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
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
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
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
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