Lecture 19 - Stellar Evolution- White Dwarfs and Supernovae

Lecture 19 - Stellar Evolution- White Dwarfs and Supernovae...

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Unformatted text preview: Stellar Evolution - White Dwarfs and Supernovae ASTRONOMY 3 Lesson 19 1 The "Planetary Nebula" NGC 6543 with its central white dwarf as observed by Hubble Space Telescope in the visual and the Chandra satellites in X-rays (Y.-H Chu et al.). The image on the right shows a supernova in the spiral galaxy NGC 4526, the insert on lower right the outflow from the blue supergiant Sher 25 in the star forming region NGC 3603. NATURE of the Universe Astronomy - White Dwarfs & Supernovae Review of Lesson 18 The first indirect detections of planets around other stars are based on the Doppler effect. The variation in radial velocity of the star as it orbits its common center of mass with the planet is measured. Other indirect methods include astrometry and the search for transits. The direct detection of planets around stars is much more difficult. Planetary mass objects in orbit around brown dwarfs have been directly detected using ground-based telescopes. The detection and study of Earth-like terrestrial planets and the search for signs of life requires space missions, which are currently in various planning stages. 2 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Today's Topics The H-R diagram in time (Ch. 12.6, 12.1) Future Evolution of the Sun (Ch. 12.2, 12.3) Evolution of Stars more massive than the Sun (Ch. 12.4) Novae and Supernova (Ch. 12.3, 12.5) Summary & Results of Mid-term Exam 2 3 NATURE of the Universe Astronomy - White Dwarfs & Supernovae H-R diagram in time H-R diagram with main sequence stars only How does it change with time? 4 NATURE of the Universe Astronomy - White Dwarfs & Supernovae H-R diagram in time Stars on the main sequence produce energy by hydrogen fusion in their core. O-type star (hot, high mass) L-type star (cooler, low mass) Sun 1 million years 10 billion years Main sequence life-spans 10 trillion years In a star cluster, all stars have the same age, but various masses. We can use star clusters to study the evolution of stars. 5 NATURE of the Universe Astronomy - White Dwarfs & Supernovae H-R diagram in time NGC 2264 is young a star cluster, which formed less than 2 million years ago. Even its most massive stars are still on the main sequence, while the lowest mass stars are still in their pre main sequence phase. 6 NATURE of the Universe Astronomy - White Dwarfs & Supernovae H-R diagram in time The Pleiades (located in Taurus) are a star cluster, which formed 100 million years ago. The most massive stars are no longer on the main sequence. 7 NATURE of the Universe Astronomy - White Dwarfs & Supernovae H-R diagram in time M55 is a star cluster which formed 12 billion years ago. Only star with masses less than the Sun are still on the main sequence. Other stars have already evolved off the main sequence. 8 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun What causes stars to move off the main sequence? Their luminosity increases while their surface temperature drops. Hence they must drastically increase in size! Equilibrium (main sequence) core temperature and pressure increase Expanision new Equilibrium 9 NATURE of the Universe Astronomy - White Dwarfs & Supernovae main sequence star (for 10 billion years) giant star (for 1 billion years) Evolution of the Sun Summary of the Sun's future evolution Planetary nebula white dwarf 10 NATURE of the Universe Astronomy - White Dwarfs & Supernovae giant star Evolution of the Sun After 10 billion years, the Sun has used up all the hydrogen in the core. hydrogen shell fusion helium fusion in the core helium and hydrogen shell fusion As the hydrogen fusion moves into a shell around the helium core, the Sun extends to become a giant stars. As the core contracts, it becomes hot and dense enough to start helium fusion. Once all helium in the core is fused to carbon, helium fusion moves into a shell around the carbon core. 11 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun As a red giant star, the Sun will grow to 100 times its present size. Earth Mercury Venus Today Earth Venus In the process of becoming a red giant, the Sun will swallow Mercury and extend out to almost the orbit of Venus. 5 billion years into the future 12 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun Future Earth as the Sun evolves into a red giant (from Carl Sagan's "COSMOS") In the final stage, oceans on Earth will have evaporated, and the atmosphere will have escaped into space. 13 Paintings by A. Schaller, created for "COSMOS" by C. Sagan. NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun In the red giant phase the solar wind is strong and the Sun will loose more than 40% of its present mass. At the end of this phase, the hot core becomes exposed and starts to ionize the material blown off by the solar wind. The Helix nebula observed with the Hubble Space Telescope For historical reasons, astronomers call such nebulae a "Planetary Nebulae" as they appear "disk-like" and fuzzy like some of the outer planets when viewed through small telescopes. Animation modeling the formation of the Helix nebula The planetary nebulae phase lasts only a few 10,000 years. 14 See http://hubblesite.org/newscenter/archive/releases/2003/11/ and http://hubblesite.org/ newscenter/archive/releases/1996/13/ NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun In the mid 1800s, a faint companion to Sirius, the brightest star in the sky, was detected. The companion, named Sirius B, is 2.5 times hotter and 1,000 times less luminous than Sirius A. Sirius B 15 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun From observations of the orbits of Sirius A and Sirius B around their common center of mass, the mass of both stars can be computed using Newton's laws. Sirius B has about the same mass as the Sun and the same size as Earth. White dwarfs continue to cool down and to get fainter. 16 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Evolution of the Sun Finally, the hot, exposed carbon core of the Sun is all that left. The Sun has turned into a white dwarf. As a white dwarf the Sun will have about the same size as Earth. As it contains 54% of the mass of the present Sun, its density is very high (1 million times the density of water). How much could one "bench press" on a white dwarf? If a bench presser could lift 140 kg (300 lbs) on Earth, (s)he would be able to lift 1 gram (1/28 oz) on the surface of the white dwarf. 17 NATURE of the Universe Evolution of massive stars Astronomy - White Dwarfs & Supernovae Like low mass stars, high mass star loose matter by stellar winds. Light echos illuminating previously ejected layers around the red supergiant V838 Mon as observed by the Hubble Space Telescope over a period of 4 years. 18 See http://hubblesite.org/newscenter/archive/releases/2006/2006/50/ NATURE of the Universe Evolution of massive stars Astronomy - White Dwarfs & Supernovae Stars more massive than the Sun initially evolve quite similar, though they become blue and subsequently red supergiants. For stars with more than 4 times the mass of the Sun, nuclear fusion reactions lead to the creation of elements like oxygen, neon, magnesium, silicon, etc. 19 NATURE of the Universe Evolution of massive stars Astronomy - White Dwarfs & Supernovae How far does the fusion process go? (A little bit of nuclear physics) Fe C He O Cu Ag energy gain from fusion H For the most massive stars, the final outcome of the fusion process is iron. Fusion leading to elements beyond iron requires energy to be added. 20 NATURE of the Universe Evolution of massive stars Astronomy - White Dwarfs & Supernovae The most massive stars can derive energy from nuclear fusion leading up to the formation of iron (Fe) Where do the heavier elements come from? 21 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Novae & Supernovae The term "Nova" traces back to "stella nova", i.e. a new star on the sky, where previously no star had been observed. DQ Herculis This happens when a white dwarf lights up again, getting several 100 to up to 10,000 times brighter for a brief amount of time. during outburst: 1935 2 months later Where does the white dwarf find the energy for this sudden brightening? DQ Herculis Sirius Novae occur in binary systems consisting of a white dwarf and a giant or main sequence star. We can observe ejected material around the nova. May 1989 22 NATURE of the Universe Astronomy - White Dwarfs & Supernovae As the second star turns into a giant, hydrogen from its outer layers is falling on the white dwarf. When sufficient hydrogen has accumulated, temperatures and density get high enough to start hydrogen fusion. Novae & Supernovae Like a hydrogen bomb, the fusion creates a run-away explosion, which can be seen as a nova. As the explosion is taking place on the surface of the white dwarf, the white dwarf does not get destroyed. 23 NATURE of the Universe Astronomy - White Dwarfs & Supernovae What are Supernovae? Novae & Supernovae 24 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Novae & Supernovae There are two types of supernovae: A white dwarf supernova happens once a white dwarfs reaches a mass more than 1.4 times the mass of the Sun due to mass transfer from a companion. SciAm Oct 2006 25 You can find more about the latest research on supernova explosions in an article in the Oct 2006 issue of Scientific American - see http://www.sciam.com/issue.cfm?issuedate=Oct-06 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Novae & Supernovae A supernova can get a million times brighter than a nova, almost as bright as an entire galaxy of stars. Only very recently was a team of astrophysicists led by W. Hillenbrandt (MPA Garching) able to model a supernova explosion on their computer (previous models simply didn't explode!) white dwarf at start of explosion after 1 sec after 10 sec Note that after 10s, the supernova has already 70 times the size of the white dwarf. 26 NATURE of the Universe Astronomy - White Dwarfs & Supernovae A massive star supernova is the end of a star with a mass more than 8 times the mass of the Sun. Novae & Supernovae The explosion happens quite unevenly, as can be traced in supernova remnants. SciAm Oct 2006 27 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Novae & Supernovae Supernova remnants - the witnesses of past explosions Tycho Brahe's type I Supernova (1572) (white dwarf supernova) Crab nebula: type II Supernova (1054) (massive star supernova) 8 light years X-ray view depicting hot gas 10 light years Visual view depicting ionized gas 28 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Novae & Supernovae "We are star stuff" (Carl Sagan) "We are children of the universe" (Hoimar v. Ditfurth) What do they mean? All elements on Earth apart from hydrogen have been created in the interior of stars. All elements heavier than iron like copper, silver, gold, or uranium have been created in supernova explosions. The material ejected by stars, novae and supernovae forms the interstellar medium and dark clouds, which leads to new generations of star and planet formation. 29 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Summary: Stellar Evolution Summary The Sun will turn into a red giant once all the hydrogen in its core has been fused into helium. Subsequent fusion results in a carbon core. At the end of the red giant phase, the Sun will have lost almost half of its mass by winds. It will then turn into a carbon white dwarf. Stars more massive than the Sun gain energy by nuclear fusion leading up to iron. All element more massive (heavier) than iron can only be produce in supernova explosions. 30 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Homework Reading assignment: Chapter 13 No online Homework due! Think about: Can anything be denser than a white dwarf? Can anything travel faster than light? Homework 31 NATURE of the Universe Astronomy - White Dwarfs & Supernovae Mid-term exam 2: dose of statistics Mid-term Exam 2 - Stats Distribution of scores 8 perfect scores (45 correct answers) Median: 41 correct answers Mean: 39.6 correct answers 41 - 45 correct answers 36 - 40 correct answers 32 - 35 correct answers 27 - 31 correct answers 23 - 26 correct answers 18 - 22 correct answers 14 - 17 correct answers Answer key can be found on the class web page under "exams" You can view your individual score at my.ucla.edu See me during office hours in case you want to discuss specific questions and answers, or like, e.g., to review your scantron. 32 Answer key to Mid-term exam 2 is available at http://web.physics.ucla.edu/class/06F/ 3_BRANDNER/exams/index.html ...
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This note was uploaded on 10/13/2008 for the course ASTR 4 taught by Professor Wolfgang during the Fall '06 term at UCLA.

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Lecture 19 - Stellar Evolution- White Dwarfs and Supernovae...

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