The Language of Astronomy

Asterism

An unofficial group of stars recognised by astronomers or the public, such as the False Cross or Diamond Cross.

Black Hole
A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. It’s formed when a massive star collapses in on itself, squeezing a ginormous amount of star stuff a tiny space, resulting in an extremely high concentration of mass. This high density creates a gravitational well so deep that escape is impossible. Many galaxies (including ours) have supermassive black holes at their centers. 

Bolide

A large meteor which explodes in the atmosphere.

Comet
A comet is a “dirty snowball” of ice and rocky debris, typically a few kilometres across, that orbits the Sun in a long ellipse. When close to the Sun, the warmth evaporates the ice in the nucleus to form a coma (cloud of gas) and a tail. Named for their discoverers, comets sometimes make return visits after as little as a few years or as long as tens of thousands of years.

Bright Nebula

An interstellar (located among the stars) cloud of gas and dust where stars are born or have died. There are some superb star-birth clouds visible in binoculars: The Great Orion Nebula, Eta Carinae Nebula and the Tarantula Nebula in the Large Magellanic Cloud.

 

Culmination 

The moment at which a celestial object lies on an observer’s meridian (the north–south line in the sky). In other words, directly overhead.

 

Dark Nebulae 

Clouds of gas and dust in space that are dense enough to obscure and block light from background stars. They’re often locations where material is coalescing to form new stars. The Coal Sack in the Southern Cross is a superb example.

 

Double Stars (Binary stars) 

A pair of stars that are gravitationally bound to each other, and orbiting a common center of mass called the barycenter. These stars are not just visually close in the sky, but are physically and gravitationally connected, influencing each other’s movement and evolution.

 

Fireball

A meteor brighter than Venus.

 

Galaxy 

A stupendously large collection of gas, dust, and billions of stars and their solar systems, all held together by gravity and isolated from similar systems by vast regions of space. We can see 3 fabulous naked eye galaxies in the Southern hemisphere – the Large Magellanic Cloud, the Small Magellanic Cloud and Andromeda (and they are superb in a pair of binoculars).

 

Globular clusters 

Globular clusters (GC) are densely packed collections of ancient stars. Roughly spherical in shape, they contain hundreds of thousands to millions of stars. They are very old – typically 12-13 billion years old which makes them almost as old as the universe itself (the universe is 13.8 billion years old). We have the two most gorgeous GCs to observe in binos – Omega Centauri (the largest GC in our sky, 10 million stars) and 47 Tuc (around 500,000 stars).

 

Light-year  

A light-year is a measurement of distance and not time (as the name might imply). A light-year is the distance a beam of light travels in a single year, which equates to approximately 9.7 trillion kilometers. Betelgeuse, for example, is 642 light-years away. It may have gone supernova yesterday… but we won’t see it until the year 2667!

 

Magnitude (apparent) 

Apparent magnitude describes how bright an object appears in the sky from Earth. An object’s apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object’s light caused by interstellar dust along the line of sight. The scale seems counter-intuitive as the brighter an object appears, the lower its magnitude. Thus, Sirius (the brightest star in the sky has an apparent magnitude of -1.46, while 13th brightest star, alpha Crucis (Acrux in the Southern Cross) has an apparent magnitude of 1.3.

 

Meteors 

Meteoroids are objects in space that range in size from dust grains to small asteroids. When a meteoroid enters Earth’s atmosphere, it is traveling at very high velocity – more than 11 km per second at minimum. Frictional heating, produced by the meteoroid’s energetic collision with atmospheric atoms and molecules, causes its surface to melt and vaporise and also heats and ionises the air around it. The result is the luminous phenomenon recognized as a meteor. The vast majority of meteoroids that collide with Earth burn up in the upper atmosphere. If a meteoroid survives its fiery plunge through the atmosphere and lands on Earth’s surface, the object is known as a meteorite.

 

Meteor shower 

A meteor shower happens when Earth passes through the path of a comet that orbited the Sun. The bits of comet debris, most no larger than a grain of sand, cause tens to hundreds of meteors each hour. Many of these meteor showers, such as the eta Aquariids, can be predicted and occur at the same time each year.

 

Open clusters 

An open cluster is a group of hundreds to thousands of stars that were formed from the same giant molecular cloud, usually within the last few hundred million years, and are still loosely gravitationally bound to each other. Over time the stars drift away from each other. The Sun must have formed in such a cluster 4.5 billion years ago; but since such clusters only last a few hundred million years at most, the Sun must have been going around the Galaxy on its own for more than 90% of its life.

 

Planetary Nebula 

A planetary nebula is created when a dying low- and intermediate-mass star blows off its outer layers after it has run out of fuel to burn. These outer layers of gas expand into space, forming a nebula which is often the shape of a ring or bubble. The core of the dead star forms an exceedingly hot little white dwarf. They cool so slowly that the Universe isn’t old enough to contain any that have cooled off completely and become a cold cinder known as a black dwarf.