An Etymological Dictionary of Astronomy and Astrophysics
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Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307).

→ luminosity; → problem.

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

→ luminosity; → size; → relation.

A measure of the amount of light that a point source radiates in a given direction. It is expressed by the luminous flux per unit leaving the source in the direction per unit of solid angle.

→ luminous; → intensity.

The eleventh of Jupiter's known satellites; it is 36 km across and orbits Jupiter at a distance of about 11,720,000 km with a period of 259 days. It was discovered by S. Nicholson in 1938.

Lysithea was a daughter of Oceanus and one of Zeus' lovers.

A vector quantity measuring the strength and direction of the magnetic field. It is the → magnetic flux per unit area of a magnetic field at right angles to the magnetic force. Magnetic flux density is expressed in → teslas. Also called → magnetic induction.

→ magnetic; → flux; → density.

Strength of a magnetic field at a point, denoted H. The force which could be exerted on unit north magnetic pole situated at that point. Measured in oersteds. Same as → magnetic field strength.

→ magnetic; → intensity.

The mass per unit area of the ring material, integrated through the thickness of the ring. Sometimes called → surface density (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ mass; → density.

The ratio of the mass of a system, expressed in solar masses, to its visual luminosity, expressed in solar luminosities. The Milky Way Galaxy has a mass-luminosity ratio in its inner regions of about 10, whereas a rich cluster of galaxies such as the Coma Cluster has a mass-luminosity ratio of about 200, indicating the presence of a considerable amount of dark matter.

→ mass; → luminosity; → ratio.

A relationship between luminosity and mass for stars that are on the main sequence, specifying how bright a star of a given mass will be. Averaged over the whole main sequence, it has been found that L = M^3.5, where both L and M are in solar units. This means, for example, that if the mass is doubled, the luminosity increases more than 10-fold.

→ mass; → luminosity; → relation.

The density of pure water occurring at 3.98 °C, which is 1.0000 g cm^-3, or 1000 kg m^-3. Water when cooled down contracts normally until the temperature is 3.98 °C, after which it expands. Because the maximum density of water occurs at about 4 °C, water becomes increasingly lighter at 3 °C, 2 °C, 1 °C, and 0 °C (→ freezing point). The density of liquid water at 0 °C is greater than the density of frozen water at the same temperature. Thus water is heavier as a liquid than as a solid, and this is why ice floats on water. When a mass of water cools below 4 °C, the density decreases and allows water to rise to the surface, where freezing occurs. The layer of ice formed on the surface does not sink and it acts as a thermal isolator, thus protecting the biological environment beneath it. This property of water liquid is very unusual; molecules pack more closely than in the crystal structure of ice. The reason is that → hydrogen bonds between liquid water are not stable, they are continuously broken and new bonds are created. In the crystal structure of ice molecules have a fixed pattern creating empty space between molecules.

→ maximum; → density; → water.

Same as → mean place.

→ mean; → position.

In → propositional logic, a → sentence containing at least one → connectives. See also → atomic proposition.

→ atomic; → proposition.

An observationally determined relationship between the → morphological classification of galaxies and the → environments in which they are located. Specifically, the morphology-density relation indicates that early-type galaxies (→ ETG) are preferentially located in high density environments, whereas late-type galaxies (→ LTG) are preferentially found in low density environments. Hence, spiral galaxies are rare in the high densities of clusters and are common in the lower density group environments. Early-type galaxies, on the other hand, are common in clusters and are rarely found in isolation.

→ morphology; → density; → relation.

1) A nebulous form, shape, or mass.
2) The state or condition of being nebulous.
3) A fuzzy celestial object, constituted of gas and dust, generally part of a larger → nebula.

→ nebulous; → -ity.

1) The fact of being necessary or indispensable.
2) Something necessary or indispensable.

→ necessary; → -ity.

A filter having a flat response over the range of wavelengths of interest. Also called neutral filter or gray filter.

→ neutral; → density; → filter.

A large area of the north polar region of Mars which is covered with alternating layers of water ice and dust. → South Polar Layered Deposits.

→ north; → polar; → layer; → deposit.

The density of an atomic nucleus (about 10¹⁴ g/cm³).

→ nuclear; → density.

A change in the → energy level or state of an atomic → nucleus involving a → quantum of energy.

→ nuclear; → transition.

Number of a particular type of object found in each unit volume.

→ number; → density.