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Stars and Galaxies
Important Vocabulary
Absolute Magnitude
• How bright a star would
look if it were a standard
position from Earth.
Astronomical UnitThe distance from the Earth to the
Sun. Usually written AU = about 93
million miles.
Apparent Magnitude
• How bright a star looks
from Earth
Light Year
The distance which a ray of light would
travel in one year. This is about
6,000,000,000,000 (6 trillion) miles.
Light travels at a speed of 186,000
miles per second
What is a star?
• Stars are balls of gases that release heat
energy produced by nuclear reaction within
their cores. Throughout the galaxy, stars are
constantly forming, evolving, and being
destroyed.
Why study stars?
• Most stars are similar to our nearest star, the
sun.
• By studying a range of stars, astronomers can
figure out how stars changed over time and
hence understand more about the past and
probable future of our sun.
Get to know our closest star…the sun!
• It is a ball of glowing gas of average size,
temperature, and brightness when compared
to other stars
• The sun’s surface is known as the photosphere
surrounded by a vapory layer called the
chromosphere
• Sunspots are relatively cool areas on the sun
More about our sun
• The sun is located approximately 93,000,000
million miles (1 AU) from Earth.
• It is estimated to reach about 27 million
degrees Fahrenheit (15 million degrees
Celsius)
• In a nuclear reaction, hydrogen is converted to
helium by fusion
How is a star born?
• Stars form in huge clouds of dust and gas
called nebula(e).
• The nebula(e) shrinks under the inward pull of
its own gravity then becoming a premature
baby star known as a protostar.
A star is born…
• The protostar continues to build in
temperature and density switching itself on
generating its own heat and light.
Main Sequence Star
This is the star’s main lifetime…billions of years are spent in this stage…our sun is in
main sequence right now. Most stars (about 90%) are Main Sequence Stars.
For these stars, the hotter they are, the brighter they are.
Main Sequence Stars can grow,
then die!
Massive stars
• Mass greater than 10 of our suns
(600 million miles across)
• Relatively old stars
• Continues to grow to a red
supergiant expanding its outer
layers.
• Betelgeuse and Rigel are super
giants. These stars are rare.
• Eventually its core collapses
causing a huge explosion known
as a supernova
Weeks after- can continue
to glow brightly but as parts
are scattered over universe
from explosion, core can be
crushed into tiny super
dense neutron star possibly
turning into a black hole
Small/Medium Stars
• Mass about 60 million miles across
• Swells into a red giant, eventually losing its outer
layers
• Forms into a gas shell called planetary nebula(e)
• Continues to cool and fade so that core is exposed
as a white dwarf for billions of years
• Core glows red (like an ember) as continues to cool
• Core stops glowing then known as a black dwarf
(none known yet; universe too young)
Main Sequence Stars
(Young Stars)
Yellow Dwarf
• Relatively small
• Can be up to 20 times larger
than our sun and up to
20,000 times brighter
• Our sun is one
Red Dwarf
• A small, cool, very faint star
whose surface temperature
is under 4,000 K.
• Most common type of star
• Proxima Centauri is one
More Star Types
White Dwarf
•
Ancient white dwarf stars shine in the
Milky Way galaxy. White dwarfs are
stars that have burned up all of the
hydrogen and helium they once
used as nuclear fuel to elements such
as carbon, nitrogen, and oxygen
Black Dwarf
•
A 'black dwarf' is a white dwarf that
has cooled down enough that it no
longer emits light.
More Star Types
Red Giant
•
•
Towards the end of a star's life, the
temperature near the core rises and this
causes the size of the star to expand. This
is the fate of the Sun in about 5 billion
years.
Stars convert hydrogen to helium to
produce light (and other radiation). As time
progresses, the heavier helium sinks to the
center of the star, with a shell of hydrogen
around this helium center core. The
hydrogen is depleted so it no longer
generates enough energy and pressure to
support the outer layers of the star. As the
star collapses, the pressure and
temperature rises. Then , the star expands
into a Red Giant.
Blue Giant
•
•
•
Because blue stars are large, and
compact, they burn their fuel quickly,
which gives them a very high
temperature. These stars often run out of
fuel in only 10,000 - 100,000 years.
A blue giant is very bright. Like a light
house, they shine across a great
distance. Even though blue giant stars are
rare, they make up many of the stars we
see at night.
Blue giant stars die in a spectacular
way. They grow larger just like the Sun
sized stars, but then instead of shrinking
and forming a planetary nebula they
explode in what is called a super nova.
Super nova explosions can be brighter
than an entire galaxy, and can be seen
from very far away.
More Star Types
Nova
• A star which suddenly
flares up to many times its
original brightness before
fading again.
Supernova
Is a super bright explosion
of a star. A supernova can
produce the same amount
of energy in one second, as
an entire galaxy.
Death of a Star
Explanation: What would you see if you went right up to a black
hole? Above are two computer generated images highlighting how
strange things would look. On the left is a normal star field containing
the constellation Orion. Notice the three stars of nearly equal
brightness that make up Orion's Belt. On the right is the same star
field but this time with a black hole superposed in the center of the
frame. The black hole has such strong gravity that light is noticeably
bent towards it. Black holes may eject a great amount of radioactive
energy known as a quasar.
More information on star types
Neutron Star
•
•
Neutron stars are created in the cores of
massive stars during supernova explosions.
The core of the star collapses, and crushes
together every proton with a matching electron
turning them into a neutron. The neutrons,
however, can often stop the collapse and remain
as a neutron star.
•
They are the most dense objects known. They
are only about 10 miles in diameter, yet they
are more massive than the Sun. One sugar cube
size of a neutron star weighs about 100 million
tons, which is about as much as a mountain.
•
Like their less massive counterparts, white
dwarfs, the heavier a neutron star gets the
smaller it gets. Imagine if a 10 pound bag of
flour was smaller than a 5 pound bag!
Spinning neutron stars that emit radio waves are
called pulsars.
Stars Life Cycle
Nebulae →Proto star→ Main Sequence Star
↙
↘
If … massive star
if …smaller or medium star
Red Giant or Blue Giant
White Dwarf →Black Dwarf
↙
↘
• Nova or Supernova
•
↓
↘
• Neutron Stars or Black Hole
The End Of Part One Stars
Constellations
• The constellations have been called humanity's oldest picture book. For
5,000 and more years, people have looked into a clear night sky and seen
the same stars we see today. They isolated groups of stars and connected
them to each other with imaginary lines, much as we play connect-thedots.
• In the past, people had an excellent knowledge of the night sky. They were
able to tell when to plant and when to harvest, and later they navigated
the seas with the stars' help. Characters of myth and legend were used to
name and tell the stories of the stars such as the group of stars that
looked like a man with a sword (or bow and arrow) was named Orion, for
the famous hunter in Greek mythology.
Stars can form patterns in the sky
called constellations
How are they helpful?
• The two brightest stars in the Big Dipper
"point" to the North Star. The North Star,
which is also called Polaris, is a star that
always points to true north, while the other
stars in the northern sky seem to circle around
it.
• Because of its seemingly fixed position in the
sky, sailors and travelers have been able to use
the north star to guide them on their travels.
Finding the North Star
Finding the Big Dipper is Key to Finding the North Star
The key to locating the North Star in the night sky is to first find the Big
Dipper, a constellation of stars known as Ursa Major.. The Big Dipper is
perhaps the best known group of stars in the northern sky and is easy to
distinguish from all others. Also known as the Great Bear, the Big Dipper
is located just north of the celestial pole. Knowing how to find the Big
Dipper makes it easy to find the north star.
The second key to finding the North Star is a similarly shaped
constellation of stars known as the Little Dipper. The Little Dipper, also
known as Ursa Minor, is smaller and more difficult to find in the night
sky. Fortunately its big brother, the Big Dipper points the way.
The Pole Star we are seeking is the brightest of the Little Dipper stars
and is located at the tip of the dipper’s handle.
Diagram of Big and Little Dippers
Locating the North Star is easy if
you follow this simple diagram.
1. Find the Big Dipper. Draw an
straight line between the two stars
of the Big Dipper as shown, toward
the Little Dipper.
2. The North Star, the brightest
star in this constellation, is located
at the end of the handle of the
Little Dipper.
Zodiac
Twelve constellations, together called the Zodiac, form a belt around the earth. As the earth
revolves around the sun, a different part of the sky becomes visible until, after a year, the earth
has completed one trip and starts again. Each month, one of the 12 constellations appears above
the horizon in the east to begin its march across the sky. Night after night, the constellation
appears to move across our sky until it disappears below the horizon in the west and the next
constellation appears in the east.
The 12 Constellations of the Zodiac
Galaxies
Our stars and planets are all part of a
galaxy
•
Galaxies are formed by massive star clumps. This process takes place anywhere
from 0.5 to 1 billion years. Usually stars are in a system of two or more stars which
are clumped together.
•
After about 1-2 billion years, the clumps grow large enough to be seen by the
Hubble Space Telescope. After 2-4 billion years, collisions and mergers occur,
making the formation of larger, irregular-looking objects
•
After 4 -13 billion years, galaxies form and take their final shapes. We can see
these galaxies today.
Sometimes Galaxies can combine
(like Andromeda and the Milky Way are predicted to do someday)
Before
AFTER
How do galaxies get their shapes?
• Galaxies form out of gigantic gas clouds that develop in early space. These
clouds collapse due to the forces of gravity. Different properties of the
collapse create different shapes of galaxies.
Andromeda is the
galaxy next door. It is
a spiral galaxy.
Our galaxy is the Milky
Way: it is a barred spiral
galaxy
Ceres is an elliptical
galaxy
A galaxy without a particular shape is known as
an irregular galaxy
THE END or is it?
• As technology continues to grow more and
more advanced we’re are sure to learn more
and more about stars and the galaxies in
which they belong.
• REMEMBER: We don’t see galaxies or stars the
way they are. We see them the way they
were!