The Cosmic Landscape
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| Define astronomical unit. | An astronomical unit is a distance unit based on the average distance of the Earth from the Sun. | |
| Define atom. | An atom is a submicroscopic particle consisting of a nucleus and orbiting electrons. It is a the smallest unit of a chemical element. | |
| Define 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. | |
| Define dark energy. | 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 matter are unknown. | |
| Define dark matter. | Dark matter is matter that emits no detectable radiation but whose presence can be deduced by its gravitational attraction on other bodies. | |
| Define electric charge. | Electric charge is the electrical property of objects that causes them to attract or repel one another. A charge may be either positive or negative. | |
| Define electric force. | Electric force is the force generated by electric charges. It is attactive between unlike charges but repulsive between like charges. | |
| Define electron. | An electron is a low-mass, negatively charged subatomic particle. Electrons orbit the atomic nucleus but may at times be torn free. | |
| Define galaxy cluster. | A galaxy cluster is a group of galaxies held together by their mutual gravitational attraction. Example: The Milky Way belongs to the Local Group galaxy cluster. | |
| Define gravity. | Gravity is the force of attraction that is between two bodies and is generated by their masses. | |
| Define light-year. | A light-year is a unit of distance equal to the distance light travels in one year. A light-year is roughly 10^13 km, or about 6 trillion miles. | |
| Define Local Group. | The Local Group is the small cluster or group of galaxies to which the Milky Way belongs. | |
| Define Milky Way Galaxy. | The Milky Way Galaxy is the galaxy to which the Sun belongs. Seen from Earth, the Galaxy is a pale, milky-white band in the night sky. | |
| Define neutrino. | Neutrinos are tiny neutral particles with little or no mass and immense penetrating power. These particles are produced in great numbers by the Sun and other stars as they fuse hydrogen into helium, and also by supernova explosions. | |
| Define neutron. | A neutron is a subatomic particle of nearly the same mass as the proton but with no electric charge. Neutrons and protons compose the nuclei of atoms. | |
| Define nucleus. | The nucleus is the core of an atom around which the electrons orbit. The nucleus has a positive electric charge and constitutes most of an atom's mass. |
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