Astronomy. concepts. Astronomy. Scale of the Universe. 300 million km (diameter) ~1 billion km (orbital path)

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1 Astronomy 1. Introduction and Observations 2. Sun and Solar System 3. Stars (Stellar Evolution) 4. Galaxies An excellent book on astronomy by Timothy Ferris (1988, 2003) Also, there are two excellent periodicals related to astronomy Astronomy and Sky and Telescope 5. Universe (Deep Space, Expanding Universe, Hubble Red Shift, Cosmology) Astronomy 1. Earth s position in the solar system 2. Origin of the Universe 3. Natural interest in observing the night sky Problems in understanding astronomy concepts 1. Scale (distance and time) 2. Frame of reference (changing and 3-D) 3. Vast distance and time (large numbers and unfamiliar units/terminology) Scale of the Universe Earth orbit Solar System Nearest Star Milky Way Galaxy Local Group of Galaxies Observable Universe 300 million km (diameter) ~1 billion km (orbital path) 12 billion km across (~ 10-3 light years*) 4.27 light years 10 5 light years 2.5 million light years 13.7 billion light years * One light year ~10 13 km, or ~10 trillion km (It takes about 1/1000 of a year, or about 9 hours for light to travel across the solar system; 4.27 years for light from the nearest star to reach Earth; 434 years for light from Polaris to reach Earth; and ~10 5 years for light from the most distant stars in the Milky Way, our galaxy, to reach Earth) Light Year A unit of distance - how far light travels in one year Calculate a light year: ~300,000 km/s the speed of light x60 s/min ~18,000,000 km/min x60 min/hr ~1,080,000, km/hr x24 hr/day ~25,920,000,000 km/day x365 days/yr ~9,460,800,000,000 km/yr speed of light (units = km/yr) ~9,460,800,000,000 km One light year (units = km) So, one light year is approximately 10,000,000,000,000 km, or, km or, 10 trillion km (~60 MB jpeg file) 1

2 Close-up of area in lower right hand corner of previous slide Hubble Space Telescope (HST) Ultra Deep Field Image ( ) Can image 30 th magnitude objects. Required 400 orbits, 11.3 days or recording. Image contains about 10,000 galaxies. Area covers 1/12.7 million of the entire sky. Like looking through an 2 ½ m (8 ft) long soda straw. With this view, astronomers would need about 50 Ultra Deep Fields to cover the entire Moon. Hubble's keen vision (0.085 arc seconds) is equivalent to standing at the U.S. Capitol and seeing the date on a 400 m (1/4 mile) away at the Washington monument Deep space image (after repair/upgrade of Hubble Space Telescope (HST) The Universe's Most Ancient Object The farthest and one of the very earliest galaxies ever seen in the universe appears as a faint red blob in this ultra-deep field exposure taken with NASA's Hubble Space Telescope. This is the deepest infrared image taken of the universe. Based on the object's color, astronomers believe it is 13.2 billion light-years away. The most distant objects in the universe appear extremely red because their light is stretched to longer, redder wavelengths by the expansion of the universe. This object is at an extremely faint magnitude of 29, which is 500 million times fainter that the faintest stars seen by the human eye. The dim object is a compact galaxy of blue stars that existed 480 million years after the Big Bang, only four percent of the universe's current age. It is tiny and considered a building block of today's giant galaxies. Over one hundred such mini-galaxies would be needed to make up our Milky Way galaxy. The Hubble Ultra Deep Field infrared exposures were taken in 2009 and 2010, and required a total of 111 orbits or 8 days of observing. The new Wide Field Camera 3 has the sharpness and near-infrared light sensitivity that matches the Advanced Camera for Surveys' optical images and allows for such a faint object to be selected from the thousands of other galaxies in the incredibly deep images of the Hubble Ultra Deep Field. Credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz), R. Bouwens (University of California, Santa Cruz and Leiden University), and the HUDF09 Team

3 Large Numbers Number of stars in the universe (just recently updated), in at least 3 trillion galaxies. 300 sextillion (300x10 21 or, 3 trillion times 100 billion) ,000,000 Million ,000,000,000 Billion Stars in Milky Way (spiral) galaxy (left) and in an elliptical 1,000,000,000,000 Trillion galaxy (right) which contains large numbers of red dwarf 1,000,000,000,000,000 Quadrillion stars. 1,000,000,000,000,000,000 Quintillion ,000,000,000,000,000,000,000 Sextillion USAToday, December 2, 2010 Frame of Reference 3D and constantly changing (due to Earth s rotation and seasons caused by tilt of axis of rotation) North star (Polaris) North star (Polaris) Meteor Time lapse photo Time lapse photo Time lapse photo centered on Polaris (~ 8 hours) Sun Ursa Major (the big bear, or the big dipper) We view the stars in the sky as if they were all equal distance from Earth (as on a flat or spherical surface). However, the distance to stars varies greatly. 3

4 Moons All of the objects that we see with the naked eye in the night sky are within the Milky Way galaxy (can see two other galaxies with excellent viewing or binoculars and many galaxies with a telescope) The constellation Vega Jupiter Galileo Galilei Galileo s observations of Jupiter s moons demonstrated that moons revolved about a planet providing support for the Copernican theory that the Sun was the center of the solar system. Jupiter and the Galilean Moons as viewed through a modern amateur telescope (25 cm Meade). Callisto Jupiter Europa Ganymede Io Io Europa Ganymede Callisto Jupiter has at least 63 moons. The largest (and mostly closest to the planet) were discovered by Galileo in 1610 and are called the Galilean moons Kepler s 3 rd law: P 2 ~ D 3 Period squared is proportional to Distance cubed. The exact equation can be used to calculate the mass of a planet: Johannes Kepler Nights Distance (km) Ganymede Jupiter Period (days) Sky and Telescope javascript Jupiter s moons orbit calculator: 4

5 Close-up of Galilean Moons positions relative to Jupiter on the date and time shown (C = Callisto, E = Europa, I = Io, G = Ganymede). With the calculator (below) you can step through time (use 10 minute or 1 hour time steps) to see the orbits of the moons about Jupiter (viewed from Earth, approximately along the plane of the ecliptic. Sky and Telescope javascript Jupiter s moons orbit calculator: Moon rotating (animation from multiple images from the NASA LRO (Lunar Reconnaissance Orbiter): The moon does rotate about once every 27 days so that we always see the same side of the Moon. The rotation rate is cause by gravitational locking of the Moon to the Earth. It tool 4 years of observations to gather the images for the animation. The Solar System (Sun and planets not to scale; Figure 15.17, text) The Moon as it appears from Earth (northern hemisphere): Orbits to scale Orrery Orrery An excellent online Orrery (for viewing the planets in orbit) can be found at: - main controls are Solar speed, system orbit brightness, model planet (Orrery) size and note zoom. that (also: it is 29 not at true scale in distances diameters! An excellent online Orrery (for viewing the planets in orbit) can be found at: - main controls are speed, orbit brightness, planet size and zoom. (also:

6 A Brief Tour of the Solar System Another view of the Solar System The Sun and planets drawn to scale (orbital positions not to scale) Figure 15.18, text) The Sun and planets drawn to scale (orbital position not to scale) ( 32 The terrestrial planets The gas giant planets (Jovian planets) Note sunspots The Sun and planets drawn to scale (orbital position not to scale) ( 33 The Sun and planets drawn to scale (orbital position not to scale) ( 34 Figure 15.19, text Earth s Moon (Figure 15.23, text) 6

7 Mars (Figures and 15.30, text) Olympus Mons (and outline of Arizona for scale) Jupiter (and great red spot) (Figure 15.33, text) Saturn (and rings) (Image from Hubble Space telescope; Figure 15.36, text) On July 19, 2013, in an event celebrated the world over, NASA's Cassini spacecraft slipped into Saturn's shadow and turned to image the planet, seven of its moons, its inner rings -- and, in the background, our home planet, Earth. ( mission_pages/cassini/whycassini/jpl/cassini html#.uothpd8wlpf) Moon Zoom in to see Earth and Moon Neptune (and great dark spot) (Figure 15.40, text) 7

8 Asteroid Eros (Figure 15.42, text) Comet (dust and ion tail) orbiting the Sun (Figure 15.43, text) Planets and Solar System Websites Viewing the Night Sky Observing Stars Apparent Magnitude (brightness; what we see without adjusting for distance to the star) and Absolute Magnitude (brightness adjusted for distance) The Sun has a brightness (apparent magnitude of -27; note that smaller magnitudes are brightest). Faintest stars: Apparent Magnitude Naked Eye viewing 6 Binoculars 10 Amateur Telescopes 15 Modern Large Telescopes 25 8

9 Measuring Distance to Stars: 1. The Stellar Parallax method Example of lunar parallax: Occultation of Pleiades by the Moon This is the method referred to by Jules Verne in From the Earth to the Moon: (Figure 16.2, text) Stellar Parallax distance determined from parallax angle, the smaller the parallax angle the greater the distance to the star. A parallax angle of 1 second of arc (1/3600 degrees angle) corresponds to a distance of 3.09 x km and is called one Parsec. Distant stars are measured in MegaParsecs. Another method is to take two pictures of the Moon at exactly the same time from two locations on Earth and compare the positions of the Moon relative to the stars. Using the orientation of the Earth, those two position measurements, and the distance between the two locations on the Earth, the distance to the Moon can be triangulated: This is the method referred to by Jules Verne in From the Earth to the Moon. Measuring Distance to Stars: 1. The Stellar Parallax method Measuring Distance to Stars: 2. Brightness method Stellar Parallax Animation available at: (Figure 16.2, text) Light spreads out with distance such that the brightness varies by 1/r 2, where r is the distance. For example in the diagram above, the brightness at 4 m distance would be only 1/16 th of the brightness at 1 m. Variable Stars Certain stars that have used up their main supply of hydrogen fuel are unstable and pulsate. RR Lyrae variables have periods of about a day. Their brightness doubles from dimest to brightest. Cepheid variables have longer periods, from one day up to about 50 days. Their brightness also doubles from dimest to brightest. Period ~3 days Measure the period of the variable star, then see next slide Empirical relationship (determined by observations of stars having a known distance) between period of pulsation for variable stars and luminosity. Absolute magnitude for a star whose distance is unknown can be calculated from the determined luminosity and then the distance to the star calculated from the brightness method using the absolute and apparent magnitude and 1/r 2 relationship. Period ~3 days indicates a luminosity (proportional to absolute magnitude) of about

10 American astronomer Henrietta Leavitt observed many Cepheid variables in the Small Magellanic Cloud (a satellite galaxy to ours). She found the period-luminosity relation (below; reported in 1912). ( ) Measuring Distance to Stars: 2. Brightness method Period ~3 days indicates a luminosity (proportional to absolute magnitude) of about 500. Brightness Demo Absolute Brightness = 1, Absolute Brightness = 4 Observer Distance Stars o Star A Star B Star B is twice as far as A The apparent brightness of A and B will be the same Star C Star D Star D is twice as far as C The apparent brightness of C will be 16 times as great D The Sun A Typical Star Main composition of the Sun Hydrogen 73.46% Helium 24.85% Oxygen 0.77% Carbon 0.29% Iron 0.16% Spectacular loops and prominences are often visible on the Sun's limb. The sun is one of over 100 billion stars in the Milky Way Galaxy. It is about 25,000 light-years from the center of the galaxy, and it revolves around the galactic center once about every 250 million years. The sun is a star with a diameter of approximately 1,390,000 km, about 109 times the diameter of Earth. The largest stars have a diameter about 1,000 times that of the sun. Fewer than 5 percent of the stars in the Milky Way are brighter or more massive than the sun. But some stars are more than 100,000 times as bright as the sun, and some have as much as 100 times the sun's mass. At the other extreme, some stars are less than 1/10,000 as bright as the sun, and a star can have as little as 7% of the sun's mass. There are hotter stars, which are much bluer than the sun; and cooler stars, which are much redder. Comprising about % of the total mass of the Solar System most of the remainder is Jupiter. Nuclear Fusion in the Sun (and other stars) the proton-proton chain reaction: Note that the process starts with 4 protons and ends with one atom of helium-4 (4 protons are fused into one helium-4 atom) plus large energy release the gamma particles that eventually convert to photons (electromagnetic radiation, or light)

11 Nuclear Fusion in the Sun (and other stars) the proton-proton chain reaction: The decrease in mass (from 4 protons to 2 protons and 2 neutrons) is only 0.7% but the energy release is large because of the equation e=mc 2. Also note that at the end of the reaction, there are still two protons remaining, so the reaction continues as a chain reaction always releasing energy. Nuclear Fusion in the Sun (and other stars) the protonproton chain reaction: The proton proton chain occurs around times each second in the core of the Sun. The Sun releases energy at the mass-energy conversion rate of 4.26 million metric tons per second, ( W) or megatons of TNT per second. ( Nuclear Fusion in the Sun (and other stars) the proton-proton chain reaction: Energy from nuclear fusion in stars larger than the Sun (hotter and higher pressure core) is generated by the Carbon-Nitrogen-Oxygen (CNO), or other, chain reaction. Tools of Astronomy: 1. Samples meteorites (abundance of elements). 2. Photography telescopes (Earth-based and satellite; visible and invisible wavelengths) 3. Distance measurement stellar parallax, brightness method (variable stars), Hubble red shift. 4. Brightness absolute and apparent magnitude (classification). 5. Spectroscopy continuous spectrum (amount of radiation at different wavelengths; provides temperature of surface of star), bright line and dark line spectra (provides composition) Life Cycle of the Sun In about another 5 billion years, the hydrogen will be nearly depleted and the Sun s core will collapse and heat up resulting in helium fusion and the Sun will become a Red Giant with a size that will probably extend out to the present orbit of Mars

12 Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. Note 11 year cycle of sunspots Very bright stars have more shorter-wavelength radiation and higher temperatures. These measurements are from spectrometers. The Hertzsprung-Russell (H-R) Diagram The Hertzsprung-Russell (H-R) Diagram Hotter stars have shorter wavelength radiation and shorter lifetimes Luminosity (Sun = 1) (Figure 16.7, text) Sun Absolute Magnitude Luminosity (Sun = 1) Classification of Stars (Figure 16.7, text) Sun Cooler stars have longer wavelength radiation and longer lifetimes Absolute Magnitude Surface Temperature (K) Surface Temperature (K) 12

13 Low Mass (Figure 16.12, text) Medium Mass (Figure 16.11, text) High Mass H-R diagram illustrating life cycle of a main sequence star (such as the Sun). Most of lifetime, star is on Main Sequence, then in dwarf stage. Life cycle of stars 74 Supernova remnant Cassiopeia A located about 10,000 light years from Earth. Trifid Nebula (birthplace of Stars, mostly H and He gases illuminated by hot young stars). Neutron Star (Figure 16.8, text) (Figure 16.6, text) Horsehead (mostly dark) Nebula in constellation Orion. The Helix Planetary Nebula (formed during a star s collapse from a red giant to a white dwarf). (Figure 16.10, text) (Figure 16.13, text) 13

14 The Crab Nebula (in the constellation Taurus; the remains of the supernova explosion of A.D. 1054). The Veil Nebula (in the constellation Cygnus; the remains of a supernova implosion). (Figure 16.14, text) (Figure 16.15, text) Galaxies Typically consist of 1 billion to over 100 billion stars. Most are relatively flat. Stars in the galaxy revolve around a central area, and thus don t collapse from gravity (similar to the solar system). Most galaxies (including the Milky Way) probably have black holes in the center of the galaxy accounting for a substantial part of its mass. Spiral Galaxy M81 (NASA) Types of Galaxies: Elliptical, Spiral, Irregular, Dwarf A tour of some galaxies (Chapter 16, text) (Figure 16.17, text) Spiral Galaxy Andromeda Above: The Milky Way y(panorama from Earth). Right: Spiral galaxy NGC 2997 (similar to Milky Way). (Figure 16.19, text) (Figure 16.20, text) 14

15 The Barred spiral galaxy Elliptical galaxy ESO 325-G004. Most elliptical galaxies are very old and probably result from two galaxies colliding. (Figure 16.22, text) 86 Cosmic Web (structure of the universe, how the galaxies are distributed in the universe) Videos Where the Galaxies Are Margaret Geller, 1991 Measuring Distance to Stars (and Galaxies): 3. The Hubble Red Shift (a Doppler Effect) Moving object emitting radiation NASA /Goddard Space Flight Center Scientific Visualization Center: Cosmic Origins Spectrograph: Large Scale Structure of the Universe Journey Through the Cosmic Web Motion Observer here Observer here sees longer sees shorter wavelength wavelength radiation (red radiation (blue shifted) shifted) Doppler Effect (and Red Shift) Animation available at: Measuring Distance to Stars (and Galaxies): 3. The Hubble Red Shift (a Doppler Effect) Moving object emitting radiation Example of Doppler Effect Observer senses different wavelengths (hears different sounds) from approaching and receding ambulance. Note that the ambulance moving away results in longer wavelengths. The faster the ambulance is moving away, the lower the wavelength. (Figure 16.22, text) Motion Observer here Observer here sees longer sees shorter wavelength wavelength radiation (red radiation (blue shifted) shifted) Doppler Effect (and Red Shift) Animation available at: 15

16 Hubble s Law Hubble s Law Hubble, 1929 Hubble's law is a statement of a direct correlation between the distance to a galaxy and its recessional velocity as determined by the red shift. It can be stated as: The reported value of the Hubble parameter has varied widely over the years, testament to the difficulty of astronomical distance measurement. But with high precision experiments after 1990 the range of the reported values has narrowed greatly to values in the range of: H 0 is the slope of the line Modern Hubble s Law Data Hubble s original 1929 data One MegaParsec (Mpc) = 3 million light years = 3 x km 1) Virtually all galaxies are moving away from us, e.g. they are redshifted. 2) The more distant the galaxy, the larger its redshift, that is the faster it is moving away. At all locations, objects are moving away and more distant objects have higher recession velocity. Expanding Universe As the universe expands, the galaxies get farther apart (although some are gravitationally connected and collide/merge) and increase their red shift. B 1 2 A Expanding universe modeled as the surface of a balloon - Imagine a balloon with points A, B and C labeled. After expanding the balloon, the distances change. Cosmic Microwave Background Radiation C Time 1 2 From Time 1 to Time 2, increase in distance A B to B is 1, A to C is 2. So, 4 velocity for A to C is twice is large as from A to B. More distant C objects have higher recession velocity. This is true for all locations on the surface. Distances in Red A Time 2 95 Observations of background radiation of the universe from the 1990 NASA COBE (Cosmic Background Explorer) satellite 16

17 Cosmic Microwave Background Radiation With a traditional optical telescope, the space between stars and galaxies (the background) is pitch black. But with a radio telescope, there is a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum, hence the name cosmic microwave background radiation. The CMB's serendipitous discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned them the 1978 Nobel Prize. Cosmic Microwave Background Radiation The CMBR is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe. This is the source for the term relic radiation, another name for the CMBR. Evidence for Big Bang Theory Red Shift, universe expanding Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006 for their work on precision measurement of the CMBR. The Cosmic Microwave Background Radiation measured by the COBE satellite almost precisely fits the backbody radiation curve corresponding to K ( o C) a very cold temperature due to cooling of the universe (in the space between galaxies and stars) and red shift. Distant galaxies receding faster Cosmic background radiation Abundance of elements in the universe is consistent with mathematical models and nuclear physics of Big bang evolution of the universe Einstein s general theory of relativity predicts expanding universe Formation of galaxies and large scale structure of the universe Black Holes A black hole is a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it even traps light. The fundamental descriptions of black holes are based on equations in the theory of general relativity developed by the German-born physicist Albert Einstein. The theory was published in Characteristics of black holes The gravitational force is strong near a black hole because all the black hole's matter is concentrated at a single point in its center. Physicists call this point a singularity. It is believed to be much smaller than an atom's nucleus. The surface of a black hole is known as the event horizon. This is not a normal surface that you could see or touch. At the event horizon, the pull of gravity becomes infinitely strong. Thus, an object can exist there for only an instant as it plunges inward at the speed of light. Black Holes Formation of black holes According to general relativity, a black hole can form when a massive star runs out of nuclear fuel and is crushed by its own gravitational force. While a star burns fuel, it creates an outward push that counters the inward pull of gravity. When no fuel remains, the star can no longer support its own weight. As a result, the core of the star collapses. If the mass of the core is three or more solar masses, the core collapses into a singularity in a fraction of a second. Galactic black holes Most astronomers believe that the Milky Way Galaxy -- the galaxy in which our solar system is located -- contains millions of black holes. Scientists have found a number of black holes in the Milky Way. These objects are in binary stars that give off X rays. A binary star is a pair of stars that orbit each other. 17

18 Black Holes Supermassive black holes Scientists believe that most galaxies have a supermassive black hole at the center. The mass of each of those objects is thought to be between 1 million and 1 billion solar masses. Astronomers suspect that supermassive black holes formed several billion years ago from gas that accumulated in the centers of the galaxies. There is strong evidence that t a supermassive black hole lies at the center of the Milky Way. Astronomers believe this black hole is a radio-wave source known as Sagittarius A* (SgrA*). The clearest indication that SgrA* is a supermassive black hole is the rapid movement of stars around it. The fastest of these stars appears to orbit SgrA* every 15.2 years at speeds that reach about 3,100 miles (5,000 kilometers) per second. The star's motion has led astronomers to conclude that an object several million times as massive as the sun must lie inside the star's orbit. The only known object that could be that massive and fit inside the star's orbit is a black hole. Because black holes are invisible, they are mostly detected (inferred) form the gravitational forces required for objects which revolve around them and from radiation from nearby regions (outside the event horizon) that are energized by the rapid motion of objects and gases and emit x-ray radiation. Some useful references: Chaisson, E., Relatively Speaking: Relativity, Black Holes, and the Fate of the Universe, W.W. Norton & Company, New York, 254 pp., Ferris, T., Coming of Age in the Milky Way, Anchor Books, New York, 495 pp.,