Presentation on theme: "A105 Stars and Galaxies Today’s APOD ROOFTOP TONIGHT AT 9 PM"— Presentation transcript:
1A105 Stars and Galaxies Today’s APOD ROOFTOP TONIGHT AT 9 PM HAND IN HOMEWORKExam coming on Nov. 2
2Upcoming EventsOrionid meteor shower peaks Saturday night, view from 11:45 onward – if weather is clear, watch for at least 20 minutes from a dark siteTransit of MercuryNov. 8, 2:15 PM – SunsetFrom Sample Gate
5Star formation brings stars to the main sequence …What happens next?
6Explaining the HR Diagram EnergyGravityEnergy Transport
7Review: Why was the Sun’s energy source a major mystery? Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million yearsWhy does the Sun shine?The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.
8How does nuclear fusion occur in the Sun? The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chainGravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature
9Stellar Mass and Fusion The mass of a main sequence star determines its core pressure and temperatureStars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-livedStars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes
11Star Clusters and Stellar Lives Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observationsStar clusters are particularly useful because they contain stars of different mass that were born about the same time
12Evolution of a Very Low Mass Star (~0.3 solar masses) The entire star is convective.As hydrogen is consumed, the core shrinks and heats, the luminosity rises along the main sequence.Since convection occurs through the whole star, all the star’s hydrogen is burned.Leaves a helium remnant๏Lifetime: 300 Billion Years
13What are the life stages of a Sun-like star? A star remains on the main sequence as long as it can fuse hydrogen into helium in its coreWhat happens next?
14Life Track after Main Sequence Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over
15Sun-like stars become red giants When the helium core contracts, the surroundinghydrogen puffs up and the star becomes a red giant.
16Broken ThermostatAs the core contracts, H begins fusing to He in a shell around the coreLuminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting
17Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsionFusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon
18Once helium burning begins the “thermostat” starts to work again Once helium burning begins the “thermostat” starts to work again. Helium burning stars neither shrink nor grow because core thermostat is temporarily fixed.
19End of FusionFusion progresses no further in a Sun-like star because the core temperature never grows hot enough for fusion of heavier elementsElectron pressure from quantum mechanics supports the core against further gravitational contraction
20The End of Solar-type Stars MainSequencePlanetaryNebulaRedGiantWhiteDwarfWhen the carbon core reaches a density that is high enough, the star blows the rest of its hydrogen into space.The hot, dense, bare core is exposed!Surface temperatures as hot as 100,000 degreesThe hot core heats the expelled gas and makes it glow
21Planetary NebulaeFusion ends with a pulse that ejects the H and He into space as a planetary nebulaThe core left behind becomes a “white dwarf”
24Earth’s FateSun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth
25Earth’s FateSun’s radius will grow to near current radius of Earth’s orbit
26Summary The life stages of a Sun-like star H fusion in core (main sequence)H fusion in shell around contracting core (red giant)He fusion in coreHow does a Sun-like star end?Ejection of H and He in a planetary nebula leaves behind an inert white dwarf
27Life Stages of High-Mass Stars Late life stages of high-mass stars are similar to those of low-mass stars:Hydrogen core fusion (main sequence)Hydrogen shell burning (supergiant)Helium core fusion (supergiant)
28What about Massive Stars? Massive stars continue to generate energy by nuclear reactions until they have converted all the hydrogen and helium in their cores into iron.Once the core is iron, no more energy can be generatedThe core collapses and the star explodesSUPERNOVA!
29Core then suddenly collapses, creating supernova explosion Iron builds up in core until degeneracy pressure can no longer resist gravityCore then suddenly collapses, creating supernova explosionDeath_seq_of_high-mass_star.swfDownload a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)
30A “Recent” Supernova in Our Galaxy A new star in Taurus observed by the Chinese in 1054 A.D.Visible in the daytimeGradually faded; gone after about two yearsThe Crab Nebula is a supernova remnant
31The Crab Nebula Continues to Expand The Crab Nebula is about 7000 LY awayThe Nebula is about 10 LY acrossExpanding at a speed of about 1,400 kilometers per secondThe Crab Nebula - Then and NowImages taken in 1973 and recently
32The Large Magellanic Cloud Distance: about 150,000 LYPart of the Local Group“Irregular” galaxyLots of star formation
33Super- nova 1987a Feb. 1987 Star previously known – 18 solar masses Study formation of supernova remnant
34Rings around Supernova 1987A The supernova’s flash of light caused rings of gas around the supernova to glow
35SummaryThe life stages of a high-mass star are similar to the life stages of a low-mass starHigher masses produce higher core temperatures that enable fusion of heavier elementsA high-mass star ends when the iron core collapses, leading to a supernova
36Is life on Earth safe from harm caused by supernovae? Earth is safe at the present timebecause there are no massive starswithin 50 light years of the Sun.
37Sun-like Star Summary Main Sequence: H fuses to He in core Red Giant: H fuses to He in shell around He coreHelium Core Burning:He fuses to C in core while H fuses to He in shell4. Planetary Nebula leaves white dwarf behindNot to scale!
38Life Stages of High-Mass Star Main Sequence: H fuses to He in core Red Supergiant: H fuses to He in shell around He coreHelium Core Burning:He fuses to C in core while H fuses to He in shellMultiple Shell Burning:Many elements fuse in shells5. Supernova leaves neutron star behindNot to scale!
39Role of MassA star’s mass determines its entire life story because it determines its core temperatureHigh-mass stars with >8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosionsSun-like stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfsIntermediate mass stars can make elements heavier than carbon but end as white dwarfs
43Abundance of Elements in the Galaxy Goals:Know how chemical elements are createdin the Early Universein Starsin SupernovaeKnow how the Galaxy is enriched in chemical elements
44The Origin of ElementsThe process by which elements (nuclei) are created (synthesized) is called nucleosynthesisNucleosynthesis has occurred since the creation of the universe and will essentially go on foreverThe elements created come together to form everything material we know, including us
45Primordial Nucleosynthesis Hydrogen and helium were created during the BigBang while the Universe was cooling from its initialhot, dense state.About 10% of the lithium in the Universe today wasalso created in the Big Bang. We’re still not surewhere the rest comes from.The first stars formed from this material.
46Hydrogen Burning Stars burn hydrogen in their interiors to produce helium.Hydrogen burning also rearranges carbon,nitrogen, and oxygen.
47HeliumBurningThree helium atomscombine to form carbon
49The Iron Peak Metals In the cores of massive stars just before supernova explosions, atomic nucleiexchange protons and neutrons to formthe iron peak metals.
50Making Elements Up to Iron Hydrogen – from big bang nucleosynthesis.Helium – from big bang and from hydrogen burning via the p-p chain and CNO cycle.Nitrogen – from CNO cycle.Carbon, Oxygen – from helium burning.Light elements (Neon, Magnesium, Calcium – from carbon and oxygen burning.Iron metals – from the final burningMaking Elements Up to Iron
51Heavy Metals All heavier elements are formed when iron peak elements captureneutrons
52Elements Heavier than Iron … Once iron is formed, it is no longer possible to create energy via fusion.Elements heavier than iron require a different process (Iron is atomic number 26.)The heaviest naturally occurring nucleus is uranium (atomic number 92). How do we get to uranium then?Elements heavier than iron are created byneutron captureThe neutron is converted into a proton and added to the nucleus, increasing the atomic number to make the next element in the periodic table.
53Making Heavy Metals in Stars In low mass stars like the Sun, heavy metals are created when the star is a giantMassive stars make heavy metals when they become supernovae
54Stellar Nucleosynthesis We know now that all chemical elements heavier than atomic number 5 (Boron) were produced in stars.The light elements are essentially ashes of nuclear burning during the normal stellar evolution process.The heavier elements are produced in the envelopes of giants and during explosive nucleosynthesis that occurs during supernovae.
55Chemical Enrichment of the Universe We know now that massive stars act as factories for creating heavy elementsMassive stars end their lives in supernova explosionsThe explosion scatters the new elements into interstellar spaceElements synthesized inside stars are also brought to the surface and expelled via stellar windsA new generation of stars recycle this material, enriching it further
56The Galaxy (and the universe) is gradually enriched in heavy elements Despite all the nucleosynthesis that has occurred since the creation of the universe, only 2% of the ordinary matter in the universe is now in the form of heavy elements. Most is still hydrogen and helium
57EXAM NOV. 2nd Star Death – Units 67, 68, 69 News Quiz on Tuesday For Next Week...Star Death – Units 67, 68, 69News Quiz on TuesdayHomework Due EACH THURS.EXAM NOV. 2nd