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Stars & Constellations Astronomy 2 What is a Star? Star: A body of gas that gives off a tremendous amount of radiant energy in the form of light and heat Stars close to home… The nearest star to Earth is our Sun It takes approximately 8 minutes for the light from the sun to reach Earth The next nearest star is called Proxima Centauri It takes approximately 4.2 years for the light to reach Earth (traveling at light speed) The sun and Proxima Centuri are only two of the billions of stars in the Milky Way galaxy What is Light? A form of electromagnetic radiation which is energy that travels in waves Electromagnetic radiation is arranged in a continuum called the electromagnetic spectrum Light and the Universe Light Year—used as a measurement of the great distances in space A light year is the distance that a ray of light can travel in a year, or: A light year is 5,865,696,000,000 miles (9,460,800,000,000 kilometers). A star or a star system’s distance from earth is measured in light years. Parallax A change in an object’s direction due to a change in the observer’s position. Astronomers also use parallax to calculate the distance to the stars. Parsec—short for “parallax second”— is a special unit of distance for a star’s distance from earth. > 1 parsec = 3.258 light years Why Do Stars Twinkle?? The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation) Stars (except for the Sun) appear as tiny dots in the sky As their light travels through the many layers of the Earth's atmosphere, the light of the star is bent (refracted) many times and in random directions This random refraction results in the star winking out (it looks as though the star moves a bit, and our eye interprets this as twinkling) Characteristics of Stars Size: anywhere from 20km to 1 trillion km in diameter Mass: the amount of matter Color: stars can be red, blue, white, orange or yellow Composition: contain different elements determined using spectra Characteristics of Stars Temperature: Blue is the hottest (35000°C) Red is the coolest (3000°C) Yellow (our sun) (5500°C) Characteristics of Stars Absolute Magnitude—A measure of how bright a star would be if all Stars were at the same distance— ten parsecs—from Earth • Apparent Magnitude—A measure of how bright a star appears to be on earth • • Does not measure how bright a star actually is Luminosity—the actual brightness of a star Depends on size and temperature The lower the number, higher the brightness Magnitudes -26.5 -30 We can not see dimmer than +6 with naked eye -2 -20 -10 3 6 0 10 Brighter 20 30 Dimmer Our Sun Sirius Brightest in the sky from Earth Polaris/North Star Apparent Magnitude v. Luminosity Star 1 Star 2 High luminosity Low luminosity Far from earth Close to earth = lower apparent magnitude = higher apparent magnitude Hertzsprung-Russel Diagram Relationship between the absolute magnitude (luminosity) and the temperature of stars. Spectral Class—designates the color of stars >depends on temperature >Hottest to the coolest >Blue to White to yellow to orange to Red Kinds of Stars Super giants- largest of all stars, very luminous Red giants- cooler, large, very luminous Dwarf stars- less luminous, red, orange or yellow White dwarf- very faint, small and dense Variable Stars- vary in brightness over regular periods or cycles Kinds of Stars 2 types of variable stars: A. pulsating change in brightness as they contract (brighter)expand (dimmer) Pulsars star that releases light and radio waves in pulses may be the neutron star formed in a supernova it looks like it pulses because it is rotating, like a searchlight Kinds of Stars 2 types of variable stars: B. non-pulsating Eclipsing Binary 2 stars of unequal brightness that revolve around each other brightness depends on which one is in front of the other Life Stages of Stars & Constellations Astronomy 2 Nebulae Seen only in infrared Huge clouds of dust (1%) & gas (99%) This is where most stars are born. Stellar nursery is a nebula ( a large cloud of hydrogen gas in space) in which star formation is occurring Life Cycle of a Star A star begins it’s life in a cloud of cold gas and tiny-grained dust called a NEBULA. Parts of the Nebula begin to condense due to some outside force—a shockwave—which acts upon it. Due to gravity, the gas and dust pockets continue to condense and their temperature increases. Eventually, parts of the Nebula begins to glow—PROTOSTARS are formed. Gravitational contraction of the Protostars continues causing them to become hotter and brighter. Finally, fusion takes place in the center of a protostar, halting gravitational condensation, and a star is born. Life Cycle of a Star Main Sequence Star (Our Sun)— 90% of all stars The star is stable Burns Hydrogen gas to Helium in its core through nuclear fusion The energy released causes the star to shine. Stars spend about 90% of their active lifetime as main sequence stars. Life Cycle of a Star Red Giant— The star begins to run out of fuel and the core begins to shrink Helium turns into Carbon Rapid burning of helium causes outer layer to puff out, cooling the star The star turns red Life Cycle of a Star Planetary Nebula Outer layers are ejected as core continues to shrink Shell of hot gas Core is exposed White Dwarf— Low mass core continues to shrink creating a white dwarf Surrounded by the Planetary Nebula Stars Evolution Hydrogen fuses more quickly and when a star starts to die, iron nuclei are formed Star swells to 100 times diameter of the sun—Super Giant Iron nuclei absorbs energy and core quickly and suddenly collapses If large enough it explodes into a brilliant burst of light—Super Nova then Either a neutron star forms - dense mass of neutrons Or a Black Hole (depending on the size) - a concentration of mass great enough that the force of gravity will not allow anything to escape Novas & Supernovas Believed to only happen in binary-stars systems. Gases from one star hit the surface of another and cause a nuclear type explosion. Supernova is a brilliant burst of light that follows the collapse of the iron core of a massive star. Super Nova Quasars Quasars: give off radio & X-waves. They are the most distant objects in space. Give off tremendous amounts of energy. Quasars give off enormous amounts of energy - they can be a trillion times brighter than the Sun! Quasars are believed to produce their energy from massive black holes in the center of the galaxies in which the quasars are located. Because quasars are so bright, they drown out the light from all the other stars in the same galaxy. Where did the Constellations come from? Farmers invented the constellations It is known that crops are planted in the spring and harvest in the fall. But in some regions, there is not much differentiation between the seasons Since different constellations are visible at different times of the year, you can use them to tell what month it is. For example, Scorpius is only visible in the northern hemisphere's evening sky in the summer. Where did the Constellations come from? Some historians suspect that many of the myths associated with the constellations were invented to help the farmers remember them - when they saw certain constellations, they would know it was time to begin the planting or the reaping Looking at the Constellations Constellations are not real – they are only as a way to remember where stars are in the sky Constellations are a group of stars that appears to form a pattern as seen from Earth. The constellations help by breaking up the sky into more manageable bits Example: if you spot three bright stars in a row in the winter evening, that's part of Orion – the star Betelgeuse is Orion's left shoulder and Rigel is his foot Looking at the Constellations Constellations There are 88 constellations or sky divisions The constellations change position with each season Some constellations only come up during certain seasons, some not at all Some never change position due to their position to the poles— circumpolar Constellation of Orion is seen during winter Constellations Polaris (North Star) Found at the end of the handle of the Little Dipper (asterism)-which makes up the constellation Ursa Minor (Little Bear) Also can be found for using the pointer stars in the Big Dipper (asterism)-which makes up the constellation Ursa Major (Big Bear) Also used for navigational purposes The Constellation, Orion