NSCI 314

LIFE IN THE COSMOS

2 – BASIC ASTRONOMY, AND STARS AND THEIR EVOLUTION

Dr. Karen KolehmainenDepartment of Physics

CSUSB

COURSE WEBPAGE:http://physics.csusb.edu/~karen

SOME BASIC ASTRONOMY

STAR: A VERY LARGE , HOT BALL OF GAS THAT EMITS LARGE AMOUNTS OF LIGHT. THE LIGHT AND HEAT ARE PRODUCED BY NUCLEAR FUSION (i.e., SMALL NUCLEI COMBINING TO PRODUCE LARGER NUCLEI) OCCURRING IN THE CENTER.

PLANET: A FAIRLY LARGE OBJECT (BUT MUCH SMALLER THAN A STAR) THAT ORBITS AROUND A STAR. IT CAN BE ROCKY OR GASEOUS. THERE IS NO NUCLEAR FUSION OCCURRING INSIDE.

MOON: A SMALLER OBJECT THAT ORBITS AROUND A PLANET.

SOME BASIC ASTRONOMY EARTH: THE PLANET UPON WHICH WE LIVE.

SUN: THE STAR AROUND WHICH THE EARTH ORBITS.

SOLAR SYSTEM: OUR SUN, THE 8 PLANETS AND SMALLER BODIES (DWARF PLANETS, COMETS, ASTEROIDS, ETC.) THAT ORBIT IT, AND THE MOONS THAT ORBIT THE PLANETS.

GALAXY: A LARGE CLUSTER OF STARS (1 MILLION TO 1 TRILLION STARS). MANY OF THESE STARS HAVE THEIR OWN SOLAR SYSTEMS.

THE MILKY WAY: THE GALAXY IN WHICH OUR SOLAR SYSTEM IS LOCATED. IT CONTAINS ABOUT 400 BILLION STARS, PLUS THE PLANETS AND SMALLER BODIES ORBITING THESE STARS.

SOME BASIC ASTRONOMY UNIVERSE: EVERYTHING THAT EXISTS. WITH

POWERFUL TELESCOPES, WE CAN SEE ABOUT 100 BILLION OBSERVABLE GALAXIES. THE TOTAL NUMBER OF GALAXIES IN THE UNIVERSE IS LIKELY TO BE MUCH HIGHER, PROBABLY AT LEAST 1 TRILLION.

BIG BANG: AN EXPLOSION THAT STARTED THE UNIVERSE APPROXIMATELY 13.7 BILLION YEARS AGO. ALL OF THE MATTER IN THE UNIVERSE WAS EXPELLED OUTWARD FROM THE EXPLOSION. GALAXIES ARE STILL MOVING APART FROM EACH OTHER AS A RESULT. (BY COMPARISON, OUR SOLAR SYSTEM IS ONLY 4.6 BILLION YEARS OLD.)

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 1012 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

NAME Distance From SunMERCURY 0.4 AUVENUS 0.7EARTH 1.0MARS 1.5JUPITER 5.2SATURN 9.5URANUS 19NEPTUNE 30

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 FLATTENED DISK.

PLANETS IN OUR SOLAR SYSTEM

SHAPE FLATTENED DISK

DIAMETER 100,000 LY

THICKNESS 2,000 LY

NUMBER OF STARS 400 BILLION

ROTATION PERIOD 250 MILLION YEARS

SUN’S DISTANCE FROM CENTER 30,000 LY

AVERAGE DISTANCEBETWEEN STARS 5 LY

TYPICAL STAR 0.5 SOLAR MASSES

MILKY WAY GALAXY

ACROSS UNITED STATES 0.02 SECONDS

EARTH TO MOON 1.3 SECONDS

EARTH TO SUN 8 MINUTES

ACROSS SOLAR SYSTEM FEW HOURS

NEAREST STAR (BEYOND SUN) 4 YEARS

ACROSS MILKY WAY GALAXY 100,000 YEARS

NEAREST OTHER MAJOR GALAXY 2 MILLION YRS

FARTHEST GALAXIES 10 BILLION YRS

LIGHT TRAVEL TIMES

WHY EXAMINE STARS? WHETHER OR NOT A PLANET IS

SUITABLE FOR LIFE DEPENDS PARTLY ON WHAT KIND OF STAR IT ORBITS. WE NEED TO DECIDE WHICH STARS MIGHT HAVE PLANETS THAT ARE SUITABLE FOR LIFE

WE SHOULD THINK ABOUT WHETHER STARS THEMSELVES, AT ANY STAGE OF THEIR “LIFETIMES,” MIGHT BE SUITABLE LOCATIONS FOR LIFE.

TEMPERATURE SCALESSCALE ABSOLUTE WATER WATER

ZERO FREEZES BOILS

FAHRENHEIT -459 32 212

CELSIUS -273 0 100

KELVIN 0 273 373

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

TC = T – 273 TC IS CELSIUS TEMPERATURE

TF = (9/5) TC + 32 TF IS FAHRENHEIT TEMPERATURE

PROPERTIES OF STARS THE SUN:

MASS: SOLAR MASS = 2 x 1030 kg = 330,000 EARTH MASSES = 1,000 JUPITER MASSES

SIZE: SOLAR RADIUS = 7 X 105 km = 110 EARTH RADII BRIGHTNESS: SOLAR LUMINOSITY = 4x1026 W “SURFACE” TEMPERATURE: 6000 K (10,000 oF) COMPOSITION:

OTHER STARS: MASS RANGES FROM 1/10 TO 20 SOLAR MASSES SIZE RANGES FROM 1/100 TO 500 SOLAR RADII BRIGHTNESS RANGES FROM 0.000001 (10-6 or 1

MILLIONTH) TO 1,000,000 (106 or 1 MILLION) SOLAR LUMINOSITIES

TEMPERATURE RANGES FROM 2,500 K TO 30,000 K (OR 4000 oF TO 50,000 oF), AND IS RELATED TO COLOR

90.99% HYDROGEN8.87% HELIUM0.08% OXYGEN0.03% CARBON0.02% NEON0.01% 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 IN ORDER OF HOTTEST TO COOLEST IN ORDER OF 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. SO FAR, THE ELAPSED TIME IS A FEW MILLION YEARS.

OUTLINE OF STAR FORMATION

OUTLINE OF STAR FORMATION- THE STAR STABILIZES (STOPS CONTRACTING).

THIS HAPPENS BECAUSE THE HIGH TEMPERATURE CAUSES A HIGH OUTWARD PRESSURE WHICH “BALANCES” THE INWARD PULL OF GRAVITY. (THE HIGH TEMPERATURE IS MAINTAINED BY THE ENERGY RELEASE FROM THE NUCLEAR REACTIONS). 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 oF).

HYDROGEN NUCLEI (PROTONS) FUSE TO FORM HELIUM.

•FOUR 1H (PROTONS) FUSE TO ONE 4He + ENERGY.

•TWO OF THE PROTONS ARE CONVERTED TO NEUTRONS.

WHERE DOES THE ENERGY COME FROM?

•ONE 4He IS 0.7% LIGHTER THAN FOUR 1H 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 STARSALL 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)

SpectralType

Number of Stars in MW

Percent ofTotal

100,000

500

10

2

0.9

0.2

0.005

O

B

A

F

G

K

M

5 million

10 million

500 million

1 billion

10 billion

100 billion

1 trillion

80,000

360 million

2.4 billion

12 billion

28 billion

60 billion

290 billion

0.00002 %

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 CARBON AND 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: THE OUTER PART OF THE STAR IS EJECTED.

THE EJECTED MATERIAL EXPANDS, COOLS, AND ENRICHES THE INTERSTELLAR MATERIAL WITH HEAVIER ELEMENTS (FUSION PRODUCTS). THE CORE OF STAR REMAINS IN THE CENTER OF THE NEBULA, AND BECOMES A …

WHITE DWARF: CORE OF THE ORIGINAL STAR LEFT AFTER OUTER PARTS ARE EJECTED. IT SHRINKS TO ABOUT EARTH-SIZE, AND IS VERY DENSE. (A TEASPOONFUL OF WHITE DWARF MATERIAL WOULD WEIGH ABOUT A TON.) THERE IS NO MORE FUSION, SO THE WHITE DWARF COOLS AND DIMS (VERY, VERY SLOWLY) UNTIL EVENTUALLY IT BECOMES A …

BLACK DWARF: NO MORE LIGHT EMITTED. STELLAR CORPSE IS A DENSE SOLID BALL OF CARBON, NITROGEN, AND OXYGEN. THE UNIVERSE ISN'T OLD ENOUGH FOR BLACK DWARFS TO HAVE FORMED YET.

LATE STAGES OF STELLAR EVOLUTION FOR MASSIVE STARS

SUPERNOVA (TYPE II): THE CORE OF THE RED SUPERGIANT COLLAPSES UNTIL ATOMIC NUCLEI COLLIDE WITH EACH OTHER AT HIGH SPEED. NUCLEI PUSH EACH OTHER APART VIOLENTLY, CAUSING STAR TO EXPLODE. THERE IS ENOUGH ENERGY NOW AVAILABLE FOR FUSION OF HEAVY ELEMENTS (EVEN HEAVIER THAN IRON). THESE ARE EJECTED IN THE EXPLOSION AND 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

EXTREMELY DENSE – 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 THE 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.