November 3, 2016
Astronomy 122
Prof. J. Brau

Outline

• 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
• Summary

Star-Forming Regions

The birthplace of stars are dense, violent nurseries. Characterized by outbursts of activity and interstellar shock waves. Our Sun and the Solar System have survived such a violent environment of 4.5 billion years ago. Most of the stars in our immediate cosmic neighborhood probably formed together in a dense cloud of gas and dust. Accompanied by massive stars that died out long ago, and bright emission nebula in the neighborhood excited by these bright stars. Examples within the Milky Way today. Star-forming region in Orion Nebula. 1400 light-years distant. Scores of young stars and protostars embedded in nebula. Infrared image of the so-called Elephant Trunk Nebula. 2500 light-years distant. Scores of young stars and protostars embedded in nebula. Examples of Extragalactic Star Formation. Stellar Nursery in Tarantula Nebula in Large Magellanic Cloud. 170,000 light-years away. 100 light-years across. Less than 2 million years old. About 90,000 solar masses. NGC 604 In nearby galaxy M33. Roughly 500 pc across.

Orion Nebula       and Infrared Spectrum

Gravitational Competition

• How do stars from?
  • Region of interstellar medium collapses under its own weight.
    • Pulled by gravity.
  • Cloud heats as it shrinks.
    • Heating due to release of gravitational energy.
  • Gravity only weakly influences interacting particles.
    • Heat generated in collapse opposes pull of gravity.
    • 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.
• Eventually, if mass is sufficient to produce concentration, it becomes hot enough for nuclear burning (thermonuclear fusion).

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.

Path of Newborn Star






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). Gliese 229 (right image). Difficult to observe. Number of brown dwarfs in the Milky Way may be comparable to the number of stars. Infrared image of star cluster near Orion Nebula. Faint specks are brown dwarf candidates. 10-15 percent of "stars" in Orion are brown dwarfs. 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.

IR Image near Orion       with numerous brown dwarfs

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 Stages of development. Formation of bipolar jets due to intense heating and strong outflows. Jets fan out as the disk of nebular gas is blown away. New star appears with spherical wind. Actual image of hit young 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.

Trifid Nebula (M20)       Illustrates birth phases

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.

Generation of Star Formation

Star Clusters

End result of collapse of cloud is a group of stars known as a star cluster. NGC 3603 Newborn star cluster. Around 20,000 light-years away. Contains about 2000 bright stars. Pleiades or Seven Sisters Example of an open cluster. About 450 light-years away. H-R diagram reveals this is young cluster, less than 100 million years. Globular cluster Omega Centauri No massive stars. About 16,000 light-years away. H-R diagram shows age is about 10 billion years - oldest stars in the galaxy.

The Pleiades




• Protostellar Collisions
  • Physical interactions (close encounters and collisions) between protostars are important in determining outcome of formation.
  • Supercomputer simulations of star formations.
  • Large protostars may grow by "stealing" gas from the smaller ones, for example.
  
  
  
• Carina emission nebula and Eta Carinae
  • The innermost core, Eta Carinae, has a mass of about 100 times the Sun, and a luminosity of 5 million 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 hundred thousand years ago, since its lifetime can only be about a million years.
  
  
  

  Star formation in Orion Nebula The Trapezium, 4 bright O-type stars responsible for ionizing the nebula, are clearly seen. The infrared image shows an extensive cluster of young stars. Image probably includes many brown dwarfs. 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.

  Star formation in Orion Nebula

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, essentially "failed stars") 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".