Lecture 1: Astrophysics - School Of Physics

Lecture 1: AstrophysicsSenior Astrophysics2018-03-06Senior AstrophysicsLecture 1: Astrophysics2018-03-061 / 35

Outline1Introduction2What is astrophysics?3Website of the Week4Radiation5Intensity and brightness6Next lectureSenior AstrophysicsLecture 1: Astrophysics2018-03-062 / 35

Course structure18 sessions — last lecture ADV only14 lectures 4 computational labsFrom week 2, Friday 12pm lectures will be in SNH Learning Studio 4003Need to remember your Matlab!Extra lab session on Wednesdays at 10 am; SNH Learning Studio. Onlyneed to attend one per week.Also: plan to spend 10 minutes each week telling you about importantweb research tools in astronomy: Website of the Week [WOTW]Will be the basis for at least one assignment question, so pay attention!Lecture 1: AstrophysicsIntroduction3 / 35

TimetableWeekweek 1week 2week 3week 4week 5week 6week 7week 8Date6 Mar7 Mar9 Mar13 Mar14 Mar16 Mar20 Mar21 Mar23 Mar27 Mar28 MarDayTue 1pmWed 9amFri 12pmTue 1pmWed 9amFri 12pm/Wed 10amTue 1pmWed 9amFri 12pm/Wed 10amTue 1pmWed 9am1011131718202430Tue 1pmWed 9amFri 12pm/Wed 10amTue 1pmWed 9amFri 12pm/Wed 10amTue 1pmMon 5pmAprAprAprAprAprAprAprAprLecture 1: AstrophysicsTopicLecture 1IntroductionLecture 2Radiation processesLecture 3Emission and absorptionLecture 4Sources of radiationLecture 5Stellar structureLab 1Lab 1: Line formationLecture 6The main sequenceLecture 7Stellar evolution 1Lab 2Lab 2: Stellar evolution 1Lecture 8Stellar evolution 2Lecture 9SupernovaeMid-semester breakLecture 10Stellar remnantsLecture 11Binary starsLab 3Lab 3: Stellar evolution 2Lecture 12Accretion energyLecture 13Binary evolutionLab 4Lab 4: Roche lobesLecture 14ADV-only lectureAssignment 1 due 9 AprAssignment 2 due 30 AprilIntroduction4 / 35

AssignmentsAssignments will be released two weeks before the due date.All assignments will be submitted through Turnitin.Either typeset (LATEX/Word etc.) or handwritten is acceptableBUT quality must be legible (no unreadable photos from your phone!)Deadline is 11:59 pm on the due date.DO NOT put your name on the assignment, or in the file name; justyour SID. Assignments will be marked anonymously.Lecture 1: AstrophysicsIntroduction5 / 35

Course outlineRadiation — lectures 1–4 lab 1How is radiation produced? How is it affected as it propagates?Stellar structure and evolution — lectures 5–10 labs 2 & 3Modelling stars; the evolution of stars; stellar remnantsBinary stars – lectures 11–13 lab 4Binary stars; evolution of binaries; applications in modern astronomyFinal lecture (lecture 14): Adv-only: Problems in binary evolution(examinable)Lecture 1: AstrophysicsIntroduction6 / 35

TextbooksNo required text.Highly recommended:Carroll & Ostlie, An Introduction to Modern AstrophysicsHilditch, An introduction to close binary starsBoth on closed reserve in SciTech library.Lecture 1: AstrophysicsIntroduction7 / 35

AstrophysicsAstrophysics is a big subject.Sometimes will just flag where a whole (possibly large!) field branchesoff, e.g. won’t talk about how stars formSometimes will give qualitative description (with lots of pretty pictures);unless I say otherwise, this is non-examinableA few topics are amenable to analysis at our level in time available. Iwill walk through these in class, and these will most naturally be thesubject of assignments and exams.Lecture 1: AstrophysicsIntroduction8 / 35

What is astrophysics?Application of laws of physics tounderstand the behaviour of astronomical objectspredict new phenomena that could be observedPhysics used can include electromagnetism, fluid mechanics, plasma physics,general relativity, nuclear physics, and more.Lecture 1: AstrophysicsWhat is astrophysics?9 / 35

What is astrophysics?Main difference between astrophysics and other branches of physics:controlled experiments are (almost) never possible!Observational science: only information we have is via study of EMradiation from objects (and, very recently, high energy particles andgravitational waves)Lecture 1: AstrophysicsWhat is astrophysics?10 / 35

What is astrophysics?Consequences:complex systems: can’t change variables separately to isolate theireffects, e.g. galaxiestimescales: oftenhuman lifetimes, so can’t follow many processes allthe way, e.g. stellar evolutionrare events: knowledge is limited, and will usually not occur nearby;e.g. SNebest way to add knowledge is to use new wavelength domains, e.g.radio, X-raystatistical arguments vital! more so than in many areas of lab physics,e.g. -ray burstsLecture 1: AstrophysicsWhat is astrophysics?11 / 35

Physical conditionsPhysical conditions we are studying are very diverse:temperature:3K(CMB) T10 20 kg/m3(ISM) 1012 Kgas near BHdensity 1018 kg/m3(neutron star)speeds:v 0.99c in jets from BHLecture 1: AstrophysicsWhat is astrophysics?12 / 35

UnitsAstronomers use many non-SI units. Partly historical, like magnitudes,but we’ll try to avoid those in this course.Other main problem is scale of astronomy: inconvenient orders ofmagnitude involved with using SI units.Principal ones we will worry about:distance:parsec (pc); 1 pc 3.26 ly 3.086 1016 mNearest star is 1.3 pc awaysolar radius: 1R 6.955 108 mmass:solar mass: 1M 1.989 1030 kgluminosity (energy/unit time)solar luminosity: 1 LLecture 1: Astrophysics 3.839 1026 WWhat is astrophysics?13 / 35

Website of the Week:APOD: Astronomy Picture of the Dayhttp://apod.nasa.gov/apod/Annotated daily astronomical imageLecture 1: AstrophysicsWebsite of the Week14 / 35

Part 1: RadiationThe physics of radiation (4 lectures 1 lab)12How is radiation affected as it propagates to the observer?Mechanisms that produce radiationBlackbody radiationTransitions within atoms3Use results to understand spectra of stars, nebulaeLecture 1: AstrophysicsRadiation15 / 35

RadiationBecause all of astronomy is dependent on electromagnetic radiation, we needto understand radiation itself: how it is produced, how it propagates, andhow it is affected as it propagates.Lecture 1: AstrophysicsRadiation16 / 35

Interstellar gasLecture 1: AstrophysicsRadiation17 / 35

Photons escaping starLecture 1: AstrophysicsRadiation18 / 35

Basic properties of radiationEM radiation: frequency , wavelength cIndividual photons have energyE h where h Planck’s constantCommon to measure energies in eV, where1 eV 1.602 1019J(energy of visible light photon few eV)Lecture 1: AstrophysicsRadiation19 / 35

Basic properties of radiationSimplification: astronomical objects are normally much larger than , soneglect diffractionlight travels along straight linesLecture 1: AstrophysicsRadiation20 / 35

Need to be careful about direction of photon travel:Far from source (e.g. light from distant star) can assume radiation istravelling radiallyBut e.g. inside a star, photons move in many different directions, so haveto worry about angles and how much radiation is moving in eachdirectionLecture 1: AstrophysicsRadiation21 / 35

Flux and luminosityDefine the radiant flux F from a source as the total amount of lightenergy (of all wavelengths) that crosses a (perpendicular) unit area perunit timedE F dA dtLecture 1: AstrophysicsRadiation22 / 35

Flux and luminosityFor a spherically symmetric steady source with luminosity L ( totalamount of energy emitted per unit time): energy conservation meansL4 r2inverse square law: measured flux drops off assquare of distanceF (r) Lecture 1: AstrophysicsRadiation23 / 35

Flux and luminosityL is total luminosity emitted at all wavelengths, so F is also integratedover all wavelengths.In practice, often measure only part of spectrum, so need to considerhow radiation is distributed over frequencyZF F ( )d total energy flux integral of F over all frequenciesThendE F dA dt d Lecture 1: AstrophysicsRadiation24 / 35

For astronomers onlyUnits of F are (energy)/(time) per area per frequency bin so W mHz 1Special unit: 1 Jy 10 26 W m 2 Hz 12F called “flux density” — in order to get power received, need tomultiply by the area and by the bandwidthLecture 1: AstrophysicsRadiation25 / 35

Solid angleFirst we define a solid angleConsider a sphere of radius r.The area dS of a patch on the surface isdS r d r sin d r2 d d is the solid angle subtended by the area dS atthe centre of the sphereUnit of solid angle is the steradian sr4 sr cover the whole sphereLecture 1: AstrophysicsIntensity and brightness26 / 35

Specific intensityConsider an area dA normal to a light ray, and consider all rays thatpass through dA which lie within a small solid angle d Amount of energy dE passing through dA and into d in time dt andfrequency range d isdE I dA dt d d I is the specific intensity of the radiationLecture 1: AstrophysicsIntensity and brightness27 / 35

Specific intensityUnits of specific intensity are: W m2Hz1sr1Another, more intuitive name for specific intensity is brightnessLecture 1: AstrophysicsIntensity and brightness28 / 35

BrightnessWe can show that brightness is constant if there is no emission orabsorption.Consider two points on a ray; construct areas dA1 and dA2perpendicular to the ray at those points. How much energy is carried bythose rays that pass through both dA1 and dA2 ?Lecture 1: AstrophysicsIntensity and brightness29 / 35

BrightnessLet d 1 be the solid angle subtended by dA2 at dA1 ; then the energypassing through dA1 into d 1 isdE1 I 1 dA1 dt d 1 d 1and similarlydE2 I 2 dA2 dt d 2 d 2Lecture 1: AstrophysicsIntensity and brightness30 / 35

No energy loss ! dE1 dE2No frequency change ! d 1 d 2Use the definition of solid angle: d 1 dA2 /r2Find:I 1 I 2Lecture 1: AstrophysicsIntensity and brightness31 / 35

BrightnessIn other words, specific intensity remains the same as radiationpropagates through space.The brightness of an object1 stays the same no matter the distance: asit gets further away, amount of flux goes down, but amount of area goesdown at same rate.If we define the distance along the beam to be s, then we can writedI 0dsi.e. brightness doesn’t change along beam1Only matters for extended objects (not point sources).Lecture 1: AstrophysicsIntensity and brightness32 / 35

BrightnessThe Sun in three imaginary photos taken from a long distance (left), medium distance (center), and short distance (right)would have a constant brightness but increasing angular size ess.html)Lecture 1: AstrophysicsIntensity and brightness33 / 35

Brightnessapod.nasa.gov/apod/ap150102.htmlLecture 1: AstrophysicsIntensity and brightness34 / 35

Next lectureRadiation processesEmissionAbsorptionCross-sectionOptical depthLecture 1: AstrophysicsNext lecture35 / 35

Carroll & Ostlie, An Introduction to Modern Astrophysics Hilditch, An introduction to close binary stars Both on closed reserve in SciTech library. Lecture 1: Astrophysics Introduction 7 / 35. Astrophysics Astrophysics is a big subject. Sometimes will just flag where a whole (possibly large!) field branches