Characteristics of Stars

1. Characteristics of Stars Chapter 3 Lesson 4 Page 110

2. How are Star Classified? • All stars are huge spheres of glowing gas • They are made mostly of hydrogen • They produce energy through nuclear fusion • They are classified according to their physical characteristics • Characteristics used to classify stars include: color, temperature, size, composition, and brightness

3. Color and Temperature • A star’s color reveals its surface temperature. • Coolest stars (3,200◦ C or 5,792◦ F) appear red • Yellow stars like the sun are about 5,500◦ C or 9,932◦) • The hottest stars appear blue (over 20,000◦ C or 36,032◦ F)

4. Size • Many stars are actually about the same size as the sun • Some are much larger called giant stars or supergiant stars • Most stars are smaller than the sun • White dwarfs are about the size of Earth • Neutrons stars are only about 20 km in diameter

5. White Dwarf Stars • About 1,600 light-years away, two dense white dwarfs in the J0806 binary star system orbit each other once every 321 seconds. When they reach the end of their long evolutions, smaller stars typically become white dwarfs

6. Neutron Stars • Despite their small diameters—about 12.5 miles (20 kilometers)—neutron stars boast nearly 1.5 times the mass of our sun, and are thus incredibly dense. Just a sugar cube of neutron star matter would weigh about one hundred million tons on Earth.

7. Chemical Composition • Stars very in chemical composition • The chemical composition of most stars if • 73%-hydrogen • 25%- helium • 2% other elements • This is close to the composition of the sun

8. Chemical Composition • Astronomers use spectrographs to determine elements found in stars • Spectrograph- a device that breaks light into colors and produces an images of the resulting spectrum. • Most large telescopes have spectrographs to analyze light • Read page 112

9. Brightness of A Star • Stars also differ in brightness or the amount of light they give off • The brightness of a star depends on both its temperature and size • A larger and hotter stars tends to be brighter • This is also determined by its distance from Earth and how bright it truly is

10. Brightness of Stars • The brightness of a star is described in two ways: • Apparent brightness • Absolute brightness

11. Apparent Brightness • Apparent brightness (apparent magnitude)- is its brightness as seen from Earth • Fairly easy to measure • Not really accurate because you can’t see how much light a star is really giving off • The sun looks brighter than the other stars simply because it is closer to Earth

12. Absolute Brightness • Absolute brightness(luminosity)- is the brightness the star would have is it were at a standard distance from Earth • Finding luminosity is more complicated, why? • First a scientists must find its apparent brightness and its distance away from Earth • Then they can calculate its absolute brightness • This can vary tremendously

13. What is an HR diagram? • Read page 114 • Hertzsprung-Russell (HR) diagram- graphs that find out if the temperature and the absolute brightness of stars are related. • Surface temperature on the x-axis • Absolute brightness on the y-axis • Look at the bottom of page 114

14. H.R. Diagrams- How Astronomers use them • Astronomers use H.R. diagrams to classify stars and to understand how stars change over time • Main sequence- a diagonal area on an Hertzsprung-Russell diagram that includes more than 90 percent of all stars • Surface temperature increases as absolute brightness increases • look at the diagram and read page 115