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Stars Star Field as seen through the Hubble Space Telescope 2 Objectives • Describe how a protostar becomes a star. • Explain how a main-sequence star generates energy. • Describe the evolution of a star after its main-sequence stage. Review • Stars are essentially all made of the same material! – Hydrogen and – Helium • Luminosity differences are due to the differences in the temperatures of stars. • Hotter stars appear Blue • Cooler Stars appear Red • Stars are divided into seven major spectral or temperature classes • O, B, A , F, G , K, M (Oh Be A Fine Girl (Guy) Kiss Me • O – Hottest Stars • M – Coolest Stars One Exception • Since 1995 Astronomers have found new stars with surface temps even lower than spectral class M. – These are called Brown Dwarfs, and emit light in mostly the infrared range. • Classifying Stars • H-R Diagram (Hertzsprung + Russel) • In 1912 classification scheme for stars invented • Stars are plotted according to: • Luminosity (Absolute Brightness) and Temperature (Spectral Class) • Brightness increases as you move up the y-axis • Temperature decreases as you move to the right on the x-axis Classifying Stars • Notice that about 90% of stars fall within a band which runs diagonally through the middle of the diagram. These are Main Sequence stars. • This band extends from the hot, luminous blue stars to the cool, dim red stars – Ex- our Sun is a Main sequence star • Main sequence: the location on the H-R diagram where most stars lie; it has a diagonal pattern from the lower right to the upper left Understanding the H-R Diagram • Recall that two factors determine the luminosity (brightness) of a star: • Temperature • Size • So, a cooler star may still appear bright if it is very large like…. • Giants • Upper right hand side of diagram • Approximately 10 to 100 times larger than our Sun • Ex- Aldebaran 9 Relative Size of some Well Known Stars H-R Diagram of some Nearby stars Star Formation • A star forms in a dense, cold cloud of dust and gas made up of mostly Hydrogen and Helium called a Nebula. – Gravity pulls particles of the nebula closer together. – These regions spin, shrink, and form a flattened disk. Star Formation • The disk eventually forms a central concentration of matter called a protostar • The protostar continues to heat up due to further contraction, begins to glow, and develops Bipolar Outflow • No Fusion Yet…heat generated only by contraction Protostar and Bipolar Jets Protostar 14 Protoplanetary Disk- Photo taken by Hubble Space Telescope 15 The Eagle Nebula – Possible formation of Many stars. 16 Star Formation • Eventually, contraction produces a high enough temperature at the core and Nuclear Fusion Begins. • At this time, Hydrostatic Equilibrium is reached • Hydrostatic Equilibrium: when the outward force of pressure caused by nuclear fusion balances the inward force of gravity • ***Once fusion begins, Hydrostatic Equilibrium is reached and A MAIN SEQUENCE STAR IS BORN Hydrostatic Equilibrium – The outward pressure of Nuclear Fusion is EQUAL to the inward Pull of Gravity Our Sun- A Main Sequence Star Hydrogen Vs. Helium Concentrations over the Life of the SUN Time Frame for Star Formation • More mass = more heat = faster star formation • Less mass = less heat = slower star formation • Makes sense, right? • Our sun probably took about 50 million years to form Stellar Evolution • More Massive Stars have to burn hotter and faster to resist gravitational contraction and therefore use up their fuel quicker. • Are massive, hotter stars more or less luminous than others? • Less massive stars burn cooler and therefore can last longer • Our Sun will fuse hydrogen for about 10 billion years • Once a star’s Hydrogen supply runs out, fusion stops and the core begins to contract • At this time, the outer layers of hydrogen fuse at an incredible rate and the star expands to become a RED GIANT…. Demo Red Giants • Red Giants are very luminous due to their large size, but the heat is spread out over a larger area so stars in this stage are cooler than they were before burning through their supply of Hydrogen – That’s why it turns red! • Low mass stars such as our sun will become Red giants • Higher Mass stars will expand much further to become Red Supergiants Artist’s view of Earth and the Sun as a Red Giant Star 24 Leaving the Main Sequence Reading Check Where are giants and supergiants found on the H-R diagram? Giants and supergiants appear in the upper right part of the H-R diagram. Death of a Star • Once the hydrogen supply is depleted, the Red Giant contracts, heats up further, and begins fusing helium at a very high rate. • When Helium runs out, fusion stops, the core contracts further, and the outer layers expand again. • These outer layers of gas drift away to form a Planetary Nebula: a huge shell of gas and dust illuminated by the very hot exposed core of a star Butterfly Nebula in Ophiucus 27 Eskimo Nebula in Draco 28 Cat’s Eye Nebula in Draco 29 Final Stages of a Star • As the planetary nebula disperses, gravity causes the remaining matter in the star to collapse inward until it cannot be compressed further. • A hot, extremely dense core of matter - a white dwarf - is left. • White dwarfs – very hot but not luminous because of their small size – shine for billions of years before they cool completely and become a black dwarf. Sirius B is a white dwarf star shown next to a much brighter companion star, Sirius A. Supernova • Massive stars eventually become hot enough to fuse heavier elements in their core • These shrink very rapidly and rebound with a tremendous shock wave which blows apart the entire shell of the star in an explosion called a… Supernova!!! Star field seen before supernova Supernova • The explosion causes a dramatic increase in brightness • Energy released is more than 100 times what our sun will radiate over its entire lifetime • Supernovas outshine ALL the stars in its own galaxy COMBINED!! • May even be visible on earth during daylight hours • very rare After Supernova explosion 1987 Supernova in the Large Magellanic Cloud – Hubble Space Telescope 33 Veil Nebula – Remnant of a supernova that exploded about 15,000 years ago 34 Crab Nebula- A Remnant of a Supernova Explosion observed in 1054 AD which was bright enough to be seen during the day for over three weeks and during the night for 6 months 35 Supernova • A white dwarf which orbits a Red Giant may also become a supernova • The explosions involved completely destroy the white dwarf star and may destroy much of the red giant. White Dwarf and Companion Star will probably lead to a Supernova Neutron Stars • Neutron Star: a star which has collapsed to the point where electrons and protons have compressed enough to form neutrons • Only massive stars can become Neutron stars • More massive (heavy) than a white dwarf BUT only the size of a large city!!!!! • A paper clip made from a Neutron star would outweigh Mt. Everest Final Stages of Massive Stars Pulsars A rapidly spinning neutron star which emits pulses of radio and optical energy Black Holes • An object so massive and dense that even light cannot escape its gravity • Occur when the compression of a star’s core is so great that even neutrons cannot withstand the weight of their own gravity Black Hole’s Effect on a Planet