a 100 K cloud, roughly 10⁵⁷ atoms are required
      • much more than all the atoms contained in the Earth
• Complications of Spin and Magnetism
  • spin competes with inward pull of gravity
  • effect of magnetism - hinders contraction

Formation of Stars Like the Sun

The H-R diagram is a useful way to summarize the observed properties of prestellar objects, just as it is for stars

The evolution of a star can be thought of as passing through seven evolutionary stages

Star formation begins when gravity begins to dominate over heat, causing a cloud to lose its equilibrium and start to contract




STAGE 1: AN INTERSTELLAR CLOUD

A very large interstellar cloud (spanning tens of parsecs) provides the initial stage of star formation

The cloud might contain thousands of times the Sun's mass

• the mass is dominately in the form of cold atomic and molecular gas, with some dust

Approximate Time to Next Stage:	2 x 106 yr
Central Temperature:	10 K
Surface Temperature:	10 K
Central Density:	109 particles/m3 103 particles/cm3
Diameter:	1014 km



• the collapsing cloud will break up into tens, hundreds, or thousands of fragments



STAGE 2: A COLLAPSING CLOUD FRAGMENT

Approximate Time to Next Stage:	3 x 104 yr
Central Temperature:	100 K
Surface Temperature:	10 K
Central Density:	1012 particles/m3 106 particles/cm3
Diameter:	1012 km



At this phase the cloud is very diffuse, so heat is not trapped (except near the center) and temperature does not build much ( except at the center)




STAGE 3: FRAGMENTATION CEASES

Approximate Time to Next Stage:	105 yr
Central Temperature:	10,000 K
Surface Temperature:	100 K
Central Density:	1018 particles/m3 1012 particles/cm3
Diameter:	1010 km



Several tens of thousands of years after the stage 2 fragment began to collapse, it becomes opaque at the center, and the central temperature rises significantly
• Cloud has now shrunk to region the size of our Solar System
• Protostar appears at the center of the fragment



STAGE 4: A PROTOSTAR

Approximate Time to Next Stage:	106 yr
Central Temperature:	1,000,000 K
Surface Temperature:	3000 K
Central Density:	1024 particles/m3 1018 particles/cm3
Diameter:	108 km



• approximate evolutionary track of the collapse of an interstellar cloud fragment prior to its reaching stage 4, a protostar
  • this early evolutionary track is known as the Kelvin-Helmhotz contraction phase
• the collapsing cloud heats as it contracts



STAGE 5: PROTOSTELLAR EVOLUTION

Approximate Time to Next Stage:	107 yr
Central Temperature:	5,000,000 K
Surface Temperature:	4000 K
Central Density:	1028 particles/m3 1022 particles/cm3
Diameter:	107 km



• As the protostar moves beyond stage 4, it becomes a T Tauri star, moving toward the main sequence
  • this path from stage 4 to 6 is known as the Hayashi track
    • Characterized by violent surface activity and strong protostellar winds
• Central temperature is still not hot enough for thermonuclear fusion
  • repulsion of two positively charged protons (Hydrogen nuclei) cannot be overcome



STAGE 6: A NEWBORN STAR

Approximate Time to Next Stage:	3 x 107 yr
Central Temperature:	10,000,000 K
Surface Temperature:	4500 K
Central Density:	1031 particles/m3 1025 particles/cm3
Diameter:	2 x 106 km



• Thermonuclear fusion begins
• larger, but cooler, than Sun, resulting in somewhat lower luminosity



STAGE 7: THE MAIN SEQUENCE AT LAST

Approximate Time to Next Stage:	1010 yr
Central Temperature:	15,000,000 K
Surface Temperature:	6000 K
Central Density:	1032 particles/m3 1026 particles/cm3
Diameter:	1.5 x 106 km



• Star contracts a bit and comes into hydrostatic equilibrium

Summary of the Evolution of a Solar-type Star

Stars of Different Masses

• Stars of different masses appear at different points on the H-R diagram
  How do they get there?
• Stars of different masses from the Sun follow different evolutionary tracks on the H-R diagram
  • arrive at different points on the main sequence
• zero-age main sequence (ZAMS)
  • main-sequence band predicted for stars
  • stars will be slightly off-set from this band depending on the concentration of the small fraction of heavy elements
  • star "stays put" on the main-sequence, spending most of its life in one place
• "failed" stars
  • brown dwarfs
    • mass less than 0.08 solar masses
      • but more than .012 solar masses (12 times Jupiter's mass)
    • Jupiter-like objects
      • Jupiter is technically not a brown dwarf (0.001 solar masses)
      • examples of candidates: Gliese 623 (left image) and Gliese 229b (2 right images)
    • difficult to observe, number of brown dwarfs in the Milky Way may be comparable to the number of stars
      • Hubble Space Telescope image in infrared of brown dwarfs in Trapezium of Orion Nebula
        • about 50 brown dwarfs seen at distance of 1,500 light years

Observations of Cloud Fragments and Protostars

• period of cloud contraction is far too long for us to "watch"
  • our computer calculations predict it
  • we can observe it at various stages of evolution
• cloud contraction
  • M20 (The Trifid Nebula), evidence for three broad phases of star formation
    • parent cloud (stage 1)
    • contracting fragment (between stages 1 and 2)
    • emission nebula (stages 6 and 7)
• cloud fragments
  • Orion Nebula
• protostars
  • Protostars within the Orion Molecular Cloud imaged by the Hubble Space Telescope
• protostellar winds
  • formation of bipolar jets, as the disk of nebular gas is blown away by the new star
  • HH30, illustrating a real example of the bipolar flow, along with an artist's conception of a young star system
  • Another pair of outflow jets, HH1 and HH2, in the Orion molecular cloud

Shock Waves and Star Formation

• Shock waves driven out by high temperatures and pressures in an emission nebula may compress interstellar clouds to greater densities, triggering star formation
  • "Contracting fragment" may have been triggered by M20
  • Interstellar shock waves, which can trigger star formation, may come from several sources
    • Emission nebulae
    • Planetary nebulae
    • Supernova explosions
    • Spiral-arm waves of galaxy
    • Interactions between galaxies
  • Star formation might resemble a chain reaction
  • Star-forming region in galaxy NGC 4214 (13 million light-years distant), which may represent several generations in a chain of star formation.

Star Clusters

• End result of collapse of cloud is a group of stars known as a star cluster
  
  
• Newborn cluster NGC 3603
  
  
• Pleiades or Seven Sisters
  • example of an open cluster
  • H-R diagram reveals this is young cluster, less than 100 million years
  
  
  
• Globular cluster Omega Centauri
  • no massive stars
  • about 10 billions year old - oldest stars in the galaxy
  
  
  
• Protostellar Collisions
  • physical interactions (close encounters and collisions) between protostars are important in determining outcome of formation
  • Large protostars may grow by "stealing" gas from the smaller ones, for example
  
  
  
• Carina emission nebula
  • the innermost core, Eta Carinae, has a mass of about 100 times the Sun, and a luminosity of 5 milion times, one of the most massive stars known
  • Has brighten significantly several times in last few hundred years
  • Brightening mechanism unknown
  • Probably formed only a few hunderd thousand years ago, since its lifetime can only be about a million years
  
  
  
• Star formation in Orion Nebula
  • The Trapezium, 4 bright stars responsible for ionizing the nebula, are clearly seen
  • The infrared image shows an extensive cluster of young stars
  • The bright yellow object (BN object) is probably a dust-shrouded B-type star just beginning to form own emission nebula
  
  
  
• Eventually, emission nebulae will give rise to large open star clusters
  • Most open clusters tend to disperse over a few hundred-million (100,000,000) years
  • More massive clusters will persist for somewhat longer
    • being more tightly bound by the gravitational pull of the cluster
  • The Sun must have been a member of a cluster at one time, but now is a lonely, isolated star
  
  
  • Over time, these clustering stars will become isolated stars, like the Sun, or isolated small groups, such as binary star systems

Summary

Many examples of star forming regions are observed in space

Stars form when an interstellar cloud collapses under its own gravity, heats, and breaks into pieces of star-sized masses

The evolution of the protostar can be represented as a path on the Hertzsprung-Russell diagram

The resulting protostar, which emits primarily infrared radiation is known as a T Tauri star

Process takes about 50 million years for a star like the Sun, as a disk forms, and jets of matter are expelled in the direction perpendicular to the disk

More massive stars form more quickly, while less massive stars take longer, all the result of the cloud's self gravity

Some objects (brown dwarfs) are not massive enough to reach nuclear ignition
vast numbers (difficult to observe) may populate the Milky Way galaxy

Many stages of the stellar evolution are observed by radio and infrared telescopes

Very massive stars can trigger a chain reaction of star formation

A contracting cloud can lead to hundreds or thousands of stars (a star cluster)

Open clusters
• in plane of Milky Way
• hundreds to thousands of stars
• many bright blue stars
• "recently" formed

Globular clusters
• out of plane of Milky Way
• millions of stars
• no main-sequence stars more massive than the Sun
• formed "long ago"