STARS: LIFE CYCLE AND

ITS KINDS

STAR

• LUMINOUS SPHERE OF PLASMA HELD TOGETHER BY ITS OWN GRAVITY

• PRODUCES HEAT, LIGHT, ULTRAVIOLET RAYS, X-RAYS, AND OTHER FORMS OF RADIATION

• COMPOSED LARGELY OF GAS AND PLASMA• RESPONSIBLE FOR THE MANUFACTURE AND DISTRIBUTION

OF HEAVY ELEMENTS SUCH AS CARBON, NITROGEN, AND OXYGEN

• THE MOST PROMINENT STARS WERE GROUPED INTO CONSTELLATIONS AND ASTERISMS, AND THE BRIGHTEST STARS GAINED PROPER NAMES

• THE NEAREST STAR TO EARTH IS THE SUN

INTERNATIONAL ASTRONOMICAL UNION

• ALSO KNOWN AS IAU• COLLECTION OF PROFESSIONAL ASTRONOMERS, AT THE

PHD LEVEL AND BEYOND, ACTIVE IN PROFESSIONAL RESEARCH AND EDUCATION IN ASTRONOMY

• ACTS AS THE INTERNATIONALLY RECOGNIZED AUTHORITY FOR ASSIGNING DESIGNATIONS TO CELESTIAL BODIES (STARS, PLANETS, ASTEROIDS, ETC.)

CONSTELLATION

• SPECIFIC AREA OF THE CELESTIAL SPHERE AS DEFINED BY IAU

• CURRENTLY, THEY ARE 88 CONSTELLATIONS• FAMOUS EXAMPLES OF CONSTELLATIONS ARE ORION

AND CASSIOPEIA

ORION

CASSIOPEIA

CONSTELLATION VS ASTERISM

• THERE IS NO SHARP DISTINCTION BETWEEN THE TWO• CONSTELLATION IS AN AREA AND IS USUALLY RECOGNIZED BY

THE PATTERN OF STARS IN THE SAID AREA WHEREAS ASTERISM ITSELF IS A GROUP OF RECOGNIZED STARS

• ASTERISM MAY BE A SUB GROUP OF STARS IN A CONSTELLATION • SOMETIMES, ASTERISM MAY BE A SUB GROUP OF MULTIPLE

CONSTELLATIONS• THE MOST FAMOUS EXAMPLE OF ASTERISM IS THE BIG DIPPER,

WHICH IS A PART OF THE CONSTELLATION URSA MAJOR

BIG DIPPER (ASTERISM)

URSA MAJOR (CONSTELLATION)

LAYERS OF A STAR

• PHOTOSPHERE - DEEPEST LAYER OF THE SUN THAT WE CAN OBSERVE DIRECTLY.

• CHROMOSPHERE - THE CHROMOSPHERE IS A LAYER IN THE SUN BETWEEN ABOUT 250 MILES (400 KM) AND 1300 MILES (2100 KM) ABOVE THE SOLAR SURFACE (THE PHOTOSPHERE). THE TEMPERATURE IN THE CHROMOSPHERE VARIES BETWEEN ABOUT 4000 K AT THE BOTTOM (THE SO-CALLED TEMPERATURE MINIMUM) AND 8000 K AT THE TOP (6700 AND 14,000 DEGREES F, 3700 AND 7700 DEGREES C), SO IN THIS LAYER (AND HIGHER LAYERS) IT ACTUALLY GETS HOTTER IF YOU GO FURTHER AWAY FROM THE SUN, UNLIKE IN THE LOWER LAYERS, WHERE IT GETS HOTTER IF YOU GO CLOSER TO THE CENTER OF THE SUN.

• TRANSITION REGION - THE TRANSITION REGION IS A VERY NARROW (60 MILES / 100 KM) LAYER BETWEEN THE CHROMOSPHERE AND THE CORONA WHERE THE TEMPERATURE RISES ABRUPTLY FROM ABOUT 8000 TO ABOUT 500,000 K (14,000 TO 900,000 DEGREES F, 7700 TO 500,000 DEGREES C).

• CORONA - THE CORONA IS THE OUTERMOST LAYER OF THE SUN, STARTING AT ABOUT 1300 MILES (2100 KM) ABOVE THE SOLAR SURFACE (THE PHOTOSPHERE). THE TEMPERATURE IN THE CORONA IS 500,000 K (900,000 DEGREES F, 500,000 DEGREES C) OR MORE, UP TO A FEW MILLION K. THE CORONA CANNOT BE SEEN WITH THE NAKED EYE EXCEPT DURING A TOTAL SOLAR ECLIPSE, OR WITH THE USE OF A CORONAGRAPH. THE CORONA DOES NOT HAVE AN UPPER LIMIT.

LIFE CYCLE OF STARS

INTERSTELLAR MEDIUM AND NEBULA SPACE MAY SEEM EMPTY, BUT ACTUALLY IT IS FILLED WITH THINLY SPREAD GAS AND DUST. THIS GAS AND DUST IS CALLED INTERSTELLAR MEDIUM. THE ATOMS OF GAS ARE MOSTLY HYDROGEN (H2), AND THE GAS ATOMS ARE TYPICALLY ABOUT A CENTIMETER APART. THE DUST IS MOSTLY MICROSCOPIC GRAINS AND COMPRISES ONLY A FEW PERCENT OF THE MATTER BETWEEN STARS. THE DUST IS MOSTLY CARBON AND SILICON. IN SOME PLACES, THIS INTERSTELLAR MEDIUM IS COLLECTED INTO A BIG CLOUD OF DUST AND GAS KNOWN AS A NEBULA (NEBULAE- PLURAL). THIS IS THE BIRTHPLACE OF STARS BECAUSE THE GAS AND DUST IS WHAT MAKES UP A STAR. IN FACT, OUR SUN WAS PROBABLY BORN IN A NEBULA NEARLY 5 BILLION YEARS AGO.

PHASES OF THE LIFE CYCLE

PROTOSTAR

• [HUMAN- FETUS]• EARLIEST PHASE• VERY YOUNG STAR

THAT IS STILL GATHERING MASS FROM ITS PARENT MOLECULAR CLOUD

MAIN SEQUENCE STAR

• [HUMAN- INFANCY THROUGH ADULTHOOD]

• MAIN SEQUENCE STARS FUSE HYDROGEN ATOMS TO FORM HELIUM ATOMS IN THEIR CORES. ABOUT 90 PERCENT OF STARS, INCLUDING THE SUN, ARE MAIN SEQUENCE STARS. THESE STARS CAN RANGE FROM ABOUT A TENTH OF THE MASS OF THE SUN TO UP TO 200 TIMES AS MASSIVE

RED GIANT/ SUPERGIANT• [HUMAN- MIDDLE AGE]• AN AGING GIANT STAR

THAT HAS CONSUMED ITS CORE'S SUPPLY OF HYDROGEN FUEL. HELIUM HAS ACCUMULATED IN THE CORE, AND HYDROGEN IS NOW UNDERGOING NUCLEAR FUSION IN THE OUTER SHELLS. THESE SHELLS THEN EXPAND, AND THE NOW COOLER STAR TAKES ON A RED COLOR. THEY ARE THE LARGEST KNOWN STARS.

WHITE DWARF > BLACK DWARF

• WHITE DWARF THAT HAS COOLED SUFFICIENTLY THAT IT NO LONGER EMITS SIGNIFICANT HEAT OR LIGHT

RED GIANT > WHITE DWARF• STELLAR REMNANT

COMPOSED MOSTLY OF ELECTRON-DEGENERATE MATTER

• ITS MASS IS COMPARABLE TO THAT OF THE SUN

• ITS VOLUME IS COMPARABLE TO THAT OF EARTH.

RED SUPERGIANT > SUPERNOVA

• RARE HAPPENING, IT IS THE EXPLOSION OF A STAR

• IT IS THE LARGEST EXPLOSION THAT TAKES PLACE IN SPACE

• THESE OCCUR AT THE END OF A STAR'S LIFETIME, WHEN ITS NUCLEAR FUEL IS EXHAUSTED AND IT IS NO LONGER SUPPORTED BY THE RELEASE OF NUCLEAR ENERGY

(1) SUPERNOVA >BLACK HOLE

• PLACE IN SPACE WHERE GRAVITY PULLS SO MUCH THAT EVEN LIGHT CAN NOT GET OUT. THE GRAVITY IS SO STRONG BECAUSE MATTER HAS BEEN SQUEEZED INTO A TINY SPACE

• MADE WHEN THE CENTER OF A VERY BIG STAR FALLS IN UPON ITSELF, OR COLLAPSES

(2) SUPERNOVA >NEUTRON STAR

• TYPE OF COMPACT STAR THAT CAN RESULT FROM THE GRAVITATIONAL COLLAPSE OF A MASSIVE STAR AFTER A SUPERNOVA. NEUTRON STARS ARE THE DENSEST AND SMALLEST STARS KNOWN TO EXIST IN THE UNIVERSE; WITH A RADIUS OF ONLY ABOUT 11–11.5 KM (7 MILES), THEY CAN HAVE A MASS OF ABOUT TWICE THAT OF THE SUN

KINDS OF STARS

MAIN SEQUENCE

A STAR IS SAID TO BE BORN ONCE NUCLEAR FUSION COMMENCES IN ITS CORE. AT THIS POINT IT IS, REGARDLESS OF MASS, CONSIDERED A MAIN SEQUENCE STAR. THIS IS WHERE THE MAJORITY OF A STAR'S LIFE IS LIVED. OUR SUN HAS BEEN ON THE MAIN SEQUENCE FOR ABOUT 5 BILLION YEARS, AND WILL PERSIST FOR ANOTHER 5 BILLION YEARS OR SO BEFORE IT TRANSITIONS TO BECOME A RED GIANT STAR.

RED GIANT

ONCE A STAR HAS USED UP ALL OF ITS HYDROGEN FUEL IN ITS CORE IT TRANSITIONS OFF THE MAIN SEQUENCE AND BECOMES A RED GIANT. DEPENDING ON THE MASS OF THE STAR IT CAN OSCILLATE BETWEEN VARIOUS STATES BEFORE ULTIMATELY BECOMING EITHER A WHITE DWARF, NEUTRON STAR OR BLACK HOLE. ONE OF OUR NEAREST NEIGHBORS (GALACTICALLY SPEAKING), BETELGEUSE IS CURRENTLY IN ITS RED GIANT PHASE AND IS EXPECTED TO GO SUPERNOVA AT ANY TIME.

WHITE DWARF

WHEN LOW-MASS STARS, LIKE OUR SUN, REACH THE END OF THEIR LIVES THEY ENTER THE RED GIANT PHASE. BUT THE OUTWARD RADIATION PRESSURE OVERWHELMS THE GRAVITATIONAL PRESSURE AND THE STAR EXPANDS FARTHER AND FARTHER OUT INTO SPACE. EVENTUALLY, THE OUTER ENVELOPE OF THE STAR BEGINS TO MERGE WITH INTERSTELLAR SPACE AND ALL THAT IS LEFT BEHIND IS THE REMNANT OF THE STAR'S CORE. THIS CORE IS A SMOLDERING BALL OF CARBON AND OTHER VARIOUS ELEMENTS THAT GLOWS AS IT COOLS. WHILE OFTEN REFERRED TO AS A STAR, A WHITE DWARF IS NOT TECHNICALLY A STAR AS IT DOES NOT UNDERGO NUCLEAR FUSION. RATHER IT IS A STELLAR REMNANT, LIKE A BLACK HOLE OR NEUTRON STAR. EVENTUALLY IT IS THIS TYPE OF OBJECT THAT WILL BE THE SOLE REMAINS OF OUR SUN BILLIONS OF YEARS FROM NOW.

NEUTRON

A NEUTRON STAR, LIKE A WHITE DWARF OR BLACK HOLE, IS ACTUALLY NOT A STAR BUT A STELLAR REMNANT. WHEN A MASSIVE STAR REACHES THE END OF ITS LIFE IT UNDERGOES A SUPERNOVA EXPLOSION, LEAVING BEHIND ITS INCREDIBLY DENSE CORE. A SOUP-CAN FULL OF NEUTRON STAR MATERIAL WOULD HAVE ABOUT THE SAME MASS AS OUR MOON. THEY’RE ONLY OBJECTS KNOWN TO EXIST IN THE UNIVERSE THAT HAVE GREATER DENSITY ARE BLACK HOLES

BLACK HOLE

BLACK HOLES ARE THE RESULT OF VERY MASSIVE STARS COLLAPSING IN ON THEMSELVES DUE TO THE MASSIVE GRAVITY THEY CREATE. WHEN THE STAR REACHES THE END OF ITS MAIN SEQUENCE LIFE CYCLE, THE ENSUING SUPERNOVA DRIVES THE OUTER PART OF THE STAR OUTWARD, LEAVING ONLY THE CORE BEHIND. THE CORE WILL HAVE BECOME SO DENSE THAT NOT EVEN LIGHT CAN ESCAPE ITS GRASP. THESE OBJECTS ARE SO EXOTIC THAT THE LAWS OF PHYSICS BREAK

BROWN DWARF

BROWN DWARFS ARE NOT ACTUALLY STARS, BUT RATHER "FAILED" STARS. THEY FORM IN THE SAME MANNER AS NORMAL STARS, HOWEVER THEY NEVER QUITE ACCUMULATE ENOUGH MASS TO IGNITE NUCLEAR FUSION IN THEIR CORES. THEREFORE THEY ARE NOTICEABLY SMALLER THAN MAIN SEQUENCE STARS. IN FACT THOSE THAT HAVE BEEN DETECTED ARE MORE SIMILAR TO THE PLANET JUPITER IN SIZE, THOUGH MUCH MORE MASSIVE (AND HENCE MUCH DENSER)

VARIABLE STAR

• MOST STARS WE SEE IN THE NIGHT SKY MAINTAIN A CONSTANT BRIGHTNESS (THE TWINKLING WE SOMETIMES SEE IS ACTUALLY AN ATMOSPHERIC EFFECT AND NOT A VARIATION OF THE STAR), BUT SOME STARS ACTUALLY DO VARY. WHILE SOME STARS OWE THEIR VARIATION TO THEIR ROTATION (LIKE ROTATING NEUTRON STARS, CALLED PULSARS) MOST VARIABLE STARS CHANGE BRIGHTNESS BECAUSE OF THEIR CONTINUAL EXPANSION AND CONTRACTION. THE PERIOD OF PULSATION OBSERVED IS DIRECTLY PROPORTIONAL TO ITS INTRINSIC BRIGHTNESS. FOR THIS REASON, VARIABLE STARS ARE USED TO MEASURE DISTANCES SINCE THEIR PERIOD AND APPARENT BRIGHTNESS (HOW BRIGHT THEY APPEAR TO US ON EARTH) CAN BE SUED TO CALCULATE HOW FAR AWAY THEY ARE FROM US.

• A FAMOUS EXAMPLE IS V838 MONOCEROTIS