The Cosmic Landscape
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| About how much bigger in radius is the Sun than the Earth? | The radius of the Sun is 200 times that of the Earth. | |
| How big is an astronomical unit? | The distance from the Earth to the Sun, or about 150 million kilometers. | |
| Roughly how big across is the Milky Way Galaxy? | The Milky Way Galaxy is about 100,000 light-years across. | |
| How is a light-year defined? | A light-year is defined as the distance light travels in empty space in one year, about 10 trillion kilometers. | (revised) |
| What force holds the different astronomical systems described in this section together? What other forces exist in nature? | Gravity holds the astronomical systems together. Other forces include the electromagnetic force, the strong force, and the weak force. | |
| What was the Big Bang? | The Big Bang was the event that, according to many astronomical theories, created the Universe. It occurred about 13.7 billion years ago and generated the expanding motion that we observe today. | |
| What are dark matter and dark energy? | Dark matter is matter that emits no detectable radiation but whose presence can be deduced by its gravitational attraction on other bodies. Dark energy is a form of energy (or a cosmological constant) detected by its effect on the expansion of the Universe. It causes the expansion to speed up. The nature and properties of dark energy are unknown. | (revised) |
| What particles make up an atom? | Electrons, protons, and neutrons. | |
| What force holds the electrons to an atom's nucleus? | The electric force. | (revised) |
| What is meant by the scientific method? | The scientific method is the process of observing a phenomenon, proposing a hypothesis on the basis of the observations, and then testing the hypothesis. | |
| What is the difference between a hypothesis and a theory? | A hypothesis is an explanation proposed to account for some set of observations or facts; a theory is a set of ideas that have achieved wide acceptance by successful testing. | (revised) |
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