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Liquid Methane lakes on Titan Liquid Methane lakes on Titan Announcements Lecture Series Extra-Credit Write-up due today. Mid-Term Exam #3 next Monday, Nov. 17 (review later). Course grade updated with HW #3. Haven’t done old homework? Now’s a good time to go back and pick up some points. Last Time: Lives of Stars Stars are born in cold molecular clouds, visible only in infrared and radio light. Cloud collapses, proto-star heats up. If hot enough, nuclear fusion of hydrogen will ignite, and a star is born. A disk of material coalesces around the star: it’s the solar nebula, from which planets form. Jets are sometimes launched from protostars to many thousands of AU. Last Time: Lives of Stars Stars form in groups, sometimes large groups, which can go on to be star clusters. Orion is a nearby stellar nursery, in which many stars are currently being born. Last Time: Lives of Stars Stars are in a constant struggle of gravity vs. pressure. Gravity wants to collapse the star, pressure wants to expand it. Normally, pressure is provided by the heat of the gas inside the star. In certain situations, however, another type of pressure can take over: Degeneracy pressure (electron or neutron musical chairs). Last Time: Lives of Stars Stars spend most of their lives burning hydrogen in the core on the main sequence, a narrow track in the HR diagram. More massive main sequence stars are bluer (hotter), larger, and more luminous that the much more common low mass stars. A star begins to die when it uses up its hydrogen fuel. How it dies depends on its mass. Last Time: Lives of Stars All stars: As H is burned up in core, it leaves behind He “ash”. When H is used up, core begins contracting, and H is burned in a shell around the core. Star’s outer layers expand, cooling and getting much brighter, becoming a Red Giant star. Eventually core contracts enough to become so hot it can fuse Helium to Carbon ( 3 He ⇒ 1 C): star is stable again (for a short while). Last Time: Low mass Stars Low mass stars (< 2.25 Msun) are supported by degeneracy pressure when their cores collapse at this stage. The core gets hotter and hotter but the pressure doesn’t increase! Broken thermostat! A “Helium Flash” occurs, in which a huge amount of helium is burned up in a few seconds. Last Time: Low mass Stars Once core helium runs out, it begins burning in a shell around the contracting core. Star becomes a Red Giant again. strong mass loss occurs via stellar winds. Lower mass stars (< 8 Msun) can’t burn carbon: degeneracy pressure stops the collapse, and their life is nearly over. Last Time: Low mass Stars They swell up and blow off all outer layers, forming a beautiful “planetary nebula”, and leaving behind a small hot remnant of the core called a “white dwarf”. This is the sun’s fate. White Dwarfs have a maximum mass of 1.4 Msun. Any larger white dwarf collapses into a neutron star or black hole. Last Time: High Mass Stars Higher mass stars (> 8 Msun) burn hydrogen much faster by using Carbon, Nitrogen, and Oxygen as a “go between”. Consequently, they live much shorter lives. Higher mass stars don’t have a helium flash, but start burning helium quickly after the core contracts. When burning H and He in shells, they swell up to Red Supergiants. Last Time: High Mass Stars When they run out of core helium, they can burn Carbon in their cores, and more. They add Helium to Carbon to make oxygen, burning in shells and in the core all the way to Iron (Fe). After iron, making larger atoms consumes rather than produces enery: bad news. Core collapses out of control, electrons are squeezed into protons forming neutrons, causing the star to explode as a supernova. Last Time: High Mass Stars When they run out of core helium, they can burn Carbon in their cores, and more. They add Helium to Carbon to make oxygen, burning in shells and in the core all the way to Iron (Fe). After they get to iron, making larger atoms consumes rather than produces enery: bad news. Last Time: High Mass Stars When all fuel is spent a massive star’s core collapses out of control, electrons are squeezed into protons forming neutrons, causing the star to explode as a supernova. All naturally occurring elements heavier than iron are produced during the fast supernova explosion. Artist’s Conception of Supernova A supernova can be as bright as a galaxy! Artist’s Conception of Supernova A supernova can be as bright as a galaxy! Cassiopeia A Supernova Remnant LIFE OF A HIGH-MASS STAR (25MSun) Low Mass star’s life in review (1 Msun) Protostar Red supergiant Blue main-sequence star Helium-burning supergiant Main sequence lifetime: 5 million years Multiple sh burning sup S Later Endpoint: Neutron star or black hole TIM Main sequence lifetime: 10 billion years Protostar Yellow main-sequence star LIFE OF A LOW-MASS STAR (1MSun) 1 Protostar: A star system forms when a cloud of interstellar gas collapses under gravity. 2 Yellow main-sequence star: In the core of a low-mass star, four hydrogen nuclei fuse into a single helium nucleus by the series of reactions known as the proton-proton chain. 3 Red giant star: After core hydrogen is exhausted, the core shrinks and heats. Hydrogen shell burning begins around the inert helium core, causing the star to expand into a red giant. 4 m-burning rgiant core d, d the sing oa Low Mass star’s life in review (1 Msun) Multiple shellburning supergiant Supernova Later stages: < 1 million years TIME Later stages: 1 billion years Endpoint: White dwarf Red giant star 4 Helium-burning star: Helium fusion begins when the core becomes hot enough to fuse helium into carbon. The core then expands, slowing the rate of hydrogen shell burning and allowing the star's outer layers to shrink. 5 Double shell-burning red giant: Helium shell burning begins around the inert carbon core after the core helium is exhausted. The star then enters its second red giant phase, with fusion in both a hydrogen shell and a helium shell. 6 Helium-burning star Double shell-burning red giant Planetary nebula: The dying star expels its outer layers in a planetary nebula, leaving behind the exposed inert core. 7 Planetary nebula White dwarf: The remaining white dwarf is made primarily of carbon and oxygen because the core of the low-mass star never grows hot enough to produce heavier elements. High Mass star’s life in review (25 Msun) !"#$%&'(('))*+,-$./0122-34-43"22-$5-#26722689'2:!; COSMIC CONTEXT FIGURE 12.22. STELLAR LIVES All stars spend most of their time as main-sequence stars and then change dramatically near the ends of their lives. This figure illustrates the life stages of a high-mass star and a low-mass star and shows how long a star spends in each stage of life. Notice that the lifetime of a low-mass star is far longer than that of a high-mass star. 1 Protostar: A star system forms when a cloud of interstellar gas collapses under gravity. 2 Blue main-sequence star: In the core of a high-mass star, four hydrogen nuclei fuse into a single helium nucleus by the series of reactions known as the CNO cycle. 3 Red supergiant: After core hydrogen is exhausted, the core shrinks and heats. Hydrogen shell burning begins around the inert helium core, causing the star to expand into a red supergiant. 4 Helium-burning supergiant: Helium fusion begins when the core temperature becomes hot enough to fuse helium into carbon. The core then expands, slowing the rate of hydrogen shell burning and allowing the star’s outer layers to shrink. 5 LIFE OF A HIGH-MASS STAR (25MSun) Protostar Blue main-sequence star Red supergiant Helium-burning supergiant Multiple sh burning sup S Main sequence lifetime: 5 million years Later Endpoint: Neutron star or black hole TIM Main sequence lifetime: 10 billion years usion s hot m into rate ning s m-burning rgiant High Mass star’s life in review (25 Msun) 5 Multiple shell-burning supergiant: After the core runs out of helium, it shrinks and heats until fusion of heavier elements begins. Late in life, the star fuses many different elements in a series of shells while iron collects in the core. 6 Supernova: Iron cannot provide fusion energy, so it accumulates in the core until degeneracy pressure can no longer support it. Then the core collapses, leading to the catastrophic explosion of the star. 7 Neutron star or black hole: The core collapse forms a ball of neutrons, which may remain as a neutron star or collapse further to make a black hole. Multiple shellburning supergiant Supernova Later stages: < 1 million years TIME Later stages: 1 billion years Endpoint: White dwarf White Dwarfs, Neutron Stars, and Black Holes The bizarre Stellar Graveyard The Stellar Graveyard All stars burn H to He on the main sequence. After H runs out, gravity or pressure will dominate, causing the star to change. After all fuel has been used, stars end their lives as either white dwarfs, neutron stars, or black holes, depending on their mass. White Dwarfs Earth white dwarf The exposed core of a dead low mass star which has shed it’s outer layers as a red giant. About the size of the earth. Very dense: 1tsp= several tons! White Dwarfs White dwarfs are supported by electron degeneracy pressure, which is why they don’t contract and burn their core material. They have no fusion going on; they cool and fade away. Usually made of carbon, but sometimes helium (very low mass stars can’t even burn helium). Later evolution of a White Dwarf: Nova We keep saying “super”nova, so what is a nova? A dying, bloated red giant gives its white dwarf binary companion mass. What is a Nova? When the surface temperature hits 107 K, this hydrogen ignites on the surface. Novae are as bright as 105 Suns. In contrast, supernovae are as bright as 1010 Suns! White Dwarf Supernova! If a white dwarf grows larger than 1.4Msun, electron degeneracy pressure cannot hold back the star’s gravity. it begins to collapse, temperature gets hotter, and it can burn fusion. A “carbon flash” occurs. The star is destroyed in an instant white dwarf supernova. Neutron Stars The remnant of a massive star’s supernova which ends its life can be a neutron star. Electrons are driven into protons to form neutrons: essentially a gigantic single nucleus. Neutron degeneracy pressure supports it against the intense gravity. Neutron stars are the size of a city (10 km). Neutron stars are even denser than white dwarfs because they are smaller! A sugar cube of a neutron star would weigh as much a all of humanity, 10 billion tons! What if you brought some home? A tiny Ball Bearing made of neutron star matter would instantly plunge through the earth, heading straight to towards the core. It would surface again on the opposite side of the planet, and fall again. Again and again it would fall until it settled at the center of the earth. What if you brought an entire Neutron star home? The entire Earth and solar system would be crushed onto it’s surface, forming a thin layer no more than 1 inch thick! How were neutron stars discovered? Jocelyn Bell, a 24 year-old graduate student, noticed a regular radio pulse coming from a certain position on the sky in 1967. The period is 1.337301 seconds. Pulsars are neutron stars that give off regular pulses. Neutron stars spin fast because they are so small (conservation of angular momentum). The Crab Nebula Pulsar The Pulsar at the center of the Crab Nebula pulses thirty times a second. Pulsars Pulsars are rotating neutron stars that act like lighthouses. Beams of radiation coming from the poles look like pulses as they sweep past Earth. Cartoon of a Pulsar The Crab Pulsar in X-rays How massive can a neutron star be? When a neutron star is larger than about 3 Msun, it’s gravity is so strong, even neutron degeneracy pressure can’t support it. Some massive star cores are this large, so after a supernova, the core continues to collapse further, and further. Nothing can stop it. Not nuclear fusion, not degeneracy pressure. Nothing. It becomes..... ...A Black Hole A black hole is an object whose gravity is so strong, not even light can escape it. To escape the earth, you have to go 25,000 mph. To escape a black hole, you have to travel faster than the speed of light. Thought Question What happens to the escape velocity of the Earth if you shrink it? a) it increases b) it decreases c) It stays the same Hint: Thought Question What happens to the escape velocity of the Earth if you shrink it? ✪ a) it increases b) it decreases c) It stays the same Hint: Black Holes If you shrank the Earth to a ball 3 cm in diameter, light could not escape it. The escape speed from earth is 25,000 mph. The escape speed from a black hole is faster than light. Nothing travels faster than light: NO ESCAPE! The radius around a black hole where the escape speed reaches the speed of light is called the EVENT HORIZON. It is the point of no return. We can’t observe what happens inside the event horizon. Space Time Consider this 2D rubber mat as a representation of space. Mass deforms space and what we perceive as gravity is a curvature of space. Space Time The planets orbit the Sun because the Sun’s mass curves space. Black Holes The “well” for a black hole is infinitely deep. Nothing, not even light, can get out. Black Hole If something/ someone falls beyond the event horizon, it/he/she can never communicate with the outside world again. What is it like to visit a black hole? In order to escape the gravity of the black hole, photons have to give up energy. The photons become gravitationally redshifted. This is similar to the Doppler shift, but has nothing to do with the motion of the light source. What is it like to visit a black hole? Time passes more slowly near the event horizon (as seen by an outside observer). Death By Black Hole How do we know black holes exist Black holes are invisible, but the material around them isn’t! As material is sucked down onto a black hole (from a mass losing binary companion, for instance), it emits strong X-rays. Objects can happily orbit a black hole; only when they get close are they in trouble. It does not “suck everything around it in”. Binary orbits give you an estimate of the mass of the “unseen” object. If greater than 3Msun, very likely a black hole. Gamma Ray Bursts Formation of black holes produces extreme amounts of energy, including “Gamma Rays”, higher energy than X-Rays. Artist’s rendition The most powerful luminous events in the universe! Discovered by satellites in the early 1960’s, searching for (illegal) nuclear bomb testing in the atmosphere. Gamma Ray Bursts The cover the sky in all directions: not associated with our Milky Way Galaxy. Giant Cosmic Explosions Hubble Space Telescope Image Exam #3 Mon, Nov 17th, in Class. 50 multiple choice problems Keep your exam copy for Buy-Back extracredit. Covering Chapter 5, 10–13, on light, spectra, telescopes, the sun, stars, stellar evolution, and the stellar graveyard. Exam #3, review Use: end of chapter guides, online “Study Area” quizzes. Also Study: Workbook Exercises (Blackbody Radiation, Analyzing Spectra, Apparent and Absolute magnitudes of stars, stellar evolution). Exam #3 Topics The nature of light: wavelength, frequency, energy, color. Thermal (Blackbody) radiation, and the impact of temperature on peak wavelength. The electromagnetic spectrum (of which visible is just a tiny part). Elements: Atomic mass Exam #3 Topics Spectra, emission, absorption, scattering of light. The (fixed) speed of light. Electron transitions in atoms, and the resulting emission spectrum. Absorption vs. emission spectra. Magnitude system: Apparent, absolute. Telescopes: advantages. Exam #3 Topics The Sun, it’s composition, layers, surface and core temperature, lifetime, etc. How the sun is powered, how long that power takes to get out, and to reach us. Sunspots: definition, impact on earth. Exam #3 Topics Parallax, and its use for measuring the distance to stars. Mass of a star, in controlling type. Luminosity (meaning). The spectral sequence of stars, and luminosity class. Exam #3 Topics Formation of stars in molecular clouds. Range of stellar masses, temperatures, luminosity. Clusters of stars, and their appearance on the HR diagram. Degeneracy pressure, and what it does. Exam #3 Topics Evolutionary paths of low-mass and highmass stars. Planetary Nebulae. Novae and binary stars. Supernovae: what types, which stars? Exam #3 Topics White dwarfs, neutron stars (& pulsars), and black holes. Basic characteristics (mass, size, etc.). How do they fit into the evolutionary picture? Gamma Ray Bursts. Where/what? Reminders Turn in Astronomer’s Lecture Series Extra Credit TODAY. Exam #3 next Mon. Tell your friends! Bring a #2 pencil and your rocket-ID.