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Video Field TripStars: Life and Death
1. What happens when stars run out of fuel?
2. What will happen when to the sun when it dies?
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Properties of Stars
Chapter 25, Section 1
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Characteristics of Stars
• Color is a clue to a star’s temperature• Very hot (30,000 K) stars emit their light in the
blue spectrum, red stars are much cooler, stars with temperatures between 5000 and 6000 K appear yellow
• Binary Stars – pairs of stars, pulled together by gravity, that orbit each other
• Binary stars are used to determine the star property most difficult to calculate – its mass
• The mass of a body can be calculated if it is attached by gravity to a partner
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Star Temperature
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Binary Stars
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Concept Check
• What is a binary star system?
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Measuring Distance to Stars
• Parallax is determined by taking a picture of a star at one time, and another picture six months later; using the angle between its apparent shift, astronomers can determine how far away it is
• The nearest stars have large parallax angles, while those of distant stars are too small to calculate
• Light-Year – unit used to express stellar distance, the distance light travels in one year (~9.5 trillion kilometers)
• Our closest star (besides the sun), Proxima Centauri, is about 4.5 light-years away from the sun
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Parallax
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Stellar Brightness
• Apparent Magnitude – a star’s brightness as it appears to 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 – how bright a star actually is
• To determine absolute brightness, astronomers measure how large the star is, what temperature it is, and what its apparent brightness would be at 32.5 light-years
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Stellar Brightness
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Concept Check
• What is the difference between apparent magnitude and absolute magnitude?
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Hertzsprung-Russell Diagram• A Hertzsprung-Russell diagram shows the
relationship between the absolute magnitude and temperature of stars
• Main-Sequence Star – This category contains the majority of stars and runs diagonally from the upper left to the lower right on the H-R diagram
• Red Giants – a large, cool star of high luminosity
• Supergiants – a very large, very bright red giant star
• Cepheid Variables – A star whose brightness varies periodically because it expands and contracts, a type of pulsating star
• Nova – A star that explosively increases its brightness
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Hertzsprung-Russell Diagram
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Concept Check
• The H-R diagram shows the relationship between what two factors?
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Interstellar Matter
• Nebulae – clouds of dust and gases in space
• Emission nebulae consist largely of hydrogen, they absorb ultraviolet radiation emitted by nearby stars
• Reflection nebulae merely reflect the light of nearby stars
• Astronomers like to study nebulae because stars and planets form from them
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Dark Nebula – Horsehead Nebula
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Assignment
• Read Chapter 25 Section 1 (pg. 700-706)
• Do Section 25.1 Assessment #1-7 (pg. 706)