1. Chapter 21, section 2: Characteristics of stars
Key concepts: How are stars classified? How do astronomers measure distances to the stars? What is an H-R diagram and how do astronomers use it?
Key terms: constellation, spectrograph, apparent brightness, absolute brightness, light-year, parallax, Hertzsprung-Russell diagram, main sequence

2. Stars Constellations: imaginary patterns made by stars
Example: Orion (seen in winter), Big Dipper, Little Dipper,

3. Classifying stars
Color, temperature, size, composition, and brightness

4. Color and temperature
If you look reeeeeeally carefully, you can see slight differences in the colors of stars.
Example: Betelgeuse, bright star in Orion’s shoulder, looks reddish. Rigel, star in Orion’s heel, is blue-white.
Color reveals the star’s surface temperature.
Hotter – 20,000 degrees C – glows blue
Middle – about 5,500 degrees – glows yellow
Cooler – about 3200 degrees C - reddish

5. Size
Many stars are about the size of the sun, which is a medium sized star. Many are much larger than that.
Very large are called giant stars or supergiant sars.
If a supergiant star (Betelgeuse) were where our sun is, it would be large enough to fill the solar system out to Jupiter (meaning, it would take the place of Mercury, Venus, Earth, and Mars and their orbits).
Most stars are smaller than the sun
White dwarfs – size of Earth
Neutron stars – smaller – only about 20 km in diameter

6. Chemical composition
Varies, but usually about 73% Hydrogen, 25% Helium, and 2% other elements.
Astronomers use spectrographs to determine the elements found in stars. A spectrograph is a device that breaks light into colors and produces an image of the resulting spectrum.
Gases in stars absorb some light. When the star’s light is seen through a spectrograph, each absorbed wavelength is shown as a dark line. Each element absorbs particular light. By comparing a star’s spectrum with that of known elements, astronomers can infer how much of each element is found in the star.

7. Brightness of stars Depends on both its size and temperature.
Remember: photosphere is the layer of a star that gives off light. Betelgeuse is cool, so its photosphere doesn’t give off much light per square meter. However, because it is so big, it shines quite brightly.
Rigel is very hot, so each square meter gives off a lot of light. Even though it is smaller, it shines more brightly.

8. Apparent brightness and absolute brightness
Apparent brightness is the brightness as seen from Earth. Astronomers measure it using electronic devices.
Absolute brightness is the brightness a star would have if it were at a standard distance from Earth. This is more complex than apparent brightness. Astronomers calculate apparent brightness and its distance from Earth. Astronomers can then calculate the absolute brightness.

9. Measuring distances to stars
The light year – the distance that light travels in 1 year, about 9.5 million million km. (it is distance, not time)
Parallax – often used – the apparent change in position of an object when you look at it from different places. Look at figure 9 on page 757. Astronomers measure how much stars have moved, over time, in reference against other stars that are much farther away. The less the star appears to move, the farther away it is. This is useful for stars less than a few hundred light years away.

10. Hertzsprung-Russell Diagram
About 100 years ago, those two guys made graphs to determine if temperature and absolute brightness of a star are related. They plotted surface temps on the x-axis against absolute brightness on the y-axis. It formed a pattern. The diagram (page 758) is still used today to classify stars and to understand how they change over time.

11. Main sequence stars
A diagonal area within the HR diagram. More than 90% of stars, including the sun.
Within the main sequence, surface temperature increases as absolute brightness increases.
High bluish stars are located at the left of an HR diagram and cooler reddish stars are located at the right.