Astronomy Final Exam Study Guide

# Astronomy Final Exam Study Guide ### Quasars 🌟 - Most luminous objects in the night sky - Stuff orbits black holes, experiencing friction, accretion of matter -> gravitational potential energy, releasing frequency in the x - ray - HUGE redshift - rapid fluctuations in their electromagnetic output indicative of their relatively small size of emitting regions - Probably already faded with time and have now evolved into relatively normal galaxies - We just see their light from millions of years ago because they are **so far away** - Powered by ==material falling into a central super-massive black hole==, an accretion disk - AGN (Active Galactic Nuclei) are all the same type of object, just ==viewed from different angles== - The primary wavelength of jets are ==radio==, and mostly imaged in the radio part of the spectrum - A supermassive black hole has 2 jets and an accretion disk - The jets are created on either side by the rotation of the disk - They can cause ==superluminal== sources when relativistic jets are beaming nearly towards us, causing an illusion of motion faster than light - Can only be seen if material is within 5 degrees of the line of sight to earth - They cannot actually travel faster than the speed of light - Jets eject material with respect to the accretion disk at a 90 degree angle - Jets are formed by pressure in the accretion disk forcing out a small fraction perpendicular to the disk - turbulence in the plasma of the accretion disk - We can **measure** the size of a black hole in a quasar using the **Eddington radiation limit** - The sun would have to increase its luminosity by 30,000 to hit the Eddington limit - As plasma falls inwards, its motion is slowed by the conservation of **angular momentum** - Prevents most of the material in the disk from actually falling in - AGN magnetic field affects ==charged particles== - Dusty Torus covers the accretion disk - AGNs are powered by the release of gravitational energy of inward falling material - Closer the material is to the black hole - the faster it moves, explained by Kepler's 3rd Law - Extremely hot - material in the accretion disk reaches up to 100,000k - Seyfert Galaxies: spiral galaxies with nuclei that resemble quasars - Type 2 Seyfert galaxies have narrow emission lines, spiral - Type 1 have broad emission lines - radio galaxies are elliptical and have narrow emission lines - the energy source of a galaxy is always located in its center - Active Galaxies - wide spectral lines because rapid thermal or rotational motion of the sources of radiation broaden the spectral lines because of the **Doppler Shift** - Blazars are seen by observers looking at AGN at intermediate viewing angles AND radio-loud quasars - Seen straight down the jet of an AGN - Blazars have no emission lines in their spectra - Radio-loud quasars have broad emission lines, found in elliptical galaxies - Radio Galaxies are probably quasars with jets that are beaming into the plane of the sky - Seen by observers looking at an AGN edge on - quasars lie at the center of ellipticals and spirals - 10% of quasars emit intense radio radiation in addition to their shorter wavelength spectra - Galactic collision enhances the accretion disk, enhancing the radiation output in some galaxies - Only difference between radio sources, quasars, and blazars is the angle between the line of sight and the jets of matter - Unified Model : a galaxy with a supermassive black hole and accretion disk but *no jets* is a "radio quiet AGN" - Quasars die once the black hole consumes all nearby fuel, and become a dormant black hole called a dead quasar - the center of a still-functioning galaxy <hr /> ### Planets 🪐 - Hydrogen is detected in the Sun's atmosphere by analyzing the absorption lines in it's spectrum - Spectroscopy tells us the chemical composition of planets - inner planets are terrestrial, outer are Jovian (Gas Giants and Ice Giants) - More metals and heavy elements in the inner planets, more lighter gases and ice in outer elements - Higher density - Hydrogen molecules escape Earth's atmosphere because while the average speed is below, many will have speeds above escape speed, and they will escape - GG's, Saturn and Jupiter, have a composition similar to the Sun, mostly hydrogen and ice - In the outer part of the solar system, CO2 and methane are in ice form - Temp of a gas is related to the average speed of the gas particles - Lower density - Geoactivity transforms a planet by covering up craters - Mars has cratered highlands and smooth lowlands - so the highlands are older than the lowlands and more geologically active - Crater at top of Olympus Mons is called a caldera - Similarly the moon is not active, so it can't hide its craters (erased by **erosion, weather, and geological activity**) - Planetary mass is measured by **gravitational influences** by a planet on satellites - Greater the mass, the higher the pressure and temp at center of planet, and greater gravitational - More gravity -> easier to retain an atmosphere of light elements - The **escape** speed of a planet is the speed needed by a molecule to leave the planet without being pulled back in by gravity - The mass and the diameter of a planet affect the escape speed - We observe planetary composition through spectroscopy - effects from other influences, however -> the spectral lines of something, as viewed from earth, will contain characteristics of ==their atmosphere, the Earth's atmosphere, and the cooler outer layers of the sun's atmosphere== - Pluto deemed no longer a planet because it **has not cleared most other objects from its neighborhood** - Trans-Neptunian Object - Jupiter and Saturn radiate more energy from the sun -> due to ==gravitational contraction== - Orbits - Most orbits are circular - All planets orbit sun counterclockwise - all lie in nearly the same plane - Satellites - All planets but Mercury and Venus have moons - Jupiter Moons - Io - most geologically active world in the solar system - Europa - worldwide ocean of liquid water beneath icy surface - **Titan** is the only moon with a substantial atmosphere <hr> ### Solar System 🌕 - Most abundant is hydrogen, then helium - All hydrogen originated in the big bang - Ratio of helium to hydrogen, by mass, is 1 : 2.5 - Carbon and other heavy elements formed in earlier generations of stars in fusion - Silicon has been said to be an alternative to carbon for life - for every 10,000 carbon atoms we have 100 - 1000 atoms of silicon - Carbon is a "recycled" material from stars found in the human body - H and O are relative abundant, so water should be relatively abundant - All planets in solar system fit within the three properties of solar system comparing jovian vs terrestrial planets - Terrestrial planets are much smaller because heavy elements are rare - ==Nebular Hypothesis==: all planets and Sun formed from a rotating cloud of gas and dust called the solar nebula - Gravitational pulls made the nebula contract, formed the ==protosun== at the center, a buildup of mass -> contracted more and more until a true star was born - Kelvin - Helmhotz Contraction (gravitational energy into thermal energy) - Protoplanetary Disk - Slight rotation of the nebula - rotating** flattened disk** around central star, region where solar systems form - Regions formed, more **ice and snow in outer region helped build up larger planets** (colder too) - Planetesimals - clumps began to merge, gravity grew, forming larger and larger - Core Accretion Model - gained more gas and dust as their gravitaitonal pull increases more and more - Jovian Planets - More materials were able to condense at the jovian locations, and light gasses were swept there, starting condensation earlier - Ices (water, methane, ammonia), metals, and rocks - Galilean satellites of Jupiter are believed to have formed by the accretion of material in a disk around jupiter - Neptune's gravity moved icy objects out of the way into the Kuiper Belt - Migrated inwards and outwards, ==deflecting planetesimals inward to form the asteroid belt== - **Late Heavy Bombardment** - period of time where all planets were subject to intense period of large impacts - Exoplanets - Habital zones are locations where it is just close enough to have liquid water and thus life - Different techniques for finding exoplanets: - Microlensing - Used to find 8-7 ExtraSolar Planets - bending of light from a background star, causing the star to appear brighter - Astrometric - Measures wobble of exoplanet in an orbital plane perpendicular to our line of sight - Transits - Most successful, can find ==smaller planets==, observes when planet passes between us and parent star, like a partial solar eclipse - We can figure out the mass of the planet with Kepler 3rd law - We can also figure out the absorption spectrum of its atmosphere against the star - We can tell the size of the star based on the dimming - Radial Velocity - Measures the doppler shift of a ==star's== spectrum - Most found are so close to their parent stars that they complete a full orbit in only a few days - Doppler effect "wobble" - need to be quite massive and orbiting close to star - Looking at the redshifting and blueshifting - Knowing the orbital period of an exoplanet tells us the semimajor axis using Kepler's third law - Small bodies of rock and ice orbit about everywhere in the solar system - Stopped forming planets after hundreds of millions of years (depleted much of planet building material) - Titius-Bode Law says that the semimajor axis an orbit from one planet to the next increases by 1.4 to 2 <hr> ### Asteroids 🪨 - Rocky objects - hundreds of thousands of kilometer-sized asteroids are known - Covered with rubble and small craters - Most just burn up in our atmosphere - All orbit Sun in the same direction of planets - Asteroid Belt between Mars and Jupiter, 2 - 3.5 au from Sun - Were it not for Jupiter, an Earth-size planet would have formed in the asteroid belt - Trans-Neptunian Objects - Small bodies beyond Neptune - Pluto, 1800 of them - ==Kuiper Belt== - 30au to 50au outside Sun, debris left over from formation of solar system - Trojan asteroids share the orbit of **Jupiter**, 60deg ahead or behind, usually - Albedo - how much light reflects off an asteroid - Low albedos mostly emit ==infrared light== - Magnetic Fields - Tells us internal structure of a planet - Caused by dynamo motion of liquid iron in the outer portion of cores, conducting electricity - Kirkwood Gaps - rings where asteroids with multiples of Jupiter's orbit get pulled outward - Happen at 1/3, 1/5, and 2/5 of Jupiter's orbital period - Hayabusa spacecraft of Japan got the first ever sample return from an asteroid in 2010 - NEO's Near Earth Objects are capable of creating a life-threatening impact every few tens of millions of years - The combined matter of all objects in the asteroid belt is equivalent to a single object of 1500km in diameter - Not sufficient to be the missing planet predicted through Titius-Bode law <hr> ### Meteors ☄️ - Meteoroid -> Meteor -> Meteorite - We 100 tons of meteors a day - Meteor Showers are phenomenon that occur when ==Earth passes through== a swarm of dust particles in space - Puffy groups we pass through - Almost all meteorites are 4.54 billion year's old - the age of the solar system - Bolide - extremely bright meteor, when meteoroid gets all the way to Earth <hr> ### Comets 💫 - Most originate beyond the Solar System in the ==Oort Cloud== (or Kuiper belt, for short term comets), when objects in Kuiper belt collide - Inner and outer limits of Kuiper belt determined by Neptune's gravity - Kuiper belt objects have orbits steeply inclined to the ecliptic - Most objects found between orbit of pluto and 50au from the sun - Composed of nucleus (head) and tail - nucleus described "dirty snowball" - The tails don't exist when at aphelion (farthest from sun) - Two tails, one ion (gas) tail, and one dust tail - ==Gas tail== is always blown away from the comet in the anti-Sun direction by solar wind - Atoms and molecules ionized by sunlight then pushed away by the solar wind - Dust Tail points the other way, looking white / yellow - Tails are **very** low density - Fuzzy glow is the **coma** - Hydrogen spheres envelope comets but only seen in ultraviolet region of EM spectrum - Highly elliptical orbits with huge semi-major axes, so they do not appear in our sky very often - Long-term ones originate in the Oort cloud - Kuiper belt extends to 50au and is in ecliptic, Oort cloud is spherical and extends to 50,000au from sun <hr> ### Cosmology 🌌 - Hubble Law describes continuing expansion of space - 14 billion ly in every direction from Earth - Universe is 13.82 billion years old - Not a constant value throughout all of time! - Cosmic Microwave Background Radiation - Evidence for the great inflationary period - the remarkable uniformity suggests that all parts of universe had to be very close, then suddenly carried car apart - the polarization of the cosmic background provides a test of cosmic inflation - same everywhere we look - greatly redshifted remnant of the hot universe - accidentally discovered using a large ground-based communications antenna - almost completely isotropic - implies that early matter was smoothly distributed - Isotropy problem - two points in space separated by a distance farther than light can have traveled in the lifetime of the universe can have exactly the same temperature, as if they had been contact earlier in history - Would have to go back 10^-35 (start of inflationary epoch) to find that the isotropy problem doesn't exist - Hydrogen makes up 75% of the universe, Helium 24% - Population 3 stars, the first generation of stars, exploded and sent their heavy elements into space to enrich the raw material for the next generation of stars - Pop 3 stars were much more massive, 1000 mass of sun, and much hotter and more luminous - Expansion - We see, and every other galaxy sees every other galaxy ==moving away from the observing, with more distant galaxies moving faster== - If the universe was infinite, we reach ==Olber's Paradox==, at night every line of sight should eventually intersect a star in an infinite universe - But the night sky is dark (**evidence for a finite universe**) - ==Dark energy== acts as a long-range repulsive effect, causing the expansion to accelerate - With Dark matter: - if cold: star forming regions form first, then galaxies -> clusters -> super clusters - if hot: super clusters -> fragmenting -> galaxies - If the universal expansion always accelerates, the Universe would expand forever - As it continues to accelerate, **less and less galaxies will be visible in the sky** (too far for their light to reach us) - Eddington Limit - The ==luminosity== beyond which the outward force due to radiation pressure on matter exceeds the inward force due to gravity (max luminosity of a star) - Sound Waves existed in the early universe because the spectrum of light emitted from the compressions and rarefactions of the sound waves still exist - Timeline - 3 min after -> "primordial nucleosynthesis" stopped, leaving a Universe with matter consisting mostly of hydrogen, helium, and a little bit of lithium - 20 min after -> fusion stoppe , making 3x hydrogen and 1x helium, structures began to start to form - 380,000 Years (Recombination) -> temperature fell, and the universe became ==transparent== for the first time (electrons could finally combine with protons) - At some point, inflation - crazy fast boom where it became trillions of times bigger - Happened when the space in the universe transitioned from a false vacuum to a true vacuum - Big Bang - Gravity emerged as an independent force in Planck time, 10^-43 seconds - everything in Universe used to be much closer together and hotter than now, it has drifted a bit - there is ==no center== of the universe where the Big Bang happened - All time and space began with the big bang - ==Cosmic Horizon== - We can see light 45 billion years away (because it got moved, it was originally closer, just redshifted) - The most distant objects in the universe can no longer be observed because their light hasn't reached Earth yet, and they lie outside our cosmic light horizon - The universe is flat or nearly so, looking the same in every direction - The lookback time of a galaxy is just how many ly away it is - If something has redshift Z, the universe became z + 1 times larger while that light traveled to earth - We measure with redshift to describe distance because the distance of a galaxy is changing with Hubble constant over time, but its redshift is a consistent observational number - things get redshifted while traveling through expanding space - The local group, and thousands of other galaxies, are being pulled in one direction because of the immense gravity from a collection of visible galaxies and dark matter - The universe had to have some fluctuations in density because it is required by the Heisenberg uncertainty principle - Jean's Length - density fluctuations cannot evolve into permanent objects if their diameter is smaller than the Jean's length - If higher dimensions exist, deflections by a force can be explained as straight lines of motion in the higher dimensional space - Edward Witten String Theory - seven dimensions are coiled tightly like coiled strings - If the average mass density was less than the critical density in the early universe, it would have expanded very rapidly without clumping - no galaxies, stars, planets, and humans due to big collapse - Four Forces - in decreasing strength: strong nuclear, electromagnetic, weak nuclear, gravitational - The exchange of the intermediate vector boson describes the weak nuclear force - strong force pulls protons and neutrons together - weak force acts to turn neutrons into protons - Gravity plays a more significant role in the universe over long distances because there is no balancing of charges <hr> ### Extraterrestrials 👽 - Stars which are likely to have planets supporting our own kind of life forms are those like the sun, **low-mass main-sequence stars** - A habitable planet is less likely to exist far from the Galactic center because there are ==less metals to form planets in the outer disk== - SETI - Search for Extraterrestrial Intelligence - Mostly an effort of ==radio astronomy== - NASA stopped funding it because it costed so much money - Seeks out target stars that have a greater potential to support life - Liquid Water is a requirement for life - Earth's original atmosphere was water vapor, nitrogen, and carbon dioxide - Drake Equation - Tells us number of technologically advanced civilizations in the universe based on certain factors - Enceladus, with an underground ocean, is a moon of Saturn - Disputed evidence for ancient microorganisms on Mars - Amino Acids are the building blocks of proteins, foundation of Earth-based life - Carbon is building block of life because of its versatility - Easy to form structures - Miller-Urey experiment made organic molecules in a primitive Earthlike atmosphere - Organic molecules have been found on primitive meteorites, molecular clouds in space, and comets - Space-based telescopes search for life in the infrared spectra of exoplanets looking for water vapor, carbon dioxide, and ozone - We use infrared satellites to find stars with Earthlike planets - We hope that an intelligent civilization does not not use a broad band of radiation to target more receivers - we want focused beams, frequencies near the water hole, and repeating signals - A planet around another star can probably be detected most easily by the transit method