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Astronomy Final Exam Study Guide

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Astronomy 2021A/B

Astronomy Final Study Guide Habitable zones - Not likely for the earth to burn or freeze if it moves closer or further from the sun o In reality, it moves 147.1 million km each January and 152.1 million km each July o Northern hemisphere has summer when earth is farthest from the sun and winter when it is closest to the sun - Habitable zone: range of distances from the sun within earth could move but still be suitable for life o A zone in which it is possible to have liquid water on its surface o Being within a habitable zone does not make the world habitable (example: the moon) o Habitable zones evolve with time. Sun brightens with time so we expect a habitable zone to move outward with time - Surface liquid water: key factor - It is possible for life to exist outside of the habitable zones o These worlds could not have complex life (micro) o Subsurface oceans or in little pockets of water o Liquids other than liquid water  Ex. Ethane (Titan is the only planet that meets this criteria) Venus - Runaway greenhouse effect produced by atmospheric carbon dioxide - Has 200,00 times more of atmospheric co2 than earth o However they have the same total amount of Co2 (just in different locations) o Venus = all in atmosphere o Earth = in carbonate rocks or dissolved in oceans  Uses Co2 cycle  Venus lacks this cycle as it has no water in which Carbon dioxide can dissolve - Water on Venus (mostly in atmosphere) o Earth has 10,000 more water than venus o Water on earth came from planetesimals that originated in more distant parts of the universe and crashed on ours o Leading hypothesis is that UV light broke apart water molecules, allowing the hydrogen to escape into space ensuring that the water molecules could never reform  Has no magnetic field so solar wind strips away gas easily  This is proven through deuteurium atoms • Rare deuteurium atoms are more common as they are twice as heavy and couldn’t escape to space • 100 times higher on venus than on earth • - Entire surface of venus was repaved 750 million years ago - Has ongoing volcanic activity o Clouds contain sulfuric acid which is made of sulfur dioxide which can only enter the atmosphere through volcanic outgassing o Venus express revealed recent lava flows Greenhouse Effects - UV radiation can’t penetrate the oceans on earth so the water was protected - Venus is 30% closer to sun than earth is o Not within habitable zone today - If earth was moved to venus’s location the global average temp would increase to 45 celsius (30 degree change)  This would lead to increased evaporation more water in atmosphere (water is a greenhouse Gas)  strengthens greenhouse effect more ocean evaporation as temperatures increase  repeat loop (positive feedback loop) o Runaway greenhouse effect - Venus may have once been habitable and well within the habitable zone o 4 billion years ago the sun was 40% as strong Habitable zone today - Factors influencing Surface Habitability 1. Distance from star a. Greenhouse effects can occur because of distance b. Only 10% of stars are as luminous as the sun so a lot of stars are different i. Brighter star = wider habitable zone and dimmer star = narrower ii. However brightest stars have very short lives (millions of years) and short stars have longer years 2. Planetary size a. Small size means interior cools quickly (no magnetic field) i. Must retain internal heat b. Plate tectonics is necessary for any planet to be habitable long-term (planets must be big enough for plate tectonics to occur) 3. Role Of an atmosphere a. Without sufficient atmosphere pressure, liquid can’t be stable i. Mars doesn’t have a stable atmosphere ii. May require a magnetic field for atmosphere to occur (prevents from solar wind stripping) • Inner boundary (anywhere closer = runaway greenhouse effect) – Somewhere between Venus (0.72 AU) and Earth (1 AU) – Optimistic model, 0.84 AU: runaway greenhouse effect – Pessimistic model, 0.95 AU: moist runaway greenhouse effect (water vapor circulating higher in the atmosphere) • Atmosphere becomes moist with water causing oceans to be lost • Outer boundary (anywhere further = water freezes) – Where the atmosphere of an Earth-size planet has enough greenhouse effect – Optimistic model, 1.7 AU (cf., Mars 1.52 AU): enough greenhouse effect – Pessimistic model, 1.4 AU: middle atmosphere too cold  CO sn2w  CO 2oss from atmosphere  less greenhouse effect • Habitable zone – There is a habitable zone around the Earth – Optimistic = .84 AU – 1.7 AU – Pessimistic = .95 AU – 1.4 AU Changing Habitable zone • Dependence on solar luminosity – Sun less luminous in the past  habitable zone moves in – Sun more luminous in the future  habitable zone moves out – Continuous habitable zone • An area which has been habitable since the heavy bombardment (4 billion years ago) • Under optimistic assumptions, both earth and mars fall in this zone • Stellar evolution – H to He  fewer particles at the core  less pressure  squeezing by layers above  temperature rise  luminosity rise – Quantitative stellar structure and evolution is well modeled – Checked against observations of stars of all masses and ages • Evolving habitability – Habitable also until the death of the Sun: optimistic 3-4 billion years, pessimistic at most another billion years • Earth will succumb to the moist greenhouse gas effect • Water will circulate in atmosphere allowing hydrogen to be broken apart and escape into space. Eventually all the oceans will evaporate Death of the Sun - Is already occurring and will be complete in 5 billion years (will run out of hydrogen) - Red giant star (occurs within first hundred million years after 5 billion years) o Will swell 100 times larger (engulfs Venus) o Surface temperature on Earth 700C o no life will survive o Sun is smaller but pumps more energy - Planetary nebula o Outer part of the Sun expelled into the interstellar medium o Leaves a core known as a white dwarf star - White dwarf o Radius: ~ Earth radius o Slowly losing energy over many byr  stellar death Global Warming - Increase in average temperatures on whole planet - Warming: o 0.8 C in the last century o o 0.6 C in the last 30 years o Stronger in northern hemisphere (more land) - Burning fossil fuels expels more Co2 into space allowing the greenhouse effect to speed up - CO 2easurements: o Since 1958: directly measured in the atmosphere o Past: tree rings and Antarctic ice sheet - Isotopic abundances measure past temperatures: o Antarctic ice sheet (trapped bubbles in ice) o Sediments on ocean floor - Correlation of temperature with CO 2 o Strong correlation  Hiher temperature has been easily correlated with higher CO2 levels o Current slope very steep Consequences of Global warming - Temperature rise by 3-5 C by 2100 - Shift in regions of food production - Polar regions warm most - Northern hemisphere warms more - More moisture → stronger storms - Rise ins sea level by up to ~1 m o Sea levels have increased 20 cm in last century - Ecological changes (agricultural land may turn into deserts) Chapter 11: Habitability Outside the Solar System - Star = large ball of gas that generates energy by nuclear fusion in its hot central core o Our sun fuses hydrogen into helium into its core o Star is born when nuclear fusion begins in its central core o Star dies when it ceases to produce energy by any kind of fusion o Brighten as they age - Our sun will someday produce carbon by fusing helium in its core o Massive stars can create all the other elements - Higher mass Stars die in explosions called supernovae in which their cores collapse to form neutron stars or black holes Properties of stars • Evolution – Protostar  main sequence  giant or supergiant  white dwarf, neutron star, or black hole • Luminosity is the most obvious characteristic of a star – The brightest stars in our sky aren’t actually the brightest though – Example Sirius is brightest in our sky but is only moderately bright  it just happens to be closer than other brighter stars (8.6 light years away) • Lifetimes (stars luminosity and temperature and related to its mass) – All stars are born with 98% or more hydrogen and helium • Mass tells us how much hydrogen it has for fusion, while luminosity tells us how brightly the star shines • The more massive the sun, the shorter its lifetime Spectral Sequence - Edward Pickering (1846 – 1919) invented spectroscopy - Hired women as his “computers” - Used visibility to classify stars o Type A = strongest o Type B = weaker o Type c = weaker (up until type O) - In 1896 hired Annie Jump Cannon (1836 – 1941) o Classified more than 400,000 stars o She Concluded there were only 7 major specteral types (Types: OBAFGKM (politically incorrect mnemonic: “Oh, Be A Fine Girl, Kiss Me”))  Subdivided these into 10 different categories (so B0,B1,B2,B3…B9) where B9 almost was as bright as A0  System adopted in 1910 - IN 1925 Cecilia Payne-Gaposchkin showed the difference in spectral types reflected differences in surface temperatures Which suns make good stars? • O: much too short – With lifetimes less than a million years earth like planets haven’t been able to form • B: little life past accretion phase – 50 mill. Lifetime – Stars death will occur too quickly • A and F: – Life short but manageable – Higher UV light a potential problem, but maybe more ozone – Hotter than sun so habitable zone is further – Life on these planets would be much more simpler as there is not enough time for them to become complex (took earth 4 billion years) • G: – OUR SUN IS A G STAR – Make up 7% of suns while A and F make up 3% • K and M: – Long lifetimes • Every m star born is still shining – Very common (90% of all stars) – Less luminous  smaller habitable zone (planets would have to orbit star much closer) – Two problems for thought of M planets being able to hold life 1. Synchronous orbit: problem, unless the atmosphere rotates • Planets may get stuckwith only one side facing sun making the other side freeze 2. Flares: main danger is UV, but it also produces more ozone • Sun may emit flares burning any life on nearby planets • Recent studies have shown that both of these aren’t fatal • Flares can create an atmosphere and underwater creatures are safe • Co2 would heat up the dark side! • Brown dwarfs: – Stars that have masses less than 8% of sun – Gravity isn’t able to compress core to high enough temperatures to sustain fusion • As a result brown dwarfs appear • Very common with very hot surfaces • Have masses 10 to 80 times that of Jupiter – Brown dwarfs can have planets with moons which may have life on them Multiple Star Systems (2 or more stars orbit each other closely in same star system) - Alpha centurai has a triple star system o Has a G,K and M star - Binary star systems = 2 stars • Orbit – Planet pulled by two or more stars can have a complicated orbit, moving in and out of the habitability zones of the stars • Stable cases – Stars close, planet orbiting both at a safe distance – Stars far, planet orbiting close to one of them • Unstable cases – Any other combination • Planets in binary star systems can be habitable!!!!! Detecting Extrasolar Planets - First planet around another star was in 1995 – 51 Pegasi - 2 methods of detecting extrasolar planets o Directly: Pictures or spectra of the planets themselves  Direct detection can tell us way more about a planet o Indirectly: Precise measurements of a stars properties may reveal the effects of orbiting planets  Majority of detections are indirect - Indirect techniques: Astrometric and Doppler techniques use gravitational tugs on stars - from orbiting planets to identify them - Everything in a star system orbits the system’s Center of mass which is the balance point of all the mass in the solar system o Example: Jupiter affects the suns orbit o If a more massive planet was located where Jupiter was, it would pull the center of mass farther from the suns center giving the sun a larger orbit and faster orbital speed Astrometry (gravitational tugs) - If a star wobbles around center of mass then there must be unseen planets - 2 main difficulties: 1) we are looking for changes that are very small 2) stellar motions are largest for planets orbiting far from their stars but long long orbital periods mean it can take decades to notice the motion Doppler Effect (Gravitational tugs) - Studies a stars spectrum, looking for telltale signs that the star is moving o Appear in small shifts in wavelengths of spectral lines caused by Doppler Effect o BEST SUITED FOR IDENTIFYING PLANETS THAT ORBIT RELATIVELY CLOSE TO THEIR STAR (quicker to notice orbit of planet closer to its star) - Can tell us a planets mass o Doppler shift reveals only a stars motion directed to or away from us o If we view a planets orbit straight on it is impossible to detect the the planet with the Doppler technique  can only view the planet at some angle other than face-on o Doppler technique gives us a lower limit on stars true orbital speed which means mass is also a lower estimate (planets true mass will be no more than double that mass) - Can affect sound and light o When an object emitting sound is moving toward you then you get shorter wavelengths and higher frequencies. After the object passes you (you are now behind it) the wavelengths are stretched out giving a lower frequency\ o Same with light. If light is moving toward you, the wavelengths are shorter creating Blueshift – blue light. If it is moving away from you the light emits longer wavelengths so it looks red – A Redshift. o Current effects can measure a star’s wavelength to 1 meter/second Transits and Eclipses - Transit: a third indirect way of detecting distant planets o Changes in a stars brightness that occur when a planet passes in front of or behind the star o Example: star HD189733  Transit occurs every 2.2 days telling us the orbital period  2.5% dip in stars brightness tells us jhow the planets radius compares to the stars radius - Eclipse: half an orbit after a transit when the planet passes behind its star o Example star HD189733  Infrared brightness drops about .3% during an eclipse  Allows us to calculate planets temperature - We expect fewer than 1% of stars to undergoe a transit as it requires a planet to pass perfectly in front of and behind its star in line with earth o Kepler is currently watching many stars and requires a dip in brightness to occur 3 times before we can infer a transit and that there is another planet Direct Detection • Angular resolution – Need to see dim planet near a bright star – Angular resolution limited by diffraction and atmosphere – Overcome atmosphere by going to space • Infrared observations – Improve the luminosity ratio between star and planet by observing in the infrared – Diffraction blurring , stronger in the infrared Gravitational lensing - Effect predicted by einsteins theory of relativity - When an objects gravity bends or affects the light of a more distant object - Only occurs once as planets have to be lined up for it to occur o Rarely ever happens again Observational Summary About Planets - Have masses like other jovian planets and are large with Low densities - Have discovered “super earths” which are very rocky with large amounts of metal - Hot Jupiters  jovian planets which orbit close to the sun o Have highly elliptical orbits o Form far out and endure planetary migration which is when gravitational waves pull them closer - Terrestrial planets: o Migration of Jovian planets disrupts terrestrial planets in the habitable zone during the migration o Final elliptic orbits  long-term disruption Signatures of habitability and life - Distance from star o Is it in the habitable zone? - Imaging o Clouds o Diurnal changes (oceans v. continents) o Seasonal changes (snow and/or ice) - Spectroscopy o Surface temperature o Surface composition o Atmospheric composition (from IR spectra)  Absorption and emission of gases in the atmosphere  O 2  CH 4 (methane) Frequency of Earth like Planets - Rare earth hypothesis: the processes that created earth are one of a kind o Contribute our climate to 2 pieces of “luck”:  Plate tectonics • Affects the CO2 cycle and earths atmosphere • Nothing unique about plate tectonics on Earth: depends on liquid mantle and convection due to radioactive heating  The moon • Regulates earths axis tilt - Galactic Habitable zone: area of galaxy whixh may be habitable o Outer regions of galaxy are unlikely to have terrestrial planets because of low abundances of elements other than hydrogen and helium o Inner regions are more likely to encounter supernovae and be exposed to radiation as they occur more often o Galactic habitable ring contains about 10% of all stars in galactic disk • Need heavy elements – We think that terrestrial planets are formed from rocky planetessimals • Bottom line – Earth-size planets are very likely, unless we are unaware of something special in the formation process of our solar system Jupiter – Responsible for aligning asteroids along circular orbits between Mars and Jupiter – Essentially protects earth from impacts – Responsible for ejecting comets to the Oort Cloud – Do other stars have such a “Jupiter”, and what about migration? Stable Climate - Stable climate for several byr o Essential for life o Has to adapt to the rising luminosity of the star over several byr Hertzsprung-Russel (H-R) Diagram - Used to classify the different types of stars - Stars used to be only judged based on brightness which consisted of: 1. Its distance 2. Its intrinsic luminosity 3. Dimming effects of interstellar dust in between it and us - By the end of the nineteenth century we could identify them by colour o Were anywhere from blue to red - Danish astronomer Ejnar Hertzsprung arranged stars by colour o Blue-white to red o Hotter stars (blue stars were more luminous than cooler stars) o Further red stars were more luminous than closer ones  2 categories of red stars (1. Small ones the intrinsically fainter ones 2. giants  intrinsically more luminous o Divided all stars into 2 groups i. The main sequence (stars in a diagonal line ii. The giants: all other stars Chapter 12: The Search For Extraterrestrial Intelliegence - SETI (search for extraterrestrial intelligence) searches ONLY for technologically advanced life Drake Equation (Discovered by frank Drake) - Shows number of civilizations in our galaxy or in the universe from which we could get a signal of extraterrestrial life o Doesn’t give us a definitive answer but lays out the important factors in determining the answer o Can find the answer for the universe by multiplying our answer by 100 billion the amount of observable galaxies in the universe - Equation o # civilizations = Nhp F lite Fcivf now - NHP number of habitable planets in the Milky Way Galaxy - lifefraction that actually have life o (if flite is 1 then all planets have life but if it is 1/100 then one inn a hundred do) - civ fraction that have a civilization at some time o if you have 1/1,000 fciv then that means out of 1,000 planets with life only one has had a civilization while the other 999 were not developed enough - now: fraction that have a civilization now o if a civilization stays technologically ative for a billion years then the chance of a signal reaching us is 1 in 10 - The lifetime of a civilization was determined to be the critical factor in the success of SETI efforts Question of Intelligence -
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