The Mysteries of the Night Sky
For thousands of years, human beings have
looked into the night sky and wondered about what they saw. What were the sparkling
objects above? How did they move? How far away were they? Were they all the
same? Over time, stories were told about the night sky. The stories turned
into myths and legends. Observations were also made about the night sky. These
observations were the beginnings of astronomy.
The night sky is filled with stars. Have you ever watched the stars
at night?
©M.Durham/GLOBIO.org Many of the objects you can see in the night sky are stars, large objects
in space that produce light.
Stars are so far away that their light does not reach us for years. As light
from so far away passes through our atmosphere,
it is moved around so that the star seems to twinkle.
Count Them if You Can
Scientists believe that our galaxy is a barred spiral galaxy like the one
pictured above. A barred spiral galaxy is a rotating, flat disk of stars,
gas and dust with a bar-shaped center made of stars.
©NASA/ESA Our galaxy, the Milky Way, contains more
than 100 billion stars. Our own Sun is
one of them. It’s the only star in our solar
system, and the star closest to Earth. But many of those other 100 billion
stars are the centers of their own solar systems.
Beyond the Milky Way, there are many more galaxies.
In fact, there are so many more galaxies that scientists estimate they contain
a billion billion stars! Just as Earth rotates
on its own axis, a galaxy rotates around a center
of gravity.
What Are Stars Made Of?
Most stars are made of gases and a gas-like
substance called plasma. Some stars are made of tiny particles.
These subatomic particles
can be closely packed together, which makes some stars extremely dense.
Scientists call it the Mountains of Creation. It is a region in outer space
where hundreds of new stars are born. Some of these new stars are bigger
than our sun!
©California Institute of Technology A Star Is Born
Every star starts out in a mass of clouds and gas called a nebula.
Gravity causes hydrogen gas
in the nebula to spin. As the hydrogen gas spins, it becomes hotter. It begins
to glow. This is a protostar.
Small protostars do not make enough heat to shine and become “real” stars.
But medium and larger protostars spin until they reach extremely hot temperatures.
Through the process of nuclear
fusion, hydrogen in the stars begins to turn into helium. This is the same
process that goes on in the Sun, and makes it able to give light and heat to
Earth.
Big and Hot = Bright and Blue
Earth’s most important star is the sun. It’s the energy source
that lets plants make food to feed us all.
© SOHO (ESA & NASA) A star’s color comes from its surface
temperature. All stars are extremely hot, but the stars with the hottest surface
temperatures are blue. Ones with surface temperatures that aren’t quite
as hot are red. Our Sun is somewhere in between the hottest and coolest stars.
It glows yellow.
A star’s brightness also comes from its surface temperature. The hottest
stars are the brightest. The coolest ones are the dimmest. Again, our Sun is
somewhere in between.
How does one star come to have a hotter surface temperature than another?
It’s a matter of size. The more mass a
star has, the hotter the surface temperature. That means an enormous star would
be hot on the surface, blue in color, and shine brightly.
A Star Dies
Stars don’t last forever. When the
hydrogen in a star runs low, the star can no longer sustain nuclear fusion.
It no longer has the energy to turn hydrogen into helium. That’s when
a star’s fire begins to cool and the star begins to die. As most stars
cool, they turn red.
What Is a Nova?
The word “nova” means “new” in Latin. But in star
terms, a nova is not new. A nova is an explosion that happens on a white dwarf,
which is a very old star. When a white dwarf is close to another star, it can
attract matter off
the surface of the other star. When the matter builds up on the surface of
the white dwarf, the surface may actually explode!
This swirl of colors is made of material left from Kepler’s Supernova
located in the Milky Way Galaxy. Kepler’s Supernova was the last supernova
seen in our galaxy.
©HST/NASA/ESA A Supernova Gets at the Core
A supernova is like a nova, except the explosion takes place at the core of
the star, not the surface. Every supernova and nova is important, because each
explosion creates dust. This dust forms the building blocks for nebulae, where
new stars are born.
Black Holes
After a supernova, a star may leave behind a burned part. This part can collapse
and absorb the light around it. This is a black hole, the end of a giant star.
A star must be 10 to 15 times larger than the Sun in order to end as a black
hole. A smaller star will simply cool rather than collapsing.
This illustration shows different stages of a black hole ripping apart and
then swallowing a star. Starting at the left, the star moves too close to
the black hole but the star’s gravity is weaker than the black hole’s.
As seen in the middle, the star stretches apart and starts breaking apart.
To the right, some of the pieces disappear into the black hole but some pieces
get really hot and light up before they disappear into the hole.
©NASA/JPL-Caltech Clusters of Stars
Pleiades has many bright blue stars. Since these stars shine brightly and
are blue, do you think their temperatures are cool or hot?
©NASA Stars are often found in huge groups called
clusters. There are two types of clusters, open and globular.
Open clusters have mostly young, bright, blue stars that
were born together. They usually have irregular shapes. The most famous open
cluster is the Pleiades. At 400 light years away, the Pleiades is Earth’s
closest open cluster. You can see the Pleiades without a telescope.
Globular clusters can have as many as a million stars – more
than open clusters. They tend to have a sphere-like shape, with many stars
at the center. There are 160 globular clusters within the Milky Way. A globular
cluster named M15 is the closest of its kind to Earth.
Stars within a globular cluster can be made up of different materials and
may have formed at different times and have different colors.
© NASA, ESA, G. Piotto (U of P) and A. Sarajedini (U of F) Globular clusters orbit the center of the Milky Way in a region known as the
galactic halo. Stars in globular clusters in the galactic halo are the oldest
structures in our galaxy. Studying them has helped scientists figure out the
age of the galaxy.
Constellations: Starry Shapes in the Sky
Certain stars appear to move in shapes and
patterns across the night sky. These patterns are constellations.
Some look like animals, humans, or objects. Constellations seem to travel in
the sky from east to west every night, but they’re really not moving.
The earth is moving, rotating on its axis. Constellations also seem to change
with the seasons. Again, they’re really not changing. They just look
that way because Earth is revolving around the Sun.
Studying the Stars
This star chart shows the location of several different constellations such
as Pegasus and Phoenix. Can you find the constellation that looks like a
fish?
©Dana Berry/NASA A star chart is very helpful when identifying
stars and constellations. With practice, you can become familiar with the night
sky and the changes you will see. The best place to view stars at night is
a place far from city lights.
To use a sky map or a star chart, first find where north is at your nighttime
location. Then point the star chart toward north. Adjust the star chart for
the month and time of night. Look for patterns in the stars in the sky. Try
to match them to the star chart. Bright stars will look like large dots on
the chart. Dim stars will be shown as small dots.
Stars of different colors and brightness are visible to the human eye at
night.
© Daita Saru Without a sky map, it may be hard to tell a planet from a star. Keep in mind
that planets will seem to wander among constellations throughout the year.
Stars stay in the same place in their constellations.
Astronomy and Astrology
Occasionally astronomy is confused with astrology because
the words sound the same. Both astronomy and astrology are fields that use
constellations. However, astronomy is a science dealing with everything in
outer space. Astrology is not a science. It is based on predictions by birth
date, corresponding to constellations. Many people find astrology to be fun,
but it is not a science.