Guiding Questions 1 Why do astronomers think that stars evolve bad use of term this is about the birth life and death of stars and that is NOT evolution 2 What kind of matter exists in the spaces between the stars 3 In what kind of nebulae do new stars form 4 What steps are involved in forming a star like the Sun 5 When a star forms why does it end up with only a fraction of the available matter 6 What do star clusters tell us about the formation of stars 7 Where in the Galaxy does star formation take place 8 How can the death of one star trigger the birth of many other stars The Birth of Stars 1 2 Understanding how stars evolve requires both observation and ideas from physics Because stars shine by thermonuclear reactions they have a finite life span That is they fuse lighter elements into heavier elements When the lighter elements are depleted there is nothing left to fuse The theory of stellar evolution not in the same sense as biological evolution but more like life cycle development like growing up describes how stars form and change during that life span 3 4 Interstellar gas and dust is ubiquitous the Galaxy Interstellar gas and dust which make up the interstellar medium ISM are concentrated in the disk of the Galaxy Clouds within the interstellar medium are called nebulae Dark nebulae are so dense that they are opaque They appear as dark blots against a background of distant stars Emission nebulae or H II regions are glowing ionized clouds of gas Emission nebulae are powered by ultraviolet light that they absorb from nearby hot stars Reflection nebulae are produced when starlight is reflected from dust grains in the interstellar medium producing a characteristic bluish glow 5 6 1 7 8 9 10 11 12 2 Protostars form in cold dark nebulae Star formation begins in dense cold nebulae where gravitational attraction causes a clump of material to condense into a protostar As a protostar grows by the gravitational accretion of gases KelvinHelmholtz contraction causes it to heat and begin glowing 13 14 Protostars evolve into main sequence stars A protostar s relatively low temperature and high luminosity place it in the upper right region on an H R diagram Further evolution of a protostar causes it to move toward the main sequence on the H R diagram When its core temperatures become high enough to ignite steady hydrogen burning it becomes a main sequence star 15 16 During the birth process stars both gain and lose mass The more massive the protostar the more rapidly it evolves In the final stages of pre main sequence contraction when thermonuclear reactions are about to begin in its core a protostar may eject large amounts of gas into space Low mass stars that vigorously eject gas are called T Tauri stars 17 18 3 A circumstellar accretion disk provides material that a young star ejects as jets Clumps of glowing gas called Herbig Haro objects are sometimes found along these jets and at their ends 19 20 21 22 Young star clusters give insight into star formation and evolution 23 Newborn stars may form an open or galactic cluster Stars are held together in such a cluster by gravity Occasionally a star moving more rapidly than average will escape or evaporate from such a cluster A stellar association is a group of newborn stars that are moving apart so rapidly that their gravitational attraction for one another cannot pull them into orbit about one 24 another 4 25 26 Star birth can begin in giant molecular clouds 27 Star forming regions appear when a giant molecular cloud is compressed This can be caused by the cloud s passage through one of the spiral arms of our Galaxy by a supernova explosion or by other mechanisms 29 The spiral arms of our Galaxy are laced with giant molecular clouds immense nebulae so cold that their 28 constituent atoms can form into molecules O and B Stars and Their Relation to H II Regions The most massive protostars to form out of a dark nebula rapidly become main sequence O and B stars They emit strong ultraviolet radiation that ionizes hydrogen in the surrounding cloud thus creating the reddish emission nebulae called H II regions Ultraviolet radiation and stellar winds from the O and B stars at the core of an H II region create shock waves that move outward through the gas cloud compressing the gas and triggering the formation of more protostars 30 5 31 Supernovae can compress the interstellar medium and trigger star birth 33 32 Life After the Main Sequence 34 A star s lifetime on the main sequence is proportional to its mass divided by its luminosity Guiding Questions 1 How will our Sun change over the next few billion years 2 Why are red giants larger than main sequence stars 3 Do all stars evolve into red giants at the same rate 4 How do we know that many stars lived and died before our Sun was born 5 Why do some giant stars pulsate in and out 6 Why do stars in some binary systems evolve in unusual ways 35 The duration of a star s main sequence lifetime depends on the amount of hydrogen in the star s core and the rate at which the hydrogen is consumed N B The more massive a star the shorter is its main36 sequence lifetime 6 The Sun has been a main sequence star for about 4 56 billion years and should remain one for about another 7 billion years 37 When core hydrogen fusion ceases a main sequence star becomes a red giant During a star s main sequence lifetime the star expands somewhat and undergoes a modest increase in luminosity 38 Red Giants Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main sequence star This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star The core shrinks and becomes hotter while the star s outer layers expand and cool The result is a red giant star 39 40 As stars age and become giant stars they expand tremendously and shed matter into space 41 When the central temperature of a red giant reaches about 100 million K helium fusion begins in the core A process called the triple alpha process converts 42 helium to carbon and oxygen 7 After the helium flash a low mass star moves quickly from the red giant region of the H R diagram to the horizontal branch In a more massive red giant helium fusion begins gradually In a less massive red giant it begins suddenly in a process called the helium flash 43 44 H R diagrams and observations of star clusters reveal how red giants evolve The age of a star cluster can be estimated by plotting its