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Large Distances in Space The light from the stars we see at night travel great distances to reach Earth. Other than the sun, Proxima Centauri is the next closest star. The light that we see from it takes over 4 years to reach our eyes. Other light from stars has taken thousands, millions, or even billions of years to reach Earth. The stars of Orion's belt: Mintaka, Alnilam and Alnitak. (800-1300 ly) When ancient people looked at the sky, they saw pictures of mythic heroes (Hercules ), animals (Draco ) and objects (Lyra ). They used to tell stories about the groups of stars in the sky, which are now called constellations . 88 different constellations can be seen from the Earth’s northern & southern hemispheres. Constellations are not natural; they are man’s grouping of stars as we view them from Earth. The Big Dipper is probably the best known asterism – a small grouping of stars from a larger constellation. The Big Dipper is part of the constellation Ursa Major or the Great Bear. The dipper can be used to find other constellations. Draw a line through the two stars farthest from the dipper’s handle. The line through these “pointer stars ” leads to the last star in the handle of the Little Dipper . This is Polaris , or the North Star . The movement of the stars in the sky is due the Earth’s rotation and revolution . 1. The Earth rotates from West to East (rule of thumb ) So the whole sky appears to move from East to West . The sun and the stars and the moon rise in the east & set in the west. 2. One star seems to be stationary – the North Star called Polaris . This is because it lies directly above the North Pole . All other stars appear to revolve around it. 3. Stars or constellations close to the North Star can be seen all year . They are called circumpolar stars or constellations . 4. A constellation’s position in the sky changes with the seasons . Some constellations can only be seen during certain seasons . During the winter months, Orion can be seen from the Northern Hemisphere. • But six months later, the Earth has moved to the other side of the sun. • Orion cannot be seen because it is facing the daytime side of the Earth. Now people on Earth can see Lyra , a summer constellation. Summer - The Teapot, part of Sagittarius, crosses the night sky during the summer. When we look at the Teapot, we are looking towards the center of our Milky Way galaxy Fall - Pegasus and Andromeda fly near Cassiopeia the Queen. Looking south of and far beyond the Milky Way, we see the distant Andromeda galaxy Winter - Looking out through the spiral arms of the Milky Way galaxy we see Orion the Hunter chasing Taurus the Bull. Spring - When Leo the Lion crosses the sky at night, we see galaxies north of our Milky Way galaxy, like the group called the Leo Triplet Ursa Minor – the little dipper Distances in Space Units we use to measure distances on Earth are way too small for the extremely large distances in space. The distance between the Earth and the Sun for example is 150 million kilometers or 93 million miles . Now that’s a BIG number! Here’s something easier to remember… 1 AU (Astronomical Unit ) = 93 million miles = the distance from the Earth to the Sun It is easy to measure distances in our solar system with AU’s, but what about for far away stars and galaxies? Our closest star, Proxima Centauri is… 24.8 trillion miles or 260,000 AU away from Earth!! Now that’s another BIG number! ☺ let’s make it easy again… Proxima Centauri is 4.2 light years away What is a light year ? A unit that measures the distance light travels in one year Light travels 9.5 trillion kilometers in one year (5.8 trillion miles) Light travels 300,000 km/sec so… Sunlight reaches Earth in 8 minutes Sunlight reaches Pluto: 4 hours Light from Proxima Centauri reaches Earth: 4.2 years How far away is Proxima Centauri? 24.8 trillion miles or 260,000 AU or 4.2 light years It is much simpler to speak & write in light-years !! How do astronomers measure the distance to stars? One method astronomers use to measure distances to stars is called parallax . You try it… If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it. Parallax Stars that are close to Earth show parallax as Earth moves from one side of the sun to the other . Astronomers can then determine the distance using geometry ! Luminosity is another word for the brightness of a star . Ex: different watt light bulbs. A star’s luminosity depends on size and . The BIGGER the star - the brighter the star. The hotter the star - the brighter the star. When you look up at the stars at night, it is obvious that some are brighter than others. However this is deceptive since the observed brightness of a star depends on how far away you are from it . Astronomers express luminosity of a star through a scale of magnitudes. is the measure of the brightness of a star. Every star needs 2 different magnitudes Magnitude and Magnitude. How bright a star appears to us from Earth (comparing which star looks brighter in the night sky) We still use the Greek system of apparent . The brightest stars were said to be 1st magnitude. The dimmest stars visible in a dark sky far from a city are 6th magnitude. The dimmest stars visible in a city are often only 3rd magnitude. Later, it became necessary to assign to some stars a magnitude brighter than 1.0. Ex: Vega has magnitude of 0 and Sirius has a magnitude -1.4, the sun has a magnitude of -26.74. So, the more negative the value of apparent magnitude, the brighter the star appears. Conversely, the larger the value of magnitude (more positive) the fainter the star appears. How bright a star would be if all stars were the same distance from Earth (32.5 light years). The star that is actually brighter comparing them side-by-side. The absolute magnitude of the Sun as about +5 . The absolute magnitude of Rigel is about -7, meaning it is really incredibly bright. Color Surface Temperature (C) Examples Blue over 25,000 10 Lacertae Rigel Blue 11,000 - 25,000 Spica Blue 7,500 - 11,000 Sirius, Vega Blue to White 6,000 - 7,500 Canopus, Procyon White to 5,000 - 6,000 Sun, Capella Yellow Arcturus, Orange to Red 3,500 - 5,000 Aldebaran Betelgeuse, Red under 3,500 Antares A light echo around the star V838 Monoceros. • Not quite yet understood. Red Giants White dwarfs HighTemperature Low The H-R diagram plots surface temperatures of stars against their absolute magnitudes . Main Sequence Stars Where a star begins its life (fusion of H to He) Majority of all stars lie in a band that runs diagonally through the diagram Theme: hotter the temp – brighter the star Giants and Super Giants A group of cool stars that are very bright because of the large size Found in the upper right corner of the diagram White Dwarfs Near the end of their lives A group of hot stars that are dim because of their small size Found in the lower left corner of the H-R diagram.
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