NSCI 314 LIFE IN THE COSMOS 2 – BASIC ASTRONOMY, AND STARS AND THEIR EVOLUTION Dr. Karen Kolehmainen Department of Physics CSUSB COURSE WEBPAGE:

MOTIONS IN THE SOLAR SYSTEM EACH PLANET SPINS OR ROTATES ON ITS OWN AXIS, PRODUCING DAY AND NIGHT. THE EARTH SPINS ONCE EVERY 24 HOURS (1 DAY). EACH PLANET ORBITS OR REVOLVES AROUND THE SUN. THE EARTH COMPLETES ONE ORBIT IN 365 DAYS (1 YEAR). THE FARTHER A PLANET IS FROM THE SUN, THE LONGER IT TAKES TO COMPLETE ONE ORBIT (i.e.,THE LONGER ITS YEAR). MOONS ORBIT AROUND PLANETS. THE EARTH'S MOON TAKES ABOUT A MONTH TO COMPLETE ONE ORBIT AROUND THE EARTH. SEASONS ARE CAUSED BY THE FACT THAT THE EARTH'S ROTATIONAL AXIS (THE LINE IT ROTATES AROUND) IS TIPPED.

UNITS FOR MEASURING DISTANCES ASTRONOMICAL UNIT (AU) –THE AVERAGE DISTANCE BETWEEN THE EARTH AND THE SUN –ABOUT 150,000,000 KM –A UNIT OF DISTANCE USED WITHIN THE SOLAR SYSTEM LIGHT YEAR (LY) –THE DISTANCE LIGHT TRAVELS IN ONE YEAR –ABOUT 9.5 x KM OR 6333 AU –A UNIT OF DISTANCE USED FOR STARS PARSEC (pc) –ABOUT 3.26 LIGHT YEARS –ANOTHER UNIT OF DISTANCE USED FOR STARS

NAMEAverage Distance From Sun MERCURY0.4 AU VENUS0.7 EARTH1.0 MARS1.5 JUPITER5.2 SATURN9.5 URANUS19 NEPTUNE30 ALL PLANETS ORBIT THE SUN IN THE SAME DIRECTION, AND THEIR ORBITS ARE NEARLY IN THE SAME PLANE. THUS, THE SOLAR SYSTEM IS SHAPED LIKE A DISK. PLANETS IN OUR SOLAR SYSTEM

SHAPEFLATTENED DISK DIAMETER100,000 LY THICKNESS2,000 LY NUMBER OF STARS400 BILLION ROTATION PERIOD250 MILLION YEARS SUN’S DISTANCE FROM CENTER30,000 LY AVERAGE DISTANCE BETWEEN STARS:5 LY TYPICAL STAR0.5 SOLAR MASSES MILKY WAY GALAXY

LIGHT TRAVEL TIMES ACROSS UNITED STATES0.02 SECONDS EARTH TO MOON1.3 SECONDS EARTH TO SUN8 MINUTES ACROSS SOLAR SYSTEMFEW HOURS ACROSS MW GALAXY100,000 YEARS NEAREST OTHER GALAXY2 MILLION YRS. FARTHEST GALAXIES10 BILLION YRS.

WHY EXAMINE STARS? WHETHER OR NOT A PLANET IS SUITABLE FOR LIFE PROBABLY DEPENDS A LOT ON WHAT TYPE OF STAR IT ORBITS. WE SHOULD THINK ABOUT WHETHER STARS THEMSELVES, AT ANY STAGE OF THEIR “LIFETIMES,” MIGHT BE SUITABLE LOCATIONS FOR LIFE.

TEMPERATURE SCALES SCALE ABSOLUTE WATER WATER ZEROFREEZESBOILS FAHRENHEIT CELSIUS KELVIN THE HIGHER THE TEMPERATURE, THE FASTER THE RANDOM MOTION OF INDIVIDUAL PARTICLES. ABSOLUTE ZERO IS THE LOWEST POSSIBLE TEMPERATURE, AT WHICH THIS RANDOM MOTION STOPS. CONVERSION:T IS KELVIN TEMPERATURE T C = T – 273T C IS CELSIUS TEMPERATURE T F = (9/5) T C + 32 T F IS FAHRENHEIT TEMPERATURE

PROPERTIES OF STARS THE SUN: MASS: SOLAR MASS = 2 x kg = 330,000 EARTH MASSES = 1,000 JUPITER MASSES SIZE: SOLAR RADIUS = 7 X 10 5 km = 110 EARTH RADII BRIGHTNESS: SOLAR LUMINOSITY = 4x10 26 W “SURFACE” TEMPERATURE: 6000 K (10,000 o F) COMPOSITION: OTHER STARS: MASS RANGES FROM 1/10 TO 20 SOLAR MASSES SIZE RANGES FROM 1/100 TO 500 SOLAR RADII BRIGHTNESS RANGES FROM (10 -6 or 1 MILLIONTH) TO 1,000,000 (10 6 or 1 MILLION) SOLAR LUMINOSITIES TEMPERATURE RANGES FROM 2,500 K TO 30,000 K (OR 4000 o F TO 50,000 o F), AND RELATED TO COLOR 90.99% HYDROGEN 8.87% HELIUM 0.08% OXYGEN 0.03% CARBON 0.02% NEON 0.01% NITROGEN <0.01% EVERYTHING ELSE

BLACKBODY RADIATION TYPE OF LIGHT THAT IS EMITTED BY A STAR OR ANY OTHER HOT GLOWING OBJECT THE COLOR OF LIGHT DEPENDS ONLY ON THE TEMPERATURE OF THE OBJECT: –HOTTER OBJECTS ARE BLUER –COOLER OBJECTS ARE REDDER –ROOM TEMPERATURE OBJECTS EMIT IN INFRARED THE BRIGHTNESS OF THE LIGHT DEPENDS ON BOTH TEMPERATURE AND SIZE OF THE OBJECT: –FOR OBJECTS OF THE SAME SIZE, HOTTER OBJECTS ARE BRIGHTER COOLER OBJECTS ARE FAINTER –FOR OBJECTS OF THE SAME TEMPERATURE, BIGGER OBJECTS ARE BRIGHTER SMALLER OBJECTS ARE FAINTER

Blackbody Spectra

MAIN SEQUENCE: THEY FUSE HYDROGEN INTO HELIUM FOR ENERGY. 90% OF STARS ARE THIS TYPE. THEIR SIZE, TEMPERATURE, AND BRIGHTNESS REMAIN RELATIVELY CONSTANT FOR A LONG PERIOD OF TIME (MILLIONS TO BILLIONS OF YEARS). GIANTS AND SUPER GIANTS: THEY FUSE HEAVIER ELEMENTS FOR ENERGY. THEY ARE MUCH LARGER AND MORE EVOLVED THAN MAIN SEQUENCE STARS. MOST ARE RED. WHITE DWARFS, NEUTRON STARS, & BLACK HOLES: “DEAD” STARS, END STAGES OF STELLAR EVOLUTION. SPECTRAL TYPES: O B A F G K M HOTTEST TO COOLEST BLUEST TO REDDEST SUN IS A G-TYPE MAIN SEQUENCE STAR HERTZSPRUNG-RUSSELL DIAGRAM: PLOT OF BRIGHTNESS vs. SPECTRAL TYPE OR TEMPERATURE TYPES OF STARS

DWARFS AND GIANTS RED GIANTS AND SUPERGIANTS ARE BRIGHTER THAN MAIN SEQUENCE STARS OF THE SAME TEMPERATURE. THEREFORE THEY MUST BE LARGER. WHITE DWARFS ARE FAINTER THAN MAIN SEQUENCE STARS OF THE SAME TEMPERATURE. THEREFORE THEY MUST BE SMALLER.

THE INTERSTELLAR MEDIUM  SPACE IS NOT TOTALLY EMPTY!  ATOMS, MOLECULES, AND DUST PARTICLES ARE PRESENT  MOSTLY HYDROGEN, HELIUM IS THE NEXT MOST ABUNDANT ELEMENT  CONCENTRATED IN CLOUDS OR NEBULAE  INTERSTELLAR MATTER IS VERY LOW DENSITY, EVEN IN MOST NEBULAE  STARS FORM FROM NEBULAE, AND THEREFORE THEY CONTAIN THE SAME MIX OF ELEMENTS AS THE INTERSTELLAR MEDIUM OUT OF WHICH THEY FORMED (MOSTLY HYDROGEN)

- A SUFFICIENTLY DENSE NEBULA (TYPICALLY ABOUT 1 LIGHT YEAR IN DIAMETER) STARTS TO COLLAPSE DUE TO GRAVITY, PULLING MATERIAL TOWARDS THE CENTER.  AS THE CLOUD CONTRACTS, ITS ROTATION SPEEDS UP, IT FLATTENS, AND ITS TEMPERATURE INCREASES, ESPECIALLY IN THE CENTER.  THE INNER PART OF THE CLOUD GETS HOT ENOUGH FOR MOLECULES TO BREAK APART INTO ATOMS, THEN FOR ATOMS TO IONIZE, AND FINALLY FOR HYDROGEN NUCLEI TO START TO FUSE TO FORM HELIUM NUCLEI.  ONCE FUSION OF H TO He BEGINS, IT BECOMES A MAIN SEQUENCE STAR. OUTLINE OF STAR FORMATION

- THE STAR STABILIZES (STOPS CONTRACTING) BECAUSE OF THE HIGH TEMPERATURE, AND THUS HIGH PRESSURE, IN THE CENTER (MAINTAINED BY THE ENERGY RELEASE FROM THE NUCLEAR REACTIONS). THIS PROVIDES AN OUTWARD PUSH THAT BALANCES THE INWARD PULL OF GRAVITY. THE STAR REMAINS STABLE (NOT MUCH CHANGE IN SIZE, BRIGHTNESS, OR TEMPERATURE) FOR A LONG TIME. - IN MANY CASES, PLANETS AND SMALLER BODIES FORM FROM SMALLER LUMPS IN THE CLOUD THAT WERE NOT PULLED INTO THE CENTER. - SOMETIMES THE CLOUD BREAKS INTO SEVERAL LARGE FRAGMENTS AS IT CONTRACTS, EACH OF WHICH FORMS A SEPARATE STAR. THE RESULT IS A DOUBLE OR MULTIPLE STAR OR A SMALL CLUSTER OF STARS.

OUTLINE OF STAR FORMATION

HIGH TEMPERATURE IS NECESSARY FOR NUCLEAR FUSION. THE NUCLEI MUST BE MOVING FAST ENOUGH TO COLLIDE DESPITE THEIR ELECTRICAL REPULSION. IN THE CORE OF A MAIN SEQUENCE STAR, TEMPERATURE IS ABOUT 15 MILLION K (27 MILLION o F). HYDROGEN NUCLEI (PROTONS) FUSE TO FORM HELIUM. FOUR 1 H (PROTONS) FUSE TO ONE 4 He + ENERGY. TWO OF THE PROTONS ARE CONVERTED TO NEUTRONS. WHERE DOES THE ENERGY COME FROM? ONE 4 He IS 0.7% LIGHTER THAN FOUR 1 H COMBINED. THE LOST MASS IS CONVERTED TO ENERGY. E = m c 2 ENERGY = MASS x (SPEED OF LIGHT) ² NUCLEAR REACTIONS IN MAIN SEQUENCE STARS

THE SUN CONVERTS 4 BILLION kg OF MATTER INTO ENERGY EACH SECOND. THE SUN HAS SUFFICIENT HYDROGEN TO DO THIS FOR 100 BILLION YEARS, BUT WILL STOP AFTER ABOUT 10 BILLION YEARS BECAUSE ONLY THE CORE UNDERGOES FUSION. (THE OUTER PARTS OF THE STAR AREN’T HOT ENOUGH FOR FUSION.) WHEN HYDROGEN IN THE CORE RUNS OUT, THIS MARKS THE END OF THE MAIN SEQUENCE LIFETIME. THE MORE MASSIVE THE STAR, THE SHORTER THE MAIN SEQUENCE LIFETIME. EVEN THOUGH THERE IS MORE HYDROGEN TO FUSE, FUSION PROCEEDS SO MUCH MORE RAPIDLY (BECAUSE IT IS HOTTER) THAT THE HYDROGEN IN THE CENTER DOESN’T LAST AS LONG. NUCLEAR REACTIONS IN MAIN SEQUENCE STARS

MAIN SEQUENCE STARS ALL PROPERTIES OF A MAIN SEQUENCE STAR DEPEND ON ITS MASS. –MORE MASSIVE STARS ARE LARGER. –MORE MASSIVE STARS ARE HOTTER. –MORE MASSIVE STARS ARE BLUER. –MORE MASSIVE STARS ARE BRIGHTER. –MORE MASSIVE STARS HAVE SHORTER LIFETIMES. EVEN THOUGH THEY HAVE MORE NUCLEAR FUEL, THEY USE IT UP MORE QUICKLY BECAUSE OF THEIR HIGHER TEMPERATURES.

Brightness (Sun = 1) Spectral Type Number of Stars in MW Percent of Total 100, OBAFGKMOBAFGKM 5 million 10 million 500 million 1 billion 10 billion 100 billion 1 trillion 80, million 2.4 billion 12 billion 28 billion 60 billion 290 billion % 0.09 % 0.6 % 3 % 7 % 15 % 73 % Lifetime (Years) PROPERTIES OF MAIN SEQUENCE STARS

AT THE END OF THE MAIN SEQUENCE LIFETIME… THE OUTER PART OF THE STAR EXPANDS BY UP TO 200 TIMES, BRIGHTENS BY UP TO 100 TIMES, AND COOLS TO 3,000 K. THE STAR BECOMES A RED GIANT. IN MASSIVE STARS, A SECOND STAGE OF EXPANSION AND COOLING PRODUCES A SUPERGIANT. MEANWHILE, THE CORE CONTRACTS AND HEATS UP UNTIL HELIUM BEGINS TO FUSE TO FORM HEAVIER ELEMENTS. THE STAR GOES THROUGH OCCASIONAL EPISODES OF INSTABILITY, WITH RAPID OSCILLATIONS IN SIZE, TEMPERATURE, AND BRIGHTNESS.

He FUSES INTO CARBON, NITROGEN AND OXYGEN + ENERGY. IN LOW MASS STARS, FUSION STOPS HERE BECAUSE IT NEVER GETS HOT ENOUGH TO FUSE C, N, AND O TO EVEN HEAVIER ELEMENTS. IN MASSIVE STARS, CARBON FUSES INTO SILICON + ENERGY, THEN SILICON FUSES INTO IRON + ENERGY. FUSION OF IRON TO YET HEAVIER ELEMENTS WOULD REQUIRE ENERGY AS AN INPUT RATHER THAN RELEASING IT. (THE NUCLEUS THAT WOULD RESULT FROM FUSION IS HEAVIER THAN THE NUCLEI THAT WOULD FUSE TO FORM IT.) THIS ENERGY IS UNAVAILABLE, SO FUSION DOESN’T PROCEED BEYOND IRON, NO MATTER HOW MASSIVE THE STAR. NUCLEAR REACTIONS IN RED GIANTS AND SUPERGIANTS

LATE STAGES OF STELLAR EVOLUTION FOR SOLAR-TYPE (LOW MASS) STARS PLANETARY NEBULA : OUTER PART OF STAR IS EJECTED. EJECTED MATERIAL EXPANDS, COOLS, AND ENRICHES INTERSTELLAR MATERIAL WITH HEAVIER ELEMENTS (FUSION PRODUCTS). CORE OF STAR REMAINS IN CENTER OF NEBULA, AND BECOMES A … WHITE DWARF : CORE OF ORIGINAL STAR LEFT AFTER OUTER PARTS ARE EJECTED; NO MORE FUSION; SHRINKS TO ABOUT EARTH-SIZE, VERY SLOWLY COOLS AND DIMS UNTIL EVENTUALLY IT BECOMES A … BLACK DWARF : NO MORE LIGHT EMITTED. STELLAR CORPSE IS A DENSE SOLID BALL OF CARBON, NITROGEN, AND OXYGEN.

LATE STAGES OF STELLAR EVOLUTION FOR MASSIVE STARS SUPERNOVA (TYPE II): CORE OF RED SUPERGIANT COLLAPSES UNTIL NUCLEI COLLIDE WITH EACH OTHER AT HIGH SPEED, NUCLEI PUSH EACH OTHER APART VIOLENTLY, CAUSING STAR TO EXPLODE. FUSION OF HEAVY ELEMENTS (EVEN HEAVIER THAN IRON) OCCURS, AND THESE ARE THEN ADDED TO THE INTERSTELLAR MEDIUM. NEUTRON STAR OR BLACK HOLE: CORE OF ORIGINAL STAR LEFT OVER AFTER THE SUPERNOVA EXPLOSION, EXTREMELY DENSE

NEUTRON STAR BALL OF NEUTRONS (LIKE A BIG NUCLEUS) RADIUS ABOUT 10 km MASS OF SEVERAL SOLAR MASSES HIGH DENSITY – ONE TEASPOONFUL OF NEUTRON STAR MATERIAL WOULD WEIGH A BILLION TONS MANY NEUTRON STARS EMIT PULSES OF RADIO WAVES AND OTHER ELECTROMAGNETIC RADIATION, AND ARE OBSERVED AS “PULSARS”

BLACK HOLE SO MASSIVE THAT NOTHING, NOT EVEN LIGHT, CAN ESCAPE. (ESCAPE VELOCITY EXCEEDS THE SPEED OF LIGHT.) “EVENT HORIZON” SEALS OFF THE INTERIOR FROM THE REST OF THE UNIVERSE. EVENT HORIZON HAS RADIUS OF SEVERAL km FOR STELLAR MASS BLACK HOLES. MATTER INSIDE EVENT HORIZON PROBABLY CONTRACTS TO A “SINGULARITY” – INFINITE DENSITY! CAN SOMETIMES BE DETECTED VIA GRAVITATIONAL EFFECTS ON OTHER OBJECTS (E.G., IN A DOUBLE STAR SYSTEM) AND/OR FROM X-RAYS EMITTED BY MATTER FALLING IN. IN ADDITION TO STELLAR-MASS BLACK HOLES FORMED AT END OF LIVES OF MASSIVE STARS, LARGER BLACK HOLES EXIST IN THE CENTERS OF MOST GALAXIES (INCLUDING MILKY WAY).

ESCAPE VELOCITY SPEED NEEDED WHEN TAKING OFF FROM THE SURFACE OF A PLANET OR OTHER OBJECT TO ESCAPE THE OBJECT’S GRAVITY (TECHNICALLY, TO ESCAPE TO AN INFINITE DISTANCE AWAY, SLOWING TO ZERO SPEED IN THE PROCESS) EARTH'S ESCAPE VELOCITY IS 11 km/s THE MORE MASSIVE THE OBJECT, THE LARGER THE ESCAPE VELOCITY. THE SMALLER THE RADIUS OF THE OBJECT, THE LARGER THE ESCAPE VELOCITY. THEREFORE, A MASSIVE BUT SMALL OBJECT WILL HAVE A LARGE ESCAPE VELOCITY.