Download Stars and the Sun

Stars and the Sun
• a star is a ball of hot plasma
(like a gas with an electrical
charge)
• 90% of the matter in the
universe is plasma
• it is not “on fire”
• it does not “burn” like a fire
does
Why is a star hot?
• Nuclear fusion
– 4 hydrogen atoms are pushed
together to make 1 helium atom
– left over particles released as
energy
– the opposite of what happens in
nuclear power plants (fission)
where atoms are split up
Anatomy of a star
• Has layers just as
the Earth does
• Core: center where
fusion takes
place (15 mil K)
• Radiative zone:
energy moves
out from center
• Convective zone:
energy rises and
sinks, some
escapes
Anatomy of a star
• Photosphere: light
we see comes
from here
• Has atmosphere
just as the
Earth does
• Chromosphere: first
layer of
atmosphere …
is pink!
• Corona: extends out
into space
Brightness of a star
• depends on:
– how big the star is (bigger = brighter)
– how far from Earth it is (closer =
brighter)
– how hot it is (hotter = brighter)
• called the magnitude (2 kinds)
• as seen from Earth is called the
apparent magnitude
Brightness of a star
• Kinds of brightness:
– Luminosity: measure of the amount of
energy given off. Measured as compared
to the Sun (1), logarithmic 102, 104
– Apparent magnitude: as seen from Earth,
lower (including negative) is brighter!
– Absolute magnitude: if all stars were same
distance from Earth, lower (including
negative) is brighter!
Spectrum
• the visible light waves that a star
gives off or absorbs
• Different for each element…like a
fingerprint
• can tell us:
– what star is made of
how fast moving
– how far away it is
how fast spinning
– if its moving to/away
Spectrum
Star color
• Related to temperature
• Stars give off all wavelengths of
light, but usually more of one
wavelength (color)
• Red = ~ 3,500K
Orange = ~ 5,000K
• Yellow = ~ 6,000K
White = ~ 10,000K
• Blue = ~ 15,000K +
HR Diagram
• Plots brightness and temperature
HR Diagram
• Groups:
– Main sequence: diagonal line, 90% of stars
– White dwarfs: hot, but small, lower left
– Red giants/supergiants: cool, but big,
upper right
– Blue Giants: hot and big, upper left
• Grouped by spectrum:
–OBAFGKM
– Oh, Be A Fine Girl (Guy) Kiss Me
Stars Life Cycle
• Goes through different stages,
depending on mass: more mass =
bigger and hotter
• All stars start as a cloud of gas and dust
(nebula)
• Condenses and eventually fusion starts,
but very cold star (protostar) and
usually can’t be seen in nebula
Nebulae and protostars
Stars like the Sun
• Become Main Sequence stars next as
gas and dust continue to condense
and the nebula disappears
– Fuse H  He for about 10 billion years
• Runs out of H, core shrinks, outer
layers cool and expand (red giant)
– Big enough to swallow first 3 planets
– Uses He other elements for about 10
million years
Stars like the Sun
• Runs out of He, core shrinks, outer
layers float into space (planetary
nebula)
• Core left over, small and hot (white
dwarf)
• Eventually fuses up to carbon, ends as
small cold ball of carbon (black
dwarf)
Stars like the Sun
Stars bigger than the Sun
• 8x or more massive
• Nebula  protostar
• A star with much more mass than the
Sun will be bigger and brighter (blue
giant) but not last as long
• Then it will get bigger and cooler (red
supergaint)
• It can fuse elements up to Iron (Fe #26)
Stars bigger than the Sun
• When core is made of iron, it can’t
condense anymore. The great gravity
pulls the outer layers in quickly and
they hit the iron core and rebound,
exploding in a supernova
• In that explosion elements heavier than
iron are formed
Stars bigger than the Sun
• The core is usually left over.
– If the core isless than 3 times the mass of
the Sun, it will become a neutron star …
something so dense that electrons are
pushed into the nucleus and cancel out
protons and make all neutrons! Some give
out regular pulses of radio waves (pulsar).
– If the core is 3x or more massive than the
Sun it collapses again into a black
hole…so dense that not even light escapes!
Stars bigger than the Sun
Relative star sizes
Relative star sizes
Galaxies
• Collections of millions or billions of
stars
• Also interstellar matter … gas and
dust
• Grouped together in clusters ours is
called the Local Group
• Clusters form superclusters and so
on
Galaxies
• 3 basic shapes:
– Spiral
• Central bulge with spiral arms
• Older stars towards center, younger ones on outside
• Sometimes a bar instead of a bulge … barred spiral
– Elliptical
•
•
Oval or round, no arms
Mostly older star , little interstellar matter
– Irregular
•
•
No regular shape
No pattern to kinds of stars or amount of
interstellar matter
Spiral Galaxies
Elliptical Galaxies
Irregular Galaxies
Galaxies
• Milky Way
― Most likely a spiral galaxy…strip of
stars is the spiral arm
― Hard to tell without leaving it
• Quasar
― Most distance and radiant object i
space
― Very young galaxy
• Change over time … as stars change
and run into each other
Colliding Galaxies