Download Star Formation

Star Formation
Star Chapter 19:
A Traumatic Birth
Standards
 Understand the scale and contents of
the universe, including
• objects such as stars
 Understand the concept of equilibrium
 Stars evolve as they consume
their fuel supply
Formation of stars like the sun
 Gravitational competition
• Star formation begins when a nebula
in interstellar space starts to collapse
under its own weight
• The nebula shrinks and heats up
• The center becomes hot enough to
start fusion
• Contraction stops and a star is born
Formation of stars like the sun
 A star is in equilibrium: gravity pulling in
exactly balances fusion pressure
pushing out
Stages of Star Formation
 Stage 1: An Interstellar Cloud
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Cloud of cold atomic & molecular gas
Thousands of times mass of sun
Starts to collapse
Begins to fragment
Stages of Star Formation
 Stage 2: A Collapsing Cloud Fragment
• Sun-like star: fragment is 1 – 2 solar
masses
• ~100 x size of solar system
• Temperature & pressure increase and
fragmentation stops
Stages of Star Formation
 Stage 3: Fragmentation Ceases
• Several tens of thousands of years
after initial contraction
• Size of solar system
• Heats up & begins to resemble a star
– called a protostar – has surface
Stages of Star Formation
 Stage 4: Protostar
• Shrinks, density & temperature
increase
• Size of Mercury’s orbit
• Can be plotted on H-R diagram
• Violent surface activity
• Strong winds
• Called T Tauri star
Stages of Star Formation
 Stage 5: Protostellar Evolution
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•
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~10 x size of sun
Central temperature = 5,000,000 K
Gas is ionized
Evolution slows
Stages of Star Formation
 Stage 6: Newborn Star
• 10 million years = true star
• Central temperature = 10,000,000 K =
fusion starts: H2  He
Stages of Star Formation
 Stage 7: Main Sequence at Last
• Endpoint of prestellar evolution, no
matter what mass star has
• When fusion starts & star becomes
stable
Stars of Other Masses
• The time required for an interstellar
cloud to become a main sequence
star depends strongly on its mass
• The most massive O stars reach the
10 million Kelvin needed to start
fusion in a million years (1/50 time
taken by sun)
• An M-type star less massive than our
sun takes one billion years to form
Stars of Other Masses
 Whatever the mass, the endpoint of the
prestellar evolutionary track is the main
sequence
• A star is considered to have reached
the main sequence when hydrogen
burning (fusion) begins in the core
and its properties settle down to
stable values (i.e., it’s in equilibrium)
Stars of Other Masses
• Zero-age main sequence (ZAMS) –
main sequence band predicted by
theory. Agrees well with observed
main sequence stars, which provides
strong support for the modern theory
of star formation and stellar structure
Stars of Other Masses
 The composition of a star affects its
internal structure, which affects both its
temperature and luminosity, and thus its
place on the main sequence
• Stars with more heavy elements are
cooler and slightly less luminous than
stars of the same mass with fewer
heavy elements
Stars of Other Masses
 The main sequence is not an
evolutionary track – stars do not evolve
along it
• It is a way station where stars stop
and spend most of their lives
• Once on the main sequence, a star
stays in the same location on the H-R
diagram until it runs out of fuel and
begins to die
Failed Stars
 Some cloud fragments are too small to
become stars
• Jupiter is a good example
• There is not enough mass to start
fusion. They continue to cool,
eventually becoming compact, dark,
cold fragments of unburned matter
• They are known as brown dwarfs
• Note: Jupiter would need to be 80
times more massive to start fusion
Failed Stars
• The minimum mass needed to start
fusion is about 0.08 solar masses
• There may be vast numbers of these
objects throughout the universe, but
they are difficult to detect
Observations of Cloud Fragments
and Protostars
 The age of our entire civilization is much
shorter than the time needed for a cloud
to contract and form a star.
• We can never observe individual
objects proceed through a full
panorama of star birth
• We can, however, observe many
different objects at different stages of
stellar evolution