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Download Stars Unit 1-2: Stars
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Stars Unit 1-2: Stars How Far to a Star? • The closest star to Earth is the Sun. • The average distance from the Earth to QuickTime™ and a the Sun (since it varies greatly) is decompressor are needed150 to seemillion this picture. approximately kilometers. – This distance is designated as 1 Astronomical Unit, or AU. • The next closest star is Alpha Centauri, which is 4x1013km away, or 270000AU. How Far to a Star? • As you can imagine, if our neighboring star is almost 300000AU away, this unit doesn’t work too well for interstellar distances. • A second unit is the light-year, measures the distance that light travels in one year. – One light-year is approximately 9500000000000km. How Far to a Star • Using light-years, Alpha Centauri can be expressed as being 4.3 light-years away. – Sirius is 9 light-years away. – Vega, the brightest summertime star, is 27 lightyears away. – Polaris, the North Star, is 680 light-years away. – The Milky Way Galaxy is about 100,000 lightyears across. QuickTime™ and a decompressor are needed to see this picture. Physical Properties of Stars • The physical properties of a star include: – Size – Density – Mass – Color &Temperature – Brightness & Luminosity Size • Stars vary in size a great deal. – The sun has a diameter of approximately 1,380,000km. – Some stars are smaller than the Earth! – The largest observed stars have diameters some 2,000 time the size of the sun! • That’s approximately 2,760,000,000km. • For the record, there are just about 11 football fields in one km. Density • Although it seems that stars vary greatly in size, they vary even more in density! – Our sun has a density about 1.4 times greater than water. – Betelgeuse (don’t say it two more times!) is one-millionth the density of the sun. – Sirius is so dense, that one teaspoon of it would weigh more than a ton on earth! QuickTime™ and a decompressor are needed to see this picture. Mass • The difference between mass in stars in significantly less than the difference in density. – The sun’s mass is about 300,000 times that of Earth. Approx 2x1030kg. – Most stars are fairly close to the mass of the sun. Color & Temperature • The color of a star tells us the star’s approximate temperature. – Cooler stars are more red. QuickTime™ and a decompressor – Hotter stars are more blue. are needed to see this picture. – The sun is a yellow star, so it’s actually on the cooler side of the color spectrum. Brightness & Luminosity • Brightness is the perceived strength of the star’s light when observed from earth. – This is more commonly called apparent magnitude. • The actual brightness of a star is called its luminosity. Brightness & Luminosity • Luminosity depends on the stars temperature and size. • Apparent magnitude depends on the distance from the star to the Earth. • Example: – A 100W light bulb has a higher luminosity than a 20W flashlight bulb. – But if that flashlight bulb is right in front of your eye, it will appear brighter than the 100W bulb far away. Brightness & Luminosity • Both the brightness and luminosity of the stars is used to categorize them. • This is the purpose of studying star magnitudes. Elements in Stars • Scientists use spectral analysis to determine the composition of stars. – Spectral analysis is a process that uses a tool called a spectroscope to separate the different wavelengths of light coming from a star. – By observing which wavelengths are stronger, missing, or weaker, scientists can determine the composition of the star. Elements in Stars • Through spectral analysis, scientists have found stars are primarly made of the following elements: – Hydrogen – Helium – Rarely: • • • • • Iron Titanium Calcium Sodium Etc. Elements in Stars • Spectral analysis of the sun: – 70% Hydrogen – 28% Helium – 2% heavier elements • Every star has slightly different spectra, and every star has slightly different composition. • Almost every star powers itself by fusing hydrogen atoms together and making helium.