Download the lives of stars

THE LIVES OF STARS
LESSON OBJECTIVES:
o Define constellation
o Classify stars based on their color and temperature
o Outline the stages of a star (stellar evolution)
INTRODUCTION
When you look at the sky on a clear night, you can see hundreds of
stars. Stars are giant balls of glowing gas that are very, very hot.
Most of these stars are like our Sun, but some are smaller than our
Sun, and some are larger. Except for our own Sun, all stars are so
far away that they only look like single points, even through a
telescope.
Orion Constellation
CONSTELLATIONS
Long before electric lights, people
studied patterns of stars in the night
sky. People named these patterns,
called constellations and told stories
about them. This picture shows one
of the most easily recognized
constellations. The ancient Greeks
thought this group of stars looked like
a hunter from one of their myths, so
they named it Orion after a great
hunter.
The patterns we see in constellations
stay the same night after night.
However, each night, these patterns
move across the sky. They seem to
move because the Earth is spinning
on its axis. The patterns we see
change with the seasons, too, as
Earth revolves around the Sun. As a
result, you can see different
constellations in the winter than in the
summer. For example, Orion is a prominent constellation in the
winter sky, but not in the summer sky. The crew names at REALMS
(Ursa, Draco, Aquila, Cygnus, Delphinus, Pegasus) are all different
constellations.
ENERGY OF STARS
Only a tiny bit of the Sun’s light reaches Earth; yet that light is enough
to keep us warm and gives energy for all living things on Earth. The
Sun is an average, ordinary star. The reason the Sun appears so
much bigger and brighter than any of the other stars we see is
because it is very close to us. Some other stars produce much more
energy than our Sun. How do stars make so much energy?
NUCLEAR FUSION
Stars are made mostly of hydrogen and helium. These are both very
lightweight elements that are gases. There is so much hydrogen and
helium in a star that the weight of these gases is enormous. In the
center of a star, the pressure is great enough to heat the gases and
cause nuclear fusion reactions. In a nuclear fusion reaction, the
nuclei, or centers of two atoms join together and create a new atom
from the two original atoms. In the core of a star, the most common
reaction turns two hydrogen atoms into a helium atom. Nuclear
fusion reactions require a lot of energy to get started, but once they
are started, they produce even more energy. We see and feel this
energy as heat and light.
HOW STARS ARE CLASSIFIED: COLOR AND TEMPERATURE
If you watch a piece of metal, such as a coil of an electric stove as it
heats up, you can see how color is related to temperature. When you
first turn on the heat, the coil looks black, but you can feel the heat
with your hand held several inches from the coil. As the coil gets
hotter, it starts to glow a dull red. As it gets even hotter, it becomes a
brighter red, then orange. If it gets extremely hot, it might look
yellow-white, or even blue-white. Like a coil on a stove, a star’s color
is determined by the temperature of the star’s surface.
Cool Stars
Red
Warmer Stars
Orange & Yellow
Extremely Hot Stars
Blue & White
Cooler stars
Hotter stars
For most stars, surface temperature is related to size. Bigger stars
produce more energy, so their surfaces are hotter, but sometimes a
very small star can be very hot or a very big star can be cool. In the
picture above, class O stars are much larger and hotter than the class
M stars which are smaller and cooler.
LIFETIME OF STARS
We could say that stars are born,
change over time, and eventually
die. Most stars change in size, color
and class (M-O) at least once during
this journey.
1. Formation of Stars
Stars are born in clouds of gas and
dust called nebulas, like the one
shown to the right. Stars form when
gravity starts to pull gas and dust in
the nebula together. As the gas and
dust comes together, the pressure
and the temperature get higher and
higher until nuclear fusion starts
happening in the center. At this
point, the ball of gas becomes a star.
Nebula
2. The Main Sequence
For most of a star’s life, the nuclear fusion in the core combines
hydrogen atoms to form helium atoms. A star in this stage is said to
be a main sequence star. The hotter a main sequence star is, the
brighter it is. A star remains on the main sequence as long as it is
fusing hydrogen to form helium. Our Sun, which is a medium-sized
star, has been a main sequence star for about 5 billion years. It will
continue to shine without changing for about 5 billion more years.
Really large stars burn through their supply of hydrogen very quickly,
so they ‘live fast and die young’! These very large stars may only be
on the main sequence for 10 million years or so. Very small stars
may be main sequence stars for tens to hundreds of billions of years.
3. Red Giants and White Dwarfs
A star like our Sun will become a red giant star in the next stage.
When a star uses up its hydrogen, it begins to fuse helium atoms
together into heavier atoms like carbon. The star’s core starts to
collapse inward while the outer layers spread out and cool. The star
becomes larger, cooler on the surface and red in color. When a red
giant burns up all of the helium in its core, what happens next
depends on its mass. A star like the Sun, stops fusion and shrinks to
a hot white, glowing object about the size of Earth, called a white
dwarf star. Eventually, a white dwarf cools down and its light fades
out.
4. Supergiants and Supernovas
A massive star ends its life in a more
dramatic way. Very massive stars become
red supergiants, like Betelgeuse
(pronounced beetle juice). In a red
supergiant, fusion does not stop. The star
continues fusing atoms into heavier atoms,
until it produces iron atoms. Then the star’s
iron core explodes violently in a supernova.
A supernova explosion contains so much
energy that it fuses heavy atoms together,
producing heavier elements such as gold,
silver, and uranium. A supernova can shine
as brightly as an entire galaxy for a short
time, as shown in the picture to the right.
Supernova
5. Neutron Stars and Black Holes
After a large star explodes in a supernova, the leftover material in the
core is very dense. If the core is less than about four times the mass
of the Sun, the star will be a neutron star, as shown below. A neutron
star is made almost entirely of neutrons. Even though it is more
massive than the Sun, it is only a few kilometers in diameter! If the
core remaining after a supernova is more than about 5 times the
mass of the Sun, the core will collapse to become a black hole. Black
holes are so dense that not even light can escape their gravity. For
that reason, black holes can’t be seen. We know they are there
because of the effect they have on objects around them and by the
radiation that leaks out around the edges of black holes. A black hole
is a very dense core of a supermassive star.
REVIEW QUESTIONS
1. What is the difference between a nebula and a star?
2. How does a star produce energy?
3. Which is hotter: a blue star or a red star?
4. List the seven main classes of stars, from hottest to coolest.
5. How do stars form?
6. What will happen to our Sun after it leaves the main sequence?
7. Suppose a large star explodes in a supernova, leaving a core that
is 10 times the mass of the Sun. What would we call this?