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?

Properties of Stars

Chapter 25, Section 1

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

Star Temperature

Binary Stars

Concept Check

• What is a binary star system?

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

Parallax

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

Stellar Brightness

Concept Check

• What is the difference between apparent magnitude and absolute magnitude?

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

Hertzsprung-Russell Diagram

Concept Check

• The H-R diagram shows the relationship between what two factors?

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

Dark Nebula – Horsehead Nebula

Assignment

• Read Chapter 25 Section 1 (pg. 700-706)

• Do Section 25.1 Assessment #1-7 (pg. 706)