Universe, galaxies, geocentric and heliocentric notes.

1.  Astronomy-the study of space and everything outside the Earth.

2.  The Big Bang Theory  the universe was created sometime between 10 billion and 20 billion years ago when a cosmic singularity exploded and hurled matter and in all directions.

3. Universe-space and all the matter and energy in it.

4. Milky Way-we are on Earth which is in our solar system and it is in our galaxy

5.  Galaxies are large systems of stars and interstellar matter held together by gravity, contains millions of stars

6. irregular galaxies-no real shape, contain very hot newer stars and have lots of dust and gases for new stars.

7.  spiral galaxy-shaped like a pinwheel.  Has a central bulge with many older red stars and the spirals or arms contain material for new stars.  

8.  elliptical galaxies-ball or oval shaped, contain mostly older stars, very little material for new stars.

9. Milky Way Galaxy- is our galaxy; it is a spiral galaxy. Is about 100,000 light years across.

10. Our solar system is located inside one of its spiral arms

11.  galaxies contain gigantic cloud of gases and dust called nebula, the birthplace of stars

12. nebular hypothesis-idea that our solar system formed when the gases and dust of a nebula began to spin.

13. AU or astronomical unit-the distance from the sun to the Earth

14. light year-the distance light travels in a year (9.5 trillion kilometers, 1 light year equals 63,240 AU's)

15. The solar system is made up of the sun, nine planets, moons that orbit some of the planets, comets, asteroids, and big and small space rocks.  Our Solar system is about 26,000 light-years from the galaxy’s center in one of the spiral arms.

16. Aristotle-famous, ancient Greek scientist, lived over 2300 years ago.  Believed Earth was center of Solar System.  Believed by kings and Catholic Church.

17. Ptolemy-Ancient Greek scientist who proposed the Geocentric model of the solar system, lived 2,000 years ago.

18.  geocentric model-early belief that the Earth was the center of the solar system and the Sun, the planets, and the stars revolved around it.


  1. heliocentric solar system-has the Sun in the center and the Earth is orbiting around it.

20.  Nicolaus Copernicus-Polish astronomer proposed the heliocentric view of the solar system, published a book in 1543, was dying.

21. idea that the sun was the center of the solar system is called the heliocentric theory

22. Johannes Kepler-German scientist who wrote a book, around 1609, supporting heliocentric model, also suggested idea of gravity, he inspired Galileo.

23.  Galileo Galilei-Italian scientist, inspired by Copernicus and Kepler, used the telescope to prove that the heliocentric idea of the solar system was correct, he also gave us the scientific method.

24.  Sir Isaac Newton- 1700's, English scientist who defined gravity, explained how gravity made the heliocentric model work.

25.  gravity-the force that all objects exert on each other because of their mass.  all matter has gravity and everything attracts everything else.  

26.  law of universal gravitation-if two objects are moved twice as far apart as they were, their gravitational attraction will be 4 times less.

27.  Black Hole- massive star that has collapsed and its gravity is so powerful that it even pulls light into it.