1. Astronomy

2. What is Astronomy? 1. Astronomy is the study of the universe. This includes planets, stars, galaxies, black holes, moons, meteors, comets, asteroids and all of the matter that exists in space. 2. Astronomers are people who observe and study space.

3. Modern Calendar 1. Our modern calendar is based on the observations of bodies in our solar system. 2. A year is the time it takes for the Earth to orbit the sun. 3. A month is the time it takes for the moon to orbit the Earth. 4. A day is the time it takes for the Earth to rotate once on its axis.

4. The Size and Scale of our Universe 1. It is important to consider scale when we think about the universe. 2. Stars appear to be very small when viewed from Earth, but they are actually very large, some, like our sun, are bigger than Earth.

5. The Scale of our Solar System Earth

6. Scale of the Universe Mercury SUN

7. Our Galaxy and Scale Our Milky Way galaxy is simply huge. Just how big is our galaxy? Let's now pretend that our galaxy is a kid's sandbox, and our sun is a grain of sand in a sandbox. The Earth is a miniscule dust speck near the grain of sand, too small to be seen without a microscope. If our sun were a grain of sand in this sandbox representing the Milky Way galaxy, the sandbox would be somewhat oval and yet flat, and would be about 20 feet in diameter. The sand would be about 12 inches thick in the center, and thinner towards the edges.

8. Milky Way Galaxy

9. Distance in Space 1. Distance in space is measured in light years. 2. A light year is used to measure large differences in space. 3. A light year is the distance light travels in a year, 5.88 trillion miles. 4. Light travels at 186,000 miles per second.

10. Each swirling object you see is a galaxy in our universe. 1. We can estimate that there are about 100 billion stars in our galaxy. 2. Andromeda is the nearest major galaxy to our own Milky Way Galaxy.galaxyMilky Way Galaxy 3. Most scientists believe that Andromeda is about 2 million light years away from the Milky Way. THE MILKY WAY GALAXY

11. What is a Galaxy? 1. A large group of stars, gas, and dust held together by gravitational attraction. 2. A system of about 100 billion stars. 3. There are billions of galaxies in the observable universe.

12. 3 Types of Galaxies 1. Spiral 1. Spiral 2. Elliptical 2. Elliptical 3. Irregular 3. Irregular

13. Spiral Galaxy 1. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. disk starsgas and dustbulgedisk starsgas and dustbulge 2. The Milky Way galaxy is a spiral galaxy.

14. Irregular Galaxy 1. These are galaxies that feature neither spiral nor elliptical morphology. spiral elliptical morphologyspiral elliptical morphology 2. They are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.

15. Elliptical Galaxy 1. Elliptical galaxies have smooth, featureless light- profiles. 2. There is very little interstellar matter (neither gas nor dust), which results in low rates of star formation. interstellar matterstar formation interstellar matterstar formation 3. Elliptical galaxies are dominated by old stellar populations, or old stars. old stellar populations old stellar populations

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