Astro Glossary

Denebola

Denebola is the third-brightest visible star of Leo, the lion. Also known as Beta Leonis, it represents the lion's tail. It is roughly 75 percent more massive than the Sun and about 15 times brighter. It is also much wider than the Sun, and it spins so rapidly that it's wider through the equator than through the poles. Denebola is 36 light-years away and less than one-tenth the Sun's age. Its surface is much hotter than the Sun's so Denebola shines pure white.

Discovery of Planets and Moons

Double Stars

See binary and multi-star systems

Draco, the Dragon

An ancient but faint constellation that winds around Polaris, the North Star. Its brightest star, Thuban, once marked the north celestial pole, and was used as a celestial marker for the construction of the pyramids of Giza.

Dumbbell Nebula, M27

A planetary nebula, which is a glowing cloud of gas and dust expelled by a dying star. The Dumbbell is named for its shape, which resembles the handheld weights. It is 1,360 light-years away, in the constellation Vulpecula.

Dwarf Galaxies

Dwarf Planet

A body in our solar system that is larger than a comet or asteroid but not large enough to qualify as a major planet. This category was created by the International Astronomical Union in 2006. A dwarf planet is large enough that its gravity can pull it into a roughly spherical shape, but not large enough to clear its orbit around the Sun of other objects. As of 2011, there are five dwarf planets in the solar system: Pluto, Ceres, Eris, Makemake, and Haumea (which is not spherical, but is shaped more like an egg or a potato). Ceres is in the asteroid belt, while the others are beyond the orbit of Neptune, the eighth planet from the Sun.

Dwarf Stars

Most stars are classified as dwarfs, including the Sun. These stars lie on the main sequence, the phase of life in which a star steadily converts the hydrogen fuel in its core to helium. When the core hydrogen has been depleted, the star gets larger, cooler, and brighter. It becomes first a subgiant, then a giant. Especially massive stars may become bright giants or supergiants. Despite the "dwarf" moniker, many main-sequence stars are quite large, with some spanning several times the Sun's diameter. The Sun is classified as a G2V star; G2 indicates its surface temperature (around 10,000 degrees Fahrenheit/5,500 C), while the Roman numeral V designates its dwarf status.Some classes of stars that are small have special dwarf classifications. Red dwarfs are only a fraction the size and mass of the Sun, and can be as little as one ten-thousandth the Sun's brightness. Because they are so cool, their surfaces are orange or red. White dwarfs are the dead cores of once-normal stars. They typically are less massive than the Sun, and only about the size of Earth. They are extremely hot and dense, although they no longer produce nuclear reactions.