Candidates should be able to: Describe the principal contents of the universe, including stars, galaxies and radiation; Describe the solar system in terms of the Sun, planets, planetary satellites and comets; Describe the formation of a star, such as our Sun, from interstellar dust and gas; Describe the Sun’s probable evolution into a red giant and white dwarf; Describe how a star much more massive than our Sun will evolve into a super red giant and then either a neutron star or black hole; The observable universe is the space around us bounded by the event horizon - the distance to which light can have travelled since the universe originated. The edge of the observable universe at about 46–47 billion light-years away (1ly = 4.5 x 1015m) There are definite total numbers of everything; about 1011 galaxies, 1021 stars, 1078 atoms, 1088 photons Hubble’s Universe A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter. There are probably more than 170 billion galaxies in the observable universe. The majority of galaxies are organized into a hierarchy of associations known as galaxy groups and clusters, which, in turn usually form larger superclusters. Our own Milky way is about 100,000ly across. Our solar system is about 28,000ly from the galactic centre and takes about 230 million years to complete one revolution. • The Sun accounts for 99.8% of the total matter in the solar system. • There are eight planets • There are 5 (so far) dwarf planets • 146 known moons (and more awaiting confirmation) • Asteroids (in a belt between Mars and Jupiter) • Comets (found in the Kuiper belt and Oort cloud) Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "starforming regions", collapse to form stars. If a cloud is massive enough that the gas pressure is insufficient to support it, the cloud will undergo gravitational collapse. Clumps of matter will group together in the nebula until they are gigantic clumps of dust and gas. By the time the clumps reach sun-like sizes, the gas is dense enough that it no longer loses heat to the surrounding nebula. It starts to heat up. At this stage, the clump is called a protostar. From the start of the collapse to this stage, typical time scales are on the order of a few hundred thousand years. As the protostar becomes larger, gravity squeezes it tighter, causing pressure to build and the heat to increase. Then, when the pressure in the center causes the core to reach a temperature of 10,000,000 K , hydrogen fusion is initiated. Now, the protostar has become a star. It shines with its own light. Its solar wind quickly pushes away the rest of the dust and gas in its vicinity. After a low mass star like the Sun exhausts the supply of hydrogen in its core, there is no longer any source of heat to support the core against gravity. Hydrogen burning continues in a shell around the core and the star evolves into a red giant. When the Sun becomes a red giant, its atmosphere will envelope the Earth and our planet will be consumed in a fiery death. Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core. At this stage, the Sun will have an outer envelope extending out towards Jupiter. During this brief phase of its existence, which lasts only a few tens of thousands of years, the Sun will lose mass in a powerful wind. Eventually, the Sun will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star. Once stars that are 3 times or more massive than our Sun reach the red giant phase, their core temperature increases as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull carbon atoms together as the temperature increases and additional fusion processes proceed, forming oxygen, nitrogen, and eventually iron. These hefty stars (above) reside in one of the most massive star clusters in the Milky Way Galaxy. The “crown jewels” are 14 massive stars on the verge of exploding as supernovae. Each red supergiant is about 20 times the Sun's mass. The Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory joined forces to probe the expanding remains of “Kepler's supernova remnant”, first seen 400 years ago by sky watchers, including famous astronomer Johannes Kepler. The combined image unveils a bubble-shaped shroud of gas and dust that is 14 light-years wide and expanding at 2,000 km/s. Observations highlight a fast-moving shell of iron-rich material from the exploded star, surrounded by an expanding shock wave that is sweeping up interstellar gas and dust. Each colour in this image represents a different region of the electromagnetic spectrum, from X-rays to infrared light. When the core contains essentially just iron, fusion in the core ceases, since iron is the most stable of all the elements. Creating heavier elements through fusing of iron thus requires an input of energy rather than the release of energy. Since energy is no longer being radiated from the core, in less than a second, the star begins the final phase of gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity, and the core recoils out from the heart of the star in a shock wave, which we see as a supernova explosion. As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. While many of the more common elements are made through nuclear fusion in the cores of stars, it takes the unstable conditions of the supernova explosion to form many of the heavier elements. The shock wave propels this material out into space. The material that is exploded away from the star is now known as a supernova remnant. The hot material, the radioactive isotopes, as well as the leftover core of the exploded star, produce X-rays and gamma-rays. February 19, 2004: The brightest supernova explosion ever seen since the one observed by Johannes Kepler 400 years ago. SN 1987A, the titanic explosion blazed with the power of 100,000,000 suns for several months following its discovery on Feb. 23, 1987. This image, taken Nov. 28, 2003 by the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope, shows many bright spots along a ring of gas, like pearls on a necklace. These are being produced as a supersonic shock wave unleashed during the explosion slams into the ring at more than a million miles per hour. The collision is heating the gas ring, causing its innermost regions to glow. If the remnant of the explosion is 1.4 to about 3 times as massive as our Sun, it will become a neutron star. In Chandra's image (above), the colours of red, green, and blue are mapped to low, medium, and high-energy X-rays. At the centre, the bright blue dot is likely the neutron star that astronomers believe formed when the star exploded. Neutron stars are extremely dense remnants of exploded stars consisting of tightly packed neutrons. These stars end their lives in a spectacular explosion called a supernova. The outer layers of the star are hurtled out into space at thousands of miles an hour, leaving a debris field of gas and dust. Where the star once was located, a small, collapsed, incredibly dense object, a neutron star, is often found. While only 10 miles or so across, the tightly packed neutrons in such a star contain more mass than the entire Sun. The result of the final implosion is an unimaginably compacted core: atoms would be crushed together with their electrons squeezed into the nucleus, forming neutrons and a neutron star, with a core so dense that a single spoonful would weigh 200 billion pounds. The core of a massive star that has more than roughly 3 times the mass of our Sun after the explosion will do something quite different. The force of gravity overcomes the nuclear forces which keep protons and neutrons from combining. The core is thus swallowed by its own gravity. It has now become a black hole which readily attracts any matter and energy that comes near it. This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped disk of dust fuelling a massive black hole in the centre of galaxy, NGC 4261, located 100 million light-years away in the direction of the constellation Virgo. By measuring the speed of gas swirling around the black hole, astronomers calculate that the object at the centre of the disk is 1.2 billion times the mass of our Sun, yet concentrated into a region of space not much larger than our solar system.