Atmosphere of Venus

The Atmosphere of Venus

Venus’s atmosphere has been probed by American and Russian spacecraft, theoretical calculation, and Earth based telescopes to yield many pictures. The dense clouds that obscure the surface do not reach to the ground, but end several tens miles above it. The atmosphere above the clouds consists of simple molecules – mostly carbon monoxide – while inside the clouds more complex molecules, such as sulfuric acid, may have formed. Below the clouds in a region of increasing density, temperature and pressure, the major component of the atmosphere appears to be carbon dioxide.

Venus

Craterlike shapes show on the radar image (blue) of a 900 mile section of Venus’ surface. Shown in shades and dark is the amount of radar power returned to Earth by the Venus surface. On the radar altimetry map (orange), shaded contours show a raised crater rim. The largest craters in the scene are about 100 miles in diameter, but only about 0,3 miles deep. Below is a rendition of the surface as it might appear if Venus’ clouds allowed us to see it.

Venus Landscape

Venus landscape

Venus appears to have a trough extending nearly the length of the radar image (900 miles) and measuring 75 miles wide. It is shallow – about one mile deep – and shows evidence of branching at the south (bottom) and possible “tributaries” at the northern end. The feature is interesting because it is not really a groove, but rather a crack at the top of a bulge in the surface of the planet:

Planet Venus

Below: with the atmosphere removed, you might expect a trough as shown. Artwork by Victor Costanzo:

Venus Landscape

Venus radar images present a complex puzzle to the scientist attempting to understand the nature of the planet’s surface. Familiarity of the geology of Earth, the Moon, Mercury and Mars, as well as an understanding of how geologic processes may differ in different planetary environments, is necessary before a realistic interpretation of such images can be attempted. Further complicated interpretation is the number of radar artifacts, such as the dark lane up the middle of each image, blurring near the dark lane, and the strange appearance the radar image lends to the planetary surface detail. Dr. Michael Malin of the Jet Propulsion Laboratory has given the interpretations below. Radar imagery are provided by doctor Richard Goldstein, Jet Propulsion Laboratory.

Picture of Venus

Rough mountains appear in the upper quarter of this image separated from a smooth plain by a nearly straight line indicating the possible presence of a fault. A linear feature located slightly left and above center, pointing north, may be another fault. At the approximate center of the lower half of the image lies what seems to be a cluster of mountains containing perhaps a dozen peaks in a region some 200 miles across. The individual peaks stand about one mile above the plain. This is suggestive of a region of volcanic activity.

Atmosphere of Venus, color photo

In the complex and confusing imagery of this frame several features can be deduced after careful analysis. The rough blotchy area in the right of the image is a plateau, raised about 6/10 of a mile above the bright area to the left. A thin, dark and light line appears to separate this region from areas to the left, and may be as escarpment (cliff). Altimetry data confirms that the ground is lower to the left of this feature. The bright spot in the lower right is the image of a mountainside that happened to be tipped directly toward Earth on the day this image was obtained. The average slope of the mountainside – about six degrees – can be determined from this reflection.

Greater slops may exist at higher resolution. Not very obvious is a feature located halfway between the mountain and the image center – canyon-like winding feature appearing as a depression on the altimetry data. Very little more can be distinguished until higher resolution radar images can be obtained.

Prominent in this image is the large, round, crater-like feature in the lower half of the frame. Analysis of the image reveals it to be a summit caldera atop a volcanic pile some 350 miles across. Several lines of evidence favor the caldera interpretation: 1) it has a steep inner slope and a shallow outer slope; 2) It has multiple scarps only along one side (impact craters tend to be symmetrical); 3) The large protruding ridge to the right is typical of volcanic features, and 4) the ration of its interior and exterior diameters is typical of calderas, but not impact craters. The volcanic pile can be distinguished by its texture – somewhat rougher than the surrounding terrain.

(Atmosphere of Venus) Photos: Jet Propulsion Laboratory

Mercury

Mercury

Mercury is easily the most difficult of all the naked-eye planets to see. This is mainly because it is the closest planet to the sun and therefore tends to get lost in the bright glow of the morning and evening sky. In itself, and despite its small diameter of 3,010 miles, it can be quite bright, outshining Sirius and equalling even Jupiter on favorable occasions.

Mercury cannot stay bright for long. Its orbital period is only 88 days, so its motion in the sky is quite rapid. No sooner does it swing well clear of the sun than it moves sunward again. As it does so it changes in apparent size and phase and therefore in brightness. Sometimes it passes directly between the sun and the earth and then, seen through a telescope, looks like a small round black spot. Eleven crossings or transits took place in 20th century. The last five were on May 9, 1970; November 10, 1973; November 13, 1986; November 6, 1993’ November 15, 1999.

Mercury. Photograph in colors. Source : NASA, photograph of public domain

A Hostile World

Mercury’s orbit is decidedly elliptical in shape. Its distance from the sun ranges from 29 million miles 40 43 million miles. An average value is about 36 million. At these distances the sun will appear some nine times brighter and hotter than it does to us. For this reason alone Mercury must be extremely hostile world. Astronomers once thought the planet turned one and the same face perpetually toward the sun. In fact, one side was intensely hot and the other extremely cold. Recent observations, however, made with the giant 1,000-foot radio-radar telescope at Arecibo, Puerto Rico, indicate that the planet rotates once in about 59 days. It rotates in the same sense or direction as does the earth. Even so, Mercury must be a world of great extremes in surface temperature for, like the moon, its small mass and weak gravitational pull prevent it holding on to a protective blanket in the form of a dense atmosphere.

Exploring the Milky Way

Exploring the Milky Way

(The August sky)

On warm summer nights far from the lights of the city, the sky is dominated by an awe inspiring band of light stretching across the sky – our own galaxy, which we call the Milky Way. The Milky Way reveals more detail to the casual observer than other galaxies do through the world’s most powerful telescopes – yet surprisingly few star atlases and observers’ handbooks devote more than a short amount of space to it. And no maps seem to exist that can give much help to the binocular user or astronomy enthusiast with a portable telescope. In short, the Milky Way has been ignored by observing books and charts. You’ll find however, if you take the time to look, that the Milky Way is an extremely rewarding subject for study with a very wide range of instruments – from your own dark adapted eyes, to binoculars, small telescopes, and richest field telescopes. It is an exhaustible source of objects foe viewing with even the largest telescopes.

The first thing you notice when you look carefully at the Milky Way is that it is by no means a simple object. It has variations in brightness, dark spots, bright spots, places where there are too many stars to count – and other places where there are no stars at all! We couldn’t expect it to be simple, of course; it’s a whole galaxy spread out before us, with all the detail and complexity, you’d find in a swarm of 100 billion stars.

Galaxy Rising. The obvious is sometimes false, the unexpected is sometimes true. We have come from the Cosmos and we were born ultimately of the stars and now for while inhabiting a world called Earth, we have begun our long voyage home.

When the Milky Way rises in the northern hemisphere it comes up almost parallel to the horizon. Cygnus and Cassiopeia rise first; then Aquila, Scutum and the claws of Scorpius, and finally the rich brilliant central regions of the galaxy in Sagittarius. By August, it’s high in the sky by the time darkness falls, and the southern parts in Scorpius and Sagittarius cross the meridian just after the end of twilight. By midnight the brilliant starclouds of Cygnus pass through the zenith. For campers, vacationers and city dwellers (perhaps getting a rare chance to be out of the city and under clear, dark country skies). August is the time to observe the Milky Way.

We’ll begin of the galaxy in the south, not because it’s brighter and contains more objects, but it is somehow fitting to start near the center of the galaxy. For the benefit of those living in the northern states, we’ll only go as far as 35 degrees declination.

For visual observers and binocular users, Sagittarius’ teapot forms a valuable starting point, much as the Big Dipper does for the north circumpolar stars. Looking south, the handle of the teapot (on the left) is composed of four stars, while the spout (on the right) contains three stars. Looking above the spout, you’ll see a puff of celestial “steam” about five degrees long. This is the richest and most brilliant starcloud in the Milky Way, known as the Great Sagittarius Starcloud. From northern latitudes it may not look as bright as some of the starclouds in Cygnus, but from latitudes where it passes overhead (and it isn’t dimmed by the atmosphere), it is brighter. Surrounding it are darker areas where the light from stars is absorbed by dust lying along the plane of the galaxy. The Great Sagittarius Starcloud shines through a “hole” in the dust. In fact, if the dust wasn’t there absorbing light, the entire Milky Way would shine more brightly than this starcloud does now.

If you use binoculars – or even just your eyes if they’re sharp – you’ll have already noticed the Lagoon nebula (M-8), located only a few degrees north of the starcloud. The Lagoon is a cluster of stars surrounded with a delicate glow of haziness – actually an H – II region where hydrogen fluoresces in the intense ultraviolet glare of the newly born star cluster in its center. Just a bit farther north, and quite a bit fainter, is the Trifid nebula (so called because it is divided into three parts) and a tiny open star cluster, M-21. But let’s not get carried away indicating specific sights: these fields are so rich that the slightest shake of bump of the telescope will bring new objects into the field.

Let’s start again from the spout of the teapot. This time let’s begin at the star where the spout joins the pot (Delta). Swing past the tip of the spout (Gamma) and about twice as far again; there, a bright open cluster (M-6) will be waiting for you. Nearby are four or five smaller and fainter clusters, easily seen in a small telescope or richest field telescope. Swing south and east a few degrees, and M-7, a really brilliant naked eye cluster, will dazzle you. In a big telescope, both M-6 and M-7 fill the whole field, as the Pleiades do in binoculars.

Where in the area is the exact galactic center located? In visible light there is nothing to mark it, although it can be “seen” in the radio and infrared that penetrate the galactic dust lying between us and the center: the spot is actually about two degrees west of the tip of the teapot’s spout. What a sight it would be if we could see it!

Another object that’s easy to find is the globular cluster M-22. The top of the teapot’s lid is marked by Lambda; a bit to the north and east you’ll find the globular. It’s a knot of about 100,000 stars, resolvable in a six inch telescope under good conditions; with binoculars is a fuzzy ball. M-22 is one of the biggest and brightest globulars in this part of the sky, but if you use a chart you can easily find many more including M-54, M-55, M-69, M-70 and M-28, along with lots of NGC objects.

Skipping over to Scorpius, let’s start at the obvious place – the red giant star Antares. Another globular cluster, M-4, found 1- ½ degrees to the west, is bright and easily resolved in a six inch scope. About midway between M-4 and Antares, and a bit to the north, is the much more difficult globular NGC – 6144. You shouldn’t have any trouble seeing it with a tree inch of larger aperture if sly conditions are good. North of Scorpius, spread through the dark expanses of Ophiuchus and Serpens, are quite a few globular clusters, including bright ones like M-5 and many smaller, fainter ones. But there we are getting farther from the Milky Way.

As you return from Scorpius toward the plane of the galaxy, you’ll notice a complex, patchy area far too complicated to describe accurately. This area of stars and starless patches is marked by dark nebulae of astounding complexity. Long dark lanes run from the area near Antares across about 15 degrees of sky into northern Sagittarius. This dust complex is not far from the Earth (as galactic distances go). It spreads along the inner edge of the sun’s spiral arm, seemingly stretched along the direction of galactic rotation. Whether it really does so or not, it certainly looks that way.

Just about 10 degrees north, along the galactic plane from the Great Sagittarius Starcloud , is the Lasser, or small, Sagittarius Starcloud. The open cluster M-24 is embedded in this cloud so that the cluster is hard to see against the starry background. On photographs, the Lesser Starcloud sometimes looks like a wide mouthed beast wearing dark glasses; hence it is also called the Google-eyed Monster (so much for nomenclature!). For degrees to the east is the brilliant open cluster M-25. Although it doesn’t have many stars, those it does contain are quite bright. Six degrees west is another open cluster, M-23. This much “smoother” cluster holds several hundred fairly bright stars. Direct your sight another degree north, and you’ll be looking at M-17, an open cluster bathed in nebulosity. This entire complex is called the Omega nebula. The nebula itself is a H-II region glowing primarily in the red light of hydrogen.

Another three degrees north is an open cluster with nebulosity: M-16, also known as the Eagle nebula. Both M-16 and M-17 are beautiful, softly glowing objects with sprinklings of stars in a six or eight inch telescope.

The fields we have crossed are filled with many more nebulae and clusters than mentioned. You can scan binoculars up and down this area and watch them go by. And it’s hardly possible to point a richest field telescope (or even a six inch at 40 power) at these areas of the Milky Way without having something interesting in the field.

When we look toward the center of the galaxy, lots of obscuring dust in our own spiral arm blocks the view in several places; but where that dust is thin, we can see right across a gap to the next spiral arm inward from ours. M-8, M-20, M-16 and M-17, for instance, all lie in that spiral arm (called the Sagittarius arm), while our sun lies in the Cygnus arm. As we look toward Cygnus (at right angles to the center of the galaxy), we’re looking down the length of our own arm at objects that generally lie closer to us. Just a quick visual scan of the Milky Way in Sagittarius compared to the Cygnus section will convince you that something is rather dramatically different. Where Sagittarius is complex, Cygnus is simple; where Sagittarius is full of incredible numbers of very faint stars, Cygnus has many more bright stars, and generally a smoother appearance.

The overwhelming naked eye feature that extends all the way from Scutum and Serpens Cauda up to Deneb in Cygnus is the Great Rift. Lying very close to the galactic plane, the rift is a huge lane of dust of dust obscuring everything lying beyond it. We see this remarkable dark cloud of dust against the starclouds of the galaxy that silhouette it on either side. Its length is nearly 1/6 of a full circle, spanning 60 degrees of sky. But in spite of its darkness, the Great Rift is brighter than the sky well away from the band of the Milky Way – an effect due mainly to the rich layer of stars between us and this dust band.

Near the southern end of the Rift, cloth to the galactic plane, is a starcloud called the “gem of the Milky Way” by E.E. Barnard, pioneering photographer of the Milky Way. The Scutum Starcloud is brilliant, though not as bright as the Great Sagittarius Starcloud. But northern observers have a better view of it because it is higher in their sky. Tucked away along the starcloud’s northern edge is the impressively dense open cluster, M-11. Small telescopes can barely resolve this cluster, but a six or eight inch reveals hundreds upon hundreds of stars crowded into a tight little ball. On the southern edge of the Scutum Starcloud nestles another small cluster, M-26. It isn’t nearly as rich as M-11, but is still an easy mark for binoculars.

Moving north across the Great Rift, you’ll find several more large clusters in Aquila and Serpens embedded in a branch of the Milky Way that pushes out into the dark expanse of Ophiuchus. NGC-4756 and NGC-6709 are both easy to spot in a small telescope; the former sprawls across a full degree of sky.

As you follow the Great Rift north toward Cygnus, you’ll pass some dense dark nebulae when you reach the declination of Altair, the bright star in Aquila. Dark nebulae, however, are hard to see; even the slightest bit of moonlight or nearby town-light will blow them out. The Rift may have a side branch here because the western lobe of the Milky Way is missing entirely in Vulpecula, presumably obscured by the omnipresent dustlanes that lie along this stellar band.

But as we enter the constellation Cygnus, we encounter a stunning revival: the Cygnus Starcloud, a rich oval nearly 20 degrees long and six degrees wide. Here is one of the most fabulous fields in the sky for an RFT, this cloud can only be called mind boggling. The various over-layers of brilliant young blue stars are stellar associations – young star groups recently born and shining brilliantly against the deeper, uniform background of faint stars too numerous to count. On color photographs of this area, you can easily see this concentration of hot blue stars. Toward the northern end of the star-cloud, near Gamma Cygni, you may also detect faint wisps of nebulosity.

Hop over the Rift to the less spectacular eastern branch a bit south of Epsilon Cygni, and find the star 52 Cygni. Careful scrutiny of this field with a six inch telescope scope reveals the western arc of the Cygnus loop nebula. This supernova remnant is a faint wisp of nebula a degree long. More care is needed to see the other half of this loop of gas because there’s no guide star. A few degrees south, you can see NGC-6940 – a small oval cluster of stars.

The main thrust of the Great Rift it toward the Deneb, but it turns suddenly to the west, and a starry region lies in your path. The North America nebula (NGC-7000) lies in the region. The nebula itself is very hard to separate from the rich stellar under-layer which follows the same outlines. In an RFT you can certainly spot the nebula if the sky is clear and very dark, and many people can see the nebula in large binoculars. With your eye, you’ll see the stellar background, but probably not the nebula itself. A few degrees farther long the Milky Way in the cluster M-39, composed of a very few bright stars. It’s a good sight in binoculars, but is lost in most telescopic views of the area.

With M-39, we’ll end our tour of the summer Milky Way. Have we omitted any objects? Absolutely! The fields are so rich, and so many smaller, dimmer, and more subtle structures of dust, gas and stars lie waiting to be seen that the job can never be finished. Come back for another look – for many more looks – and don’t forget to bring your camera next time!

Binoculars will bring you close to the Milky Way, but without color. Human eyes are insensitive to color at the faint light levels typical of Milky Way objects –such as this view of M-8 (the Lagoon nebula) and parts of the nearby Sagittarius Starcloud. Visually, the nebula has a slight tinge of green, the brighter stars gleam with a touch of bluish color, and the star-clouds with a colorless pale glimmer.

(By Richard Berry, Astronomy)

The artist can render best the visual appearance of the Milky Way shown here stretching from Sagittarius to through Cepheus, encircling 1/3 of the sky. The combination of delicacy and detail, seen by the eye from a site of superb quality, is never captured on film. The key locates some of the major features that you can easily see by eye or with slight optical aid.  One of the richest parts of the Milky Way stretches from the tip of Scorpius’ tail to Scutum. M-6, M-8, M-20, M-16 and M-17 lie very close to the galactic plane, almost defining it. The Small Sagittarius Starcloud shows a “monastery” aspect in shades of delicate blue – the result of young hot stars in it. Note that along the galactic plane, dense obscuring dustclouds block our view of the galactic core. Galaxy Rising. Artwork by Adolf Schaller. Source of the image : omnicosm.com

Viking Orbits Mars

Viking Orbits Mars

Although most press coverage of the Viking mission will stress the landing attempt, exploration started weeks earlier with both the basic research and attempt to find a safe landing site.

Viking discoveries began even before the spacecraft reached orbit. Far encounter pictures, made at about the moon`s distance from Earth, showed Mars half illuminated (a view never seen from Earth). Viking approached from the morning side of Mars, and photos showed light, hazy veils and bands over more of the planet than expected by some scientists. Low-floored impact basins, such as Hellas and Argyre, showed startlingly bright patches of frost and/or low haze as they emerged from Martian night into morning. By midday (the phase most clearly seen from Earth), such frost and haze “burns off” in the Martian sun.

Source of the photo: Nasa

Other photos showed unexpected details among the four huge volcanic mountains of the Tharsis region. All four mountains showed up as very dark spots – similar to early Mariner 9 views when the dark, lava cove red volcanos protruded through bright clouds of the 1971 Martian dust storm. But this year there was no dark storm! Why where the mountains so dark? They may have been protruding through morning mists that lighten the general tone of lower topography. Alternatively, they may have been stripped of light dust or coated with dark dust as a result of Mars’ active winds. Arsia Mons, the southernmost volcano, was especially altered since 1971-1972, with lobes of dark material extending north-east and south-west from its base. Again, there are probably products of dust removal or deposition.

The far encounter pictures were especially useful in filling a gap in Mars data: Previous missions had produced either “postage stamp” close-ups too detailed to show global patterns, or global images from too far away to reveal relations between dark marking and geologic detail.

Following For encounter studies, Viking I entered Mars orbit on June 19, 1976, with a 38 minute rocket burn. This slowed the spacecraft into a 43 hour orbit ranging from 930 miles (1,500 km) to 31,000 miles (50, 600 km). A three minute burn on June 21, 1976, brought the vehicle into an orbit that passes over the landing site every 24.6 hours, ranging from the same low point to a high of 20, 400 miles (32,800 km).

Early orbiter measurements indicated a greater quantity and diversity of water vapor that had been expected in the Martian atmosphere. Viking scientists interpreted this as evidence of daily exchange of water from soil to atmosphere, as might happen with diurnal melting of ice. The finding suggested that Viking may have arrived at a good time to look for biological activity, which is presumed to require water.

The prime choice for a Viking I landing site – selected many months before Viking neared the planet – was a relatively smooth plain known as Chryse (rhymes with icy). It is several thousand feet lower than most Martian terrain, meaning that the air pressure is higher than average. Though this pressure may be only one percent of that found at Earth’s sea level (less than that outside the cabins of the highest commercial jets), it is high enough to allow liquid water, should the temperature rise high enough. Liquid water, of course, would favor biological activity if there is any life on Mars – and the search for life is a prime motivation of the Viking mission.

A second favorable aspect of the Chryse region is that it is only 20 degrees north of Martian equator. On warm afternoons, temperatures should exceed the melting point of water. (The highest temperatures actually recorded by Viking through June 21 were around -31 degrees Fahrenheit, but these were morning temperatures).

Third and most important, Chryse lies at the mouths of four of the largest channels of Mars, named Shalvatana Vallis, Siums Vallis, Tiu Vallis, and Ares Vallis. These channels are believed to be beds of ancient rivers that ran with water perhaps 100 million years ago. If so, the water must have emptied into the Chryse region, perhaps forming a temporary sea or possibly evaporating rapidly. Thus, the Chryse region might have a good chance of presenting ancient bio-chemical effects that required water.

For these reasons, as Viking approached Mars, there was intense interest in seeing what detailed, close-up pictures of the Chryse landing site would reveal. Would Chryse be as smooth as had been hoped? The maximum slope that Viking could land on was about 20 degrees, and the maximum size rock that would clear the undercarriage without punching a hole in the lander was about 10 inches. Could such obstacles be avoided?

The first pictures to help provide an answer came on the night of June 22-23, after Viking went into orbit around Mars. Due to improved camera optics and electronics, they showed more detail than anything provided by Mariner 9. What they showed was somewhat frightening to Viking mission planners. Scientists are concerned, on the one hand, that the small scale structure of Mars is unexpectedly rough – but, on the other hand, there is great excitement at the new detail discovered. Some pictures have about 10 times the resolution of Mariner 9 frames. Cinder cones and other volcanic features have been found. Unknown craters have been recognized in regions that were obscured by haze during previous missions. Most exciting is confirmation of Mariner 9’s finding features such as apparent ancient shore lines along Martian channels, indicating that abundant water flow once occurred and carved out major landforms in some regions. Explanations of the geologic processes or climate changes that produced water in Mars’ ancient past will be a major goal of Viking explorations.

This color rendition of Mars as it would appear to a person approaching the planet was made from three separate black and white pictures taken through color filters – red, violet and green – on June 17, 1976 A Viking 1 closed to within 348,000 miles (560,000 km) of the red planet. The black and white frames were taken just seconds apart by one of the Viking orbiter`s two television cameras and radioed back to Earth. Corrections were made in a computer for the color response of the camera; the color photograph was then reconstituted on a TV screen. The Tharsis Mountains are clearly seen – a row of three huge volcanos standing about 12 – ½ miles (20 km) above the surrounding plain. Toward the top of this picture is Olympus Mons, Mars’ largest volcano. The circular whitish feature at the bottom of the disk is a large impact basin, Argyre. The area around Argyre is slightly brighter than elsewhere, probably because of the presence of discontinuous thin carbon dioxide ice on the surface. Several atmospheric features are faintly visible. West of the southernmost Tharsis volcano (left) is an irregular white area (which has been seen on two successive days by Viking 1) that has been interpreted as a water ice surface frost or ground fog; the faint, curved bands in the lower half of the picture probably are thin cirruslike clouds. The yellos at the edge of the planet’s limb is somewhat artificial, caused by extreme variation of brightness in the violet photograph. Source of the photo: NASA

By William K. Hartmann (Viking Orbits Mars, a text in Astronomy magazine, July, 1976)

The Sun

The Sun

The Sun, a great ball of immensely hot gases about 865,000 miles in diameter, is at once the central body of the solar system, the sustainer of life on the Earth, and the nearest star. It contains over 99 per cent of all the material in the solar system, and if it were divided into a million equal parts, each part would be larger than the Earth. Even Jupiter, the largest and most massive of the planets, is a tiny dwarf compared to the Sun.

As a star the sun is relatively small, faint and cool. Although much larger than many stars, it is only a speck compared with supergiant stars like Antares and Betelgeuse. One of the nearest stars is 500,000 less luminous than the sun. On the other hand, some stars are thousands of times more brilliant. True, the sun appears to be much larger than other stars, but these appear like pin-points because they are so far away. The sun is distant 93 million miles, so its light takes only about eight minutes to reach us. Proxima Centauri, the nearest star, is so remote that its light takes just over four years.

Prominences of the Sun. Prominences of the edge of the sun.

Sunspots

The sun’s bright disk, called the lightsphere or photosphere, is often marked by dark spots. These invariably change in shape, size and number as they move across the disk. They are seldom found near the sun’s poles. Studies of their movements show that the sun rotates once in about 25 days at its equator, and then more slowly with increasing latitude to reach about 34 days near the poles. The photosphere cannot therefore be a solid surface. It merely represents the level through which, owing to the opacity of the solar gases, we cannot see.

The spots are really hot, bright regions. They appear dark because we see them in contrast to the hotter, brighter surrounding regions of the photosphere. The spots have temperatures of approximately 3,500 degrees centigrade, but the temperature of the photosphere is nearly 6,000 degrees centigrade. The number of spots varies from a minimum though to a maximum and then to a minimum again in an average period of 11 years. This period, known as the sunspot cycle, has been as short as six years and as long as 17. Large sport usually appear at sunspot maxima. The largest seen so far occurred in 1947. It grew from a few small spots to a group visible to the unaided eye, At its maximum development covered some 6,200 million square miles, or about one per cent of the sun’s disk.

Prominences

The hot gases above the level of the photosphere form a tenuous and fairly transparent “atmosphere”. This region consists largely of hydrogen and helium, but also contains a small percentage of other gases representing probably all 92 permanent elements from hydrogen to uranium. Here are found flame-like clouds of shining gases named prominences. Some form shapes like pyramids, trees and arches and keep fairly still for hours and even days. Others surge outwards with explosive violence to form spectacular steamers, loops and plumes or cascade sunwards like fountains of fire. Their observation used to be restricted to the times of total solar eclipses. But they now can be studied in full daylight with the coronograph, a telescope that artificially eclipses the sun, and also with telescopes equipped with special filters.

More violent than even the most active prominences are sudden explosive disturbances known as solar flares. Large ones can be seen directly with the telescope, The great majority of them are more readily detected when the sun is photographed in the red light of hydrogen. A large flare not only emits liberal doses of x-rays and ultraviolet rays. It also ejects streams of fast moving electrified particles (protons and electrons). If the flare occurs on the sun’s near side, both the rays and the particles have a good chance of reaching the earth. The rays make the journey in just over eight minutes, disturb the earth’s ionosphere, or electrified upper atmosphere, and upset radio communications. The particles take much longer. They eventually stream into the earth’s atmosphere to upset the directions of compass needles. They bring about vivid displays of northern and southern lights.

The Corona

During the total eclipse of the Sun, when the photosphere is completely hidden by the dark body of the moon, the sun’s atmosphere is seen to extend far into space. It then appears as a pearly-white aureole, often with a delicate structure of tufts and curved wisps of light. It consists of extremely thin gases, whose temperature, measured by the velocity of their atoms, is about one million degrees centigrade near the sun. Most of its light is sunlight scattered by atomic particles. It also continuously emits radio waves. These sometimes come in intense bursts usually associated with active sunspots or large solar flares.

The corona is no static medium, nor is it limited to the sun in the way that the earth’s atmosphere’s limited to the earth. Its form and appearance are definitely influenced by the sun’s activity. At sunspot’s minimum, long curved streamers of light reach to great distances from the sun’s equatorial regions, while relatively short brush-like streaks appear at the poles. At sunspot maximum, the long streamers disappear and the corona takes on a more regular and uniform appearance.

We now know that the corona, and therefore the sun, extends in a very real sense far beyond the earth. Hot hydrogen gas flows from the sun in all directions and rushes past the earth at about 900,000 miles an hour. It blows through the solar system like a swift wind, sweeping any fine particles, meteoric dust, and gases into interstellar space. The streams of particles blasted off by solar flares are like temporary gales in the outward flow.

How the Sun Shines

Sunspot, prominences, flares, and the corona are all outward visible signs of activity deep in the sun’s interior. The mainspring of the sun’s energy output is a process known as a thermonuclear reaction. Under the enormous pressures and temperatures of several million degrees which exist in the sun’s central part of core, hydrogen is transmitted into helium. Vast quantities of energy are released in the process. They pass through the photosphere and, in the form of light and heat, pour into space. Calculations show that the sun converts about four million tons of its mass into energy every second. But since the total mass of the sun is so enormous, the loss is only a few percent over several thousand million years.

Considerations based on theory indicate that the sun has been shining for some 6,000 to 8,000 million years. Also, that it has enough hydrogen in its core to keep it going without much change for at least another 2,000 to 3,000 million years. But when the available hydrogen has been used up, the sun will gradually swell into a reddish star several times larger than its present size. The change will probably take many millions of years, As the sun approaches giant status its immense heat could boil away our seas and oceans and bring about the end of life on earth.

Just as man is destined to destruction, so also is the sun. Its nuclear fire cannot burn forever. Modern theories of the evolution of the stars suggests that the sun will dwindle into a white dwarf. It will become a hot, small, and extremely dense star, the last stage towards final extinction. So if intelligent life on the earth escaped a hot death, it eventually would have to face death through darkness and extreme cold.