Astronomy

The nearest spiral galaxy to the Milky Way.

• A relatively small and rocky object that orbits a star;
• asteroids are officially considered part of a category known as "small solat system bodies."

The average distance (semimajor axis) of Earth from the Sun, which is about 150 million km.

The amount by which a planet's axis is tilted with respect to a line perpendicular to the ecliptic plane.

The name given to the event thought to mark the birth of the universe.

A collection of a few dozen or more galaxies bound together by gravity; smaller collections of galaxies are simply called groups.

A relatively small, icy object that orbits a star. Like asteroids, comets are officially considered part of a category known as "small solat system bodies."

An alternative name for the universe.

An object that orbits the Sun and is massive enough for its gravity to have made it nearly round in shape, but for that does not qualify as an official planet because it has not cleared its orbital neighborhood. The dwarf planets of our solat system include the asteroid Ceres and the Kuiper belt objects Pluto, Eris, Haumea, and Makemake.

The Sun's apparent annual pathe among the constellations.

The plane of Earth's orbit around the Sun.

The idea that the space between galaxies or clusters of galaxies is growing with time.

A huge collection of anywhere from a few hundred million to more than a trillion stars, all bound together by gravity.

A collection of a few dozen or more galaxies bound together by gravity; smaller collections of galaxies are simply called groups.

The idea that Earth is the center of the entire universe.

Any system of objects, such as a star system or a galxy, that is held together by gravity.

A few to a few dozen galaxies bound together by gravity.

One of two small, irregular galaxies (the other is the Small Magellanic Cloud) located about 150,000 lightyears away; it probably orbits the Milky Way Galaxy.

The distance that light can travel in 1 year, which is 9.46 trillion km.

The group of about 40 galaxies to which the Milky Way Galaxy belongs.

The portion of the Milky Way Galaxy that is located relatively close (within a few hundred to a couple thousand light-years) to our Sun.

The supercluster of galaxies to which the Local Group belongs.

Used both as the name of our galaxy and to refer to the band of light we see in the sky when we look into the plane of the Milky Way Galaxy.

An alternative name for asteroids.

An object that orbits a planet.

The process in which two (or more) smaller nuclei slam together and make one larger nucleus.

The portion of the entire universe that, at least in principle, can be seen from earth.

The path followed by a celestial body because of gravity; an orbit may be bound ( elliptical) or unbound (parabolic or hyperbolic),

A moderately large object that orbits a star and shines primarily by reflecting light from its star. More precisely, according to a definition approved in 2006, a planet is an object that (1) orbits a star (but itself neither a star nor a moon); (2) is massive enough for its own gravity to give it a nearly round shape; and (3) has cleared the neighborhood around its orbit. Objects that meet the first two criteria but not the third, including Ceres, Pluto, and Eris, are designated dwarf planets.

The orbital motion of one object around another.

The spinning of an object around its axis.

Any object orbiting another object.

• One of two small, irregular
• galaxies (the other is the Large Magellanic Cloud) located about 150,000 light-years away; it probably orbits the Milky Way Galaxy.

An asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet.

A star (sometimes more than one star) and all the objects that orbit it.

A large, glowing ball of gas that generates energy through nuclear fusion in its core. The term star is sometimes applied to objects that are in the process of becoming true stars (e.g., protostars) and to the remains of stars that have died (e.g., neutron stars).

A star (some times more than one star) and all the objects that orbit it.

The largest known structures in the universe, consisting of many clusters of galaxies, groups of galaxies, and individual galaxies.

A type of radio emission that occurs when electrons moving at nearly the speed of light spiral around magnetic field lines.

The sum total of all matter and energy.