DOCSLIB.ORG

11. Dead Stars

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Astronomy 110: SURVEY OF ASTRONOMY 11. Dead Stars 1. White Dwarfs and Supernovae 2. Neutron Stars & Black Holes Low-mass stars fight gravity to a standstill by becoming white dwarfs — degenerate spheres of ashes left over from nuclear burning. If they gain too much mass, however, these ashes can re-ignite, producing a titanic explosion. High-mass stars may make a last stand as neutron stars — degenerate spheres of neutrons. But at slightly higher masses, gravity triumphs and the result is a black hole — an object with a gravitational field so strong that not even light can escape. 1. WHITE DWARFS AND SUPERNOVAE a. Properties of White Dwarfs b. White Dwarfs in Binary Systems c. Supernovae and Remnants White Dwarfs in a Globular Cluster Hubble Space Telescope Finds Stellar Graveyard The Companion of Sirius Sirius weaves in its path; has a companion (Bessel 1844). 1800 1810 1820 1830 1840 1850 1860 1870 “Father, Sirius is a double star!” (Clark 1862). P = 50 yr, a = 19.6 au a3 = M + M ≃ 3 M⊙ P2 A B MA = 2 M⊙, MB = 1 M⊙ Why is the Companion So Faint? LB ≈ 0.0001 LA Because it’s so small! DB = 12000 km < D⊕ ≃ 6 3 ρB 2 × 10 g/cm The Dog Star, Sirius, and its Tiny Companion Origin and Nature A white dwarf is the degenerate carbon/oxygen core left after a double-shell red giant ejects its outer layers. Degeneracy Pressure Electrons are both particles and waves. h λ = The wavelength λ of an electron is me v e Rules for electrons in a box: 1. Waves must fit evenly. 2. Each wave must be different. (Note: the same rules apply to electron orbits around nuclei.) Degeneracy Pressure Electrons are both particles and waves. h λ = The wavelength λ of an electron is me v e Rules for electrons in a box: 1. Waves must fit evenly. 2. Each wave must be different. Suppose we compress the box: • all λ decrease ⇒ all v increase! • energy cost implies pressure Sizes of White Dwarfs Planet Earth 1 M⊙ White Dwarf 1.3 M⊙ White Dwarf surface gravity: 1 G surface gravity: 3.8×105 G surface gravity: ~2×106 G The higher a white dwarf’s mass, the smaller its radius! This trend continues to lower masses; Jupiter is about as large as degenerate objects get. White Dwarf Structure Visible surface: normal gas ~50 km thick; pure H or He Interior: degenerate matter typically C/O mixture Center: degenerate matter C/O nuclei in crystal form Galaxy's Largest Diamond Star gradually crystalizes from inside out as it cools. White Dwarfs in Binary Systems A white dwarf orbiting another star can become active when the other star becomes a red giant... Animation of Interacting Stars Accretion Disks Mira: "Wonderful" Star Reveals its Hot Nature Because it has angular momentum, the transferred gas orbits around the dwarf, forming an accretion disk. Friction in the disk moves angular momentum outward and mass inward. The disk becomes incandescent. Classical Novae H and He from the companion build up on the white dwarf’s surface. When enough has accumulated, the H burns violently, producing a thermonuclear explosion. Explosions from White Dwarf Star RS Oph Classical Novae: RS Ophiuci Classical Novae: RS Ophiuci 16 Feb, 2006 Recurrent Nova RS Ophiuci Explosions repeat every ~20 yr; about 10% of accreted mass is retained. Current mass: MWD ≃ 1.35 M⊙. The Limits of Degeneracy c We can add electrons if they have smaller wavelengths, λ. Smaller wavelengths imply higher velocities, v. elocity As v gets near the speed of light, v c, electrons behave like photons. Light (radiation) pressure makes stars unstable, so no white dwarf can weigh more than ~1.4 M⊙. 0 White Dwarf Supernovae If a white dwarf’s mass reaches 1.4 M⊙, carbon ignites in its degenerate center, and a thermonuclear explosion completely destroys the star. Illustration of Mira System White Dwarf Supernova Simulation 3-D Simulation of Type Ia Supernova Origin of the Elements Explosion makes ~0.8 M⊙ of Fe-group elements. Iron-Group Elements Most of the iron in the universe is made by white-dwarf supernovae. Supernovae Compared Two different scenarios produce titanic explosions: Massive Star SN White Dwarf SN Evolved star with initial White dwarf in binary mass > 8 M⊙. with nearby giant star. Degenerate Fe core Degenerate C/O star reaches 1.4 M⊙. reaches 1.4 M⊙. Gravitational collapse Nuclear burning of C/O 2 2 yields ~ 0.2 M⊙c ; yields ~ 0.002 M⊙c . 99% escapes as neutrinos. Supernovae Light Curves Ni56 → Co56: half-life 6 days Co56 → Fe56: half-life 77 days Massive-star and white-dwarf supernovae reach similar peak luminosities and fade gradually with time. Supernovae Remnants Inner remnant Outer shock wave (iron lines) (hydrogen lines) X-ray Visible White-Dwarf Supernova Remnant DEM L71: Supernova Origin Revealed Debris from supernova explosions expand at thousands of km/s in all directions, slamming into interstellar gas. Remnant of Tycho’s Supernova (1572) supernova relatavistic debris electrons Shock velocity 3000 km/s ~ 0.01 c Tycho's Supernova Remnant Provides Shocking Evidence for Cosmic Rays Remnants accelerate cosmic rays to near light-speed. The Crab Nebula (1054) The Crab Nebula from Hubble The Veil Nebula The Veil Nebula Unveiled 2. NEUTRON STARS AND BLACK HOLES a. Neutron Stars and Pulsars b. A Brief Introduction to Black Holes Nearest Known Neutron Star Hubble Sees a Neutron Star Alone in Space Origin and Nature Animation of Star Collapse A neutron star is the degenerate sphere of neutrons left after the iron core of a massive star collapses. Birth of a Neutron Star In the core, nuclei are smashed into protons & neutrons; the protons combine with electrons to make neutrons & neutrinos. At birth, the temperature of a neutron star is ~1011 K, but neutrino emission cools it to ‘only’ 106 to 107 K. Sizes of Neutron Stars Google Maps: Oahu Sizes of Neutron Stars ~20 km surface gravity: ~1011 G density: ρ ≈ 1015 g/cm3 Artist's impression of a neutron star Why Are Neutron Stars So Small? White dwarfs are supported by electron degeneracy; the electron wavelength is h λ = me v Neutron stars are supported by neutron degeneracy; the neutron wavelength is h λ = mn v Now, mn = 1840 me, so we expect neutron stars to be about 1840 (say, 2000) times smaller than white dwarfs. Why Do Neutron Stars Spin So Fast? Conservation of angular momentum: before collapsing, the star’s core probably rotates once every few hours. Collapse by a factor of x decreases the rotation period by a factor of x2. The core collapses by roughly a factor of 1000, so it spins about 10002 = 106 times more often. Final rotation period is a few hundredth’s of a second! Pulsars A pulsar is a spinning neutron star with a magnetic field tilted at an angle to its rotation axis. Pulsars Particles accelerated by the spinning field create two beams of radiation aligned with the magnetic axis. As the pulsar turns, these beams sweep through space. Discovery of Pulsars • Using a radio telescope in 1967, Jocelyn Bell noticed very regular pulses of radio emission coming from a single part of the sky. • The pulses were coming from a spinning neutron star —a pulsar. Copyright © 2009 Pearson Education, Inc. As Small As Why Pulsars Must Be Neutron Stars Circumference of Neutron Star = 2π (radius) ~ 60 km Spin Rate of Fast Pulsars ~ 1000 cycles per second Surface Rotation Velocity ~ 60,000 km/s ~ 20% speed of light ~ escape velocity from NS Anything else would be torn to pieces! Copyright © 2009 Pearson Education, Inc. The Crab Nebula Pulsar The Crab Nebula from Hubble The Crab Nebula Pulsar The Crab Nebula and Pulsar Crab Nebula: a Dead Star Creates Celestial Havoc Neutron Stars in Binary Systems X-Ray Bursts • Matter accreting onto a neutron star can eventually become hot enough for helium to fuse. • The sudden onset of fusion produces a burst of X rays. Copyright © 2009 Pearson Education, Inc. Degeneracy’s Limits, Again! c Just as with white dwarfs, there’s a maximum allowed mass for a neutron star, roughly 3 M⊙. elocity A neutron star which somehow v gains more mass presumably collapses to form a black hole. 0 A Brief Introduction to Black Holes Black Hole Images A Brief Introduction to Black Holes A black hole is an object with a gravitational field so strong that not even light can escape. Black Hole Images Thought Question What happens to the escape velocity from an object if you shrink it? A. It increases. B. It decreases. C. It stays the same. Hint: Copyright © 2009 Pearson Education, Inc. Escape Velocity initial kinetic final gravitational = energy potential energy (escape velocity)2 G × (mass) = 2 (radius) Copyright © 2009 Pearson Education, Inc. The Schwarzschild Radius Let’s insert the speed of light, c, into the escape-velocity equation: c2 G M = 2 R The result is a relationship between the mass, M, and radius, R, of a black hole. Solving for R, we get 2 G M R = c2 R is called the Schwarzschild Radius. Any object of mass M becomes a black hole if its radius is less than or equal to than R, because light is unable to escape. Sizes of Black Holes 18 km M = 3 M⊙ Google Maps: Oahu A black hole’s mass strongly warps space and time in the vicinity of the event horizon. Note: ‘event horizon’ is another term for the Schwarzschild Radius. These diagrams show how space becomes warped near a massive object or black hole.
Recommended publications
  • White Dwarfs

    White Dwarfs

    Chandra X-Ray Observatory X-Ray Astronomy Field Guide White Dwarfs White dwarfs are among the dimmest stars in the universe. Even so, they have commanded the attention of astronomers ever since the first white dwarf was observed by optical telescopes in the middle of the 19th century. One reason for this interest is that white dwarfs represent an intriguing state of matter; another reason is that most stars, including our sun, will become white dwarfs when they reach their final, burnt-out collapsed state. A star experiences an energy crisis and its core collapses when the star's basic, non-renewable energy source - hydrogen - is used up. A shell of hydrogen on the edge of the collapsed core will be compressed and heated. The nuclear fusion of the hydrogen in the shell will produce a new surge of power that will cause the outer layers of the star to expand until it has a diameter a hundred times its present value. This is called the "red giant" phase of a star's existence. A hundred million years after the red giant phase all of the star's available energy resources will be used up. The exhausted red giant will puff off its outer layer leaving behind a hot core. This hot core is called a Wolf-Rayet type star after the astronomers who first identified these objects. This star has a surface temperature of about 50,000 degrees Celsius and is A composite furiously boiling off its outer layers in a "fast" wind traveling 6 million image of the kilometers per hour.
  • R-Process Elements from Magnetorotational Hypernovae

    R-Process Elements from Magnetorotational Hypernovae

    r-Process elements from magnetorotational hypernovae D. Yong1,2*, C. Kobayashi3,2, G. S. Da Costa1,2, M. S. Bessell1, A. Chiti4, A. Frebel4, K. Lind5, A. D. Mackey1,2, T. Nordlander1,2, M. Asplund6, A. R. Casey7,2, A. F. Marino8, S. J. Murphy9,1 & B. P. Schmidt1 1Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT 2611, Australia 2ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia 3Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, AL10 9AB, UK 4Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 5Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden 6Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany 7School of Physics and Astronomy, Monash University, VIC 3800, Australia 8Istituto NaZionale di Astrofisica - Osservatorio Astronomico di Arcetri, Largo Enrico Fermi, 5, 50125, Firenze, Italy 9School of Science, The University of New South Wales, Canberra, ACT 2600, Australia Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis1–3. However, in Galactic chemical evolution models, neutron-star mergers alone cannot reproduce the observed element abundance patterns of extremely metal-poor stars, which indicates the existence of other sites of r-process nucleosynthesis4–6. These sites may be investigated by studying the element abundance patterns of chemically primitive stars in the halo of the Milky Way, because these objects retain the nucleosynthetic signatures of the earliest generation of stars7–13.
  • The Sun, Yellow Dwarf Star at the Heart of the Solar System NASA.Gov, Adapted by Newsela Staff

    The Sun, Yellow Dwarf Star at the Heart of the Solar System NASA.Gov, Adapted by Newsela Staff

    Name: ______________________________ Period: ______ Date: _____________ Article of the Week Directions: Read the following article carefully and annotate. You need to include at least 1 annotation per paragraph. Be sure to include all of the following in your total annotations. Annotation = Marking the Text + A Note of Explanation 1. Great Idea or Point – Write why you think it is a good idea or point – ! 2. Confusing Point or Idea – Write a question to ask that might help you understand – ? 3. Unknown Word or Phrase – Circle the unknown word or phrase, then write what you think it might mean based on context clues or your word knowledge – 4. A Question You Have – Write a question you have about something in the text – ?? 5. Summary – In a few sentences, write a summary of the paragraph, section, or passage – # The sun, yellow dwarf star at the heart of the solar system NASA.gov, adapted by Newsela staff Picture and Caption ___________________________ ___________________________ ___________________________ Paragraph #1 ___________________________ ___________________________ This image shows an enormous eruption of solar material, called a coronal mass ejection, spreading out into space, captured by NASA's Solar Dynamics ___________________________ Observatory on January 8, 2002. Paragraph #2 Para #1 The sun is a hot ball made of glowing gases and is a type ___________________________ of star known as a yellow dwarf. It is at the heart of our solar system. ___________________________ Para #2 The solar system consists of everything that orbits the ___________________________ sun. The sun's gravity holds the solar system together, by keeping everything from planets to bits of dust in its orbit.
  • Brown Dwarf: White Dwarf: Hertzsprung -Russell Diagram (H-R

    Brown Dwarf: White Dwarf: Hertzsprung -Russell Diagram (H-R

    Types of Stars Spectral Classifications: Based on the luminosity and effective temperature , the stars are categorized depending upon their positions in the HR diagram. Hertzsprung -Russell Diagram (H-R Diagram) : 1. The H-R Diagram is a graphical tool that astronomers use to classify stars according to their luminosity (i.e. brightness), spectral type, color, temperature and evolutionary stage. 2. HR diagram is a plot of luminosity of stars versus its effective temperature. 3. Most of the stars occupy the region in the diagram along the line called the main sequence. During that stage stars are fusing hydrogen in their cores. Various Types of Stars Brown Dwarf: White Dwarf: Brown dwarfs are sub-stellar objects After a star like the sun exhausts its nuclear that are not massive enough to sustain fuel, it loses its outer layer as a "planetary nuclear fusion processes. nebula" and leaves behind the remnant "white Since, comparatively they are very cold dwarf" core. objects, it is difficult to detect them. Stars with initial masses Now there are ongoing efforts to study M < 8Msun will end as white dwarfs. them in infrared wavelengths. A typical white dwarf is about the size of the This picture shows a brown dwarf around Earth. a star HD3651 located 36Ly away in It is very dense and hot. A spoonful of white constellation of Pisces. dwarf material on Earth would weigh as much as First directly detected Brown Dwarf HD 3651B. few tons. Image by: ESO The image is of Helix nebula towards constellation of Aquarius hosts a White Dwarf Helix Nebula 6500Ly away.
  • Supernovae Sparked by Dark Matter in White Dwarfs

    Supernovae Sparked by Dark Matter in White Dwarfs

    Supernovae Sparked By Dark Matter in White Dwarfs Javier F. Acevedog and Joseph Bramanteg;y gThe Arthur B. McDonald Canadian Astroparticle Physics Research Institute, Department of Physics, Engineering Physics, and Astronomy, Queen's University, Kingston, Ontario, K7L 2S8, Canada yPerimeter Institute for Theoretical Physics, Waterloo, Ontario, N2L 2Y5, Canada November 27, 2019 Abstract It was recently demonstrated that asymmetric dark matter can ignite supernovae by collecting and collapsing inside lone sub-Chandrasekhar mass white dwarfs, and that this may be the cause of Type Ia supernovae. A ball of asymmetric dark matter accumulated inside a white dwarf and collapsing under its own weight, sheds enough gravitational potential energy through scattering with nuclei, to spark the fusion reactions that precede a Type Ia supernova explosion. In this article we elaborate on this mechanism and use it to place new bounds on interactions between nucleons 6 16 and asymmetric dark matter for masses mX = 10 − 10 GeV. Interestingly, we find that for dark matter more massive than 1011 GeV, Type Ia supernova ignition can proceed through the Hawking evaporation of a small black hole formed by the collapsed dark matter. We also identify how a cold white dwarf's Coulomb crystal structure substantially suppresses dark matter-nuclear scattering at low momentum transfers, which is crucial for calculating the time it takes dark matter to form a black hole. Higgs and vector portal dark matter models that ignite Type Ia supernovae are explored. arXiv:1904.11993v3 [hep-ph] 26 Nov 2019 Contents 1 Introduction 2 2 Dark matter capture, thermalization and collapse in white dwarfs 4 2.1 Dark matter capture .
  • Chapter 16 the Sun and Stars

    Chapter 16 the Sun and Stars

    Chapter 16 The Sun and Stars Stargazing is an awe-inspiring way to enjoy the night sky, but humans can learn only so much about stars from our position on Earth. The Hubble Space Telescope is a school-bus-size telescope that orbits Earth every 97 minutes at an altitude of 353 miles and a speed of about 17,500 miles per hour. The Hubble Space Telescope (HST) transmits images and data from space to computers on Earth. In fact, HST sends enough data back to Earth each week to fill 3,600 feet of books on a shelf. Scientists store the data on special disks. In January 2006, HST captured images of the Orion Nebula, a huge area where stars are being formed. HST’s detailed images revealed over 3,000 stars that were never seen before. Information from the Hubble will help scientists understand more about how stars form. In this chapter, you will learn all about the star of our solar system, the sun, and about the characteristics of other stars. 1. Why do stars shine? 2. What kinds of stars are there? 3. How are stars formed, and do any other stars have planets? 16.1 The Sun and the Stars What are stars? Where did they come from? How long do they last? During most of the star - an enormous hot ball of gas day, we see only one star, the sun, which is 150 million kilometers away. On a clear held together by gravity which night, about 6,000 stars can be seen without a telescope.
  • OLLI: the Birth, Life, and Death Of

    OLLI: the Birth, Life, and Death Of

    The Birth, Life, and Death of Stars The Osher Lifelong Learning Institute Florida State University Jorge Piekarewicz Department of Physics [email protected] Schedule: September 29 – November 3 Time: 11:30am – 1:30pm Location: Pepper Center, Broad Auditorium J. Piekarewicz (FSU-Physics) The Birth, Life, and Death of Stars Fall 2014 1 / 12 Ten Compelling Questions What is the raw material for making stars and where did it come from? What forces of nature contribute to energy generation in stars? How and where did the chemical elements form? ? How long do stars live? How will our Sun die? How do massive stars explode? ? What are the remnants of such stellar explosions? What prevents all stars from dying as black holes? What is the minimum mass of a black hole? ? What is role of FSU researchers in answering these questions? J. Piekarewicz (FSU-Physics) The Birth, Life, and Death of Stars Fall 2014 2 / 12 The Birth of Carbon: The Triple-Alpha Reaction The A=5 and A=8 Bottle-Neck 5 −22 p + α ! Li ! p + α (t1=2 ≈10 s) 8 −16 α + α ! Be ! α + α (t1=2 ≈10 s) BBN does not generate any heavy elements! He-ashes fuse in the hot( T ≈108 K) and dense( n≈1028 cm−3) core 8 −8 Physics demands a tiny concentration of Be (n8=n4 ≈10 ) Carbon is formed: α + α ! 8Be + α ! 12C + γ (7:367 MeV) Every atom in our body has been formed in stellar cores! J. Piekarewicz (FSU-Physics) The Birth, Life, and Death of Stars Fall 2014 3 / 12 Stellar Nucleosynthesis: From Carbon to Iron Stars are incredibly efficient thermonuclear furnaces Heavier He-ashes fuse to produce: C,N,O,F,Ne,Na,Mg,..
  • A Review on Substellar Objects Below the Deuterium Burning Mass Limit: Planets, Brown Dwarfs Or What?

    A Review on Substellar Objects Below the Deuterium Burning Mass Limit: Planets, Brown Dwarfs Or What?

    geosciences Review A Review on Substellar Objects below the Deuterium Burning Mass Limit: Planets, Brown Dwarfs or What? José A. Caballero Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain; [email protected] Received: 23 August 2018; Accepted: 10 September 2018; Published: 28 September 2018 Abstract: “Free-floating, non-deuterium-burning, substellar objects” are isolated bodies of a few Jupiter masses found in very young open clusters and associations, nearby young moving groups, and in the immediate vicinity of the Sun. They are neither brown dwarfs nor planets. In this paper, their nomenclature, history of discovery, sites of detection, formation mechanisms, and future directions of research are reviewed. Most free-floating, non-deuterium-burning, substellar objects share the same formation mechanism as low-mass stars and brown dwarfs, but there are still a few caveats, such as the value of the opacity mass limit, the minimum mass at which an isolated body can form via turbulent fragmentation from a cloud. The least massive free-floating substellar objects found to date have masses of about 0.004 Msol, but current and future surveys should aim at breaking this record. For that, we may need LSST, Euclid and WFIRST. Keywords: planetary systems; stars: brown dwarfs; stars: low mass; galaxy: solar neighborhood; galaxy: open clusters and associations 1. Introduction I can’t answer why (I’m not a gangstar) But I can tell you how (I’m not a flam star) We were born upside-down (I’m a star’s star) Born the wrong way ’round (I’m not a white star) I’m a blackstar, I’m not a gangstar I’m a blackstar, I’m a blackstar I’m not a pornstar, I’m not a wandering star I’m a blackstar, I’m a blackstar Blackstar, F (2016), David Bowie The tenth star of George van Biesbroeck’s catalogue of high, common, proper motion companions, vB 10, was from the end of the Second World War to the early 1980s, and had an entry on the least massive star known [1–3].
  • Chapter 1: How the Sun Came to Be: Stellar Evolution

    Chapter 1: How the Sun Came to Be: Stellar Evolution

    Chapter 1 SOLAR PHYSICS AND TERRESTRIAL EFFECTS 2+ 4= Chapter 1 How the Sun Came to Be: Stellar Evolution It was not until about 1600 that anyone speculated that the Sun and the stars were the same kind of objects. We now know that the Sun is one of about 100,000,000,000 (1011) stars in our own galaxy, the Milky Way, and that there are probably at least 1011 galaxies in the Universe. The Sun seems to be a very average, middle-aged star some 4.5 billion years old with our nearest neighbor star about 4 light-years away. Our own location in the galaxy is toward the outer edge, about 30,000 light-years from the galactic center. The solar system orbits the center of the galaxy with a period of about 200,000,000 years, an amount of time we may think of as a Sun-year. In its life so far, the Sun has made about 22 trips around the galaxy; like a 22-year old human, it is still in the prime of its life. Section 1.—The Protostar Current theories hold that about 5 billion years ago the Sun began to form from a huge dark cloud of dust and vapor that included the remnants of earlier stars which had exploded. Under the influence of gravity the cloud began to contract and rotate. The contraction rate near the center was greatest, and gradually a dense central core formed. As the rotation rate increased, due to conservation of angular momentum, the outer parts began to flatten.
  • Size and Scale Attendance Quiz II

    Size and Scale Attendance Quiz II

    Size and Scale Attendance Quiz II Are you here today? Here! (a) yes (b) no (c) are we still here? Today’s Topics • “How do we know?” exercise • Size and Scale • What is the Universe made of? • How big are these things? • How do they compare to each other? • How can we organize objects to make sense of them? What is the Universe made of? Stars • Stars make up the vast majority of the visible mass of the Universe • A star is a large, glowing ball of gas that generates heat and light through nuclear fusion • Our Sun is a star Planets • According to the IAU, a planet is an object that 1. orbits a star 2. has sufficient self-gravity to make it round 3. has a mass below the minimum mass to trigger nuclear fusion 4. has cleared the neighborhood around its orbit • A dwarf planet (such as Pluto) fulfills all these definitions except 4 • Planets shine by reflected light • Planets may be rocky, icy, or gaseous in composition. Moons, Asteroids, and Comets • Moons (or satellites) are objects that orbit a planet • An asteroid is a relatively small and rocky object that orbits a star • A comet is a relatively small and icy object that orbits a star Solar (Star) System • A solar (star) system consists of a star and all the material that orbits it, including its planets and their moons Star Clusters • Most stars are found in clusters; there are two main types • Open clusters consist of a few thousand stars and are young (1-10 million years old) • Globular clusters are denser collections of 10s-100s of thousand stars, and are older (10-14 billion years
  • Brown Dwarfs: at Last Filling the Gap Between Stars and Planets

    Brown Dwarfs: at Last Filling the Gap Between Stars and Planets

    Perspective Brown dwarfs: At last filling the gap between stars and planets Ben Zuckerman* Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 Until the mid-1990s a person could not point to any celestial object and say with assurance known (refs. 5–9), the vast majority of that ‘‘here is a brown dwarf.’’ Now dozens are known, and the study of brown dwarfs has brown dwarfs are freely floating among come of age, touching upon major issues in astrophysics, including the nature of dark the stars (2–4). Indeed, the contrast be- matter, the properties of substellar objects, and the origin of binary stars and planetary tween the scarcity of companion brown systems. dwarfs and the plentitude of free floaters was totally unexpected; this dichotomy Stars, Brown Dwarfs, and Dark Matter superplanet from a brown dwarf, such as now constitutes a major unsolved problem in stellar physics (see below). lanets (Greek ‘‘wanderers’’) and stars how they formed, so that the dividing line Surveys for free floaters, both within have been known for millennia, but need not necessarily fall at 13 Jovian P ϳ100 light years of the Sun and in (more the physics underlying their differences masses. distant) clusters such as the Pleiades (The became understood only during the 20th Low-mass stars spend a lot of time, tens Seven Sisters), are still in their early century. Stars fuse protons into helium of billions to trillions of years, fusing pro- stages. But the picture is becoming clear. nuclei in their hot interiors and planets do tons into helium on the so-called ‘‘main Currently, it is estimated that there are not.
  • Lecture 15: Stars

    Lecture 15: Stars

    Matthew Schwartz Statistical Mechanics, Spring 2019 Lecture 15: Stars 1 Introduction There are at least 100 billion stars in the Milky Way. Not everything in the night sky is a star there are also planets and moons as well as nebula (cloudy objects including distant galaxies, clusters of stars, and regions of gas) but it's mostly stars. These stars are almost all just points with no apparent angular size even when zoomed in with our best telescopes. An exception is Betelgeuse (Orion's shoulder). Betelgeuse is a red supergiant 1000 times wider than the sun. Even it only has an angular size of 50 milliarcseconds: the size of an ant on the Prudential Building as seen from Harvard square. So stars are basically points and everything we know about them experimentally comes from measuring light coming in from those points. Since stars are pointlike, there is not too much we can determine about them from direct measurement. Stars are hot and emit light consistent with a blackbody spectrum from which we can extract their surface temperature Ts. We can also measure how bright the star is, as viewed from earth . For many stars (but not all), we can also gure out how far away they are by a variety of means, such as parallax measurements.1 Correcting the brightness as viewed from earth by the distance gives the intrinsic luminosity, L, which is the same as the power emitted in photons by the star. We cannot easily measure the mass of a star in isolation. However, stars often come close enough to another star that they orbit each other.