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Lecture 1: Astrophysics Senior Astrophysics 2017-03-07 Senior Astrophysics Lecture 1: Astrophysics 2017-03-07 1 / 26 Outline 1 Introduction 2 What is astrophysics? 3 Website of the Week 4 Radiation 5 Next lecture Senior Astrophysics Lecture 1: Astrophysics 2017-03-07 2 / 26 Course structure 18 sessions — last lectre ADV only 14 lectures + 4 computational labs From week 2, Friday 12pm lectures will be in SNH Learning Studio 4003 Need to remember your Matlab! Extra lab session on Wednesdays at 10 am; SNH Learning Studio. Only need to attend one per week. Also: plan to spend 10 minutes each week telling you about important web research tools in astronomy: Website of the Week [WOTW] Will be the basis for at least one assignment question, so pay attention! Lecture 1: Astrophysics Introduction 3 / 26 Timetable Week week 1 week 2 week 3 week 4 week 5 week 6 week 7 week 8 Date 7 Mar 8 Mar 10 Mar 14 Mar 15 Mar 17 Mar 21 Mar 22 Mar 24 Mar 28 Mar 29 Mar 31 Mar 4 Apr 5 Apr 7 Apr 11 Apr 12 Apr Day Tue 1pm Wed 9am Fri 12pm Tue 1pm Wed 9am Fri 12pm/Wed Tue 1pm Wed 9am Fri 12pm/Wed Tue 1pm Wed 9am Fri 12pm/Wed Tue 1pm Wed 9am Fri 12pm/Wed Tue 1pm Wed 9am 26 Apr 1 May Wed 9am Mon 5pm Lecture 1: Astrophysics 10am 10am 10am 10am Topic Lecture 1 Introduction Lecture 2 Radiation processes Lecture 3 Emission and absorption Lecture 4 Sources of radiation Lecture 5 Stellar structure Lab 1 Lab 1: Line formation Lecture 6 The main sequence Lecture 7 Stellar evolution 1 Lab 2 Lab 2: Stellar evolution 1 Lecture 8 Stellar evolution 2 Lecture 9 Supernovae Lab 3 Lab 3: Stellar evolution 2 Lecture 10 Stellar remnants Lecture 11 Binary stars Lab 4 Lab 4: Roche lobes Lecture 12 Accretion energy Lecture 13 Binary evolution Mid-semester break Lecture 14 ADV-only lecture Assignment 1 due Assignment 2 due Introduction 4 / 26 Assignments All assignments will be submitted through Turnitin. Either typeset (LATEX/Word etc.) or handwritten is acceptable BUT quality must be legible (no unreadable photos from your phone!) No more coversheets. Deadline is 11:59 pm on the due date. Lecture 1: Astrophysics Introduction 5 / 26 Course outline Radiation — lectures 1–4 + lab 1 How is radiation produced? How is it affected as it propagates? Stellar structure and evolution — lectures 5–10 + labs 2 & 3 Modelling stars; the evolution of stars; stellar remnants Binary stars – lectures 11–13 + lab 4 Binary stars; evolution of binaries; applications in modern astronomy Final lecture (lecture 14): Adv-only: Problems in binary evolution (examinable) Lecture 1: Astrophysics Introduction 6 / 26 Textbooks No required text. Highly recommended: 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 / 26 Astrophysics Astrophysics is a big subject. Sometimes will just flag where a whole (possibly large!) field branches off, e.g. won’t talk about how stars form Sometimes will give qualitative description (with lots of pretty pictures); unless I say otherwise, this is non-examinable A few topics are amenable to analysis at our level in time available. I will walk through these in class, and these will most naturally be the subject of assignments and exams. Lecture 1: Astrophysics Introduction 8 / 26 What is astrophysics? Application of laws of physics to understand the behaviour of astronomical objects predict new phenomena that could be observed Physics used can include electromagnetism, fluid mechanics, plasma physics, general relativity, nuclear physics, and more. Lecture 1: Astrophysics What is astrophysics? 9 / 26 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 EM radiation from objects (and, very recently, high energy particles) Lecture 1: Astrophysics What is astrophysics? 10 / 26 What is astrophysics? Consequences: complex systems: can’t change variables separately to isolate their effects, e.g. galaxies timescales: often human lifetimes, so can’t follow many processes all the way, e.g. stellar evolution rare events: knowledge is limited, and will usually not occur nearby; e.g. SNe best way to add knowledge is to use new wavelength domains, e.g. radio, X-ray statistical arguments vital! more so than in many areas of lab physics, e.g. γ -ray bursts Lecture 1: Astrophysics What is astrophysics? 11 / 26 Physical conditions Physical conditions we are studying are very diverse: temperature: T 3K < (CMB) < 1012 K gas near BH density 10−20 kg/m3 (ISM) < ρ < 1018 kg/m3 (neutron star) speeds: v ∼ 0.99c in jets from BH Lecture 1: Astrophysics What is astrophysics? 12 / 26 Units Astronomers 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 of magnitude involved with using SI units. Principal ones we will worry about: distance: parsec (pc); 1 pc = 3.26 ly = 3.086 × 1016 m Nearest star is 1.3 pc away solar radius: 1R = 6.955 × 108 m mass: solar mass: 1M = 1.989 × 1030 kg luminosity (energy/unit time) solar luminosity: 1 L = 3.839 × 1026 W Lecture 1: Astrophysics What is astrophysics? 13 / 26 Website of the Week: APOD: Astronomy Picture of the Day http://apod.nasa.gov/apod/ Annotated daily astronomical image Lecture 1: Astrophysics Website of the Week 14 / 26 Part 1: Radiation The physics of radiation (4 lectures + 1 lab) 1 2 How is radiation affected as it propagates to the observer? Mechanisms that produce radiation Blackbody radiation Transitions within atoms 3 Use results to understand spectra of stars, nebulae Lecture 1: Astrophysics Radiation 15 / 26 Radiation Because all of astronomy is dependent on electromagnetic radiation, we need to understand radiation itself: how it is produced, how it propagates, and how it is affected as it propagates. Lecture 1: Astrophysics Radiation 16 / 26 Interstellar gas Lecture 1: Astrophysics Radiation 17 / 26 Photons escaping star Lecture 1: Astrophysics Radiation 18 / 26 Basic properties of radiation EM radiation: frequency ν , wavelength λ λν = c Individual photons have energy E = hν where h = Planck’s constant Common to measure energies in eV, where 1 eV = 1.602 × 10−19 J (energy of visible light photon ∼ few eV) Lecture 1: Astrophysics Radiation 19 / 26 Basic properties of radiation Simplification: astronomical objects are normally much larger than λ, so neglect diffraction light travels along straight lines Lecture 1: Astrophysics Radiation 20 / 26 Need to be careful about direction of photon travel: Far from source (e.g. light from distant star) can assume radiation is travelling radially But e.g. inside a star, photons move in many different directions, so have to worry about angles and how much radiation is moving in each direction Lecture 1: Astrophysics Radiation 21 / 26 Flux and luminosity Define the radiant flux F from a source as the total amount of light energy (of all wavelengths) that crosses a (perpendicular) unit area per unit time dE = F dA dt Lecture 1: Astrophysics Radiation 22 / 26 Flux and luminosity For a spherically symmetric steady source with luminosity L (= total amount of energy emitted per unit time): energy conservation means L 4πr2 inverse square law: measured flux drops off as square of distance F (r) = Lecture 1: Astrophysics Radiation 23 / 26 Flux and luminosity L is total luminosity emitted at all wavelengths, so F is also integrated over all wavelengths. In practice, often measure only part of spectrum, so need to consider how radiation is distributed over frequency Z F = Fν (ν)dν total energy flux = integral of Fν over all frequencies Then dE = Fν dA dt dν Lecture 1: Astrophysics Radiation 24 / 26 For astronomers only Units of Fν are (energy)/(time) per area per frequency bin so W m−2 Hz−1 Special unit: 1 Jy = 10−26 W m−2 Hz−1 Fν called “flux density” — in order to get power received, need to multiply by the area and by the bandwidth Lecture 1: Astrophysics Radiation 25 / 26 Next lecture Radiation processes Intensity and brightness Emission Absorption Lecture 1: Astrophysics Next lecture 26 / 26