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STARS Revised June 2009 By the Queensland Wilderness Adventure Club (Sponsored by the Youth Ministries of the South Queensland Conference of SDA Church) Edited by Dr Bruce Hands M Astron [JCU] MB BS [U of Q] CONTENTS 1 a What is the earth's nearest celestial neighbour ? Page 3 b What is the distance from the earth ? 3 c What governs the tide ? 4 d What causes an eclipse ? 5 e What is a shooting star ? 6 f How fast does light travel ? 6 2 Make a diagram showing relative positions and movements of the earth, sun and moon. Show positions and movements for eclipses of the sun and moon. 6 3 Make a diagram of our solar system and be able to name the planets in order from the sun. 8 4 What is the difference between planets and stars ? 5 What is a constellation? Name and point out six. visible throughout the year. 6 Draw a chart of the Southern Cross, Orion and Scorpio. 35 7 What is the Milky Way ? 36 8 What is the morning and evening star ? 9 Explain zenith and Nadir. 10 What is the Aurora Australis? Identify in the sky eight stars. 11 Name two constellations 12 Observe the Milky Way in the night sky. Why does it carry both names ? 37 38 What causes it ? Stars 1 Honour Trainer's Notes_Extra Info.doc 39 Page 2 of 39 STARS 1.a What is the earth's nearest celestial neighbour ? Apart from the components in our solar system, the nearest celestial neighbour is commonly quoted as Alpha Centauri. Alpha Centauri is that one of the two pointers to the Southern Cross that is furthest from the Southern Cross and is also the brighter of the two pointers. Alpha Centauri is a binary star which means there are two stars orbiting around each other. To complicate matters, Proxima Centauri has a third star that orbits around the binary. Which of the three stars in the complex is closest to earth varies with the orbital positions in the complex. Currently, Proxima Centauri is closest to earth and is about 4.2 light years from earth. 1.b What is the distance from the earth ? There are different ways of measuring interstellar distances. A light year is the distance that light travels in one year. In light years, Alpha Centuri is 4.2 light years away. The speed of light is about 3 x 10 to the 8th power m/s or 186 000 miles per second. That means that the distance that light travels in one year is 2.99792458 x 100 000 000 x 60 x 60 x 24 x 365.25636 or about 1 followed by 16 zeros with the answer in meters. A parsec is another unit of stellar distance. It is measured by the apparent movement of a star against its background when measured from earth by two measurements taken from opposite sides of earths orbit. That is, by two measurements taken six months apart. In those units, Proxima Centauri is 1.3 pc away. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 3 of 39 1.c What governs the tide? The major influencing factor is the moon. The sun has a smaller influence. Moons Gravitational Pull on oceans Moon Sun Earth High tide 12 hours apart When sun & moon pull in the same alignment King tide as sun & moon pull in same alignment Stars 1 Honour Trainer's Notes_Extra Info.doc Page 4 of 39 1.d What causes an eclipse ? Sun’s radiation Moon In Earth’s shadow Sun Earth Solar eclipse As seen by someone on earth’s surface in the moons shadow looking at the sun Lunar eclipse as seen by someone on earth’s surface at night looking at the moon Moon’s shadow is on part of earth’s surface Stars 1 Honour Trainer's Notes_Extra Info.doc Page 5 of 39 1.e What is a shooting star? Meteoroids are dust particles left in the orbit of a comet. They are believed to originate from comets. When meteoroids intersect with the earth’s atmosphere, they produce visible streaks of light as they heat, burn and vaporise. Then they are called meteors. This streak of burning debris is commonly called a shooting star. Asteroids are larger and generally follow a different orbit. Asteroids (or small planets) are small, naturally formed solid bodies that orbit the sun. They are airless and no outflow of gas or dust can be detected. 1.f How fast does light travel? The speed of light is 2.99792458 x 108 m/s – generally taken as 3 x 108 m/s or 186 000 miles per second. This is roughly 660 million miles per hour. 2. Make a diagram showing relative positions and movements of the earth, sun and moon. Show positions and movements for eclipses of the sun and moon. Earth’s axis Earth’s North pole - S moon E Earth’s South pole Earth’s rotation Stars 1 Honour Trainer's Notes_Extra Info.doc Page 6 of 39 During an eclipse: Sun’s radiation Moon In Earth’s shadow Sun Earth Solar eclipse As seen by someone on earth’s surface in the moons shadow looking at the sun Lunar eclipse as seen by someone on earth’s surface at night looking at the moon Moon’s shadow is on part of earth’s surface Stars 1 Honour Trainer's Notes_Extra Info.doc Page 7 of 39 3. Make a diagram of our solar system and be able to name the planets in order from the sun. The planets in order from the sun are: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune The relative sizes of the http://antwrp.gfsc.nasa.gov/apod planets are illustrated 28 Aug 2006 Stars 1 Honour Trainer's Notes_Extra Info.doc on the Page 8 of 39 web site The relative sizes of the orbits are illustrated on the web site www.nineplanets.org The small print names in this diagram are Venus, Earth, Mars, Jupiter and Saturn. innermost orbit of Mercury is red and not labelled.] The orbits named below are Jupiter, Saturn, Uranus, Neptune and Pluto. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 9 of 39 [The If we use the distance from the sun to earth as one unit [= one astronomical unit = 1 AU] then the relative distances of the planets from the sun can be compared by studying the table below. Semi major axis AU Eccentricity Inclination degrees Period years Mercury 0.38710 0.205631 7.0048 0.2408 Venus 0.72333 0.006773 3.3947 0.6152 =225 days Earth 1.00000 0.016710 0.0000 1.0000 Mars 1.52366 0.093412 1.8506 1.8807 Jupiter 5.20336 0.048393 1.3053 11.856 Saturn 9.53707 0.054151 2.4845 29.424 Uranus 19.1913 0.047168 0.7699 83.747 Neptune 30.0690 0.008586 1.7692 163.723 Pluto 39.4817 0.248808 17.1417 2488.02 For years, Pluto irritated Astronomers because it did not seem to fit into the order of things very well. This was because of its size, orbit shape and inclination and the unusually large moon that it possessed. Various suggestions were proposed but now the truth is becoming apparent. British astronomer Kenneth Edgeworth and Dutch-American Gerard Kuiper suggested during 1943 to 1950 something that has only been seen to exist in the 1990s. Pluto is the King of millions of Edgeworth Kuiper Objects (EKO), also known as Kuiper Belt Objects (KBO]. These constitute a whole band of objects that have been detected. Most are in the size range of 100-500km. About 90% are in a roughly circular orbit beyond Neptune. Another scattered band exists in an inclined elliptical orbit and extends out to about 200AU. These are more than another asteroid belt as is between Mars and Jupiter. EKOs have a combined mass in the order of 300 times the total mass of all asteroids. They have a very diverse range of colours, surfaces, sizes, surface activities and some possess satellites and atmospheres. Several binary pairs are known to exist. The short period comets are believed to come from this band. Pluto is 1485 miles [2390 km] in diameter. The next known largest EKO is Quaoar. [“kwa-whar”] which has a diameter of 800 miles [1300 km]. Pluto’s moon Charon has a diameter of 737 miles [1186 km]. Quaoar is about 10 AU beyond Pluto. Varuna is another EKO and has a diameter of 560 miles [900 km]. Beyond the EKOs may be more planets – even more giant planets like Jupiter. Such planets are beginning to become within detectable range with the advent of giant telescopes. Dark giants way beyond the present imagined limit of our solar system would be difficult to see as they creep ever so slowly around our sun. They are suspected to exist because of distortions in the paths of comets. There are also planetary mass objects [PMO]. These are not easily categorized. PMOs are planet sized objects beyond the reach of nearby stars. They are bigger than Jupiter but too small to be a star. To be a star the object has to be at least 75 Jupiter masses. This is the mass required to squeeze the core to maintain hydrogen fusion. If deuterium fusion is the energy source, 13 Jupiter masses would be sufficient to maintain a star for awhile. PMOs so far detected are less than 12 Jupiter masses. Mass is not easily calculated when the object is being gravitationally influenced by nearby objects. But if they are a planet, what are they doing drifting around beyond the reach of any star? An alternative name to PMO is “planetar”. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 10 of 39 4. What is the difference between planets and stars? stars. Identify in the sky eight To be able to distinguish between a planet and a star: Be familiar with the constellations in the zodiac (the path through which the sun & moon travel) so as to be able to recognise a “star” that is out of place. That works for the brighter planets like Jupiter, Venus, Mars and Saturn but is a bit hard for the fainter planets. The planets give off a more steady light and the stars give off a twinkling light. That is because the light from the stars travel a far greater distance and will be distorted and bent a little somewhere in its travels to earth. When viewed over several nights, a planet will be noticed to steadily migrate through the stars in the constellation where the planet is observed. The word “planet” comes from a Greek word that means “wanderer” because the Greeks observed that some “stars” were not fixed in position with the rest of the stars but wandered along independently. Planets are found only in the band of sky where the sun and moon are seen. The twenty brightest stars are: Sirius Alpha Canis Majoris Canopus Alpha Carinae Rigel Kent Alpha Centauri Arcturus Alpha Bootis Vega Alpha Lyrae Capella Alpha Aurigue Rigel Beta Orionis Procyon Alpha Canis Minoris Achernar Alpha Eridani Hadar Beta Centauri Altair Alpha Aquilae Betelgeuse Alpha Orionis Aldebaran Alpha Tauri Acrux Alpha Crucis Spica Alpha Virginis Antares Alpha Scorpio Pollux Beta Geminorium Fomalhaut Alpha Pisces Austrini Denub Alpha Cygni Mimosa Beta Crucis Stars 1 Honour Trainer's Notes_Extra Info.doc Page 11 of 39 The stars are named by use of the Greek alphabet. The brightest star in each constellation is named alpha followed by the constellation name. Then the next brightest star is called beta followed by the constellation name. Thus Sirius is the common name for the brightest star in the constellation of Canis Major and the alternative name for the star Sirius is Alpha Canis Majoris. You need to be able to identify at least eight stars. star constellation charts that follow will do. Any of the stars above or any in the The Greek alphabet is [diagram from Wikipedia]: 5. What is a constellation ? visible throughout the year. Name and point out six. Name two constellations The word “constellation” means in Latin ”set with stars”. A constellation is a grouping of stars that have been arbitrarily considered to be a group and usually named after some object, animal or mythological being. Some significant constellations are illustrated below. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 12 of 39 Crux This is the smallest constellation. Acrux is a triple system and the brightest pair of the three are about 3000 and 1900 times more luminous than our sun. The distance between the pair is about 6 times the distance across our solar system. Alpha, Beta, Delta, Zeta plus some stars in Scorpius and Centaurus all form a group moving in similar motion. Dcrux is a variable star. The coal sack is a dark nebula filled with inert dust & gas. The jewel box (NGC 4755 or KCrux) is an open cluster of many & coloured stars, the most luminous of which is about 80 000 times that of our sun. Gamma Crucis Delta Crucis Beta Crucis Epsilon Crucis The Jewel Box The Coalsack Alpha Crucis & Zeta Crucis Stars 1 Honour Trainer's Notes_Extra Info.doc Page 13 of 39 Centaurus Alpha Centauri is the closest binary to our sun and Proxima Centauri orbits this binary. Omega Centauri (NGC 5139) is globular cluster. The two pointers Apha & Beta Centauri point to Crux. (NGC is the abbreviation for New General Catalogue which is a listing of star clusters, nebulae and galaxies.) Omega Centauri was named as a fourth magnitude star almost 2000 years ago. In 1677 Halley observed it as a cluster and not as a star. Now there are known to be over a million stars in the cluster with some 1000 times more bright than our sun. The cluster is about 17 000 light years away. (Remember that the Milky Way Galaxy is about 100 000 light years in diameter and about 3 000 light years thick.) NGC 5128 is a galaxy with a dark band through it that gives the impression of a galaxy with one very big explosion blowing it into two pieces – or possibly two separate galaxies colliding. Measurements place it about 25 million light years away. NGC 5128 Omega Centauri Beta Centauri Alpha Centauri Crux Stars 1 Honour Trainer's Notes_Extra Info.doc Page 14 of 39 Musca Crux Beta Muscae Alpha Muscae Alpha Muscae means the Southern Fly. Beta Muscae is a slowly rotating binary of about 400 or 500 years. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 15 of 39 The twelve constellations in the Zodiac are Taurus, Aries, Pisces, Aquarius, Capricornus, Sagittarius, Scorpio, Libra, Virgo, Leo, Cancer and Gemini. Taurus Zeta Tauri Nath or Beta Tauri M1 Crab Nebula Aldebaran Alpha Tauri Pleiades or M45 Ecliptic (the projection of earths orbit onto the celestial Sphere = the path of the sun across the sky) Stars 1 Honour Trainer's Notes_Extra Info.doc Page 16 of 39 Aldebaran is Alpha Tauri. It means the Follower as it follows the Pleiades across the sky. It was involved in numerous Roman, Greek , Persian and Eastern stories. Arcturus in the book of Job is thought to be Aldebaran. It is about 40 times our sun’s diameter and is about 68 light years away. Aldebaran is one of the few first magnitude stars that may be occulted by the moon. The disappearance of the star is startlingly abrupt, particularly so at the moons dark edge. The V shaped central group is the Hyades star cluster group and represents the bull’s head – Aldebaran being the eye and Zeta and Beta being the horns. (Aldebaran is not actually in the star cluster.) Numerous old stories associate the Hyades with rainy weather. The whole of the Hyades group is moving towards Orion and there are several hundred stars in the cluster. Zeta Tauri has no English common name but translation of it’s Chinese name means the Gate of Heaven. It is a closely paired binary. Zeta is a shell star, having a permanent shell of gas around it. The Crab Nebula is about 6 light years in diameter and expanding at 600 miles per second (= 50 million miles per day). This is believed to be the supernova explosion of July 4 1054 AD. The explosion would have been giving the light of about 400 million suns. If the law of averages prevales, we are over due for another supernova. The Pleiades or Seven Sisters is the most famous star cluster in the heavens. In Greek this means To Sail. The rising of the Pleiades was the sign of the opening of the navigational season in the Mediterranean. There are many legends associated with the Pleiades. Eta Tauri or Alcyone is the brightest star in the Pleiades. The whole group is leisurely drifting along at about 25 miles per second. The entire cluster is enveloped in a vast faint nebulosity. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 17 of 39 Taurus Pleiades Aries The Ram is named in Greek mythology as having golden fleece. Alpha Arietis or Hamar Ecliptic Beta Arietis or Sheratan Stars 1 Honour Trainer's Notes_Extra Info.doc Page 18 of 39 Beta Arietis Pisces Alrisha Alpha Piscium M74 Eta Piscium Ecliptic Stars 1 Honour Trainer's Notes_Extra Info.doc Page 19 of 39 Alrisha means the cord in Arabic. This is the cord that ties the two fish together. It is fine binary of strange colours that are described differently by different observers. M74 or NGC 628 is a face on spiral galaxy very close to Eta Piscium and about the same size as our galaxy (80 000 light years in diameter) but quite a lot less luminous. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 20 of 39 Aquarius Pisces Zeta Pi Gamma Sadalmelik (alpha) Helix Nebula NGC 7293 M2 NGC 7089 Sadalsud (beta) M73 Ecliptic Stars 1 Honour Trainer's Notes_Extra Info.doc Page 21 of 39 Gamma, Eta, Zeta and Pi make the water carrier or jug of Aquarius. M2 is a globular cluster easily seen with a small telescope, say 2 to 10 inch. It is about 50 000 light years away. There are more than 100 000 stars in the cluster. The cluster shines with about half a million times the luminosity of our sun. If we were at M2 and looking at our sun, we would be able to see our sun with only the greatest of telescopes. M72 is a globular cluster about 60 000 light years away and about 85 light years in diameter. A 10 inch telescope will see the cluster but a much larger one is needed to see it’s interesting characteristics. With better than a 10 inch telescope, the Saturn nebula is seen with a fluorescent green tint. It’s shape is reminescent of Saturn. Binoculars will pick up the Helix nebula as a circular hazy spot. Estimations on it’s distance vary a lot but if a distance of 450 light years is accepted, then the diameter of the bluish green nebula is about 1.75 light years across. The helix nebular is so named because the nebula is made of two rings easily seen forming a “two ring coil”. It is made of tenuous gas illuminated by a tiny but hot central star. Although the dwarf star is only about 2% the diameter of our sun, it has a temperature of about 100 000 degrees Kelvin. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 22 of 39 Capricornus Alpha Aquarius Delta M30 Gamma Beta M2 M73 Beta Alpha Ecliptic Alpha Capricorni is made of two stars Alpha 1 and Alpha 2. They are close together but not a binary. Each has a small companion star. Beta Capricorni is a binary and Beta B is a close double itself. Close to Delta Capricorni is where Neptune was first observed in 1846. M30 is a globular cluster about 40 000 light years away. Capricornus was associated with the lowest point reached by the sun below the celestial equator (= 23.5 degrees). It gave it’s name to the Tropic of Capricorn for latitude 23.5 degrees south. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 23 of 39 Sagitarrius Archer Beta Capricorni Alpha Capricorni M55 M22 M25 Kaus Australis M24 M17 Gamma Sagittarii M8 M20(Trifid) M23 Ecliptic (the dark line part is the bow and gamma is the tip of the arrow being shot at the scorpion) Stars 1 Honour Trainer's Notes_Extra Info.doc Page 24 of 39 Sagittarius contains a wide range of objects. Also, the Greek lettering is quite out of order. The brightest star is Epsilon (Kaus Australis). Behind Gamma Sagittarii are some bright star clouds in the Milky Way and this region marks the central hub of our galaxy. There are some curious winding lanes of dust in that area. Gamma has a yellow tint. Connected by a solid line in the diagram above is the “milk dipper”, the end of the handle being Epsilon. Below Epsilon is M8, the Lagoon Nebula – so named because of a dark swirl running through the middle of the nebula. It is about 3 000 light years away and can be easily seen with the naked eye. It is possible that M20 (the Trifid nebula) is also part of the same nebulosity. M17 is also known as the Swan nebula, Omega nebula and Horseshoe nebula. Although it is very faint, it is a large nebula being about 40 light years across. M24 is also visible to the naked eye. M55 can be seen by binoculars. It is a globular star cluster about 20 000 light years away and about 80 light years in diameter. The luminosity is about 100 000 that of our sun. As we look towards this area of the Milky way, we are looking towards the Galactic centre and there are millions of stars and associated structures. We are about 30 000 light years from the galactic centre. The Milky way disc is about 80 000 (100 000 if some far flung traces are counted) in diameter and about 10 00 light years thick at the hub. The disc is about 3 light years thick. The overall shape is a barred spiralled galaxy. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 25 of 39 Scorpius Scorpion Sagitarrius M7 M23 Antares (alpha) M4 M80 delta Stars 1 Honour Trainer's Notes_Extra Info.doc Page 26 of 39 beta Antares (alpha) in Greek means “the rival of Mars”, so called because of its red color. In Roman the translation means the “heart of the scorpion” so called because of its position in the constellation. It is the 15th brightest star in the sky. Some think that “the chambers of the south” in the book of Job refer to this constellation. In ancient times, this constellation was given a larger area of the night sky and what is now alpha and beta Librae were previously in Scorpio. The translation of the two common names for alpha and beta Librae is the northern claw and the southern claw. In the Middle Ages, the scorpion became the symbol of unyielding fortitude, from the legend that if this creature was surrounded by enemies, it would sting itself to death rather than to submit to capture. Lord Byron refers to this when he wrote “The mind that broods o’er guilty woes Is like the scorpion girt by fire“ Antares is a supergiant star about 600 million miles in diameter or about 700 times the size of our sun. It is somewhat egg shaped and is about 9 000 more luminous than our sun. The density of Antares is about a millionth that of the sun and therefore is only about 10 to 15 more in mass than our sun. Antares varies in both its diameter and luminosity. There is a small green companion star to Antars. This companion star can be seen with a 6 inch telescope when conditions are right. There are about 100 stars in what is called the “Scorpio-Centaurus Association”. These stars are in Scorpio, Crux and Centaurus and are a part of the Local Star Cloud which shows a common space motion. They form a part of one of the spiral arms of our galaxy. Beta Scorpii has two small companions, one of which is itself a double. M4 (NGC6121) is a globular cluster that is visible to the naked eye. A 4 inch refractor starts to show the individual stars. M6 and M7 are open star clusters seen by the naked eye. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 27 of 39 Beta Scorpio Libra Delta Scorpii 5897 Beta Alpha Beta Librae is the Northern Claw of Scorpius and some observers say it is greenish. Star colours are strangely elusive and vary from observer to observer. If one sees it to be greenish, then it is the only star to be green without a red companion. Alpha Librae is the Southern Claw of Scorpius. 48 Librae is the classic example of a shell star. The magnitude is 4.85. The star is surrounded by an outer shell of turbulent gases which occasionally show violent expansion velocities up to 60 miles per second. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 28 of 39 Alpha Librae Virgo Spica M104 Sombrero Virgo galaxy cluster Beta Virginis Stars 1 Honour Trainer's Notes_Extra Info.doc Page 29 of 39 Spica is Alpha Virginis and is a close, massive binary. The brighter of the two is also a pulsating variable. Antares and Spica are the 15th and 16th brightest star in the heavens and have magnititudes very close to 1. Spica is about 2 300 times more luminous than our sun. It is about 275 light years away from us. In ancient writings this constellation represented a girl holding an ear of wheat because of its timing to harvest season. In other customs, it represented a girl standing for justice. With greater than a six inch telescope, thousands of galaxies can be seen in the Virgo galaxy cluster. M104 is probably part of the Virgo galaxy cluster but is seen as a galaxy edge on and has a dark ring of equatorial dust thus giving it the name of Sombrero. A ten inch telescope is advised to see the band. It is a massive galaxy about 130 000 light years in diameter (counting the faint outer portions). It has a mass of about 1.3 trillion suns. The speed of this galaxy (red shift = 700 miles per second) has been heavily implicated in calculations in the expanding nature of the universe. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 30 of 39 Beta Virginis Beta Leonis Leo Regulus Gamma Leonis Regulus lies at the base of the sickle of Leo and represents the head and mane of the lion. Beta or Denebola is the lions tail. Close to Gamma Leonis is the radiant point of the 33 year cycling Leonid Meteor showers, one of which was very spectacular on Nov 13 1833. It is due on Nov 17 or 18 1999. Towards Virgo, many galaxies are seen as part of the Virgo Cluster. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 31 of 39 Regulus Cancer Alpha M44 Beta Several thousand years ago, this constellation was background of the sun during the summer solstice, its maximun elevation above the celestial equator. At that time, the sun would be directly overhead if the observer was at a latitude of 23.5 degrees north. This line around the earth is the Tropic of Cancer (appropriate from the point of view of causing skin cancer - Cancer is really the “Crab”). Currently, the solstice lies between Gemini and Taurus. The brightest stars in Cancer are 4th magnitude stars. M44 is known as Praesepe or the Beehive Cluster or the Manger. It is one of the largest, nearest and brightest clusters, being easily seen with the naked eye. Prior to the invention of tetescopes, it was called the Cloudy One or Little Cloud or Little Mist. Galileo counted 36 stars. Over the 14th magnitude, there are now known to be 350 stars in the cluster. This cluster and the Hyades Cluster in Taurus are moving with almost equal speed (25 miles per second) and parallel to each other. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 32 of 39 Beta Cancri Gemini Pollux Castor Orion M35 These are the tips of the two horns in Taurus Stars 1 Honour Trainer's Notes_Extra Info.doc Page 33 of 39 Castor is alpha although it is (currently) fainter visually (photographically it is brighter). It is made up of three binaries Castor is also known as the Horseman. The pair of twinkling stars have been known for centuries as the heavenly twins. Castor was the first object beyond our solar system in which the force of gravitation was shown to be in operation. The main pair of stars in Castor are only about the distance across our solar system apart. Neptune and Pluto were discovered when passing through Gemini. The constellations around the south celestial pole are visible throughout the year (for those who live in the southern hemisphere) Orion The hunters head Betelgeuse Bellatrix The hunters shoulders The hunters belt Zeta The hunters sword Sword of Orion M42 & M43 Rigel The hunters legs Saiph (the solid line section is commonly called the “saucepan”) Stars 1 Honour Trainer's Notes_Extra Info.doc Page 34 of 39 Stars around the Southern Celestial Pole. Crux Pointers Musca Triangulum Australe South Celestial Pole Canopus (in Carina) Larger Magellanic Cloud Smaller Magellanic Cloud Hydrus Archernar 6. Draw a chart of the Southern Cross, Orion and Scorpio. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 35 of 39 7. What is the Milky Way ? Observe the Milky Way in the night sky. The Milky Way is the galaxy in which our solar system exists. It is a barred spiral galaxy about 100 000 light years across and about 3 000 light years thick at the centre. We are about 25 000 light years from the centre. When we look towards Sagittarius, we are looking towards the centre. It seems that there are countless galaxies in the universe. The Milky Way has a spherical bulge at the centre. Young stars are in that bulge. A flat pancake about 600 light years thick of young stars surrounds the central bulge and is known as the thin disk. The Milky Way that we see at night is in the thin disk. Older stars are in a thick disk that is about three times thicker than the thin disk. A sphere of old spherical star clusters surrounds the flat disks. The spiral arms of the galaxy revolve around the galaxy at a different speed to our galaxy. That means our sun is slowly crossing the spiral arms. The speed of crossing those arms determines how many comets are disturbed from the Oort cloud and sent in towards our solar system. Solar systems further out than the Milky Way cross the spiral arms at a faster rate and are not protected as well from cosmic collisions. The centre of the Milky Way contains a tangle of strong magnetic fields. The galaxy that is closest to the Milky Way is a dwarf, irregular galaxy in the constellation Canis Major and is 42,000 light years away. The Larger Magellanic cloud [at 170 000 light years away] and the Smaller Magellanic cloud [close to 200 000 light years away] are next closest. All three galaxies appear to be being sucked into the Milky Way. Considering the diameter of the Milky Way, all these three galaxies are very close. There are 12 dwarf galaxies within 500,000 ly of the Milky Way. Halo of globular clusters Bulge Thin disk Thick disk Milky Way Moving groups are collections of stars that share the same motion through space. An example of a moving group is the group of about 40 stars that includes Sirius, Alpha Coronae Borealis, Beta Eridani and Zeta Leonis. An open cluster such as the Pleiades is more compact than a moving group but not as compact as a globular cluster. Calculations of the energy required to sustain the known visible constituents of the universe tell us that something in excess of 50% of what is immediately around us cannot be detected. That is called dark matter. It seems that something in excess of 50% of the universe is invisible. Furthermore, the number of stars in space increases by the cubed power of the distance from us but the luminosity decreases by the squared power as distance is increased. The reason that the night sky is not brighter than the day sky is that the light from distant stars has not yet reached us. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 36 of 39 The Milky Way is that blurry, hazey white smudge that extends roughly from east to west through the night sky. As you study more carefully, you notice that the Milky Way does not follow the course of the moon and planets. When looking at a star map that shows the Milky Way and the course of the sun through the stars, you will notice that there is a significantly different course. This is because the disc of our galaxy is is not parallel to the disc of our solar system. The constellations that the sun passes through are those constellations that are in the same plain as our earth and they are the twelve signs of the zodiac. The mass of stars in the disc of our galaxy will be above the path of the sun (ecliptic) for six months and below the ecliptic for the other six months. 8. What is the morning and evening Star ? Why does carry both names ? Venus is called both the morning and the evening star. Venus is cloud covered and therefore reflects a lot of the sun’s radiation. That makes Venus shine very brightly. When Venus is near the western horizon, it is called the evening star. At sunset it is commonly the first “star” to be seen in the semi darkened sky as it is commonly the brightest object in that part of the twilight sky. Likewise at sunrise, when Venus is in the eastern sky it is the last “star” to disappear into the brightening sky as it is commonly the brightest object in the predawn sky. Because the orbit of Venus is inside earths orbit, Venus has to be seen between the sun and the tangent at earths orbit Sun Venus Orbit Earth Orbit To an observer on earth, Venus has to be seen to the left of the tangent. This means Venus has to be seen towards the east in the hours between midnight and dawn or towards the west in the hours between sunset and midnight. Very rarely [every eight years], the morning and evening star can be seen in the same 24 hour period. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 37 of 39 9. Explain zenith and nadir. As defined on the web http://en.wikipedia.org , Zenith is the vertical direction pointing away from the direction of the force of gravity. Nadir is the direction pointing in the same direction as gravity. 10. What is the Aurora Australis? Energetic solar flares eject streams of charged particles into space as part of the solar wind. These interect with the region governed by the earth’s magnetic field and give rise to showers of energetic particles that follow the earths masgnetic field down towards the earths surface. In the atmosphere between 100-300km these excite atoms of gas which emit light and this is seen as curtains and bands of coloured light. They are seen more vividly closer to midnight. In the southern hemisphere these are called the Aurora Australia. They are seen usually in latitudes over 50 degrees. In the northern hemisphere, the same phenomena is called the Aurora borealis and below is a sample of the aurora borealis as recorded on the web site http://antwrp.gfsc.nasa.gov/apod 9 April 2007. Stars 1 Honour Trainer's Notes_Extra Info.doc Page 38 of 39 Stars 1 Honour Trainer's Notes_Extra Info.doc Page 39 of 39