Chapter 25

1. Stars and galaxies Chapter 25

2. Constellations • Ancient Greeks, Romans and other cultures saw patterns of stars in the sky called constellations • They imagined they represented mythological characters, animals or familiar objects. • Stars in constellations have no relationship to each other in space.

3. Modern constellations • The sky is now divided into 88 constellations • Many were named by ancient astronomers

4. Circumpolar constellations • Many constellations circle the North star, Polaris – Big Dipper, Little Dipper, Cepheus, etc. • Polaris is directly above the North pole • Constellations appear to move because of the Earth’s rotation • As Earth orbits the Sun, different constellations come into view. While others disappear (Orion is only seen in winter) • Circumpolar const. are visible all year

5. Absolute and apparent magnitude • some stars seem brighter than others, but that doesn’t mean they are closer to us or are really brighter • A star that is very dim might appear bright in the sky if it’s close to Earth, and a star that is very bright might appear dim if it is far away

6. Measurement in space • Measuring a star’s parallax tells us the distance to stars from our solar system • Scientists measure the apparent shift of an object from two different positions. • The closer an object is to Earth, the greater the parallax

7. We use light years to measure distance in space because space is so huge • Light travels 300,000 km/s or about 9.5 trillion km in one year • The nearest star to Earth, other than our Sun, is Proxima Centauri, which is 4.3 light years away, or 40 trillion km

8. Properties of stars • The color of a star tells us its temperature • Astronomers study star properties by looking at their spectra. • Light from a star passes through a spectroscope which breaks the light into all the colors of the spectrum • Dark lines in the spectra tell scientists which elements are in a stars atmosphere