Transcript Interstellar clouds

OBJECTIVES:
1. Know the phases of star formation
starting with the interstellar cloud.
2. Understand how the life cycle of both
low and high mass stars contribute to
the makeup of the universe.
3. Know some of the particular stages of
the life of low and high mass stars.
The Interstellar Medium
• The interstellar medium of the milky way
consists of clouds containing 90% Hydrogen,
9% Helium, and 1% dust grains.
• It has an extremely low density of 1 atom per
cm3, about 10 billionth the density of gas
created by our best vacuums here on earth.
Interstellar clouds
• Interstellar clouds: dense regions where stars
may form.
• They are not uniform, some areas are denser
and collapse faster.
• The collapsing cloud fragments into dense star
forming cores called molecular cloud cores.
Protostars
• Protostars: gravitational energy is being
converted into thermal energy, and material
falls on the accretion disk.
• The protostar is thousands of time more
luminous than the star it will form, and
hundreds of times larger.
From Protostar to Star
• A balance between gravity and pressure is
always maintained.
• As a protostar radiates out thermal energy it
shrinks, and the center becomes more dense.
• As more material falls on the protostar it
contracts, temperature and pressure in the
core climb until fusion of hydrogen begins.
• A star is cable of nuclear fusion.
Star Clusters
• A closely knit collection of stars, such as
Pleiades (seven sisters).
• It would take approximately 30 million years
for the collapse of the molecular cloud to form
a star similar to sun.
Low-Mass Stellar Evolution
• Our sun is typical low-mass star with a size of
1 M (M stands for Mass of the sun)
• Low mass stars are 3M or smaller.
Luminosity of a Star
• Luminosity is determined by the amount of
fuel that it is burning.
• Our sun fuses 4 billion kilograms of hydrogen
per second.
• In some 5 billion years from now the sun’s life
will come to an end.
• The more massive a star the shorter its life
time, since increasing the mass/gravity
increases the rate of nuclear fusion.
Subgiant
• When a low mass star fuses the majority of its
hydrogen, it begins to expand and cool
forming a subgiant.
Red Giant
• When the star can cool no further it becomes
a red giant.
• Helium builds up in the core creating a
degenerate helium core.
• As the remaining hydrogen fuses, in the Hburning shell, the Helium core grow in density
but not in volume.
Asymptotic giant branch (AGB)
• Helium begins to fuse in the core creating
carbon-12 mass amounts of alpha particle
radiation.
• This carbon core drives up the strength of
gravity and pressure in the core.
• The radius may now be 100 of times an typical
1 M star.
White Dwarf
• Within 50,000 years a post-AGB star burns all
fuel on its surface leaving a tiny cinder of
carbon with a remaining mass of less than
70% of the original star.
• White dwarfs have immense gravity, some
with the mass of sun and a volume of the
earth.
Planetary Nebula
• Planetary nebula may form around low-mass
dying stars
• Form when the mass ejected by the AGB star
piles up in a dense expanding shell.
• Planetary nebula are visible for about 50,000
years of so, and can be illuminated by a white
dwarf.
nova
• A possible fate of a white dwarf that is part of
a binary star system.
• A white dwarf’s immense gravity can pull
matter from a companion star forming an
accretion disk around the white dwarf.
• More material more gravity, more gravity the
white dwarf shrinks, temperatures increase
and any hydrogen gas explodes in a nova.
• This cycle can repeat itself many time.
Type 1 supernovae
• Through millions of years of mass transfer and
countless nova outbursts, the mass of the
white dwarf increases.
• There comes a point when the pressure can
no longer balance gravity and the temperature
of 6 X 108 K fuse carbon nuclei up to element
26 Iron (Fe).
• At that point the energy of fusion is liberated
in an explosion that sends the material across
the cosmos destroying the white dwarf.