1. Astronomy Star Notes

2. Our Sun
Our sun, Sol, is a fairly average star although it is very special to us. It is a glowing ball of gas held together by gravity and powered by nuclear fusion at its core. It is very similar to most other stars. The sun’s core fuses atoms together creating ever larger and heavier elements within it. When the atoms are fused together they give off energy. Stars will fuse hydrogen atoms to form helium and then helium to form lithium and so on until the star consists of only iron. At that point the star will die and stop giving off energy.

3. Milky Way
We live in a very small solar system within the galaxy known as the Milky Way. The Milky Way is a collection of stars, planets, black holes, nebulae, gas, dust,…, held together by its own gravity. Our galaxy contains billions of stars as do all other galaxies.

4. Sun’s Layers
The sun’s layers are shown here. The core is the area where the nuclear energy (fusion) is produced. The photosphere is where most of the light we see comes from. The chromosphere is the layer that gives the sun its color. The corona is the outer atmosphere that extends far beyond the sun’s surface.

5. Sunspots
Sunspots are dark spots on the sun that are cooler than the surrounding sun. They are usually as larger or larger than the Earth. They follow an 11 year cycle – getting more prevalent over the 11 years and then very rare.

6. Prominences
Prominences are large sheets or loops of glowing gas that are ejected from the sun’s surface.

7. Filaments
Dark lines across the sun’s surface that actually indicate where prominences are shooting out from sun’s surface. They look dark against the hotter brighter surface.

8. Main Sequence of Stars
Stars tend to fall into groups. Most of them are very similar in size and temperature. They are found in the middle of the HR (Hertzsprung Russell) diagram. This diagram shows the relationship between the temperature, color and brightness (luminosity) of a star.

9. Nebula
Stars begin in large clouds of gas and dust called nebulae. These regions of space are called star nurseries. The gas and dust slowly condense down because of gravity. When enough mass and gravity occurs (critical mass), nuclear fusion begins. This is the birth of a new star.

10. Protostar
When nuclear fusion begins, the baby star is called a protostar. It is not stable yet and is not very bright.

11. Main Sequence Star
Once a star has gone through its adolescence where it is unstable it settles down into the size it will remain until old age – its adult years called main sequence. Main sequence stars are stable adult stars.

12. Red Giant
When stars reach middle age, they tend to become red giants. At this stage, most of their fuel has been used. The pull of gravity becomes less allowing the internal pressure of the star to push itself outward expanding its size dramatically. This actually helps the star to cool which makes it become red. Our sun will become a red giant in about 5 billion years extending its surface to Venus and maybe a bit beyond.

13. After a star’s middle age (red giant) comes the unstable years of old age when they again increase and decrease in size and/or brightness periodically. At this stage they are called variable stars.
Variable Star

14. White Dwarf
Stars die in several different ways that depend on the mass of the original main sequence star. When a red giant that was originally close to the size of our sun runs out of fuel, its outer shell is ejected into space leaving a very small dense star known as a white dwarf. White dwarfs are small, about the size of Earth, and are very dim because the only light given off is due to stored heat. It no longer sustains nuclear reactions.

15. Black Dwarf
Eventually white dwarfs give off all their heat to space whereupon they become black dwarfs – cold, dense, burned-out embers in space.

16. Nova
Occasionally white dwarfs give off dying gasps before becoming black dwarfs. When a white dwarf’s surface undergoes an explosion resulting in a rapid, temporary increase in the star’s brightness, a nova has occurred.

17. Supernova
When a star much more massive than our sun dies, it will not nova. It will supernova. A supernova is a star that increases dramatically in brightness before slowing dimming from view. Supernova a extremely bright and energetic.

18. Neutron Star
When a massive star goes supernova, its outer layers are exploded into space. The left over remnant is a very tiny and extremely compressed ball of neutrons called a neutron star although it is not really a star since all nuclear reactions have stopped.

19. Black Hole
When an extremely massive red supergiant goes supernova, its outer layer is exploded into space. The remaining core is so large that its gravity collapses the star down upon itself. In this type of star death, the star has so much mass that when the gravity collapses it down nothing can escape from it – even light. This is how we get a black hole.