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Notes - Stars Chapter 12, Lesson 1 Stars • Stars are balls of gas, mostly hydrogen, that produce light by nuclear reactions in their cores. Star Structure • Stars have layered structures. • Energy is produced in the core. • Temperatures range from 5,000,000 K to 100,000,000 K in the core. • Atoms separate from their nuclei, forming plasma. Star Structure Star Types • Stars have many diameters, masses, and surface temperatures. • Our Sun is a mediumsized star with a surface temperature of about 5800 K. Star Types Properties of Different Types of Stars Type Supergiant Diameter (1=Sun’s dia.) 100 – 1,000 Mass (1=Sun’s mass) 8-17 Surface Temp. (K) Variable Red Giant 10-100 1-4 3,000 – 4,000 Main Sequence White Dwarf 0.1 – 15 0.1 – 60 2,400 – 50,000 0.01 0.5 – 1.44 6,000 – 100,000 Neutron Star 0.00 1-4 variable Star Distances • One AU is the average distance between the Sun and Earth. • A light-year is the distance light travels in one year: 9,500,000,000,000 km or 63,000 AU. Star Distances Star Composition • Stars can only be studied by the light they emit. • A spectroscope is an instrument that is used to study light. • Astronomers can determine what elements are present in a star. Star Composition • Light split by a prism into a rainbow is a continuous spectrum. • A continuous spectrum is emitted by hot, dense materials, such as the gas of the Sun’s photosphere. Star Composition • Dark lines are sometimes seen in a spectrum, called an absorption spectra. • Absorption spectra are produced when light passes through cooler gases that absorb certain wavelengths. Star Composition • Each element absorbs only certain wavelengths. Star Composition • Absorption lines help astronomers identify elements in stars. Star Temperature • As metal gets hotter, it changes from red to yellow to white. • The color of stars also depends on temperature. Star Temperature • Every object emits electromagnetic radiation. • The wavelength emitted depends on the temperature of the object. - Objects at room temperature emit long, infrared waves. - As temperature rises, wavelengths become shorter. Star Temperature Star Temperature Oh Boy, Another F's Gonna Kill Me. The temperature of a star is indicated by the color it glows. (The Harvard Stellar Classification System) Type of Star Color Surface Temp (oC) O Blue Above 25,000 B Blue-White 10,000 – 25,000 A White 7,500 – 10,000 F Yellow-White 6,000 – 7,500 G Yellow 5,000 – 6,000 K Orange 3,500 – 5,000 M Red Below 3,500 Star Brightness • The brightness of stars depends on two things: energy and distance. • Light looks brighter as you move closer to the source. • Luminosity is measured by how much energy in joules is released per second. • One joule per second is called a watt. Star Brightness • Apparent magnitude is the apparent brightness of a star as measured on Earth. - Apparent magnitude depends on the star’s actual brightness and distance. - The smaller the magnitude number, the brighter the star. (http://spaceweather.com/flybys ) Star Brightness • Absolute magnitude is the apparent magnitude it would have if it were 32.6 light years away from Earth. Star Brightness Star Classifications • Two astronomers independently developed diagrams of how absolute magnitude, or luminosity is related to the temperature of stars. • The diagram that plots temperature vs. absolute magnitude is called the Hertzsprung-Russell Diagram. Star Classifications • 90% of stars fall on a diagonal, curved line, called the main sequence. • The remaining stars fall into one of three other groups. - Red giants - Supergiants - White dwarfs Star Classifications Star Classifications The apparent brightness of a star depends on what two things? A magnitude and distance B distance and temperature C distance and absolute brightness D absolute brightness and temperature 12.1 Stars A light-year is a unit of ____. A time B temperature C brightness D distance As a star increases in absolute magnitude, it appears ____ on Earth. A larger B hotter C brighter D more dense The average distance between Earth and the Sun is called a(n) ____. A light-year B astronomical unit C angstrom D solar unit SCI 4.d A star that is blue in color is ____ than a star red in color. A hotter B cooler C larger D smaller