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Stars Chapter 30 Pages 775- Star • A ball of gases that gives off a tremendous amount of electromagnetic energy • Notice that I did not say light • Stars emit all wavelengths of the electromagnetic spectrum Analyzing Starlight • Astronomers analyze stars by looking at the light they emit • They use a spectrograph • A spectrograph separates light into different colors or wavelengths • Stars produce a display of colors and lines called a spectrum Three types of spectra • Emission or bright line • Absorption or dark line—dark line shows composition and temperature • Continuous Sources of Spectra Star classification • If we look at a star with a spectroscope we see dark lines called absorption spectra • These lines indicate the star’s chemical composition The most common element in stars is hydrogen Each chemical element has a characteristic spectra in a given range of temperatures The colors and lines indicate the elements that make up a star Chemical Composition Lab • Use the transparency and the paper copy to identify the elements found in the “Spectra of Unknown Composition” • Slide the transparency up and line the unknown spectra just above the Calcium lines – Are all lines present? – Then calcium is in the star • List results in two columns on your 3x5 card – Elements Present and Elements Not Present Color and Stars Think of a candle • Hottest part is blue • Red is the cool part at the top Examples • Temperature? • Composition? Apparent Motion of Stars • Time lapse photography show a circular pattern revolving around the star Polaris • We know Polaris as the North Star • It is located directly above the North Pole so as the Earth spins the stars appear to move Doppler Effect • The apparent shift in the wavelength of light emitted by a light source moving toward or away from a viewer Which is closest? • See how the lines shift farther to the red end of the spectrum as a star is farther away • The lines are from Ca, H, Na, and Mg parallax The apparent shift in a stars location when viewed from different locations in orbit The closer the star is, the more it appears to move Another view of parallax Absolute vs. Apparent Magnitude • Absolute—how bright a star actually is • Apparent—how bright it appears from Earth http://www.astronomynotes.com/starprop/s4.htm Stellar Evolution Section 2 Page 781 The Hertzsprung Russell diagram – HR Diagram – 90% of stars are found on a diagonal line called the main sequence – Hotter on the left cooler on the right – Brighter at top dimmer at bottom Another View of the HR Diagram Birth of Stars • Nebula—interstellar clouds of gas and dust • Drawn together by gravitational attraction called accretion • Begins to spin as it condenses • This huge ball of gas is called a protostar Birth of a Star Part 1 Birth of a star • This ball continues to be drawn together by gravitational attraction • When temperatures inside reach about 10million Kelvins nuclear fusion begins • This produces a huge amount of radiant and thermal energy • Ignition of nuclear fuel marks the change from protostar to star A Red Giant you know--Betelgeuse Main sequence star • Most of a star’s life is spent as a main sequence star • Its size, temperature, and color are relatively stable • During this time it burns up its supply of hydrogen • What happens next depends on the star’s size Red Giant • When the sun has burned all its hydrogen, the helium core will contract because of gravity • The rise in temperature will ignite the helium in the core and the core expands to become a red giant • Our sun will reach this stage in about 5 billion years Famous Red Giants • Antares The fate of stars differs according to their size (Low mass stars) • Small stars will shrink and throw off the outer layers in rings becoming a planetary nebula • The shrunken core that remains is a white dwarf – There is no nuclear fusion, it is only glowing hot – If it is a binary star it may form a nova, or, • When the core cools it becomes a black dwarf Medium Mass stars—like the sun Become RED GIANTS • Starts to burn helium-contracts because of gravity • This raises the temperature and the star expands • Our sun will reach this stage about 5 Billion years from now White Dwarf • What remains after the star swells and ejects the outer layers • What remains no longer burns fuel • It slowly cools • It is now a stellar remnant • It can change color as it cools Black Dwarf • A cold, dark lump of matter that remains when a star has burned out and cooled off Life cycle of a low mass star • • • • Nebula Protostar Main sequence Red Giant – Planetary nebula • White Dwarf • Black Dwarf Binary star interaction High Mass Stars have a different fate They burn faster And Die differently High mass stars • After the main sequence, stars with a mass much greater than the sun can burn and create larger and larger elements • When it gets to iron, it takes too much energy to create other elements so it collapses • This causes a supernova, this is when heavier elements are made • After a supernova a high mass star may become a • neutron star or a • black hole if it is VERY massive Life Cycle of a High Mass Star • • • • • Protostar Main sequence Red giant or red supergiant Supernova Neutron star or black hole Black Holes Singularity— density is infinite So how does the life cycle of a high mass star differ from a small mass star? • High mass burn quicker and brighter and burn out faster • They also have a different fate Groups of Stars • • • • Main sequence—diagonal line Red Giants– bright and cool Supergiants—brighter and cooler White Dwarfs—dim and hot Spectral Class • OBAFGKM • Oh Be A Fine Girl Kiss Me Revisit the HR diagram • Where are stars most of their lives? • Where are they when they begin to die? • What are they after they use up their fuel? How is the Life cycle of a high mass different???? • Starts the same but burns faster and ends differently • Either – Supernova then • Neutron star • Black hole Large Mass Stars have a different fate • They end in a supernova • Generate the elements of life • The inner part implodes to form a super dense neutron star The Hertzsprung –HR Diagram • 90% of stars are found on a diagonal line called the main sequence • Hotter on the left cooler on the right • Brighter at top dimmer at bottom Russell diagram Lab—HR diagram • • • • • • • • Study lists and answer 21.1 &21.2 Temp on horizontal Absolute magnitude on vertical Note graph lines are not equal Chart nearest stars with a (+) sign Chart brightest stars with a Show stars on both as circled + Use an * to show the sun on the diagram Word bank for paragraph • • • • • • • • • Black holes Nebula 1 million 1 thousand Light years Parsec White dwarf Magnitude Red giant • • • • • • • • • Black dwarf Fusion Fission Temperature Massive Neutron star Condense Closer Super nova Pulsars • emit low frequency radio transmissions – A few emit X-rays or visible light – The picture to the left is gamma ray burst animation a Extremely Massive stars • Extremely massive stars that undergo gravitational collapse may become a Event horizon—point of no return Gravity Lensing— or how we know where they are Image copyright © 1998 by John Chang. http://www.rdrop.com/users/green/school/detect.htm NASA Life Cycle of Stars Review Lab—HR diagram • • • • • • • • Study lists and answer 21.1 &21.2 Temp on horizontal Absolute magnitude on vertical Note graph lines are not equal Chart nearest stars with a (+) sign Chart brightest stars with a Show stars on both as circled + Use an * to show the sun on the diagram