Astronomy Ch. 1

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.

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.

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

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.

Ranking Task 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 & 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.

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 out solar system (to Neptune) - the average distance from Earth to the Sun & one astronomical unit (AU) *Be sure to remember that 1 AU is the average distance from Earth to the Sun of about 150 million kilometers, while 1 light-year is nearly 10 trillion kilometers. In other words, a light-year is a distance more than 60,000 times as large as an astronomical unit.

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 *

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.

Which of the following correctly lists our "cosmic address" from small to large?

Earth, solar system, Milky Way Galaxy, Local Group, Local Supercluster, universe

Let's start with an example from history. Listed below are a series of claims regarding United States President John F. Kennedy (1917-1963). Classify each statement according to whether or not it is falsifiable.

[Falsifiable:could be proven false] - Kennedy was the 35th president of the United States - Kennedy died from a bullet in his brain [Not falsifiable:could not be proven false] - Kennedy's death was the will of God - The murder of John F. Kennedy was an act of evil - If he'd lived, Kennedy would have ended the Vietnam War - Kennedy's murder was orchestrated by an undetectable shadow government of the United States *Note that both of the falsifiable claims in this example happen to be true. The claim about Kennedy being the 35th President is falsifiable because it can be checked against historical records. The claim that Kennedy died from a bullet in his brain is falsifiable because it could have been shown false by the medical examiner. The remaining claims are not falsifiable: Statements that call on any type of supernatural being are by definition out of the realm of science. Similarly, a claim of something being "undetectable" could not be falsified, and a claim about what Kennedy would have done if he had lived is a conjecture that cannot be disproven.

Let's now consider possible scientific claims. Recall that a scientific claim is falsifiable if it could in principle be shown to be false by observations or experiments, even if those observations or experiments have not yet been performed. Classify each claim according to whether or not it is falsifiable.

[Falsifiable:could be proven false] - The chemical content of the universe is mostly hydrogen and helium - Earth is at the center of the solar system - The Sun is at the center of the solar system - The observable universe contains approximately 100 billion galaxies [Not falsifiable:could not be proven false] - We are all playthings in a computer program created by advanced aliens - The laws of nature are magnificent and beautiful - The universe was created by God *Note that falsifiability alone does not make something science. However, scientific models must make predictions that can be tested, and in general we can only test claims or predictions that are falsifiable.

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 *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).