1. Chapter 19 – Stars, Galaxies and the Universe
Stars – A ball of hot gas undergoing fusion to produce heat, light and other forms of radiation.
Color can be used to determine the temperature and composition of a star: Blue – Hot Red – Cool
Spectrum – the component colors of light when passed through a prism. Used to determine a stars composition.
Spectrograph – An instrument used to analyze star light.
Emission Lines – Colors given off by the hot gases of a star and can determine the elements present in that star.

2. Absorption Spectrum – The black areas of a stars spectrum that represents the light that gets absorbed by that stars atmosphere.
Stars are classified by how hot they are O-class stars are the bluest & the hottest M-class stars are the reddest & the coolest. Astronomers classify stars by their brightness or magnitude. Positive numbers are dim stars and negative numbers are bright stars.
Apparent Magnitude – How bright a star appears to be when viewed from the Earth.
Absolute Magnitude – The actual brightness of star calculated at a distance of 32.6 light years from the Earth.
Our sun (Sol) has an absolute magnitude of +4.8 but an apparent magnitude of because it is so close to us.

3. Distances to the stars:
A light year is the unit of measurement astronomers use and is the distance that light travels in a vacuum in one year (approx. 9,500,000,000,000 km.). Measurements of stellar distances are based on the apparent movement of a star when viewed from the Earth and is called the parallax effect. To view a parallax shift, a telescope must view a star from two different locations and then the star is compared to other stars in the background image.
The stars, just like the sun and the moon, appear to move across the night sky due to the rotation of the Earth. However, in reality, the stars are actually shifting very slowly so that in time, the constellations will not appear as they do now.

4. The Life Cycle of Stars:
Early Life – A stars begins life as a ball of gas & dust. Gravity begins to draw everything in towards the center creating a denser mass which increases friction hence temperature. The temperature continues to increase until it’s hot enough for fusion to occur then the star begins to glow.
Mid-Life – The star slowly consumes it’s hydrogen fuel.
The End – The fuel is used up and the star begins to cool, enlarging first then shrinking or it could blow itself apart distributing its elements into the universe where it could possibly recombine into a new star.

5. Types of Stars
Main Sequence Stars – A typical star in the middle of it’s life.
Red Giant – A stars that has enlarged and cooled as the result of its using up all of its hydrogen. The core shrinks and the atmosphere expands to become 10 times larger than our sun or a super-giant expands to 100 times larger.
White Dwarfs – A small, hot star that is actually the remaining core of a red giant or super-giant once its gases have blown away in a planetary nebula. There is no hydrogen remaining and the star is at the end of it’s life.

6. Hertzsprung-Russell (H-R) Diagrams
A graph that shows the relationship between a stars temperature and it’s absolute magnitude (brightness). Used to study the lifespan of a star. A stars temperature will determine its color. The brightest stars are at the top left and the dimmest stars are at the bottom right of the H-R diagram. The diagonal pattern where most stars lie is called the Main Sequence. Our sun is a main sequence star.

7. Old Stars
Supernovas – A gigantic explosion in which a massive star collapses and throws its outer layers of gas into space. A supernova will result in a black hole or a neutron star.
Neutron Star – A star with such a huge gravitational pull, that the electrons & protons of its component elements are crushed into neutrons.
Pulsar – A rapidly spinning neutron star that emits rapid radio pulses picked up as clicks on radio telescopes.
Black Hole – A star that collapses its center so completely all that is left is a hole in space/time. A black hole is so gravitationally massive that even light cannot escape.

8. Galaxies
A large grouping of stars, gas and dust that are classified by their shapes; spiral, elliptical or irregular.
Our galaxy is called the Milky Way and is a spiral galaxy.
Spiral galaxies have a dense, bulging center with arms spiraling out composed of gas, dust and stellar nurseries.
Stellar nurseries are found in some nebulae, a few elliptical galaxies and in the arms of spiral galaxies.

9. Nebula – A cloud of gas and dust where new stars form and old ones have exploded.
Globular Cluster – A group of tightly packed, older stars that looks like a ball. These form a halo surrounding the central bulge (Shapley Center) of a spiral galaxy and are also common near elliptical galaxies.
Open Cluster – groups of closely grouped stars found in the spiral discs of galaxies.
Astronomers need to look at the farthest objects in the universe to help them determine what early galaxies looked like.
Quasars – one of the most distant objects that are very bright and powerful releasing energy at a very high rate. A quasar is actually an active black hole. The energy released is a result of too much matter falling in at a high rate of speed. The U.S. Air Force calibrates the position of it’s G.P.S. satellites to 350 distant quasars because the do not move relative to the Earth.

10. Cosmology
The study of the origin, structure and future of the universe. To determine the age of the universe, cosmologists measure the distance from the Earth to various galaxies.
The Big Bang Theory – States that all of the matter in the universe was once a microscopic point of infinite density that exploded approx billion years ago. This matter at first formed subatomic particles, then atoms and eventually nebulae, stars and continues to expand today. Cosmic background radiation fills the farthest points of the universe and continues to expand outwards with the universe. Scientists believe that the universe will always exist and continue expanding.