4Any of the celestial bodies (other than comets or satellites) that revolve around the sun in the solar system (6)
5One celestial body obscures another (7)
6Originally called black dwarfs, these substellar objects were first conceived of in the early 1960s as dark bodies floating freely in space. Stellar models had suggested that a true star must have a m ass at least 80 times that of Jupiter to kindle the stable fusion of hydrogen. Objects with less than 80 Jupiter masses were believed to exist, but it was recognized that they would be extremely difficult to find because they would emit very little light. Two words _ _ _ _ _ / _ _ _ _ _ (5,5)
8Short-lived bursts of gamma-ray photons, the most energetic form of light. At least some of them are associated with a special type of supernovae, the explosions marking the deaths of especially massive stars. Three words _ _ _ _ _ / _ _ _ / _ _ _ _ _ (5,3,5)
12(astronomy) the luminous cloud of particles surrounding the frozen nucleus of a comet; forms as the comet approaches the sun and is warmed (4)
13active galactic nuclei (abbreviation). A class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies. Many astronomers believe supermassive black holes may lie at the center of these galaxies and power their explosive energy output. (3)
14The (usually elliptical) path described by one celestial body in its revolution about another (5)
15The natural satellite of the Earth (4)
16Any celestial body visible (as a point of light) from the Earth at

night (4)
17Any star around which a planetary system evolves (3)
19The closest of Jupiter's moons; has active volcanoes (2)
20A grouping of a number of similar things (7)
21A starlike object that may send out radio waves and other forms of energy; large red shifts imply enormous recession velocities (6)


1A _ _ _ _ _/ _ _ _ _ _ is what stars like our Sun become when they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, such a star expels most of its outer material (creating a planetary nebula), until only the hot core remains, which then settles down to become a very hot (T > 100,000K) young _ _ _ _ _/ _ _ _ _ _ (5,5)
2A diffuse mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation. (7)

3The motion of a spinning body (as a top) in which it wobbles so that the axis of rotation sweeps out a cone. The act of preceding in time or order or rank (10)
7(astronomy) the precise date that is the point of reference for which information (as coordinates of a celestial body) is referred (5)
8The largest of Jupiter's satellites (8)
9Stony or metallic object that is the remains of a meteoroid that has reached the earth's surface (9)
10A system of two stars that revolve around each other under their mutual gravitation. Two words _ _ _ _ _ _ / _ _ _ _ (6,4)
11A constellation on the equator east of Taurus; contains Betelgeuse and Rigel (5)
17The international scientific organisation searching for extra terrestrial life using radio telescopes (4)
18The 4th planet from the sun. (Roman mythology) Roman god of war and agriculture; father of Romulus and Remus; counterpart of Greek Ares (4)

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