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A Journey Through the Universe

Course Outline. Introduction: Light

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A Journey Through the Universe

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    1. A Journey Through the Universe

    2. Course Outline Introduction: Light & Distance Tools & Techniques of Modern Astronomy The Sun, Our Standard Star Neighbours of Rock and Gas: The Solar System The Life of Stars I: Characteristics of Normal Stars

    3. Course Outline The Life of Stars II: Birth & the Creation of Planets The Life of Stars III: Ending with a Bang or a Whimper Our Milky Way Galaxy: Structure & Composition The Dynamic Life of Galaxies: Starbursts, Collisions, & Supermassive Black Holes The Evolution of the Universe: From the Big Bang to the End of Time

    5. Course Objectives Understanding: Stars, planets and galaxies Our Galaxy in the Universe Telescopes, instruments, and experimental techniques used by astronomers Modern astronomy

    6. My Research Dark Matter Galaxies Clusters Black holes

    7. What is Astronomy? Astronomy is basically the study of LIGHT It is purely an observational science We cannot run experiments. Can do ‘thought experiments’, including mathematical/physical models and simulations (‘theory’), but these can only be verified or discarded on the basis of actual data (‘observation’). Like all science, nothing is certain.

    8. Light: The Visible Spectrum Visible light can be split into a rainbow: a spectrum. Visible light is a form of electromagnetic radiation. In fact, it is only a very small fraction of the full electromagnetic (EM) spectrum. All EM radiation can be referred to as ‘light’; we observe astronomical objects in nearly all types of light. The Sun is our primary source for all radiation.

    9. The Full Radiation Spectrum The characteristic quantities of light are wavelength and frequency (which are directly related to each other) Both quantities are also directly related to energy Short wavelength, high frequency => high energy radiation Long wavelength, low frequency => low energy radiation Energy is also related to temperature: the higher the energy, the hotter the radiation

    10. EM Radiation: Examples Radio from low frequency to higher: aircraft/shipping bands; MW radio, shortwave, TV & FM radio, microwaves, radar Infrared (IR): basically ‘heat’ – e.g. night vision goggles Ultraviolet (UV): the radiation that causes sunburn X-rays: used to look through soft tissue to bones, teeth Gamma-rays: ‘radioactive’ natural elements, nuclear reactors/power plants – high energy, exposure to which is dangerous to living things.

    11. Radiation from Space to the Earth’s Surface

    12. Multiwavelength Astronomy What determines the type of EM radiation emitted by an astronomical object? Temperature! To observe at different wavelengths of light is to look at different temperatures & energies, which gives us different types of information about an object. Most objects radiate over a range of wavelengths but generally they have a characteristic frequency/temperature where they radiate most strongly. Living things radiate most strongly in the IR – body heat!

    13. Characteristic Radiation

    14. The Dual Nature of Light Is light a Particle or a Wave? It is both This is called the ‘wave-particle duality’ of light EM radiation can be described in two ways: As a stream of ‘photons’ – massless particles As a continuous wave Can think of this as a stream of photons travelling in a wave at the speed of light (not really accurate but the best way to reconcile the paradox) Whether light looks like a wave or a particle depends on the type of experiment. This might seem odd.

    15. The Dual Nature of Light Higher energy light behaves like a particle, while lower energy light behaves more like a wave. This is crucial for designing detectors & telescopes, as different types of instruments are needed to see radiation of different energies In fact, we generally choose whichever description of light we need for our study: Imaging/measuring brightness: photons (particles) Splitting light into a spectrum: waves (but other parts of spectroscopy depend on the particle nature of light.)

    16. Measuring Distance Astronomers use a variety of units in the measurement of distance. They choose a characteristic scale. Basic unit: the meter (and kilometer). Used to describe (for example) the Earth-Moon distance. The Astronomical Unit (AU) = 1.496 x 1011 m. This is the average Earth-Sun distance, useful for solar system distance measurements. Light year (ly) = 63240 AU. The distance that light travels in a year. Used for nearby stars. Parsec (pc) = 3.26 ly. The most commonly used unit in astronomy. Also kiloparsec (kpc) = 1000 pc; used for galactic distances & megaparsec (Mpc) = a million (106) pc; used for extragalactic distances.

    17. Distance = Time The speed of light is the fastest thing there is. Nonetheless a light photon takes time to cover a large distance. Distance is also a measure of time because all information comes in the form of light. Thus the farther away an object is (spatial distance), the more time the light has taken to reach us. So we observe astronomical objects not as they are ‘right now’, but as they were when the light left the object and began its journey through space. Therefore in astronomy we are looking back in time – in some cases almost all the way back to the beginning of the universe at the Big Bang.

    18. The Cosmic Distance Scale First accurate measurement of Earth’s size made in 200 BC by Eratosthenes in Egypt Earth is an ‘oblate spheroid’ – slightly flattened at the poles It takes ~90 min to orbit the Earth (at an altitude of 200 miles – shuttle, Hubble, space station)

    19. The Solar System Average Earth-Moon distance is 400,000 km = 2 light-seconds It takes ~8 minutes for light from the Sun to reach Earth (8 light-minutes = 93 million miles)

    20. The Next Nearest Star

    21. The Solar Neighborhood Sirius, Procyon, and Altair are all very bright stars easily visible to the naked eye. Different types of stars have different intrinsic brightnesses, so we cannot simply assume that bright stars are nearby while fainter ones are further away. Epsilon Eridani has at least one planet. (Jupiter-sized, 3.2 AU)

    22. What is a Galaxy? Galaxies are large systems of stars and gas, typically containing several million to several trillion stars. They come in a variety of shapes and sizes. They are separated by vast distances. There are trillions of galaxies in the universe. Mostly contain dark matter.

    23. The Milky Way Galaxy Travelling at the speed of light, it would take 26,000 years to get from the Sun to the centre of the Milky Way. Stars are closer together the closer you get to the Galactic Centre.

    24. The Orion Arm Our corner of the Milky Way! Our spiral arm is named the ‘Orion Arm’ for the Orion star cluster (seen in the constellation Orion). The Sun is ~8.5 kpc from the Galactic Centre – about 2/3 of the way out from the centre to edge of the Milky Way.

    25. The Milky Way from Above This is an accurate representation of what our Galaxy looks like if we were able to see it from outside. A ‘barred spiral’ galaxy. Diameter ~100,000 ly = 30 kpc

    28. The Nearest Galaxies These two ‘satellite’ galaxies of the Milky Way are easily visible with the naked eye from the Southern hemisphere They are irregular galaxies in orbit around the Milky Way. They will eventually be ‘eaten’ by our Galaxy – galactic cannibalism!

    29. The Local Group This composite image of the Local Group shows the galaxies in a ~correct orientation. There are at least 40 galaxies in the Local Group, within an area of ~5 million ly. The Milky Way is one of the 3 largest; most of the others are ‘dwarf’ galaxies. In order of size: Andromeda, Milky Way, Triangulum – all spirals!

    30. The Local Supercluster Called the ‘Virgo Supercluster’ after the largest nearby cluster. Covers ~100 million ly, contains ~28000 galaxies. Galaxies tend to cluster in groups. About 75% of all galaxies are in clusters.

    31. The Nearest Superclusters Coma has a diameter of ~9 Mpc and is 92 Mpc away. Perseus is at about the same distance = 300 million ly. There are ~100 superclusters within 1 billion ly of us = 250,000 trillion stars!

    32. Superclusters around Virgo

    33. Sheets and Voids Each of the 9325 points in this map is a galaxy! The Universe has a ‘bubbly’ structure – sheets & filaments of galaxies in a web punctuated by voids. Typical diameter of a void ~25 Mpc and fill about 90% of space. The ‘Great Wall’ is a sheet of galaxies measuring 200x70 Mpc, 100 Mpc from us.

    34. The Farthest Reaches of Visible Space The visible Universe has a radius of ~14 billion years, because the Universe is ~14 billion years old (time since the Big Bang). The Universe is expanding. The Universe contains 10 million galaxy superclusters – 20 billion trillion stars!

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