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Star Formation

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Tags: Astronomy, Star Formation

This note is for Chapter 19 of Astronomy Today, 8th Edition.

Star-Forming Regions

  • star-forming regions are observed in many regions of the universe far beyond our own Galaxy
  • main reason that the Sun and other stars don’t collapse: the outward pressure of their heated gases exactly balance gravity’s inward pull

The Formation of Stars Like the Sun


  1. an interstellar cloud: the core of a dark dust cloud or perhaps of molecular cloud.
    • once the collapse begins (might triggered by some external event), fragmentation into smaller and smaller clumps of matter
  2. a collapsing cloud fragment
  3. fragmentation ceases
    • for the first time, the contracting cloud fragment is beginning to resemble a star, and the dense, opaque region at the center is called a protostar
  4. a protostar
    • as the stage-3 fragment contracts, it spins faster (to conserve angular momentum) and flattens into a rotating protostellar disk (for solar system, the disk is also referred to solar nebula) perhaps 100AU in diameter, surrounding the central stage-4 protostar.
  5. protostellar evolution
  6. a newborn star
    • the protostar becomes a true star
  7. the main sequence at last

Stars of Other Masses

The temperatures, densities, and radii of prestellar objects of other masses exhibit similar trends, but the numbers and the tracks differ, in some cases considerably.

  • A star is considered to have reached the main sequence when hydrogen burning begins in its core and the star’s properties settle down to stable values.
  • The main-sequence line predicted by theory is called the zero-age main sequence.
  • the main sequence is itself not an evolutionary track–stars do not evolve along it. It is just a “way station” on the H-R diagram where stars stop and spend most of their lives.

Observations of Cloud Fragment and Protostars

Evidence of Cloud Contraction

  • stage-1: the huge, dark molecular cloud surrounding the visible nebula. both its density and temperature are low
    • regions labeled A and B are denser, warmer fragments
  • between stage 1 and 2: the region labeled “contracting fragment”
  • stage 6 and 7: the emission nebula (M20 itself) results from the formation of one or more massive stars

Evidence of Cloud Fragments

  • way to stage 3: (d) and (e)

Evidence of Protostars

for objects at more advanced stages of star formation, radio techniques become less useful, because stage 4, 5, 6 have increasingly higher temperatures. Their emission shifts toward shorter wavelengths, so these objects shine most strongly in the infrared.

Protostellar Winds

Strong heating within the turbulent disk and a powerful protostellar wind (may be related to the violent surface activity associated with many protostars) combine to produce a bipolar flow, expelling two “jets” of matter in the directions perpendicular to the disk. As the protostellar wind gradually destroys the disk, blowing it away into space, the outflow widens until, with the disk gone, the wind flows away from the star equally in all directions.


image image

Shock waves and Star formation

shock wave: a shell of gas, rushing rapidly through space, can push ordinarily thin matter into dense sheets

  • many astronomers regard the passage of a shock wave through interstellar matter as the triggering mechanism needed to initiate star formation in a galaxy.

Star Clusters

the end result of the collapse of a cloud is a group of stars (star cluster), all formed from the same parent cloud and lying in the same region of space.

clusters and associations

  • open cluster: loose, irregular cluster, found mainly in the plane of the Milky Way
    • typically contain many bright blue stars, indicating that they formed relatively recently
  • associations: less massive, but more extended, clusters
  • globular cluster: roughly spherical, generally found away from the Milky Way plane
    • include no main-sequence stars much more massive than the Sun, indicating that they formed long ago.


clusters and nebulae

At present, the main stages in the formation of individual stars (stages 3-7) are becoming clearer, the answers to the general questions involving stages 1 and 2 are still sketchy.

The precise number of stars of any given mass or spectral type likely depends in a complex (and poorly understand) way upon conditions within the parent cloud.

the cluster environment

physical interactions–close encounters and even collisions–between protostars within a star cluster may be very important in determining the properties of the stars that eventually form.


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