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- Data Type
- lets the computer know what type of data to expect, whether they are numbers, or sets of numbers or if they are words.
- Dark Energy
- a hypothesized exotic form of energy that makes up the majority of the universe. This form of energy is causing the expansion of the universe to accelerate.
- Dark Matter
- a hypothesized, exotic form of matter that does not emit nor absorb electromagnetic energy (light) and is, currently, only detectable by the gravitational influence it has on surrounding matter and energy.
- Dark Side
- the part of the Moon the sun is NOT currently shining on.
- Data
- the factual information a scientist collects that is related to the hypothesis she or he is testing. Data can be direct measurements of properties, for example, the height of plants one month after seeds have been planted. Data can also be observations of patterns, for example, the behavior of animals when they encounter a predator. In order to determine whether a particular measurement or observation is a rule rather than an exception, a scientist will often repeat measurements or observations to gather additional data.
- Declination (Dec)
- Used in the equatorial coordinate system to measure the angular position of a celestial object above or below the plane of the celestial equator. Declination is typically measured in degrees, with positive angles to the North and negative angles to the South.
- Degree
- a coordinate on the sky corresponding to latitude on the Earth.
- Density
- a measure of how much matter is packed into a certain amount of space. Denser objects have more material packed into a given space than lower density objects.
- Detector
- a device, or devices, used to detect photons, or in some cases other particles like protons, electrons, etc.
- Deuterium
- an isotope, or a variation, of the hydrogen atom with a neutron in its nucleus. Most hydrogen atoms (~99%) have only a proton in its nucleus. Deuterium, which is about 1% of naturally occurring hydrogen, has a proton and a neutron.
- Diffuse X-ray Emission
- a source of x-ray that is not a point source, but rather generated from a large region or area.
- Digital Image
- a picture created by a digital detector such as a CCD.
- Dipole
- an object (such as a magnet) that has two opposite poles separated by some distance.
- Direct Evidence
- uses measurable phenomena (data) to support a hypothesis. If an archaeologist, say, were to discover a clay jar from an ancient civilization that would be direct evidence that the civilization knew how to make ceramics.
- Doppler Effect
- the apparent change in the wavelength and frequency of sound or light depending on whether the source is moving towards or away from you. The faster you or the source is moving, the more profound the impact. You may have experienced an example of this with sound waves if you have ever heard a higher pitch sound from a car moving towards you, and a lower pitch noise when it is moving away.
- Dwarf Galaxy
- a galaxy that contains somewhere around several billion stars, as opposed to the hundreds of billions of stars found in a galaxy like our own.
- Dwarf Star
- a low-mass main sequence star that is not quite as luminous or hot as more massive stars. Because of their low masses these types of stars can steadily burn (fuse) hydrogen for several billions of years. Our own Sun is a dwarf star.
- Dyson Sphere
- a hypothetical monumental construction where a hollow sphere is constructed around a star to harness all of its radiant energy.
