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The Solar System

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1) What do we mean by a geocentric universe? Contrast a geocentric view with our modern view of the universe.

Geocentric describes the idea that everything revolved around Earth, compared to modern knowledge that everything revolves around the Sun (our star).

2) Briefly describe the major levels of structure (such as planet, star, galaxy) in the universe.

Planet: (a) Orbits a star, (b) large enough for its own gravity to make it round, (c) has cleared most other objects from its orbital path. Sun: The star of our solar system.
Star: Large, glowing ball of ignited gas that generates heat and light through nuclear fusion in its core. Galaxy: A great island of stars in space, containing a few hundred million or trillion stars held together by gravity, orbiting a common center.

3) What do we mean when we say that the universe is expanding? How does expansion lead to the idea of the Big Bang?

Observations of distant galaxies show that the universe is expanding by an average distance increase between galaxies. We are able to trace back at this rate to determine what we were all one universe and where the Big Bang might have started.

4) What did Carl Sagan mean when he said that we are “star stuff”?

Star stuff refers to the knowledge that all the elements in the universe are created from stars, including ourselves. The bigger the star, the heavier the elements.

5) How fast does light travel? What is a light-year?

Light travels at a speed of 300,000 km/sec. From moon to Earth, it takes about 1 second for light to travel. From the sun to the Earth is takes about 8 minutes. On light-year’s travel = 10 trillion km (6 trillion miles).

6) Explain the statement: The farther away we look in distance, the farther back we look in time. Because light takes so long to travel these long distances, the lights we are seeing are actually trillions of years old.

7) What do we mean by the observable universe? Is it the same thing as the entire universe?

The observable universe includes everything that we can potentially see, anything less than 14 billion light-years from Earth’s position). It is not the same thing as the entire universe, just the portion that we can see.

8) Describe the solar system as it looks on the 1-to-10 billion scale used in the text. How far away are the other stars on this same scale?

The nearest star system to our own, Alpha Centauri, is about 4.4 light-years away. That distance is roughly 4400 km (2700 mi) on the 1-to-10 billion scale, or roughly equivalent to the distance across the U.S.

9) Describe at least one way to put the scale of the Milky Way Galaxy into perspective and at least one way to put the size of the observable universe into perspective.

If you reduce our solar system by a scale factor of 1 billion, the diameter of the Milky Way Galaxy become 100 meter, (a football field), and our microscopic solar system is located on the 20 yard line. If you stood at our position, millions of star systems would lie within the reach of your arms.

10) Use the cosmic calendar to describe how the human race fits into the scale of time.

The entire human civilization falls into just the last half minute on the cosmic calendar, where one month is more than 1 billion years.

11) Define astronomical unit, ecliptic plane, and axis tilt. Explain how each is related to Earth’s rotation and/or orbit.

Astronomical unit: Earth’s average orbital distance, equivalent to about 150 million kilometers or 93 million miles. Ecliptic plane: Earth’s orbital plane, flat path Axis tilt: 23 1/2 degrees perpendicular to the ecliptic plane, points almost exactly to Polaris (current Northern Star)

12) What is the shape of the Milky Way Galaxy? Describe our solar system’s location and motion.

The shape of the Milky Way Galaxy is a rotating, pinwheel-like disk. Our solar system is located in a 230-million-year orbit, about 28,000 light-years from the center of the Galaxy.

13) Distinguish between our galaxy’s disk and halo. Where does the mysterious dark matter seem to reside?

Most of the mass of the galaxy lies outside of the visible disk in what we call the halo. The matter outside is called dark matter because we have not detected any light coming from it.

14) What key observation leads us to conclude that the universe is expanding? Use the raisin cake model to explain how these observations imply expansion.

An expanding raisin cake shows that if someone was living in one of the raisins inside the cake, they could figure out that the cake is expanding by noticing that all the other raisins are moving away, with more distant raisins moving away faster. In the same way, we know that we live in an expanding universe because all galaxies outside our Local Group are moving away from us, more distant ones moving faster.

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