Celestial Bodies

A Short Encyclopedia of Celestial Bodies

Look up on a dark, clear night. The sky is scattered with points of light; some dim and others bright. There are stars in a range of colours, and wispy streaks of Milky Way. Do you know everything you are seeing when you look up? And what about the countless, far-out objects lurking in the skies that are invisible your eyes; the myriad extra-galactic objects that fill the universe?

This short encyclopedia to celestial bodies only covers a fraction of the beautiful and interesting heavenly bodies we have discovered so far, but every small bit counts when working toward the infinite.


Asteroids are minor planets, a term sometimes used to describe objects in orbit around the Sun which do not have discs like planets do, and are not active comets. The dwarf planet Ceres was the first asteroid discovered, and was originally classified as a planet. Some scientists also refer to asteroids as planetoids or small Solar System bodies.

These rocky bodies may have formed in several ways, such as early collisions between planets and their moons. Asteroids are mainly composed of rock and minerals, though the composition from asteroid to asteroid can be rather varied and is therefore not well understood. Some are known to have rocky cores well others have metallic cores. The mantles and crusts of the asteroids may also differ.

Asteroids have some interesting characteristics. They are usually irregular in shape and vary greatly in size from only one meter across to over 1000 kilometres in diameter.  As they age, they often become darker and redder. Several are shown to have their own moons – probably smaller bits of the asteroid which have broken off. There even appears to be binary asteroids: asteroids of similar size orbiting around a common centre of mass.

Most asteroids lie in the asteroid belt, a region between the orbits of Mars and Jupiter. The asteroid belt is thinly spread out and largely empty. No spacecraft that have passed the orbits of Mars and Jupiter have collided with asteroids. The only dwarf planet located within the asteroid belt is Ceres. The belt does contain three other large asteroids: 4 Vesta, 2 Pallas, and 10 Hygiea.

Scientists pay special attention to near-earth asteroids. Though smaller asteroids can enter the earth’s atmosphere leaving nothing more than a spectacular show, an impact with near-earth asteroids can be devastating.

See also: Comet, Meteoroid

Black hole

Black holes have a super power: they stretch the realms of space, science and the imagination.

Black holes are regions of space that cannot be detected or observed optically, because they emit no light. Instead, we see them in X-ray and see the effects of their immense gravity. The gravity of a black hole is so strong it can bend space and thus distort time.

To understand just how powerful this is, consider the Earth’s own gravity. To escape the Earth’s gravitational pull, you have to move at just over 11 kilometres per second. This is the escape velocity of Earth. The gravitational pull of a black hole makes it so that not even light, which travels at 300 000 kilometres a second, can reach the required escape velocity. Once a black hole gets hold of light, it is trapped forever. This point of no return is called the event horizon.

There are three broad kinds of black holes: stellar black holes, supermassive black holes, and intermediate black holes.

When a star exhausts its supply of hydrogen, it becomes unstable and the core collapses into a dense ball. For the most massive stars, the collapse and compression are so intense that they become infinitely dense black holes.

The origins of supermassive black holes are not well understood. They may form when several thousand stellar black holes merge into one. They could perhaps be the result of a large star system collapsing simultaneously into one mammoth black hole. These types of black holes are the centres of all galaxies. They can be billions of times more massive than the Sun, but with a radius only a couple of light years across. The supermassive black hole at the Milky Way’s centre is Sagittarius A * (pronounced Sagittarius A – star).

No one can know for sure what happens at the event horizon. Some believe that black holes could be wormholes – portals into other dimensions. There are also theories that the black hole’s force would stretch a human into a long thin piece of agonized spaghetti, while time slowed down this cruel death. Others believe the force would incinerate a person. It is safe to say either way to stay away.

See also: Neutron star, Quasar, Supernova


Comets are small celestial bodies of rock and ice which can put on a spectacular display. A particularly grand sighting of a comet can become so bright it can be seen during daylight, with a tail that arcs across the sky.

Comets and asteroids differ in a number of ways. Both are relatively small rocky bodies, but comets are composed of dust, ice, and frozen gases. Active comets are also distinguished by having an unbound, two part atmosphere: the coma and the tail.

The coma surrounds the nucleus of dust and ice, and is caused by the frozen gases being released as the comet heats up. These gases are pushed into space by stellar wind and pressure, causing the tail. Though the nuclei of comets are small, the coma and tails can be great. Comas can reach 15 times the diameter of the Earth, and tails can stretch as far as Earth is from the Sun (150 million kilometres).

The orbital periods of comets can range from a couple of decades to potentially millions of years. Short period comets originate in the Kuiper belt; a band of comets lying beyond Pluto’s orbit. It is believed that long-period comets come from the Oort cloud, which is a theatrical cloud of myriad comets surrounding the Sun from the far reaches of the Solar System.

Periodic comets leave trails of debris. These bits of debris enter the Earth’s atmosphere when the planet passes through the trail, causing annual meteor showers.

There are over 5000 known comets in the Solar System, but the number of actual comets may be exponentially higher. The Oort cloud is thought to contain billions, if not trillions of comets.

See also: Asteroid, Meteoroid


Galaxies are large systems of stars (and everything which orbits them), interstellar gas and dust, and dark matter; all held together by gravity. Galaxies are some of the oldest things in the universe. The oldest galaxies formed only a few hundred million years after the Big Bang – a fraction of the universe’s 13.8 billion year time line. The estimations on how many galaxies there are in our universe lie between 200 billion and 2 trillion.

Galaxies come in a variety of shapes and sizes. A galaxy can have one of three general shapes: elliptical, spiral and irregular and each can have certain variations. The smallest galaxies are called dwarf galaxies, and may contain only a hundred million or so stars. Giant galaxies on the other hand could potentially contain trillions of stars. A small galaxy could be only a few thousand light years across, where a giant one can span 300 000 light years. The Milky Way is quite large, measuring 100 000 light years in diameter.

The majority of galaxies have supermassive black holes at their centres. Supermassive black holes are extremely dense objects which are millions and even billions of times more massive than the Sun. Everything contained in the galaxy orbits about this centre of mass, and the orbital period is called a galactic year. The Sun’s galactic year is 225 – 250 million years.

Millions of light years separate the galaxies, but that does not mean a quiet existence. Interactions are frequent. Galaxies have close encounters, collide and merge. The Milky Way and its neighbour Andromeda are currently heading straight for one another, and will eventually merge.

See also: Quasar

Messier objects

The Messier objects are 110 varying celestial bodies, 105 of which were published in a list by Charles Messier over a ten year period in the late 1700s.

Messier had a passion for hunting comets. The trouble was that many nebulae and galaxies can have fuzzy appearances similar to those of comets – until taking a closer look. Frustrated, Messier listed these objects to avoid wasting any further time on them!

The list is a standard today, and includes diffuse nebulae, planetary nebulae, open clusters, globular clusters, and galaxies. Each object has an assigned M-number. For example, the Butterfly Cluster in Scorpius is also known as M6.

Objects on the list are some of the closest and brightest deep sky objects from Earth. They are also exceptionally beautiful to view, even through small aperture instruments. It should be noted though that Messier compiled the list based on his observations north of the celestial equator, so it is not necessarily comprehensive.

There is a time during spring when all Messier objects are visible in a single night. Amateur astronomers make an all-night party of this (naturally): a Messier marathon.

See also: Galaxy, Nebula


Meteoroids are small rocky bodies in space, mainly the fragments of asteroids and comets. Some are the debris of violent impacts; matter ejected by a planet or moon when an asteroid or comet hits the surface at great speeds and temperatures. They range in size from tiny grains to just a meter across. Meteoroids fall in one of three main classes: iron, stone, and stony-iron.

Meteoroids that enter the Earth’s atmosphere are known as meteors. Though meteoroids mainly come from the asteroid belt between the orbits of Mars and Jupiter, others are associated with periodic comets. The debris these comets leave behind enters the Earth’s atmosphere in great numbers causing annual meteor showers.

The meteoroid enters the atmosphere at rapid speeds, and the interaction with Earth’s air molecules causes it to heat up and glow. The material the meteor sheds also lights up as it plummets toward Earth.

If a meteor survives the impact without disintegrating, the body is then known as a meteorite. Some meteors are so big they cause an impact crater upon impact. Millions of meteoroids enter the Earth’s atmosphere daily, most as small as a grain of sand.

See also: Asteroid, Comet

Natural satellite

Natural satellites are moons in orbit around planets. Of the eight major planets in the Solar System, only two (Mercury and Venus) lack moons. Earth is the only one among these six planets to have a single permanent moon. Mars has two. The giant planets combined have over a hundred.

The relationship between Earth and our Moon is unique and important in a number of ways. The Moon is just over 1 light second away from the Earth, or 300 000 kilometres away. Its apparent size in our sky is nearly exactly the same as the Sun’s, meaning the Moon’s disc can perfectly cover that of the Sun’s in a total solar eclipse. The Moon’s gravity acts on the Earth’s ocean’s causing tides, and also acts on the Earth’s crust. The satellite’s synchronous rotation means that we always see the same side of the Moon.

Earth also had a temporary Moon for Just over nine months in 2006 – 2007 called 2006 RH 120. These type temporary moons are asteroids with highly eccentric orbits.

There are many lunar features, from extinct volcanoes to evidence of once having an atmosphere and running water. Other natural satellites still have geological activity: volcanic, tectonic, geysers, and weather. Planetary scientists believe that some of these moons may even harbour life.

See also: Asteroid, Planet


Nebulae are huge clouds of interstellar dust and gases (mainly hydrogen and helium). They are vast regions, stretching across hundreds of light years. The three main types of nebulae are diffuse nebulae (HII Regions), planetary nebulae, and supernova remnants.

Diffuse nebulae are extensive and have no well-defined boundaries. This type of nebula is often a birthplace of stars. Diffuse nebulae can be divided into three furthers types of nebulae: emission nebulae, reflection nebulae, and dark nebulae. Emission nebulae contain excited or ionized gases. Reflection nebulae reflect the light of nearby stars, and dark nebulae neither emit nor reflect light, but instead block the light of objects behind them.

A planetary nebula is formed when a dying low to medium massed star ejects its outer shells of gas. Hydrogen fusing at a star’s core creates an outward pressure which keep the star in equilibrium. This state of equilibrium cannot be sustained when a star exhausts its hydrogen, and the star expands its gases into space. The core of such a star can rise to temperatures of 100 million Kelvin. The ultraviolet radiation emitted from the core ionizes the gas, creating a beautiful planetary nebula. Planetary nebula are short lived; dissipating within a few thousand years.

Supernova remnants are caused by the violent expulsion of gases from a massive star exploding.

They are not very bright objects. If you moved a nebula closer to the Earth it would appear bigger but not much brighter. One of the brightest nebulae from Earth is the Orion Nebula, which can be seen with the naked eye. Still, nebulae can be very beautiful to view with the right setup.

See also: Supernova

Neutron star

The extremely dense cores of some collapsed massive stars are called neutron stars. The mass of a neutron star can be 10 – 29 times that of the Sun, but occupying a space no bigger than a city. A teaspoon of neutron star material would weigh over a billion tonnes.

Neutron stars are the remains of massive stars which have gone supernova. The core of such a massive star cannot support its own weight, crushing all its atomic matter into one dense ball. The core of a neutron star is composed entirely out of neutrons. When a star collapses even further than this state it becomes the densest object known to science – a black hole.

This kind of intense environment is naturally very hot. The surface temperature of a neutron star can reach 600 000 Kelvin. Their magnetic fields are hundreds of millions of times that of Earth’s, and they have gravity 200 billion times stronger than gravity on our planet.

They rotate at incredibly high speeds – one of the fastest spinning neutron stars has a rotation of 716 times per second. A neutron star which emits beams of electromagnetic radiation as it rotates is called a pulsar, and the light house effect of these spinning bursts of energy makes them easily detectable.

It is believed that there may be around 100 million neutron stars in the Milky Way; but many do not emit beams of visible radiation, and most are old, cold and undetectable.

There is evidence of binary neutron star systems and of neutron stars hosting exo-planets.

See also: Black hole, Supernova


Planets are celestial bodies in orbit around stars and which meet several criteria. A planet must be massive enough to be rounded by its own gravity but not so massive that nuclear reactions take place in its core. The planet must also be gravitationally dominant in its orbital zone.

The Solar System has eight major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Each is very distinct geologically; having mountains, valleys, volcanoes, polar ice caps, a variety of weather and much more. The Solar system’s planets can be divided into small rocky planets (the first four planets), followed by large gas planets (Jupiter and Saturn are known as gas giants and Uranus and Neptune are ice giants, which are still gaseous but freezing.)

The solar system also contains dwarf planets. Though there are over a hundred known objects in the Solar System which may eventually be classed as dwarf planets, and though the true number of these planets may be thousands, there are only five officially recognized dwarf planets. They are Ceres, Pluto, Haumea, Makemake and Eris. Ceres is located in the asteroid belt while all others lie outside the orbit of Neptune.

Our Sun is not unique in having planets. Studies estimate that there is at least one planet per star in our galaxy. There are also 3797 confirmed exo-planets. Exo-planets can be tricky to detect because of their small size, great distance, and the fact that a planet does not emit its own light. Most scientists agree that there is a possibility of these planets harbouring organic life, and possibly even intelligent life.

See also: Natural Satellite


Space is not dull, and supernovae events may be the greatest evidence of this! Supernova events can happen in several ways. A very massive star becomes increasingly unstable once its supply of hydrogen runs out. One of the consequences of such a volatile star is a high energy explosion that tears the star’s outer layers apart. Some supernova events are so intense that the entire star – core and all – is blown to smithereens. Other supernova leave behind neutron stars or black holes.

Another type of supernova occurs with white dwarves in a binary star system. White dwarves are the remaining cores of low and medium massed stars which have lost their gases at the end of their lives. The dwarf’s companion star can shed its own matter onto the white dwarf until it becomes overwhelmed and detonates.

So much energy is released that a star which goes supernova can outshine its entire galaxy. The shockwave of a supernova travels through space at 10% the speed of light, bathing any nearby objects in deadly radiation. Scientists keep a close eye on nearby stars which are candidates for supernova.

On the other side of a supernova’s deadly effects is its potential to create. The star’s ejected gases and metals racing through space fill interstellar medium with seeds crucial to stellar evolution.

Only three naked eye supernovae have been recorded in the Milky Way in the last 1000 years, but many more have been observed throughout the galaxy using telescopes and advanced technology.

See also: Black hole, Neutron star


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