Astro Notes - Fall 2021 PDF

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Fall 2021...

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ASTR 142 Planet- a moderately large object that orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition Moon (or Satellite)- an object that orbits a planet Asteriod- a rel. small and rocky object that orbits a star Comet- a relatively small and icy object that orbits a star Solar (star) system- a star and all the material that orbits it, including its planets and moons Nebula- a interstellar cloud of gas and/or dust Galxy- a great island of stars in space, all held together by gravity and orbiting a common center Universe- a sum total of all matter and energy that is, everything within and between all galaxies Light travels at a finite speed (300,000 km/s) The farther away we look in distance, the further back we look in time. Light-year-The distance light can travel in 1 yr At great distances, we see objects as they were when the universe was much younger What is our place in the universe? Earth is part of the solar system, which is the Milky Way Galaxy, which is a member of the Local Group of galaxies in the Local Supercluster The distances between planets are huge compared to their sizes—on a scale of 1-to-10-billion, Earth is the size of a ball point and the Sun is 15 meters away. – On the same scale, the stars are thousands of kilometers away. – It would take more than 3000 years to count the stars in the Milky Way Galaxy at a rate of one per second, and they are spread across 100,000 light-years. – The observable universe is 14 billion light-years in radius and contains over 100 billion galaxies with a total number of stars comparable to the number of grains of sand on all of Earth's beaches. Stars at different distances all appear to lie on the celestial sphere. The 88 official constellations cover the celestial sphere. The Ecliptic is the Sun's apparent path through the celestial sphere. North celestial poleis directly above Earth's North Pole. South celestial poleis directly above Earth's South Pole. Celestial equatoris a projection of Earth's equator onto sky. The Local Sky An object's altitude (above horizon) and azimuth (direction along horizon) specify its location in your local sky. Meridian: line passing though zenith and connecting N and S points on horizon -Horizon all points 90 degrees away from zenith -Zenith: the point directly overheard Right ascension: like longitude on celestial sphere (measured in hours with respect to spring equinox)

Declination: like latitude on celestial sphere (measured in degrees above celestial equator) Right Ascension: Vega’s RA of 18h 35.2m out of 24h places it most of the way around celestial sphere from spring equinox. Declination: Vega’s dec of +338degree 44’ puts it almost 39 degrees N of celestial equator (negative dec would be S of equator). The Sun’s RA and dec change along the ecliptic during the course of a year. Sun’s declination is negative in fall and winter and positive in spring and summer. Earth rotates from W to E, so stars appear to circle from east to west Stars near the North Celestial pole are circumpolar and never set. We cannot see stars near the S celestial pole. All other stars (and Sun, Moon, planets) rise in E and set in W. As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky. We can see over 2000 stars and the Milky Way with our naked eyes, and each position on the sky belongs to one of 88 constellations We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon. Why do stars rise and set? B/c of Earth’s rotation Why do the constellations we see depend on time of year? The time of year determines the location of the Sun on the celestial sphere Seasons depend on how Earth’s axis affects the directness of sunlight Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun Summers occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth Variation of Earth-Sun distance is small- abt 3%; this small variation is overwhelmed by the effect of axis tilt. Variation in any season of each hemisphere- Sun distance is even smaller! Summer (June) solistice Winter (December) solstice Spring (March) equinox Fall (September) equinox Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years - Polaris won’t always be the North Star - Positions of equinoixes shift around orbit, e.g., spring equinox, once in Aries, is now in Pisces - Earth’s axis precesses like the axis of a spinning top What causes the seasons? - The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter - We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon

The summer and winter solstices are when the N. Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight How does the orientation of Earth’s axis change with time? - The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000-year precession cycle that slowly and subtly changes the orientation of Earth’s cycle Sidereal day: Earth rotates once on its axis in 23 hrs, 56 mins, and 4.07 secs Solar day: The Sun makes one circuit around the sky in 24 hrs A solar day is longer than a sidereal day by 1/360 because Earth moves about 1 degree in orbit each day. Sidereal month: Moon orbits earth in 27.3 days. Earth and Moon travel 30 degrees around Sun during that time (30/36=1/12) Synodic month: A cycle of lunar phases; takes about 29.5 days, 1/12 than a sidereal month Sidereal year: Time for Earth to complete one orbit of Sun Tropical year: Time for Earth to complete one cycle of seasons. Tropical year is about 20 mins (1/26,000) shorter than a sidereal year b/c of precession Planetary periods can be measured with respect to stars (sidereal) or to apparent position of Sun (synodic) Differences between a planet’s orbital (sidereal) and synodic period depends on how far planet moves in 1 earth year Po= Psync (1yr/Psync-1yrs) The length of a tropical year is about 365.25 days In order to keep the calendar year synchronized with the szns, we must add one day every 4 years (Feb 29) For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years unless they are divisible by 400 (e.g., 2000) Lunar Phenomena First spacecraft to fly past Moon: January 1959 First spacecraft to (crash) land on Moon: September 1959 First pic of far side of Moon: October 1959 The US is (so far) the only country to send people to the Moon First person on Moon: July 1969 Last person on Moon: December 1972 Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth Half of Moon is illuminated by Sun and half is dark We see a changing combo of the bright and dark faces as Moon orbits Moon takes about 29.5 days to go through the whole cycle of phases (synodic month) Phases are due to different amounts of sunlit portion being visible from Earth Time to make full 360 degrees rotation around Earth, sidereal month, is about 2 days shorter Waxing- moon visible in afternoon/evening Gets “fuller” and rises later each day Waning- moon visible in late night/morning Gets “less full” and sets later each day -

Synchronous rotation: the Moon rotates exactly once with each orbit That is why only one side is visible from Earth The Earth and Moon cast shadows When either passes through the other’s shadow, we have an eclipse Lunar eclipses can occur only at full moon Lunar eclipses can be penumbral, partial, or total. Solar eclipses can occur only at new moon Solar eclipses can partial, total, or annular Eclipses occur when Earth, Moon, and Sun form a straight line Eclipses don’t occur every month b/c Earth’s and Moon’s orbits are not in the same plane So, we have about 2 eclipses szns each year, with a lunar eclipse at new moon and solar eclipse at full moon 1. must be a full moon or new moon 2. The Moon must be at or near one of the 2 points in its orbit where it crosses the ecliptic plane (its nodes) Eclipse recurs with the 18 years, 11 1/3-day saros cycle, but type (e.g., partial, total) and location may vary Moon has large dark flar areas due to lava flow or maria Moon also has many craters from meteorite impacts Far side of Moon has some craters but no maria Meteoroid strikes Moon, ejecting material; explosion ejects more material, leaving crater Craters are typically about 10 x as wide as the meteoroid creating them, and 2x as deep Rock is pulverized to a much greater depth Most lunar craters date to at least 3.9 bill years ago; much less bombardment since then Regolith: Thick layer of dust left by meteorite impacts Moon is still being bombarded, esp. by very small micrometeoroids soften featers More than 3 bill yrs ago, the moon was volcanically active; the tille here was formed then Why do we see phases of the Moon? – Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth. What causes eclipses? – Lunar eclipse: Earth's shadow on the Moon – Solar eclipse: Moon's shadow on Earth – Tilt of Moon's orbit means eclipses occur during two periods each year Why do we only see one side of the Moon? – The Moon is tidally locked with the Earth and hence it makes one rotation as it revolves around the Earth. What are some of the surface features of the Moon? – The Moon is heavily cratered

– The craters come in a wide range of sizes – There are also large ancient lava flows called Maria. Greeks were the first people known to make models of nature They tried to explain patterns in nature without resorting to myth or the supernatural Underpinnings of the Greek geocentric model: Earth at the center of the universe Heavens must be “perfect”: Objects moving on perfect spheres or in perfect circles. Did not explain retrogrades The most sophisticated geocentric model was that of Ptolemy (AD 100-170) – Ptolemaic model: -sufficiently accurate to remain in use for 1,500 years Arabic translation of Ptolemy’s work name Almagest “the greatest compilation” So how does the Ptolemaic model explain retrograde motion? Planets really do go backwards in this model The Muslim world preserved and enhanced the knowledge they received from the Greeks Al-Mamum’s House of Wisdom in Baghdad was a great center of learning around AD 800 With the fall Constantinople (Istanbul) in 1453, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance Why does modern science trace its roots to the Greeks? They developed models of nature and emphasized that the predictions of models should agree with observations How did the Greeks explain planetary motion? The Ptolemaic model had each planet move on a small circle whose center moves around Earth on a larger circle. Proposed a Sun-centered model (published 1543) Used model to determine layout of solar system (planetary distances in AU) But. The model was no more accurate than the Ptolemaic model in predicting planetary positions, b/c it still used perfect circles. Compiled the most accurate (one arcminute) naked eye measurements ever made of planetary Positions Still could not detect stellar parallax, and thus still thought Earth must be at center of solar system (but recognized than other planets go around Sun). Hired Kepler, who used Tycho’s observations to discover the truth about planetary motion. Kepler first tried to match Tycho’s observations with circular orbits But an 8-arcminute discrepancy led him eventually to ellipses. Ellipse – elongated circle Kepler's First Law: The orbit of each planet around the Sun is an ellipse with the Sun

at one focus. Peri-Greek for near Apo-Greek for away, apart Kepler’s Second Law: As a planet moves around its orbit, it sweeps out equal areas in equal times This means that a planet travels faster when it is nearer to the Sun and slower when it farther from the Sun Kepler’s Third Law: More distant planets orbit the Sun at slower average speeds, obeying the relationship P^2=a^3 P= orbital period in years A= avg. distance from Sun in AU Copernicus created a sun-centered model; ; Tycho provided the data needed to improve this model: Kepler found a model that fit Tycho’s data What is Kepler’s three laws of planetary motion? 1. The orbit of each planet is an ellipse with the Sun at one focus. 2. As a planet moves around its orbit it sweeps out equal areas in equal times. 3. More distant planets orbit the Sun at slower average speeds: p^2= a^3 Galileo overcame major objections to the Copernican view. Three key objections rooted in Aristotelian view were: 1.Earth could not be moving because objects in air would be left behind. 2.non-circular orbits are not “perfect” as heavens should be. 3.If Earth were really orbiting Sun, we’d detect stellar parallax Galileo’s experiments showed that objects in air would stay with Earth as it moves. – Aristotle thought that all objects naturally come to rest. Galileo showed that objects will stay in motion unless a force acts to slow them down (Newton’s first law of motion Tycho’s observations of comet and supernova already challenged this idea. Using his telescope, Galileo saw: Sunspots on Sun ("imperfections") Mountains and valleys on the Moon (proving it is not a perfect sphere) Tycho thought he had measured stellar distances, so lack of parallax seemed to rule out an orbiting Earth. Galileo showed stars must be much farther than Tycho thought in part by using his telescope to see the Milky Way is countless individual stars. If stars were much farther away, then lack of detectable parallax was no longer so Troubling Galileo’s observations of phases of Venus proved that it orbits the Sun and not Earth 1 watt= 1J/s Colors of Light: White light is made up of many different colors How do light and matter interact? Emission Absorption

Transmission Transparent objects transmit light Opaque objects block(absorb) light Reflection/scattering Interactions of Light with Matter Interactions between light and matter determine the appearance of everything around us Light is a form of energy Light comes in many colors that combine to form white light Matter can emit light, absorb light, transmit light, and reflect light Interactions between light and matter determine the appearance of everything we see Light can act either like a wave or like a particle Particles of light are called photons A wave is a pattern of motion that can carry energy without carrying matter along with it. Wavelength is the distance between 2 wave peaks Frequency is the number of times per second that a wave vibrates up and down. Wave speed- wavelength x frequency A light wave is a vibration of electric and magnetic fields Light interacts with charged particles through these electric and magnetic fields Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field and vice versa 1 cm=30 GHz ½ cm= 60 Ghz Wavelength x frequency=speed of light =constant Particles of Light Called photons Each photon has a wavelength and a frequency The energy of a photon depends on its frequency E= h x f h= 6.626 x10 ^-34 J/s (Planck’s constant) Light can behave like either a wave or a particle A light wave is a vibration of electric and magnetic fields Light waves have a wavelength and a frequency Human eyes cannot see most forms of light The entire range of wavelengths of light is known as the electromagnetic spectrum Matter is made up of atoms in an electron cloud with a nucleus Atomic number= # of ______ in nucleus Atomic mass number = # of protons and neutrons Molecules: consists of 2+ atoms ______: same # of protons but different # of neutrons ______ in atoms are restricted to particular energy levels The only allowed changes in energy are those corresponding to a transition between energy levels Matter is made of atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons

The energies of electrons in atoms correspond to particular energy levels. Atoms gain and lose energy only in amounts corresponding to particular changes in energy Levels Spectra of astrophysical objects are usually combinations of these 3 basic types Absorption Emission Continuous Continuous: The Spectrum of a common (incandescent) light bulb spans all visible wavelengths, without Interruption Emission: A thin or low density cloud of gas emits light only at specific wavelengths that depends on its composition and temperature, producing a spectrum with bright emission lines Absorption: A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum Each type of atom has a unique set of energy levels Each transition corresponds to a unique photon energy, frequency, and wavelength Emission transitions produce a unique pattern of emission lines Because those atoms can absorb photons with those same energies, absorption transitions produce a pattern of absorption lines at the same wavelengths Each type of atoms has a unique spectral fingerprint Chemical Fingerprints- observing the fingerprints in a spectrum tells us which kinds of atoms are present Molecules have additional energy levels because they can vibrate and rotate The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated Many of these molecular transitions are in the infrared part of the spectrum Thermal Radiation-nearly all large or dense objects emit thermal radiation, including stars, planets, and you An object’s thermal radiation spectrum depends on only one property: its temperature All thermal motion ceases at 0K Water freezes at 273K and boils at 373K 1. Hotter objects emit more light at all frequencies per unit area 2. Hotter objects emit photons with a higher average wavelength Wien’s Displacement Law λmax = 0.0029 (meters Kelvin)/T(Kelvin) Reflected sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed – objects must look red

Thermal radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225K Carbon dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere UV emission lines: indicate a hot upper atmosphere Measuring the Doppler Shift Stationary Moving Away Away faster Moving forward Toward faster The amount of blueshift or redshift tells us an object’s speed forward or away from us Doppler shift tells us ONLY about the part of an object’s motion towards or away from us V= shift -rest/ rest Measuring Redshift Measuring Velocity Continuous spectrum, emission line spectrum, absorption line spectrum Each atom has a unique fingerprint We can determine which atoms something is made of by looking for their fingerprint in the Spectrum Nearly all large or dense objects emit a continuous spectrum that depends on temperature The spectrum of that thermal radiation tells us the object's temperature The Doppler effect tells us how fast an object is moving toward or away from us 9/28/2021 Telescopes Portals of Discovery How do eyes and cameras work? Refraction is the bending of light when it passes from one substance into another. Your eye uses refraction to focus light Sun appears distorted at sunset because of how light bends in Earth’s atmosphere Refraction can cause parallel light rays to converge to a focus The focal plane is where light from different directions comes into focus The image behind a single (convex) lens is actually upside-down A camera focuses light like an eye and captures the image with a detector The CCD detectors in digital cameras are similar to those used in modern telescopes Astronomers often use computers software to combine, sharpen, or refine images This image of Saturn’s moon has been processed to highlight the plume of water ice coming from its surface Eyes use ref...