Download 3. Stellar Formation and Evolution

Formation of Stars
• Stars are formed within extended regions of higher density
in the interstellar medium.
• These regions are called molecular clouds mainly
composed of hydrogen plus helium.
• As massive stars are formed from molecular clouds, they
powerfully illuminate those clouds, ionizing hydrogen and
creating an H II region.
Protostar formation
1.
The star forming clouds are initially in hydrostatic equilibrium (정수압 평형).
2.
The formation of a star begins with a gravitational instability inside a
molecular cloud
3.
It is triggered by shock waves from supernovae or the collision of two
galaxies. Stellar wind and radiation pressure from massive young stellar
objects may compress interstellar medium.
4.
Once a region reaches a sufficient density of matter when the internal gas
pressure is not strong enough to prevent gravitational collapse (gravitational
instaility), it begins to collapse under its own gravitational force.
hydrostatic equilibrium
• As the cloud collapses, dense dust and gas form 'Bok
globules'.
• As a globule collapses and the density increases, the
gravitational energy is converted into heat. When the
protostellar cloud has approximately reached the stable
condition of hydrostatic equilibrium, a protostar forms at
the core.
• These pre-main sequence stars are often surrounded by a
protoplanetary disk (explain later).
M 83, a barred spiral galaxy
NGC 3603, an open cluster of stars
surrounded by massive cloud
The Antennae Galaxies, very high starburst galaxy occurring from the collision of two galaxies
• Early stars of <2 solar masses are called T Tauri
stars, while those with greater mass are Herbig
Ae/Be stars.
• These newly born stars emit jets of gas along their
axis of rotation, which may reduce the angular
momentum of the collapsing star and result in small
patches of nebulosity known as Herbig-Haro objects.
• These jets, in combination with radiation from
nearby massive stars, may help to drive away the
surrounding cloud in which the star was formed.
Upper limit: 150 M
Radiation pressure too great.
Lower limit: 0.08 M
Too cool for H-fusion to begin
Brown dwarf
(Jupiter mass = 0.001M)
Main Sequence
Main Sequence
• Stars spend about 90% of their lifetime at this stage, fusing
hydrogen to produce helium near the core. Such stars are
said to be on the main sequence.
• Once a star is born, the proportion of helium in a star's core
will steadily increase. As a consequence, in order to
maintain the required rate of nuclear fusion at the core, the
star will slowly increase in temperature and luminosity.
For example, the Sun is estimated to have increased in
luminosity by about 40% since it reached the main
sequence 4.6 billion years ago.
• Fusion process differs by mass.
• For low mass stars, protonproton chain is dominant
process for nuclear fusion at
the core, whereas for high
mass stars, carbon-nitrogenoxygen cycle is dominant.
Electron
Electron
Proton
Proton
1H
2H
(Hydrogen)
Proton
Neutron
Neutron
3H
(Tritium)
Neutron
(Deuterium)
Post-Main Sequence
Massive stars process up to iron
 explode in Supernova events
Low Mass stars stop before iron
 gently blow themselves to death forming planetary nebulas
Red Giant
• When stars > 0.4 M run out their hydrogen fuel in their
core, their outer layers expand and cool to form a red giant.
• In a red giant of up to 2 M, hydrogen fusion proceeds in a
shell-layer surrounding the core. Eventually the core is
compressed enough to start helium fusion. Stars shrinks in
radius and increases its surface temperature.
• After the star has consumed the helium at the core, fusion
continues in a shell around a hot core of carbon and oxygen.
• Eventually the outer layers of the star will be shed, creating
a planetary nebula, with only a white dwarf left behind.
Planetary nebula
Red Supergiant (High Mass Star)
• After a helium-burning runs out of helium fuel in its core,
the star's core starts to collapse and heat up. This causes the
outer layers of the star to expand and cool, similar to the
process that occurred after the star ran out of hydrogen fuel
and left the main sequence. As the star becomes larger and
larger, it eventually becomes a red supergiant.
• Extremely massive supergiants can generate high enough
pressure and temperature to fuse elements even heavier than
carbon and oxygen. Near the end of the red supergiant phase,
a high mass star will develop several "onion layers" of
heavier and heavier elements.
• Eventually stars this massive die …
Death of High Mass Stars: Supernova (Type II)
• The "Type II" supernovae are the result of a massive star
consuming all of its nuclear fuel and then exploding.
• These stars have large H-rich envelopes, hence the presence
of H in the spectra
• Elements heavier than Mg produced during explosion.
Lighter
elements produced during preceding stellar
evolution
Example spectra of Type Ia and Type II SNe
H
H
No H or He
H
S
Ca
Typical Type II Supernova
Si
Typical Type Ia supernova
The Sun
• The Sun is a Population I, or heavy element-rich, star.
– Population I: metal rich
– Population II : metal poor
– Population III: metal free, which is believed to form in the early universe
• The formation of the Sun may have been triggered by
shockwaves from nearby supernovae. This is suggested by a high
abundance of heavy elements in the Solar System, such as
uranium, relative to the abundances of these elements in
Population II stars.
• These elements could most plausibly have been produced by
endergonic nuclear reactions during a supernova, or by
transmutation through neutron absorption inside a massive
second-generation star.
Photosphere (6000K)
Core (13,600,000 K) with
0.25 solar radi:
Thermonuclear reaction
Summary
Gas and Dust
Main Sequence
Large Mass Red Giant
Medium Mass Red Giant
Small Mass Red Giant
Iron Core
Iron Core
Carbon Core
Black Hole +
Supernova Remnant
Neutron Star +
Supernova Remnant
White Dwarf +
Planetary Nebula