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Stars • Patterns of stars in the night sky • Looks like spots of light arranged in a particular shape against the dark night sky • Stars in the sky can be found at specific locations within a constellation. • Modern astronomy divides the sky into 88 constellations • The Big Dipper is part of the Ursa Major constellation. • The two front stars of the big dipper point to Polaris the North Star. • Polaris is located at the end of the handle of the little dipper. Part of Ursa Minor • As Earth rotates, Ursa Major, Ursa Minor, and other constellations in the northern sky circle around Polaris • Because of this, they are called circumpolar constellations. • It appears that the constellations complete one full circle in the sky in about 24 hr. as Earth rotates on its axis. • As Earth orbits the Sun, different constellations come into view while others disappear. • Circumpolar constellations are visible all year long. Other constellations are not. • Orion is visible in the winter but can’t be seen in the summer because the daytime side of Earth is facing it. • Some stars appear brighter than others. • When you refer to the brightness of a star, you can refer to its absolute magnitude or apparent magnitude. • Absolute magnitude is a measure of the amount of light it gives off. • Apparent magnitude is a measure of the amount of light received on Earth. • A star that’s dim can appear bright if it’s close to Earth, and a star that is bright may seem dim if it’s far away. • One way is to measure parallax, the apparent shift in the position of an object when viewed from two different positions. • A special unit of measure is needed to record distances in space. • Distances between stars and galaxies are measured in light-years. • Light-Year is the distance that light travels in one year. 178,000 miles per second. • The color of a star indicates its temperature. • Astronomers study the composition of stars by observing their spectra. • Scientists use a spectroscope in a telescope to break the spectrums of stars into their component colors and elements in the star. • Like a fingerprint, the patterns of lines can be used to identify the elements in a star’s atmosphere. Evolution of Stars Classifying Stars • In early 1900’s, Ejner Hertzsprung and Henry Russell made some important observations. • They noticed that, in general, stars with higher temperatures also have brighter absolute magnitudes. • Hertzsprung and Russell developed a graph that shows this relationship called a Hertzsprung-Russell diagram Hertzsprung – Russell Diagram The Main Sequence • On the H-R diagram most stars fit into a diagonal band that runs from the upper left to the lower right of the graph. • These stars are called main sequence stars. • This band contains hot, blue, bright stars in the upper left and cool, red, dim stars in the lower right of the graph. Dwarfs and Giants • About 90% of all stars are main sequence stars. • The 10% that are not are either white dwarfs or red giants. • White dwarfs are hot stars, but not bright. • Giants are usually red in color and cool stars. The largest giants are called super-giants. • Ex: Antares is a super-giant and is 300 times larger than the sun. It is more than 11,000 times as bright as the sun. How do stars shine? • Temperatures in the center of the Sun are high enough to cause hydrogen to fuse to make helium. • This process is called fusion. • Fusion occurs in the core of stars • Normally atoms would repel each other, but the core of stars is so hot that the atoms fuse together. • Temperatures of a star can exceed 15,000,000 K. Atoms move so fast that some of them fuse upon colliding. Evolution of Stars • Stars begin as a large cloud of gas and dust called a Nebula. • Gas and dust exert a gravitational force on each other and the nebula begins to contract. • Gravitational forces cause instability within the nebula. The nebula begins to break apart into smaller and smaller pieces. • Each piece eventually might collapse to form a star. A star is born • As particles in the smaller pieces of nebula move closer together, the temperatures in each piece increases. • When the temperature inside the core reaches of a nebula piece reaches 10 million K, fusion begins. • Energy radiates outward into space and a star is born. Main Sequence to Giant Stars • Fusion causes pressure to increase. This pressure balances the attraction due to gravity and the star becomes a main sequence star. • When hydrogen in the core is depleted, a balance no longer exists between pressure and gravity. • The core contracts causing the core temp. to increase and the outer layers of the star to cool and expand. At this point the star becomes a giant. • Our sun will become a giant in about 5 billion years. White Dwarfs • When a star uses much of it helium, it contracts even more and its outer layers escape into space. • This leaves behind the hot, dense core and the star becomes a white dwarf. • A white dwarf is about the size of Earth. • Eventually, the white dwarf will cool and stop giving off light. Supergiants and Supernovas • Stars that are about 8 times more massive than the sun have a more violent star evolution. • The core heats up to much higher temperatures. Heavier and heavier elements form by fusion and the star expands into a supergiant. • Iron forms in the core. Iron cannot release energy and the star’s core collapses violently and a shock wave occurs. The outer portion of the star explodes producing a supernova. • A supernova can be millions of times brighter than the sun Neutron Stars • If the collapsed core of a supernova is between 1.4 and 3 times as massive as the sun, it will shrink to approximately 20 km in diameter. • Only neutrons can exist in the dense core and the star becomes a neutron star. • The dense material of a neutron star is so heavy that a teaspoonful would weigh about 600 million metric tons in Earth’s gravity. Black Holes • When a supernovas core collapses, the gravity near this mass is so strong that it becomes a black hole and nothing can escape from it, not even light. • Black holes are not like giant vacuums. It has an event horizon, a region inside of which nothing can escape. Anything that crosses the event horizon will be sucked into it. Recycling Matter • Matter from collapsed stars form nebulas where new stars are formed. • Elements from old stars can become parts of new stars. • In fact, your body contains many atoms that were fused in the cores of ancient stars.