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STELLAR EVOLUTION NOTES
* Birth of Stars *
 What is a star?
a celestial body undergoing fusion
 What is stellar evolution?
the changes that occur to a star as it ages
 How is a star’s life like a human life? birth, mature life & death
 Why is the Sun a star while the Earth and Moon are not?
Earth & moon don’t
undergo fusion
 Ejnar Hertzsprung of Denmark and Henry Russel of the USA independently
discovered a basic link between the temperature and luminosities of stars. We
still refer to their graph of that relationship as the
H-R diagram
 Most stars lie on the band known as the main-sequence, the position of a star on
this band is a result of the star’s mass . The larger the star on the main-sequence
the brighter and hotter the star will be.
 Exceptions to the main-sequence of stars are the red giants, super giants, white
dwarfs
 The H-R diagram was important in our understanding of stellar evolution because
it capture stars in different stages of development and astronomers were able to
piece together the life history of a star using the current laws of physics.
 Where are stars born? A stars life begins in the inter-stellar medium (space
between stars) as a cloud of gas and dust (nebula)
 What is the composition of the nebula? The nebula is made of all the known
elements, but most of the nebula is composed of hydrogen and helium
 What is the origin of a nebula? The origin of the nebula is either a supernova
remnant (atoms heavier than helium) or the Big Bang (atoms of hydrogen &
helium)
 Does this nebula become a star on it’s own? NO , thermal motion (heat) &
turbulence (from nearby stars) keep the nebula from contracting.
 What happens to start the process of condensing this nebula into a star? A
shockwave acts as a catalyst (from a distant supernova) to compress & disrupt
the nebula into fragments.
 Gravity then causes the cloud to collapse.
 Friction builds as the particles collide, the cloud grows denser, radiates heat and
radio waves into space and it is not called a protostar.
 When the temperature inside the protostar reaches 20 million degrees F, the
atoms smash together so violently that they over come their natural repulsion &
the protons combine and a process known as fustion begins and a star is born.
 Hydrogen is fused into Helium with a great release of energy. (E = Mc2)
 (E = Mc2) is the relationship between matter & energy, which shows that the two
are interchangeable and that small amount of mass can produce lots of energy.
 This formula helped determine the process a star uses to live for a long time.
 What happens to a body that never reaches 20 millions degrees? A nebula never
reaching the fusion stage is a called a brown dwarf also smaller bodies that
revolve around stars are called planets.
 The protostar stage is short and when a protostar is still surrounded by its original
nebulosity it is called a cocoon star.
* Main Sequence Stars *
 When the expansion due to hydrogen fusion in a star’s core balances the stars
own gravity, a stable state of equilibrium exists and the star is called a
main-sequence star
 The size of a MS star is determined by it’s
birth mass . The smaller stars are
much more numerous.
 The color of a MS star is determined by its surface temperature .
 Stars spend 90% of their lives as a hydrogen fusing MS star.
 There are seven major categories of MS stars. Stars are classified by their
temperature and they are designated by letters O B A F G K M .
 Oh Be A Fine Guy (or Girl) Kiss Me , is a silly little pneumonic verse used by
astronomers to remember the order of the classification letters. Originally, they
were in alphabetical order but due to errors they were reassigned into these letter
classifications.
 Class O blue giants consume their hydrogen quickly & die in only 7 millions
years. Class G stars like our Sun will live over 10 billion years. Small red
dwarfs ( class M ) may last 300 billion to trillion years. Since the universe is
believed to be only 15 billion years old, none of the class M stars have yet died a
natural death .
 Eventually the usable hydrogen in a star’s core becomes exhausted .
 The helium ash is NOT fusible at these temperatures so the fusion stops and
the core collapses under the weight of the outer layers.
 Frictions from the collapse raises the internal temperature & the unfused
hydrogen in the stars outer layers begins to fuse. The stars grows, reddens, &
becomes a red giant .
 Does this process happen to all stars? Almost, only the low mass stars cannot
generate enough internal heat for this stage and burns out .
 The core continues to collapse and the gravitational energy becomes great
enough to eventually fuse helium some stars do this in a helium “flash”.
 Helium fusion produces carbon & oxygen ash and the core contracts & grows
hotter. Subsequent contractions fuse oxygen, silicon, and other heavier
elements. This is known as a pulsating star .
 Stars that pulsate in a regular pattern are very important to astronomers as they
can be used to determine distances in space and are called Cepheid variable
stars.
* Death of Stars *
 How a star dies is dependent upon its mass at birth
 Stars .4 solar masses or less ( red dwarfs ) are completely mixed and have very
little hydrogen when they die. They cannot ignite helium, so they slowly contract
& heat up, becoming white dwarfs. Eventually they burn themselves out
 Stars between .4 & 4 solar masses evolve the same way, they can fuse helium but
cannot fuse carbon. After the pulsating stage the star ejects its outer layers
forming a planetary nebula. The remaining core of the star becomes a white
dwarf . A WD star’s name is misleading since they cool down through white,
yellow, orange, & finally red.
 A WD does not shine by fusion anymore but because it is slowly leaking it’s own
heat into outer space. A WD is very dense , one cubic foot of a WD is 1500
tons!
 In a binary system when one star is WD and the other is a MS star that is
expanding, the MS star can lose mass to the WD. The WD will ignite the mass in
an eruption that cause the WD to temporarily “flair up” (10 magnitudes higher
or more), this is called a nova and the process can be reoccurring.
 The most massive stars can get hot enough to fuse carbon and other elements.
After they go through the giant stage they eventually fuse elements through iron.
Iron cannot release energy through fusion reactions, as a result the core becomes
unstable and produces a supernova explosion.
 The final result of a supernova explosion can be that it blows off all or most of
the star’s material at high speeds. A small proportion of stars that reach an
unstable state late in their evolution will blow away it’s outer layers but leave its
collapsed core, which will turn out to be a neutron star or a black hole .
 A neutron star occurs following a supernova explosion in which the surface of a
star is ejected and the remaining core then collapses under its own weight &
the subatomic particles are compressed together, forcing the protons to merge
with the electrons, producing a star that is completely composed of neutrons.
The resulting neutron star is very dense, has a strong magnetic field, and is
rapidly rotating. As neutrons stars spin they emit beams of radiation from their
magnetic poles. If the beams sweep over the Earth we detect a pulse or the
blinking of its light. The neutron stars are known as a pulsar . They have
diameters of 15-30 miles. They may spin up to 1,000 times per second. They
eventually slow & burn out.
 The largest stars also go through the supernova stage. However the remaining
core is massive enough to have the gravitational attraction to keep light from
escaping. This occurs because the collapsing star contracts to such a small size
that the concentrated force of gravity has an escape velocity greater than the
speed of light . The resulting object is called a black hole . A black hole’s
event horizon marks its boundary inside of which all information is trapped.
There is an infinitely dense singularity (point of gravitation) in the middle of a
black hole.