1. Organizing the cosmos Galaxies & our Universe There is still uncertainty on exactly how the universe and our solar system formed. There are many theories but no one knows for sure. By studying distant galaxies astronomers may one day determine how the universe was formed

2. What is a Galaxy? A large group of stars, dust, and gases held together by gravity A large group of stars, dust, and gases held together by gravity Interstellar Matter- dust and gases found between stars Interstellar Matter- dust and gases found between stars

3. Our Milky Way Our Milky Way Size: large spiral disk 100,000 light year wide & 10,000 thick Structure: Spiral disk-like shape like our solar system Galaxy is Greek for milk **Our Sun is 1 of 100 Billion stars in the Milky Way Galaxy  The sun rotates around the galaxy once about every 240 million years

4. Pinwheel Galaxy Largest galaxy known to date The giant spiral disk of stars, dust and gas is 170,000 light- years across or nearly twice the diameter of our Milky Way. The galaxy is estimated to contain at least one trillion stars. Approximately 100 billion of these stars alone might be like our Sun in terms of temperature and lifetime. The giant spiral disk of stars, dust and gas is 170,000 light- years across or nearly twice the diameter of our Milky Way. The galaxy is estimated to contain at least one trillion stars. Approximately 100 billion of these stars alone might be like our Sun in terms of temperature and lifetime.

5. Andromeda Galaxy Our closest neighbor It is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen even from urban areas with binoculars 2.3 million ly away

6. Did You Know ? The Milky Way galaxy, and its closest neighbor, the Andromeda galaxy, are on a collision course with each other! This collision between these two spiral galaxies will likely occur in about 3 billion years, and the resulting galaxy is expected to be an elliptical. The Milky Way galaxy, and its closest neighbor, the Andromeda galaxy, are on a collision course with each other! This collision between these two spiral galaxies will likely occur in about 3 billion years, and the resulting galaxy is expected to be an elliptical.

7. Galaxy Classification In 1924, Edwin Hubble divided galaxies into different “classes” based on their appearance. Galaxies differ in size, shape, and ages of their stars Hubble classification serves as the basic language of the field.

8. Types of Galaxies Spirals & Ellipticals & Irregulars (oh my!)

9. 1. Spiral Galaxy Disk shaped (looks like a pinwheel) Disk shaped (looks like a pinwheel) Contain mixture of young/old stars Contain mixture of young/old stars Greater concentration of stars near the middle (older stars) Greater concentration of stars near the middle (older stars) Arms (younger stars) are seen extending from central nucleus Arms (younger stars) are seen extending from central nucleus Orderly rotational motion Orderly rotational motion Our own Galaxy is a spiral. Our own Galaxy is a spiral.

10. Types of Spiral Galaxies Normal Spiral Galaxies- a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars winding out from its center or nucleus Barred Spiral Galaxies- a galaxy having straight arms extending from its center Spiral arms attach to the bar instead of the center disk

11. 2. Elliptical Galaxies (not a disk) and often football shaped (not a disk) and often football shaped Smooth round shape Smooth round shape No arms No arms Very little interstellar matter leads to minimal star formations Very little interstellar matter leads to minimal star formations Composed of old stars Composed of old stars Random star motion Random star motion 60% of all galaxies 60% of all galaxies “Space footballs”

12. 3. Irregular Galaxies No exact size or shape No exact size or shape Composed of mostly young stars Composed of mostly young stars Lacks symmetry Lacks symmetry Small and faint Small and faint

13. Final Galaxy vocabulary…. 1. Most Galaxies are located in groups called a cluster. 2. Local Group – the name for the cluster of galaxies the Milky Way is in 3. Super cluster – Gigantic Cluster of galaxies Skip Quasars

14. Hubble’s Tuning Fork diagram classifying different galaxies by shape Use your Hubble’s Tuning Fork Diagram to help you identify these Galaxies: