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Stars & Their Characteristics. Constellations. A group of stars that appear to form a pattern in the sky. Only appear together from Earth, actually at dif. Distances. Far distance of stars make any movement not apparent in a constellation for thousands of years.
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Constellations • A group of stars that appear to form a pattern in the sky. • Only appear together from Earth, actually at dif. Distances. • Far distance of stars make any movement not apparent in a constellation for thousands of years. • Asterism- small grouping of stars. Ex. Big Dipper part of constellation Ursa Major (Big Bear). Can find Polaris (North Star) using Big Dipper.
Star Movement/Sky Appearance • Movement of stars in sky caused by Earth’s rotation & revolution. • Earth turns West to East so sky appears to move from East to West to us on Earth, that’s why the sun, moon, & stars rise in the East & set in the West. • Sky directly above poles seems stationary as Earth turns on it’s axis, so Polaris appears fixed in the sky while all other stars seem to move counter-clockwise around it. These stars are Circumpolar Stars.
Earth’s Movement & Constellations • Constellation positions in the sky change with the seasons. Ex. Big Dipper: Fall-near North horizon, Spring- overhead. • Some constellations can only be seen during certain seasons. Ex. Lyra in Summer & Orion in Winter. • These changes are due to Earth’s position as it orbits the Sun.
Apparent Magnitude • A measure of how bright a star appears to be to an observer on Earth. • Magnitude is measured on a scale from 1 (brightest)- 6 (faintest). • The lower a stars magnitude #, the brighter the star.
Distances to stars 3 units of measurements
1. AU (Astronomical Unit) • Distance from the Sun to the Earth. • Equal to 1.5 x 108 Km or 150 million Km.
2. Light-Year • Measures the distance that a ray of light travels in one year. • In one year, light travels 9.5 x 1012 Km or 9.5 trillion Km. • Ex: Proxima Centauri is 4.2 light years from Earth.
3. Parsec (Parallax Second) • Measures distances between celestial objects (nearest stars). • One parsec = 3.258 light-years or 3.086 x 1013 Km. • Parallax- change in objects direction due to change in observers position. Occurs because Earth orbits the sun. • Using parsec measurement accounts for the shift by knowing the angel between the 2 points.
Elements in Stars Each star is different, but they are mostly composed of H, He & some heavier elements. A stars spectrum is used to determine it’s composition. Each star has a unique spectrum.
Mass, Size & Temp. of Stars • Vary more in size than in mass. • Mass= total amount of material in a body. Determined by gravitational influence on other bodies. • Stellar (star) masses are expressed as multiples of the sun or One Solar Mass.
Temperature & Color of Stars • Range of color a star emits depends on it’s surface temp. • To us on earth, stars look mostly white with a bit of color. • Cooler stars= reddish, Hotter stars= Bluish-white. • Ex: Iron when heated, 1st turns red, changes to orange, to yellow to white. Very Hot objects glow Bluish. • The color/temp. of a star helps astronomers determine the elements present.
Luminosity & Absolute Magnitude Luminosity Absolute Magnitude • Actual brightness of a star. • Depends on size & temp. Ex: • If 2 stars had the same surface temp, but 1 was bigger, the bigger is more luminous. • If 2 stars were same size, but dif. temps, the hotter is more luminous • True measure of how bright a star is. What’s the difference between apparent & absolute magnitude?
Life Cycles of Stars Stars are born from great clouds of gas & dust. They mature, grow old, & die. When they die, they produce new clouds of dust, from which new stars may arise, along w/planets to orbit them. The more massive a star, the shorter its life will be.
What is the H-R diagram? • Hertzsprung-Russell diagram – a diagram that plots the stars surface temperature against their absolute magnitude.
Groups of the H-R Diagram: Groups represent stages in the life cycles of the stars. 90% of stars fall in the Main Sequence band & range in size & temp. these are the stars that are fusing H to He. Giant Stars are above main sequence & are more luminous & larger. Supergiants are even more luminous & large. White Dwarfs are near the end of their lives, were once red giants but lost atmosphere & now only glowing stellar core.
Where in the graph are the bright, cool stars? Which axis represents temperature? What area would blue stars be located?
Birth of a Star • A star begins its life in a cloud of gas & dust called a Nebula. • A nebula condenses when an outside force (shockwave) acts upon it. • Parts of the nebula then condense due to gravity & the temp. increases. • Large enough nebulas will begin to glow (Protostars) • Protostars get hotter & brighter eventually fusion occurs & a star is born.
How would the amounts of H & He in the cores of 2 identical stars compare if one star was older than the other?
Death of a star like the Sun(main sequence star) • Balance of fusion & gravity occurs until H is used up & the core shrinks & releases new heat that expands the star into a Giant. • The star begins to die when temps rise & He (helium) fuses to heavier elements (C-O core) • Gas layers are blown away leaving C-O core (white dwarf) w/halo glow= Planetary Nebula. • Eventually gases dissipate into space & white dwarf is left.
Death of a Massive Star • Same start as before, but fusion occurs until Fe (iron) nuclei form & star swells more than 100 times the diameter of our sun, becoming a Supergiant. • Fe cores absorb energy & suddenly collapse sending shockwaves & outer layers out in a huge burst of light= Supernova and produces new elements. • Left behind is either a Black Hole (if original star was 15 x or more massive than our sun) or a Neutron Star.
Remnants of Massive Stars • After a massive star “goes supernova,” it leaves behind its core. • Neutron Star: super dense, all e- are pushed into the nuclei they orbit, result is a dense mass of neutrons that are trillions times more dense than the sun. At first, it spins & emits pulsing beams of radio waves out (Pulsar). If star was at least 15 x more massive than the sun: • Black Hole: Intense gravitational field that not even light can escape. Detected by strong sources of x-ray (given off by atoms being ripped apart by gravity)
Self-Quiz • What determines which life-cycle course a star takes? • How does cloud-like nebula form protostars? • What process marks the transition from protostar to star? • What remains balanced during the long period when a star’s H is fusing into He? • At what point is this balance lost?
What Are Galaxies? • Systems that contain millions or billions of stars. • Look like hazy patches of light in the sky. • Estimated to be 50-100 billion galaxies in observable universe. • Most are millions of light years apart.
To what galaxy do we belong? • Milky Way Galaxy • It’s a spiral galaxy (pinwheel-shaped) • Diameter of about 100,000 light-years • Our sun is about 26,000 light-years from it’s center
Types of galaxies • Spiral: pinwheel-shaped; nuclei of stars bulging at center & wound spiral arms. • Elliptical: nearly spherical to oval-shaped; stars concentrated at their centers, little interstellar gas & dust for new stars to form so few to no young stars. • Irregular: irregular in shape; much smaller & fainter than other 2 galaxy types, stars spread unevenly.
Active Galaxies • Emit more energy than their stars can give off. • Some emit radiation, or change largely in brightness in short periods of time. • Possible powered by a supermassive black hole centers that jet out hot gas in opposite directions. • Quasars: very distant, extremely luminous celestial object believed to be a type nuclei in an active galaxy.
