1. Chapter 9: Earth and Space
Lesson 2: What is a star?

2. Lesson 2 Objective:
Students will describe the physical characteristics of the sun and understand why the star Polaris is important.

3. Writing Prompt:
Read My Planet Diary in your book on page 368. Answer the question below.
Do all stars look the same? Explain your answer.
Write your answer in your Lesson 2 Notes Packet.

4. Stars
Stars are gigantic balls of very hot gases that give off radiation.
The sun is a medium-sized star.
Stars known as giants may be eight to 100 times as large as the sun.
Supergiants are even larger- they may be up to 300 times as large as the sun!
A star at the end of its life can collapse and become very small- about the size of earth.

5. Stars
The sun is a medium-sized star but is the largest in our solar system.
Scientists have been able to calculate the sun’s mass from the speeds of the planets and the shapes of their orbits around the sun.
The sun’s mass is nearly two million trillion trillion kilograms- written as a two followed by 30 zeros!
The sun has nearly 100% of the mass in the solar system.

6. How can scientists determine the mass of the sun?
Stars
The sun is huge when compared to Earth- the sun has more than one million times the volume of Earth.
If the sun was a gumball machine, it would take over one million Earth gumballs to fill the sun gumball machine!
Teacher Questions:
Compare the size of the sun to the size of other stars. Compare its size to Earth.
How can scientists determine the mass of the sun?

7. Characteristics of the Sun
The sun is a fiery ball of hot gases and has no hard surface.
It gives off enormous amounts of light and heat.
The outer part of the sun is 5,500 degrees Celsius.
The inner core could be as hot as 15,000,000 degrees Celsius!

8. Characteristics of the Sun- The Sun’s Atmosphere
The sun has an atmosphere- just like Earth.
The innermost layer is called the photosphere.
The photosphere gives off the light energy that you see.
The outermost layer is called the corona.

9. Sunspots
When scientists look at the sun using special equipment, they can see dark spots, called sunspots, moving on the surface of the sun.
Sunspots are part of the photosphere. They may be the size of Earth or larger.
They look dark because they are not as hot as the rest of the photosphere.
The number of sunspots increases and decreases in cycles of about 11 years.

10. Characteristics of the Sun- Solar Eruptions
A prominence looks like a ribbon of glowing gases that leaps out of the chromosphere into the corona. May appear and disappear in a few days or months.
A solar flare is an eruption of waves and particles into space - similar to volcanoes here on Earth.
Solar flares cause bright spots in the chromosphere that may last a few minutes or hours.
Solar flares also give off other forms of energy. The energy is powerful enough to interrupt radio and satellite communication on Earth.

11. Teacher Questions:
3. What are the physical characteristics of the sun?
4. Why might space agencies not want to send astronauts into space during solar flares?
5. What forms of energy does a solar flare give off?
6. Starting with the core, name the layers of the sun from innermost to outermost.

12. Constellations Scientists divide the night sky into 88 constellations.
A constellation is a group of stars that form a pattern.
The Little Dipper is a constellation that contains the star Polaris or the North Star.
Polaris is a very large, very hot yellow-white star. It is almost 2,500 times brighter than the sun.

13. Constellations
Polaris is a very important star in navigation because it is almost directly above the North Pole.
Polaris doesn’t seem to move as Earth rotates.
If you can find Polaris in the sky, you can tell which direction is north.
Early explorers used Polaris as a guide to direct them in their travels.

14. Stars on the Move
Stars are not always in the same place in the sky. They move in predictable ways.
If you locate the Big Dipper one evening and looked two hours later, it would seem that the Big Dipper moved toward the west- but it is you who actually moved.
The spinning of Earth make stars appear to move from east to west across the sky.

15. Teacher Questions: Why is Polaris called the North Star?
How might constellations help scientists study the sky?
If other stars are brighter and larger than the sun, why does the sun appear so large?