DOCSLIB.ORG

ASTR 565 Stellar Structure and Evolution Fall 2019 New Mexico State University

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
ASTR 565 Stellar Structure and Evolution Fall 2019 New Mexico State University 1 ASTR 565 Stellar Structure and Evolution Fall 2019 New Mexico State University Jason Jackiewicz Department of Astronomy December 5, 2019 2 Contents I Stellar Structure 7 1 Equilibrium and time scales 9 1.1 Energy equilibrium .......................................... 9 1.2 Local thermodynamic equilibrium .................................. 10 1.3 Are stars a one-fluid plasma? .................................... 10 1.4 Simple time scales of stars ...................................... 11 1.4.1 Dynamical timescale ..................................... 11 1.4.2 Thermal timescale ...................................... 12 1.4.3 Nuclear timescale ....................................... 12 1.5 Problems ............................................... 12 2 Nuclear interactions 15 2.1 Coulomb barrier ........................................... 15 2.2 Nuclear reaction rates ........................................ 15 2.3 Gamow peak ............................................. 17 2.4 Problems ............................................... 19 3 Energy release in nuclear reactions 21 3.1 Mass excess .............................................. 21 3.2 Binding energy ............................................ 22 3.3 Hydrogen burning .......................................... 23 3.3.1 PP-I chain .......................................... 24 3.3.2 PP-II and PP-III chains ................................... 25 3.3.3 CNO cycle .......................................... 26 3.4 Problems ............................................... 27 4 Distribution functions 29 4.1 An example .............................................. 29 4.2 Partition function ........................................... 30 4.2.1 Distinguishable particles ................................... 30 4.2.2 Identical fermions ...................................... 30 4.2.3 Identical bosons ....................................... 31 4.3 Derivation ............................................... 31 5 Mean molecular weight 33 6 Equation of state: Ideal gas 37 6.1 Preliminaries ............................................. 37 6.2 Maxwell-Boltzmann statistics .................................... 38 6.3 Ideal monatomic gas ......................................... 39 3 4 CONTENTS 7 Equation of state: Degenerate gas 41 7.1 Completely degenerate gas ...................................... 41 7.2 Partially degenerate gas ....................................... 44 8 Density-temperature equation of state landscape 45 8.1 Radiation pressure .......................................... 45 8.2 Putting it all together ........................................ 46 9 Hydrostatic Equilibrium 49 9.1 Derivation ............................................... 49 9.2 Simple solutions ........................................... 52 9.2.1 Linearized density ...................................... 52 9.2.2 Isothermal atmosphere .................................... 52 10 Polytropes 53 10.1 Motivation and derivation ...................................... 53 10.2 Lane-Emden equation ........................................ 54 10.3 Polytrope solutions .......................................... 55 10.4 Usefulness of polytropes ....................................... 58 11 Thermodynamics of an Ideal Gas 63 11.1 First law of thermodynamics .................................... 63 11.2 Adiabatic process ........................................... 64 12 Thermodynamics with Photons 67 12.1 Mixture of ideal gas and radiation: pressure effects ........................ 67 12.2 Mixture of ideal gas and radiation: ionization effects ....................... 69 12.3 Useful ideal gas equations ...................................... 72 13 Energy Transport: Radiation 73 13.1 Basics ................................................. 73 13.2 Diffusion ................................................ 74 13.3 Frequency dependence of radiation ................................. 75 14 Opacity sources 77 14.1 Kramer’s Laws ............................................ 77 14.2 Consequences ............................................. 79 15 Convection 83 15.1 Temperature gradients (“dels”) ................................... 83 15.2 The convective instability ...................................... 84 16 Convection 2 87 16.1 Other formulations of the instability ................................ 87 16.2 Physical conditions for convection onset .............................. 88 16.3 Depth of outer convection zones ................................... 89 16.4 Semiconvection ............................................ 89 16.5 Mixing length theory ......................................... 91 16.6 Convective overshoot ......................................... 93 16.7 An entropy formulation ....................................... 93 16.8 An energy formulation ........................................ 93 II Stellar Evolution 97 17 Overview of Asteroseismology 99 CONTENTS 5 18 Theory of the Main Sequence 101 18.1 Summary of stellar structure ....................................101 18.2 Homology relations for stars in radiative equilibrium . 102 18.3 Dependence on mass .........................................103 19 Evolution on the main sequence 107 19.1 Low-mass stars ............................................107 19.2 High-mass stars ............................................108 19.3 Summary of main-sequence properties ...............................111 20 The Terminal-Age Main Sequence and Subgiant Branch 115 20.1 Description of evolution movies ...................................115 20.2 TAMS .................................................115 20.3 Sch¨onberg-Chandrasekhar Limit ..................................117 20.4 The subgiant branch .........................................119 21 Towards and up the Red-Giant Branch 121 21.1 High-mass stars ............................................121 21.1.1 Low-mass stars ........................................122 21.2 Helium flash ..............................................124 22 Red-Giant Branch Morphology 127 22.0.1 RGB properties ........................................127 22.1 RGB location .............................................127 22.2 RGB bump luminosity ........................................127 22.3 RGB tip luminosity .........................................128 23 The Horizontal Branch 131 23.1 Quick tour of non-hydrogen nuclear reactions . 131 23.2 The horizontal branch properties ..................................132 23.3 Horizontal branch evolution .....................................133 23.3.1 High-mass stars ........................................133 23.3.2 Low-mass stars ........................................134 24 Asymptotic Giant Branch 137 24.1 General overview ...........................................137 24.2 Double-shell burning .........................................137 24.3 AGB evolution ............................................138 24.4 Thermal pulses ............................................139 24.5 Production of s elements .......................................141 25 Last Stages of Evolution: Low-Mass Stars 143 25.1 Planetary Nebula ...........................................143 25.2 White Dwarfs .............................................144 25.3 Futher WD properties ........................................146 25.4 Type Ia supernovae ..........................................147 26 Last Stages of Evolution: High-Mass Stars 149 26.1 Nuclear burning ...........................................149 26.2 Type II supernova - core collapse ..................................150 26.3 Neutron star .............................................152 26.4 Black hole ...............................................152 6 CONTENTS 27 Instability Strip and Pulsations 153 27.1 Background ..............................................153 27.2 Pulsation mechanisms ........................................154 27.3 Ionization zones ............................................156 Appendix A Conduction 157 A.1 Eddington Luminosity ........................................157 A.2 Conduction ..............................................158 Appendix B The Virial Theorem 159 Bibliography 163 Part I Stellar Structure 7 Unit 1 Equilibrium and time scales 1.1 Energy equilibrium Consider a thin layer at location r and of width dr in the nuclear burning region of a star. For energy • equilibrium, the net flow of energy crossing this region’s boundaries should be equal to the energy generation in the region. Let ε be the energy generated per gram of material per second, so the energy generated per second in • (r, dr) is εdm. Consider the luminosity (energy per second) L that carries energy away. In the layer, there should • be a balance between energy gains and energy losses (plus and minus denotes outgoing from center or toward center, respectively): + − − + εdm + Lr + Lr+dr = Lr + Lr+dr (1.1) εdm = [L+ L− ] [L+ L−]= L L (1.2) r+dr − r+dr − r − r r+dr − r εdm/dr = dL/dr, (1.3) by dividing by dr and letting it go to zero (derivative). This standard relation will appear many times: • dm ρ = , (1.4) dV or, dm = 4πr2ρ dr (1.5) If ρε is the energy produced per second in each cm3 of material, then by unit analysis the energy • generated per second in (r, dr) is also given by εdm = (4πr2dr)ρε. (1.6) Therefore, we can finally state • dL = 4πr2ρε. (1.7) dr Sometimes we will use m instead of r as the independent coordinate, and so • dL = ε. (1.8) dm 9 10 UNIT 1. EQUILIBRIUM AND TIME SCALES This is one of the main equations of stellar structure. • Note that in regions where ε = 0, the luminosity
Load more

Recommended publications
  • Hydrostatic Shapes
    7 Hydrostatic shapes It is primarily the interplay between gravity and contact forces that shapes the macroscopic world around us. The seas, the air, planets and stars all owe their shape to gravity, and even our own bodies bear witness to the strength of gravity at the surface of our massive planet. What physics principles determine the shape of the surface of the sea? The sea is obviously horizontal at short distances, but bends below the horizon at larger distances following the planet’s curvature. The Earth as a whole is spherical and so is the sea, but that is only the first approximation. The Moon’s gravity tugs at the water in the seas and raises tides, and even the massive Earth itself is flattened by the centrifugal forces of its own rotation. Disregarding surface tension, the simple answer is that in hydrostatic equi- librium with gravity, an interface between two fluids of different densities, for example the sea and the atmosphere, must coincide with a surface of constant ¡A potential, an equipotential surface. Otherwise, if an interface crosses an equipo- ¡ A ¡ A tential surface, there will arise a tangential component of gravity which can only ¡ ©¼AAA be balanced by shear contact forces that a fluid at rest is unable to supply. An ¡ AAA ¡ g AUA iceberg rising out of the sea does not obey this principle because it is solid, not ¡ ? A fluid. But if you try to build a little local “waterberg”, it quickly subsides back into the sea again, conforming to an equipotential surface. Hydrostatic balance in a gravitational field also implies that surfaces of con- A triangular “waterberg” in the sea.
    [Show full text]
  • X-Ray Sun SDO 4500 Angstroms: Photosphere
    ASTR 8030/3600 Stellar Astrophysics X-ray Sun SDO 4500 Angstroms: photosphere T~5000K SDO 1600 Angstroms: upper photosphere T~5x104K SDO 304 Angstroms: chromosphere T~105K SDO 171 Angstroms: quiet corona T~6x105K SDO 211 Angstroms: active corona T~2x106K SDO 94 Angstroms: flaring regions T~6x106K SDO: dark plasma (3/27/2012) SDO: solar flare (4/16/2012) SDO: coronal mass ejection (7/2/2012) Aims of the course • Introduce the equations needed to model the internal structure of stars. • Overview of how basic stellar properties are observationally measured. • Study the microphysics relevant for stars: the equation of state, the opacity, nuclear reactions. • Examine the properties of simple models for stars and consider how real models are computed. • Survey (mostly qualitatively) how stars evolve, and the endpoints of stellar evolution. Stars are relatively simple physical systems Sound speed in the sun Problem of Stellar Structure We want to determine the structure (density, temperature, energy output, pressure as a function of radius) of an isolated mass M of gas with a given composition (e.g., H, He, etc.) Known: r Unknown: Mass Density + Temperature Composition Energy Pressure Simplifying assumptions 1. No rotation à spherical symmetry ✔ For sun: rotation period at surface ~ 1 month orbital period at surface ~ few hours 2. No magnetic fields ✔ For sun: magnetic field ~ 5G, ~ 1KG in sunspots equipartition field ~ 100 MG Some neutron stars have a large fraction of their energy in B fields 3. Static ✔ For sun: convection, but no large scale variability Not valid for forming stars, pulsating stars and dying stars. 4.
    [Show full text]
  • Stars IV Stellar Evolution Attendance Quiz
    Stars IV Stellar Evolution Attendance Quiz Are you here today? Here! (a) yes (b) no (c) my views are evolving on the subject Today’s Topics Stellar Evolution • An alien visits Earth for a day • A star’s mass controls its fate • Low-mass stellar evolution (M < 2 M) • Intermediate and high-mass stellar evolution (2 M < M < 8 M; M > 8 M) • Novae, Type I Supernovae, Type II Supernovae An Alien Visits for a Day • Suppose an alien visited the Earth for a day • What would it make of humans? • It might think that there were 4 separate species • A small creature that makes a lot of noise and leaks liquids • A somewhat larger, very energetic creature • A large, slow-witted creature • A smaller, wrinkled creature • Or, it might decide that there is one species and that these different creatures form an evolutionary sequence (baby, child, adult, old person) Stellar Evolution • Astronomers study stars in much the same way • Stars come in many varieties, and change over times much longer than a human lifetime (with some spectacular exceptions!) • How do we know they evolve? • We study stellar properties, and use our knowledge of physics to construct models and draw conclusions about stars that lead to an evolutionary sequence • As with stellar structure, the mass of a star determines its evolution and eventual fate A Star’s Mass Determines its Fate • How does mass control a star’s evolution and fate? • A main sequence star with higher mass has • Higher central pressure • Higher fusion rate • Higher luminosity • Shorter main sequence lifetime • Larger
    [Show full text]
  • Astronomy 201 Review 2 Answers What Is Hydrostatic Equilibrium? How Does Hydrostatic Equilibrium Maintain the Su
    Astronomy 201 Review 2 Answers What is hydrostatic equilibrium? How does hydrostatic equilibrium maintain the Sun©s stable size? Hydrostatic equilibrium, also known as gravitational equilibrium, describes a balance between gravity and pressure. Gravity works to contract while pressure works to expand. Hydrostatic equilibrium is the state where the force of gravity pulling inward is balanced by pressure pushing outward. In the core of the Sun, hydrogen is being fused into helium via nuclear fusion. This creates a large amount of energy flowing from the core which effectively creates an outward-pushing pressure. Gravity, on the other hand, is working to contract the Sun towards its center. The outward pressure of hot gas is balanced by the inward force of gravity, and not just in the core, but at every point within the Sun. What is the Sun composed of? Explain how the Sun formed from a cloud of gas. Why wasn©t the contracting cloud of gas in hydrostatic equilibrium until fusion began? The Sun is primarily composed of hydrogen (70%) and helium (28%) with the remaining mass in the form of heavier elements (2%). The Sun was formed from a collapsing cloud of interstellar gas. Gravity contracted the cloud of gas and in doing so the interior temperature of the cloud increased because the contraction converted gravitational potential energy into thermal energy (contraction leads to heating). The cloud of gas was not in hydrostatic equilibrium because although the contraction produced heat, it did not produce enough heat (pressure) to counter the gravitational collapse and the cloud continued to collapse.
    [Show full text]
  • Stellar Structure and Evolution
    Lecture Notes on Stellar Structure and Evolution Jørgen Christensen-Dalsgaard Institut for Fysik og Astronomi, Aarhus Universitet Sixth Edition Fourth Printing March 2008 ii Preface The present notes grew out of an introductory course in stellar evolution which I have given for several years to third-year undergraduate students in physics at the University of Aarhus. The goal of the course and the notes is to show how many aspects of stellar evolution can be understood relatively simply in terms of basic physics. Apart from the intrinsic interest of the topic, the value of such a course is that it provides an illustration (within the syllabus in Aarhus, almost the first illustration) of the application of physics to “the real world” outside the laboratory. I am grateful to the students who have followed the course over the years, and to my colleague J. Madsen who has taken part in giving it, for their comments and advice; indeed, their insistent urging that I replace by a more coherent set of notes the textbook, supplemented by extensive commentary and additional notes, which was originally used in the course, is directly responsible for the existence of these notes. Additional input was provided by the students who suffered through the first edition of the notes in the Autumn of 1990. I hope that this will be a continuing process; further comments, corrections and suggestions for improvements are most welcome. I thank N. Grevesse for providing the data in Figure 14.1, and P. E. Nissen for helpful suggestions for other figures, as well as for reading and commenting on an early version of the manuscript.
    [Show full text]
  • STARS in HYDROSTATIC EQUILIBRIUM Gravitational Energy
    STARS IN HYDROSTATIC EQUILIBRIUM Gravitational energy and hydrostatic equilibrium We shall consider stars in a hydrostatic equilibrium, but not necessarily in a thermal equilibrium. Let us define some terms: U = kinetic, or in general internal energy density [ erg cm −3], (eql.1a) U u ≡ erg g −1 , (eql.1b) ρ R M 2 Eth ≡ U4πr dr = u dMr = thermal energy of a star, [erg], (eql.1c) Z Z 0 0 M GM dM Ω= − r r = gravitational energy of a star, [erg], (eql.1d) Z r 0 Etot = Eth +Ω = total energy of a star , [erg] . (eql.1e) We shall use the equation of hydrostatic equilibrium dP GM = − r ρ, (eql.2) dr r and the relation between the mass and radius dM r =4πr2ρ, (eql.3) dr to find a relations between thermal and gravitational energy of a star. As we shall be changing variables many times we shall adopt a convention of using ”c” as a symbol of a stellar center and the lower limit of an integral, and ”s” as a symbol of a stellar surface and the upper limit of an integral. We shall be transforming an integral formula (eql.1d) so, as to relate it to (eql.1c) : s s s GM dM GM GM ρ Ω= − r r = − r 4πr2ρdr = − r 4πr3dr = (eql.4) Z r Z r Z r2 c c c s s s dP s 4πr3dr = 4πr3dP =4πr3P − 12πr2P dr = Z dr Z c Z c c c s −3 P 4πr2dr =Ω. Z c Our final result: gravitational energy of a star in a hydrostatic equilibrium is equal to three times the integral of pressure within the star over its entire volume.
    [Show full text]
  • 1. A) the Sun Is in Hydrostatic Equilibrium. What Does
    AST 301 Test #3 Friday Nov. 12 Name:___________________________________ 1. a) The Sun is in hydrostatic equilibrium. What does this mean? What is the definition of hydrostatic equilibrium as we apply it to the Sun? Pressure inside the star pushing it apart balances gravity pulling it together. So it doesn’t change its size. 1. a) The Sun is in thermal equilibrium. What does this mean? What is the definition of thermal equilibrium as we apply it to the Sun? Energy generation by nuclear fusion inside the star balances energy radiated from the surface of the star. So it doesn’t change its temperature. b) Give an example of an equilibrium (not necessarily an astronomical example) different from hydrostatic and thermal equilibrium. Water flowing into a bucket balances water flowing out through a hole. When supply and demand are equal the price is stable. The floor pushes me up to balance gravity pulling me down. 2. If I want to use the Doppler technique to search for a planet orbiting around a star, what measurements or observations do I have to make? I would measure the Doppler shift in the spectrum of the star caused by the pull of the planet’s gravity on the star. 2. If I want to use the transit (or eclipse) technique to search for a planet orbiting around a star, what measurements or observations do I have to make? I would observe the dimming of the star’s light when the planet passes in front of the star. If I find evidence of a planet this way, describe some information my measurements give me about the planet.
    [Show full text]
  • Stellar Coronal Astronomy
    Stellar coronal astronomy Fabio Favata ([email protected]) Astrophysics Division of ESA’s Research and Science Support Department – P.O. Box 299, 2200 AG Noordwijk, The Netherlands Giuseppina Micela ([email protected]) INAF – Osservatorio Astronomico G. S. Vaiana – Piazza Parlamento 1, 90134 Palermo, Italy Abstract. Coronal astronomy is by now a fairly mature discipline, with a quarter century having gone by since the detection of the first stellar X-ray coronal source (Capella), and having benefitted from a series of major orbiting observing facilities. Several observational charac- teristics of coronal X-ray and EUV emission have been solidly established through extensive observations, and are by now common, almost text-book, knowledge. At the same time the implications of coronal astronomy for broader astrophysical questions (e.g. Galactic structure, stellar formation, stellar structure, etc.) have become appreciated. The interpretation of stellar coronal properties is however still often open to debate, and will need qualitatively new ob- servational data to book further progress. In the present review we try to recapitulate our view on the status of the field at the beginning of a new era, in which the high sensitivity and the high spectral resolution provided by Chandra and XMM-Newton will address new questions which were not accessible before. Keywords: Stars; X-ray; EUV; Coronae Full paper available at ftp://astro.esa.int/pub/ffavata/Papers/ssr-preprint.pdf 1 Foreword 3 2 Historical overview 4 3 Relevance of coronal
    [Show full text]
  • White Dwarfs - Degenerate Stellar Configurations
    White Dwarfs - Degenerate Stellar Configurations Austen Groener Department of Physics - Drexel University, Philadelphia, Pennsylvania 19104, USA Quantum Mechanics II May 17, 2010 Abstract The end product of low-medium mass stars is the degenerate stellar configuration called a white dwarf. Here we discuss the transition into this state thermodynamically as well as developing some intuition regarding the role of quantum mechanics in this process. I Introduction and Stellar Classification present at such high temperatures are small com- pared with the kinetic (thermal) energy of the parti- It is widely believed that the end stage of the low or cles. This assumption implies a mixture of free non- intermediate mass star is an extremely dense, highly interacting particles. One may also note that the pres- underluminous object called a white dwarf. Obser- sure of a mixture of different species of particles will vationally, these white dwarfs are abundant (∼ 6%) be the sum of the pressures exerted by each (this is in the Milky Way due to a large birthrate of their pro- where photon pressure (PRad) will come into play). genitor stars coupled with a very slow rate of cooling. Following this logic we can express the total stellar Roughly 97% of all stars will meet this fate. Given pressure as: no additional mass, the white dwarf will evolve into a cold black dwarf. PT ot = PGas + PRad = PIon + Pe− + PRad (1) In this paper I hope to introduce a qualitative de- scription of the transition from a main-sequence star At Hydrostatic Equilibrium: Pressure Gradient = (classification which aligns hydrogen fusing stars Gravitational Pressure.
    [Show full text]
  • A Teaching-Learning Module on Stellar Structure and Evolution
    EPJ Web of Conferences 200, 01017 (2019) https://doi.org/10.1051/epjconf/201920001017 ISE2A 2017 A teaching-learning module on stellar structure and evolution Silvia Galano1, Arturo Colantonio2, Silvio Leccia3, Emanuella Puddu4, Irene Marzoli1 and Italo Testa5 1 Physics Division, School of Science and Technology, University of Camerino, Via Madonna delle Carceri 9, I-62032 Camerino (MC), Italy 2 Liceo Scientifico e delle Scienze Umane “Cantone”, Via Savona, I-80014 Pomigliano d'Arco, Italy 3 Liceo Statale “Cartesio”, Via Selva Piccola 147, I-80014 Giugliano, Italy 4 INAF - Capodimonte Astronomical Observatory of Naples, Salita Moiariello, 16, I-80131 Napoli, Italy 5 Department of Physics “E. Pancini”, “Federico II” University of Napoli, Complesso Monte S.Angelo, Via Cintia, I-80126 Napoli, Italy Abstract. We present a teaching module focused on stellar structure, functioning and evolution. Drawing from literature in astronomy education, we identified three key ideas which are fundamental in understanding stars’ functioning: spectral analysis, mechanical and thermal equilibrium, energy and nuclear reactions. The module is divided into four phases, in which the above key ideas and the physical mechanisms involved in stars’ functioning are gradually introduced. The activities combine previously learned laws in mechanics, thermodynamics, and electromagnetism, in order to get a complete picture of processes occurring in stars. The module was piloted with two intact classes of secondary school students (N = 59 students, 17–18 years old) and its efficacy in addressing students’ misconceptions and wrong ideas was tested using a ten-question multiple choice questionnaire. Results support the effectiveness of the proposed activities. Implications for the teaching of advanced physics topics using stars as a fruitful context are briefly discussed.
    [Show full text]
  • Astronomy 241: Review Questions #1 Distributed: September 20, 2012 Review the Questions Below, and Be Prepared to Discuss Them I
    Astronomy 241: Review Questions #1 Distributed: September 20, 2012 Review the questions below, and be prepared to discuss them in class on Sep. 25. Modified versions of some of these questions will be used in the midterm exam on Sep. 27. 1. State Kepler’s three laws, and give an example of how each is used. 2. You are in a rocket in a circular low Earth orbit (orbital radius aleo =6600km).Whatve- 1 locity change ∆v do you need to escape Earth’s gravity with a final velocity v =2.8kms− ? ∞ Assume your rocket delivers a brief burst of thrust, and ignore the gravity of the Moon or other solar system objects. 3. Consider a airless, rapidly-spinning planet whose axis of rotation is perpendicular to the plane of its orbit around the Sun. How will the local equilibrium surface temperature temperature T depend on latitude ! (measured, as always, in degrees from the planet’s equator)? You may neglect heat conduction and internal heat sources. 3 4. Given that the density of seawater is 1025 kg m− ,howdeepdoyouneedtodivebefore the total pressure (ocean plus atmosphere) is 2P0,wherethetypicalpressureattheEarth’s 5 1 2 surface is P =1.013 10 kg m− s− ? 0 × 5. An asteroid of mass m M approaches the Earth on a hyperbolic trajectory as shown " E in Fig. 1. This figure also defines the initial velocity v0 and “impact parameter” b.Ignore the gravity of the Moon or other solar system objects. (a) What are the energy E and angular momentum L of this orbit? (Hint: one is easily evaluated when the asteroid is far away, the other when it makes its closest approach.) (b) Find the orbital eccentricity e in terms of ME, E, L,andm (note that e>1).
    [Show full text]
  • News & Views Research
    RESEARCH NEWS & VIEWS measure the dwarf planet’s size, shape and density more accurately than ever before. Compared with other bodies in the Solar System, Haumea rotates quickly (making one rotation in about four hours), and is strangely shaped, like an elongated egg. Ortiz et al. calculate that the object’s longest axis is at least 2,300 km, which is larger than earlier estimates6–8. In turn, given known values for Haumea’s mass and brightness, the dwarf planet’s density and reflectivity are both lower CSI DE ANDALUCIA DE ASTROFISICA INSTITUTO than the unusually high values previously considered. The authors’ results suggest that Haumea might not be in hydrostatic equilibrium, and this touches on the still-sensitive topic of how planets and dwarf planets should be defined. Remarkably, the authors show that blinks in the starlight, detected at multiple observa- tion sites both before and after a distant star was blocked by Haumea, are consistent with a 70-km-wide ring of material encircling the body, approximately 1,000 km away from Haumea’s surface (Fig. 1). Saturn’s are the most studied of all rings, Figure 1 | Artist’s impression of Haumea and its ring. Ortiz et al.4 have measured the size, shape and and yet they remain enigmatic. Data from the density of the dwarf planet Haumea with unprecedented accuracy and found that it has a planetary ring. Cassini spacecraft revealed that gravitational This is the first time that a ring has been discovered around a distant body in the Solar System. interactions between the planet’s rings and moons shepherd the ring material, and that Ring systems represent microcosms of the transition from the outer Solar System, one of the rings is produced entirely from larger-scale rotating structures, such as including close encounters with the giant matter that spews from the moon Encela- galaxies and proto-planetary disks — the disks planets14.
    [Show full text]