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UA / Astronomy / ay 101 / Why do the constellations we see depend on latitude and time of year?

Why do the constellations we see depend on latitude and time of year?

Why do the constellations we see depend on latitude and time of year?


School: University of Alabama - Tuscaloosa
Department: Astronomy
Course: Intro To Astronomy
Professor: Jeremy bailin
Term: Spring 2015
Cost: 50
Description: Study Guide for the final 12/7/2016
Uploaded: 12/01/2016
16 Pages 164 Views 5 Unlocks


Why do the constellations we see depend on latitude and time of year?

Basic Astronomical Definitions 

∙ Astronomical Unit- Earth’s average distance from the sun (around 93 million miles or 150  million kilometers)

o usually use to describe distances within our solar system

∙ Light Year- the distance that light can travel in a year (around 10 trillion kilometers, or 6 trillion  miles)

o Generally used to describe distances of stars and galaxies

∙ Star – a large, glowing ball of gas that generates heat and light through nuclear fusion in its core ∙ Planet – a moderately large object that orbits a star and shines primarily by reflecting light from  its star. An object can be considered a planet ONLY if:

How does the orientation of earth’s axis change with time?

o It orbits a star

o It is large enough for its own gravity to make it round

o It has cleared most other objects from its orbital path

▪ An object that meets the first two criteria but has not cleared its orbital path  (i.e. Pluto) is designated a Dwarf Planet

∙ Moon – an object that orbits a planet (also commonly called a satellite)

∙ Asteroid – a relatively small, rocky object that orbits a star

∙ Comet – A relatively small and icey object that orbits a star

∙ Solar System – the sun and all of the material that orbits it

How does newton’s law of gravity extend kepler’s laws?

∙ Star System – a star (or more than one) and any planets and other materials that orbit it ∙ Galaxy – a great island of stars in space, containing millions to trillions of stars, all held together  by gravity and orbiting a common center If you want to learn more check out What are pieces of music?

∙ Cluster of Galaxies – a collection of galaxies bound together by gravity We also discuss several other topics like How do the types of statistical test differ from one another?

o Small collections are usually called “groups” where larger are called “clusters” ∙ Supercluster – a gigantic region of space in which many groups and clusters of galaxies are  packed more closely together than elsewhere in the universe

∙ Universe (or cosmos) – the sum total of all matter and energy (all galaxies and everything in  between them)

∙ Observable Universe – the portion of the entire universe that can be seen from earth, at least in  principle. It is only a tiny portion of the entire universe

Chapter 1  

∙ Earth is a planet orbiting the sun

∙ Our sun is one of over 100 billion stars within the Milky Way Galaxy

∙ Our galaxy is one of over 70 galaxies in our local group

∙ The local group is one small part of the local supercluster

∙ The observable universe contains around 100 billion galaxies

o The total number of stars is comparable to the number of grains of dry sand on all  beaches of planet Earth

∙ The universe began in the Big Bang and has been expanding ever sinceIf you want to learn more check out What does consumption culture imply especially back in 1999?

o Except in some localized regions where gravity has caused matter to collapse into  galaxies and stars We also discuss several other topics like What pertains to the cost added by producing one extra item of product?
If you want to learn more check out How does the ozone layer protect the planet?

o The Big Bang produced ONLY two chemical elements: hydrogen and helium ▪ The rest have been produced by stars and recycled within galaxies

∙ The Cosmic Calendar compresses the history of the universe down to a single year o Human civilization is just a few seconds old and a human lifetime only lasts a fraction of  a second

∙ Earth rotates on its axis once each day, and orbits the sun once a year

∙ At the same time, we move with our sun in random directions relative to other stars in our local  solar neighborhood, while the galaxy’s rotation carries us around the center of the galaxy every  230 million years

∙ Galaxies move, for the most part, at random with local groups

o All galaxies beyond the local group are moving away from us  

o More distant galaxies are moving faster, telling us we are living in an expanding  universe

Chapter 2 

∙ Stars and other celestial objects appear to lie on a great celestial sphere surrounding Earth ∙ We divide the celestial sphere into constellations with well-defined borders o From any location on Earth, we see half of the celestial sphere at any given time as the  dome of our local sky If you want to learn more check out What are the types of antifungal drugs?

o The zenith is the point directly overhead

o The meridian runs from due south to due north through the zenith

∙ Earth’s rotation makes stars appear to circle around Earth each day

∙ A star whose complete circle lies above our horizon is said to be circumpolar o Other stars have circles that cross the horizon, so they rise in the east and set in the  west each day

∙ Why do the constellations we see depend on latitude and time of year?

o The visible constellations vary with time of year because our night sky lies in different  directions in space as we orbit the sun

o Constellations vary with latitude because your latitude determines the orientation of  your horizon relative to the celestial sphere

▪ The sky does not vary with longitude

∙ Seasons are caused by Earth’s tilted axis

∙ How does the orientation of Earth’s axis change with time?

o Earth’s 26,000-year cycle of precession changes the orientation of its axis in space o The changing orientation of the axis does not affect the pattern of the seasons, however  it changes the identity of the North Star and shifts the locations of the solstices and  equinoxes in Earth’s orbit

∙ The phase of the moon depends on the position relative to the sun as it orbits earth o NOTE: know the names of phases in relation to appearance

∙ What causes eclipses?

o We see a lunar eclipse when Earth’s shadow falls on the moon

o We see a solar eclipse when the moon blocks our view of the sun

▪ We do not see an eclipse at every new and full moon because the moon’s orbit  is slightly inclined to the ecliptic plane

∙ Apparent Retrograde Motion – occurs when Earth passes by (or is passed by) another planet in  its orbit  

o Posed a major mystery to those who thought the Earth was the center of the universe  ∙ Greeks rejected the idea that the earth moves around the sun because of constellations, as well  as the sun appearing to be moving around earth (rising and setting)

Chapter 3 

∙ Ancient astronomers were accomplished observers who learned to tell the time of day and the  time of year, to track cycles of the moon, and to observe planets and stars

∙ The Greeks developed models of nature and emphasized the importance of agreement between  the predictions of those models and observations of nature

∙ The Greek geocentric model reached its culmination with the Ptolemaic model, which explained  apparent retrograde motion by having each planet move on a small circle whose center moves  around Earth on a larger circle

∙ Copernicus created a Sun-centered model of the solar system to replace the Ptolemaic model,  but it wasn’t any more accurate because Copernicus used perfect circles

∙ Tycho’s accurate, naked eye observations provided the data needed to improve on Copernicus’s  model  

∙ Kepler developed a model of planetary motion that fit Tycho’s data

∙ 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 a slower average speeds

▪ p2 = a3  

∙ Galileo’s experiments and telescopic observations overcame remaining objections to the  Copernican idea of Earth as a planet orbiting the sun

∙ Science generally exhibits three hallmarks:

1. Modern science seeks explanations for observed phenomena that rely solely on natural  causes

2. Science progresses through the creation and testing of models of nature that explain the  observations as simply as possible

3. A scientific model must make testable predictions about natural phenomena that would  force us to revise or abandon the model if the predictions did not agree with  


∙ A scientific theory is a simple yet powerful model that explains a wide variety of observations  using just a few general principles, and that has survived repeated and varied testing

Chapter 4 

∙ Speed is the rate at which an object is moving

∙ Velocity is a speed in a specific direction

∙ Acceleration is a change, meaning a change in either speed or direction

∙ Momentum = Mass x Velocity

∙ A force can change can change an object’s momentum, causing it to accelerate ∙ An object’s mass is the same no matter where it is located, but its weight varies with the  strength of gravity or other forces acting on an object

∙ An object becomes weightless when it is in free-fall, even though its mass is unchanged ∙ Newton showed that the same physical laws that operate on Earth also operate in space,  making it possible to learn about the universe by studying physical laws on Earth ∙ Newtons Three Laws of Motion

1. An object moves at constant velocity if there is no net force acting upon it 2. Force = mass x acceleration ( F = ma)

3. For any force, there is always an equal and opposite reaction force

∙ Conservation of Angular Momentum means that a planet’s rotation and orbit cannot change  unless it transfers angular momentum to another object (responsible for keeping planets  rotating and orbiting around the sun)

o The planets in our solar system do not exchange substantial angular momentum  with each other or anything else, so their orbit and rotation rates remain fairly  steady

∙ Energy is always conserved—it can neither be created nor destroyed

∙ Objects received whatever energy they now have from exchanges of energy with other objects ∙ Energy comes in three basic categories –

o Kinetic – energy of motions

▪ i.e. Falling rocks, orbiting planets, moving molecules

o Radiative – energy carried by light

▪ All light carries energy, which is why light can cause changes in matter

o Potential – stored energy that might later be converted into kinetic or radiative  energy

▪ i.e. a rock perched on a ledge has gravitational potential energy because it  will fall if it slips off the edge, and gasoline contains chemical potential  

energy than can be converted into the kinetic energy of a moving car

∙ According to the universal law of gravitation, every object attracts every other object with a  gravitational force that is directly proportional to the product of the objects’ masses and  declines with the square of the distance between their centers

∙ How does Newton’s law of gravity extend Kepler’s laws?

o Newton showed that Kepler’s first two laws apply to all orbiting objects, just not  planets

o He showed that elliptical bound orbits are not the only possible orbital shape— orbits can be unbound (taking the shape of a parabola or a hyperbola)

o Newton’s version of Kepler’s third law allows us to calculate the masses of orbiting  objects from their orbital periods and distances

∙ How do gravity and energy allow us to understand orbits?

o Gravity determines orbits, and an object cannot change its orbit unless it gains or  loses orbital energy, the sum of its kinetic and gravitational potential energy,  through energy exchange with other objects.

o If an object gains enough orbital energy, it may achieve escape velocity and leave  the gravitational influence of the object it was orbiting.

∙ How does gravity cause tides?

o The moon’s gravity creates a tidal force that stretches Earth along the Earth-Moon  line, causing Earth to bulge booth toward and away from the Moon

o Earth’s rotation carries us through the two bulges each day, giving us two daily high  tides and two daily low tides

Chapter 5 

∙ What is light?

o Light is an electromagnetic wave, but also comes in individual “pieces” called photons o Each photon has a precise wavelength, frequency, and energy

o In order of increasing wavelength, forms of light are:

▪ Gamma Rays

▪ X Rays

▪ Ultraviolet Rays

▪ Visible Light

▪ Infrared

▪ Microwaves

▪ Radio Waves

∙ NOTE: increasing FREQUENCY is the opposite order, and all have the  

same speed

∙ What is matter?

o Ordinary matter is made of atoms, which are made of protons, neutrons, and electrons o Atoms of different chemical elements have different numbers of protons

o Isotopes of a particular chemical element all have the same number of protons but  different number of neutrons

o Molecules are made from two or more atoms

∙ Matter can emit, absorb, transmit, or reflect (or scatter) light

∙ What are the three basic types of spectra?

o Continuous spectrum (which looks like a rainbow)

o Absorption line spectrum (which specific colors are missing from a rainbow) o Emission line spectrum (which we see lines of specific colors on a black background) ∙ Emission/Absorption lines occur only at specific wavelengths that correspond to particular  energy level transitions in atoms or molecules

∙ Every kind of atom, ion, and molecule produces a unique set of spectral lines, so we can  determine an object’s composition by identifying these lines

∙ Objects such as planets and stars produce thermal radiation spectra, the most common type of  continuous spectra

o We can determine temperature from these spectra because hotter objects emit more  total radiation per unit area and emit photons with a higher average energy

∙ How does light tell us the speed of a distant object?

o The Doppler Effect tells us how fast an object is moving toward/away from us

▪ Spectral lines are shifted to shorter wavelengths (blueshift) for objects moving  toward us

▪ Lines are shifted to longer wavelengths (redshift) for objects moving away

∙ How do telescopes help us learn about the universe?

o Light-collecting area describes how much light a telescope can collect

o Angular resolution determines the amount of detail in telescopic images

▪ It is better (smaller) for larger telescopes and, for a given telescope size, better  for shorter-wavelength light

o Multiple telescopes can sometimes be used together through interferometry to achieve  the angular resolution of a much larger telescope

∙ Why do we put telescopes in space?

o Telescopes in space are above Earth’s atmosphere and therefore not subject to  problems caused by light pollution, atmospheric distortion and emission of light, or the  fact that most forms of light do not penetrate through the atmosphere to the grounf

▪ However, for visible light, it is now possible to overcome some of the blurring  effects of Earth’s atmosphere through adaptive optics

Chapter 6 

∙ Four major features provide clues to how the solar system formed:

o The sun, planets, and large moons generally rotate and orbit in a very organized way o The planets clearly divide into two groups: Jovian and Terrestrial

o The solar system contains vast numbers of asteroids and comets

o There are some notable exceptions to these general patterns

∙ The nebular theory holds that the solar system formed from the gravitational collapse of a great  cloud of gas and dust, which itself was the product of recycling of gas through many generations  of stars within our galaxy

∙ What caused the orderly patterns of motion?

o As the solar nebula collapsed under gravity, natural processes caused it to heat up, spin  faster, and flatten out as it shrank. The orderly motions we observe today all came from  the orderly motion of this spinning disk

∙ Why are there two major types of planets?

o The inner regions of the solar nebula were relatively hot, so only metal and rock could  condense into tiny, solid grains; these grains accreted into larger planetesimals that  ultimately merged to make terrestrial planets

o Beyond the frost line, cooler temperatures also allowed more abundant hydrogen  compounds to condense into ice, building ice-rich planetesimals; some of these grew  large enough for their own gravity to draw in hydrogen and helium gas, forming the  Jovian planets

∙ Where did asteroids and comets come from?

o Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are  the ice-rich leftover planetesimals of the outer solar system

o They still occasionally collide with planets or moons, but majority of impacts occurred  during the heavy bombardment in the solar system’s first few hundred million years ∙ How do we know the age of our solar system?

o We can determine the age of a rock through radiometric dating, which is based on  knowing the half-life of various radioactive isotopes and carefully measuring the  proportions of these isotopes and their decay products within a rock

o The oldest rocks are meteorites, and their age tells us that the accretion began in the  solar nebula about 4.55 billion years ago

Chapter 7 

∙ Why is earth geologically active?

o Internal heat drives geological activity, and earth retains internal heat because of its  relatively large size for a terrestrial world

o This heat causes mantle convection and keeps earth’s lithosphere thin, ensuring active  surface geology

▪ Also keeps part of earth’s core melted, and circulation of this molten metal  creates earth’s magnetic field

∙ What processes shape earth’s surface?

o Impact cratering

o Volcanism

o Tectonics

o Erosion

▪ Most craters have been erased by other processes

▪ We owe the existence of our atmosphere and oceans to volcanic outgassing ▪ Plate tectonics shape much of earth’s surface

▪ Ice, water, and wind drive rampant erosion on our planet

∙ Crucial effects of the earth’s atmosphere to the planet:

o Protecting the surface from dangerous solar radiation

▪ UV is absorbed by ozone

▪ X rays are absorbed high in the atmosphere

o Greenhouse Effect

∙ Was there ever geological activity on the moon or on Mercury?

o Both the moon and mercury had limited volcanism and tectonics when they were  young. However, because of their small sizes, their interiors cooled too much a long  time ago for any ongoing geological activity

∙ What geological features tell us there was once water on Mars?

o Dry riverbeds

o Eroded craters

o Studies of martian minerals

▪ Any periods of rainfall seem to have ended at least 3 billion years ago

▪ Mars today still has water ice underground and in polar caps

∙ Is Venus geologically active?

o Venus almost certainly remains geologically active

o Surface evidence shows:

▪ Major volcanic/tectonic activity in the last billion years

o Venus differs from earth in that:

▪ Lack of erosion

▪ Lack of plate tectonics

∙ Why is Venus so hot?

o Result of its thick carbon dioxide atmosphere

▪ Creating a very strong greenhouse effect

▪ Thick atmosphere comes from its distance from the sun

∙ Too close to the sun to develop liquid oceans like those on earth, where  

most of the outgassed carbon dioxide dissolved in water and became  

locked away in carbonate rock

∙ Carbon dioxide remained in Venus’s atmosphere, creating a runaway  

greenhouse effect

∙ Unique features of earth that are important for life:

o Surface liquid water

o Atmospheric oxygen

o Plate tectonics

o Climate stability

▪ A result of the carbon dioxide cycle

∙ What makes a planet habitable?

o Relatively large size

▪ Maintains internal heat allowing volcanic outgassing

o Distance from the sun

Chapter 8 

∙ What are Jovian planets made out of?

o Jupiter and Saturn are made up almost entirely of hydrogen and helium

o Uranus and Neptune are made of mostly hydrogen compounds mixed with metal and  rock

o Lack solid surfaces but have very high internal pressures and densities

o Each Jovian has a core at least 10 times as massive as Earth

▪ Consisting of hydrogen compounds, rock and metals

∙ What is the weather like on Jovians?

o All have multiple cloud layers that give them specific colors, fast winds and large storms ▪ Some stores (i.e. Great Red Spot) can rage for centuries or longer

∙ Why are Jupiter’s Galilean moons geologically active?

o Io is the most volcanically active object in the solar system

▪ Due to a hot interior kept hot by tidal heating, which occurs because Io’s close  orbit is made elliptical by orbital resonance with other moons of Jupiter

o Europa may have a deep, liquid water ocean under its icy crust

▪ Due to tidal heating

o Callisto is the least geologically active

▪ Has no orbital resonance or tidal heating

▪ However, might still have a subsurface ocean

∙ Many medium-size and large moons show a surprisingly high level if past/present volcanism or  tectonics

∙ Jovian moons are more geologically active than small rocky planets because ices deform and  melt at much lower temperatures than rock, allowing icy volcanism and tectonics at surprisingly  low temperatures

∙ Saturn’s rings are made up of a ton of individual particles, each orbiting Saturn independently  like a tiny moon

o Lie in Saturn’s equatorial plane, and are extremely thin

∙ Why do Jovian planets have rings?

o Ring particles probably come from the dismantling of small moons formed in the disks of  gas that surrounded Jovians billions of years ago

o Small ring particles come from countless tiny impacts on the surfaces of these moons,  while larger ones come from impacts that shatter the moons

Chapter 9 

∙ What are asteroids like?

o Rocky leftovers from the era of planetary formation

o Most are small, and despite their enormous numbers their total mass is less than that of  any terrestrial planet

∙ Why is there an asteroid belt?

o The belt is all that remains of the swarm of planetesimals that once lay between Mars  and Jupiter

o Orbital resonances nudged orbits in this region, leading to collisions and gradually  ejecting material, so that the region lost most of its original mass

∙ How are meteorites related to asteroids?

o Most meteorites are pieces of asteroids

o Primitive meteorites are essentially unchanged since the birth of the solar system o Processed meteorites are fragments of larger asteroids that underwent differentiation ∙ Most comets orbit far from the sun

∙ If a comet approaches the sun, its nucleus heats up and its ice vaporizes into gas ∙ The escaping gases carry along some dust, forming a coma and two tails:

o A plasma tail of ionized gas

o Dust tail

▪ Larger particles can also escape, causing meteor showers on earth

∙ Comets come from two reservoirs:

o Kuiper Belt

▪ Comets still reside in the region beyond Neptune where they formed

o Oort Cloud

▪ Form between Jovians and were kicked out to a great distance by gravitational  encounters with those planets

∙ How big can a comet be?

o Kuiper Belt – grow up to hundreds/thousands of kilometers in size

∙ Did an impact kill the dinosaurs?

o It may not have been the sole cause, but a major impact clearly coincided with the mass  extinction around 65 million years ago

o Sediments from this era contain iridium and other evidence of an impact

Chapter 10 

∙ How do we detect planets around other stars?

o We can look for a planet’s gravitational effect on its star through the astrometric  method, which looks for small shifts in stellar position

o Also the Doppler Method, which looks for the Doppler shifts that reveal the back-and forth motion of stars

o For the small fraction of planetary systems with orbits aligned edge-on to Earth, we can  search for transits, in which a planet blocks a little of its star light as it passes in front of  it

∙ What properties of extrasolar planets can we measure?

o All detection methods allow for us to determine a planet’s orbital period and distance  from its star

o Astrometric and Doppler methods can provide masses (or minimum masses) o Transit can provide sizes

▪ Some cases when transit and Doppler are used together we can determine  average density

▪ Some cases we can use transits and eclipses to prove limited data about  

atmospheric composition and temperature

∙ How do extrasolar planets compare with planets in our solar system?

o Much wider range of properties than ones in our system

o Many orbit much closer to their stars with more eccentric orbital paths

o We have also observed properties indicating planetary types, such as water worlds, that  don’t fall neatly into our two planet categories

∙ Do we need to modify our theory of solar system formation?

o our basic theory seems sound, but we have had to modify it to allow for planetary  migration and a wider range of planet types than we find in our system

o unlikely to require major change

∙ Planetary systems like ours seems to be very common, Earth-like planets may also be common

Chapter 11 – Our Star 

∙ Why does the sun shine?

o Began to shine around 4.5 billion years ago when gravitational contraction made its core  hot enough to sustain nuclear fusion

o It has shined steadily ever since because of two types of balance:

▪ Gravitational equilibrium

∙ A balance between the outward push of pressure and the inward pull of  


▪ Energy balance

∙ Between the energy released by fusion in the core and the energy  

radiated into space from the sun’s surface

∙ Sun’s structure

o Interior layers from the inside out:

▪ Core

▪ Radiation zone

▪ Convection zone

▪ Photosphere

▪ Chromosphere

▪ Corona

∙ How does nuclear fusion occur in the sun?

o The core’s extreme temperature and density are just right for fusion of hydrogen and  helium

▪ Which occurs via the proton-proton chain

o Because the fusion rate is so sensitive to temperature, gravitational equilibrium and  energy balance act together as a thermostat to keep that rate steady

∙ How does the energy get fusion out of the sun?

o Energy moves through the core and radiation zone of the sun in the form of randomly  bouncing photons

o After energy emerges from the radiation zone, convection carries it the rest of the way  to the photosphere, where it is radiated into space as sunlight

o Energy produced in the core takes hundreds of thousands of years to reach the  photosphere

∙ To study the sun, we use theoretical models of the solar interior with the information we know  and then check models against observations of the sun’s size, surface temperature, and energy  output. We also use studies of solar vibrations and solar neutrinos

∙ Convection combined with the rotation pattern of the sun—faster at the equator than the  poles—causes solar activity because the gas motions stretch and twist the sun’s magnetic field o These contortions of the magnetic field are responsible for:

▪ Sunspots

▪ Solar flares

▪ Solar prominences

▪ Coronal mass ejections

▪ Heating the gas in the chromosphere and corona

∙ The sunspot cycle, or variation in the number of sunspots on the sun’s surface, has an average  period of 11 years

o The magnetic field flip-flops every 11 years or so, resulting in a 22-year magnetic cycle Chapter 12 – Stars 

∙ How do we measure stellar luminosities?

o The apparent brightness of a star in our sky depends on both its luminosity, the total  amount of light it emits into space, and its distance from earth, as expressed by the  inverse square law for light

▪ We can therefore calculate luminosity from apparent brightness and distance;  we can measure the latter through stellar parallax

∙ How do we measure stellar temperatures?

o Color spectrums

▪ Classify stars according to sequence of spectral types: OBAFGKM

∙ Runs from hottest to coolest

∙ Cool, red stars are spectral type M (more common than O)

∙ How do we calculate stellar masses?

o We can calculate the masses of stars in binary star systems using Newton’s version of  Kepler’s third law if we can measure the orbital period and separation of the two stars ∙ Hertzsprung-Russell Diagram (H-R)

o Plots stars according to their spectral types and luminosities

∙ What is significant of the main sequence?

o Stars on the main sequence are all fusing hydrogen into helium in their cores o A star’s position along the main sequence depends on its mass

▪ High-mass are at the upper left and progressively get smaller moving toward the  upper right

▪ Lifetimes vary in the opposite way because high mass stars have shorter  


∙ Giants and supergiants are stars that have exhausted their central core of supplies of hydrogen  for fusion and are undergoing other forms of fusion at a prodigious rate as they near the ends of  their lives

∙ White dwarfs are the exposed cores of stars that have already died, meaning they have no  further means of generating energy through fusion

∙ What are the two types of star clusters?

o Open clusters

▪ Contain up to several thousands of stars and are found in the disk of the galaxy o Globular clusters

▪ Contain hundreds of thousands of stars all closely packed together, found  

mainly at the halo of a galaxy

Chapter 13 – Star Stuff 

∙ How do stars form?

o Born in cold, relatively dense molecular clouds

▪ As a cloud fragment collapses under gravity, it becomes a rapidly rotating  

protostar surrounded by a spinning disk of gas in which planets may form

▪ The protostar may also fire jets of matter outward along its poles

∙ How massive are newborn stars?

o On the upper end, the most massive newborn stars are around 150 mass of sun o On the lower end, stars cannot be less massive than about .08 mass of sun

▪ Below this mass, degeneracy pressure prevents gravity from making the core  hot enough for efficient hydrogen fusion, and the object becomes a brown  


∙ Stages of a low mass star:

o Spends most of its life generating energy by fusing hydrogen in its core

o After exhaustion, the core begins to shrink while the star as a whole expands to become  a red giant, with hydrogen shell fusion occurring around an inert helium core

o When the core becomes hot enough, a helium flash initiates helium fusion in the core,  which fuses helium into carbon

o The star shrinks somewhat in size and luminosity

o The core shrinks again when helium core fusion ceases, while both helium and hydrogen  fusion occur in shells around the inert carbon core and cause the outer layers to expand  once more

∙ How does a low mass star die?

o A low-mass star like the sun never gets hot enough to fuse carbon in its core, because  degeneracy pressure stops the gravitational collapse of the core

o The star expels its outer layers into space as a planetary nebula, leaving its exposed core  behind as a white dwarf

∙ Stages of a high mass star:

o Lives a much shorter life than a low mass

o Fusing hydrogen into helium via the CNO cycle

o After exhaustion, it begins hydrogen shell fusion and then goes through a series of  stages fusing successively heavier elements

o The furious rate of this fusion makes the star swell in size to become a supergiant ∙ How does a high mass star die?

o They die in a cataclysmic explosion called a supernova, scattering newly produced  elements into space and leaving behind a neutron star or black hole

Chapter 14 – Stars 

∙ A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star  supernova

o Pulsars provided the first direct evidence for their existence

∙ A black hole is a place where gravity has crushed matter into oblivion, creating a hole in the  universe from which nothing can escape, not even light

∙ Event horizon marks the boundary between the observable universe and the inside of the black  hole

Chapter 15 – Galaxies and Beyond 

∙ What does our galaxy look like?

o The Milky Way Galaxy is a spiral galaxy consisting of a thin disk about 100,000 light years  in diameter with a central bulge and a spherical halo surrounding the disk

o The disk contains an interstellar medium of gas and dust, while the halo contains only a  small amount of hot gas and virtually no cold gas

∙ How do stars orbit in our galaxy?

o Stars in the disk all orbit the galactic center in about the same plane and in the same  direction

o Halo stars also orbit the center of the galaxy, but their orbits are randomly inclined to  the disk of the galaxy

o Some bulge stars orbit like halo stars, while others orbit more like disk stars o Orbital motions of stars allow us to determine the distribution of mass in our galaxy ∙ How is gas recycled In our galaxy?

o Stars form from the gravitational collapse of gas clumps in molecular clouds o Massive stars explode as supernovae when they die, creating hot bubbles in the  interstellar medium that contain the new elements made by the stars

o This gas cools and mixes with the interstellar medium, forming what we call atomic  hydrogen gas

o The gas can then cool further to make molecular clouds in which new stars form ∙ Where do stars tend to form in our galaxy?

o Mostly in spiral arms

∙ What is the evidence for a black hole at our galaxy’s center?

o Orbits of stars near the center of our galaxy indicate that it contains a black hole about 4  million times as massive as the sun

o The black hole appears to be powering a bright source of radio emission known as Sgr  A*

Chapter 16 – A Universe of Galaxies 

∙ Three major types of galaxies:

o Spiral galaxies  

▪ Prominent disks and spiral arms

o Elliptical galaxies

▪ Rounder and redder than spirals and contain less cool gas and dust

o Irregular galaxies

▪ Neither disklike or rounded in appearance

∙ How do we measure the distances to galaxies?

o Radar ranging

o Standard candles

o Cepheids

o Period-luminosity relation

o White dwarf supernovae

∙ Hubble’s Law tells us that more distant galaxies are moving away faster

∙ Hubble’s constant allows us to determine a galaxy’s distance from the speed at which it is  moving away from us. Which we can measure from the redshift of its spectrum ∙ Why do galaxies differ?

o Probably stem from conditions in their protogalactic cloud systems and from collisions  with other galaxies

∙ What is the evidence for supermassive black holes at the centers of galaxies? o Active galactic nuclei

▪ Most luminous are called quasars

o Rapidly orbiting stars and gas clouds

Chapter 17 

∙ What were conditions like in the early universe?

o Filled with radiation and elementary particles

o It was so hot and dense that the energy of radiation could turn into particles of matter  and antimatter, which then collided and turned back into radiation

∙ Eras of the universe:

o Planck era

▪ Four fundamental forces may have behaved as one

o GUT era

▪ Gravity became extinct

o Electroweak era

▪ Electromagnetism and the weak force became distinct

o Particle era

▪ Matter particles annihilated all antimatter particles by the end of this era

o Era of Nucleosynthesis

▪ Fusions of protons and neutrons into helium ceased

o Era of nuclei

▪ Hydrogen nuclei captured all the free electrons, forming hydrogen atoms at the  end of it

o Era of atoms

▪ Galaxies began to form

o Era of galaxies

▪ Continues to present day

∙ Key features of the universe explained by inflation:

o The density enhancements that led to galaxy formation

o The smoothness of the cosmic microwave background

o The flat geometry of the observable universe

Chapter 18 

∙ Dark matter – unseen mass whose gravity governs the observed motions of stars and gas clouds ∙ Dark energy – form of energy thought to be causing the expansion of the universe to accelerate ∙ Role of dark matter in galaxy formation

o Because most of a galaxy’s mass is in the form of dark matter, the gravity due to that  dark matter is probably what formed protogalactic clouds and then galaxies from slight  density enhancements in the early universe

∙ What are the largest structures in a universe?

o Voids

▪ Large-scale structures whose origins trace directly back to the regions of slightly  enhanced density in early time

∙ What is the fate of the universe?

o If dark energy is what is driving the accelerated expansion, we expect the expansion to  continue accelerating, as long as the effects of dark energy do not change with time and  there are no other factors that affect the fate of the universe

Chapter 19

∙ When did life arise on earth?

o At least 3.85 billion years ago

∙ How did life arise on earth?

o Evolved from one common ancestor, which may have resembled microbes that live  today in hot water near undersea volcanic vents

o Diversified and evolved through natural selection

∙ Necessities of life:

o Source of nutrients

o Source of energy

o Liquid (aka water)

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