Name any four heavenly bodies found in the universe.
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galaxies
comets
asteroids
satellites of planets
comets
asteroids
satellites of planets
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Stars
Stars are huge luminous balls of hot gas that act like the chemical factories of the universe. The closest star to Earth is the sun. Made up mainly of hydrogen and helium atoms, stars are driven by nuclear fusion reactions, which give off the huge amounts of energy that makes them shine. As the temperature of their core rises, to the order of several million degrees celsius, some of their atoms fuse together to form new elements - including the 92 elements that make up our bodies and everything around us.
Planets
 Former planet: Pluto and its moon Charon
You would think that it was pretty easy to define a planet - a particularly large lump of rock orbiting a star. But in recent years the definition of planets has courted controversy, with the reclassification of Pluto from being the ninth planet of our solar system to that of a mere "dwarf planet" (a large asteroid). This is because although Pluto satisfies the requirements of being large enough to be pulled into a sphere by the force of its own gravity, while simultaneously orbiting a star, it fails on the last planetary requirement: it has not cleared the neighbourhood around its orbit - it is not big enough to dominate the other celestial bodies in its path.
Galaxy
Just like planets, moons and satellites, stars orbit in collections around a common centre of mass to form galaxies. The number of stars in a galaxy can range from as few as tens of millions to more than a million million, and these can be anywhere from 20,000 trillion to 20 quintillion (18 zeros) miles across. The term "galaxy" stems from the Greek, meaning milky, hence the name of our own galaxy: the Milky Way.
Red dwarfs
Red dwarfs are small stars that never really managed to get fired up. With masses of about 40% that of our sun, they are relatively cool, with a surface temperature of less than 3,200C, so they have a dimmer, reddish appearance. Red dwarfs collectively make up the vast majority of all stars in the universe. Stars that are smaller and dimmer still are known as brown dwarfs. These generally have a mass of less than 7% of our sun, making them too small to sustain hydrogen-burning fusion reactions at their core.
Depending upon its size, any non-planetary lump of metal or rock that floats in orbit around a star is known as either an asteroid or a meteoroid. The latter is anything from the size of a boulder, while asteroids can be as large as minor or dwarf planets. Most of those that exist in our solar system are within an orbit known as the asteroid belt, between the orbits of Mars and Jupiter. In contrast, comets are loose collections of ice, dust or rock measuring just a few kilometres across that have entered a solar system and orbit the star, often appearing to have a forward-facing "tail" or coma when coming close to the sun.
Supernovae
If stars are the universe's factories then supernovae are the delivery trucks. Depending upon their size, when some stars reach the end of their lives and their nuclear fuel begins to run out, they start to lack sufficient outward forces to counter their own gravity. Because of this they can start to implode. But before they do so completely the atoms at their core are brought so close together that the repulsive forces between them suddenly exceed the gravitational forces pulling them together. In an instant it all recoils, exploding dramatically, spewing out newly formed elements into space in the process.
Black hole
A black hole is what's left behind when a particularly large star dies. The name comes from the fact that these celestial objects have such immense gravitational fields that, within a certain boundary, known as the event horizon, nothing - not even light - can escape their pull. They occur when the energy of a massive star is exhausted leaving no forces to counterbalance gravity, causing them to collapse until, theoretically, the object is so infinitely dense it has no volume. Theoretical because, despite being predicted to exist as far back as 1916 by Einstein, there is still no firm proof that they actually exist. However, the scientific consensus is that they do, and since 1979 there have been many observations of the indirect evidence of their existence.
Solar wind
Space is not a perfect vacuum. Apart from the dust, debris and larger objects floating about there is also solar wind. This is a stream of plasma or charged particles given off by the outer regions of a star, consisting mainly of free electrons and protons. These streams cause aurorae, for example the northern lights, and illuminate the tails, or comas, of comets. Solar wind has even been known to knock out power grids on Earth
Stars are huge luminous balls of hot gas that act like the chemical factories of the universe. The closest star to Earth is the sun. Made up mainly of hydrogen and helium atoms, stars are driven by nuclear fusion reactions, which give off the huge amounts of energy that makes them shine. As the temperature of their core rises, to the order of several million degrees celsius, some of their atoms fuse together to form new elements - including the 92 elements that make up our bodies and everything around us.
Planets
 Former planet: Pluto and its moon Charon
You would think that it was pretty easy to define a planet - a particularly large lump of rock orbiting a star. But in recent years the definition of planets has courted controversy, with the reclassification of Pluto from being the ninth planet of our solar system to that of a mere "dwarf planet" (a large asteroid). This is because although Pluto satisfies the requirements of being large enough to be pulled into a sphere by the force of its own gravity, while simultaneously orbiting a star, it fails on the last planetary requirement: it has not cleared the neighbourhood around its orbit - it is not big enough to dominate the other celestial bodies in its path.
Galaxy
Just like planets, moons and satellites, stars orbit in collections around a common centre of mass to form galaxies. The number of stars in a galaxy can range from as few as tens of millions to more than a million million, and these can be anywhere from 20,000 trillion to 20 quintillion (18 zeros) miles across. The term "galaxy" stems from the Greek, meaning milky, hence the name of our own galaxy: the Milky Way.
Red dwarfs
Red dwarfs are small stars that never really managed to get fired up. With masses of about 40% that of our sun, they are relatively cool, with a surface temperature of less than 3,200C, so they have a dimmer, reddish appearance. Red dwarfs collectively make up the vast majority of all stars in the universe. Stars that are smaller and dimmer still are known as brown dwarfs. These generally have a mass of less than 7% of our sun, making them too small to sustain hydrogen-burning fusion reactions at their core.
Depending upon its size, any non-planetary lump of metal or rock that floats in orbit around a star is known as either an asteroid or a meteoroid. The latter is anything from the size of a boulder, while asteroids can be as large as minor or dwarf planets. Most of those that exist in our solar system are within an orbit known as the asteroid belt, between the orbits of Mars and Jupiter. In contrast, comets are loose collections of ice, dust or rock measuring just a few kilometres across that have entered a solar system and orbit the star, often appearing to have a forward-facing "tail" or coma when coming close to the sun.
Supernovae
If stars are the universe's factories then supernovae are the delivery trucks. Depending upon their size, when some stars reach the end of their lives and their nuclear fuel begins to run out, they start to lack sufficient outward forces to counter their own gravity. Because of this they can start to implode. But before they do so completely the atoms at their core are brought so close together that the repulsive forces between them suddenly exceed the gravitational forces pulling them together. In an instant it all recoils, exploding dramatically, spewing out newly formed elements into space in the process.
Black hole
A black hole is what's left behind when a particularly large star dies. The name comes from the fact that these celestial objects have such immense gravitational fields that, within a certain boundary, known as the event horizon, nothing - not even light - can escape their pull. They occur when the energy of a massive star is exhausted leaving no forces to counterbalance gravity, causing them to collapse until, theoretically, the object is so infinitely dense it has no volume. Theoretical because, despite being predicted to exist as far back as 1916 by Einstein, there is still no firm proof that they actually exist. However, the scientific consensus is that they do, and since 1979 there have been many observations of the indirect evidence of their existence.
Solar wind
Space is not a perfect vacuum. Apart from the dust, debris and larger objects floating about there is also solar wind. This is a stream of plasma or charged particles given off by the outer regions of a star, consisting mainly of free electrons and protons. These streams cause aurorae, for example the northern lights, and illuminate the tails, or comas, of comets. Solar wind has even been known to knock out power grids on Earth
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