Common terms every astrophotographer should know — now sorted alphabetically
An asteroid is a small rocky object orbiting the Sun. Asteroids are much smaller than planets and are sometimes called minor planets or small Solar System bodies. They are leftovers from the formation of the Solar System about 4.6 billion years ago. Most asteroids orbit in the Asteroid Belt between Mars and Jupiter. This region contains millions of asteroids. Asteroids range enormously in size. Some are tiny rocks only a few metres wide, others are hundreds of kilometres across. The largest asteroid is Ceres which is about 940 km in diameter. Asteroids range enormously in size. Some asteroids have orbits that bring them near Earth. These are called Near-Earth Asteroids (NEAs). Astronomers track them carefully to check for future impact risks.
An Astronomical Unit (AU) is a unit of distance used in astronomy. It is defined as the average distance between the Earth and the Sun. One AU equals approximately 1 AU ≈ 149.6 million km or about 93 million miles. Light from the Sun takes about 8.3 minutes to travel 1 AU to Earth. For nearby Solar System distances AU is convenient. For stars and galaxies astronomers usually use light-years or parsecs because AU becomes too small. For example the nearest star Proxima Centauri is about 268,000 AU away or 4.23 light years.
A comet is a small icy object that orbits the Sun. Comets are often described as dirty snowballs or icy dirtballs because they contain ice, dust, rock and frozen gases. Far from the Sun, a comet is just a dark frozen object. As it gets closer sunlight heats the ice, the ice turns directly into gas, dust and gas stream away into space. This creates a glowing cloud called the coma and often a long tail. The tail is made up of tiny dust particles, is often curved and reflects sunlight. The tail always points away from the Sun pushed by solar winds. Short-period comets usually come from the Kuiper Belt beyond Neptune and have an orbital period of less than 200 years. A modern short-period designation looks like P/YYYY Xn name where P/=periodic comet, X=half month of discovery, n=order of discovery in that half month, each month is split into two parts so A=Jan 1-15, B= Jan16-31, C=Feb 1-15 etc. For example P/2023 Q2 (Pan-STARRS) is P/=periodic comet, 2023=year of discovery, Q=second half of August, 2=second comet in that interval Pan-STARRS= survey that found it. The nomenclature for long period comets is C/YYYY Xn name. C/ comets=long period or one time visitors.
A conjunction in astronomy is when two celestial objects appear close together in the sky from our point of view on Earth. It is an apparent alignment, not necessarily a real physical closeness in space. They could be millions or billion of miles apart in reality. Common types of conjunction: 1. Planet-planet conjunction - two planets appear close together eg. Jupiter and Venus side by side in the evening or morning sky. 2. Moon-planet conjunction - moon passes near a planet, I was lucky enough to capture the Moon-Saturn conjunction on 17 Sept 2024. 3. Planet-star conjunction - a planet passes near a bright star. 4. Solar conjunction - two types - superior conjunction for outer planets like Mars, Jupiter or Saturn, the planet is on the far side of the Sun, - inferior conjunction for inner planets like Mercury and Venus, planet passes between Earth and Sun. They are not visible due to the sun glare.
A deep-sky object DSO is any astronomical object outside our Solar System that is not a single star or planet. These are the faint fuzzy things you see in telescopes, such as galaxies, nebulae and star clusters. They are called deep sky because they lie far beyond nearby stars in the night sky. Astronomers organise DSOs using catalogues. Messier Catalogue bright, famous objects; NGC Catalogue thousands of deeper, fainter objects. These help observers find and identify them.
An eclipse is when one celestial object blocks the light of another, causing it to become partially or fully hidden. Unlike a conjunction which is just a close appearance, an eclipse involves a physical alignment that causes shadowing. 1. Solar eclipse - a solar eclipse happens when the Moon moves between the Earth and the Sun. Total eclipse - Sun completely blocked. Partial eclipse - only part of the Sun covered. Annular eclipse - Moon appears too small to fully cover the Sun, leaving a ring of fire. 2. Lunar eclipse - a lunar eclipse happens when the Earth moves between the Sun and the Moon. Earth's shadow falls on the Moon. The Moon darkens and often turns red - blood moon. It is visible from the entire night side of Earth. This can only happen during a full moon. Total lunar eclipse - Moon fully enters Earth's shadow. Partial lunar eclipse - only part of the Moon is darkened. Penumbral eclipse - very subtle shading.
It is the angular distance between a celestial object and the Sun as seen from the Earth. Elongation helps astronomers and observers know when a planet is easiest to see, how long it stays above the horizon after sunset or before sunrise, how far a comet or asteroid appears from the Sun and whether a thin crescent Moon may be observable
A galaxy is a huge collection of stars, gas, dust, planets, nebulae, and dark matter all held together by gravity. They can contain anywhere from millions to trillions of stars. Our galaxy is called the Milky Way and contains about 100-400 billion stars, huge clouds of gas and dust and a supermassive black hole at the center called Sagittarius A. Our galaxy is 100,000 light years across. Galaxies are classified by their shape: 1. Spiral Galaxies - these have rotating arms, our galaxy is one of them. 2. Elliptical galaxies - round or oval shaped and usually contain older stars, little gas or dust and less star formation. Some are giant galaxies containing trillions of stars. 3. Irregular galaxies - no clear shape. Their strange appearance is often caused by collisions or gravitational interactions with other galaxies. Galaxies are not fixed in space — they move and sometimes collide. The Andromeda Galaxy is moving toward the Milky Way and the two are expected to merge in about 4–5 billion years. The Large and Small Magellanic Clouds are both dwarf galaxies and satellite galaxies of the Milky Way.
A globular cluster is a large, tightly packed group of very old stars held together by gravity. They are almost perfectly spherical and can contain tens of thousands to millions of stars in one cluster. Through a telescope, a globular cluster looks like a bright fuzzy ball with stars becoming more crowded toward the centre. The Milky Way has about 150–180 known globular clusters. Globular clusters are among the oldest objects in the universe — many are 12 to 13 billion years old. They mainly contain old, red stars, low mass stars and red giants. They usually have little gas and dust left, so very few new stars form there today. Omega Centauri is the largest globular cluster in our galaxy and is visible to the naked eye from southern skies. The best known in the northern hemisphere is Messier 13, the Great Hercules Cluster.
A star’s magnitude is a way astronomers measure how bright a star appears. There are two main kinds: Apparent magnitude — how bright the star looks from Earth, and Absolute magnitude — how bright the star really is if all stars were placed the same distance away. A smaller number means a brighter object. For example, the Sun is about -26, full Moon about -13, Sirius the brightest star -1.46 and the faintest stars visible under dark conditions is 6. Negative numbers are brighter than positive numbers. A difference of 5 magnitudes equals exactly 100 times in brightness. This means that a magnitude 1 star is about 2.5 times brighter than a magnitude 2 star and a magnitude 1 star is 100 times brighter than a magnitude 6 star.
The Messier objects are a famous catalogue of astronomical objects created by the French astronomer Charles Messier in the 1700s. The catalogue contains nebulae, star clusters and galaxies. There are 110 Messier objects, abbreviated to M1 to M110. Messier created the list because he was hunting for comets but he kept finding faint fuzzy objects that looked like comets but do not move. To avoid confusing them with real comets, he made a list of these permanent objects.
A nebula is a giant cloud of gas, dust, plasma in space. Nebulae are some of the most beautiful objects in astronomy and are often called the "clouds of space." Most nebulae contain hydrogen, helium and tiny dust particles. These materials are the raw ingredients for forming stars and planets. Nebulas can be classified into: 1. Emission nebulae - glow because nearby hot stars energise the gas, often red from glowing hydrogen (Orion Nebula). 2. Reflection nebulae - do not produce their own light, reflect light of nearby stars, often appearing blue (Pleiades). 3. Dark Nebulae - dense clouds of dust that block light from stars behind them (Horsehead Nebula). 4. Planetary nebulae - form when a dying Sun-like star blows off its outer layers (Ring Nebula). 5. Supernova remnants - expanding clouds left after a massive star explodes (Crab Nebula). Nebulae can be enormous — some dozens or even hundreds of light-years across.
The NGC Catalogue stands for the New General Catalogue. It is one of the largest and most important catalogues of deep-sky objects in astronomy. The catalogue contains galaxies, nebulae, star clusters and other deep-sky objects. There are over 7,800 NGC objects. The catalogue was compiled in 1888 by John Louis Emil Dreyer. Later, astronomers added even more objects in supplements called the Index Catalogue (IC). Together, the NGC and IC catalogues contain many thousands of deep-sky objects.
A nova is a sudden brightening of a star caused by a thermonuclear explosion on the surface of a white dwarf star. A nova occurs in a binary star system, where two stars orbit each other closely. One of the stars is a white dwarf, the dense leftover core of a Sun-like star. The white dwarf’s strong gravity pulls hydrogen gas from its companion star. Over time hydrogen builds up on the white dwarf’s surface. The gas becomes extremely hot and compressed. Nuclear fusion suddenly ignites. A huge burst of energy is released. The star rapidly brightens, sometimes by thousands or even millions of times. Unlike a supernova, the white dwarf usually survives and the process can happen again later.
A planet is a large object in space that orbits a star, is rounded by its own gravity and does not produce its own light. Planets shine because they reflect light from their star. The modern definition used by astronomers says a planet must: 1. orbit a star 2. be large enough for gravity to make it round 3. have cleared its orbital neighbourhood of most other objects. This is why Pluto is now classified as a dwarf planet. Some planets have rings like Saturn, Jupiter, Uranus and Neptune. Exoplanets are planets orbiting other stars. Planets form from disks of gas and dust around young stars. Tiny particles collide and gradually build larger bodies over millions of years.
A supernova is a gigantic stellar explosion — one of the most powerful events in the universe. For a short time, a single exploding star can shine brighter than an entire galaxy containing billions of stars. There are two main ways supernovae happen. 1. Massive star supernova - very large stars (more than 8 times the mass of the Sun) explode. A massive star supernova may leave behind a neutron star or, if the star is extremely massive, a black hole. 2. White dwarf supernova (Type Ia) - if a white dwarf pulls too much material from a nearby companion star, it can become unstable. Once it reaches a critical mass, runaway nuclear fusion occurs and the entire star explodes. Unlike massive star supernovae, this type usually leaves nothing behind.
A transit in astronomy is when a smaller object passes directly in front of a larger object, as seen from an observer. It’s similar to an eclipse, but usually the word transit is used when a small object crosses the face of a star or planet and does not fully cover it. Examples are Venus transit and Mercury transit. Again I was fortunate to witness and capture both transits of Venus. The next transit of Venus will occur on 10-11 December 2117 and 8 December 2125.