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Galaxies

Chapter 9. Galaxies. What are they?. A huge group of stars, gas and dust held together by gravity. Occur in clusters Found throughout the universe The Solar System is a tiny part of the Milky Way Galaxy Thought to be our “Universe”.

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Galaxies

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  1. Chapter 9 Galaxies

  2. What are they? • A huge group of stars, gas and dust held together by gravity. • Occur in clusters • Found throughout the universe • The Solar System is a tiny part of the Milky Way Galaxy • Thought to be our “Universe”

  3. William Herschel maps out the distribution of stars and gets: The Sun at the Centre The “universe” of Herschel

  4. Where are we? The Milky Way as seen from above Could we see the centre of our galaxy from Earth?

  5. The Milky Way – seen from Earth • It was believed that Hera spilled milk in the sky • Collection of 400 billion stars

  6. “Dust Happened” At visible wavelengths, the center of our galaxy suffers ~ 30 mag of extinction by dust!! Even with big modern telescopes, we cannot see very far in the plane of our galaxy at visible wavelengths

  7. Mapping the Galaxy • Visible Light • Strong interstellar absorption • We can see only 4000 - 5000 pcs • Radio Light • Can observe entire galaxy

  8. Types of Galaxies • Elliptical • Spiral • Irregular

  9. Elliptical Galaxies • Most common • Composed of old stars, inter-stellar gas and dust

  10. Spiral Galaxies • Flat pin-wheels with arms spiralling out from a central region • "Arms" are composed of dust, gas and young stars. • The Milky Way is a spiral galaxy

  11. Irregular Galaxies • Mix of young and old stars with dust and gas • No shape but generally smaller • Generally as a result of galaxies that have collided or a newer galaxy that is still evolving a shape

  12. Activity 9-3 • How would you count the number of blades of grass on a soccer field? • Sampling technique • Count the amount in a manageable area • Multiply the small area to fit the whole field (7500m2) • What’s a manageable area?

  13. Counting Galaxies by Sampling – pg. 366 • Groups of 3 • Each group is responsible for one assigned square • Count the galaxies (dots) • Ignore the stars (spikes) • Average your group’s count of the Galaxies

  14. Calculation • Use the equation in the textbook to calculate the number of galaxies in the universe Total Area of the sky x Average Number Area of one Section per section 4.13x104 square degrees__ x Average 2.2 x 10-4 square degrees Count Answer Learning Check Questions & # 1 – 3 on pg. 366

  15. How much of the sky does this picture cover? Put a dime in your friend’s hand. Ask your friend to walk 20 m away from you and hold up the dime THAT’s the size of the sky this picture represents!

  16. Tomorrow • Prepare the following chart to fill in:

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