astronomy chapter 1

If the 14 billion year history of the universe were compressed to one year, and "now" is exactly midnight December 31, approximately how long ago were your grandparents born?

0.15 second ago This question is using the scale of the cosmic calendar discussed in Section 1.2 of The Cosmic Perspective; if you do the math, you'll find that 0.15 second on the cosmic calendar represents about 67 years of real time.

Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.

1. Our entire solar system orbits around the center of the Milky Way Galaxyabout once every 230 million years. 2. The Milky Way and Andromeda galaxies are among a few dozen galaxies that make up our Local Group. 3. The Sun appears to rise and set in our sky because Earth rotatesonce each day. 4. You are one year older each time Earth orbitsabout the Sun. 5. On average, galaxies are getting farther apart with time, which is why we say our universeis expanding. 6. Our solar systemis moving toward the star Vega at about 70,000 km/hr.

Suppose we wanted to make a scale model of the Local Group of Galaxies, in which the Milky Way Galaxy was the size of a marble (about 1 cmcm in diameter). How far would it be from the Milky Way Galaxy to the most distant galaxies in the observable universe on this scale?

1.4km

Based on observations of the universal expansion, the age of the universe is about __________.

14 billion years In other words, current evidence suggests that the universe was born in the Big Bang about 14 billion years ago.

Suppose we wanted to make a scale model of the Local Group of Galaxies, in which the Milky Way Galaxy was the size of a marble (about 1 cmcm in diameter). How far from the Milky Way Galaxy would the Andromeda Galaxy be on this scale?

25cm

Suppose we wanted to make a scale model of the Local Group of Galaxies, in which the Milky Way Galaxy was the size of a marble (about 1 cmcm in diameter). How far would the Sun be from Alpha Centauri on this scale?

4.4×10−5cm

cluster of galaxies (or group of galaxies)

A collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters.

supercluster

A gigantic region of space in which many groups and clusters of galaxies are packed more closely together than elsewhere in the universe.

galaxy

A great island of stars in space, all held together by gravity and orbiting a common center, with a total mass equivalent to millions, billions, or even trillions of stars.

star

A large, glowing ball of gas that generates heat and light through nuclear fusion in its core. Our Sun is a star.

planet

A moderately large object that orbits a star and shines primarily by reflecting light from its star. According to the current definition, an object can be considered a planet only if it (1) orbits a star, (2) is large enough for its own gravity to make it round, and (3) has cleared most other objects from its orbital path. An object that meets the first two criteria but has not cleared its orbital path, like Pluto, is designated a dwarf planet.

comet

A relatively small and ice-rich object that orbits a star.

asteroid

A relatively small and rocky object that orbits a star.

star system

A star (sometimes more than one star) and any planets and other materials that orbit it.

On a scale where the Sun is about the size of a grapefruit and the Earth is about 15 meters away, how far away are the nearest stars besides the Sun?

About the distance across the United States Using a scale of 1 to 10 billion, on which Earth is about the size of a ballpoint and located about 15 meters from a grapefruit-size Sun, the distance to the nearest stars is more than 4,000 kilometers (about 2,500 miles).

small solar system body

An asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet.

moon (or satellite)

An object that orbits a planet. The term satellite is also used more generally to refer to any object orbiting another object.

The following statements describe ways in which the analogy might apply to the real universe. Which statements are correct?

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 average distance increases with time both between raisins in the cake and between galaxies in the universe. 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.

What do we mean when we say that the universe is expanding?

Average distances between galaxies are increasing with time. Note that while distances between galaxies are increasing (on average), galaxies themselves are not expanding because they are held together by gravity. (Many clusters of galaxies are also held together by gravity and hence not expanding, which is why we say that average distances between galaxies are increasing with time.)

Why do the patterns of the stars in our sky look the same from year to year?

Because the stars in the constellations are so far away. Although nearby stars move relative to us at typical speeds of tens to hundreds of thousands of kilometers per hour, they are so far away that it would take thousands of years for their motion to be noticeable to the eye.

Why do scientists suspect that most of our galaxy's mass consists of dark matter?

Dark matter is invisible to our telescope, because it does not emit light. But we can detect it because of its gravity that affects observable objects. Starts orbit the galactic center significantly faster than they should if we could see most of our galaxy's mass. Large amount of unseen mass is present in our galaxy, mostly residing in the halo of the galaxy.

1 astronomical unit

Earth's average distance from the Sun Jupiter lies about 5.2 of these from the Sun about 150 million kilometers

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.

How do the speeds at which we are moving with Earth's rotation and orbit compare to the speeds of more familiar objects?

Earth's rotation carries most people around the axis faster than a commercial jet travels, and Earth's orbit carries us around the Sun faster than the Space Station orbits Earth

How are galaxies important to our existence?

Galaxies recycle material from one generation of stars to the next, and without this recycling, we could not exist. Except for hydrogen, all the elements of life were produced by stars and scattered into space when the stars died. Without galactic recycling, these elements would have dispersed into the expanding universe rather than being available to build planets and life around later generations of stars.

Relative to the age of the universe, how old is our solar system?

It is about one third the age of the universe. It is about one third the age of the universe.

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 lookback 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 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.

measurements

Mars is about 1.5 AU from the Sun. Jupiter is about 5 AU from the Sun. The star Sirius is about 8 light-years from the Sun. The diameter of the Milky Way Galaxy is about 100,000 light-years. The distance from Earth to the Moon is less than 0.01 AU. Keep these distances in mind as you study astronomy, and it will help you put other distance scales into perspective by comparison.

Using the ideas discussed in your text, in what sense are we "star stuff?"

Nearly every atom from which we are made was once inside of a star. According to present understanding, the early universe contained only the elements hydrogen and helium. All other elements, including carbon, oxygen, and other major ingredients of life, were produced by stars.

Could we see a galaxy that is 20 billion light-years away?

No, because it would be beyond the bounds of our observable universe.

Could we see a galaxy that is 20 billion light-years away? (Assume that we mean a "lookback time" of 20 billion years.)

No, because it would be beyond the bounds of our observable universe. The universe is about 14 billion-years old, so we cannot observe objects more than about 14 billion light-years away.

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 Stars can shine brightly for billions of years. 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. Explanations The universe is expanding because it began with a Big Bang. Stars shine with energy generated by nuclear fusion in their cores. The orderly orbits of planets arose from their formation in a rotating gas cloud. The Andromeda and Milky Way galaxies formed in the same way. 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).

Notice that Earth's axis is shown with an arrowhead in this diagram. What does the arrow point to?

Polaris, the North Star

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.

solar system

The Sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system (solar means "of the Sun"), it is often applied to other star systems as well.

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.

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 galaxy's distance of 13 billion light-years means that its light traveled through space for 13 billion years to reach us. Therefore, the light left on its journey when the universe was only about 1 billion years old - which means the galaxy had already formed by that time.

expansion (of the universe)

The increase in the average distance between galaxies as time progresses.

orbit (or revolution)

The orbital motion of one object around another due to gravity. For example, Earth orbits the Sun once each year.

Suppose we make a scale model of our solar system, with the Sun the size of a grapefruit. Which of the following best describes what the planets would look like?

The planets are all much smaller than the Sun. Four planets are within about 20 meters of the Sun, while the rest planets are spread much farther apart. You can see this idea clearly in the figures that relate to the Voyage model solar system in Section 1.2 of The Cosmic Perspective.

observable universe

The portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the entire universe.

rotation

The spinning of an object around its axis. For example, Earth rotates once each day around its axis, which is an imaginary line connecting the North and South Poles.

Choose the correct explanation why does the statement make sense (or is clearly true) or does not make sense (or is clearly false): NASA will soon launch a spaceship that will photograph our Milky Way Galaxy from beyond its halo.

The statement doesn't make sense. Even if NASA could build a spaceship that traveled close to the speed of light, it would take tens of thousands of years to get into the halo and then the same time for image to get back to Earth.

universe (or cosmos)

The sum total of all matter and energy—that is, all galaxies and everything between them.

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.

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.

When did humans learn that the Earth is not the center of the universe?

Within the past 500 years. It all began with Copernicus, who published his work in 1543.

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.

Which of the following has your "cosmic address" in the correct order?

You, Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe Be sure to look at the relative scales, which are shown in Figure 1.1 of The Cosmic Perspective.

The number of stars in the Milky Way Galaxy is approximately __________.

a few hundred billion We generally say that there are more than 100 billion stars in the Milky Way Galaxy; we do not have a precise count, but the actual number is probably a few hundred billion to a trillion or so.

A light-year is __________.

about 10 trillion kilometers A light-year is the distance that light can travel in one year.

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.

When we look at an object that is 1,000 light-years away we see it _________.

as it was 1,000 light-years ago It takes 1,000 years for light to travel 1,000 light-years, so we see the object as it was 1,000 years ago.

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.

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.

look at 1.2 question 1

Shown here are astronomical objects located at different distances from Earth. Rank the objects based on their distances from Earth, from farthest to nearest.

look on 1.1 question 13 and 14

Consider each of the following statements that you might hear in everyday life. Classify each statement as either an observation or an explanation.

observation james is wearing a yellow shirt. The plastic melted when I held it near the light bulb. The defendant's DNA was found at the crime scene. 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.

On a scale where the distance from Earth to the Sun is about 15 meters, the distance from Earth to the Moon is __________.

small enough to fit within your hand Using a scale of 1 to 10 billion, Earth is about the size of a ballpoint and located about 15 meters from a grapefruit-size Sun, while the Moon (about a fourth the size of Earth) is a mere 4 centimeters (about an inch and a half) from Earth.

An astronomical unit (AU) is __________.

the average distance between Earth and the Sun This distance is about 150 million kilometers. (More precisely, it is 149.6 million kilometers.)

Neither 1 AU nor 1 light-year

the diameter of Earth Mars's average distance from the Sun

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 the distance across our solar system (to Neptune) the average distance from Earth to the Sun=one astronomical unit (AU)

1 light year

the distance light travels in 1 year the star Sirius lies about 8 of these from the Sun about 10 trillion kilometers light-year is defined to be the distance that light can travel in one year, which is about 10 trillion kilometers (more precisely, 9.46 trillion kilometers).

What do astronomers mean by the Big Bang?

the event that marked the beginning of the expansion of the universe Based on careful study of the current expansion of the universe (and of the cosmic microwave background, which we'll discuss later in the text), the Big Bang occurred about 14 billion years ago.

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 are marble-size or smaller and the nearest stars are thousands of miles away.

The total number of stars in the observable universe is about _________.

the same as the number of grains of sand on all the beaches on Earth It's a remarkable fact. There are some 1011 (100 billion) stars in a typical galaxy and 1011 galaxies in the observable universe, making a total of some 1011 ( 1011 = 1022 stars, comparable to the total number of (dry) grains of sand on all beaches on Earth combined.

Which of the following best describes what we mean by the term universe?

the sum total of all matter and energy In other words, the universe consists of all the galaxies and everything within and between them.

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 WayGalaxy our solarsystem the Sun Jupiter Earth

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.

Suppose we look at two distant galaxies: Galaxy 1 is twice as far away as Galaxy 2. In that case _________.

we are seeing Galaxy 1 as it looked at an earlier time in the history of the universe than Galaxy 2 Remember that the farther away we look in distance, the further back we look in time. Because Galaxy 1 is farther away, we are seeing it at a time further into the past, which means earlier in the history of the universe.

What makes dark matter and dark energy so mysterious and so important?

we don't exactly know what they are, and they make up the majority of the total energy content of the universe

The star Betelgeuse is about 600 light-years away. If it explodes tonight,

we won't know about it until about 600 years from now.

If we imagine the history of the universe compressed into one year, with the present as the stroke of midnight at the very end of that year, dinosaurs became extinct _________

yesterday morning Dinosaurs went extinct only about 65 million years ago in real time, which is indeed yesterday morning on a cosmic calendar that compresses the 14-billion-year history of the universe into one year.