Astronomy Exam 3 Study Guide
Astronomy Exam 3 Study Guide ASTR 101
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This 12 page Study Guide was uploaded by Abby Geiger on Wednesday March 9, 2016. The Study Guide belongs to ASTR 101 at Clemson University taught by Dr. Sean Brittain in Spring 2016. Since its upload, it has received 1809 views. For similar materials see Solar System Astronomy in Science at Clemson University.
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Date Created: 03/09/16
ASTRONOMY EXAM 3 STUDY GUIDE CHAPTER 7: True statements about our Sun: Sun is made mostly of hydrogen and helium, the Sun is a star, the Sun contains more than 99 percent of all the mass in our solar system Venus has the highest average surface temperature All the jovian planets have rings Kuiper Belt: a region of the solar system beginning just beyond the orbit of Neptune that contains many icy comets Pluto has more in common with comets in the Kuiper belt than it does with terrestrial planets like Earth All the planets orbit counterclockwise around the Sun Hydrogen compounds: water, ammonia, and methane Oort cloud: it’s not really a cloud, but rather refers to the trillion or so comets though to orbit the Sun at great distances Our Moon is about the same size as moons of the other terrestrial planets (FALSE) On average, Venus is the hottest planet in the solar system- even hotter than Mercury (TRUE) The mass of the Sun compared to the mass of all the planets combined is like the mass of an elephant compared to the mass of a cat (TRUE) The weather conditions on Mars today are much different than they were in the distant past (TRUE) Pluto orbits the Sun in the opposite direction of all other planets (FALSE) Moons cannot have atmospheres, active volcanoes, or liquid water (FALSE) Saturn is the only planet in the solar system with rings (FALSE) ALL terrestrial planets have had volcanic activity at some point in their histories. Large moons orbit their planets in the same direction the planet rotates most of the time. ALL of the planets orbit the Sun in the same direction that Earth does MOST of the moons of the planets orbit in the same direction that their planets rotate What is unusual about our Moon? It is surprisingly large relative to the planet it orbits What are the four major features of the solar system o Several exceptions to the general trends stand out Uranus rotates with an axis tilt that lies nearly in the ecliptic plane Venus rotates in a direction opposite to the rotation of the other terrestrial planets Our Moon has a diameter more than ¼ the diameter of the Earth o Swarms of asteroids and comets populate the solar system o Planets fall into two major categories (terrestrial and jovian) o Large bodies in the solar system have orderly motions Patterns of motion in the Solar System: o All the planets (not counting Pluto) orbit the Sun in nearly the same plane o All the planets (not counting Pluto) have nearly circular orbits o Planets closer to the Sun move around their orbits at higher speeds than planets farther from the Sun Jovian planets: o Have more moons than terrestrial planets o Orbit farther from the Sun than terrestrial planets o Are larger in mass than terrestrial planets o Are larger in size than terrestrial planets Formation of planets: Inner parts of disk are hotter than outer parts Rock can be solid at much greater temperatures Frost line Inside: too hot for hydrogen compounds to form ice Outside: cold enough for ice to form The Solar System contains a vast number of small bodies, which we call asteroids when they are rocky and comets when they are icy. These small bodies are concentrated in the region(s) of the solar system that we call: o The Kuiper belt o The Oort Cloud o The asteroid belt The planets were NOT each born in a separate, random event BECAUSE if they had been born in separate random events: planetary orbits would have many different orientations and directions rather than all being in the same direction and in the same plane We expect a theory to not only explain the observed characteristics of our solar system but also to make testable predictions about other solar systems Terrestrial Planets: o Small size o Located within the inner solar system o Solid, rocky surface o Accreted from planetesimals of rock and metal o Surfaces dramatically altered during the heavy bombardment Jovian Planets o Numerous orbiting moons o Extensive ring systems o Primarily composed of helium, hydrogen and hydrogen compounds o Low average density o Ejected icy planetesimals that are now Oort cloud comets o Accreted from icy planetesimals o Formed in a region of the solar system with lower orbital speeds o Large moons formed in surrounding disks of material o Formed in regions cold enough for water to freeze The Law of conservation of angular momentum: cloud will rotate rapidly as it collapses The Law of conservation of energy: why the cloud heats up as it collapses Colliding cloud particles exchange angular momentum and on average end up with the rotation pattern for the cloud as a whole, which is why the collapsing cloud forms a disk Terrestrial planets formed in the inner solar system and jovian planets formed in the outer solar system because ices condensed ONLY in the outer solar system, where some icy planetesimals grew large enough to attract gas from the nebula, while only metal and rock condensed in the inner solar system making terrestrial planets What should we expect to be true for other star systems based on the nebular theory as it explains our own solar system? o Jovian planets always form farther from their star than terrestrial planets o Planetary systems should be common o Other planetary systems should have the same two types of planets: terrestrial and jovian o Planetary systems should generally have all planets orbiting in nearly the same plane Jupiter’s main ingredient: HYDROGEN AND HELIUM Steps of solar system formation o Collapse, condensation, accretion Leftover ice-rich planetesimals are called comets Why didn’t a terrestrial planet form at the location of the asteroid belt? o Jupiter’s gravity kept planetesimals from accreting HIGHEST abundance to LOWEST abundance o Hydrogen and helium gas o Hydrogen compounds o Rock o Metals HIGHEST temperature to LOWEST temperature (would condense into a solid) o Metals o Rock o Hydrogen compounds o Hydrogen and helium gas DISTANCE FROM SUN from FARTHEST to CLOSEST that they could condense into a solid o Hydrogen compounds o Rock o Metals Substances found within the inner 0.3 AU of the solar system before planets began to form: o Rocks, metals, hydrogen compounds, hydrogen, and helium, all in GASEOUS form NO substances existed as solid flakes within the inner 0.3 of the solar system before planets began to form Formation of solar system (FIRST to LAST) o Large cloud of gas and dust o Contraction of solar nebula o Condensation of solid particles o Accretion of planetesimals o Clearing the solar nebula Nebular Theory: our modern theory of the formation of our solar system o Consistent with theory: A star is surrounded by a disk of gas but has no planets A star has 20 planets Of a star’s 5 terrestrial planets, 1 has a moon as large as Earth’s moon Beyond its jovian planets, a star has two ice-rich objects as large as Mars o Not consistent with theory: All 6 of a star’s terrestrial planets have a moon as large as Earth’s moon A star’s 5 terrestrial planets orbit in the opposite direction of its 3 jovian planets A star has 9 planets, but none orbit in close to the same plane A star’s 4 jovian planets formed in its inner solar system and its 4 terrestrial planets formed father out How did the Moon form? It formed from the material ejected from Earth in a giant impact How old is the solar system? 4.5 billion years Neptune is about 30 times as far from the Sun as our own planet Venus is the planet with the highest average surface temperature The planet with the lowest average density is Saturn The planet that orbits closest to the Sun is Mercury The only rocky planet to have more than one moon is Mars Jupiter is the jovian planet that orbits closest to the Sun Uranus has a rotational axis that is tilted so much it lies nearly in the plane of its orbit Most of the surface of Earth is covered with liquid water CHAPTER 8: Nebular Theory: Our solar system formed from the collapse of an interstellar cloud of gas and dust Three major changes that occurred in the solar nebula as it shrank in size: It got hotter Its rate of rotation increased It flattened into a disk Giant Impact Hypothesis for the origin of the moon states that the moon formed from material blasted out of Earth’s mantle and crust by the impact of a Mars-size object Doppler Technique Detected the most extrasolar planets so far Used for most of the first 200 extrasolar planet detections Currently best suited to find Jupiter-sized extrasolar planets orbiting close to their stars Requires obtaining and studying many spectra of the same star o Compare many spectra of the star taken over a period of many months or years If a planet is discovered by the D.T. and is discovered to have transits, we can determine density, precise mass, orbital period, and physical size (radius) Can find planet’s orbital distance Why do we say this technique gives us the planet’s “minimum mass”? o The size of the Doppler shift that we detect depends on whether the planet’s orbit is tilted Transit Method Can tell us size of a planet Can be used on a backyard telescope Looks for very slight, periodic dimming of a star First to identify Earth-sized extrasolar planets Can potentially detect planets in only a few percent of all planetary systems Allows for the extrasolar planet’s radius to be determined Planet detection strategy of NASA’s Kepler Mission Used to find extrasolar planets by carefully monitoring changes in a star’s brightness with time Allows us to find planets around only a small fraction of stars that have planets o Kepler Mission overcomes this limitation by studying a large number of stars Can find planet’s orbital distance Astrometric Technique Can detect a planet in an orbit that is face-on to the Earth Measures precise changes in a star’s position in the sky, in fractions of arcseconds Used to find a Jupiter-sized planet through careful measurements of the changing position of a star Can find planet’s orbital distance Extrasolar Planets Most extrasolar planets discovered so far most resemble jovian planets Most known extrasolar planets are more massive than Jupiter because current detection methods are more sensitive to larger planets Why is it so difficult to take pictures of extrasolar planets? o Their light is overwhelmed by the light from their star o A sun-like star is about a billion times brighter than the light from a Jupiter-size planet orbiting it What is an extrasolar planet?? o A planet that orbits a star that is not our own First confirmed detections of extrasolar planets occurred in the 1990s As a result of the discoveries of extrasolar planets, the new idea “Jovian planets can migrate from the orbits in which they are born” has been added into our theory of solar system formation HAS been observed among extrasolar planets: o Orbits that are much more eccentric than the orbits of planets in our own solar system o Orbits that take the planets much closer to their star than Mercury orbits the Sun Has NOT been observed among any known extrasolar planets o Orbital speeds that are so slow that we cannot explain them o Orbits that are not elliptical Orbital Period Orbital radius can be inferred from a star’s orbital period You can determine a planet’s mass from the star’s velocity curve by measuring both the star’s orbital period and its change in velocity over the orbit If a planet has a shorter orbital period than another planet, it must be closer to its star Why don’t we expect to find life on planets orbiting high-mass stars?? o The lifetime of a high-mass star is too short Hot Jupiters Very close to their stars Kepler Mission Currently searching for planet transits around some 100,000 stars GAIA Astrometry Mission Being designed to measure very small changes in stellar positions, which should allow it to discover many extrasolar planets Terrestrial Planet Finder Mission (Proposed plans) Will someday provide us with the first actual images and spectra of terrestrial worlds orbiting other stars “Evolution is a theory” because it explains a great deal about life and is supported by an enormous body of evidence Drake Equation: f(now) is the fraction of planets with civilizations at the present time (as opposed to only in the past or future) The Drake equation is designed as a way of estimating the number of intelligent civilizations in the Milky Way Which type of planet would you expect to cause the largest Doppler shift in the spectrum of its star? A massive planet that is close to its star Doppler shifts measure velocity o Star will move faster if it experiences a greater gravitational tug from its planet. o A massive, close-in planet will cause the strongest gravitational tug Earth’s History: (FIRST TO LAST) Giant impact forms moon End of heavy bombardment Early life (based on fossil evidence) o Oldest fossil evidence of life dates to about 3.5 to 4 billion years ago Oxygen build-up in the atmosphere Animals colonize land Earliest mammals Dinosaurs go extinct o About 65 million years ago Earliest humans Earth was born about 4.5 billion years ago Moon formed a few tens of million years later Solar System is 4.5 billion years old Likely to be habitable: Subsurface ocean on Europa Moon with atmosphere orbiting jovian planet 1 AU from 1 M (sun) star Underground on Mars o We have detected water ice on Mars, and Mars still has some volcanis heat Unlikely to be habitable: Surface of terrestrial planet 10 AU from 0.5M (sun) star Volcanoes on Io Surface of Mars As the mass of the central star increases, the distance to the habitable zone increases and the size of the habitable zone increases High-mass main-sequence stars have a more distant and wider habitable zone than low-mass stars Sky surveys looking for radio signals generated by technology are part of the search for extraterrestrial intelligence The notion that living organisms with advantages that give them greater reproductive success (in some local environment) will survive while others perish is an example of what Darwin called natural selection One of the key premises for the theory of evolution is that living organisms are able to produce far more offspring than their environment can support The different time periods of the geological time scale are defined by changes in fossils found from those time periods Incoming ultraviolet light from the Sun can cause a mutation in a living organism’s DNA that can affect its ability to survive and reproduce “Frost Line” is a circle at a particular distance from the Sun, beyond which the temperature was low enough for ices to condense Hydrogen Compounds can condense into what we call ice at low temperatures Asteroids and comets are leftover planetesimals that never accreted into planets Suppose you start with 1kg of a radioactive substance that has a half- life of 10 years. After 20 years pass, you’ll have .25kg of the radioactive substance remaining. How do we use velocity curves to show that some extrasolar planets are close to their host stars? The velocity curve of the host star shows periods much shorter than a year CHAPTER 9 Why is it so hard to detect planets around other stars? Planets are dim, stars are bright o Sun-like star is 10 billion times brighter than the light reflected from its planets o Like being in San Francisco and trying to see a pinhead 15m from a grapefruit in D.C. How do we detect planets around other stars? Radial Velocity Orbits of Extrasolar Planets Many detected planets have greater mass than Jupiter Many of these are much closer to their star than Mercury is to the Sun Highly elliptical orbits Massive planets called “Hot Jupiters” orbit very close to their stars Many systems are NOT co-planar Theory: explanatory framework for making sense of observations and predicting new phenomena Finding MORE New Worlds How will we search for Earth-like planets? o GAIA: use astronomy to measure precise motions of a billion stars (spatial shifting) o SIM: use interferometry to measure star motions even more precisely Nebular Theory Do we need to modify our theory of solar system formation? o The discovery of hot Jupiters has forced reexamination of the theory Planetary Migration or Gravitational encounters may explain hot jupiters Gravitational Encounters Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit. Encounters can increase inclination of orbits Multiple close encounters with smaller planetesimals can also cause inward migration Planetary Migration A young planet’s motion can create waves in a planet-forming disk Matter in these waves can tug on a planet o This causes its orbit to move inward Direct Detection Determining whether Earth-mass planets are really Earth-like Missions capable of blocking enough starlight to measure the spectrum of an Earth-like planet and being planned NASA’s Terrestrial Planet Finder 1 in 6 stars has an Earth-like planet The list of Kepler candidates contains most of the knowledge about exoplanets but it is not complete or pure Issue of false positives (purity): o Observed periodic transit signal could be due to: Transiting planet Eclipsing binary CHAPTER 10 Aliens The Drake equation N= number of civilizations in our galazy with which communication might be possible R= the average rate of star formation per year in our galaxy p = the fraction of those stars that have planets ne= the average number of planets that can potentially support life per star that has planets ℓ = the fraction of the above that actually go on to develop life at some point i = the fraction of the above that actually go on to develop intelligent life c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space L = the length of time such civilizations release detectable signals into space SETI experiments look for deliberate signals from extraterrestrials Cannot detect signals as weak as our own broadcasts (TV/radio) Difficulties of Interstellar Travel Far more efficient energies are needed Energy requirements are enormous Ordinary interstellar particles become like cosmic rays Social complications of time dilation When did life arise on Earth? Earliest life form: 3.85 BYA (shortly after end of heavy bombardment) o Evidence comes from fossils, carbon isotopes Fossils in sedimentary rock o Relative ages: deeper layers formed earlier o Absolute ages: radiometric dating Theory of Evolution Tells us HOW evolution occurs: through natural selection Evolution proceeds through mutations o Supported by DNA All life on Earth shares a common ancestry Cyanobacteria -> origin of oxygen Necessities for Life: A nutrient source Energy o Sunlight, chemical reactions, internal heat Liquid water Life on Mars?? Mars had liquid water in the past Now it is very dry, but there is water that comes to the surface on occasion o Underground water source
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