H

Neutral and ionized hydrogen regions located in interstellar space. H I regions are vast, cold clouds of hydrogen that line the spiral arms of our own galaxy. Astronomers detect dim but distinct H I radiation at the "21-cm line" with radio telescopes. This is the wavelength of energy radiated during a hydrogen quantum spin flip. H II regions are visible as a part of nebulae, where hot young stars ionize their nearby hydrogen womb. As hydrogen recombines, it radiates at several specific wavelengths as electrons descend through atomic energy levels. From the 3rd to 2nd energy level, an electron radiates a photon (light) at 656 nanometers (red light) giving such H II regions a red glow. Astronomers use the luminosity of H II regions to gauge the rate of star formation within a nebula. Similarly, astronomers gauge the mass and structure of our galaxy and other galaxies with the luminosity of H I regions.

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The outermost extent of our Galaxy or another, containing a sparse distribution of stars and globular clusters in a more or less spherical distribution.

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A theory first proposed by British physicist Stephen Hawking, that due to a combination of properties of quantum mechanics and gravity, under certain conditions black holes can seem to emit radiation.

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The temperature inferred for a black hole based on the Hawking radiation detected from it.

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Helio-

Prefix referring to the Sun.

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Sun-centered solar system theories.

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The point at which the solar wind meets the interstellar medium or solar wind from other stars.

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The point at which the solar wind meets the interstellar medium or solar wind from other stars.

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Second lightest atom after hydrogen. Two electrons fly around two protons and two neutrons (atomic nucleus). Atomic number (number of protons in atomic nucleus) is 2. Atomic weight (sum of number of protons and neutron in atomic nucleus, index of weight) is 4. Two helium atoms stick together to form non-odorous colorless gas.

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The "throat" of a black hole in one universe connecting up with one in a different universe; in theory, it is a bridge from one universe to another.

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The derived SI unit of frequency, defined as a frequency of 1 cycle per second.

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A plot of stellar luminosity versus effective temperature that reveals an orderly pattern of stellar evolution. The HR Diagram provides astronomers the stellar "big picture," a handful of conceptual hooks on which to hang billions of stars. The most prominent feature is the main sequence where stars spend the majority of their luminous life. This band represents a stable temperature-luminosity range where the lifetime of the star is dependent on its mass. Another group stretching horizontally off the main sequence is the giant branch: A zone populated with stars whose outer atmosphere has expanded close to the orbital radius of Mercury. Supergiants live above the giants on the H-R diagram. These are stars of enormous size. Betelgeuse, an orange supergiant in Orion's left shoulder, would swallow the Earth if it were our solar system's star. Finally another important group of stars called white dwarfs reside in the low-luminosity, high-temperature corner of the H-R diagram. These stellar remnants glow with the heat generated by collapsing from the size of the Sun down to Earth size.

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1: The point at which an object entering a black hole cannot return; also the edge of a black hole.

2: The apparent junction of earth and sky.

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Hubble Space Telescope.

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A classification of galaxies based on their visual appearance in a telescope. Hubble introduced a "tuning fork" diagram of galaxy types in a 1936 paper called "The Realm of the Nebulae" that contains the following galaxy types:

Spiral

Pinwheel-shaped galaxies with a bright compact center, or bulge, surrounded by graceful spiral arms. Our galaxy is considered a spiral, as is the Andromeda galaxy.

Barred Spiral

Contains two sickle shaped arms extending from a central bulge.

Elliptical

Smooth football-shaped galaxies with little gas and dust between stars. These look like glowing cotton balls in a telescope. Most of the Local Group galaxies are elliptical.

Lenticular

Similar to spirals, but without any spiral structure.

Irregular

Galactic blobs without a shape that fits neatly into Hubble's tuning fork diagram of galaxies. These could represent casualties of galactic collisions. Two nearby examples are the Large and Small Magellan Clouds.

De Vaucouleurs' Extension

Dr. Gerald de Vaucouleurs suggested expanding the classification of spiral galaxies to incorporate a greater variety of spirals than Hubble allows in his tuning fork diagram. He also organized galaxy types in a three dimensional scheme that formed a continuous boundary for all galactic types, instead of the two dimensional tuning fork form. This three dimensional shape looks like a lemon, with elliptic and irregular ends and spirals in the center.

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This is the proportional constant (H) used in Hubble's law. 1/H has the unit of time, and describes the age of universe in the theory of expanding Universe. It is the most important constant in cosmology, and its value affects the size of universe and age. But because of the difficulties in determining distance to far galaxies, there is a large error. The biggest theme in present cosmology is how much you can make the error smaller. The original aim of Hubble Space Telescope is, to decrease the error of far galaxy's distance calculation, and to increase the accuracy of this constant. The value of 1/H that we know now is about 15 plus minus 5 billion years.

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A relationship between a galaxy's distance from us and its velocity through space. The farther away a galaxy is from us, the faster it is receding from us. The constant of proportionality is the Hubble constant, H_o, named after Edwin P. Hubble who discovered the relationship. Hubble's Law is interpreted as evidence that the Universe is expanding.

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The lightest atom in the matter. One electron is flying around 1 proton (atomic nucleus). Atomic number (number of protons in atomic nucleus) is 1. Atomic weight (sum of protons and neutrons in an atomic nucleus, index of weight) is 1. When two hydrogen atoms stick together, it exists as non-odorous, colorless gas.

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A balance between the weights of various layers, as in a star or the Earth's atmosphere, and the pressures that support them.

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I

Used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system.

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First, let's think of a condition of space where "temperature is high". If temperature is high, this means the space's thermal energy is big. And this amount of thermal energy is nothing other than the vigorousness of active molecules. Molecules are always moving, and more vigorous the action, the higher the temperature of that space.
Next, let's think about an enclosed system (a space where no exchange of matter or energy takes place). Density is how much matter is stuffed in a certain space. If density is high, it means a lot of matter is stuffed in that space. The higher the density, matter tends to crash into each other more often. Same with active molecules, if there are a lot of molecules around and crash vigorously into each other, the movement naturally becomes radical. Thus temperature increases.

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In astronomy, a picture of the sky.

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Time measured using imaginary numbers (i.e. the square root of negative one).

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A violent inward collapse. An inward explosion.

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The inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.

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The ratio of the speed of light in a vacuum to the speed of light in a medium under consideration.

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A to the power of "a". It means you multiply A by itself "a" times.
E.g.) 10 to the fourth power = 10 * 10 * 10 * 10 = 10000 (10 thousand) If "a" is a negative value, you divide 1 by A multiplied absolute value of "a" times (e.g. 10 to the fourth power = 1 / (10 * 10 * 10 * 10) = 1/10000).

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The planets Mercury and Venus are called inferior planets because their orbits are closer to the Sun than is Earth's orbit. (The other planets are called superior planets.)

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1: A subject to no limitation or external determination.

2: Extending indefinitely; endless.

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A period of rapid expansion occurring within less than a second of the Big Bang.

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A theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion during the first 10^-30 s. After this period of rapid expansion, the Big Bang and inflationary models are identical.

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Lying outside the visible spectrum at its red end - used of thermal radiation of wavelengths longer than those of visible light.

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Patches of interstellar dust, which emit infrared radiation and look like cirrus clouds on the images of the sky produced by the Infrared Astronomy Satellite.

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Between galaxies.

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The magnetic field carried with the solar wind.

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Among or between the stars.

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Pockets of micron-size grains of carbon, iron, and iron-magnesium silicates scattered at varying densities between the stars in our galaxy and other galaxies.

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Composed mainly of hydrogen and helium, in proportions similar to the Sun interstellar gas is scattered at varying densities between the stars in our galaxy and other galaxies. Interstellar gas supplies the raw material for star formation. Carbon monoxide and hydroxyl molecules (CO and OH) have also been detected within interstellar gas, along with highly ionized oxygen, nitrogen, carbon and silicon. These are likely remnants of past supernovae.

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Regions of space between stars populated by gas and dust. Astronomers presently estimate that the interstellar medium accounts for 10 percent of galactic mass.

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Atom with a partial electric charge. Positive ions with plus charge, and negative ions with minus charge exist. According to characteristics of atoms, some are likely to become an ion while some are not.

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Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose electrons.

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A region of charged particles in a planet's upper atmosphere; the part of the earth's atmosphere beginning at an altitude of about 400 kilometers (25 miles) and extending outward 400 kilometers (250 miles) or more.

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A galaxy without rotational symmetry; neither a spiral nor an elliptical galaxy.

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J

Beams of particles, usually coming from an active galactic nucleus or a pulsar. Unlike a jet airplane, when the stream of gas is in one direction, astrophysical jets come in pairs with each jet aiming in opposite directions.

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The metric unit of energy; the work done by a force of 1 Newton (N) acting through a distance of 1 m.

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Any of the four outer, gaseous planets: Jupiter, Saturn, Uranus, and Neptune.

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