ASTR 101 Ch: 1
The following statements describe ways in which the analogy might apply to the real universe. Which statements are correct?
-The average distance increases with time both between raisins in the cake and between galaxies in the universe. -An observer at any raisin sees more distant raisins moving away faster, just as an observer in any galaxy sees more distant galaxies moving away faster.-The raisins stay roughly the same size as the cake expands, just as galaxies stay roughly the same size as the universe expands. Like any scientific model, the raisin cake analogy has limitations, but it gives us a good overall picture of how the universe is expanding.
If you wanted to show the distance between Earth and the Moon on the same scale, about how far apart would you need to place the two photos above?
1 meter (about the length of your arm)
1 light-hour in kilometers
1.08×10^9 km
1 light-minute in miles
1.12×10^7 mi
1 light-day in miles
1.61×10^10 mi
1 light-minute in kilometers
1.80×10^7 km
1 light-second in miles
1.9×10^5 mi
Shown here are six galaxies, each labeled with its approximate distance from Earth. Rank the galaxies from left to right based on the amount of time it has taken their light to travel to Earth, from the longest time to the shortest time.
10 billion light years, 5 billion light years, 2 billion light years, 800 million light years, 230 million light years, 70 million light years. In fact, distances in light-years correspond directly to light-travel times. For example, light takes 100 million years to reach us from a galaxy that is 100 million light-years away. This fact has important implications to the study of the evolution of the universe, as you'll see in Part B.
Light received from a galaxy at the far end of the Local Supercluster was emitted roughly ____________.
108 years ago.
An astronomer looks at the Andromeda galaxy (the other large galaxy in the Local Group) through her telescope. How long ago did that light leave Andromeda?
2.5 million years.
1 light-day in kilometers
2.59×10^10 km
Each box in this figure represents a different level of structure in our universe. Each box is labeled with one of the numbers 1-5. Which box represents the Milky Way Galaxy?
3
1 light-second in kilometers
3.00×10^5 km
Suppose you wanted to show the Sun on the same scale. About how big would it need to be?
3.5 meters in diameter (about 11.5 feet across)
About how far away from Earth would the Sun be located on this scale?
375 meters (about the length of four football fields)
1 light-hour in miles
6.72×10^8 mi
Consider again the galaxies you ranked in Part A. This time, rank them from left to right based on the age of the universe at the time these galaxies emitted the light we receive from them today, from oldest (closest to today) to youngest (furthest back in time).
70 million light years, 230 million light years, 800 million light years, 2 billion light years, 5 billion light years, 10 billion light years. By looking to objects at a variety of distances, we can actually see what the universe was like at different ages. This fact helps astronomers to reconstruct the history of the universe.
Which of the following are real motions of our "spaceship Earth"?
Earth orbits the Sun, Earth moves with the Sun on an orbit around the center of the Milky Way galaxy, Earth and our solar system moves with the Milky Way galaxy relative to other galaxies in our Local Group. It may feel like the surface of the Earth is stationary, but we are all moving relative to other objects in the universe in numerous ways on our "spaceship Earth."
An astronomical unit is?
Earth's average distance from the Sun.
Which of the following lies in the ecliptic plane?
Earth's orbital path around the Sun. The ecliptic plane is defined by Earth's orbital path around the Sun.
Which sequence is correctly ordered from smallest to largest?
Earth, Moon's orbit, the Sun, Neptune's orbit.
Which of the following correctly lists our "cosmic address" from small to large?
Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe.
By how many orders of magnitude is the distance to the nearest stars greater than the orbital radius of Neptune?
Four.
Rank the following objects from largest to smallest.
Galaxy, Solar System, Sun, Earth. Galaxies are gigantic collections of millions, billions, or even trillions of stars, and stars are far larger than the planets that orbit them. It's important to understand the relative sizes of common astronomical objects so you'll have the proper context for everything we'll learn from now on.
In the rest of this activity, we will consider the timing of events using a type of time line known as the "cosmic calendar." Originally developed by Carl Sagan, the cosmic calendar compresses the history of the universe into a single year, with the Big Bang occurring at the first instant of January 1 and the present being the stroke of midnight on December 31.
In the rest of this activity, we will consider the timing of events using a type of time line known as the "cosmic calendar." Originally developed by Carl Sagan, the cosmic calendar compresses the history of the universe into a single year, with the Big Bang occurring at the first instant of January 1 and the present being the stroke of midnight on December 31.
What does this photograph show?
It is a picture of the Andromeda galaxy, located about 2.5 million light-years away
What is the significance of this photograph?
It shows a person standing on the most distant world ever visited by a human being.
Because we live in an expanding universe, distant galaxies are farther away from us today than they were when the light we see from them started on its journey to us. So what do we mean when we say that a galaxy is 7 billion light-years away?
Its light has taken 7 billion years to reach us. As explained at the end of the video, distance statements can be ambiguous in an expanding universe, because we don't know if they refer to an object's distance at the time we see it, at the present time, or some time in between. Therefore, in this book and in the video, we use distances based on the actual time it took light from distant objects to reach us, which astronomers call look back times. So when we say that a galaxy is 7 billion light-years away, we mean its light traveled through space for 7 billion years to reach us.
The earliest radio broadcasts on Earth were emitted about 100 years ago. Approximately where are these initial radio waves now? (Radio waves are a type of radiation, so they travel at the speed of light)
Just passing relatively local stars in our galaxy.
Regarding the history of the universe, which of the following is true?
Key elements of which Earth and life are made, including carbon, oxygen, and iron, did not exist when the universe was born and were created later in stars. Evidence indicates that the Big Bang produced essentially only the elements hydrogen and helium, and all other elements were made subsequently by stars.
The table in the video shows you the speeds of Raisins 1, 2, and 3 as measured from the Local Raisin. Suppose instead that you measured speeds as seen from Raisin 2. An observer at Raisin 2 would measure __________.
Local Raisin speed = 4.0 cm/hr; Raisin 1 speed = 2.0 cm/hr; Raisin 3 speed = 2.0 cm/hr. Note that the observer at Raisin 2 sees essentially the same thing as the observer at the Local Raisin: The nearest raisins (Raisins 1 and 3 in this case) are moving away at 2 cm/hr and the next nearest (the Local Raisin in this case) is moving away at 4 cm/hr. If the cake were bigger, the pattern would continue. The key point is this: It does not matter which raisin you observe from; in an expanding raisin cake — or an expanding universe — all observers see all the other raisins (galaxies) moving away, with more distant raisins (galaxies) moving faster.
Could we see a galaxy that is 50 billion light-years away?
No, because it would be beyond the bounds of our observable universe.
Could you use the same scale to represent the distances to nearby stars? Why or why not?
No, the nearest stars would not fit on Earth with this scale.
These photos show two different astronomical objects. Which object is bigger, and by about how much?
Object 2 is more than a trillion times as large as Object 1. Object 1 is the planet Saturn and Object 2 is an entire galaxy, and the diameter of a galaxy is indeed more than a trillion times the diameter of a planet like Saturn.
Notice that Earth's axis is shown with an arrowhead in this diagram. What does the arrow point to?
Polaris, the North Star. Indeed, Polaris is the North Star because Earth's rotation axis points very close to it.
Note that an observer located at the Local Raisin would see Raisins 1, 2, and 3 all move away from her during the video. What would an observer located at Raisin 2 see?
Raisin 1 and Raisin 3 both move away from her. In fact, observers at any raisin would see all the other raisins moving away. Note also that the farther away a raisin is located from the observer, the faster it moves away — which is just what Hubble discovered for galaxies in the universe.
These diagrams show a raisin cake before it is put in the oven and again one hour later after it has expanded during baking. Suppose you lived in Raisin 3 (the raisin labeled "3"). What would you have noticed about Raisin 2 during baking?
Raisin 2 is moving away from you at a speed of 2 cm/hr. Notice that Raisin 2 begins 1 cm away from Raisin 3 and ends 3 cm away from Raisin 3. It therefore appears to move 2 cm from Raisin 3 during the hour, so someone on Raisin 3 will see Raisin 2 moving at 2 cm/hr.
Which of the following is a general difference between a planet and a star?
Stars glow in infrared and visible light, while planets glow only in the infrared.
Consider the indicated events in the history of the universe that have helped make human life possible. Rank the events based on when they occurred, from longest ago to most recent. To rank items as equivalent, overlap them. Note: If two events occurred within seconds of each other, rank them as equivalent.
The Big Bang <same> the universe begins to expand, elements such as carbon and oxygen first exist, nuclear fusion begins in the Sun, earliest life on Earth, dinosaurs go extinct, earliest humans. Now that you know the order in which these events occurred, continue on to check whether you know approximately when they occurred.
Suppose we made a scale model of our Milky Way Galaxy in which the disk of the galaxy is the size of a football field. Which (if any) diagram represents the Sun on the same scale?
The Sun on this scale would be microscopic and too small to see on the screen. Because a football field is about 100 meters long and our galaxy is about 100,000 light-years in diameter, scaling the galaxy to a football field means that 1 meter represents about 1,000 light-years, so 1 millimeter represents 1 light-year, or about 10 trillion (1013) kilometers. The Sun's diameter of a little over 1 million (106) kilometers, therefore is only 1 ten millionth of a millimeter on this scale, which is about the size of a single atom!
When we say the universe is expanding, we mean that:
The average distance between galaxies is growing with time.
Which of the following statements best describes what astronomers mean when they say that the universe is expanding?
The average distance between galaxies is increasing with time. Keep in mind that while the universe as a whole is expanding, individual galaxies (and gravitationally bound groups or clusters of galaxies) and their contents do not expand. That is why the average distances between galaxies grow with time, but objects such as people, Earth, the solar system, and the Milky Way Galaxy remain stable in size.
This painting represents the Sun and planets (and two dwarf planets) of our solar system. What is not to scale in this painting?
The distances between the planets are not shown to scale.
This diagram represents Earth's rotation and orbit. What do we call the flat blue plane shown in this diagram?
The ecliptic plane. Notice that the ecliptic plane is the plane defined by Earth's orbit around the Sun.
Suppose we observe a galaxy that is 13 billion light-years away. Which of the following can we conclude?
The galaxy formed before the universe was 1 billion years old.
The fact that nearly all galaxies are moving away from us, with more distant ones moving faster, helped us to conclude that:
The universe is expanding.
Rank the following items according to their size (diameter) from left to right, from largest to smallest.
The universe, The local supercluster, The local group, The Milky Way Galaxy, Our Solar System, The Sun, Jupiter, Earth.
In what sense are telescopes like time machines?
They allow us to see distant objects as they were long in the past. The farther away we look in space, the further back we look in time. Therefore, when we look to very distant objects, we see them as they were long in the past, when the universe was much younger than it is today.
In January, Earth's rotation axis points in the direction of the star Polaris. Where does it point in July?
Toward the star Polaris. Earth's axis does not noticeably change its orientation in space over the course of each year, so it remains pointed toward Polaris at all times. Note: The axis is not quite perfectly pointed at Polaris, but points within 1° of it. Moreover, as you'll learn in Chapter 2, the orientation of Earth's axis in space gradually changes due to a motion known as precession. However, the rate of precession is so slow that it would take several centuries before the change was noticeable to the naked eye.
If stars existed but galaxies did not:
We would not exist because we are made of material that required recycling in galaxies.
Today, the evidence that we live in an expanding universe is extremely strong because astronomers have measured the motions of millions of galaxies. Nevertheless, in science, we must always remain open to the possibility that some future observation could call even our most strongly supported theories into question. Which of the following hypothetical observations would not be consistent with what we expect in an expanding universe?
You discover an extremely distant galaxy that is moving toward us. In an expanding universe, extremely distant galaxies must be moving away from us, just as distant raisins in the raisin cake must be moving away. It would make no sense to find an extremely distant galaxy that was moving toward us, and such an observation would, therefore, cause us to question the very idea of an expanding universe.
Consider each of the following statements that you might hear in everyday life. Classify each statement as either an observation or an explanation.
[Observations] James is wearing a yellow shirt, The defendant's DNA was found at the crime scene, The plastic melted when I held it near the light bulb, The defendant's calendar says he was at lunch when the crime occurred. [Explanations] James wears a yellow shirt when his other shirts are dirty, Plastic melts when it reaches a temperature that breaks bonds that hold its structure intact, The defendant only intended to scare the victim. Be sure you understand all these everyday examples before you continue to the astronomical examples in Part B. For example, the statement "James is wearing a yellow shirt" is an observation because it is something you can see directly, while the statement "James wears a yellow shirt when his other shirts are dirty" is an explanation because it is an attempt to explain why James is wearing a yellow shirt.
Consider the following astronomical statements. Classify each statement as either an observation, meaning something we have directly observed or measured, or an explanation, meaning something that we infer from observational or experimental evidence even though we cannot observe it directly.
[Observations] The Andromeda Galaxy is similar in structure to our own Milky Way, All planets orbit the Sun in the same direction and nearly the same plane, More distant galaxies are moving away from us faster than nearby galaxies, Stars can shine brightly for billions of years. [Explanations] The universe is expanding because it began with a Big Bang, The Andromeda and Milky Way galaxies formed in the same way, The orderly orbits of planets arose from their formation in a rotating gas cloud, Stars shine with energy generated by nuclear fusion in their cores. It's not always easy for a beginning student of astronomy to tell the difference between a statement describing an observation and a statement describing an explanation. But it's important to learn to distinguish between the two in order to understand how scientific reasoning helps us makes sense of the physical universe. The distinction between an observation and an explanation is very important in science, but keep in mind that it can sometimes be a bit fuzzy. For example, we have classified the statement "Stars can shine brightly for billions of years" as an observation because we have observational techniques (covered later in your textbook) that allow us to measure the ages of stars; in contrast, although the statement "Stars shine with energy generated by nuclear fusion in their cores" is consistent with many observations, our conclusion that the energy source is fusion depends on our understanding of the physics that should occur inside stars. Note also that our ideas of observation and explanation can change with time. For example, the statement "Earth orbits the Sun" is something we now consider an observed fact, but in the past it would have been considered only one potential explanation for what we see in the sky (and an alternate explanation of "everything orbits Earth" was favored for thousands of years).
According to current scientific estimates, when did the Big Bang occur?
about 14 billion years ago. Scientists estimate the age of the universe by measuring the expansion rate, from which they can determine how long it has taken the universe to reach its current size. Current estimates put this age at about 14 billion years.
The Andromeda Galaxy is faintly visible to the naked eye. When you look at the Andromeda Galaxy, the retina of your eye is absorbing light that has traveled through space for ______ to reach you.
about 2 ½ million years. The distance to the Andromeda galaxy is about 2 ½ million light-years, which means light takes about 2 ½ million years to travel from the Andromeda galaxy to us. Therefore, the light that you see from Andromeda traveled through space for 2 ½ million years before being absorbed by your retina.
Suppose that someone in the Andromeda galaxy had a super-telescope through which they were looking at Earth right now. They would see Earth ______.
as it was about 2 ½ million years ago. The approximately 2 ½-million-light-year distance between Earth and the Andromeda galaxy means that light takes about 2 ½ million years to travel this distance. Therefore, just as we see the Andromeda galaxy as it was about 2 ½ million years ago, someone looking right now from Andromeda would see us as we were about 2 ½ million years ago.
Nearly all of the objects that you can see in this photograph are _____________.
galaxies.
On the cosmic calendar, which compresses the history of the universe into a single year, about when did life arise on Earth?
in September. From Part C, Earth formed in early September on the cosmic calendar. Life apparently arose on Earth within less than a billion years after that, which means it was still in September (because each month on the cosmic calendar represents about 1.2 billion years).
On the cosmic calendar, which compresses the history of the universe into a single year, about when did Earth form?
in early September. The cosmic calendar compresses the 14-billion-year history of the universe into 1 year, which means that each month represents about 1.2 billion years. The solar system's age of 4 ½ billion years therefore puts its birth in early September.
On the cosmic calendar, which compresses the history of the universe into a single year, about when did early humans first walk on Earth?
just a few hours before midnight on December 31. The cosmic calendar compresses the 14-billion-year history of the universe into 1 year, which means that each day represents about 1/365 of 14 billion years, or about 40 million years. Early humans arose just a few million years ago, which therefore means just a few hours before the present moment (the stroke of midnight on December 31) on the cosmic calendar.
The age of our solar system is about:
one-third of the age of the universe.
Look once more at the objects you ranked in Parts A and B. This time, rank the objects from left to right based on how much they have aged since they emitted the light we see today, from greatest to least.
star on far side of Andromeda Galaxy, star on near side of Andromeda Galaxy, star on far side of Milky Way Galaxy, star near center of Milky Way Galaxy, Orion Nebula, Alpha Centauri, Pluto, The Sun. As you found in Part B, light from more distant objects takes a longer time to travel to Earth. This means that we see more distant objects as they were longer ago. For example, if an object is 10 light-years away, then we see it as it was 10 years ago, but if it is 20 light-years away, we see it as it was 20 years ago. In other words, more distant objects have aged more since their light left on its way to Earth.
Consider again the objects you ranked by distance in Part A. Suppose each object emitted a burst of light right now. Rank the objects from left to right based on the amount of time it would take this light to reach Earth, from longest time to shortest time.
star on far side of Andromeda Galaxy, star on near side of Andromeda Galaxy, star on far side of Milky Way Galaxy, star near center of Milky Way Galaxy, Orion Nebula, Alpha Centauri, Pluto, The Sun. Notice that light from more distant objects takes a longer time to travel to Earth. Now continue to Part C to be sure you understand the implications for what we actually see in the night sky.
Shown here are astronomical objects located at different distances from Earth. Rank the objects based on their distances from Earth, from farthest to nearest.
star on far side of Andromeda Galaxy, star on near side of Andromeda Galaxy, star on far side of Milky Way Galaxy, star near center of Milky Way Galaxy, Orion Nebula, Alpha Centauri, Pluto, The Sun. Now continue to Part B to explore the light-travel times for the light we see from objects such as these.
Our solar system consists of _________.
the Sun and all the objects that orbit it
Rank the following items that describe distances from longest distance (left) to shortest distance (right). (If two distances are equal, drag the second item on top of the first item.)
the distance from the Milky Way Galaxy to the Andromeda Galaxy, The distance from the Sun to the center of the Milky Way Galaxy, The distance from Earth to Alpha Centauri, One light-year, he distance across our solar system (to Neptune), the average distance from Earth to the Sun <- (the same) -> one astronomical unit (AU).
The total number of stars in the observable universe is roughly equivalent to:
the number of grains of sand on all the beaches on Earth.
If we represent the solar system on a scale that allows us to walk from the Sun to Pluto in a few minutes, then:
the planets would all be marble size or smaller and the nearest stars would be thousands of miles away.
Which of the following best describes what we mean by the universe?
the sum total of all matter and energy.
Based on what you've learned from the raisin cake analogy, which two properties of distant galaxies do astronomers have to measure to show that we live in an expanding universe?
their distances and speeds. The analogy shows that if you lived in an expanding raisin cake, you would see all other raisins moving away from yours, with more distant ones moving faster. This is exactly what we observe for galaxies outside our Local Group, which is why we conclude that we live in an expanding universe.
The star Betelgeuse is about 600 light-years away. If it explodes tonight,
we won't know about it until 600 years from now.
