A protostar forming in the Orion Nebula. This also has protoplanetary discs, and will probably become a planetary system.

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1 A protostar forming in the Orion Nebula. This also has protoplanetary discs, and will probably become a planetary system. These are also called stellar nurseries. Consisting almost entirely of hydrogen, they are enormous, and can be light years across. Particles in the cloud get pulled together by their own gravitational attraction. The clouds merge together. They become more and more concentrated to form a protostar. Objects may also form that are too small to become stars, and these become planets. Sun facts: Our Sun was a protostar for about 100,000 years. The Rosette Nebula is not the only cosmic cloud of gas and dust to evoke the imagery of flowers -- but it is the most famous. At the edge of a large molecular cloud in Monoceros, some 5,000 light years away, the petals of this rose are actually a stellar nursery whose lovely, symmetric shape is sculpted by the winds and radiation from its central cluster of hot young stars. The stars in the energetic cluster, cataloged as NGC 2244, are only a few million years old, while the central cavity in the Rosette Nebula, cataloged as NGC 2237, is about 50 light-years in diameter. The nebula can be seen firsthand with a small telescope toward the constellation of the Unicorn (Monoceros).

2 When the gravity of a protostar pulls all the gas close enough together, the pressure is large enough that the hydrogen undergoes nuclear fusion, and turns into helium. When nuclear fusion of hydrogen begins, the star is born. Sun facts: The Sun is turning 700 million tons of hydrogen into 695 million tons of helium every second. It is this nuclear fusion that generates the Sun s energy, and is why you feel warm in the heat of the Sun. This radiation of energy is also what stops the Sun s (and other star s) gravity from making it collapse, as it pushes outwards, balancing the gravity pulling inwards. This is Proxima Centuari, the closest star to the Sun. It is about 0.8 the mass of the Sun. This is our sun. It is a fairly average star, as stars go, and is about 4.57 billion years old. About 80% of stars have a smaller mass than our Sun, and about 10% have a mass roughly equal to our Sun.

3 When a star s hydrogen in its core runs out (as it has all become helium), the gravity crushes the core more (because helium is heavier than hydrogen). This extra gravitational force allows helium to undergo nuclear fusion. This releases more energy than hydrogen fusion (and creates elements up to oxygen). This extra energy makes the gas of the sun hotter, and it expands to become a red giant. Galaxies are made up of stars, but are all stars found within galaxies? Using the Hubble Space Telescope, researchers exploring the Virgo Cluster of galaxies have found about 600 red giant stars adrift in intergalactic space. Above is an artist's vision of the sky from a hypothetical planet of such a lonely sun. The night sky on a world orbiting an intergalactic star would be a stark contrast to Earth's - which features a spectacle of stars, all members of our own Milky Way Galaxy. As suggested by the illustration, a setting red sun would leave behind a dark sky flecked only with faint, fuzzy, apparitions of Virgo Cluster galaxies. Possibly ejected from their home galaxies during galaxy-galaxy collisions, these isolated suns may well represent part of a large, previously unseen stellar population, filling the space between Virgo Cluster galaxies. The size of the current Sun compared to its estimated size during its red giant phase in the future

4 Planetary nebula are the ejected outer shell of red giants. They only last a few tens of thousands of years, and aren t actually anything to do with planets. They got their name because, before telescopes became more powerful, they looked like the gas giants of the Solar System (Jupiter, Saturn, Neptune and Uranus). This particular planetary nebula, pictured above and designated Shapley 1 after the famous astronomer Harlow Shapley, has a very apparent annular ring like structure. Although some of these nebulas appear like planets on the sky (hence their name), they actually surround stars far outside our Solar System. This pretty planetary nebula, cataloged as NGC 6369, was discovered by 18th century astronomer William Herschel (who lived in Bath) as he used a telescope to explore the constellation Ophiucus. Round and planet-shaped, the nebula is also relatively faint and has acquired the popular moniker of Little Ghost Nebula. Surprisingly complex details and structures of NGC 6369 are revealed in this tantalizing image composed from Hubble Space Telescope data. The nebula's main ring structure is about a light-year across and the glow from ionized oxygen, hydrogen, and nitrogen atoms are colored blue, green, and red respectively. Over 2,000 light-years away, the Little Ghost Nebula offers a glimpse of the fate of our Sun, which could produce its own planetary nebula about 5 billion years from now.

5 When a star has no more light elements in its core, nuclear fusion stops and no more radiation is released. Due to its own gravity, the star collapses in on itself. As it collapses, it heats up and turns from red to yellow to white. It becomes a white dwarf. This is a hot, dense, white star much smaller in diameter than it was. Eventually, it will fade out, go cold, and become a black dwarf. However, black dwarfs are only theoretical, because they would take longer than the current age of the universe to form. White dwarfs are very dense. For comparison, a white dwarf and Earth are shown next to each other for scale. However, the white dwarf has as much mass as the Sun. It is over 1,000,000 times more dense than water. Diminutive by stellar standards, white dwarf stars are also intensely hot... but they are cooling down as time goes on. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages; of the age of their parent globular cluster; and even the age of our universe.

6 Not all stars are the same size. A star is formed when its core undergoes nuclear fusion of hydrogen into helium. Only 10% of the stars are high mass stars, with masses at least 1.5 times that of the sun. The most massive star known is about times the mass of the sun. The more massive a star, the more quickly it will use up all its fuel, and the shorter its lifespan will be. So far 29 planets have been discovered orbiting high mass stars. Planets outside our Solar System are called exoplanets. Nearly 1,000 exoplanets are known. Pictured is Fomalhaut b, the first photograph of an exoplanet. The star it orbits, Fomalhaut, is 2.1 times the mass of the Sun. The picture was taken by removing the direct light from the star, so that the reflected light from the planet remained. The highlighted star is Eta Carinae, the most massive star known. This whole image takes in a span of 300 light years, and is 7,500 light years away.

7 These are, by volume, the largest stars in the universe. When the star s hydrogen in its core runs out (as it has all become helium), the gravity crushes the core more (because helium is heavier than hydrogen). The gravitational forces are high enough for nuclear fusion to produce elements up to iron the heaviest element that can be produced by nuclear fusion in a star. This produces much more energy than hydrogen fusion, and the star expands into a red supergiant. The Sun compared to VY Canis Majoris, the largest known star. Betelgeuse really is a big star. If placed at the centre of our Solar System it would extend to the orbit of Jupiter. But like all stars except the Sun, Betelgeuse is so distant it usually appears as a single point of light, even in large telescopes. Still, astronomers have resolved the surface of Betelgeuse and reconstructed this image of the red supergiant. The intriguing picture shows two, large, bright, star spots. The spots potentially represent enormous convective cells rising from below the supergiant's surface. They are bright because they're hotter than the rest of the surface, but both spots and surface are cooler than the Sun. Also known as Alpha Orionis, Betelgeuse is about 600 light-years away.

8 These are the most violent explosions in the universe. Not all stars turn into a supernova, only stars whose core is 1.5 or more times as massive as the sun. All the elements in the universe heavier than iron (Fe) are produced in a supernova they are the only events to provide enough energy for nuclear fusion to create the heavier elements. On July 4, 1054 Chinese astronomers recorded a new star in the sky, one so bright it could be seen during the day. It was, in fact, a supernova, and the Crab Nebula above are the remains of this massive explosion. A star goes supernova when it can no longer undergo nuclear fusion in its core. Once a star has stopped fusing elements in its core, it stops producing radiation, which pushes against the force of gravity. Once the fuel of nuclear fusion runs out, the core collapses. Fast. In a thousandth of a second, the core of the star goes from being thousands of miles across, to just a few miles across. The matter falls at 45,000 miles per hour. The core heats up to a billion degrees. The energy and nuclear forces that build up are enormous, and the core explodes. Sun fact: The Sun s entire lifetime is expected to be 12 billion years. A supernova can release as much energy in a few weeks as the Sun produces in its entire 12 billion year life. You can see just how violent a supernova is. The bright light in the bottom left of and is brighter than the centre of the galaxy.

9 When a star has no more light elements in its core, nuclear fusion stops and no more radiation is released. Due to its own gravity, the star collapses in on itself. As it collapses, it heats up and turns from red to yellow to white. It becomes a white dwarf. This is a hot, dense, white star much smaller in diameter than it was. Eventually, it will fade out, go cold, and become a black dwarf. However, black dwarfs are only theoretical, because they would take longer than the current age of the universe to form. White dwarfs are very dense. For comparison, a white dwarf and Earth are shown next to each other for scale. However, the white dwarf has as much mass as the Sun. It is over 1,000,000 times more dense than water. Diminutive by stellar standards, white dwarf stars are also intensely hot... but they are cooling down as time goes on. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages; of the age of their parent globular cluster; and even the age of our universe.

10 Black holes are weird, and the laws of physics as we know them break down. However, we do know that as they form, they spin. This means that matter falling into a black hole doesn t fall in straight, it spirals in. As the matter spirals around the black hole, but before it enters it, it forms what is called an accretion disk. The matter closest to the black hole spirals almost at the speed of light, and this can create massive friction, heat and magnetic fields. If energy and matter isn t too close to the black hole, the accretion disk can send it flying off into space, as in this artist s impression of a black hole. If a star 3 times the mass of the Sun, or more, goes supernova, the gravity is so strong that it will collapse in on itself and create a black hole. The gravity of a black hole is so strong that not even light can escape, hence their name. This also means they can t be seen, but they can be detected by their effects to the surrounding space. A mere 11 million light-years away, Centaurus A is a giant elliptical galaxy - the closest active galaxy to Earth. Centaurus A's central region is a jumble of gas, dust, and stars in optical light, but both radio and x-ray telescopes trace a remarkable jet of high-energy particles streaming from the galaxy's core. The cosmic particle accelerator's power source is a black hole with about 10 million times the mass of the Sun coincident with the x-ray bright spot at the galaxy's center. Blasting out from the active galactic nucleus toward the upper left, the energetic jet extends about 13,000 light-years. A shorter jet extends from the nucleus in the opposite direction. Other x-ray bright spots in the field are binary star systems with neutron stars. Active galaxy Centaurus A is likely the result of a merger with a spiral galaxy some 100 million years ago.