1. 25.1 Properties of Stars Objectives:
What can we learn by studying star
properties?
How does distance affect parallax
What factors determine a star’s
apparent magnitude?
What relationship is shown on a
Hertzprung-Russell Diagram?
25.1 Properties of Stars

2. 25.1 Properties of Stars Characteristics of Stars
 Star Color and Temperature
• Color is a clue to a star’s temperature.
A constellation is an apparent group of stars originally named for mythical characters. The sky contains 88 constellations.

3. The Constellation Orion
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4. Constellations of the Winter Sky
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5. 25.1 Properties of Stars Characteristics of Stars
 Binary Stars and Stellar Mass
• A binary star is one of two stars revolving around a common center of mass under their mutual gravitational attraction.
• Binary stars are used to determine the star property most difficult to calculate—its mass.

6. Common Center of Mass
For stars of equal mass, the center of mass lies in the middle.
A star twice as massive as its partner is twice as close to the center of mass. It therefore has a smaller orbit than its less massive partner.
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7. 25.1 Properties of Stars Measuring Distances to Stars  Parallax
• Parallax is the slight shifting of the apparent position of a star due to the orbital motion of Earth.
• The nearest stars have the largest parallax angles, while those of distant stars are too small to measure.
To us, the clock reads as if it is 12 after the hour, but to an observer in line with the light that is responsible for the minute hand’s shadow, the clock would appear as if it is 14 past the hour.

8. Photo taken 6 months later
Parallax
Original Photo
Photo taken 6 months later
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The parallax angle shown here is exaggerated to illustrate the principle. Because the distances to even the nearest stars are huge, astronomers work with very small angles.

9. Light Year
Light-Year
Due to the tremendous distances between stars, measuring distances in kilometers or astronomical units just isn’t practical.
A light-year is the distance light travels in a year about 9.5 trillion kilometers.
Name
Distance (light-years)
Sun —-
Alpha Centauri A
4.3
Alpha Centauri B
Alpha Centauri C
4.2
Barnard's Star
6.0
Wolf 359
7.7
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10. 25.1 Properties of Stars Stellar Brightness  Apparent Magnitude
• Apparent magnitude is the brightness of a star when viewed from Earth.
• Three factors control the apparent brightness of a star as seen from Earth: how big it is, how hot it is, and how far away it is.
 Absolute Magnitude
• Absolute magnitude is the apparent brightness of a star if it were viewed from a distance of 32.6 light-years.

11. Distance, Apparent Magnitude, and Absolute Magnitude of Some Stars
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12. 25.1 Properties of Stars Hertzsprung–Russell Diagram
 A Hertzsprung–Russell diagram shows the relationship between the absolute magnitude and temperature of stars.
 A main-sequence star is a star that falls into the main sequence category on the H–R diagram. This category contains the majority of stars and runs diagonally from the upper left to the lower right on the H–R diagram.

13. Hertzsprung–Russell Diagram
In this idealized chart, stars are plotted according to their temperature and their absolute magnitude.
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14. 25.1 Properties of Stars Hertzsprung–Russell Diagram
 A red giant is a large, cool star of high luminosity; it occupies the upper-right portion of the H–R diagram.
 A supergiant is a very large, very bright red giant star.

15. 25.1 Properties of Stars Hertzsprung–Russell Diagram  Variable Stars
• A Cepheid variable is a star whose brightness varies periodically because it expands and contracts; it is a type of pulsating star.
• A nova is a star that explosively increases in brightness.

16. Images of a Nova Taken Two Months Apart
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These two photographs show the decrease in brightness that follows a nova flare-up

17. 25.1 Properties of Stars Hertzsprung–Russell Diagram
 Interstellar Matter
• A nebula is a cloud of gas and/or dust in space.
• There are two major types of nebulae:
1. Bright nebula
- Emission nebula
- Reflection nebula
2. Dark nebula

18. The famous Horsehead Nebula found in the constellation Orion.
Interstellar Matter
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The famous Horsehead Nebula found in the constellation Orion.

19. 25.1 Properties of Stars Wrap-up Questions
How is the wavelength of light emitted from a star related to its temperature?
What is parallax and how do scientists use it to calculate distance?
What unit of length do astronomers use to measure vast distances?
Compare absolute magnitude to apparent magnitude.
What does a Hurtzsprung-Russell diagram compare?
Name two types of variable stars.