Star Formation

1

• Star Formation

2
Gravitational Competition

• Simply stated, star formation begins when part
  of the interstellar medium one of the cold, dark
  clouds, starts to collapse under its own weight.
• The cloud fragment heats up as it shrinks, and
  eventually its center becomes hot enough for
  nuclear burning to begin. gt

3
Gravitational Competition

• At the point of nuclear burning the contraction
  stops, and a star is born gt

4
Gravitational Competition

• Why dont all clouds form stars?
• When a few atoms accidentally cluster for an
  instant, their combined gravity is insufficient
  to bind them into a lasting, distinct clump of
  matter. gt

5
Gravitational Competition

• An accidental cluster of atoms will disperse as
  quickly as it forms.
• The effect of heatthe random motion of the
  atomsis much stronger than the effect of
  gravity. gt

6
Gravitational Competition

• Imagine, for example, 50, 100, 1000, even a
  million atoms, each gravitationally pulling on
  all the others.
• If you have a large number of atoms the
  gravitational pull is still insufficient to
  prevent dispersion of the clump of atoms.gt

7
Gravitational Competition

• the temperature of a gas is simply a measure of
  the average speed of the atoms or molecules in it
  
• the higher the temperature, the greater the
  average speed, and thus the higher the pressure
  of the gas. gt

8
Gravitational Competition

• This is the main reason that the Sun and other
  stars don't collapse.
• The outward pressure of their heated gases
  exactly balances gravity's inward pull. gt

9

• How many atoms must be accumulated for their
  collective pull of gravity to prevent them from
  dispersing back into interstellar space?
• Nearly 1057 atoms are required
• This is even larger than the 1051 elementary
  particles that constitute all the atomic nuclei
  in our entire planet. gt

10
Gravitational Competition

• Rotationthat is, spincan also compete with
  gravity's inward pull.
• a contracting cloud having even a small spin
  tends to develop a bulge around its midsection. gt

11
Gravitational Competition
12
Gravitational Competition

• For material to remain part of the cloud and not
  be spun off into space, a force must be
  appliedin this case, the force of gravity.
• more mass is needed for a rapidly rotating
  interstellar cloud to contract to form a star
  than is needed for a cloud having no rotation at
  all. gt

13
Gravitational Competition

• Magnetism can also hinder a cloud's contraction.
  
• The particles tend to become "tied" to the
  magnetic fieldfree to move along the field lines
  but inhibited from moving perpendicular to them.
  gt

14
Gravitational Competition

• Theory suggests that even small quantities of
  rotation or magnetism can compete quite
  effectively with gravity and can greatly alter
  the evolution of a typical gas cloud. gt

15
Formation of Stars

• Star formation begins when gravity begins to
  dominate over heat, causing a cloud to lose its
  equilibrium and start to contract.
• There are 7 stages as the cloud becomes a star.
  gt

16
Formation of a star step 1

• The first stage in the star-formation process is
  a dense interstellar cloud
• Stage 1 clouds contain thousands of times the
  mass of the Sun, mainly in the form of cold
  atomic and molecular gas. gt

17
Formation of a star step 1

• it must become unstable and eventually break up
  into smaller pieces.
• theory suggests that once the collapse begins,
  fragmentation into smaller and smaller clumps of
  matter naturally follows gt

18
Formation of a star

• a typical cloud can break up into tens,
  hundreds, even thousands, of fragments, each
  imitating the shrinking behavior of the parent
  cloud and contracting ever faster. gt

19
(No Transcript)
20
Formation of a star

• an interstellar cloud can produce either a few
  dozen stars, each much larger than our Sun, or a
  whole cluster of hundreds of stars
• There is little evidence of stars born in
  isolation, one star from one cloud. gt

21
Formation of a star

• The Sun, which is now found alone and isolated
  in space, probably escaped from the multiple-star
  system where it formed gt

22
STAGE 2 A COLLAPSING CLOUD FRAGMENT

• A fragment destined to form a star like the Sun
  contains between one and two solar masses of
  material at this stage
• this fuzzy, gaseous blob is still about 100 times
  the size of our solar system gt

23
STAGE 2 A COLLAPSING CLOUD FRAGMENT

• the gas constantly radiates large amounts of
  energy into space. The material of the fragment
  is so thin that photons produced within it easily
  escape without being reabsorbed by the cloud gt

24
STAGE 2 A COLLAPSING CLOUD FRAGMENT

• As stage 2 fragments continue to contract, they
  become so dense that radiation cannot get out
  easily. The trapped radiation causes the
  temperature to rise, the pressure to increase,
  and the fragmentation to cease. gt

25
STAGE 3 FRAGMENTATION CEASES

• a typical stage 2 fragment has shrunk by the
  start of stage 3 to roughly the size of our solar
  system
• The inner regions have just become opaque to
  their own radiation and so have started to heat
  up gt

26
STAGE 3 FRAGMENTATION CEASES

• The central temperature has reached about 10,000
  Khotter than the hottest steel furnace on Earth.
  
• It is still able to radiate its energy into space
  and so remains cool. The density increases much
  faster in the core of the fragment gt

27
STAGE 3 FRAGMENTATION CEASES

• The dense, opaque region at the center is called
  a protostar
• the interior of a collapsing fragment of gas is
  sufficiently hot and dense that it becomes opaque
  to its own radiation gt

28
STAGE 3 FRAGMENTATION CEASES

• After stage 3, we can distinguish a "surface" on
  the protostarits photosphere gt

29
STAGE 4 A PROTOSTAR

• As the protostar evolves, it shrinks, its
  density grows, and its temperature rises, both in
  the core and at the photosphere
• 100,000 years after the fragment began to form,
  it reaches stage 4, where its center seethes at
  about 1,000,000 K gt

30
STAGE 4 A PROTOSTAR

• the temperature is still short of the 107 K
  needed to ignite the protonproton nuclear
  reactions that fuse hydrogen into helium. gt

31
STAGE 4 A PROTOSTAR

• our gassy heap is now about the size of
  Mercury's orbit
• Knowing the protostar's radius and surface
  temperature, we can calculate its luminosity gt

32
STAGE 4 A PROTOSTAR

• its total luminosity is very large
• By the time stage 4 is reached, our protostar's
  physical properties can be plotted on the
  HertzsprungRussell (HR) diagram gt

33
STAGE 4 A PROTOSTAR

• its temperature is now so high that
  outward-directed pressure has become a powerful
  countervailing influence against gravity's
  continued inward pull gt

34
(No Transcript)