hands on

The skyby Ray Forma

Go to the end of this article to find more detailed information about the Star Chart, and howto use it, and a table of Moon phases. Use the table to help you with the timing of successfulastronomy evenings for students. The best time for an astronomy evening is usually six daysafter New Moon.

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Chart centre: 19h 26,6m -35^ 00' Universal time: Aug 23, 2006 10:00 LMT: 08:00 pm Location: 140° 00' E 35° 00' S

Fig 1. The Star chart

4 0 TEACHING SCIENCE I VOLUME 52 NO 2 \ WINTER 2006

During evenings at this tifneof the year, it is obvious thatthe centre of out Milky Way

galaxy passes through local zenith.The star chart shows the position of ourGalactic Centre, labelled GC, lying inthe constellation of Sagittarius, veryclose to the boundary with Ophiuchus,the Serpent Bearer. I wonder, how big isthe black hole that evidence indicatesis at the centre? Clouds of dust totallyobscure the optical view of our Galaxy'scentre, and we should perhaps bethankful for this. Events at the centreprobably generate lots of light, and thiswould keep us awake at this time ofthe year if that light could penetrate thedust.

There are many lovely nebulae in thearea around the galactic centre, andit is worth scanning for them throughbinoculars with a decent aperture, on atripod or similar support.

South of the Galactic Centre lies the bigwintertime constellation of Scorpius,the celestial scorpion, one ofthe fewconstellations that actually looks likethe creature it represents. Antares, oro-Scorpii, is an obvious red supergiantthat is 530 light-years distant. Its surfacetemperature of 3280K gives it thecolour similar to that of Mars. SinceAntares is found within the Zodiac,which contains the apparent path ofthe Sun and planets, it is commonlymistaken for the red planet, a factshown by its name, Antares, or 'Ant-Ares', which means 'like Mars', 'Ares'being the Greek name for the god ofwar. This magnificent first magnitudestar is ranked the IS"" brightest in thenight sky. It is, however, a semi-regularvariable that can change by severaltenths of magnitude over a period ofyears. Its great distance reveals that it istruly luminous; to the eye over 10,000times brighter than the Sun. Becauseit is cool, only 3280K at its surface, itradiates a considerable amount of itslight in the invisible infrared. If youtake that into account, the star becomessome 60,000 times brighter than ourSun. A low temperature coupled withhigh luminosity tells us that the star mustbe huge, luminosity and temperaturegiving a radius of about 3 AstronomicalUnits. It is so big that astronomers caneasily detect and measure the size ofits apparent disk, which gives an evenbigger radius of 3.8 AU, three-fourthsthe size of the orbit of Jupiter. Thedifference is caused by uncertainties indistance, temperature, and the actuallocation ofthe mass-losing surface, asthe star is slowly evaporating, encased

itself in a gas cloud, or nebula, thatshines by light scattered from theultraluminous star within.

Antares, with a mass of 15 to 18 solarmasses, probably does not have muchtime left to it. It is massive enoughto one day develop an iron core andto eventually explode as a brilliantsupernova. The event may be a millionyears off, an astronomical blink of aneye; or it may occur tonight, so keep awatch on one of the great stars of thenight-time sky.

Due north, near the horizon, you willfind Vega twinkling brightly. Vega, ora-Lyrae, is a star approximately25.3 light-years away in the smallconstellation of Lyra, the Harp. Sciencefiction fans will be familiar with Vega,where writers make use of the evidencethat indicates that Vega hosts a solarsystem. This system plays a role in IsaacAsimov's Eoundation trilogy, and CarlSagan's science fiction novel and film.Contact.

Vega is the fifth brightest star in thenight sky. It is a 'nearby star', andtogether with Arcturus and Sirius,one of the brightest stars in the Sun'sneighbourhood. It is twice as massiveas our Sun and about fifty times asluminous. Vega's current age is about500 million years. Since more powerfulstars use their fusion fuel more quicklythan smaller ones, Vega's estimatedlifetime is only one thousand millionyears, approximately a tenth that of ourSun. After it leaves the main sequence,Vega is likely to become a yellow giantbefore entering the red giant stage, afterwhich it will eventually shed its outerlayers to become a white dwarf.

In about 14,000 AD, Vega willbecome the North Polar Star, owingto the precession of the equinoxes.Professional astronomers used Vega forthe calibration of absolute photometricbrightness scales. When the magnitudescale was fixed, Vega happened to beclose to zero magnitude. Therefore,the visual magnitude of Vega wasdeclared to be, by definition, zero at allwavelengths. This tasted for many years,but is no longer the case, because theapparent magnitude zero point is nowdefined as a specified numerical flux.

We see Vega from within 5" of its polaraxis of rotation, but if you were lookingalong the plane of its equator, Vegawould look about 23% fatter at theequator because, with a rotation periodof about 12,5 hours, Vega rotates at 93%ofthe speed where inertial effects wouldmake it disintegrate.

Many Messier objects are visible in theevenings at this time of year. If you haveaccess to a moderate telescope or better,and have good observing environs, thenit is worth getting hold of a MessierCatalogue and having a look at theseobjects. I have included some non-stellar objects in the Sky Chart,

The Helix Nebula, labelled NGC7293,is a 6,5 magnitude planetary nebula inAquarius, well east of the zenith. TheHelix Nebula is the closest example ofa planetary nebula created at the end ofthe life of a Sun-like star. The star expelsmassive amounts of its outer gasses intospace. In the case of the Helix Nebula,from our vantage point the expelledgases look as if we are looking down ahelix. The remnant central stellar core,destined to become a white dwarf star, isso energetic that it causes the previouslyexpelled gas to fluoresce. One day ourSun may look like this.

Fig 2. The Helix Nebula, NGC 7293,as seen through the Hubble SpaceTelescope

Ml 6, the Eagle Nebula, lies some 7,000light years distant in the constellationSerpens, the serpent, close to theborders with Scutum and Sagittarius,and in the next inner spiral arm of ourMilky Way galaxy from us. It is a greatcloud of interstellar gas and dust thatis involved in a vivid episode of starformation.

It is best to use low magnification withtelescopes to see M l 6. A 100 mmtelescope reveals about 20 stars inan uneven background of fainterstars and nebulosity; three nebulousconcentrations can be glimpsed undergood observing conditions. Undervery good conditions, you can seesuggestions of dark obscuring matterto the north of the cluster. A 300 mmtelescope will start resolving the darkpillars.

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Sagittarius star cloud in our Milky Way.The Lagoon nebula is a beautiful sight inany size telescope, and takes its namefrom the dark lane that runs through thecentre ofthe nebula.

Fig 3. The Eagle nebula, h, -y, <.c ^.r.--^,,through a big telescope

The 6.0 magnitude Omega Nebula,M l 7, also called the Swan Nebula, theHorseshoe Nebula, or, especially inthe southern hemisphere, the LobsterNebula, is a region of star formation,and it shines by excited emission,caused by the higher energy radiationof young stars. However, unlike manyother emission nebulae, these starsare not obvious in optical images, buthidden in the nebula.

Distance estimates are between 5,000and 6,000 light years, thus little lessthan that of its apparent neighbour.M l 6, the Eagle nebula. Apparently,these two star forming regions areindeed close together, in the same spiralarm of our Milky Way galaxy. It is visibleto the naked eye under good observingconditions.

Fig 4, The Omega or Lobster Nebula,Ml7, as seen through a 155 mmtelescope

Erom a dark site, M8, the LagoonNebula is visible to the unaided eyeas a small bright patch below the large

Fig 5, The Lagoon Nebula, M8, witha southern hemisphere flavour.Photographed by Ian Wallis with a250 mm telescope from a suburbanlocation in Perth, Western Australia

The Zodiac that is notOne titbit of 'astronomical' informationthat most people seem to know is their'star sign'. Eew people know what itmeans, and fewer still know that itdoesn't mean that at all, or at least that ithasn't for the past few thousand years.

If you ask them, most people would saythat a 'Gemini' is a person who wasborn when our Sun was in front of thestars of the constellation Gemini, thetwins. That was true about 2,600 yearsago, but it isn't today, for two reasons:

1 When, more than two thousandyears ago, the ancients set up theastrological star sign system, theydivided our Sun's apparent annualpath into twelve equally spaced"signs," each 30" wide, with eachsign only approximately coincidingwith its constellation. Cancer, forexample, is a small constellation,and Pisces is huge, but the ancientsaccorded both one twelfth of ourSun's annual path. The constellationCancer, composed of stars, nevercoincided exactly with the sign ofCancer, which is a 30̂ ^ wide zonealong our Sun's path. About 70 yearsago the international AstronomicalUnion settled the problem of vagueconstellation boundaries when itpublished a set of specifications foreach constellation. This defining ofthe boundaries added a constellationto the zodiac. According to the

current official constellationboundaries that all astronomersuse, our Sun passes through 13constellations, not 12. The 'thirteenthconstellation' of the zodiac isOphiuchus the Serpent Bearer, andour Sun is in front of its stars duringtbe first half of December. About oneperson in twenty is an 'Ophiuchan',but few of them know it.

2 The dates that our Sun spends ineach constellation have shifted bya few weeks. For example, our Sunwas originally in front of the stars ofGemini during the first two weeks ofMay. Now, our Sun is in front ofthestars of Taurus during the same twoweeks. What happened? Our Earthis wobbling as it spins. Because ourMoon and Sun are constantly pullingon the Earth's equatorial bulge, theEarth wobbles like a spinning top.This wobbling, or precession, is soslow that our Earth takes 25,800years to complete one wobble. Theeasiest way to think of this motionis to visualize the Earth's axis assweeping out a huge circle in the skythat includes its current position atthe South Celestial Pole (SCP on theStar Chart) and Canopus,a-Carinae, the second brightest ofthe night-time stars. Canopus will bethe South Polar star in 12,000 years.Another way to think about it is toimagine that the position of our Sunat the vernal equinox, slowly driftsaround the sky. The vernal equinoxis the one that occurs in March. Fivethousand years ago our Sun was inTaurus, near the Pleiades star cluster,on that first day of the northernspring. By the time of Christ, ourEarth's wobbling had us seeing ourSun on the boundary between Ariesand Pisces on the same date. Sixcenturies from now our Sun will bein Aquarius as the northern springbegins, and the 'Age of Aquarius', along overdue age of universal peaceand brotherhood, will finally dawn.

Not only has our view of our Sunon the first day of northern springshifted over the past few thousandyears; so has our view of our Sunon every date. For example, our Sunused to enter Cancer at the momentof our winter solstice, giving riseto the term tropic of Cancer for thelatitude where our Sun is overheadon that date. Currently our Sun isin Taurus at our winter solstice, twoconstellations away from its 'original'position.

Wben, in the second century BC,

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the Greek astronomer Hipparchusdiscovered our Earth's wobble,astrologers had a problem. Our Sunwas supposed to be in Aries as thenorthern spring began, but it wasreally in front of the stars of Pisces.The astrologers took this bombshellin their stride and declared that thezodiacal signs drifted with our Sunand that our Sun was therefore still inthe sign of Aries, even though it wasactually in front of the stars of Pisces.

So, the fixed stars ofthe constellationshave stayed in the same direction, butthe signs have drifted to the west relativeto them, and they no longer coincide.If you were born during the first twoweeks of May 2600 years ago, you wereborn when our Sun was in both thesign and constellation of Taurus. Nowduring those weeks our Sun is in Aries,Astrologlcaliy speaking, you are still aTaurus; astronomically speaking you arean Aries. Likewise, most Libras are reallyVirgos, and so on. To add insult to injury,most Sagittarians are really Ophiuchi.Ofthe 366 possible birthdates, the signsthat astrologers use correspond to theastronomical constellations 14% of thetime. The astrological signs are wrongby one constellation for 84% of people,and by two constellations for the other2%.

So much for the effect that the stars haveon our lives. The astrological effect that'Aries' is supposed to exert follows thedrifting Sun, not the stars. If astrologyworks, its mysterious forces thereforecome from arbitrary directions in spacethat have nothing to do with the starsthat lie in those directions.

The table below lists the dates whenour Sun is in the real astronomicalconstellations ofthe zodiac. The datesfluctuate by a day from year to year.

Table 1. Astronomical constellations ofthe Zodiac

Significant eventsAugust 30 -The Moon passes a fewdegrees south of Jupiter

September 7 -The Moon eclipsesUranus, as seen from many parts ofAustralia, Maximum occurs at about14:48 UTC in the east of the country;and about an hour earlier in the west.Note that this is the next day local timefor Eastern Australia,

September 26 - A 4 day old Mooncrescent passes south of Jupiter, which isfading into the sunset over the next fewdays,

Octoberl 6 - Fomalhaut, a-Piscis Astrini,is about 5*̂ north ofthe zenith.

Most of October -The planet Mercury ismaking a fairly good evening apparition.Its path is always just south of Jupiter inthe twilight sky, with its high point beingon October 17. On October 24 the 2day old Moon crescent passes just southof Mercury.

Table 2. Phases ofthe Moon over the coming year. The times are UTC (UniversalTime)

How to use the Star ChartTo use the sky chart go outside at theappropriate date and time, and hold thechart, upside-down above your head.Then turn around till the direction marksaround the chart's edge are pointing inthe correct compass directions. Now thechart should give you a condensed viewof the sky that you see beside the chart.

Because no single sky chart will dofor all localities in Australia, I havegenerated one for a position about100 km east of Adelaide, It shows whatthe sky should look like at 35° southlatitude anywhere in Australia at around8:00 pm local time on 23 August, If youare situated further north then you maynot be able to see those objects in the farsouth of the chart. Similarly, observersfurther south may not be able to view thenorthernmost objects on the chart.

New Moon

Jul 25,

Aug 23

Sep22

Oct 22,

Nov 20

Dec 20

Jan 19,

Feb 17

Mar 19

Apr 17,

May 16

Jun 15,

2006 04:20

2006 19:15

2006 12:00

2006 05:30

2006 22:20

2006 13:50

2007 03:50

2007 16:10

2007 02:50

2007 11:50

2007 19:35

2007 03:05

First Quarter

Aug 2,

Aug 31

Sep30

Oct 29,

Nov 28

Dec 27

Jan 25,

Feb 24

Mar 25

Apr 24,

May 23

Jun 22,

2006 08:40

2006 22:55

2006 11:05

2006 21:25

2006 06:30

2006 14:45

2007 22:55

2007 07:50

2007 18:10

2007 06:35

2007 21:05

2007 13:15

Full Moon

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

9, 2006

7, 2006

7, 2006

5, 2006

10:50

18:45

03:15

13:00

5, 2006 00:25

3, 2007

2, 2007

3, 2007

2, 2007

May 2, 2007

Jun

Jun

13:55

05:45

23:15

17:15

10:15

1,2007 01:10

30,2007 13:50

Last Quarter

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

16, 2006

14,2006

01:50

11:15

14,2006 00:30

12,2006

12,2006

11,2007

10,2007

12,2007

10,2007

10,2007

17:55

14:40

12:50

09:55

04:00

18:15

04:35

Jun 8, 2007 11:50

Jur ',200717:00

Constellation

Capricornus

Aquarius

Pisces

Aries

Taurus

Gemini

Cancer

Leo

Virgo

Libra

Scorpius

Ophiuchus

Sagittarius

Dates

January 19 to February 15

February 16 to March 11

March 12 to April 18

April 19 to May 13

May 14 to June 19

June 20 to July 20

July 21 to August 9

August 10 to September 15

September 16 to October 30

October 31 to November 22

November 23 to November 29

November 30 to December 17

December 18 to January 18

The September equinox occurs at 05:04UTC on 23 September, Throughout

Australia that eventwill occur on 24September, local time.At that time, we enterthe spring leg of ourjourney around theSun. That journey willlast till our summersolstice occurs on 22December at 01:23UTC, ending a springthat is 89,85 dayslong. ThroughoutAustralia, summersolstice will be on 23December, local time.

Use the chart 4 minutes earlier than 8:00pm for every day before 23 August, and4 minutes later for every day after 23August. feici)

Enjoylooking

up!About the author:

R.iy Fnrm.i teaches science at MethodistLadies College in Claremont, WA.

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