PlanetaryScience

160. Describe the factors which cause seasons. Describe how sunrise and sunset locations change throughout the year. Describe how the noon sun angle changes throughout the year.
 * Seasons are caused by Earth’s revolution around the Sun, and the tilt of the Earth. When the northern hemisphere is tilted away from the Sun, it is winter in the northern hemisphere and summer in the southern. When the northern hemisphere is tilted toward the Sun, it is summer in the northern hemisphere and winter in the southern. **
 * The further north or south you are from the equator, the greater the difference in sunrise and sunset locations during the year. In the northern hemisphere, the sun will rise and set further north of the celestial equator in the summer and further south of the celestial equator in the winter. This is reversed in the southern hemisphere. **
 * The noon sun angle is calculated as 90 degrees – zenith angle. **
 * The zenith angle is the latitude where you are at + or – the latitude at which the Sun is overhead at noon. **
 * If the Sun’s latitude and your latitude are in the same hemisphere, subtract. **
 * If the Sun’s latitude and your latitude are in different hemispheres, add. **
 * So during the summer, the NSA is a larger/higher angle (more direct light, Sun is more overhead). In the winter, the NSA is a smaller/lower angle (more indirect light, Sun is lower in the sky). **
 * IB **
 * Looks good! I would like to add to “factors which cause seasons” from Mr. Gallagher’s first lecture and __The Universe__, page 13: **
 * **The sun is in the sky longer each day in the summer**
 * Together with the other factors (rises and sets farther north/reaches higher point in sky) this results in each portion of the earth’s surface receiving more energy in a given amount of time in summer. Also, sunlight passes through more atmosphere in winter than it does in summer resulting in more scattering and absorption in the atmosphere. **
 * -PM **
 * Good addition PM. All of the info looks accurate. I looked up the info in the Met I book (pg. 37-38 of The Atmosphere) and saw that the most important reasons for differences in solar energy reaching a particular location are the seasonal changes in the angle at which the sun's rays strike the Earth (more or less concentrated "beam" of light) and changes in the length of daylight. Both are obviously related to the tilt of Earth's axis and the location/angle of the sun overhead. I hope this topic is covered on the exam because we got it covered! : ) **
 * JA**

161. How did the moon form? Why does its near side and far side look different? Describe what causes the phases of the moon. Describe the rotation and revolution periods of the moon.
 * The large impact theory is the current theory that explains the formation of the Moon. According to this theory, a young Earth was hit by an object that may have been as large as Mars. Much material was thrown off of Earth as a result of the impact, began to orbit Earth, and eventually coalesced into our Moon. (The Universe, pg 172) **
 * The far side of the Moon has a thicker crust, more craters, and fewer maria (maria are thought to have formed due to volcanism where lava was forced to the surface; more likely with thinner crust). The near side of the Moon has thinner crust, more maria where lava filled in craters, creating a smoother surface. **
 * The Moon orbits Earth and, depending on where the Moon is in its orbit, the perspective from Earth determines how much of the lit portion of the Moon we see. The Moon orbits in a counterclockwise (when viewed from above the North Pole) direction around Earth. When the Moon is between the Earth and the Sun, it is a new moon; the unlit side of the Moon faces Earth. As it orbits we see the phases go from waxing crescent, first quarter, waxing gibbous, full (Earth is between the Moon and the Sun), waning gibbous, last (third) quarter, waning crescent, back to new. In the northern hemisphere, the lit side moves from right to left. In the southern hemisphere, it moves from left to right. **
 * The Moon orbits Earth once every 29.5 days (synodic period – or how long it takes the Moon to be in the same alignment with Earth) or every 27.3 days (sidereal period – the time between successive stages). In one complete orbit, the Moon rotates one time. Thus we always see the same side of the Moon. **
 * IB **
 * Looks great! **
 * -PM **
 * Good DS **


 * I would like to add:**
 * Nearside of moon less cratered. Earth may act as a gravitational shield for nearside. (from lesson 2 notes Dr. Gallagher)**


 * Maria have greater density than surrounding highland areas. Tidal forces from earth have caused this denser side of the moon to face the earth. (The Universe, p. 169)**
 * MarkW**

162. Describe what causes lunar and solar eclipses.


 * A lunar eclipse is caused when the Earth is aligned in such a way as to cast a shadow/block sunlight from reflecting off the Moon. A solar eclipse is caused when the Moon is aligned between the Sun and Earth in such a way that it casts a shadow on Earth. Lunar eclipses are more common and are seen from most of Earth. Solar eclipses are more infrequent and seen by fewer people as the path of the solar eclipse is relatively small. **
 * IB**
 * Looks good. I guess we'll have to ramble on to make this more detailed if it shows up on the final. Maybe mentioning that eclipses do not occur every new/full moon due to the tilt of the moon's orbit around the Earth. Also, we could add the umbra (dark shadow) and penumbra (partial shadow), which lead to total lunar eclipses, penumbral lunar eclipses, and partial lunar eclipses (a mixture of the 2). There are also partial (in the penumbra) and total solar eclipses (umbra). Pg. 21-22 of The Universe**
 * JA**

163. Discuss the solar system views of Ptolemy and Copernicus. How did Kepler’s Laws help clarify the heliocentric model? What evidence did Galileo provide for the heliocentric model?


 * Ptolemy presented the geocentric model, with Earth at the center of the universe. He explained the retrograde (backward) motion of the planets with the idea of epicycles (wherein each planet orbits in a small circle the center of which moves around the circumference of a larger circle). His model lasted for over 1400 years after his death.**


 * Copernicus developed the heliocentric model, placing the Sun at the center of the solar system. He noted that Mars’ changes in brightness could not be completely accounted for by Ptolemy’s model. In Copernicus’ system, Earth became just another planet orbiting the Sun. Retrograde motion of planets is explained by the perspective from Earth looking at the other planets. Inner planets complete their orbits faster than outer planets, meaning planets “pass” each other up as they orbit. This makes it appear as if a planet being observed actually reverses motion.**


 * Kepler’s Laws explain planetary motion. His laws modified Copernicus’ model by stating that the planets’ orbits are elliptical and not perfect circles, and he backed up the heliocentric model with data.**
 * His first law states that “each planet’s path around the Sun is an ellipse, with the Sun at one focus of the ellipse”. Most planets in our solar system move in nearly circular paths.**
 * His second law states that “a planet moves along its elliptical path with a speed that changes in such a way that a line from the planet to the sun sweeps out equal areas in equal intervals of time”. This means that when a planet is closer to the Sun, it moves faster than when it is further.**
 * His third law states that “the ratio of the curve of the semimajor axis of a planet’s orbit to the square of its orbital period around the Sun is the same for each planet”. In other words, the distance (the semimajor axis, or half the length of the major axis of the orbit) is very nearly the same as the planet’s average distance from the Sun.** **(The Universe, pages 58-62)**


 * Galileo was the first to use an instrument in his observations when he used a telescope. With it he observed four of Jupiter’s moons, the variations in the surface of our Moon, sunspots, and Venus’ phases. In Ptolemy’s model, we would never see more than the crescent phase of Venus. However Galileo was able to see Venus in the gibbous phase, which went against Ptolemy’s model.**
 * IB**
 * Looks good. Copernicus revived many ideas of Aristarchus, who lived 400 years before Ptolemy and introduced a moving Earth solution which was the first heliocentric model.**
 * -PM**
 * Good.** **DS**

164. Describe the characteristics of terrestrial planets. Describe the characteristics of Jovian planets.

**A terrestrial planet is a term used to describe the four planets in the solar system that are closest to the Sun, Mercury, Venus, Earth, and Mars. These four planets are composed primarily of rock and have solid surfaces. The word terrestrial is derived from the Latin word//terra//, meaning ground or soil.** **Besides a rocky, solid composition, the terrestrial planets share certain similar characteristics :** **The four other major planets, Jupiter, Saturn, Neptune, and Uranus, in the solar system are referred to as gas planets or Jovian planets , after the planet Jupiter which is the largest planet in the solar system. The gas planets are much larger than the terrestrial planets and are composed mainly of gas and liquid.** **In contrast to the terrestrial planets, some characteristics of the gas planets are:**
 * **They are close to the Sun.**
 * **They either have no moons or few moons.**
 * **They all have weak magnetic fields.**
 * **They all have closely spaced orbits.**
 * **None of the planets have rings around them.**
 * **They are all farther away from the Sun.**
 * **They have multiple moons.**
 * **They all possess strong magnetic fields.**
 * **There is a greater distances between orbits.**
 * **They all have some form of rings orbiting each planet.**
 * MCA**
 * Good. Would add:**
 * Jovian planets have faster rotation speeds which makes them more oblate. DS**

165. Describe the formation of the solar system and how it created the differences between terrestrial and Jovian planets.
 * 1) **Cloud of gas and dust, nebula, collapses due to gravity. Any slight rotation that it had at the beginning, before the collapse, results in greatly increased speed of the central portion (conservation of momentum). Material in center becomes a star.**
 * 2) **As gases begin to cool, nonvolatile elements begin to condense first into small chunks of matter. Planets begin accreting through collisions of this matter.**
 * 3) **T Tauri Stage- most H, He and other volatiles are blown away from inner solar system by solar wind. Increased temps also result in faster moving gas particles. This allows them to overcome the gravitational pull of planets. As a result, it is harder for inner planets to increase in size resulting in smaller inner planets. These inner planets are left with rocky composition and little volatiles.**
 * 4) **Outer planets have more H and He available due to solar sweeping, allowing for larger sizes, lower densities and massive H/He atmospheres. (The Universe p. 193-199) MarkW**

166. Describe the atmospheres of Venus, Earth and Mars. How are they similar? How are they different? What has happened to make them different?


 * __Venus Atmosphere __**
 * **completely cloud covered **
 * **runaway greenhouse effect (900F) **
 * **atmospheric pressure 90X that of earth’s **
 * **96% CO2, 3% N, trace amounts H2O vapor, sulfur dioxide due to abundant volcanism **
 * **lack of liquid water due to proximity to sun. Plus, H2O vapor was broken into constituent parts, H and O, from U.V. radiation from sun. This allowed H to escape atmosphere. **
 * **high wind speed aloft, 360 km/hr, but zero wind at surface. Due to slow rotation causing slow movement of sun across planet’s surface **


 * __Earth Atmosphere __**
 * **some cloud cover **
 * **mild greenhouse effect **
 * **thicker atmosphere allowed for temperature moderation between day/night**
 * **“normal” atmospheric pressure **
 * **80% N, 20% O, trace amounts H2O vapor, CO2 and ozone **
 * **variable winds at surface **
 * **because earth has oceans, most CO2 is locked in seawater and carbonate rocks (limestone) in oceans. This prevented runaway greenhouse effect **


 * __Mars Atmosphere __**
 * **very little atmospheric pressure**
 * **surface winds evident from dust storms and eolian (wind-shaped) features**
 * **<span style="font-family: "Arial","sans-serif"; font-size: 13.33px;">extreme temperature differences between day/night due to thin atmosphere **
 * **<span style="font-family: "Arial","sans-serif"; font-size: 13.33px;">95% CO2 but too little atmosphere to be able to moderate temps **
 * **<span style="font-family: "Arial","sans-serif"; font-size: 13.33px;">smaller planet means lower escape velocity. Most H2O vapor escaped atmosphere **
 * **<span style="font-family: "Arial","sans-serif"; font-size: 13.33px;">plate tectonics eventually ceased, this stopped the recycling of CO2 through volcanism. A permanent depletion of CO2 from atmosphere and destruction of greenhouse effect resulted. This made Mars get colder. **
 * **<span style="font-family: "Arial","sans-serif"; font-size: 13.33px;">little ozone in atmosphere allowed U.V. radiation to break up H2O vapor into constituent parts, H and O. Hydrogen escaped planet and oxygen reacted with iron rich minerals to form red surface color. (The Universe, p. 219-238) MarkW **

177. Describe the surface features of Mars and what they tell us about Mars’s past.
 * The polar caps of Mars consist of two parts; a water ice base that is covered during the winter by frozen carbon dioxide. In summer, the CO2 evaporates, leaving behind the ice. What appear to be dry riverbeds on Mars lead scientists to believe that water once flowed on the surface. These Martian outflow channels (extremely long, wide swathes of scoured ground on Mars) commonly contain the streamlined remnants of pre-existing topography and other linear erosive features indicating sculpting by fluids moving downslope. In addition, images that show that new deposits and gully sites appear and change on Mars’ surface over a period of a few years provide evidence that liquid water from an underground source //may// flow on the surface of Mars for short periods of time. **
 * MLS **
 * I would like to add from Mr. Gallagher’s lecture these features: **
 * **Southern Highlands- densely cratered southern hemisphere**
 * **Northern Plains- separated from highlands by a 2-3 km cliff with prominent lava flows, cinder cones and ridges**
 * **Huge shield volcanoes- Olympus Mons (largest volcano in solar system)**
 * **Global Escarpment- fault scarp which contains Vales Marinaras (Grand Canyon could fit in a side channel)**
 * **Evidence of mass wasting- scarps, cliffs, dunes**
 * **Wind shaped features- linear grooves, yardangs, loess, dune fields**
 * Mars’ past includes volcanic activity, which was thought to have ceased a long time ago, but recent evidence shows it may be more current. **
 * -PM **
 * Good DS **

178. Describe the characteristics of Europa and Io which make them so interesting to scientists and the general public. Where do they get their energy? Would they be good candidates for the search for life?
 * Europa is an icy moon of Jupiter. It is interesting to scientists because it possibly has a subsurface ocean. Scientists believe that Europa has a thick, icy crust because it has few craters and has a high albedo. The moon is pale-yellow in color and has regions of reddish-brown. The pictures of Europa taken by the voyager spacecraft in 1979 found that long cracks exist that extend thousands of kilometers over the moon’s surface. These cracks are no more than a few kilometers high, which makes Europa one of the smoothest objects in our solar system. It appears that these cracks are areas in the ice where icy waters from beneath seeped up and froze. There are also large circular features which could be the result of impacts on the moon or from the upwelling of material from below the surface. If these features are from impacts and since there are relatively few of them (compared to our moon, for example), it shows that the surface is active because recent events have evened out the surface and removed past impacts.**


 * The evidence for Europa’s active surface also includes pictures that indicate that there are blocks of crust that appear to flow like icebergs in an ocean that cannot be seen. In other locations the blocks are tilted, and twisted out of their original location. There are also bands of ice and rock that spread outward from a central ridge. The cause of this activity is gravity. As Europa revolves around Jupiter, Jupiter and Jupiter’s other moons pull on Europa. This process is called tidal flexing. Within a single day, the moon stretches and bends up to several tens of meters. Since the outer icy crust of Europa is rigid, this produces the cracks and ridges. It his thought that the expansion and contraction of the moon may produce enough heat to melt the crust under the surface forming an ocean. This opens the possibility of the existence of life. This makes Europa one of the key interests for scientists looking for life in places other than our planet.**


 * Io is the closest moon to Jupiter that his best known for its volcanic activity. Io has about 100 times more volcanic activity than there is on Earth, and is the most geologically active object in the solar system. In addition, Io has a thin atmosphere that is believed to be coupled to Jupiter’s atmosphere. Io has a yellow-orange color that is a result of sulfur covering its surface. Io has very few impact craters because the surface is constantly being changed by the volcanic activity. It is thought that the volcanic surface of Io may be like that of Earth’s in its early evolution. Like Europa, Io’s internal heat is produced by tidal flexing as a result of Jupiter’s intense gravity. Io does not generate its own magnetic field since it does not have convection inside its iron core. It does have electrical currents that connect the moon to Jupiter that produce auroral events.**
 * (Koupelis, page 254 - 255)**
 * JI**

179. Describe the atmospheres of Jupiter, Saturn, Uranus and Neptune. In what ways are they similar? 180. What are some of their differences? What might explain these differences? Describe some features visible on these planets.
 * All Jovian planets have a thick atmosphere. The majority of the atmosphere of all these planets is hydrogen and helium. Jupiter and Saturn have no other gases aside from hydrogen and helium that make up over 1% of the total. Uranus and Neptune also have about 2% methane which absorbs red light to give these planets their signature blue color. KMS**

181. What is the current status of Pluto as a planet? What characteristics led to this status? Describe the characteristics of asteroids, comets, Kuiper Belt Objects and the Oort Cloud.
 * **__Pluto__ -(from Wikipedia) According to a resolution made in 2006 by the International Astronomical Union, there are three main conditions for an object to be considered a 'planet':**
 * 1) **The object must be in orbit around the Sun.**
 * 2) **The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium**
 * 3) **It must have cleared the neighborhood around its orbit. Pluto fails to meet the third condition, since its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit). The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.**

====**__asteroids__ - metallic, rocky bodies without atmospheres that orbit the Sun but are too small to be classified as planets. Most asteroids are of the small irregular variety. None of them are large enough to have high enough gravity to pull themselves into spherical shape. Images of the same asteroid vary in brightness which confirms the irregular shape. **====


 * **__comets__ - small icy solar system bodies composed of loose collections of ice, dust, and small rocky particles. Comets are believed to have two sources. Long-period comets (those which take more than 200 years to complete an orbit around the Sun) originate from the Oort Cloud. Short-period comets (those which take less than 200 years to complete an orbit around the Sun) originate from the Kuiper Belt.**
 * **__Kuiper Belt Objects__- The Kuiper Belt is a disk-shaped region past the orbit of Neptune roughly 30 to 100 AU from the Sun. The Belt contains many icy bodies which can become comets. Occasionally the orbit of a Kuiper Belt object will be disturbed by gravitational interactions with the giant planets in such a way as to cause the object to take up an orbit that crosses into the inner solar system.**
 * **__Oort Cloud__ - Danish astronomer Jan Oort proposed that comets reside in a huge cloud at the outer reaches of the solar system, far beyond the orbit of Pluto. This has come to be known as the Oort Cloud. Statistics imply that it may contain as many as a trillion comets and may account for a significant fraction of the mass of the solar system. However, since the individual comets are so small and so far away, we have no direct evidence about the actual existence of the Oort Cloud.**
 * MLS **


 * Just wanted to add that the asteroid belt is located between the orbits of Mars and Jupiter and the Oort Cloud is also considered to be a spherical shell that surrounds the solar system.**
 * Good! KMS**
 * Looks good. Just want to add that Oort was actually Dutch, not Danish.**
 * IB**
 * Would add:**
 * Asteroids: origin - left over material that didn't form into a planet**
 * Comets: three parts - head, tail and coma**
 * DS**

182. Describe the process by which stars are formed and how they shine. Describe the life cycles of low-mass stars, medium mass stars and high mass stars. Describe the role supernovae play in “forging the elements” and “sowing the seeds” of life in the universe.


 * Stars form in cold places called giant molecular clouds. Slow moving particles come together due to gravity. The core begins to collapse, gravity increases, the speed of the molecules increase, and temperature increases. An increase of pressure at the core due to the increase in temperature causes the collapse to stop. Once collapse stops this is a protostar. Gravity is the source of energy of a protostar. Once the protstar becomes hot enough nuclear fusion begins and it is a main sequence star.**


 * Low mass star cycles: if the protostar has less than 8% of the mass of our sun it will still generate energy/heat due to gravity and glow a dull red color, but it does not have enough mass to begin nuclear fusion. It becomes a brown drawf.**


 * If the protostar is less than .4 solar masses of our sun the proto star does begin fusion and becomes a main sequence. Convection is occurring throughout the star and the entire star runs low on Hydrogen at the same time. The rate of fusion decreases causing the core to contract increasing temperature. It becomes a white dwarf.**


 * Medium Mass Star Cycle:**
 * Begin as a protostar as stated above, begins nuclear fusion and becomes a main sequence star. Once the hydrogen is depleted the fusion process breaks down. The core pressure increases caused by electron degeneracy pressure. The star begins to expand. Hydrogen is still fusing in the outer layers of the sun. It is becoming a red giant. Helium is fusing in the core, very high temperatures, due to increase in mass, forming carbon and oxygen. This causes a helium flash, up to 100,000 times brighter than our sun. it is releasing energy as the solar wind this is called the planetary nebula stage. The star is ejecting it material into space. Once most of the mass has been ejected it is a planetary nebula. The core is left behind and it is a white dwarf. Once it cools further it becomes a black dwarf.**


 * Large Mass Star Cycle**
 * Proto star with more than .4 solar masses. It does begin nuclear fusion but runs out of hydrogen very fast. It becomes a red giant and fallows the above sequence. If it becomes a super giant it can evolve into a remnant nebula or a black hole.**

**MET** **I found this website with an article written by a 15 year old student. I captured a part of it by SI- process(slow internet capture) and am adding it to an already good answer. PB** Supernovae are considered to have been “sowing the seeds” of life because they have played a part in the formation and distribution of virtually every element that exists in the universe. Only hydrogen, some helium, and some lithium, formed 13.7 billion years ago in the Big Bang, were created independently of supernovae. Other than hydrogen, some helium, and some lithium (which were formed in the Big Bang) every element in the universe was produced by a star at the end of its life and expelled into space by a supernova explosion. Supernovae are believed to be responsible for the formation of half of all the elements heavier than iron, and also the formation of virtually every element in the mass range between helium and iron. Through all of this nucleosynthesis, supernovae are what produce the elements that not only are fundamental for all known life forms, such as carbon, oxygen, and nitrogen, but also the elements that form the planets on which life evolves and develops. Supernovae are responsible for the dispersion of heavy elements, produced by both the R and S-processes (rapid neutron capture – slow neutron capture), all around the universe. Without the formation of an iron and nickel core at the very end of a massive star's life, the core of our own planet could not possibly have formed. Without this core, our planet would never have been able to form from the dust cloud that existed in the early years of our solar system. In fact, without the nucleosynthesis inside supernovae, explosions and the conditions that existed just before, as well as the dispersion of the heavy elements produced by red giants, none of the planets, asteroids, comets, and heavy mass objects that we observe in the universe would exist. The universe would be completely lifeless, containing just stars and giant clouds of gas. Then, in short, as the old saying goes: "//We are all made of stardust"//. Maybee B. The role of supernovae in the origins of life. Young Scientists J [serial online] 2011 [cited 2012 May 6];4:56-60. Available from: [] 183. Describe the creation of the universe according to Big Bang Theory. Describe the historical aspects of the development of this theory. Describe the evidence for this theory.
 * Supernovae**
 * A supernovae is the expulsion of star matter into space during the planetary nebula stage of a red giant or super giant. The elements being ejected are the building blocks of life, as we know it here on Earth. Our star, the Sun, formed as a medium mass star. Our planet Earth formed from the remnants of the Sun’s formation. We are made from stardust. The universe is “recycling”.**


 * At one time the matter in the Universe was much closer together. At one point in time the density of matter was high and an “explosion” started the expansion of the Universe. After the initial explosion temperature decreased allowing nuclear particles to form into atoms of low mass like hydrogen. These atoms came together due to gravity to begin the formation of stars, clusters of stars, galaxies. At the same time the Universe is expanding, galaxies are moving apart.**


 * Evidence:**
 * CBR- comsmic background radiation; predicted in 1948 and 1960s. Discovered by Penzias and Wilson in the 1970s by accident using Bell Laboratories horn shaped radio antenna.**


 * Red shift of the galaxies; Hubble’s law and red shift show galaxies are moving away from each other.**


 * Time dilation of distant supernova light curves. A supernova that takes 20 days to decay will appear to take 20 +1(redshift z) days to decay.**


 * The observed proportions of light elements such as deuterium, helium, and lithium. These elements are not formed in stars, but they do form during very hot nuclear fusion that occurred during the first few minutes of a hot young Universe.**
 * MET**

184. How will the universe end? What evidence causes scientists to currently expect this ending?
 * The current theory of the end of the universe is an ever expanding one. Our universe is currently expanding (open universe) and we know this thanks to Hubble who noticed the red shift of galaxies. The evidence is that current estimates show the amount of known matter in the universe is below the critical density, which points to an open universe. Also, research indicates the universe is expanding faster now than in the past.**
 * KMS**
 * I believe our universe is flat, and that flatness is a consequence of its early inflationary period (Inflationary Universe Theory). Although the critical density contribution of matter (0.3) is below 1, if you add in the contribution of dark energy (0.7), the critical density of the universe is equal to 1, which means a flat universe. Because the universe is dominated by dark energy, which acts like the cosmological constant, it is accelerating its expansion.**
 * Anyone have a better understanding? I find this all very difficult to grasp and may be wrong.**
 * DS**
 * Now I'm acutally a bit confused. Looking back at our text is says our universe has flat geometry, but will continue expanding. Clarification? KMS**
 * I actually got a question on a quiz wrong when I said the universe is flat. The correct answer should be open but nearly flat (not just open either). In studying for my Advanced Planetary Science final, I summarized that flat would eventually slow but an open actually continues forever and accelerates. See the graph on pg. 534 of the In Quest of the Universe book and look at the red line labeled flat, open. I think it is the dark energy that makes up the .7 which none of the other models have. This force causes it to keep expanding at increasing speed, thus making it open. I hope I didn't confuse you more but you both are right in a way. JA **
 * I agree. I'd like to add that in the 1990s, scientists measuring recessional velocities of Type Ia supernovae expected to see evidence that they were slowing down but instead discovered they were speeding up. This was the discovery of the "dark energy", something that had to overcome gravitational forces to cause the universe to expand at an accelerating rate, which will result in the universe ending in a cold, dark death. **
 * IB**

.185. What are the other possibilities? What would determine which possibility actually occurs?
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">All the possibilities (assuming gravity is main factor): **
 * __<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">Open __<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> – continues to grow in size indefinitely, eventually fizzles out **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">o Critical density < 1 **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">__Closed__ – someday will stop growing and begin to contract, the Big Crunch **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">o Critical density >1 **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">__Flat__ – borderline between open and closed; gradually slows but doesn’t stop/fall backward **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">o Critical density must equal 1 **
 * __<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">Oscillating __<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> – consecutive Big Bangs/Big Crunches **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">(Critical density = average mass and energy density of universe) **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">DS **
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">Good. KMS **