1. Text Definitions You were to complete these on your own. Begin to work with the terms and definitions a little each night. You were to complete these on your own. Begin to work with the terms and definitions a little each night. We will be using them in our study of the life cycle of a star and the Hertzsprung- Russell diagram. We will be using them in our study of the life cycle of a star and the Hertzsprung- Russell diagram.

2. Stars Information from NASA’s Observatorium What are stars?

3. Stars are self-luminous, gaseous spheres. “Self luminous” means that stars generate their own light. This makes stars different from planets, moons, asteroids and comets, all of which shine by reflecting sunlight.

4. Stars are different in other ways: They possess far more mass (stuff). They have different compositions. They are hotter. Most are bigger than planetary objects.

5. Let’s use the Sun, our nearest star as an example. It is a self-luminous, gaseous sphere. It has no solid surface. Its size is about 100 times the Earth and its mass is about 300,000 times that of Earth. Its core temperature is 27 million degrees Fahrenheit and its visible surface is 10,000 degrees Fahrenheit. The sun is made up of mostly hydrogen and helium. The heavier elements carbon, oxygen, nitrogen, iron, and gold make up only about 2% of the Sun’s mass.

6. The Sun’s luminosity, its brightness –referred to as one solar luminosity -- is comparable to that of 4 trillion one-hundred-watt light bulbs. Our Sun is rather average. Some stars are bigger, more massive, and brighter. Others are smaller, less massive, and less bright. Some stars are single like the Sun. Others have one or several companion stars. Some stars are much older than the Sun. Others have been formed very recently.

7. INTERESTING FACT The Earth intercepts about one- half billionth of the energy radiated by the Sun. If we could harness all of the energy the Sun radiates in one second, we’d have enough energy to satisfy the current US energy consumption for about 4 million years.

8. What makes stars shine? Stars are nuclear furnaces. Stars generate their own energy by nuclear reactions in their super-hot cores. This newly released energy flows from the stars’ hot interiors to the cooler surface layers, where the energy is radiated into space. We see that radiation and say that stars shine.

9. Back to the Sun as our example. The temperature is so hot in the core of the Sun that hydrogen atoms fuse into helium atoms with the release of large amounts of energy. It is the same process that powers a hydrogen bomb explosion, the difference is that the process in the sun is not explosive and goes on at a steady pace for billions of years. Most of the stars you see in the night sky get their energy in this way. Eventually the hydrogen core will become a helium core. The helium core will then fuse into carbon and oxygen, with still more energy being released.

10. Do stars shine forever? Stars do not shine forever. Stars are of finite size. That means that stars have only a certain amount of nuclear fuel. Eventually they will use up all their fuel and fade. How long stars shine depends on how much mass they have. Mass determines the amount of fuel stars have at birth. Mass also determines how bright stars are, or how rapidly they consume fuel.

11. The greater a star’s mass, the greater is the amount of its nuclear fuel. However, the more massive stars are fuel guzzlers. They shine much brighter than the less massive stars and use up their fuel very fast. So, the more massive stars have shorter lives. Our sun will last for about 10 billion years (we are currently about 5 billion years into that life).

12. Why are stars important to us? 1. 1. The Sun (our nearest star) gives us light and warmth. 2. 2. The Earth was formed as a by-product of the formation of the Sun. Our planet and the other planets in the solar system came into existence as by-products of the formation of the Sun. Without the Sun, there would be no Earth, and no life, as we know it.

13. 3. 3. Carbon, Nitrogen, Oxygen, and the other elements needed for life were produced by earlier generations of stars. Nearly all of the elements heavier than hydrogen and helium in today’s universe were manufactured in earlier generations of stars. The most massive of those stars ended in super-energetic explosions, called supernovae, ejecting their heavy elements into space. Some 4.6 billion years ago, our Sun and the planets formed from clouds of gas and dust enriched by the heavy elements from earlier generations of massive stars.

14. Keep reviewing nightly! Keep reviewing nightly! The Study Island #3 deadline is fast approaching. The Study Island #3 deadline is fast approaching. Meet in the computer lab tomorrow. Meet in the computer lab tomorrow.