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This set of slides
• This set of slides starts the topic of stellar
evolution, overview, protostars, main
sequence…
• Units covered: 59, 60, 61.
Stellar Evolution – Models and Observation
• Stars change very little over a human lifespan, so it is
impossible to follow a single star from birth to death.
• We observe stars at various stages of evolution, and can
piece together a description of the evolution of stars in
general.
• Computer models provide a “fast-forward” look at the
evolution of stars.
• Stars begin as clouds of gas and dust, which collapse to
form a stellar disk. This disk eventually becomes a star.
• The star eventually runs out of nuclear fuel and dies.
The manner of its death depends on its mass.
Evolution of low-mass stars
Evolution of high-mass stars
Tracking changes with the HR Diagram
• As a star evolves, its
temperature and
luminosity change.
• We can follow a star’s
evolution on the HR
diagram.
• Lower mass stars move
on to the main sequence,
stay for a while, and
eventually move through
giant stages before
becoming white dwarfs.
• Higher mass stars move
rapidly off the main
sequence and into the
giant stages, eventually
exploding in a supernova.
Interstellar Gas Clouds
• Stars begin as a cloud of cold gas
and interstellar dust, a molecular
cloud.
• The cloud begins to collapse in on
itself.
– Collapse is triggered by a variety of
phenomena.
– Stellar winds, explosions, etc.
– Collapse heats the center of the
cloud – gravitational energy is being
converted to heat.
• Rotation of the cloud forces it
into a disk-shape.
• After a million years or so, the
center of the disk develops a hot,
dense core called a protostar.
Protostars
• Once a dense core forms in
the disk, the system has
entered the protostar stage.
• Protostars are difficult to
find – they are shrouded by
gas and dust.
• Infrared telescopes can
detect them.
– Sees through the dust.
– Sees the radiation of the
“cooler” object.
The Eagle Nebula
Bipolar Flows
• Once the protostar
heats to around 1
million K, some
nuclear fusion begins.
• Narrow jets of gas can
form, flinging stellar
material more than a
light-year away.
• These jets can heat
other clouds of gas
and dust.
The birth tracks of low- and high-mass stars
High versus Low Mass
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Low mass stars are stars like our Sun.
Low mass stars are stars with mass < 8 times the mass of our Sun.
High mass stars are stars with mass > 8 times the mass of our Sun.
Most stars are 0.2 to 20 times MSun (over 30 MSun very rare)
Upper limit 150 MSun
Lower limit 0.08 MSun
Below the lower limit, not enough gravity (mass) to produce the
temp and pressure needed to sustain hydrogen fusion.
• 0.016 MSun to 0.08 MSun are brown dwarfs.
• Jupiter is about 75 times too small to have become a star. (17 times
smaller than the smallest brown dwarf.)
From Protostar to Star
• Low-mass protostars become stars very slowly.
– Weaker gravity causes them to contract slowly, so
they heat up slowly.
– Weaker gravity requires low-mass stars to compress
their cores more to get hot enough for fusion to
begin.
– Low-mass stars have higher density.
• High-mass protostars become stars relatively quickly.
– They contract quickly due to stronger gravity.
– Core becomes hot enough for fusion at a lower
density.
– High-mass stars are less dense.
Flowchart of Stellar Structure
The C-N-O cycle
• Low-mass stars rely on the protonproton cycle for their internal energy.
• Higher mass stars have much higher
internal temperatures (20 million K!), so
another fusion process dominates.
– An interaction involving Carbon,
Nitrogen and Oxygen absorbs
protons and releases helium nuclei.
– Roughly the same energy released
per interaction as in the proton-proton
cycle.
The Main-Sequence Lifetime of a Star
• The length of time a star spends fusing hydrogen into helium
is called its main sequence lifetime.
– Stars spend most of their lives on the main sequence.
– Lifetime depends on the star’s mass and luminosity.
• More luminous stars burn their energy more rapidly than
less luminous stars..
• High-mass stars are more luminous than low-mass stars.
• High mass stars are therefore shorter-lived.
• Cooler, smaller red stars have been around for a long time
• Hot, blue stars are relatively young.