Basics of Star Formation

Stars form in GMCs, large clouds of molecular gas that exist in the regions between the stars (the Interstellar Medium, ISM). Stars actually form in cold, dense gas and dust clouds that exist within the GMCs. The star formation process is amazing. It takes clouds whose properties are; (i) T ~ 10 - 50 Kelvin, (ii) density ~ thousands to millions of particles per cubic centimeter, and (iii) radii, initially around 1 parsec (3.26 light years), and shrinks them to form Main Sequence stars -- objects with central T ~ millions of Kelvin, central densities ~ 10²⁶ particles per cubic centimeter, and radii ~ million or so kilometers (~ 3 light-seconds!).

The star forming clouds are initially in hydrostatic and thermal equilibrium so that the star formation process must be triggered.

Star formation is driven by gravity.

The clouds have low pressures due to their low densities and temperatures ===> clouds can easily get pushed out of hydrostatic equilibrium.

Initially, the equilibrium clouds require some sort of trigger to start the star formation process. Triggers compress the clouds which pushes all particles closer together and so increases the inward pull of gravity throwing the clouds out of hydrostatic equilibrium causing them to collapse. Why do the clouds continue to collapse after the trigger?

Many triggers have been proposed:

As spiral arms move through the gas and dust disks of galaxies, the spiral may compress ISM clouds and trigger star formation.

Shock waves generated by the deaths (explosions) of massive stars (Type II Supernovas) generates strong shock waves which may compress ISM clouds and trigger star formation. The picture shows the Cygnus loop in our Galaxy.

Intense stellar winds and radiation pressure (because of the high luminosity) from massive young stars may compress ISM clouds and trigger star formation.

The initial mass and size of the triggered cloud collapse depends on the density and temperature of the cloud. For the typical parameters given above we expect that clumps of size of a few hunded to a thousand or so Solar masses become unstable to collapse, the limiting mass known as the Jeans Mass. Once intitiated, the process then proceeds roughly as

Initially, the collapse is very fast because the density of the cloud is low. Any energy released by the collapse (compression) freely escapes as radiation and the cloud does not heat!!--duration: ~2x10⁶ years

The collapsing cloud fragments (breaks apart) into smaller pieces each of which eventually forms a star. Fragmentation occurs because the density of the cloud increases during the collapse ===> stronger gravitational attraction between the gas particles ===> smaller Jeans masses. As long as the density of the cloud is too low to trap radiation, the temperatures remain low and collapse and fragmentation continues. duration: few 10⁴ years

Fragmentation first ceases and the collapse slows near the middle of the cloud, where it is densest--an inside-out collapse occurs. This happens when the cloud becomes dense enough to trap the heat (radiation) generated by the collapse; because heating of the core ===> internal gas pressure rises ===> collapse slows and the Jeans Mass gets larger. duration: ~10⁵ years

After the central parts of the cloud becomes dense, the core then contracts more slowly as the core is now nearly in hydrostatic and thermal equilbrium. During this time the pressure and density in the core only slowly increase and the outer part of the cloud rains down on the star forming in the center. The object is referred to as a protostar during this stage of star formation. The star shines at this point because it is shrinking (powered by gravity in the manner envisioned by Lord Kelvin for the Sun).
The protostar moves on the upward sloping (to the left) red track on the HR diagram. At the bend, it becomes convective and then drops along the nearly vertical path (the Hayashi Track). Often times on the Hayashi Track, extensive circumstellar disks (sometimes protoplanetary disks) form,

On the right are shown observations of the young star, HD 141569A.
and strong stellar winds develop (tens of thousands of times stronger than the Solar Wind), and this portion of the evolution is called the T Tauri Phase . During this time, erratic variability is often detected with evidence for starspot activity.

The slow contraction and accretion of the outer cloud continues until the core temperature reaches ~10 million Kelvin, whereupon hydrogen burning commences --duration: 10⁷ years

The protostar is now becoming a star (officially, it becomes a star after nuclear fusion--hydrogen ===> helium-- is ignited in its core. This occurs when the central temperature of the protostar reaches around 10 million Kelvin) and it settles onto its place on what is referred to as the Zero Age Main Sequence [ZAMS])--duration: 3x10⁷ years

The total time needed to form a star like the Sun is around 40 million years or so.

Some Comments on the General Process

Upper Limit to the Mass of Main Sequence Stars
It is conceivable that the Upper Mass Limit for Main Sequence Stars is due to the fact that the time required for the central portion of the cloud to collapse is so short that the protostar turns into a star before the outer layers of the cloud can fully merge with the star. In this case, the initial turn-on of the star drives off the rest of the cloud arresting the star formation process and limiting the mass of the star! This appears to occur for protostars with masses of 50-100 times the mass of the Sun.

Lower Limit to Main Sequence Star Masses
The lower mass limit arises because stellar material cannot be compressed beyond certain limits. It is very difficult to compress electrons beyond densities of several hundred thousand times the density of water. This resistance to compression is independent of the temperature of the gas as it is due to a Quantum Mechanical effect that limits how densely you can pack the particles. This leads to something known as degeneracy pressure.

The property that the degenerate pressure is independent of the temperature of the gas is quite important because it means that once a star becomes supported by degenerate pressure , it can continue to support itself even if it is cold. The onset of degeneracy before low mass protostars become hot enough to ignite nuclear fusion halts the collapse, preventing the formation of a star. This leads to the lower limit on the mass of a star--~8 % of the mass of the Sun -- and to the formation of the failed stars known as brown dwarfs!

Multiple Star Systems
The preceding discussion concerned the formation of single stars. However, more than half (and perhaps up to 80 %) of all stars are in mulitple (double star, triple star, quadruple star, ... ) star systems. In addition, over 3,000 planets in nearly 1,000 extra-Solar planetary systems have been discoverd over the last 20 years. It is recognized that in a significant number of systems where planet formation is possible, planets seem to form. Planet formation is not rare. Apparently, the formation of multiple star systems and planetary systems is an important part of the overall star formation puzzle.
Do binary star systems form by captures of random stars or do the stars in the systems form at the same time from the same cloud?

P_orb > 100 years, M₁/M₂ is roughly the same as for indiviual stars observed in the Galaxy ===> stars probably form as individual stars, either because the binary system is so large that they form essentially as individual stars or because they formed in different regions and simply were captured into the binary system.
P_orb < 100 years, M₁/M₂ ~ 1 ===> the stars in short orbital period binary star systems (and in the extreme, planetary systems) probably form as units.

The theory of how individual stars form is in fairly good shape. The details of how binary stars form is much less secure.

HUBBLE SNAPSHOT CAPTURES LIFE CYCLE OF STARS
In this stunning picture of the giant galactic nebula NGC 3603, the crisp resolution of NASA's Hubble Space Telescope captures various stages of the life cycle of stars in one single view and highlights how astronomers study the evolution of objects whose lifetimes are much longer than human lifetimes (and, in fact, humanity's lifetime).

To the upper right of center is the evolved blue supergiant called Sher 25. The star has a unique circumstellar ring of glowing gas that is a galactic twin to the famous ring around the supernova 1987A. The grayish-bluish color of the ring and the bipolar outflows (blobs to the upper right and lower left of the star) indicates the presence of processed (chemically enriched) material.
Near the center of the view is a so-called starburst cluster dominated by young, hot Wolf-Rayet stars and early O-type stars . A torrent of ionizing radiation and fast stellar winds from these massive stars has blown a large cavity around the cluster.
The most spectacular evidence for the interaction of ionizing radiation with cold molecular-hydrogen cloud material are the giant gaseous pillars to the right and lower left of the cluster. These pillars are sculptured by the same physical processes as the famous pillars Hubble photographed in the M16 Eagle Nebula.
Dark clouds at the upper right are so-called Bok globules , which are probably in an earlier stage of star formation.
To the lower left of the cluster are two compact, tadpole-shaped emission nebulae. Similar structures were found by Hubble in Orion, and have been interpreted as gas and dust evaporation from possibly protoplanetary disks (proplyds). The "proplyds" in NGC 3603 are 5 to 10 times larger in size and correspondingly also more massive.

This single view nicely illustrates the entire stellar life cycle of stars, starting with the Bok globules and giant gaseous pillars, followed by circumstellar disks, and progressing to evolved massive stars in the young starburst cluster. The blue supergiant with its ring and bipolar outflow marks the end of the life cycle.

The color difference between the supergiant's bipolar outflow and the diffuse interstellar medium in the giant nebula dramatically visualizes the enrichment in heavy elements due to synthesis of heavier elements within stars.
This true-color picture was taken on March 5, 1999 with the Wide Field Planetary Camera 2. Credit: Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (Univ. Washington), You-Hua Chu (Univ. Illinois Urbana-Champaign), and NASA.