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title: Astronomy Final Exam Study Guide

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# 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
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### 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