3Characteristics of Stars Color is a clue to a star’s temperatureVery 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 yellowBinary Stars – pairs of stars, pulled together by gravity, that orbit each otherBinary stars are used to determine the star property most difficult to calculate – its massThe mass of a body can be calculated if it is attached by gravity to a partner
7Measuring 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 isThe nearest stars have large parallax angles, while those of distant stars are too small to calculateLight-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
9Stellar BrightnessApparent Magnitude – a star’s brightness as it appears to EarthThree 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 isAbsolute Magnitude – how bright a star actually isTo 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
13Concept CheckWhat is the difference between apparent magnitude and absolute magnitude?
14Hertzsprung-Russell Diagram A Hertzsprung-Russell diagram shows the relationship between the absolute magnitude and temperature of starsMain-Sequence Star – This category contains the majority of stars and runs diagonally from the upper left to the lower right on the H-R diagramRed Giants – a large, cool star of high luminositySupergiants – a very large, very bright red giant starCepheid Variables – A star whose brightness varies periodically because it expands and contracts, a type of pulsating starNova – A star that explosively increases its brightness
16Concept CheckThe H-R diagram shows the relationship between what two factors?
17Interstellar Matter Nebulae – clouds of dust and gases in space Emission nebulae consist largely of hydrogen, they absorb ultraviolet radiation emitted by nearby starsReflection nebulae merely reflect the light of nearby starsAstronomers like to study nebulae because stars and planets form from them