Stars Chapter 11, Lesson 4 Where do stars come from? Stars form in a nebula, which is a large cloud of gas and dust in space. Gravity pulls some of the particles in the cloud together. When the center of the group has enough mass, pressure causes the temperature to increase. Stars • Stars are balls of gas, mostly hydrogen, that produce heat and light from fusion in their core. Fusion joins the nuclei of hydrogen atoms to form nuclei of helium atoms. • Scientists compare stars by their color, brightness and surface temperature. Star Classifications • Two astronomers independently developed diagrams of how absolute magnitude, or brightness is related to the temperature of stars. • The diagram that plots temperature vs. absolute magnitude is called the Hertzsprung-Russell Diagram. Star Classifications The mass of a star determines what kind of main sequence star it is. The more massive the star, the hotter it is. The brightest stars are at the top, with the dimmer ones at the bottom. The hottest stars are on the left, and the coolest ones are on the right. 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 Types • Stars have many diameters, masses, and surface temperatures. • Our Sun is a mediumsized star with a surface temperature of about 5800 K. Star Temperature • As metal gets hotter, it changes from red to yellow to white. • The color of stars also depends on temperature. 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. How Stars End • Eventually a star converts all its hydrogen to helium. • When the hydrogen is gone, the star is no longer a main sequence star. • Stars come to an end in different ways, depending on its mass. How Stars End A high mass star may use up all its hydrogen and swells into a supergiant. Then the supergiant explodes into a supernova. The core of the supernova might form a small, dense neutron star. The biggest supernovas collapse into extremely dense black holes.