The Sky at Night Phần 3 doc

This being so, it takes an effort of the imagina-tion to appreciate that some of the stars are huge enough to contain the whole orbit of the Earth round the Sun – while admittedly others

22 6 News from the Planets This is not at all Cassini showed that a high mountain ridge runs for a long distance round Iapetus, making it look rather like a table-tennis ball which has been broken in half and then unskilfully glued together The ridge is high, rising to a maximum of 8 miles above the surrounding terrain, running for 800 miles almost among the geographical (should it be the Iapetographic?) equator It is unlike any-thing else known in the Solar System, so how was it formed? Could it be that it is due to icy material which welled up from below and then solidified? Could it be that, as suggested by Paulo Frerie of Arecibo observatory, Iapetus once grazed the outer edges of the ring system, and later retreated to its present distance? It has even been suggested that Iapetus itself may have had a ring – a ringed satellite orbiting

a ringed planet Less plausibly, some UFO enthusiasts have claimed that Iapetus itself is artificial, put together by the usual nebulous aliens from afar Certainly, it may be a popular sight for future interplanetary tourists because its orbit is inclined

to the plane of Saturn’s equator by almost 16°, and travellers will see the rings well displayed – while the inner satellites, including Titan, orbit almost in the equatorial plane so that seen from them the ring system will always be edgewise-on

Perhaps, the greatest surprise of all came from Enceladus, discovered in 1787 by William Herschel It is a mere 310 miles across (about the distance between London and Penzance) and was expected to be icy and inert This is certainly true

of the even smaller Mimas, discovered by Herschel at the same time; incidentally, these were the first of the few important results coming from Hershel’s largest telescope, the 40-foot focus reflector with its 49-in mirror Mimas is dark with one vast crater, which had led to its being compared with Darth Vader’s “Death Star” Enceladus has the highest albedo of any Solar System body; there are no large craters and wide areas where there are no craters at all This must mean that these areas are young, and have been resurfaced in comparatively recent times

When Cassini flew past Enceladus on 17th of February 2005, at a range of

725 miles, it detected a tenuous but appreciable atmosphere – totally unexpected for

a world with so weak a gravitational pull; In fact no atmosphere could be retained for long, and so there must be continual replenishment from below Next came the discovery of ice geysers spouting from the south polar region; the jets rise to hundreds

of miles above the ground At NASA, they caused great excitement To quote Carolyn Porco, head of the Cassini imaging team: “I think this is important enough

to see a redirection in the planetary exploration programme We’ve just brought Enceladus to the forefront as a major target of astrobiological interest.” The readings from Enceladus’ geyser plumes indicate that all of the prerequisites for life as we know it could exist below Enceladus’ surface “Living organisms require liquid water and organic materials, and we know we have both on Enceladus now”

A few tens below the surface the temperature and pressure may be sufficient to keep water in a liquid state Further evidence comes from the so-called “tiger stripes”, which indicate cracks The ice here is a more amorphous and virtually crater-free, so that it must have welled up comparatively recently The geysers rise upward for several 100 miles, so that they are violent – and violence was the last thing to be expected on a world as small as Enceladus Most of the ice crystals fall back as snow, but some break and free altogether to become part of the wide, thin

6 News from the Planets

E-ring It is not yet clear whether the venting and the geyser activities confined to the South Pole region If so, this must be the hottest part of the whole globe Can there be hydrothermal vents below? At any rate, Enceladus is one of the only two bodies active enough for its heat to be detected by remote-sensing instruments – the other is Jupiter’s satellite Io, but Io and Enceladus are very different worlds Certainly, the past few months have been of immense interest So many new phenomena have been seen Which is the most intriguing? Make up your own mind – but I have to say that my personal vote must go to the fountains of Enceladus

P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_7,

© Springer Science+Business Media, LLC 2010

On 3 October 2005, there was annular eclipse of the Sun The track of the annularity began in the Atlantic and crossed Spain, which was convenient enough I have to admit that my travelling days are over, but the Sky at Night team led by Chris Lintott was well represented in Madrid, and was rewarded with a perfect view I had to stay at home and make do with my very small partial…

An annular eclipse occurs when the Earth, Sun and Moon line up, with the Moon

in mid position – but with the Moon in the further part of its orbit, so that its disk is not quite big enough to cover the Sun completely The Sun’s mean angular diameter is

Chapter 7

Spanish Ring

Spanish ring eclipse team (Credit: Pete Lawrence)

26 7 Spanish Ring

32 min 1 s of arc; the apparent diameter of the Moon ranges between 33 min 21 s and

<30 min, with a mean of 31 min 8 s The length of the Moon’s shadow varies between 237,000 and 227,000 miles, with a mean of 231,000 miles The Moon’s mean distance from the Earth is 238,700 miles, and from this distance the shadow is too short to reach the Earth’s surface It follows that annular eclipses are more frequent than totals in the ratio of 5:4 This is brought out by the dates of totals and annulars in Great Britain between 1800 and 2100; six annulars (1820, 1836, 1847, 1858, 1921 and 2003) and only two totals (1927 and 1999), though it is true that the track of totality on 30 June

1954 just grazed the tip of the northernmost of the Shetland Isles (I do not believe that anyone actually saw it from there) The next British totalities will be on 3 September 2081(Channel Islands) and 23 of September 2090 (Southern Ireland and Cornwall)

I saw my first annular eclipse on 29 April 1976, from the Greek island of Santorini – the site of the devastating volcanic outburst which probably destroyed the Minoan civilisation on Crete, than just about the most advanced in the whole word (The Santorini volcano, Nea Kameni, is still smouldering, though for a long time now it has been reassuringly placid.) With a party of friends, I was stationed

in the courtyard of the excellent Atlantis Hotel, under a perfect sky

Frankly, I did not know quite what to expect Annularity can last for more than

12 min so that things are less frenetic than with a total eclipse, and strict precautions must be taken all the time Of course, there is no chance of seeing the corona, though naked-eye prominences have been recorded, and so have Baily’s Beads – in fact the first description of these beads was given by Francis Baily at the annular eclipse of

15 May 1836, though they had been seen much earlier by MacLaurin at the annular eclipse of 1 March 1737 I did not know whether the sky would darken sufficiently for planets or bright stars to be seen; at the Santorini eclipse it did not, and the diminution in light was so slight that many of the locals failed to realise that anything unusual was happening Still it was enthralling to see that the jet-black disk of the Moon circled by a ring of sunlight I saw another annular from Mexico,

on 10th of May 1994; this time the sky became darker, but I could see no promi-nences and certainly no sign of corona

Eclipse-chasing is addictive, and even an annular is well worth seeing, so I was very

sorry not to be able to join the 3 October party, in Madrid At least the Sky at Night

was well represented; Chris Lintott and Mark Kidger were our commentators, and the photographers included Damien Peach, Pete Lawrence, Ian Sharp and Dave Tyler, all armed with equipment much more sophisticated than anything I could have taken to Santorini almost 30 years earlier Nowadays, good results can be obtained even with a simple digital camera, but digitals belong strictly to the twenty-first century

The track of annularity passed right through Madrid, and the eclipse took place

in the late morning, so that the Sun was pleasingly high in the sky The annular phase lasted for 4 min 11 s, and 90% of the solar surface was covered, but the drop

in the light-level was surprisingly pronounced It so happened that neither of our main commentators had seen an annular eclipse before, and they were suitably impressed, notably by the crescent-shaped shadows which were cast as the Moon crept slowly and gracefully on to the Sun

7 Spanish Ring

The sky never became dark enough for Jupiter to be seen (Venus was badly placed), but the landscape became very dim, and at Madrid was probably about the same as the light-level to the late twilight – according to those who were there; watching the picture on my television screen could not give me any real idea Baily’s Beads were well seen, and a very interesting set of observations were made

by Pete Lawrence The “beads” are produced when shafts of sunlight cast through the lower-lying parts of the Moon’s uneven limb Careful timing showed that the main “bead” was seen as the sunlight streamed past a very large, deep depression which could be identified as Mare Orientale, the Eastern Sea This is a major sea, almost all of which lies on the Moon’s far side; only a tiny section of it can ever be seen from Earth, and then only under the most favourable liberation (I first drew it

in 1949, and suggested its name, American observers rediscovered it later.) It is a huge ring structure apparently, the youngest of the principal Maria and the only one

of its kind on the far side It is so large that from Madrid it was able to produce an obvious and persistent “bead”

It cannot honestly be said that a great deal of valuable work can be done during

an annular eclipse, but what does this matter? Everyone at Madrid enjoyed it – including the town band, who came into the main city square to join the astronomers, and played with great gusto without quite matching the standard of the Royal

Philharmonic At least the Sky at Night team, their appetites whetted, could look

forward to the much grander spectacle of a total eclipse in March 2007

P Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_8,

© Springer Science+Business Media, LLC 2010

Look up into the sky, and you will see the stars as tiny, twinkling points The twinkling

is due entirely to the Earth’s atmosphere; from space (or on the Moon) stars do not twinkle (scintillate) at all, and if you have the chance of seeing stars while you are travelling in a high-flying jet you will find that the twinkling is much less then it is at sea level But with the naked eye, no star appears as anything but a dot If you use a star as an obvious disk, you may be assured that there is something wrong Almost certainly the telescope is out of focus This being so, it takes an effort of the imagina-tion to appreciate that some of the stars are huge enough to contain the whole orbit of the Earth round the Sun – while admittedly others are so tiny that they could fit com-fortably into the ring road of a small city For the last the programme of 2005 I was joined by Professors Richard Harrison and Lucy Green to say something about the Sun, the only star close enough to be examined in a great deal, and then by Drs John Mason and Barrie Jones, to discuss the sizes of the various types of stars.

Chapter 8

The Sizes of the Stars

The “Plough” in Ursa Major, photographed by Nik Szymanek

30 8 The Sizes of the Stars

If no stars show obvious disks, then how do we measure their diameters? There are various methods Of course, there is no problem at all with the Sun, which is a normal Main Sequence star of Type G; it is 865,000 miles across We may not have discovered all its secrets, but we do know a great deal about it, and it gives us a guide

to other stars – though the stars are amazingly diverse Some are much larger than the Sun, others much smaller; some are far more luminous, others remarkably feeble Let us deal first with exceptionally luminous stars – which are not necessarily the largest According to one set of measurements, the record holder is a remote celestial search light catalogued as LBV 1906-20, said to be the equal of 40 million Suns, which is about as powerful as a star could be without disrupting itself by the pressure of radiation (LBV, by the way, stands for Luminous Blue Variable.) This does seem rather dubious Then we have the Pistol Star in Sagittarius, so nicknamed

by the shape of the nebula, which it illuminates It is approximately 25,000 light years away, in the direction of the galactic centre, and it is certainly very powerful and massive Were it is not so masked by interstellar dust, it would be an easy naked-eye object; in fact, it remained undetected until the Hubble Space Telescope imaged it

in infra-red Its mass seems to be about 150 times that of the Sun, and its diameter has been given as around 300 times that of the Sun, i.e roughly 250,000,000 miles,

so that it could contain the whole of the Earth’s orbit However, the data for Eta Carinae, the erratic variable in the southern hemisphere of the sky, are more reliable The luminosity is at least 5,000,000 times that of the Sun, and it is one of the most massive stars known It is also wildly unstable; for a while around 1840 it shone as the most brilliant star in the sky apart from Sirius, though for well over a century now it has hovered on the brink of naked-eye visibility In the foreseeable future – perhaps tomorrow, perhaps not for a million years – it will explode as a supernova, ending up as either a neutron star or a black hole

The largest of all stars are red supergiants A star begins its career by condensing out of the material inside nebula; it shrinks, under the influence of gravitation, and the inside heats up When the core temperature reaches about ten million degrees, nuclear reactions are triggered off The main “fuel” is hydrogen, the most abundant element in the universe; the hydrogen atoms combine to form helium, and the star begins to shine (Yes, I know this is horribly oversimplified, but it will suffice for the moment!) When the supply of the available hydrogen runs low, different reac-tions begin, and elements heavier than helium are built up With a modest star such

as the Sun, the process is halted before it can go too far The star will briefly become a red giant (not a supergiant) and will puff off its outer layers and become

a beautiful “planetary nebula” When the outer layers are finally lost, what is left of the star collapses into what is known as the white dwarf stage It will then go on shining feebly until all its light and heat have gone, leaving it as cold, dead globe – a black dwarf; – it is quite possible that the universe is not yet old enough for any black dwarfs to have formed

But with a much more massive star, equal to (say) over ten Suns, the story is different The star evolves much more quickly, and the element-building process is not halted so soon The star heats up until its core is at a temperature of millions of degrees, and the globe is blown out to produce a supergiant The surface has

8 The Sizes of the Stars

cooled-hence the red colour – but the luminosity is immense, though without matching Eta Carinae The best known red supergiant is Betelgeux in Orion (the star marks the great hunter’s shoulder, the name can be spelt in various ways and some people pronounce it “Beetle Juice”) Its apparent magnitude varies slowly between 0.2 and 1; sometimes it equals Rigel, the other brilliant star in Orion, while at others

it is comparable with Aldebaran, in Taurus, which looks like the same colour but is

a giant rather than a supergiant Betelgeux is just over 500 light-years away, it must have a diameter of around 550 million miles, and shine 15,000 times more powerful

as the Sun In luminosity it cannot match Rigel, well over 40,000 Sun power, but Rigel is hot, bluish white star, and it is not nearly as large as a red supergiant But Betelgeux, vast though it is, is by no means the record-holder

Even larger is Mu Cephei, in the far north of the sky not far from the W of Cassiopeia; over Britain if it never sets It is variable between magnitudes 3.6 and 6, but as its seldom drops between the fifth magnitude it is almost always within naked-eye range It is so red that William Herschel christened it the “Garnet Star”, and the nickname has stuck; through binoculars it looks rather like a glowing coal

It is further away than Betelgeux (perhaps 5,000 light-years) and much larger, more massive and more luminous, since it could equal 350,000 Suns For a long time it was said to be the largest star known but we have now found that it is outmatched

by four others – VV Cephei, V354 Cephei, KW Sagittari and KY Cygni – and possibly also a fifth, VY Canis Majoris, though the various measurements used here do not agree really well

Consider KY Cygni around 5,200 light years away in the constellation of the Swan The diameter is thought to be around 1,000,000,000 miles Imagine that you could stand upon the surface and go for a walk, how long would it take you to go right around, walking at a steady 3 mph and never stopping? The answer – 150,000 years Yet, although KY is 300,000 times as luminous as the Sun, it has only 25 times the solar mass Large stars are always less dense than smaller ones; it is almost like balancing a lead pellet against a meringue

Go and look for KY Cygni by all means; its position is RA 20h 26m 52s2, dec +38° 21¢11″, but I warn you that it will not be easy It lies in a rich area, but its apparent magnitude is a modest 13.3 Rather surprisingly, the star with the largest known apparent diameter is none of these supergiants, but R Doradus in the far southern constellation of the Swordfish The distance is 200 light-years, the lumi-nosity 6,500 times that of the Sun and the diameter 150 million miles It is red, and

a variable star of the pulsating type

From incredibly large stars to very small ones, we have noticed that a modest star like the Sun will become a white dwarf when its supply of hydrogen fuel is exhausted

We know a great many white dwarfs, the most famous is the faint companion of Sirius which was also the first to be identified All the atoms are crushed and broken, and the component parts packed together with almost no wasted space; matter of this sort

is termed “degenerate”, and a cupful of it would balance the weight of an ocean liner Atoms in a normal state are mostly empty space The best analogy I can give – not a good one, I know – is to picture a snooker table upon which the balls are set out ready for a game They take up a good deal of room – but pack all the balls together, and