11 Star Formation

1. Star Forming Regions (SFR)

  • Star formation is a continuous process
  • Star forming regions are observed in all corners of universe

  • Part o ISM collapses under its own weight

➤ cloud shrinks ➤ heating up

➤ if the center is hot enough ➤ nuclear burning

➤ contraction stops

➤ star is born

  • Question: Why didn't all the matter collapse long ago?

When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion

Rotation can also interfere with gravitational collapse, as can magnetism.

Clouds may very well contract in a distorted way.

Struggle between Gravity & Heat

  • Temperature of the cloud is low but random motion exists
  • Gravitational attraction is ON
    • however, accidental clustering of atoms disappears quickly
  • Thus, as the clustering size increases
    • GRAVITY gets stronger
    • but still it is not enough; therefore HEAT > GRAVITY holds
  • So, the question is:
    • How many atoms do we need for GRAVITY to win:


    • Hint-1: Number of sands on all the beaches are 1025
    • Hint-2: In our entire planet there are 1051 elementary particles (in all nuclei)

2. Formation of Sun like stars

  • Star formation begins when gravity begins to dominates over heat.

  • This causes the cloud to lose its equilibrium and starts contracting.

  • Equilibrium can only be restored when internal structure undergoes several radical changes.

  • The table lists stages suitable for a star having a mass as that of the Sun.

Stage 1: An Interstellar Cloud

  • R ~ tens of parsecs (1014 – 1015 km)
  • T ~ 10 K
  • ρ ~ 109 particles/m3
  • M ~ x 103 M (in the form of cold atomic & molecular gas)
    • Dust will be needed to cool the cloud, but it is not enough yet)
  • Instability in the interstellar cloud causes it to collapse
  • Cloud break-up into smaller pieces
  • This usually happens in "Star Forming Regions"
  • Instability: (a) other stars, (b) magnetic field
    • Creates fragmentation
      • no turning back ➤ collapse
    • Opt-1: a few dozen stars ( Rstar > R )
    • Opt-2: hundreds of stars ( Rstar ~ R )
  • No evidence of "1 cloud ➤ 1 star"

Stage 2: A Collapsing Cloud Fragment

  • R ~ a few parsecs
  • T ~ 100 K (still cold)
    • energy escapes easily because fragment is thin
  • ρ ~ 1012 particles/m3
  • M ~ 1-2 M

  • Tcenter > Toutside
  • If "ρ increases ➤ R decreases"
    • radiation starts to get trapped
    • which increases temperature
    • and causes fragmentation to stop

Stage 3: Fragmentation Ceases

  • R ~ our solar system
  • Tcenter ~ 10 000 K
    • opaques ➤ energy kept inside ➤ causing T to increase
  • Tsurface ~ 100 K
    • it radiates into space ➤ it remains cool
  • ρcenter ~ 1018 particles/m3
  • ρcenter (change) > ρcouter (change)

  • It resembles like a star:
    • dense central region becomes protostar
  • Surface (photosphere) can be distinguished

Stage 4: A Protostar

  • Evolves: smaller, denser, hotter
  • R ~ 108 km (Mercury's orbit)
  • Tcenter ~ 106 K
    • 107 K needed for nuclear reactions
  • Tsurface ~ 3 000 K

  • T is low but R is big ➤ L is large
    • because as it shrinks it releases gravitational energy
  • It makes its first appearance on the H-R diagram.

Last Stages

  • Protostar is not in equilibrium
    • But Tcenter is hot ➤ INTERNAL PRESSURE ~ GRAVITY
  • Energy escapes
    • It diffuses out from hot center to cooler surface
    • Overall contraction slows down but doesn't stop
  • The path of decreasing luminosity (from stage 4 to stage 6) is called the Hayashi track.
    • It exhibits violent surface activity ➤ protostellar winds
  • At stage 7, the newborn star has arrived on the main sequence.

Stage 5: Protostellar Evolution

  • R ~ 10 R
  • Tcenter ~ 5 x 106 K
    • 107 K needed for nuclear reactions
  • Tsurface ~ 4 000 K
  • L ~ 10 L

  • Gas is completely ionized but protons don't have enough thermal energy
    • core is still not hot enough
    • evolution slows down due to HEAT
      • gravity cannot compress a hot object
  • Rate of energy produced internally escapes to surface:
    • is almost equal to contraction rate of the star
    • If L decreases ➤ contraction rate decreases too

Stage 6: A newborn star

  • 1 M - R ~ 106 km - T ~ 107 K ➤ Nuclear Burning
  • Bottom of Hayashi track
  • Tsurface ~ 6 000 K

Stage 7: The Main Sequence At Last

  • Star contracts a little more
  • ρ ~ 1032 particles/m3
  • Tcenter ~ 15 x 106 K
  • Tsurface ~ 6 000 K

➤ A Main Sequence star


➤ Nuclear energy generated (core) Energy radiated (surface)

  • Life-time ~ 1010 years
  • Age (Stage 1-7) ~ 40-50 x 106 years

3. Stars of Other Masses

  • The Main Sequence (MS) is a band, rather than a line, because stars of the same mass can have different compositions.
    • Most important: Stars do not move along the Main Sequence!
    • Once they reach MS, they are in equilibrium and do not move until their fuel begins to run out.

Zero-Age Main Sequence (ZAMS)

  • Massive Cloud Fragments ➤ MS approach: higherMassive Stars
    • In 106 years ➤ Tcenter ~ 107 K ➤ O-type hot stars
  • Low-mass Cloud Fragments ➤ MS approach: lowerLow-mass Stars
    • In 109 years ➤ Tcenter ~ 107 K ➤ M-type cool stars (M < 1 M )
  • ZAMS is the stage when the Hydrogen burning (in the core) stage starts
  • During the fragmentation from the same cloud (composition & elements are the same)
    • M determines the star's position on H-R diagram
    • So, difference in star's composition ➤ make star appear at different places on H-R
      • Therefore this creates a broad band on H-R (MS).

Failed Stars

  • Some fragments are too small for fusion ever to begin.
    • They gradually cool off and become dark “clinkers.”
    • A protostar must have at least 0.08 M in order to have fusion at core.
    • If Mfailed star > 12 Mjupiter it is luminous when first formed
      • A brown dwarf.

4. Observations of Cloud Fragments

Star Formation Phases

  • The M20 region shows observational evidence for three broad phases in the birth of a star.
  • The parent cloud is in stage 1.
  • The region labeled “contracting fragment” likely lies between stages 1 and 2.
  • Finally, the emission nebula (M20 itself) results from the formation of one or more massive stars (stages 6 and 7).

Orion Nebula

  • (a) The constellation Orion, with the region around its famous emission nebula marked by a rectangle.
  • (b) It suggests how the nebula is partly surrounded by a vast molecular cloud. Various parts of this cloud are probably fragmenting and contracting, with even smaller sites forming protostars.
  • The three frames at the right show some of the evidence for those protostars:
  • (c) false-color radio image of some intensely emitting molecular sites,
  • (d) nearly real-color visible image of embedded nebular “knots” thought to harbor protostars, and
  • (e) high-resolution image of one of many young stars surrounded by disks of gas and dust where planets might ultimately form.


  • (a) An edge-on infrared image of a planetary system-sized dusty disk in the Orion region, showing heat and light emerging from its center.
  • On the basis of its temperature and luminosity, this unnamed source appears to be a low-mass protostar on the Hayashi track (around stage 5) in the H–R diagram.
  • (b) An optical, face-on image of a slightly more advanced circumstellar disk surrounding an embedded protostar in Orion.

5. Shock Waves & Star Formation

Shock waves from nearby star formation can be the trigger needed to start the collapse process in an interstellar cloud.

  • (a) Star birth and
  • (b) shock waves lead to
  • (c) more star births and more shock waves in a continuous cycle of star formation in many areas of our Galaxy.
  • As in a chain reaction, old stars trigger the formation of new stars ever deeper into an interstellar cloud.
  • Expanding waves of matter:
    • They crash into surrounding molecular cloud
    • Interstellar gas tends to pile up and become compressed
    • Such a shell of gas rushing rapidly through space: shock wave
  • Other triggers:
    • Death of a nearby Sun-like star
    • Supernova
    • Density waves in galactic spiral arms
    • Galaxy collisions

6. Star Clusters

The end result of cloud collapse is a group of stars. They are all formed from the same parent cloud and lying in the same region of space.

Star Cluster:

  • the same place
  • the same parent cloud
  • formed at the same time
  • the same conditions
  • but different masses

Open Cluster

  • Found in the plane of Milky Way
  • They contain from 100s to 10 000s of stars
  • They contain stars in almost all parts of MS
  • Age: ~ 100 x 106 years (life-time of B-type stars)
  • Red stars are young too

Globular Cluster

  • Spherical in shape
  • Found away from Milky Way plane.
  • They contain 100 000s to 1 000 000s of stars
  • They spread over 50 pc
  • The lack upper MS stars
    • O-type through F-type stars have already gone from MS
  • Age: at least 1010 years (contain oldest stars of the Galaxy)