The Greatest and
Smallest Suns
So lets see what it would be like to be on another planet around a different star. For a start, we must assume that any type of life as we know it must have liquid water, at least some of the time. From that it is easy to see that it is necessary for a planet to orbit its sun in a region where the suns heat will melt ice into water without boiling it away. For a large type A, B or F star this region is very much larger and further out than for a small M type star. This shell around a star is called an ecosphere, an envelope where life can exist. The Earth is near the middle of the Sun's ecosphere zone with Mars at one edge and Venus at the other. Other factors can play apart such as atmosphere thickness, composition, angle of the planets rotation, and the amount of magnetic field. Recent calculations have put this band around our Sun at only just over 10 million km deep if such considerations as the greenhouse effect are taken into account.
Suppose the Earth was circling a giant hot star, such as P Cygni, class B, 60+ times the Suns mass. The surface temperature of P Cygni is about 50,000°C as compared to 6,000°C for our Sun. Then the ecosphere would be billions of kilometres away, 60 times the distance of Pluto from our Sun, 360 billion kilometres, 12 light days away! From this vast distance the star would be a tiny extraordinarily bright point of light in the sky, as small as any star, but not like any star we can imagine. At this distance the Earth would still receive just as much heat and light as from the Sun today. All the shadows thrown by the sun would be razor edged with even the smallest glance by the unshielded eye probability causing a temporary blind spot like the light of an arc welding torch. But not only light and heat, but also dangerous ultraviolet and x-rays would flood the planet. If we moved the Earth further away or gave it a thicker atmosphere, life may still be possible.
But the greatest objection to life round the giant star is time. It just will not last long enough for life to get going.
If large stars are not good news for life forms, how about small stars? After all there are many millions of small M type stars for every one giant.
A star 5% the mass of our Sun would only give out one-billionth the light, this is brown dwarf territory and not conducive for life. How about a star with 65 times Jupiter's mass? This is still only 6.5% the mass of the Sun and would give out only one-millionth the light. This star would be barely red hot, so we have to move the Earth in close to receive the same amount of energy as now.
At a height of 150,000 km above its surface the Earth would orbit this star in just 1.1 hours. In the sky would be a dull red sun 3,000 times larger than now, giving very little light, most of the energy in the form of infrared light. Living on such a planet with the sun covering most of the sky, giving off a dull red glow, eyes would need to be receptive in the infrared wave lengths to be able to make full use of what light was available.
This is not the main problem however. Because the star and the planet orbit so close to each other, the gravitational tidal forces would cause the planet to bulge in the direction of the stars centre of gravity. As the planet rotates this bulge will cause internal friction to heat the rocks in the crust. This heat is energy from the rotation of the planet. In a very short time the plane would slow down and become gravitational locked to the star with just one side forever pointing sunward with the other in perpetual darkness. Quickly the atmosphere and all the water on the planet would freeze out on the dark side killing any chance of life forever. Most of the satellites in the solar system are now gravitational locked to their planet.
Because of the effect of the Moon and the Sun on Earth's seas and crust, the length of the day is increasing by one second every 100,000 years.
This may not sound a lot, but it's 50,000 seconds or nearly 14 hours since formation!
Life on Earth
The very early history of Earth is not known and can only be guessed at. With the proto-planet forming a ball of semi-molten rock and iron, with just a thin crust of rock punctured constantly by the infall of meteorites and other bodies, it would be impossible to find liquid water. Then just as it was settling down to form a permanent crust, it was hit a glancing blow by another planet the size of Mars. This impact stripped off most of the atmosphere and delivered extra material and mass to the planet along with a large satellite which formed out of the remains of the splashed crust and parts of the other body thrown into orbit around Earth.
There are many things we don't know or understand about the development of life on our planet, but from the evidence we have it must have started within 500 million years of the Earth's formation 4.63 billion years ago. Even then life spent nearly 2 billion years developing before multicellular plants more advanced than blue-green algae appeared, then another billion before the earliest fossil records. Blue-green algae is able to use the energy of sunlight to convert carbon dioxide and water into tissue components by photosynthesis. In doing so they gave out small amounts of oxygen, which over time steadily built up in the atmosphere. During all this time, deadly ultraviolet radiation would have been reaching the surface because there would be no ozone layer to prevent it. Then the first cells with nuclei appeared.
These larger cells had a much more efficient chemistry, and began to change the Earth's atmosphere into the one we know today. The new cells using oxygen where 20 times more efficient for a given mass, so life was able to move more rapidly and evolved in different directions. It changed into multicellular organisms and by 600 millions years ago, had developed hard rigid tissues. By now the Earth was 4 billion years old and from then on life can be found in the fossil record. Then just 300 million years ago with an ozone layer in place, life crawled out of the sea.
But what about our super large star? Well after a mere 100 million years it will die, most, spectacularly in a super nova explosion. If the star is twice the mass of the Sun it also will not last long enough for intelligence life to appear. If we stick to the premise that a star must remain on the main sequence for about 5 billion years, then its mass must be no more than 1.4 times the Suns mass. This star will be a F2 class. By comparison Sirius will remain on the main sequence for about 500 million years and a star like Rigel for only 400. In contrast the small M type stars will last for many billions of years without much change. These stars are the longest living.
Of cause, we have no idea if the figure of 4 billion years is realistic or not for the development of intelligent life on other planets. Maybe life can get going much more quickly without any hold-ups. We don't know how many times it go going here only to be wiped out in yet another meteor bombardment. It could have had to start hundreds of times. And how many times did life get put back to the beginning by the landing of a large rock on top of the most advanced bit? None of these set backs may have happened elsewhere. Or are they necessary for life to progress? Until we can study another planet with life it can only be a guess.
In The Ecosphere
So we must increase the size of our star from the small M class type so that its ecosphere is large enough to guarantee that the tidal forces will not slow and gravitational lock the planet, so rendering it unfit for life. At 33% the mass of our Sun, a star will be class M2, bright enough for a larger Ecosphere. This then is the range of sizes we need to look to, from 1/3 to 1.4 times the mass of our Sun. In our galaxy alone it's possible that there are 75,000,000,000 planetary systems around sunlike stars.
OK, we've got our star sorted out but what about the planet? If the world is too large or small it cannot support the right conditions, this is obvious. But how big or small? Aaahh, a tricky question....
Mars has a mass of 0.11 the Earth, one-third the gravity, very little atmosphere. A small planet such as Mars will only have a small amount of water. Had it been closer to the Sun, the greater heat would have warmed the gases, speeding up the molecules in its atmosphere. Slowly the lighter gases such as hydrogen and water vapour will leak out into space from the top of the atmosphere because the weaker gravity wouldn't be able to hold onto them. Mars would then be a totally barren airless planet if it was in Earth's position. To be near the bottom of the list, a mass of 40% of Earth's could still be home for life, just. A planet half way in size between Earth and Mars would have enough water, air and gravity to hang on to them. From a semi-dry desert surface, life could change the surface into one more like Earth's. Even with planet wide deserts, enough water would still exist, after 4 billion years, near the poles to support life. The one thing that life does to a planet, is terraform it to suit.
How much larger can a planet be before it crosses over into a giant gas ball? This will depend on the nearness of its sun. The Earth and Venus started off just about the same size, but are now very different. The surface temperature of Venus is hotter then if it had no atmosphere at all because of its thick carbon dioxide gas driving up the temperature by the greenhouse effect. This did not happen to Earth as it was cooler and the pressure of the atmosphere thinner possibly from the collision of the Moons formation. Can this be the reason why we have life? If Earth had had a thicker atmosphere would the greenhouse effect have pushed up the temperature until most of the surface water had evaporated? Will we ever know?
If the Earth had been larger it may have collected more water so that it had a planet wide ocean. If we had not had the Moon we would not have had tides as strong as now. There is evidence that the early seas washing in and out of beach pools helped to provide the difference mixes of materials to form the early molecules of life. And now we have intelligent life on our planet. We are the product of millions of random changes and chances along a twisting road. But can we tell if other life is intelligent? If we get a radio message then we must conclude that the other party has reached a certain level of technology.
It all depends on what we mean by intelligent life. Intelligence is not confined to just one species on our own planet so maybe its common across other planets if given enough time. But can we talk to it and understand it? For now we won't bother with that question. But I'll point to dolphins, whales and some of the apes..... and YOU can make your own mind up: how are we going to understand the message?
Minutes of the Coventry City Council Education Committee of 2nd April 1940
by Vaughan Cooper
TECHNICAL COLLEGE - ASTRONOMICAL SOCIETY.
The Committee have approved of the erection at the Technical College, at an estimated cost of £135, of a suitable building to house a telescope and observatory which have been presented for use by the College Astronomical Society.
A note on George Smith-Clarke, 1884 - 1960
He spent three years with Daimler and designed scooters. In 1922 he
was at the Alvis and equipped its aircraft-factory in 1935. Smith-Clarke
advised on the Hurstmanceux, Sussex, 100" reflecting telescope and designed
a machine for x-ray examination of the heart and improved the iron lung.
NOTES AND QUERIES - ANSWERS
By Mike Frost
My favourite moment of the week is ten to eight on a Friday morning. That's when Friday's edition of the
Guardian lands on my doorstep, and I get my weekly dose of
"Notes and Queries" in the features section.
To explain:
"Notes and Queries" publishes a selection of reader's questions, along with answers to questions from previous weeks. Anyone can write in with either questions or answers, and sometimes the debate on a particular question can smoulder on for months. The questions range from the soberly scientific; "Do ants sleep?", "Why are dusters yellow?" to the downright whimsical; "Can I leave all my possessions to my teddy bear?", "Who gets possession of the coffee when the Gold Blend couple divorce?".
If this format sounds familiar to those unfortunates who don't take the
Guardian, perhaps it's because the N and Q idea has been widely copied by other newspapers.
The Mail on Sunday has quite a jolly version, for example, whereas the
Sunday Times's "Questions" is mind-bogglingly ponderous. Emma Freud now runs a daily
"Open to Question" on Radio 1. And
"Notes and Queries" itself transferred to BBC2, in a television version hosted by Clive Anderson and Carol Vordermann. However, the TV version seems to me to miss the charm of the original - TV'S N&Q concentrates on the presenters and the experts wheeled in to give definitive answers, whereas the newspaper version glories in the egalitarian debate between readers - where else can people spend three months discussing how much further it is round the M25 clockwise rather than anti-clockwise?
Needless to say, whenever a question appears which I think I can answer, I fire off a letter, and from time to time I even contribute a question. Not surprisingly, most of my contributions are ignored. I've never had a question published, and to date only one answer. This was just before the 1993 general election, in answer to "If the Tories win the next election, my wife says she will emigrate. Please suggest somewhere warm and stable to go." (Remember this is the Guardian). My politically correct answer eulogising Costa Rica (warm, temperate, mountainous, no army) was tucked into a comer - where nobody noticed it. Not even my parents!
So when I next submitted a question, and the
Guardian rang me up at work to check the spelling of my address, I told everyone who would listen (you can see the question below). Needless to say, the
Guardian reneged, and next Friday nothing appeared! I've nearly forgiven them...
Never mind, I'm not proud about resubmitting my work. Here are all the astronomical questions I've submitted answers for. You can read my answers further on, or perhaps in the next edition. If you think I'm wrong, or haven't explained clearly enough - write something better! Perhaps we could start a regular feature of "Mira Notes and Queries".
Do planets spin through space or tumble?
All planets spin about an axis which points in a fixed direction. Earth's points towards Polaris in Ursa Major. Usually this axis is more or less perpendicular to the plane (the ecliptic) in which the planets move around the Sun. Earth's axis is only 23 degrees from perpendicular to the ecliptic - enough to give the seasons as the Earth moves around the Sun, but basically still meaning that the Earth spins rather than tumbles in its orbit.
The same is true for all other planets - except two. The rotational axis of Uranus is 98 degrees from perpendicular, so at some times in the Uranian year, the planet tumbles, with one pole or other pointing almost directly at the Sun. At other times, the axis of spin points in the direction of motion. The double planet system of Pluto and Charon behaves similarly.
Why are these two planets so different from the rest? Nobody is really sure, but the most likely theory is a planetary collision at some time in the distant past, probably in the early history of the solar system.
Why do full and new moons cause high tides when the gravitational pull is no greater than at other times?
The gravitational pull of the moon would like to stretch the Earth into an ellipsoid - like a Rugby ball. The Earth resists, but the oceans are pulled into two tidal bulges, which follow the moon as it orbits slowly around the Earth. The Earth rotates into these two bulges, giving two high tides every day (every 26 hours, in fact, because the Earth has to catch up on the orbital motion of the moon).
So far, the phase of the moon hasn't entered the explanation - there are high tides at full moon and new moon. But there are also tides due to the gravitational attraction of the Sun, about one quarter the size of lunar tides. Once again the Earth rotates twice a day into the solar tides. At full moon and at new moon the two tides coincide and reinforce each other, giving high or spring tides. At half moon the two tides partly cancel out, giving neap tides.
Why do astronomers wait for the moon to eclipse the sun when a dinner plate held at arms length would do just as well?
The sun consists of a sphere of plasma, the photosphere, surrounded by a much more tenuous outer region called the corona. Although the corona is quite bright in itself the light from it is drowned out on Earth by the much brighter light from the photosphere. This is because of the earth's atmosphere, which scatters some of the light from the sun. What we see as blue sky is actually scattered sunlight. If you held a dinner plate against the sun the light of the corona would be drowned out by the scattered sunlight.
However, during a total eclipse, the moon moves in front of the sun and cuts off all the light from the photosphere, so none of it can reach the atmosphere to be scattered. By a happy coincidence the moon's apparent diameter is almost exactly that of the sun, so that the light from the outer regions is able to reach Earth and provide us with the beautiful soft light of the corona during an eclipse.
What happens to an object dropped into a tunnel all the way through the Earth?
Assuming it were possible to build such a tunnel, and assuming that it could be evacuated of air to give zero frictional resistance, an object dropped into a hole through the Earth along the polar axis would oscillate from pole to pole with a period of approximately eighty minutes (rather faster than Michael Palin). At first sight it might appear that deep within the Earth an object falling down the hole would be braked by the mass of Earth above it. In fact, the mass of Earth above the object is precisely cancelled by antipodean mass in the other direction. As the object reaches the Earth's centre it experiences zero force, but by then is travelling just fast enough to make it back to the surface.
Remarkably, exactly the same results hold for a straight line tunnel bored between any two points on Earth - in principle London to New York or even the Channel Tunnel could be connected by an oscillating free-falling projectile requiring no external energy source. However, tunnels not through the Earth's centre must have perfectly frictionless walls (in addition to being evacuated to give zero air resistance), so in practice projectiles dropped into any such tunnels would soon be slowed down by friction and come to rest at the lowest point of the bore.
Long-standing members of the club may remember my definitive talk on this subject (and many other related matters) "Isaac Newton and the Surrey Pumas". This talk has now been delivered in both hemispheres but the audiences will be disappointed to learn that, far from taking the hole through the pole to reach the antipodes, I travelled a more circular route. The Surrey pumas, meanwhile, seem to have migrated to become "the beasts of Bodmin Moor".
And now some Questions for you to puzzle over
None of these may be particularly complicated questions to answer, and I think I could make a stab at answering them myself. But the object of the exercise is to get you thinking... So what do you make of this lot?
When you look through a reflecting telescope, why don't you see a hole in the image corresponding to the light stopped by the flat mirror in the middle of the tube?
Why do all the planets orbit in virtually the same plane (the ecliptic) ? Why don't some of them rotate at right angles to others?
On a cloudy day, you sometimes see shafts of sunlight breaking through the clouds. How does this happen? Is the sunlight illuminating something in the atmosphere? It can't just be light reaching the observer directly through a hole in the clouds, otherwise all you would see would be a bright hole in the clouds.
This was in a recent Sunday Times Q and A spot
After the shortest day of the year (December 21), why do evenings open
up more quickly than mornings?
On January 2 the Earth is at its nearest to the Sun, which means it
is travelling most rapidly. It therefore has to turn through an extra angle
to face the sun again. The length of the day, that is, the time between
consecutive southings of the Sun, is at its greatest, about 24 hours 28 seconds. The southing of the Sun therefore gets progressively later
and by mid-February is about 12.15pm at Greenwich. The effect of this is
to push sunset later while hardly altering sunrise. Since this happens
near the winter solstice, when the length of daylight is almost constant,
the effect is obvious. A.S. Hanson, Dorset
DATE SUNRISE SUNSET
10, 12, 1993 8h, 5m, 19s 15h, 52m, 0s
15, 12, 1993 8h, 10m, 12s 15h, 51m, 56s
21, 12, 1993 8h, 14m, 21s 15h, 53m, 47s
25, 12, 1993 8h, 16m, 0s 15h, 56m, 12s
30, 12, 1993 8h, 16m, 45s 16h, 0m, 25s
2, 1, 1994 8h, 16m, 29s 16h, 3m, 34s
5, 1, 1994 8h, 15m, 42s 16h, 7m,
8s
10, 1, 1994 8h, 13m, 16s 16h, 13m, 56s
15, 1, 1994 8h, 9m, 29s 16h, 21m, 36s
20, 1, 1994 8h, 4m, 27s 16h, 29m, 57s
This shows the rising and setting times of the Sun from the Coventry
area for a period over the Christmas and New Year. As you can see sunrise
is latest about the 30th Dec. and the earliest sunset is around
the 15th Dec. So while the mornings get lighter by 12m. the
evenings get lighter by 38m in a month.
EYE ON THE SKY
By Ivor Clarke
By now everyone on the planet who is half aware of astronomical matters will know of the forthcoming impact of the comet Shoemaker-Levy 9 onto Jupiter. The eyes of all the worlds astronomers will be on the planet this July, along with every piece of hardware at their disposal. So far 12 spacecraft will be watching, with even Voyager 2, far out beyond Neptune and Pluto, looking back to the planet. From this distance Jupiter will be only two pixels across in the frame!
With all of this professional interest in this unusual event, what chance has the amateur in spotting something?
Well not much, I'm afraid, from this country.
Now don't let me put you off observing this unique event, on the contrary do try. It's most unlikely that this type of impact will happen again for a very long time, so if you miss it....
Unfortunately this impact takes place during the longest days of the year so the nights will not get totally dark from our latitudes. The first of the fragments of the comet, travelling at 60 km per sec. could impact from 16th July onwards. The average size of them are around 1 km but some 12 of the largest could be up to 5 to 8 times larger. Because they are strong out along a line it is impossible to know the exact time of impact yet. What will the conditions be like at 21h 00m on Thursday July 21st 1994 (the 25th anniversary of the Apollo 11 Moon Landing) for observing? Not only is the sky not fully dark after sunset at 20h 10m on the 21st but the Moon will be full one day later at 20h 16m on the 22nd of July. This will add its light to the sky.
The full Moon will be near Uranus and Neptune in the south south east. Jupiter's position will be at RA 14h 13m 30s and DEC 12h 14m 36s about 7° from the 0.98 mag star α Spica in Virgo, having risen at 14h 24m, Jupiter sets at 0h 13m just after midnight from a position near Coventry. The planet will be low in the south west area of the sky. So a clear horizon is necessary in the south and west, or the planet will be hidden by obstacles.
If a clear view is available, watching the planet for as long as possible before it sets will be necessary as the exact moments of any impact may not be know to the minute. And it is not at all sure what the effects of these cometary bodies will be on Jupiter. It is possible to work out what energy a 2 km body carries in various mass densities. For instance most comets are very soft objects, composed of a sponge like mixture of dust, rock and ices. So the mass of a fragment is uncertain, therefore it's impossible to estimate the explosive power in megatons. Some estimates put the impacting power at over 100 megatons for a large piece. The flash of the impact may be visible on one or more of the nearest Galilean moons if they happen to be on the far side of Jupiter.
After an impact it will be 3 to 5 hours before the impact site revolves into view. If a large mass has hit, it will have penetrated deep into Jupiter's atmosphere before exploding. Will this bring up gases from deep below the cloud layer? Your guess is as good has anyone else's. How much mixing of the atmosphere takes place will depend on many factors and some cloud disturbances may be too small to see in amateur telescopes anyway. On the other hand, planet wide effects may be easily visible. We will just have to wait and watch.
Don't think you can watch Jupiter for only a few minutes and see everything, you can't. To do any serious observing, look at Jupiter as much as possible BEFORE July so that you know how it behaves and what the cloud belts look like. The more you observe the more you'll see and when the time comes, you will know if the comet has crashed or not.
