1. Section 2 – pg 598 Characteristics of Stars
Chapter 15
Section 2 – pg 598
Characteristics of Stars

2. Constellations: imaginary patterns seen in the night sky
Pg 598
Constellations: imaginary patterns seen in the night sky
Different cultures have given different names to the constellations
The stars in constellations look to be close together but just look that way

3. Classifying Stars Pg 599 All stars are huge spheres of glowing gas
Made of hydrogen
Produce energy through nuclear fusion
Astronomers classify stars based on physical characteristics
Color, temperature, size, composition, and brightness

4. Pg 599
Color and Temperature
Stars are different colors based on their temperatures
Coolest appear reddish – 3,200 Celsius
5,500 Celsius appear yellow
Hottest appear blueish – 20,000 Celsius

5. Size Pg 599 Stars appear to be the same
size, however are different sizes
Our sun is a medium-sized star
Very large stars are called giant stars or super giant stars
Most stars are smaller than our sun
White dwarfs are about the size of Earth
Neutron stars are smaller
If Betelgeuse was at the center of our solar system, it would fill up the center out to Jupiter

6. Chemical Composition Pg 600 Stars vary by what they are made of
Most stars are 73% hydrogen, 25% helium, and 2% other elements
Astronomers use spectrographs to determine the elements in stars
Spectrograph: a device that breaks light into colors and produces an image of the resulting spectrum
Most large telescopes have spectrographs

7. The gases within a star’s atmosphere absorb some wavelengths of light produced by the star
Light seen through a spectrograph shows missing wavelengths in the form of a dark line on a spectrum
Each chemical absorbs particular wavelengths
By comparing a star’s spectrum with the spectrums of known elements, scientists can infer what elements are in each star
Pg 600

8. Pg 600
Brightness of Stars
The brightness of a star depends upon both its size and temperature
Betelgeuse is fairly cool so its photosphere does not give up much light, but b/c of its size it appears to shine brightly
How bright a star looks from earth depends on its distance and how bright it really is
Apparent brightness, absolute brightness

9. Apparent Brightness Apparent brightness: brightness as seen from earth
Pg 601
Apparent Brightness
Apparent brightness: brightness as seen from earth
Measured using electronic devices
Can’t tell how much light is being given off by star from this
The sun looks like the brightest star in our sky just because it is so close

10. Pg 601
Absolute Brightness
Absolute brightness: luminosity, the brightness the star would have if it were at a standard distance from the Earth
Calculated from the stars distance and its apparent brightness

11. Measuring Distances to Stars
Pg 602
Measuring Distances to Stars
Light from the sun takes approx. 8 minutes to reach earth
The next closest star is Proxima Centauri, 4.2 light years away
It takes 4.2 years for the light from it to reach us

12. Pg 602
The Light-Year
Astronomers use a unit called light-year to measure distances between the stars
Light-year: the distance that light travels in one year
Light travels at a speed of about 300,000 km per second

13. Pg 602
Parallax
Astronomers often use parallax to measure distances to nearby stars
Parallax: the apparent change in position of an object when you look at it from different places

14. Parallax in Astronomy Pg 603
Astronomers are able to measure the parallax of nearby stars to determine their distances
Astronomers look at the star when Earth is on one side of the sun and compare it to what the star looks like when the Earth is on the other side
Measure how much the star appears to move against background stars
The less it appears to move, the farther away it is
Can use this technique to measure
distances a few light-years away
from Earth

15. The Hertzsprung-Russell Diagram
Pg 604
The Hertzsprung-Russell Diagram
Ejnar Hertzsprung in Denmark and Henry Norris Russell in US made graphs to find if the temps and absolute brightness of stars are related
Plotted the surface temp of stars on the x-axis and their absolute brightness on the y-axis
Formed a pattern
Called the Hertzsprung-Russell Diagram or H-R diagram

16. Pg 605
Astronomers use H-R diagrams to classify stars and to understand how stars change over time
Main diagonal area, called main sequence, includes 90% of all stars (including our sun)
Absolute brightness increases as surface temperature increases

17. Chapter 15 Section 2 Homework – pg 605

18. 1A. Name three characteristics used to classify stars

19. 1B. What is the difference between apparent brightness and absolute brightness?

20. 1C. Stars A and B have about the same apparent brightness, but Star A is about twice as far from Earth as Star B. Which star has the greater absolute brightness? Explain your answer.

21. 2A. What is a light-year?

22. 2B. What is parallax?

23. 2C. Vega is 25. 3 light-years from Earth and Arcturus is 36
2C. Vega is 25.3 light-years from Earth and Arcturus is 36.7 light-years away. Which star would have a greater parallax? Explain.

24. 3A. What two characteristics of stars are shown in an H-R diagram.

25. 3B. Identify two ways in which astronomers can use an H-R diagram.

26. 3C. The star Procyon B has a surface temp of 6,600 C and an absolute brightness that is much less than the sun’s. What type of star is Procyon B?