life cycle

THE LIFE CYCLE OF STARS
Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars.

Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. Its fate is determined by the original mass of the star; it will become either a black dwarf, neutron star, or black hole.

THE BIRTH OF STARS
Accretion Disk:

The Eagle nebula, a stellar nursery illuminated by ultraviolet light which is emitted from the newborn stars.

Stars are formed in nebulae, interstellar clouds of dust and gas (mostly hydrogen). These stellar nurseries are abundant in the arms of spiral galaxies.

In these stellar nurseries, dense parts of these clouds undergo gravitational collapse and compress to form a rotating gas globule.

The globule is cooled by emitting radio waves and infrared radiation. It is compressed by gravitational forces and also by shock waves of pressure from supernova or the hot gas released from nearby bright stars. These forces cause the roughly-spherical globule to collapse and rotate. The process of collapse takes from between 10,000 to 1,000,000 years.

A Central Core and a Protoplanetary Disk:
As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a radial force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a protoplanetary disk or accretion disk). The central core becomes the star; the protoplanetary disk may eventually coalesce into orbiting planets, asteroids, etc.

Protostar:
The contracting cloud heats up due to friction and forms a glowing protostar; this stage lasts for roughly 50 million years. If there is enough material in the protostar, the gravitational collapse and the heating continue.

If there is not enough material in the protostar, one possible outcome is a brown dwarf (a large, not-very-luminous celestial body having a mass between 10²⁸ kg and 84 x 10²⁸ kg).

A Newborn Star:
When a temperature of about 27,000,000°F is reached, nuclear fusion begins. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy. This energy (radiation) production prevents further contraction of the star.

Young stars emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 500,000 miles per hour. These jets may be focused by the star’s magnetic field.

The protostar is now a stable main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die.

Life span:
The most massive stars have the shortest lives. Stars that are 25 to 50 times that of the Sun live for only a few million years. Stars like our Sun live for about 10 billion years. Stars less massive than the Sun have even longer life spans.

	CONSTELLATIONS A constellation is a group of stars that, when seen from Earth, form a pattern. The stars in the sky are divided into 88 constellations. The brightest constellation is Crux (the Southern Cross). The constellation with the greatest number of visible stars in it is Centaurus (the Centaur – with 101 stars). The largest constellation is Hydra (The Water Snake) which extends over 3.158% of the sky. There are also asterisms, smaller apparent star patterns within a constellation, like the Big Dipper (in Ursa Major), the Little Dipper (in Ursa Minor), Keystone (in Hercules), and the Pleiades (in Taurus). The 88 Constellations: The 12 Constellations of the Zodiac The zodiac is a band of 12 constellations along the ecliptic.Aquarius, the water bearer Aries, the ram Cancer, the crab Capricorn, the goat Gemini, the twins Leo, the lion Libra, the scales Pisces, the fish Sagittarius, the archer Scorpius, the scorpion Taurus, the bull Virgo, the virgin The Constellations of the Southern Hemisphere (some are seasonally visible in the Northern Hemisphere): Apus, the bird of paradise Ara, the altar Carina, the ship’s keel Centauras, the centaur Chamaeleon, the chameleon Circinus, the compass Crux, the southern cross Dorado, the swordfish Eridanus, the river Grus, the crane Hydrus, the water snake Indus, the Indian Lepus, the rabbit Mensa, the table Musca, the fly Norma, the surveyor’s level Octans, the octant Pavo, the peacock Phoenix, the phoenix Pictor, the easel Reticulum, the net Triangulum Australe, the southern triangle Tucana, the toucan Vela, the ship’s sails Volans, the flying fish The Constellations of the Northern Hemisphere (some are seasonally visible in the Southern Hemisphere): Andromeda, the princess Antlia, the pump Aquila, the eagle Auriga, the chariot driver Bootes, the herdsman Caelum, the chisel Camelopardalis, the giraffe Canes Venatici, the hunting dogs Canis Major, the big dog Canis Minor, the little dog Cassiopeia, the queen Cepheus, the king Cetus, the whale Columba, the dove Coma Berenices, Berenice’s hair Corona Australis, the southern crown Corona Borealis, the northern crown Corvus, the crow Crater, the cup Cygnus, the swan Delphinus, the dolphin Draco, the dragon Equuleus, the little horse Fornax, the furnace Hercules, the hero Horologium, the clock Hydra, the water snake Lacerta, the lizard Leo Minor, the little lion Lupus, the wolf Lynx, the lynx Lyra, the harp Microscopium, the microscope Monoceros, the unicorn Ophiuchus, the sepent holder Orion, the hunter Pegasus, the flying horse Perseus, the Medusa killer Pisces Austrinus, the southern fish Puppis, the ship’s stern Pyxis, the ship’s compass Sagitta, the arrow Sculptor, the sculptor Scutum, the shield Serpens, the snake Sextans, the sextant Telescopium, the telescope Triangulum, the triangle Ursa Major, the big bear Ursa Minor, the little bear Vulpecula, the little fox CONSTELLATION FAMILY There are many families of constellations, constellations that are either close to one another or have some other relationship. Some constellation families include the Zodiac (the 12 constellations representing the signs of the Zodiac), the Ursa Major Family (10 constellations, including Ursa Major, Ursa Minor Canes Venatici, Bootes, Coma Berenice, Corona Borealis, etc.), the Perseus Family (9 constellations, including Cassiopeia, Cepheus, Andromeda, Perseus, etc.), the Hercules Family (19 constellations, including Hercules, Sagitta, Aquila, etc.), the Orion Family (5 constellations, including Orion, Canis Major, Canis Minor, Monoceros, and Lepus), the Heavenly Waters (9 constellations, including Delphinus, Columba, Equuleus, Vela, Puppis, etc.), the Bayer Group (11 constellations, including Hydrus, Dorado, Volans, Apus,etc.), the La Caille Family (13 constellations, including Norma, Fornax, Circinus, etc.). Some Constellations: ANDROMEDA Andromeda (the princess, the daughter of Cassiopeia) is a northern hemisphere constellation. Alpha Andromeda (called Sirrah, meaning “Horse’s navel”) is the same as Delta Pegasi (from the constellation Pegasus). AQUARIUS [Abbreviation: Aqr] Aquarius (the water bearer) is a constellation of the zodiac. This faint constellation is seen along the ecliptic. The sun passes through Aquarius from mid-February until mid-March. There are many interesting celestial objects in this large constellation, including globular clusters (M2 and M72), planetary nebulae (NGC 7009 and NGC 7293), the star group M73, a double star (zeta Aqr). Aquarius is the radiant for the meteor showers: March Aquarids, Delta Aquarids, Eta Aquarids, and Iota Aquarids. Alpha Aquarii (the brightest star in Aquarius) is called Sadalmelik (meaning the Lucky One of the King) and beta Aquarii (the second-brightest star in Aquarius) is named Sadalsuud (meaning the Luckiest of the Lucky); they are twin supergiants. ARIES [Abbreviation: Ari] Aries (the ram) is a constellation of the zodiac located along the ecliptic between Pisces and Taurus. This tiny, faint constellation is seen just north of the ecliptic. The brightest star in Aries (Alpha Ari) is Hamal, the second-brightest star (Beta Ari) is Sharatan, the third-brightest star (Gamma 2 Ari) is Mesarthim, and the fourth-brightest is Botein (Delta Ari) BIG DIPPER The Big Dipper is a group of 7 stars (it is an asterism and not a constellation) contained in the Northern Hemisphere constellation Ursa major (The Great Bear). BOÖTES Boötes (the herdsman) is a large constellation in the northern hemisphere. The brightest star in Boötes is Arcturus, a red giant that is the fourth brightest star in the sky. CANCER [Abbreviation: Cnc] Cancer (the crab) is a very faint constellation of the zodiac that is shaped like a crab. Cancer is seen along the ecliptic and is located between Leo and Gemini. At the center of the crab’s shell is a huge open cluster, the Beehive cluster, also known as Praesepe (M44). The open cluster M67 is nearby. The brightest star in Cancer is Acubens (meaning claw); the second-brightest is Al Tarf (meaning tip). CANIS MAJOR Canis major (The Great Dog) is a constellation near Orion. The brightest star in Canis Major (and the brightest star in the sky) is Sirius, also known as the dog star. Canis Major is one of the constellation Orion’s hunting dogs (together with Canis Minor). CANIS VENATICI Canes Venatici (The Two Hunting Dogs or greyhounds [of the herdman Boötes]) is a small Northern Hemisphere constellation near Ursa Major and Boötes. The brightest star in Canis Venatici is Cor Caroli (alpha CVn), a blue-white star (magnitude 2.9) with a companion star (magnitude 5). The Whirlpool Nebula (M51), the Sab galaxy (M94) and the globular cluster M3 (magnitude 6) can be found in Canis Venatici. The Canes Venaticids meteor shower occurs from January 13 – January 30. Canes Venatici was named by the Polish astronomer Johannes Hevelius in 1690. CAPRICORNUS [Abbreviation: Cap] Capricornus (the goat) is a constellation of the zodiac. Capricorn is seen along the ecliptic between Sagittarius and Aquarius. The brightest stars in Capricornus are Prima Giedi (Alpha 1 Cap) and Secunda Giedi (Alpha 2 Cap). The globular cluster M30 is in Capricorn. CASSIOPEIA Cassiopeia is an easily-seen constellation that is in the far northern sky. It circles the polestar (Polaris) throughout the year and also straddles the Milky Way. The five major stars of Cassiopeia (also known as “The Lady of the Chair”) are shaped like a “W” (or an “M,” depending on your orientation). All of the stars in Cassiopeia are all less than second magnitude brightness. The brightest star in Cassiopeia is Schedar (alpha CAS), which is a multiple star that is pale rose in color and varies in magnitude from 2.2 to 2.8 magnitudes. The second-brightest, called Caph (beta CAS), is a white star of magnitude 2.4. Cassiopeia contains two open clusters, M52 (magnitude 7.3) and M103 (magnitude 7.4). The strongest radio source, Cassiopeia A, emanates from Cassiopeia; it is the remnant of a supernova which ocurred about A.D. 1660, and is 10,000 light years from us. The constellation Cassiopeia was named for Cassiopeia, the mother of Andromeda (and the wife of Cephus) in Greek mythology. CEPHEUS Cepheus is a house-shaped constellation in the Northern Hemisphere. It is a circumpolar constellation; it never dips under the horizon in the Northern hemisphere. Errai (Gamma Cephei) is a third-magnitude orange subgiant star that will become the Earth’s north polar star in 2,000 years. Cepheus is next to the constellations Andromeda and Perseus. Cepheus was named after King Cepheus in Greek mythology, and is abbreviated Cep. CETUS Cetus is a constellation that straddles the celestial equator. Cetus means whale, and is abbreviated Cet. The first-known variable star Mira, is Omicron Ceti. COLUMBA Columba (the dove, abbreviated Col) is a small constellation in the Southern Hemisphere. This constellation pictures a dove holding an olive branch in its mouth. Columba is a member of the “Heavenly Waters,” a family of constellations. Columba was named by the astronomer Bayer, but was populatized by the French astronomer Augustin Royer in 1679 (before that, Colmba’s stars have been included in the constellation Canis Major). Alpha Col (the brightest star in Columba) is called Phaet; beta Col (the second-brightest star in Columba) is called Wezn. The globular cluster NGC 1851 and the blue magnitude 5 star (mu Col, a “runaway star“) are in Columba. CRUX Crux (or Crux Australis) is the scientific name of the Southern Cross constellation. This well-known, cross-shaped Southern Hemisphere constellation is on the Australian flag. The brightest star in Crux is Acrux (alpha Cru), a double-star system at the base of the cross. The second-brightest star is Becrux or Mimosa (Beta Cru); the third-brightest is Gacrux (Gamma Cru). The Jewel Box (also known as Kappa Crucis) is an open cluster of about 100 stars in the Southern Cross. Crux lies on the Milky Way and is surrounded by the constellation Centaurus on three sides. CYGNUS Cygnus (the swan) is a constellation that is shaped like a large cross. It is also known as the Northern Cross. It is seen along the Milky Way in the northern hemisphere. At its tail is the very bright star Deneb (alpha Cygni), and at its head is the double star Albireo (beta Cygni). DRACO Draco (from the dragon in Greek mythology) is a constellation in the Northern Hemisphere. It is the radiant of the Quadrantids and Draconids (meteor showers). Draco is near the northern celestial pole; the tail of Draco is between the Big Dipper and the Little Dipper. The brightest star in the Draco is Thuban; it is located in the dragon’s tail. (Thuban means “dragon or serpent” in Arabic. Thuban was the Earth’s pole star about 5,000 years ago.) The second-brightest star in the Draco is Rastaban (which means “head of the dragon or serpent”). ERIDANUS [Abbreviation: Eri] Eridanus (the River) is a southern constellation that is may have been named for the river Nile, the river Euphrates, or for the river of tears wept by the mythical Heliades. Eridanus is located near the constellations Fornax and Lepus. The brightest star in Eridanus, alpha Eri, is Achernar (meaning “end of the river”); it is the 9th brightest star in the sky (magnitude 0.46). The second-brightest, beta Eri, is Cursa (meaning “chair/footstool of Orion” ). The third-brightest, gamma Eri, is Zaurak (meaning “boat” ). The fourth-brightest, delta Eri, is Rana (meaning “frog” ). The fifth-brightest, zeta Eri, is Zibal (part of the “Ostrich’s Nest”). Eridanus was one of the original 48 constellations first noted by Ptolemy GEMINI [Abbreviation: Gem] Gemini (also known as “The Twins”) is one of the constellations of the zodiac, located along the ecliptic between Taurus and Cancer. The brightest stars in Gemini are Castor (a sextuple star – three double stars) and Pollux (a 1st magnitude yellow star). The open cluster M35 is located in Gemini. The Geminid meteor shower seems to radiate from Gemini. HERCULES Hercules is a Northern Hemisphere constellation that is the fifth largest in the sky. It is named for Hercules, the legendary hero of Greek mythology. The brightest of its rather dim stars is Ras Algethi (alpha Her), a variable red supergiant. The four stars of the central trapezoid within Hercules, epsilon Her, zeta Her, eta Her and pi Her, form the asterism called Keystone. The globular star cluster M13 is located on the western part of the Keystone. The Tau Herculid meteor shower seems to radiate from Hercules. LEO [Abbreviation: Leo] Leo (the lion) is a constellation of the zodiac that is shaped like a lion. Leo is seen along the Milky Way in the Northern Hemisphere along the ecliptic between Virgo and Cancer. The brightest star (Alpha Leo) in Leo is Regulus (meaning “little king”). The second-brightest star (Beta Leo) is Denebola (meaning “tail of the lion”), and the third-brightest star (Gamma 1 Leo) is Algieba (meaning “forehead”). The spiral galaxies M65, M66, M95, M96, and the elliptical galaxy M105 are nearby. LIBRA [Abbreviation: Lib] Libra (the balance) is a constellation of the zodiac. Libra is seen along the ecliptic between Virgo and Scorpius. The brightest star (Alpha 2 Lib) in Libra is Zuben Elgenubi, the second-brightest star (Beta Lib) is Zuben Elschemali, the third-brightest star (Gamma Lib) is Zuben Elakrab, and the fourth-brightest star (Delta Lib) is Zuben Elakribi. LUPUS Lupus (the wolf) is a constellation in the Southern Hemisphere. It is located between Centaurus to the west, Scorpius to the northeast. The open star cluster NGC 5822 is in Lupus. LYRA Lyra (the lyre) is a small, bright constellation in the Northern Hemisphere. The brightest star in Lyra is Vega (meaning “falling eagle”) which has a white color and is the fifth brightest star in the sky. ORION Orion, also known as “The Hunter,” is a constellation. The brightest stars in Orion are Rigel. Betelgeuse, and Bellatrix. The Horsehead Nebula and the nebulae M42 and M43 (called the Orion nebula) are also in this constellation. PEGASUS Pegasus (the flying horse) is a large northern hemisphere constellation. Delta Pegasi is shared with the constellation Andromeda. 51 Pegasi (51 Peg), off the Great Square of Pegasus, is a nearby, Sun-like star that has been found to have an orbiting planet. PERSEUS Perseus is a constellation in the Milky Way in the Northen Hemisphere. It is abbreviated Per. Perseus’ major star is alpha Per, called Marfak or Algenib; it is an F5 supergiant. Beta Per is the eclipsing binary Algol. Perseus was a hero in Greek mythology who killed the monstrous Medusa. Each year in August, the Perseid meteor shower radiates from this constellation’s northern part. PISCES [Abbreviation: Psc] Pisces (the fish) is a faint equatorial constellation of the zodiac. Pisces is seen along the ecliptic between Aries and Aquarius. The brightest star in Pisces (Alpha Psc) is Al Rischa (or Alrisha), the second-brightest-star (Beta Psc) is Fum al Samakah. The spiral galaxy M74 is in Pisces. SAGITTARIUS [Abbreviation: Sgr] Sagittarius is the ninth constellation of the zodiac. To the ancients, it represented a centaur (half-man, half-horse) archer who was aiming at the Scorpion (the next constellation) which bit Orion. Its central section (the archer’s chest) also resembles a teapot. The center of the Milky Way Galaxy is in the direction of Sagittarius. Many meteor showers, including the Sagittariids, seem to radiate from Sagittarius. The brightest star in Sagittarius (Alpha Sgr) is Rukbat (which means “knee” in Arabic). SCORPIUS [Abbreviation: Sco] Scorpius (the scorpion) is a constellation of the zodiac. This constellation is seen along the ecliptic between Libra and Sagittarius. The brightest star in Scorpius is Antares, a red supergiant star that is about 500 light-years away from Earth and is about 230 times as big as the Sun. The second-brightest (Beta 1 Sco) is Graffias. SOUTHERN CROSS Crux is the scientific name of the Southern Cross constellation. This well-known, cross-shaped Southern Hemisphere constellation is on the Australian flag. The brightest star in Crux is Acrux, a double-star system at the base of the cross. This constellation lies on the Milky Way and is surrounded by the constellation Centaurus on three sides. It is abbreviated Cru. TAURUS [Abbreviation: Tau] Taurus (the bull) is a constellation of the zodiac. This constellation is seen along the ecliptic between Aries and Gemini. The brightest star (Alpha Tau) in Taurus is Aldebaran, which is one of the bull’s eyes (Aldebaran is the 13th brightest star in the sky). The second-brightest star in Taurus (Beta Tau) is Alnath, the third brightest (Gamma Tau) is Hyadum I. URSA MAJOR Ursa major (The Great Bear) is a well-known constellation in the Northern Hemisphere that contains the 7 stars of the Big Dipper. URSA MINOR Ursa minor (The Little Bear) is a Northern Hemisphere constellation that is also known as the Little Dipper. This group of stars starts at Polaris, the pole star of the Northern Hemisphere VIRGO [Abbreviation: Vir] Virgo (the virgin) is a constellation of the zodiac. It is located along the ecliptic between Leo and Libra. Virgo is seen along the ecliptic. The brightest star in Virgo is Spica (meaning “ear of wheat”). Virgo is the second-largest constellation (of the 88 constellations), after Hydra.