Stars

1. Astronomy Stars

2. Introduction • Most stars look like faint dots of light in the night sky. • But stars are actually huge, hot, bright spheres of gas that are trillions of kilometers away from the Earth.

3. Color of Stars • Although red and yellow may be thought of as “warm” colors and blue may be thought of as a “cool” color, scientists consider the opposite. • BLUE = HOT • RED (and Yellow) = COOL • Betelgeuse, which is red, and Rigel, which is blue, are the stars that form two corners of the constellation Orion. • Because the stars are two different colors, we can conclude that they have different temperatures.

4. Cool Hot

5. Composition of Stars • When you look at white light through a glass prism, you see a rainbow of colors called a spectrum – including red, orange, yellow, green, blue, indigo, and violet. Continuous Spectrum Roy G. Biv

6. Composition of Stars • Astronomers use an instrument called a spectrograph to break a star’s light into a spectrum (discontinuous). • Because some light has been absorbed by the star, the spectrum provides information about the star’s composition AND temperature.

7. Emission Lines

9. Most stars are made of: Hydrogen and Helium

10. Temperature of Stars • In the 1800’s, astronomers started to collect and classify the spectra of many stars. • At first, letters were assigned to each type of spectra. • Stars were classified according to the elements of which they were made.

11. Brightness of Stars • With only their eyes to aid them, astronomers created a system to classify stars based upon their brightness. • They called the brightest stars in the sky first-magnitude stars and the dimmest stars sixth-magnitude stars. • When they began to use telescopes, astronomers were able to see many stars that had been too dim to see before. • Rather than replace the old system, of magnitudes, they added to it.

12. Brightness of Stars • Positive numbers represent dimmer stars and negative numbers represent brighter stars. • The brightest star in the night sky, Sirius (the “dog star”), has a magnitude of -1.4.

13. Brightness of Stars

14. Brightness of Stars • The brightness of a light or star as observed from Earth is called apparent magnitude. • Absolute magnitude is the actual brightness of a star. What factors determine a star's apparent magnitude?

15. Brightness of Stars • Factors that determine a star’s brightness are: • Temperature • Size

18. Distance to Stars • A light-year is the distance that light travels in one year. • The closest star to Earth is the SUN • 93 million miles or 150 million km away • The next closest star is the Alpha Centauri triple star system. • 4.3 light-years away (Proximi Centauri 4.2?) • The North Star (Polaris), which is part of Ursa Minor is 431 light-years away.

19. Distance to Stars • Astronomers use parallax and simple trigonometry to find the actual distance to stars that are close to Earth. • A star’s apparent shift in position is called parallax. • This can only be seen through telescopes.

21. Motions of Stars • Daytime and nighttime are caused by the Earth’s rotation. • The Earth’s tilt and revolution around the sun cause the seasons. • During each season, the Earth faces a different part of the sky at night. • This is why you see a different set of constellations at different times of the year. • Because stars are so distant, their actual motion is hard to see.

22. Motions of Stars • Because of Earth’s rotation, the sun appears to move across the sky. • Likewise, if you look at the night sky long enough, the stars also appear to move. • All of the stars appear to rotate around Polaris, the North Star, which is almost directly above the Earth’s North Pole. • Because of Earth’s rotation, all of the stars appear to make one complete circle around Polaris every 24 hours.

23. Apparent Motion

24. Astronomy Life Cycle of Stars

29. How are stars classified? • Brightness (magnitude) • Distance (light years) • Temperature • Color

30. 7.3 Evolution of Stars