1. The Solar System, Galaxies, and Universe

2. The Solar System
Celestial bodies orbit the Sun, where celestial means “pertaining to the sky”, and among them are a family of eight planets.
The whole collection comprises the Solar System.

3. The Solar System
All planets orbit the Sun in the same direction and nearly the same plane.
Six of the eight planets spin in the same direction.

4. The Solar System
The inner four planets (Mercury, Venus, Earth, and Mars) are known as terrestrial planets.

5. The Solar System
The next four extend from 5 to 30 AU from the Sun, are made partly or mostly of hydrogen and helium, and they are called Jovian planets.

6. The Solar System

7. The Solar System
The asteroids are small chunks of rock or metal that orbit the Sun in paths that lie largely between Jupiter and Mars.

8. The Solar System
Comets, made of dusty ice, are typically a few kilometers in diameter and orbit the Sun in elongated paths.

9. The Solar System

10. The Solar System
As the Earth moves around its orbit of the Sun, it continually collides with cometary debris and stray asteroids that heat up in our atmosphere and streak across the sky as meteors.

11. The Solar System
The cometary dust burns up in the atmosphere, but meteors more than about a centimeter or so across can survive to strike the ground to become meteorites.

12. Stars
In the nighttime sky, the stars and planets all seem to be the same distance from the Earth, but that is an illusion.
The nearest star (called Alpha Centauri) is an astonishing 271, 000 AU away!

13. Stars

14. Stars Such large distance require the use of a large unit.
The light- year (ly) is the distance a ray of light will travel in a year at a speed of 299,792 km per second (km/s).
Therefore, alpha Centauri is 4.3 ly away.

15. Stars
Another unit of measurement used in astronomical measurements is the parsec, which equals 3.26 ly.

16. Our Galaxy
All stars are arranged in individual galaxies, vast collections of matter tied together by gravity.
Galaxies are separated by vast distances.

17. Our Galaxy
The one we live in, called simply the Galaxy, contains about 200 billion stars and about 80,000 light years across.
The name of our galaxy is the Milky Way

18. The Milky Way

19. The Milky Way
The galactic halo is a spherical collection of stars that entirely surrounds and encloses the disk.
The disk extends as a flat, circular plane forming the shape of a galaxy.
The galactic bulge is a dense collection of stars in the center of the galaxy.

20. The Milky Way
Stars tend to form into doubles, triples, multiples, and clusters that are held together by the stars’ gravity.
In the disk, there are open clusters, which contain a few hundred stars.
The galactic halo contains huge globular clusters which contain tens or hundreds of thousands of stars.

21. The Milky Way

22. Other Galaxies
Our nearest neighboring galaxy is the Andromeda Galaxy or M 31.
It is about 2.3 million light years away, or about 30 times the diameter of our galaxy.

23. Types of Galaxies Elliptical Galaxies
Have no spiral arms and highly uniform

24. Types of Galaxies Spiral
include spiral arms with galactic disk and prominent nuclear bulge.

25. Types of Galaxies Irregular
Small systems that have little form or structure.

26. The Local Group
Like stars, galaxies tend to group in multiples and clusters.
Our galaxy, along with Andromeda and a dozen other galaxies, belong to the Local Group.

27. The Universe
With technology we can see galaxies and their clusters as distances of billions of light years.
We could count over 125 billion galaxies had we had the time!

28. The Expanding Universe
The Universe, as a whole, is expanding.
All galaxies are moving away from each other at an increasing rate.

29. The Big Bang
Because the Universe is expanding, it must have been compressed into a very small volume at some time in the past.
The explosive expansion of the Universe at the beginning of time is an event called the Big Bang.*
* This is one suggested theory for the creation of the Universe as described by astronomers and physicists.