Transcript What are supernovae?

The Life Cycle of a Star
The Horsehead Nebula – one of the
most famous pictures in astronomy
Lesson Objectives
• All – To name the stages of a star
• Most – To describe the stages of a star
• Most – To describe the life cycle of a star larger
than the Sun
• Some – To explain why a star moves from one
stage to another
Interstellar medium is the total mass of
the gas and dust between the stars.
• Stars form in clouds of dust and gas called stellar nebulae.
• Gravitational forces cause the large masses of gas to be
squashed together.
• A Nebula is a stellar nursery - a region of dust and
gas where new stars are born.
• The Orion Nebula (M42) is the nearest nebula and
can be seen with the naked eye.
• The first nebula ever photographed (in 1882)
• Its proximity (1500 light years) means that we know
more about it than any other star-forming region.
• It is also in a very active stage of star formation.
• Dense regions in the clouds collapse due to gravity
• As it gets smaller the protostar at its centre gets hotter
• Once the star contracts enough that its central core can
burn hydrogen to helium, it becomes a "main sequence"
star.
• The Sun formed 4.5
billion years ago, as the
Solar System coalesced
from a cloud of gas and
dust.
• The sun is a main
sequence star.
• This is the longest, most
stable period of a star’s
life.
• It converts hydrogen to
helium in its core,
generating heat and
light.
• A stars expands as it grows old.
•As the core runs out of hydrogen and then helium, the core
contracts and the outer layers expand, cool, and become less
bright; forming a red giant.
• Our Sun will run out of fuel in ~5 billion years when it will
expand, forming a red giant engulfing Mercury and Venus.
• A planetary nebula occurs
at the end of a red giant’s
life.
• The outer layers of the red
giant start to drift off into
space.
• This is The Eskimo Nebula
The Cat’s Eye Nebula
The Helix Nebula
When a red giant
collapses, its outer
layers shed off and
what remains is white
dwarf (a very hot,
dense star).
This white dwarf is
Shapley 1 about 1000
light years away from
Earth.
• A black dwarf is a hypothetical star, created when a white dwarf
cools so it no longer emits heat or light.
• Since the time required for a white dwarf to become a black
dwarf is longer than the age of the universe (13.7 billion years),
no black dwarfs exist yet.
• A giant star is much larger
and brighter than a normal
main-sequence star of the
same surface temperature.
• Giant stars can be up to 100
times larger up to 1,000
times brighter than our the
Sun.
• After the hydrogen in a
giant star's core has been
used up, they become red
supergiants - the largest
stars in the universe in
terms of volume.
• These stars have very cool
surface temperatures
(3500–4500 K).
• Betelgeuse is a red supergiant
in the constellation of Orion.
• It is over 600 million miles in
diameter (1,000 times bigger
than the Sun but cooler).
• If Betelgeuse were at the
centre of our Solar System, it
would extend beyond the orbit
of Jupiter.
• It is 520 light-years from Earth.
• A supernova is the death of a large star. It is a
spectacular explosion
• This supernova in the Large Magellanic Cloud (LMC) is
the first to be visible to the unaided eye for almost 400
years.
How are
• In the core of a red super
supernovae formed? giant, lighter elements fuse
to form iron.
• Iron nuclei absorb energy
when they fuse and so the
process slows down.
• Decreased pressure in the
core, means the outer
layers are not held up and
so they collapse inwards.
• As the core is so dense, the
outer material collides and
bounces off, resulting in a
huge explosion.
What are supernovae?
• A dying star that
explodes violently,
producing an extremely
bright object for weeks
or months.
• They emit visible, infra
red and X ray radiation.
• Temperatures rise to 10
billion K.
• Enough energy to cause
medium weight
elements to fuse,
forming heavy elements
(up to Uranium in the
Periodic Table).
Supernovae are rare – once every century in a typical galaxy.
The remnants of a
supernova in the
constellation Cassiopeia,
all that can be seen by
astronomers.
But the core remains…
• A black hole can be created when a giant star undergoes a
supernova.
• A star with a mass greater than 20 times the mass of our
Sun may produce a black hole at the end of its life.
• Black holes are objects so dense that not even light can
escape their gravity and since nothing can travel faster
than light, nothing can escape.
If the mass of the core of a supernova is less than
2.5 solar masses, it becomes a neutron star.
Calvera, the
closest neutron
star found in the
constellation
Ursa Minor
It is made almost entirely from neutrons, compressed
like a giant atomic nucleus.