ASTRON103 Prof: Thomas Beatty Lesson 1: Our Place in the Universe Goals: 1. Sun, the stars, how the work? 2. Nebulas, how are the galaxies structured? 3. Galaxies, how are they formed? The universe is so large; we have figured out how to measure. That is a fundamental part of astronomy. What’s the difference between astronomy and astrology? Astrology is the scientific study of the universe. Astrology is the study of movement of planets and how those positions affect our lives. We don’t often use words to describe things, we use math to describe things. What do astronomers do? 1. Make observations using telescopes 2. Analyze data/observations 3. Create theories about what is seen and what might exist that is unseen (to predict) 4. Create computer models to simulate the universe 5. Invent and design instruments to see beyond the Earth 6. Use math and physics to do this Two types of astronomers: theorists and observers. Diagrams generally have correct relative sizes but wrong distances to fit them in. We use a unit of distance called light years. It is the distance that light travels in one year. The observable universe is 13 billion light years in radius. The Sun is an average star in the Milky Way, it’s about halfway it’s life. There are over 100 billion stars in the Milky Way. The nearest star is 4.2 light years away (Proxima Centauri). All of the stars we can discern are part of the Milky Way. The dark bands are dust clouds. We’re looking at the outside edge of the galaxy in the Northern hemisphere. The Milky Way is a fairly average spiral galaxy. Galaxies can group together to form even larger structures that are called clusters and super clusters. ASTRON103 Summary of distances: 1. Earth to Sun- 8 light minutes 2. Nearest Star- 4 light years 7,231,200,000.0 Count number of digits. 7.231 * 10^9 Y=10^x -→ log10(y)=x 1. Linear Plots 2. Log-Log Plot 3. Linear Plot of Logs Why are log plots not linear? - Distance: meters, light years, parsecs, solar radii(one radius of the Sun), redshift, astronomical units - Time: Seconds, years, billions of years (gigaYears) - Angles: Degrees, arcminutes, arcseconds, radians - Masses: grams, solar masses - Temperature: Kelvin - Luminosity, - Constants: Speed of light, sin(a)=a, cos(a)=1, Week 2: The Night Sky 1. How do we talk about where things are in the sky? 2. Why do stars rise and set? 3. Why do the stars we see depend on latitude and time of the year? - On earth, we can use latitude and longitude positions to give the place of something accurately. - Sirius is in the constellation of Canis Major. The dog stars. - In the sky, we use Right Ascension and Declination (like lat and long). Or we can use constellations. - In early times, without clocks and calendars, timekeeping was done using stars. This could be used in farming by understanding spring and fall for harvesting and planting. ASTRON103 - Each culture has formed their own constellation myths. We use European constellation names in the Northern Hemisphere. The Southern hemisphere constellations are named after explorers/instruments. - We use constellations to denote different regions of the sky. We have 88 official constellations. - Big blue ribbon- Milky Way has dark stretches. These are dark dust lanes that block out light coming. One of Australian Aboriginal constellations is a dark dust that looks like a bird. - The milky way is 100 ly thick but a 1000 ly long. We can’t look behind the milky way. We are in the Orion spiral arm. MW is strongest in the Southern Hemisphere. - Right Ascension (East and West) and Declination (North and South) - Stars in the constellation looks like they are close together. From a sideways, they may be very far apart. Stars are spread out in threedimensional space. Constellations looks that way from out perspective. - The Celestial Sphere: putting stars on the same plane. It’s helpful to think of the stars using the celestial plane. It is a mental construct to see what stars are visible. - A sky map is the celestial sphere unrolled. There are distortions that come out because of it closer to the poles. - Because of the Earth, we can only see half of the sky. Half of the sky is invisible to us. If the sun wasn’t there, we could notice all the stars. - The sky is moving overhead. If we are observing stars, we can’t leave the stars. Most research telescopes have small motors that help it move and keep the stars in the field of view. - In star trails, the stars look like they’re moving little towards the center. - The earth rotates counterclockwise. - Zenith is the straight overhead. - Meridian divides the sky into East and West halves. - 10 degrees of movement is 45 minutes. - At the equator, the north star is at the horizon. Stars rise and set on the equator. - Madison is between north pole and equator. We have few circumpolar stars and stars that rise and set. ASTRON103 Lecture 3: Timekeeping, the Seasons and Phases of the Moon 1. Timekeeping 2. What causes seasons? 3. What causes phases of the moon? Chronometer was the most accurate ever built. It took 30 years for the inventor to build. For centuries, we used observations of the sky to tell us the time. Stonehenge, Chaco Canyon. People needed to know what time of the day and year. We used sundials to tell the time of the day on a smaller scale. The moon goes around the earth every 29 and a half days. Sun gives time of day. Moon gives us time of month. - Until the invention of modern clocks, in the 19th century, one of the main roles of an observatory was to keep the time. The time on our clocks come from atomic clocks in US Naval Observatory. The Paris Observatory has the International Time Bureau to determine “Universal Time”. There are low frequency radio signals that the naval observatory sends time through. We have three different timescales in the sky: 1. Daily 2. Yearly: 12 months for earth to orbit the Sun. As the Earth moves about the sun, the stars we see overhead at midnight change. We have seasons because the Earth is tilted. Over the course of the year, the North pole in the year is facing away from the Sun in winter and is facing towards it in the summer for the Northern hemisphere. The seasons are not happening because we are further away/towards the Sun. Higher angle of sunlight and longer days in Summer. A) Summer Solstice, June 21 B) Autumnal Equinox, September 23 C) Winter Solstice, December 22 D) Vernal Equinox, March 21 3. Monthly: We only ever see one side of the moon from Earth. The moon is spinning like the earth is. The moon rotation and revolution take the same time. There is no dark side of the moon. There is a near and far side of the moon. The amount of moon that is illuminated changes over the course of the month. Full moon is when the it is on the opposite side of the Sun. ASTRON103 Lecture 4: Light & Matter - The nature of light - The nature of matter - Interactions between light and matter: how we learn about objects billions of light years away It could take 5 months to reach Mars. The nearest star is 4ly away. We need a different way to get information. In astronomy, we do this by looking at light from something up in the sky. The light from the star can tell us it’s velocity, temperature and chemical composition. Light also is the main way in which energy is transmitted throughout the Universe. Light is a small disturbance in an electric field which creates a small magnetic field, which in turn creates a small electric field, and so on… Therefore, light is electromagnetic radiation. Hence, light often behaves like a way. It’s like dropping a little stone into a pond; it creates ripples and causes waves to move. The waves move sidewards but the water itself doesn’t move. The energy radiates from the center and transmits energy. Light is moving in a vacuum; it doesn’t need a medium. There is a background electromagnetic field that light is interacting with. It’s not something that is physical that light interacts with. Charged particles exist in the universe. They are putting out electric field lines (positive and negative). If it’s stationary, that’s the end of it. When the charge moves, the field moves with it. Light has a constant speed: 300,000 km/h. Nothing can travel faster than the speed of light according to Einstein’s Theory of Relativity. Light can also be described as a particle. When light behaves like a particle, we call the individual packets of light photons. Photons are characterized by their energy. A light wave is described by its wavelength and frequency. Double slit experiment. V=fw - Frequency: number of wave crests that pass a given point in one second. ASTRON103 - Frequency and wavelength are related to the speed of light. Ephoton = hf. Here, h is a fundamental constant of physics (Planc constant). Long wavelength: red Short wavelength: violet Lecture 5: Astronomical Spectra and Thermal Radiation Properties of light: - Both a wave and particle - A light wave is described by frequency and the wavelength - Light moves at a constant speed. When light behaves like a particle, we call the individual packets of light photons. Photons are characterized by their energy: E = hf. (h being Planc constant) Light is part of the electromagnetic spectrum. Visible light is from 400-700 nm. Light comes from matter inside the object. Atoms are made up of protons and neutrons in the nucleus, surrounded by a cloud of electrons. Electrons fill spherical shells (orbital)around the atoms. Each orbital has a different energy. As electrons move up and down those orbitals, they gain or lose energy. Because those orbitals are specific energies, and the atom can only absorb photons of specific energies. If an electron is at a higher energy and jumps below, it’s going to only specifically emit a photon of the difference of energy between those two orbitals. Individual atoms have specific absorption and emission energies. Astronomical Spectra is separating light through an object like a prism through the telescope. We are taking visual light and breaking it up into the spectrum. We can use the spectra to see what we’re looking at by transforming into a graph, with intensity. Emission lines – spikes on the intensity graph. Absorption line- sudden dips. Continuous spectrum- smooth. ASTRON103 Atoms are not just by themselves; they’re interacting and colliding. Atom collision is interaction between electrons. They exchange energy and emit photons as they do that. Temperature is a measure of how energetically atoms are moving about. Low temperature- atoms are moving very slowly. High temperature- atoms are moving very quickly. Absolute zero- atoms are not moving. In a diffused gas, the atoms are all very spread out. They sometimes collide so they collide very rarely. You can treat them as atoms that are isolated (doing their thing). Each element has a very specific spectrum line. By taking the spectrum of a distant object, we can determine what it’s made of. A dense opaque object, or solid, we can’t treat the atoms as isolated. Photons tend to bounce around inside a dense object, being absorbed and re-emitted many times. The escaping photons have a broad range in wavelength because of the wide variety of energy levels. SCATTERING OF LIGHT. The exact shape of the spectrum depends strongly on temperature. Hotter- blue wavelengths are emitted more intensely. Less hot- red wavelengths are emitted more intensely. Wien’s Law: - Hotter bodies emit more strongly at shorter wavelength - The peak wavelength is inversely proportional to the temperature. Stefan-Boltzman Law: - The total light output increases for a hot body. - Total area under a curve is proportional to the temperature. Luminosity is the total energy output per second. Lecture 6: Astronomical Spectra and Doppler Shift - Hot dense gas: continuous spectrum ASTRON103 - Hot diffuse gas: emission line spectrum. - Cool diffuse gas: absorption line spectrum. We have very specific line spectrum because of the shells and the electron positions. As we jump out of different orbital shells, different amounts of energy are released, causing different light (photons). Difference between astronomy and astrophysics is understanding the spectrometers and spectrocity. Doppler Effect: wavelength lengthens when it’s going away (redshift); wavelength shortens when it’s coming towards (blueshift). Lecture 7: Telescopes and Detectors - Telescopes - Astronomical Cameras Astronomical telescopes are built differently. You’re not looking through the eyepiece, there is a camera at the end. You sit in a control room, you’re not even in a dome. The job of a telescope is to collect light and angle it towards a focus point, funneling light. (Basically, how the eye works). Two basic ways of funneling light is use refraction (lens at the front of the telescope that bend the light) and reflection (light comes down and it hits a curved mirror that hits another mirror to focus it). Refractions refers to the change in direction of light when it goes from one medium to the other. The problem with refractors is that they are very hard to build giant lenses. For astronomical refractors, the focal length increases as the lens size increases. As different wavelengths of light enter, lenses focus them at slightly different wavelengths. Reflection telescopes- primary curved mirror and secondary mirror. They use a curved mirror to focus the light. Mirrors can be made to have very short focal lengths. Biggest telescope- FAST Radio Telescope. The mirror for radio telescopes can be rougher. ASTRON103 Largest optical telescope- 10 meters. Not easy to make. Short wavelength and large telescopes give best images. Interforemetry with radio telescopes. Lecture 8: Dectectors and JWST CCD’s are detectors that modern cameras use. CCD’s record 98% of photons that hit it. Different colours-different wavelengths. Our eyes can perceive the difference. The CCD cannot tell any wavelength difference. It tells us how many photons are coming. We put blocking filters over the CCD and combines these filters. RBGW. In astronomy we take three separate photos and combine them through software. This also means that we can make color images using light we cannot see. Higher the telescope lower the water vapor. Use of JWST: - Learn about exoplanets - Learn about galaxies Lecture 9: The Sun, Our Star Luminosity refers to an object’s total energy output per second. The source of the Sun’s luminosity is nuclear fusion. The Sun has been stable for billions of years. Rason: balance between gravity and pressure. Hot gas causes higher pressure because the atoms move rapidly and try to push the boundaries out. Pressure = Constant x Temperature x Density. For fusion to happen, we need high temps. Surface is less dense, it doesn’t have a solid surface. Below the photosphere is the convection zone. Energy is transmitted through the core layers by radiation. At the exterior, the best way to transmit energy is through convection. Lecture 10: The Solar System, Gravity ASTRON103 Most of the mass of the solar system ended up in the last four planets (gas giants). Planets were recognized quite early by initial astronomers. We see all the planets on the ecliptic because the solar system is quite flat. On a given night, we just see them rising and setting. But they move over time. Planets don’t just move in circles. Mars moves and reverses course at one point. It’s called going retrograde in astrology. Until Copernicus, the general view that Earth was in the center. It was created because they couldn’t explain why Mars reverses course. However, Copernicus assumed that the orbits are perfectly circular. Kepler changed the Copernican model. Kepler’s Laws of Motion: 1. The orbits are ellipses 2. An individual planet moves faster when closest to the sun and slowest when further away. The planet sweeps out an equal area of the ellipse in an equal interval of time. 3. Pyears^2=Aau^3 These laws precisely describe the motions of the planets but they don’t explain them. Earth is faster than the Sun, we pass Mars. Newton was the first to explain why the Earth moves around the Sun and not vice versa. Speed: how fast an object is moving Velocity: how fast is an object moving and in what direction Acceleration: a change in velocity or direction Mass: amount of matter in a body Weight: the force that scale measure when we stand on it. Forces count. Momentum: mass x velocity Force: mass x acceleration Angular Momentum: mass x velocity x radius Newton’s first Law: Inertia Newton’s second Law: F=MA Newton’s third Law: Every action has an equal and opposite reaction ASTRON103 They come from the fundamental conservation laws. 1) Conservation of momentum: Every object wants to keep the total momentum in the system at the same. 2) Conservation of angular momentum: when your arms are further, we start spinning faster. (provides a physical explanation for Kepler’s second law) 3) Conservation of Energy; Energy can’t appear or disappear, it can only be exchanged among objects Three types of energy: i) Kinectic ii) Potential iii) Radiation Temperature is measuring the energy and motion of particles. Stars are formed by conversion of potential energy to kinetic energy. Lecture 11: Surveying the Stars Gravity is one of the fundamental forces of the Universe. It is an attractive force between masses. In an orbit, bodies are falling but the Earth is curving at the same rate. Things we would like to know about stars: 1. Distance: parallax. Doing it from opposite sides of the Earth. Doing it from opposite sides of the Earth’s orbit. Parallax decreases as the distance increases. 2. Luminosity: (energy/second) Total amount of energy per second that a star radiates into space. On earth, we measure the apparent brightness. 3. Surface Temperature. Lecture 12: Stellar Masses and Radii Most stars that exist in the Universe are on the Main Sequence. Giants and White Dwarfs are at a later stage of life. Luminosity depends on both the temperature and radius. The mass of the star is the crucial part. More massive stars are hotter. (Need to do more fusion to support the mass) We can measure masses of binary stars by using our knowledge of gravity. Most stars in the sky are binary. Stars tend to form in groups. Binary stars orbit a center of mass. Principle of moment. ASTRON103 The location of the center of mass depends on the mass ratio of the two stars. We can use eclipsing binary stars to measure the radii of stars. Using the ratio of the surface area of the two stars. How to stars form? Stars are primarily hydrogen and helium together. Stars form through molecular clouds that are dense areas of gas in interstellar gas. These clouds are dark, hence they are cold because they don’t absorb ambient light. Protostellar disc- has a protostar in the center.
