Astronomy Chapter 1 (REVISED)
What other scale model can we use to represent the galaxy, besides the Solar System scale model?
-What if we wanted a model of the galaxy to fit on a football field, about 100 (10^2) meters long? -Diameter of the disk of stars in the galaxy = 10^18 km -10^18 km = 10^21 meters -So, we want to take something that has a physical size of 10^21 meters and represent it in a model that has a size of 10^2 meters -10^21 / 10^2 = 10^19 -So, the scale factor must be 10^19
Supercluster
A gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe; a cluster of galaxy clusters
Galaxy
A great island of stars (this includes star systems and nebulae) in space, all held together by gravity and orbiting a common center
Our Solar System's Distance to the Center of the Galaxy
About 27,000 light years, a bit more than half of the radius, halfway from the center to the outer edge
Universe
All of space and everything in it
The Earth has a radius of about 6,000 km. How long would it take for an object traveling at the speed of light to circle the earth? (Recall that the speed of light is 300,000 km/s.)
Diameter of Earth (not circumference because it's not travelling around the circumference of the Earth, it's travelling the Earth's diameter) = 6,000 km x 2 = 12,000 km Time = Distance/Speed Time for object to circle the Earth = 12,000 km / 300,000 km/s = 0.04 Answer: 1/8 of a second (0.0125 s)
Ecliptic Plane
Earth's axis of orbit is perpendicular to the ecliptic plane, which is the plane of Earth's orbit around the Sun
Intergalactic/Insterstellar
Intergalactic: between galaxies Interstellar: between the stars
Intragalactic/Intrastellar
Intragalactic: within a galaxy Instrastellar: within a star system
How would someone in the Andromeda galaxy view Earth?
Just as we see the Andromeda Galaxy as it was 2.5 million years ago, the Andromeda Galaxy would see Earth as it looked 2.5 million years ago; wouldn't see any people or manmade creations
Light Second/Light Minute
Light Second: The distance that light travels in one second - 300,000 km Light Minute: The distance that light travels in one minute - (300,000 x 60=) 18,000,000 km
What does the "moderately large" size of a planet imply?
Moderately large means: 1. Enough mass to have enough gravity to pull itself (through its own gravity) into a roughly spherical shape 2. Enough gravity to "sweep out" its own orbital path -Pluto got demoted because of its "oddball status" was given a precise criterion; Pluto was never able to sweep out its own path -"Sweep out" means it has a gravitational pull, the planet can pull things to itself
How many kilometers are in one light year?
-300,000 km in one light second -300,000 km x (the number of seconds in a year) = approximately 10^13 km in one light year
What is the difference between a hypothesis and a scientific theory/model?
-A hypothesis is the discovery of a pattern in the observations, and extrapolation of that pattern to observed cases -A theory goes beyond observation to the underlying explanation; this explanation may be unobservable in principle -A hypothesis does NOT explain anything, a scientific theory is an explanation -Ex: Hubble's hypothesis was that the proportionality between a galaxy's speed and distance from us applied to all galaxies Hubble's theory was that space was expanding, which applies to the whole universe
Star
-A large, glowing ball of gas that generates heat and light through nuclear fusion -Nuclear fusion is a specific physical process for generating light and heat; a star must use this process compared to other ways to generate light and heat
Planet
-A moderately large object that orbits a star; it shines by reflected light; AKA, it does not generate its own light, like a star, but it reflects light, that's how we can see them -Planets may be rocky, icy, or gaseous in composition -Things that can only REFLECT light, like a planet, can actually GENERATE invisible light: infrared, radio waves, ultraviolet -Planets "shine" because they reflect visible light
Comet
-A relatively small and icy object that orbits a star -Like an asteroid (because both orbit stars, both are relatively small) but it's made of ice rather than rock -The tail of a comet is the ice of a comet evaporating as the comet gets closer to the Sun
Solar (Star) System
-A star and all the material that orbits it, including its planets and moons -"Star system"/"Solar system" is a general term; proper noun Solar System refers to our system -Star systems are a part of a larger structure called a GALAXY
Cosmic Calendar
-A temporal model in which we compress the entire history of the universe into 1 year -Scale factor is 1 to 14 billion -Actual intervals of time in the universe / 14 billion allow us to represent these events in their relative duration in 1 year
What formed in galaxies after the Big Bang?
-About 1 billion years after the Big Bang, matter began to coalesce into galaxies -Then, within galaxies, nebulae formed -The birth of stars had to wait for the formation of galaxies
What were the first two major events to happen after the Big Bang?
-About 3 minutes after the Big Bang, matter began to form atoms -About 1 billion years after that, matter coalesced into galaxies -In between these two events, the universe began expanding and cooling down from a high temperature at the time of its creation -These two events have continued even today -However, the milestone was at 1 billion years after the Big Bang when the universe's expansion and cooling led to the formation of clumps of matter in space that we now know of as galaxies
What is our nearest neighboring star system? How far away is it in light years and kilometers? How far away is it in the scale model?
-Alpha Centuari System is our nearest neighboring star system -It is about 4.3 light years away -There are about 10^13 km in one light year, so 4.3 light years is about 4.3 x 10^13 km -In the scale model: 4.3 x 10^13 km / 10^10 = 4.3 x 10^3 km -So, in the scale model, the Alpha Centauri system is about 4,300 km away from our solar system, which is about the distance across North America (from New York to California); all that's in between is empty space
What stage of the scientific method is an experiment?
-An experiment occurs at stage 1; it's a kind of observation in which you manipulate nature, and then observe the result of your manipulation -The data from an experiment may reveal a pattern, but the discovery of the pattern is different from performing the experiment -You can perform an experiment, collect the data, and then the experiment is complete; but you can spend years after that looking for the pattern -This pattern is NOT the experiment; experiment is stage 1, pattern is stage 2
Moon (or satellite) - what is special about Earth's Moon?
-An object that orbits a planet -"Satellite" means something that orbits something else; can be natural (like our Moon) or artificial (like NASA satellites) -Our Moon has a diameter about 1/4 of Earth's diameter: that is a VERY large ratio for a moon -A planet is a natural satellite of a star
Scientific Realism
-As scientists, we are entitled to believe in a theory (even including a theory about something that is unobservable in principle) if that theory: 1. Is logically self consistent 2. Provides the best possible explanation of a pattern we actually do observe -Ex: Hubble's theory is the best possible explanation for the pattern that he observed of the proportionality between a galaxy's speed and distance from us, so according to this theory, we are entitled to believe that it is true
Empiricism
-Belief that we should never actually believe in anything we cannot at least observe in principle -But, even though we should not actually believe such theories, we are still entitled to use them to generate testable predictions about things we can observe -Ex: the expansion of space allows us to make testable, observable predictions, that may even come true; however, we're still not entitled to believe that space is expanding, because this is unobservable
If the entire galaxy (with a diameter of 100,000 light-years) were represented by a circle with a diameter of 4,400 km (similar to the width of the continental United States), then how far apart would be the Sun and its nearest stellar neighbor, Proxima Centauri (which is 4.2 light-years away)?
-Calculate circumference of the circle using π x diameter; 4,400 km x π = 13823 km = 1.3 x 10^4 km = 1.3 x 10^7 meters -Diameter of the distribution of stars in the galaxy = 10^21 meters -How to find scale factor: 10^21 m / 1.3 x 10^7 = 1.3 x 10^14 m -4.2 light years away = 4.2 x 10^16 m -How to find distance to Proxima Centuari: 4.2 x 10^16 m /1.3 x 10^14 m = 3.2 x 10^2 m = 320 meters -Actual answer is around 200 meters
Local Group
-Consists of 50 galaxies, all swarming around each other; no clear pattern of mutual motion like orbiting a common center -Two galaxies in this group are much larger than the others (Milky Way Galaxy and Andromeda Galaxy) -The galaxies in the Local Group are loosely clustered around these two bigger galaxies, but there is no common center that they orbit around -All galaxies are held to each other by their mutual gravity, but without a single center
Earth's Axis of Rotation
-Earth rotates once each day on its axis from west to east, appears counterclockwise -Perpendicular to the equator -Over the course of 1 year, the direction of the axis of rotation remains nearly constant relative to distant objects (like other stars), but changes relative to the axis of orbit -Remains pointed in the same direction (towards Polaris) throughout the year
How is it that the universe is made up of mostly hydrogen and helium, but on Earth we mainly see much heavier elements?
-Elements such as iron, oxygen, silicon, aluminum, etc, that seem to make up the majority of our world, only make up about 2% of the universe overall -Consider just our solar system: 99.9% of the mass of our solar system is in the Sun -Planet Earth is miniscule compared to the Sun, and the Sun is primarily hydrogen and helium -Most of the mass of the universe is either in stars or in nebulae, which mainly consist of hydrogen and helium -With respect to both our Solar System and to the universe overall, our world is a collection of relatively rare materials
Why is it that everything beyond the Local Group is expanding?
-Everything from the Local Group, all the way to our individual human bodies has some sort of force keeping those objects together: gravity for the Local Group, Milky Way Galaxy, our Solar System, and Earth, and for individual objects on Earth, electric attractions between atoms -Beyond the Local Group, gravity is too weak to maintain any structure, such as superclusters of galaxies; that is why all of the matter beyond this threshold is free to hitch hike on the expansion of space -An attractive force has the ability to cause an object to maintain its structure and not expand; everything up to the Local Group is NOT expanding -Beyond this threshold, there is no force strong enough to stop the hitch hiking on the expansion of the universe -Matter only expands when there is no attractive force holding the matter together (gravity, atomic electric attraction) to overcome the expansion of space
How can we represent the size of our galaxy in the scale model of our SOLAR SYSTEM (10^10)?
-First step: convert light years to kilometers -Diameter of the disk of stars in our galaxy = 100,000 light years = 10^5 light years -1 light year = 10^13 km -10^5 ly x 10^13 km/ly = 10^18 km is the diameter of the disk of stars in our galaxy IN REALITY -10^18 km / 10^10 = 10^8 km is the diameter of the disk of stars in our galaxy IN THE SOLAR SYSTEM SCALE MODEL -Compare to the physical reality of Earth's average distance to the Sun, 1.5 x 10^8 km -The scale model of the galaxy overall would be about 2/3 the physical distance between the Earth and the Sun/radius of Earth's orbit
Expansion of the Universe
-From Earth to the Local Group, there is a threshold distance where there is no consistent correlation between the distance to a galaxy and the speed of that galaxy -However, beyond this threshold distance, beyond the Local Group, Hubble found that the distance to a galaxy is directly proportional to its speed moving away from us -In other words, the farther away a galaxy is, the faster it's moving away from us -Ex: if galaxy A and galaxy B are both outside of the Local Group, but galaxy A is twice as far away, it will also move twice as fast -However, expansion of the universe is not that the matter is moving through space, matter is not moving through space on this cosmic scale; rather, space itself is expanding, while the matter "hitch hikes" onto this expansion -Everything is moving away from everything else, and there is no common center
How does the speed of Earth's rotation vary depending on geographic location?
-If you're standing on the equator, in one day you have travelled a distance equal to the entire circumference of the Earth -Circumference of the Earth: 2π x (radius of Earth=6,400 km) = 40,000 km Speed on the equator = 40,000 km per day, which equals 1670 km/hr -That speed is different at different locations, because the equator is the only place where you travel the complete circumference of the Earth -Ex: if you're in NYC, you travel around a smaller circle of the Earth, so it's a smaller distance with the same unit of time of one day; NYC moves at a speed of about 1100 km/hr
Expansion of the Universe - Raisin Bread Analogy
-Imagine a raw, uncooked loaf of bread with raisins embedded in the dough, each exactly 1 cm apart from each other -Then, the bread is cooked, and the dough rises and expands over the course of 1 hour -Now, instead of 1 cm in between each raisin, there's 3 cm in between each raisin -The raisins moved away from each other, but they did not move through the dough; as the dough expanded, the raisin remained within the same "dough neighborhood" but moved away from its neighboring raisins because it was hitch hiking on the expansion of the dough itself -Here, the raisins represent galaxies, and the dough represents the universe
Light Year Lecture Hall Metaphor
-Imagine two students in a lecture hall, both born at the same time, so they are the exact same age -One of them sits at the front of the room, the other sits at the back of the room -If you're standing at the front of the lecture hall, the person in the back would appear to be younger, because the light that reaches you left that person further back in time -The light left further back in time, when the person was younger, so they appear to be younger -The difference in how their ages appear depends on how much longer it takes the light to travel
Why were superclusters formed to be held together, rather than held together by gravity?
-Immediately following the Big Bang, the universe was already uneven, or had an uneven/"clumpy" distribution of matter -The unevenness that we experience every day, like the "clumpiness" of structures like the Milky Way formed billions of years after the Big Bang as a result of gravity operating on matter -However, the structure of superclusters is the current residue/aftereffect of the initial unevenness of the whole universe
What is the similarity and difference between a star system and a galaxy?
-In both a star system and a galaxy, we see objects orbiting a common center -In our star system, all objects orbit the Sun, and in the Milky Way, all nebulae, stars, and star systems orbit an object at its center -However, the difference is in the distribution of matter -In our star system, 99.9% of matter is within the Sun -In the Milky Way, the distribution of matter is much more spread out
Nebula
-Interstellar (between stars, beyond star systems, always within a galaxy/intragalactic), cloud of gas and/or dust; aka always in galaxies, never in star systems -Note; gas consists of atoms moving freely or independently of each other, dust is small pieces of solid matter; a nebula can have one or the other or both, usually have both
How do we get from kilometers to meters?
-Multiply by 1000, or 10^3 -Ex: 10^18 km x 10^3 = 10^21 meters
Does the fact that the Local Group serves as a threshold, beyond which the universe expands, imply that it is the center of our universe?
-No, the Local Group is not the center of the universe -As in the raisin bread analogy, any raisin could be chosen as the Local Raisin and the same pattern of expansion will be found -The pattern of expansion can be seen from any vantage point
How much time does the cosmic calendar represent in reality?
-Number of seconds in a year: roughly 31,000,000 -One second is 1/31,000,000 year, this is the actual duration of time that we have to find a fraction of 14 billion -Each second on the cosmic calendar represents 1/31,000,000th of the cosmic year, but that represents the entire history of the universe in reality -So, we want that fraction of the actual age of the universe, so we need to multiply it by 14 billion -(Ex: what is 1/2 of 10? (1/2) x 10. what is 1/31,000,000 of 14 billion? (1/31,000,000 x 14 billion) -When we multiply them, we get 444.44 years -Each second of the cosmic calendar represents about 444 years in reality -We need to find one second of one year overall as a fraction of the timeline of the whole universe to find one second on the cosmic calendar
Distribution of Stars in Our Galaxy - Diameter and Thickness
-Once thought to be the entire size of the galaxy, but this was later found to be inaccurate, because these numbers do not tell us the size and shape of the galaxy as a whole, because the majority of the galaxy (dark matter) is not included in the stars -Stars of the galaxy are distributed in a disc -Diameter: 100,000 light years (Radius: 50,000 light years) -Thickness: 1,000 light years (outside of the bulge) -The disk is 100x wider in diameter compared to its thickness -There's also a bulge at the center of the disk, which is thicker than 1,000 light years
Edwin Hubble
-One of the first astronomers to figure out that our galaxy was one of many galaxies, and also how to collect data on other galaxies, both quantitative and qualitative; his ideas were once very controversial -Observed distant galaxies were thought to be nebulae within our own galaxy; Hubble opposed this idea -He was the first to be able to measure the distance from the Milky Way to distant galaxies; also used, but didn't invent, the doppler effect to measure the speeds of distant celestial objects -Discovered the expansion of the universe
What levels of structure are held together by gravity? Which is not?
-Planets stay in formation and are held together by gravity, star systems are held together by gravity, galaxies are held together by gravity, and local groups are held together by their shared gravitational attraction -Superclusters of galaxies are NOT held together by gravity
Asteroid
-Relatively small and rocky object that orbits a star -Unlike a planet, it doesn't have enough gravity to make itself round -Unlike a moon, asteroids orbit stars, not planets
Motion of the Local Group
-Similar to the motion in the neighborhood of any individual star in the galaxy -The Local Group doesn't move around a common center, it seems to have a more random motion with no discernable pattern -Individual interactions are deterministic, still dependent on universal laws of motion, but not systematic like the rest of the galaxy
Two Kinds of Days on Earth
-Solar day: one complete rotation of Earth relative to the Sun, can vary depending on where the Earth is in its orbit around the Sun, but averages to be about 24 hours -Sidereal day: one complete rotation of Earth relative to the stars, doesn't vary like a solar day, usually 23 hours and 56 minutes
Intrinsic Geometry
-Space and time have an intrinsic geometry -Idea that if things move, they move through something: we move through air, with air itself as a form of matter -This give rise to the idea of space as an entity -Discovery made by Einstein: Theory of General Relativity -This discovery was that space itself has a shape; it can be reshaped, bent, compressed, or expanded; this is what we mean by "intrinsic geometry" -Space is not merely a way of describing the arrangement of objects in space, rather, space itself, whether there's anything in it or not, has geometrical properties -The expansion of space is an example of the more general concept of space having an "intrinsic geometry" -Another example would be the fact that in addition to expansion, space can also change/distort its shape -Neither expanding or changing shape depend on space being inside something else -The word "intrinsic" means that expansion and changing shape refer only to space itself, not to a comparison with anything "beyond space" -So, space is NOT expanding into something else
Hubble's Discovery: Scientific Method
-Stage 1: For each individual galaxy that Hubble studied, he made individual observations and collected data about its distance from us, and its speed receding from us (he used variable stars for measuring distances, and the doppler effect for measuring speeds) -Stage 2: He discovered a two-part pattern within this collection of data: the first part is that there is a threshold distance where there is no correlation between a galaxy's speed and distance from us (AKA the existence of the threshold itself is a pattern), and the second part is that beyond that threshold there is a correlation: the further away, the faster the speed of recession -Stage 3: He offered an explanation of the pattern: the universe is expanding, and expanding from all points: there is no center of expansion
What is the difference between stage 2 and stage 3 of the scientific method?
-Stage 2 is a pattern based directly off of the observed data in stage 1 -Stage 3, however, is what brings us beyond the appearance, to the reality -So, the explanation can go beyond stage 2, even beyond something that is directly observable
Star - Metaphorical Life Cycle
-Stars have a "birth" and a "death" -Before a star is born, there is a "proto-star" or metaphorically, a "stellar embryo" -After a star dies, there is metaphorically a "stellar corpse" -A proto-star and a stellar corpse are both a "large glowing ball of gas that generate light and heat", however they don't fit the definition of a star because they don't generate light and heat through nuclear fusion -When the nuclear fusion switch is "turned on", the star is born when it's "turned off", the star dies
What is the chemical composition of the universe today?
-The Big Bang created hydrogen and helium -However, all other (non-manmade) elements were created later, inside of stars -When those stars die, all of these "heavier elements" are released back into the galaxy -The universe today is about 73% hydrogen, 25% helium, 2% everything else -What has happened from the Big Bang to today is that 2% of the hydrogen in the universe has been converted to helium, and 2% of helium was converted into everything else -So, the net result is that the overall amount of helium has remained approximately the same, 25% of the universe, but that 25% is constant because some new helium was created out of hydrogen, and some of the "old helium" was converted into other elements; so there's no net change in the amount of helium
How does the death of a star enrich the cosmos?
-The Big Bang created the elements of hydrogen and helium, but all heavier elements were later created in stars -(Iron is the heaviest element that can be created in the normal lifespan of a star, all heavier elements are created in an explosive death of a star, a supernova) -It is primarily when stars die that atoms that are formed in the core of a star (heavier elements) are released out into the wider cosmos -This happens both during the peaceful death of a star and a supernova -This means that the next generation of stars is therefore enriched with a certain percentage of heavier elements -So, as time goes on, the universe as a whole gets enriched with heavier elements -This is primarily a one way process; over time, hydrogen is gradually converted into heavier elements, and it doesn't go the other direction
What two elements were created by the Big Bang? What was the chemical composition of the universe at the time of its creation?
-The Big Bang created the first two elements on the periodic table: hydrogen (#1), and helium (#2) -(Along with a very small amount of lithium) -After the Big Bang, the universe was about 75% hydrogen, 25% helium
Major Events in the Cosmic Calendar
-The Big Bang is at the first instant of the New Year, the first second of January 1st -The present is midnight on December 31st -Galaxies began to form about 1 billion years after the Big Bang, so galaxies formed roughly 1/14 of the way through the cosmic calendar -1/12 is about one month on the cosmic calendar -So, galaxies formed roughly one month into the cosmic calendar year, around late January to early February
Earth's Axis of Orbit
-The Earth orbits around the Sun once each year, at an average distance of 150,000,000 km, or 1 AU speed of 100,000 km/hr -Perpendicular to the ecliptic plane
Motion of Our Solar System
-The Earth rotates, and orbits around the Sun: there is a third level of motion that also applies to our solar system in general -The Sun is dragging around the rest of the planets in our solar system to orbit around the center of the galaxy -The Earth and the rest of our solar system orbits the center of the galaxy with a period of about 230 million years -We do this at a speed of about 800,000 km (can calculate this based on our distance to the center of the galaxy, AKA the radius of our orbit of the center of the galaxy, about 27,000 light years) -If we compare our orbital period to the age of our solar system, we will see that we've orbited the galaxy around 20 times (4.6 billion / 230 million)
When did the Solar System form on the cosmic calendar?
-The Solar System formed about 4.6 billion years ago in reality -4.6 billion is approximately 1/3 of 14 billion, so that means the formation of our Solar System happened 2/3 of the way from the Big Bang up until the present -The universe was 2/3 of its current age when the Solar System formed -2/3 of the way through a year is around early September, this is when the Solar System formed on the cosmic calendar
Where does the recycling of star material occur?
-The cyclical pattern of the universe getting enriched with heavier elements only occurs within galaxies, NOT in the space between galaxies (AKA not in intergalactic space, always in intragalactic space) -Stars and nebulae only exist within galaxies
What is special about a nebula?
-The density of matter within a nebula is much higher than most of the galaxy -Most of the galaxy is much less dense, as it is not made up of gas or dust -This is the womb where a protostar begins to form -Stars cannot be formed outside of nebulae, as the formation of a star requires this high density of gas and/or dust
What remains constant about Earth's axis of rotation?
-The direction of the axis of rotation remains nearly constant RELATIVE TO DISTANT OBJECTS; the north side of Earth's axis of rotation is always pointed towards the star Polaris -On the other hand, the direction of the axis of rotation changes RELATIVE TO the axis of orbit -Because Earth's axis remains in a constant direction relative to Polaris, for half of the year, the northern extension of the axis of rotation points away from the axis of orbit, and therefore away from the Sun, and half a year later the axis of rotation points toward the axis of orbit, and therefore toward the Sun -This is what gives rise to seasons
Light Year
-The distance that light travels in a year -Light travels at a speed of 300,000 km/s -Thus, we see objects as they were in the past -The farther away we look in the distance, the further we look back in time -As we look into the universe, the light that comes to us has taken time to get to us -The more distant the object, the more time it took the light to get to us -So, we see these distant objects as they looked in the past, when it produced the light that is now only reaching our eyes -Ex: the Sun is 8 light minutes away, so when we look at the Sun we see it as it was 8 minutes in the past
Virgo Supercluster
-The large group of galaxy clusters that contains the Milky Way galaxy and around 50,000-60,000 others -The strength of a gravitational force gets weaker with distance; the galaxies in a supercluster are so far from each other that they are not held together by gravity -They are held together simply due to the fact that they were formed that way early in the beginning of the universe
What is the distribution of matter in the Milky Way Galaxy?
-The object at the center of the galaxy has a mass that is 1 million times the mass of our Sun (1 million = 10^6) -This may seem like a lot, until you realize that the Milky Way also has about 100 billion - 1 trillion stars/star systems (100 billion = 10^11)
Observable Universe
-The portion of the entire universe that can be seen from Earth, at least in principle -Finite -The observable universe is probably only a tiny portion of the entire universe -Not defined by limitations of technology or practical tools, rather an abstract concept in principle defined by the fundamental law of nature that says light travels at a finite speed
Earth's axis is tilted, relative to what?
-There is no absolute direction or absolute up and down in space -When we say that the Earth's axis is tilted, what we really mean is that the axis of rotation is tilted RELATIVE TO the axis of orbital motion -These axes are NOT parallel -The angle between these two axes is 23.5 degrees -The magnitude of this angle remains relatively constant (aside from cycle of precession), but the relative direction between the two axes varies as Earth orbits the Sun -The axis of rotation is tilted 23.5 degrees from a line perpendicular to the ecliptic plane, AKA from the axis of rotation -23.5 degrees is also the angle between the 2 corresponding planes: the equatorial plane, which is perpendicular to Earth's axis of rotation, and the ecliptic plane, which is perpendicular to Earth's axis of orbit
How long is the lifespan of a star?
-Very wide range: can last from 10-20 million years, to hundreds of billions of years -Because the universe is 14 billion years old, this means that there are some stars that are not even close to their entire lifespan since the beginning of the universe
Scale Model of the Solar System
-We can use a scale model to visualize the universe, using the factor 10^10 -In this scale model, the Earth is about the size of a ballpoint pen and the Sun is about the size of a grapefruit
Why is the observable universe finite?
-We know that the universe has a beginning, the Big Bang, that occurred about 14 billion years ago -So, the universe is a finite age -We also know that light travels at a finite speed (300,000 km/s) -The immediate consequence of those two facts is that light has only had the opportunity to travel a finite distance from the beginning of the universe up to the present -If it has had only a finite amount of time to travel, and in that time it travelled a finite speed, that means it has only had the opportunity to travel a finite distance -That finite distance that light has had the opportunity to travel from the beginning of the universe determines the size of the observable universe -So, we only have information about a finite region of the universe, because it is only that finite region from which that information has had the opportunity to get to us -The universe consists of everything, but the observable universe consists of that region from which we have had the opportunity to get information -Light could travel infinitely, but it would need an infinite amount of time and space to do so; there is only a finite amount of time since the beginning of the universe, so it's only had the opportunity to travel a finite distance
Motion in the Neighborhood of Any Individual Star in the Galaxy
-Within the galaxy, in addition to orbital motion around the center of the galaxy, there's also a more random, swarming motion in the neighborhood of any individual star in the galaxy, superimposed on the systematic, orbiting motion -All ordinary matter in the galaxy is orbiting its center in a systematic motion, all in the same direction with varying speeds (objects closer to the center move faster) -However, in the neighborhood of any individual star, interactions between objects are governed by the immediate interaction of neighboring stars with no discernable overall pattern -This applies to our Sun: our Sun moves randomly relative to the other stars in the local solar neighborhood at typical relative speeds of around 70,000 km/hr
Expansion of the Universe - Local Raisin, Raisin Speeds
-You can arbitrarily choose one raisin the bread to be the Local Raisin; it doesn't matter which one, it could be any -We can choose 3 raisins from the chosen Local Raisin: R#1, R#2, and R#3 -Using speed = distance/time, we can calculate the raisins' speeds -At the beginning, R#1 was 1 cm away from the LR, and at the end, after baking for 1 hour, it was 3 cm away: it moved at a speed of 2 cm/hr -At the beginning, R#2 was 2 cm away from the LR, and at the end, it was 6 cm away: it moved at a speed of 4 cm/hr -At the beginning, R#3 was 3 cm away from the LR, and at the end, it was 9 cm away: it moved at a speed of 6 cm/hr -So, we can see the distance to a raisin is directly proportional to the speed that a raisin is moving away from the LR
Intervals of Time on the Cosmic Calendar
1 Cosmic Calendar day = 37.5 million years 1 Cosmic Calendar hour = 1.56 million years 1 Cosmic Calendar minute = 26,050 years 1 Cosmic Calendar second = 434 years
What are the two ways a star can die?
1. A relatively peaceful death; this is usually more common amongst stars, and stars that die this way typically have a longer lifespan (hundreds of billions of years); these stars have less mass, so they "burn up their fuel" more slowly 2. An explosion, or a supernova; this is less common, and usually stars that have a shorter lifespan (10-20 million years) die this way; these stars also have more matter, causing them to "burn up their fuel" quicker
Two Categories of Axes
1. An axis that goes through the turning object (Earth's axis of rotation) 2. An axis that is outside of the turning object (Earth's axis of orbit)
How does Hubble's description of a pattern, and explanation (that space is expanding) go beyond the pattern he observed?
1. He used inductive reasoning, extrapolating from some set of examples to find a general rule -Hubble began with a small data set, about 20 galaxies, discovered a pattern within that data set, and then articulated that pattern that continued to apply to more galaxies -This generalization is a hypothesis: an idea about how a pattern that is observed in specific cases will continue in cases that have not yet been discovered -Stage 2: he's formulating a hypothesis of all galaxies based on the small number that he observed, making a generalization that goes beyond his observed data and applies to all galaxies 2. He went beyond the observable pattern (which includes both observed and unobserved examples) to the underlying cause or explanation -In Hubble's example, the underlying explanation for this pattern is unobservable; at least in principle, we cannot observe the expansion of the universe -Stage 3: he's proposing the explanation that space is expanding, which goes beyond his observed data, and unobservable in principle, that applies to the entire universe
Two Ways to Quantify Periodic Motion
1. How much time does it take the object to complete 1 cycle of periodic motion? AKA, what is the duration of the period? (On Earth, this number is the same for everyone: 1 day); refers to a cyclical motion 2. Quantify the motion as distance per unit of time, (ex: 65 mph; this number changes depending on geographic location); refers to a linear motion, going from point A to point B; how far do we travel in one day, and how fast is it?
How big is our galaxy? What are the two meanings behind this question?
1. Scientists (up to about 30 yrs ago) believed that our galaxy was the size of a disk with a diameter of about 100,000 light years; this only describes the distribution of stars in our galaxy, which makes sense why people would think this is the entire galaxy, because stars are what produce light and light is our main source of information about the universe; people believed that where there are stars, there's matter and celestial objects 2. However, we now know that the majority of the universe is made up of dark matter, which neither produces light nor blocks the flow of light; still a lot of information unknown about dark matter, but we know that the size and shape of the galaxy is very different from what we thought it was based on ordinary matter
What is the speed of light?
300,000 km/s 3 x 10^5 km/s 3 x 10^8 m/s
Radius of the Earth
6,400 km 6.4 x 10^3 km (x 10^3) = 6.4 x 10^6 m
Galaxy Cluster
A collection of galaxies held together by their mutual gravitational attraction. Earth's galaxy cluster is the Local Group.
How long would it take to count all the stars in the Milky Way Galaxy at a rate of one star per second?
A few thousand years; there are as many stars in the galaxy as there are grains of sand on all of Earth's beaches
Dark Matter
A form of matter that determines the size and shape of galaxies, that is thought to make up the majority of our galaxy, and neither produces light nor blocks light; stars in the galaxy are distributed in a galactic disc, but the matter of the galaxy is distributed more in a spherical, halo shape surrounding that disc; this is dark matter
Hypothesis
An idea about how a pattern that is observed in specific cases, that you hypothesize will continue in cases that have not yet been discovered; stage 2 of the scientific method
Axis
An imaginary line that passes through Earth's center and the North and South poles, about which Earth rotates
What is our nearest neighboring large galaxy? How far away is it in light years?
Andromeda Galaxy/M31, 2.5 million light years away
Our solar system is located about 27,000 light-years from the galactic center. How far does our solar system travel in one orbit?
Distance = speed x time Distance our Solar System travels: 800,000 km/hr Time of our Solar System's orbit: 230 million years 800,000 km/hr = 7 x 10^9 km/yr 7 x 10^9 km/yr x 230 million yrs = 1.6 x 10^18 km is the distance our Solar System travels in one orbit 1.6 x 10^18 km = about 170,000 light years
Equatorial Plane
Earth's axis of rotation is perpendicular to the equatorial plane, which "slices" the Earth across the equator
Earth's Cosmic Address
Earth, Solar System, Milky Way Galaxy, Local Group, Local Super Cluster
Diameter of Earth (in reality AND scale model)
Reality: (6.4 x 10^6 meters x 2=) 12.8 x 10^6 meters = 1.3 x 10^7 meters Scale Model: 1.3 x 10^7 meters / 10^10 = 1.3 x 10^-3 meters, which is about 1.3 millimeters, the size of a ballpoint pen
Radius of Earth's Orbit/Earth's Average Distance to the Sun (in reality AND scale model)
Reality: 150,000,000 km = 1.5 x 10^8 km = 1.5 x 10^11 m Scale Model: 1.5 x 10^11 meters / 10^10 = 1.5 x 10^1 = 15 meters This distance is defined as 1 AU
Radius and Diameter of the Sun (in reality AND scale model)
Reality: The Sun's radius is 700,000 km, so it's diameter is 1,400,000 km = 1.4 x 10^6 km = 1.4 x 10^9 m Scale Model: 1.4 x 10^9 meters / 10^10 = 1.4 x 10^-1 meters = 14 centimeters, about the diameter of a grapefruit
How to Calculate One Light Year Based on the Speed of Light
Speed of light = 300,000 km/s 1 light year = speed of light x 1 year We have to convert seconds in the denominator to a year (300,000 km/s x (365 days/1 year x 24 hrs/1 day x 60 mins/1 hr x 60 secs/1 min)) 1 light year = 9.5 x 10^12 km
What information can the mass of an object tell us (gravitational mass vs. inertial mass)?
The mass of an object plays two roles: 1. The mass is one factor in determining the amount of gravitational pull that this object exerts on other objects: gravitational mass 2. The mass also determines how an object responds to external force: inertial mass