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1. Uri looks through a telescope for two stars. He knows that both stars have the same absolute brightness and that the second star is twice as far from Earth as the first star. How bright will the second star appear compared to the first star?

A. It will appear brighter than the first star.B. It will appear dimmer than the first star. C. It will appear as bright as the first star.D. It will appear redder than the first star.

2. Chaya analyzes the data for two stars. Both stars are the same color, but the first star has an absolute brightness greater than the second star. What should Chaya conclude about the stars?

A. The first star is larger than the second star. B. The first star is smaller than the second star. C. The first star is hotter than the second star.D. The first star is cooler than the second star.

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3. Stars can be many different colors. Which star in the chart has the highest surface temperature?

A. Star A C. Star CB. Star B D. Star D

What are stars?What are stars?• Giant balls of flaming gases

– mostly hydrogen and helium

How big is Earth compared to our How big is Earth compared to our Star?Star?

How are stars classified?How are stars classified?

Characteristics of Stars (click for video)

Temperature

Color

Size

Absolute brightness (luminosity)

Apparent magnitude (brightness)

Characteristics of StarsCharacteristics of Stars• Magnitude (brightness)

– A measure of brightness of celestial objects

– Apparent magnitude (brightness)

• How bright a star appears to be from Earth

– Absolute brightness• How bright a star actually is

Of all the stars in the background, which is the brightest?

What type (apparent of absolute can you use to answer the question above?

Characteristics of StarsCharacteristics of Stars• Temperature & Color

– The color of a star indicates the temperature of the star (Think of a candle, the hottest colors at the bottom.)

The coolest stars are red.Medium temperature stars are orange and yellow.The hottest stars are blue.

H-R Diagram of stars help Astronomers to classify stars based on temperature, absolute brightness (luminosity), and sometimes size.

small & hot small &

cool

big & hot

big & cool

UnitsOf Sun’s

Hertzsprung-Russell Hertzsprung-Russell DiagramDiagram

Looking at stars of the same temperature, how is size related absolute brightness (luminosity)?

Hint: Compare white dwarfs to stars of the same temperature.

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1. Using the H-R Diagram above, describe the characteristics of a Class “O” Main sequence star.

2. Class “K” stars can be separated into three groups. They all share a similar temperature, but have different absolute magnitudes (brightness). How can astronomers distinguish these stars from one another?

3. Explain how a star with a low absolute brightness can have a higher apparent brightness than a star with a high absolute brightness.