Stars & Galaxies Chapter 3 DISTANCES in SPACE Enormous distances OUTSIDE the Solar System • LIGHT-YEAR – distance light travels in 1 Earth–year • NOT time! STARS •Massive balls of gas held together by gravity with a core so hot that nuclear fusion occurs BIRTH OF A STAR NEBULA – an immense cloud of dust and gas where stars are born •GRAVITY causes particles to gather around areas of higher mass, pulling in more and more particles. •Once enough matter condenses, the heat and pressure are great enough to begin nuclear fusion. NUCLEAR FUSION •Occurs under intense heat and pressure •Fuses the nuclei of multiple atoms into one •Releases a lot of energy (heat & light) TEMPERATURE •Stars are classified by their temperature •Different temperatures cause different colors •Color classifications are represented as different letters

APPARENT MAGNITUDE

•Measure of how BRIGHT a star appears from Earth •A Scale compares the brightness of all objects in the sky •The brightest objects have a negative value (Sun, Moon, Venus, Sirius) Luminosity HERTZSPRUNG-RUSSEL DIAGRAM MAIN SEQUENCE STARS •fuse HYDROGEN atoms into HELIUM atoms •@90% of stars •More mass  less time on the main sequence •What happens next depends on the star’s MASS… LOW MASS STARS After they are finished fusing H into He 1) RED GIANT • helium begins to fuse into carbon 2) WHITE DWARF • helium runs out, not hot enough to fuse carbon, fusion stops 3) BLACK DWARF • remaining heat energy gradually cools off until the star is just a cold sphere of carbon HIGH MASS STARS After they are finished fusing H into He 1) RED SUPERGIANT • Fuses heavier elements up to iron 2) SUPERNOVA • Cannot fuse iron, star collapses and explodes 3) NEUTRON STAR • Dense core of neutrons remaining after supernova 4) BLACK HOLE • Only the most massive stars collapse into a sphere so massive that their gravity prevents even light from escaping

THE SUN •Class-G Main Sequence Star •“yellow dwarf” •About ½ way through a 10 billion year life cycle on the main sequence •Will then become a red giant before fusion stops and it becomes a white dwarf

STRUCTURE OF THE SUN Interior •Core (nuclear fusion) •Radiative Zone •Convection Zone Atmosphere •Photosphere •Chromosphere •Corona FEATURES OF THE SUN • SUNSPOTS • Regions of strong magnetic activity • PROMINENCES & SOLAR FLARES • Clouds of gas and bright eruptions • CORONAL MASS EJECTIONS (CMEs) • Bubbles of gas ejected from the corona • SOLAR WIND • Charged particles streaming away from the Sun ROTATION OF THE SUN •Galileo observed the movement of sunspots across the Sun’s surface •PLASMA at the Sun’s equator rotates faster than plasma near the poles REVOLUTION OF THE SUN The Sun orbits the center of the MILKY WAY GALAXY (black hole at center?) • Located in one of the spiral arms called the “Orion Arm” GROUPS OF STARS •BINARY STAR SYSTEM •Two stars orbit each other bound by gravity •STAR CLUSTER •Groups of stars in “close” proximity •GALAXY •Hundreds of billions of stars grouped together by gravity •Galaxies exist in clusters just like stars MILKY WAY GALAXY TYPES OF GALAXIES EXPANDING UNIVERSE •Astronomers observe that all of the other galaxies are moving away from us (and one another) •What does this mean?