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Stars and Galaxies
Traveling Beyond the Earth
Chapter 21
Constellations
• Constellations are patterns of
stars in the night sky,
commonly named after
animals and characters in
Greek mythology
• Currently there are 88 that are
named
• If you look in the direction of
north, there are constellations
that appear to move in a
circle around the north star,
Polaris. These are circumpolar
constellations.
• They are visible year-round in
each hemisphere
Starlight, Starbright
• The stars that are
closer to us appear to
be brighter
• Stars shine because
of fusion in its core.
Two hydrogen atoms
fuse into one helium
atom.
• This is why a star
shines for billions of
years!
Brightness , continued
• Two types of brightness:
absolute magnitude and
apparent magnitude
• Apparent magnitude is the
brightness we see here on
Earth
• Absolute magnitude is the
actual light the star gives off.
The more negative the
number, the brighter the star
is!
• To figure out how far it is to a
star, astronomers look at it
from two different positions,
called parallax
I Can See Your True
Colors
• Not all stars are the same
color because different
elements burn different
colors. Some are red,
some are blue, etc.
• Color tells the
temperature of the star
• Hot stars are bluish/white
and cooler stars are
reddish/orange
• Astronomers call this a
star’s spectral class.
Spectral classes are O, B,
A, F, G, K, and M
Hertzsprung-Russell Diagram
• Stars are ranked on
the H-R Diagram
based on its
temperature and
absolute magnitude
• Most stars fall in the
middle of the
diagram, called the
main sequence
• Very few stars are
white dwarfs or
giants/supergiants
A Star’s Life: Normal Main
Sequence Star
•
Here is the life of a
normal sized star:
1.
2.
Nebula: starts to contract
Protostar: temperature
increases, fusion begins
Main sequence star: using
up H fuel
Giant: no more H, core
contracts, outer layers start
to expand
White dwarf: outer layers
escape into space, hot
dense core is left
Black dwarf: dead star
3.
4.
5.
6.
Life of a Massive Main
Sequence Star
•
Here is the life of a massive
main sequence star:
1.
2.
3.
4.
Nebula
Protostar
Massive main sequence star
Supergiant: core is much hotter
than a giant, heavy elements form
which causes fusion to end
Supernova: the star explodes
Neutron star or Black hole: neutron
star is the collapsed core of the
supernova; a black hole forms
when the remaining core is 3 times
more massive than the Sun
5.
6.
1.
Answer Questions on the Life of a Star
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Gravity gathers hydrogen gas and particles of dust, and the
clouds grow more dense. The hydrogen heats up to a fantastic
18 million degrees. Nuclear fusion turns hydrogen to helium and
the fire ignites.
T or F. Gravity is always at work, trying to crunch the star down.
Stellar age is determined by the size of the star.
Smaller stars burn cool and dim. Medium stars, like the Sun, will
burn steadily for some 10 billion years.
When a medium star runs out of hydrogen fuel, gravity grips the
core, the star grows hotter, and the outer layers puff up and
engulf nearby planets.
A white dwarf is a dying star.
A bigger star than the Sun lives hot and dies young.
Large stars go out with a bang called a supernova.
What is left after the supernova is much more dense than a white
dwarf, called a neutron star.
Some remains of a supernova are so dense, they become black
holes.
T or F. Supernovas are common.
We may only experience one exploding star in a century.
To the Milky Way, and
Beyond!
• Our galaxy, the Milky
Way is part of a larger
group of galaxies
called the Local
Group
• There are about 30
galaxies part of the
Local Group
• There are three types:
elliptical, spiral, and
irregular
The Andromeda galaxy is an
example of a spiral galaxy with huge
spiral arms. Astronomers believe the
Milky Way resembles this.
Galaxies, continued
Elliptical can be
football-shaped
Irregular
• Elliptical galaxies are
the most common
• Irregular are all the
galaxies that are NOT
spiral or elliptical, and
are not nearly as
common
Where Are We in the Solar
System?