Transcript stars ppt - Teachers

Star Sequence (Life Cycle)
 Born in Stellar Nebula
 Stars are large balls of hot gas.
 They look small because they are a long way
away, but in fact many are bigger and
brighter than the Sun.
 Stars are made (or “born”) in giant clouds of
dust and gas.
 Sometimes part of the cloud shrinks because
of gravity.
 As it shrinks it becomes hotter and when it is
hot enough, nuclear reactions can start in the
centre…..
 … and A Star is Born!
 Once nuclear fusion is producing heat in the
centre of the new star, this heats stops the
rest of the star collapsing.
 The star then stays almost exactly the same
for a long time (about 10 billion years for a
star like the Sun).
 The balance between gravity trying to make
the star shrink and heat holding it up is called
Thermodynamic Equilibrium.
 Depending on the star’s mass it becomes one
of three general types of stars
 Brown Dwarf – never fully develops into a star
– not enough mass (hydrogen for fusion to
continue)
 Average Star (Our Sun)
 MASSIVE Star
Average Stars
 Also known as Main Sequence stars – make
up 90% of stars in space
 Become Red Giants as hydrogen resources
are depleted
 Then can break up into a Planetary Nebula
 Remaining part of the star becomes a White
Dwarf as it cools
 Once all heat is gone it then becomes a Black
Dwarf and dies.
Massive Stars
 Become Super Red Giants as hydrogen
resources are depleted
 Then explodes into a Super Nova
 Remaining part of the star becomes either
 a Neutron Star, or
 a Black Hole and dies.
The life of a star
 During its “life” a star will not change very
much.
 However, different stars are different colour,
size and brightness.
 The bigger a star, the hotter and brighter it is.
Hot stars are Blue. Smaller stars are less
bright, cooler and Red.
 Because they are so hot, the bigger stars
actually have shorter lives than the small,
cool ones.
How does a star “die”?
 Eventually, the hydrogen (the “fuel” for the
nuclear fusion) in the centre of the star will
run out
•No new heat is made and gravity will take over
and the center of the star will shrink.
•This makes the very outside of the star “float
up” and cool down, making the star look much
bigger and redder - a Red Giant star.
The second Red Giant stage
 As the centre collapses, it becomes very hot
again, eventually getting hot enough to start a
new kind of nuclear fusion with Helium as the
fuel.
 Then the Red Giant shrinks and the star looks
“normal” again.
 This does not last very long, though, as the
Helium runs out very quickly and again the
star forms a Red Giant.
The end of a Sun-like star
 For a star like the Sun, no more nuclear
fusion can take place, so the centre of the
star will then keep collapsing.
• Eventually it can become almost as small as
the Earth, but with the same mass as a whole
star! This very dense object is called a White
Dwarf.
• A piece of White Dwarf the size of a mobile
phone would weigh as much as an elephant
on the Earth!
Hetrzsprung- Russell (H-R)
Diagram
 Illustrates important things about
stars
 Brightness
 Absolute Magnitude
 usually shown on right-hand Y-axis
 Luminosity
 usually shown on left-hand Y-axis
 Temperature/Color
 Spectral Class
 shown on X-axis
Nebulae
Emission Nebulae
Emission nebulae are clouds of high temperature gas. The atoms in the
cloud are energized by ultraviolet light from a nearby star and emit radiation
as they fall back into lower energy states (in much the same way as a neon
light). These nebulae are usually red because the predominant emission line
of hydrogen happens to be red (other colors are produced by other atoms,
but hydrogen is by far the most abundant). Emission nebulae are usually the
sites of recent and ongoing star formation. (M 42 shown)
M 42 The Orion Nebula
Reflection Nebulae
Reflection nebulae are clouds of dust which are simply reflecting the
light of a nearby star or stars. Reflection nebulae are also usually sites
of star formation. They are usually blue because the scattering is more
efficient for blue light. Reflection nebulae and emission nebulae are
often seen together and are sometimes both referred to as diffuse
nebulae. (NGC 7023 shown)
Dark Nebulae
Dark nebulae are clouds of dust which are simply blocking the light
from whatever is behind. They are physically very similar to reflection
nebulae; they look different only because of the geometry of the light
source, the cloud and the Earth. Dark nebulae are also often seen in
conjunction with reflection and emission nebulae. A typical diffuse
nebula is a few hundred light-years across. (NGC 2264 shown; see
also the Horsehead Nebula)
NGC 2264
Horsehead Nebula
Planetary Nebulae
Planetary nebulae are shells of gas thrown
out by some stars near the end of their
lives. Our Sun will probably produce a
planetary nebula in about 5 billion years.
They have nothing at all to do with planets;
the terminology was invented because they
often look a little like planets in small
telescopes. A typical planetary nebula is
less than one light-year across. (M 57
shown)
M 57
Ring Nebula
Supernovae
Supernovae occur when a massive star
ends its life in an amazing blaze of glory.
For a few days a supernova emits as much
energy as a whole galaxy. When it's all
over, a large fraction of the star is blown
into space as a supernova remnant. A
typical supernova remnant is at most few
light-years across. (M 1 shown)
M 1 Supernova Remnant
M 1 Supernova was discovered in 1054 and
was visible for 23 days during the daytime,
and easily seen for 2 years at night. It
apparently was depicted in “cave paintings”
Trifid Nebula
Lagoon Nebula
Eta Carinae
NGC 2440
Helix Nebula
Dumbbell Nebula
Cats Eye Nebula
Vela Supernova Remnant
Veil Nebula
1987A Supernova