Download Lecture 10 - University of Minnesota

Lecture 10
6/20/07
Astro 1001
Basics
• We can’t observe any star going through multiple
stages of their lifetime
– Can observe multiple stars in different phases of their
lifetime
• Two to Three new stars formed a year
• Gas and dust in between the stars is called the
Interstellar Medium
The ISM
• Can use spectroscopy to
determine the composition of
the ISM
– 70% hydrogen, 28% helium, 2%
other stuff
• Density and temperature of gas
varies greatly
• Stars form in coldest, densest
clouds of molecular gas
• Interstellar dust is also an
important part of the ISM
Why Do Stars Form?
• Gravity causes clouds to contract
– Continues until the central object
becomes hot enough to do fusion on
its own
• Star formation doesn’t happen
everywhere because gas pressure
can prevent gravity from
collapsing the cloud
– Called thermal pressure
• Exploding star might help trigger
the collapse of the cloud
Clusters and Stars
• Most stars are born in
clusters of thousands of
stars
– Average cloud mass is
1000x that of the Sun
• Several additional
sources prevent gravity
from going nuts
– Magnetic fields
– Turbulent motion
Group Work
• The mathematical insight on page 532-533
shows how the minimum mass of a star
forming cloud varies with density.
Following these examples (especially the
ones on page 533), figure out how dense the
could would have to be to form a single, 1
solar mass star. What does this say about
why stars usually form in clusters?
Fragmentation
• Collapse of a cloud results
in many smaller stars
instead of one huge star
– Clouds are turbulent and
lumpy
– Small clumps will
individually collapse
• Isolated stars can also form
– This process has been
observed
– Not fully understood
The First Generation of Stars
• Astronomers call elements other than hydrogen
and helium “metals”
– Metallicity is a measure of how much of something is
made out of metals
• Original stars had essentially 0 metallicity
– Were very large
– Didn’t live long
– Provided the metals for all prior generations of stars
Stages of Star Birth
• Protostar
– Looks a lot like a real
star
– No nuclear fusion
• Accretion
– Matter is drawn onto
the protostar by gravity
Details of Star Formation
• A protostellar disk also
forms around the
protostar
• A protostellar wind
forces particles off into
space
• Protostellar jets often
form
• Binary stars often formed
The Genesis of Nuclear Fusion
• Protostar gravitationally collapses
– About half of the energy is trapped in the star
– Raises temps from about a million degrees to
about 10 million degrees
– Might take millions of years to do
Degeneracy Pressure
• Recall that the Exclusion Principle doesn’t
allow particles to be packed too close
together
• In order for stars below about .08 solar
masses, you would need to violate the
Exclusion Principle in order to reach
necessary densities
Brown Dwarfs
• Brown Dwarfs are on the
dividing line between
planets and stars
• Would have been stars, but
degeneracy pressure halted
their collapse
• Very dim
– Shine only due to gradual
cooling of their interior
The Biggest Stars
• As stars get larger, they
create more and more
pressure
– Very large stars create
primarily Radiation
Pressure
• Radiation pressure would
blow apart a star if it was
much over 150x the mass
of the Sun
Initial Mass of Stars
• Small stars are much more likely than huge
stars
• Most stars are less massive than the Sun
Quiz Review
Mass and Fusion
• Large stars have much more gravity that has
to be balanced by more pressure
– Hence they have a greater rate of fusion and
much greater luminosities
• When a star runs out of hydrogen, it has to
do something new
– Might fuse heavier elements
– Might collapse and die
Types of Stars
• Low mass stars
– Less than 2 solar masses
– Most common type
• Intermediate mass stars
– Between 2 and 8 solar masses
– Won’t talk much about these
• High mass stars
– Greater than 8 solar masses
– Rare
– Have a very great effect on their surroundings
Low Mass Star Basics
• Spend about 10 billion years
turning hydrogen into helium
via the proton-proton chain
• Size of convective zone varies
with mass
– Low mass stars can be almost
entirely convective zones
– High mass stars have no
convective zones, but a
convective core
– No convective zone means that
the star can be a very violent
flare star
Red Giant Stage
• Core can no longer support
itself and shrinks
• Outer layers (called the
envelope) still has hydrogen,
which will start to burn
• Eventually the core will get
hot enough to burn helium
– Helium fusion stars off
violently with the helium flash
– Is now on the Horizontal
Branch
The Death of the Sun
• Through winds, the
Sun will eject its
outer layers
– The Core will be
exposed and is now
a White Dwarf
– The WD will light
up the gas around it
– Forms a Planetary
Nebula
Massive Stars
• Early life similar to that of low mass stars,
but faster
• Use the CNO Cycle to generate energy
– End result is to turn 4 hydrogen atoms into 1
helium atom
• High mass stars go through similar stages
when they run out of hydrogen fuel at first
Heavy Elements
• Massive stars can get so hot in their
core that they can fuse carbon (and
maybe other elements)
– Can produce oxygen, neon, magnesium
up to iron
• Iron can’t be fused and give you
energy
• This picture is confirmed by
observations
– Young stars have higher metallicities
– Even numbered elements much more
common than odd numbered elements
Death of a High Mass Star
• Electrons combine with
protons, so the pressure
instantly vanishes
• Star collapses, releasing
tremendous amounts of energy
• Star explodes in a supernova
– Might form a neutron star held
together by neutron degeneracy
pressure
– Might be so massive that it
forms a black hole
Supernova Observations
• Supernovae are so bright that
they can appear from nowhere
(or even shine during the day)
• A supernova helped prove that
Kepler was right about the
heavens being able to change
• In 1987, modern science got
its first look at a supernova