Download Distance, Size, and Temperature of a Star

Distance, Size, and Temperature of a Star
Objectives:
• define “magnitude”
• examine how a star’s distance, size and temperature affects its apparent magnitude
• define the term “parallax” and observe its relationship to the night sky
• discuss the life cycle of stars, using the following as examples: red dwarfs, the Sun and
blue giants
• associate the color of stars with their relative temperature and brightness
Missouri Curriculum Frameworks:
5-8:
Strand I. Scientific Inquiry (A. Processes of Scientific Inquiry)
1a. apply mathematical procedures to investigations and data sets in order to determine patterns,
relationships and predictions (1.6)
9-12:
Strand V. Universe (A. Characteristics of the Solar System)
2a. use information about a star’s characteristics to determine its age (1.6, 3.5)
Materials:
Two flashlights
Aluminum foil
Pencil
Three lamps or flashlights
3 light bulbs (1 of each of the following colors: blue, red and yellow)
“Size and Distance of A Star” Activity
1. Cover the top of one of the flashlights with foil and punch a pencil-sized hole in the
center.
2. Dim the Lights
3. Shine both flashlights on the wall from the same point in the room.
Discussion: Which star is brighter? Which one is dimmer? If we looked at these two “stars” in
the sky, we probably would have thought the dimmer one was farther away. Actually, the
dimmer star is the same distance away, but it is smaller. A star’s size, like the flashlight opening,
affects the brightness of the star. The larger the star is, the brighter the light that is seen on
Earth. Stars have varying sizes. Some are smaller than the Earth. The Sun is considered a
medium-sized star, with a diameter of 870,000 miles. Super giant stars have diameters up to
1,000 times that of the Sun. The measure of a star’s brightness as it is seen from the Earth is
known as magnitude. The higher the magnitude, the fainter the star. A star’s magnitude
depends on three things---size, distance from Earth, and temperature.
4. Remove the aluminum foil from the flashlight.
5. Shine both flashlights on the wall from an equal distance away, and observe that both
stars appear to be the same brightness.
6. Move one of the flashlights far away from the group and the other very close.
Discussion: Which star looks brighter? The light from the flashlights represents the light from
two stars that give off the same light. The light spreads out uniformly in all directions from the
flashlights, as it does from stars. The spreading of light from more distant stars results in less
light reaching Earth. Thus, two stars with the same luminosity, but at different distances from
Earth, will appear to have different magnitudes. It is hard to know how far away stars are
because we don’t know if one is really brighter than others, or just closer to us.
“Temperature of a Star” Activity
1. Put a red, yellow and blue light bulb into three flashlights or lamps.
2. Light the three lights.
Discussion: Imagine a fire. What do you see when you look at the hottest part of the flame,
which is the part closest to the wood? (Blue color) What colors you see as the flames get further
and further from the wood? (the color is first yellow, then red) As the fire gets further from the
wood, it gets cooler and that is why we see different colors. Stars work the same way. Their
temperature determines what color they will be. The blue stars are the hottest and brightest,
while red stars are the coolest and dimmest. Medium stars, like our sun are yellow because they
have a medium temperature.
Below is more information on various stars in relation to their color and temperature:
Red Dwarf Stars:
They can range in size from a hundred times smaller than the sun, to only a couple of times
smaller. Because of their small size these stars burn their fuel very slowly, which allows them to
live a very long time. Some red dwarf stars will live trillions of years before they run out of
fuel. Because red dwarf stars only burn a little bit of fuel at a time, they are not very hot
compared to other stars. Red dwarf stars are by far the most common type of star in outer space.
However, very few stars that you see in the sky are red dwarfs. This is because they are so small
and make very little light. Imagine standing on a mountain. Pretend that there are one million
kids five miles away holding flashlights and 20 miles away there is a lighthouse for ships. You
will most likely not see any of the flashlights, while you will very easily see the lighthouse. If the
flashlights all glowed as brightly as the lighthouse it would blind you. Like wise, if all the red
dwarf stars glowed as bright as the bigger stars, nighttime would be very bright.
Medium stars:
Like the Sun, these medium sized stars are yellow because they have a medium temperature.
Their higher temperature causes them to burn their fuel faster. This means they will not live as
long, only about 10 billion years or so. Near the end of their lives, these medium sized stars
swell up becoming very large. When this happens to the Sun it will grow to engulf even the
Earth. Eventually they shrink again, leaving behind most of their gas. This gas forms a beautiful
cloud around the star called a Planetary Nebula. The Sun is so hot that when it dies, it will take
along time to cool off. The sun will die in about 5 billion years, but it will still glow for many
billions of years after that. As it cools, it will be what is called a white dwarf star. Eventually,
after billions and maybe even trillions of years, it will stop glowing and at that point, it will be
what we call a black dwarf star. There are still no black dwarf stars in the Universe.
Blue Giant Stars:
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 lighthouse, 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. Because blue giant stars only
live a short time, scientists use them to find places in outer space where new stars are forming.
Remember when we talked about sun-sized stars? We said that at the end of their lives these
stars expand, taking up much more space then before. This is exactly what a Giant Star is. As a
sun sized star gets old, it starts to run out of its hydrogen fuel. When the burning of hydrogen in
the star's core begins to slow down, the core gets more compact. This means all the stuff in the
middle of the star gets really close together. As the center gets smaller and smaller, it starts to
heat up again. When it gets hot enough it will start to burn a new fuel called helium.
Once ignited, helium burns much hotter then hydrogen. The additional heat pushes the outside of
the star much further than it used to be, making the star much larger. Imagine a hot air balloon.
As the air gets hotter it stretches the outside of the balloon further and further. As the star gets
hotter its outside stretches out further and further. When our own sun begins to stretch into a
giant star it will engulf Mercury, Venus, Earth, and Mars. Many of the stars you see at night are
giant stars. This is because like a light house, giant stars glow very brightly. When the Sun
becomes a giant star it will be seen from much further than it is right now. A super giant star is
the exact same thing as a giant star, only bigger. Remember that as a star gets older it begins to
run out of fuel. As the star runs out of fuel, it will start to burn out. Just like the sun sized stars,
blue giant stars also begin to burn helium. As they do, these stars get much hotter. This extra
heat makes the outside of an old blue giant star stretch out further. Remember how hot air
balloons stretch out as the air in them gets hotter. The only difference between Giant Stars and
Super Giant Stars is their size. Super Giant Stars are much bigger. If the Sun were replaced by a
super giant star it would extend almost all the way out to Uranus. Dead super giant stars/blue
giant stars often turn into black holes. A black hole is a very compact object. As the star dies it
explodes in a huge explosion called a Super Nova. The super nova blasts away most of the star.
Anything left begins to fall in to the middle of the star. It gets more and more compact, and
smaller and smaller. If there is enough of the star left after the explosion, the star will be heavy
enough to squash it down to the size of an atom, or even smaller.