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Characteristics of Stars Notes What is a spectrograph? •Breaks up the light from an object into a spectrum •A spectrograph is a photograph of the resulting spectrum How do astronomers use spectrographs? •To gather information about a star •Ex: chemical composition and temperature What is a galaxy? • A cluster of stars • Can contains hundreds of billions of stars • We live in the Milky Way galaxy • Distance across the Milky Way = 250 million billion kilometers • Even if you could travel at the speed of light it would still take you 25,000 years to cross the galaxy The Universe: •Defined as space and everything in it •Contains billions of galaxies Distances to the Stars: •Distances are so large that kilometers are not very practical •Instead astronomers use a unit called a light-year What is a light-year? • The distance that light travels in one year • It travels about 9.5 million million km • Travels at a speed of 300,000 km per second Parallax: • The apparent change in position of an object when you look at it from different places • One of the ways astronomers measure the distance between stars How are stars classified? 1. By size 2. By color and temperature 3. By brightness 1. By Size • Stars come in all different sizes • Our sun is a medium sized star • Betelgeuse is a supergiant star and would fill our solar system all the way to Jupiter 2. By color and temperature • A star’s color reveals its temperature • The more white/blue a star is the hotter it is • The more red a star is the cooler it is • Ex: Rigel (blue-white star) ~ 15,000 degrees Celsius • Ex: Betelgeuse (reddish star) ~ 5,500 degrees Celsius 3. Brightness of Stars • Brightness: the amount of light a star gives off • Depends on size and temperature • How bright a star looks from Earth depends on how far away it is • Brightness is described in 2 different ways Brightness: Apparent Magnitude • Brightness of a star as seen from Earth • This is not the most accurate method since the closer a star is to Earth the brighter it will look • Ex: the closer you are to a flashlight the brighter the light looks Brightness: Absolute Magnitude • The brightness a star would have if it was a standard distance from Earth • This requires an astronomer to determine both the apparent magnitude and distance from Earth • Ex: calculating absolute magnitude by making all stars 1 light-year away Hertzsprung-Russell Diagram: • Shows the relationship between surface temperature and brightness • There are many different versions of the diagram but they all contain the same information • Sometimes abbreviated as an “HR Diagram” An Example of an HR Diagram: Left-side Activity: • Answer questions 1-4 on page 289 • Answer the following questions using the HR diagram on page 289 1. Which star is hotter: Rigel or Aldebaran? 2. Which star is brighter: Sirius B or Betelgeuse? 3. Which star is a main sequence star: Polaris or the sun?