Galaxy: Unexplored Continent

Galaxy: Unexplored Continent

The Galaxy is an unexplored continent filled with exotic beings of stellar dimensions. We have made a preliminary reconnaissance and have encountered some of the inhabitants. A few of them resemble beings we know. Others are bizarre beyond our most unconstrained fantasies. But we are at the very beginning of our exploration. Past voyages of discovery suggest that many of the most interesting inhabitants of the galactic continent remain as yet unknown and unanticipated.

Not far outside the Galaxy there are almost certainly planets, orbiting stars in the Magellanic Clouds and in the globular clusters that surround the Milky Way. Such enormous worlds would offer a breathtaking view of the Galaxy rising – an enormous spiral form comprising 400 billion stellar inhabitants, with collapsing gas clouds, condensing planetary systems, luminous supergiants, stable middle-aged stars, red giants, white dwarfs, planetary nebulae, novae, supernovae, neutron stars and black holes.

Nuclear fire. Stick People Figures on a Beach in a Nuclear Fire. Mosaic © Elena

It would be clear from such a world, as it is beginning to be clear from ours, how our matter, our form and much of our character is determined by the deep connection between life and the Cosmos.

The observable universe itself is only a few tens of billions of light-years across and, if there is a vast supercluster in the Virgo group, perhaps there are other such superclusters at much greater distances, which are correspondingly more difficult to detect.

For unknown ages after the explosive outpouring of matter and energy of the Big Bang, the Cosmos was without form. There were no galaxies, no planets, no life. Deep, impenetrable darkness was everywhere, hydrogen atoms in the void. Here and there denser accumulations of gas were imperceptibly growing, globes of matter were condening – hydrogen raindrops more massive than suns. Within these globes of gas was first kindled the nuclear fire latent in matter. Then, a first generation of stars was born, flooding the Cosmos with light.

The Milky Way

The Milky Way

On a clear, moonless night, the Milky Way looks like a path of pearly light among the stars. Many early peoples interpreted it as such and called it variously a celestial girdle, a road to heaven, and the celestial counterpart of the river Nile. It was first traced in detail and described in scientific terms by Ptolemy, the great Alexandrian astronomer of the second century A.D. Ptolemy commented on its patchy appearance, and described how it reached it brightest levels in the region of Sagittarius and narrowed down almost to nothing in the constellations of Auriga and Turus.

Stars, Dust, Gas

The telescope shows that the Milky Way is composed of myriads of stars, all so faint that they cannot be seen as single objects by the unaided eye. It also shows that the patchiness is produced by the way the stars are distributed. In the bright parts they are thickly scattered, whereas in the dark areas they are relatively few and far between. Photographs of the regions of Cygnus through to Sagittarius show places where the stars are so numerous and apparently so close together that their images pile into one another to form great star clouds.

Orion Nebula. Image in public domain

They also show dark irregular patches, due to clouds of interstellar dust that cut off the light of the more distant stars. The stars we do see in these dark wastes are mostly foreground objects, or stars that lie between us and the dust clouds.

Dust clouds that show fairly definite outlines are called dark nebulae. A good example is the Coalsack, a roughly oval-shaped dark patch near the Southern Cross. It appears permanent mainly because it is comparatively near to us (about 500 light-years) and therefore has hardly any foreground stars projected on it.

In many parts of the Milky Way the stars appear to be immersed in heavy folds and wreaths of bright nebulous material. The material consists of extremely rarefied gases whose light, like some of that from the tails of comets, is due to luminescence. The gases absorb the short-wave energy of the hot stars embedded in them and re-emit it at longer wavelengths. Objects of this kind are called bright nebulae or, more specifically, gaseous nebulae.

The Milky Way System – The Great Nebula In Orion

A particularly fine gaseous nebula is the Great Nebula in Orion. It can be seen quite easily without optical aid on a clear, moonless night, but there is no record that it was ever noticed before the invention of the telescope. The brightest parts, about 900 light-years away, are concentrated on a group of stars known as Theta Orionis. The group forms part of a cluster of intensely hot stars whose total radiation causes the nebula to shine over immense distances. They are thought to be comparatively young, with ages of the order of 10,000 to 40,000 years, and to have had their origin in the great gas cloud. The ragged appearance of the nebula is due largely to intervening dust clouds. In itself it is a vast spherical mass of luminescent gas which extends over practically the whole constellation of Orion.

Another fine gaseous nebula is associated with Eta Carinae, a nova-type star in southern skies. Sometimes called the “Keyhole Nebula” because of its shape, it can be traced over an area at least 25 times the apparent area of the full moon, but much of it lies hidden behind clouds of interstellar dust.

An Optical Effect

The Milky Way, as William Herschel discovered, is an optical effect. To appreciate the meaning of this, suppose the stars were evenly scattered throughout space and formed a great ball with the sun at the centre. The entire night sky would then glow with the light of innumerable faint stars. Now imagine that the hall is greatly flattened so that it becomes more like a watch in shape. From our central position we would see more stars in the direction of the rim than in directions at right angles to the rim. This would bring about the effect of a misty band of light, but the Milky Way so produced would be uniform in width and brightest in the middle of its course.

A model much nearer the actual situation is one in which the stars form a greatly flattened system shaed like a fried egg. The sun is located close to the central plane but about two-thirds of the way from the centre to the edge. The system, called the Milky Way System, or Galaxy, has no definite edges or boundaries, and its stars are concentrated in and around its central plane and also towards the centre. At the centre is the nucleus, an almost ball-like mass of highly-luminous giant stars. It lies in the direction of Sagittarius, but we cannot see it from the earth since it is hidden by obscuring clouds of dust.

Spiral Arms

We know how the Galaxy comes to have a flattened shape. The entire system, roughly 100,000 light-years across, is rotating. We also know, mainly by observations with radio telescopes, that the Galaxy has a spiral structure. The sun and its neighbours form a small knot in an immense spiral arm of stars, gas, and dust, one of two fairly continuous and almost circular arms. In their overall contours these arms curve away gently from the nucleus and, after making two or three turns, trail off into intergalactic space. Most of the stars in the Galaxy, along with the interstellar gas and dust, are confined to the galactic disk.

Two Populations of Stars

Must of the stars in the Galaxy, along with the interstellar gas and dust, are confined to the galactic disk. They travel round the centre in circular orbits similar to that of the sun, form a stellar population known as Popualtion I, and in general are believed to be comparatively young. The remainder, together with the globular clusters, are scattered around the disk to form a thinly populated spherical halo or system.

They travel round the centre in elliptical orbits, often steeply inclined to the plane of the disk. Known as Population II they are thought to represent older stars formed before the Galaxy acquired its spiral structure. Finally, both populations are immersed in extremely tenuous and ionized gas, permeated by lines of magnetic force and in itself a source of radio emission. Several other spiral galaxies have been found to be weak sources of continuous radio emission, so the Galaxy is not alone in this respect.

Galactic Cluster

Galactic Cluster

There are two kinds of star cluster. One type, known as Galactic clusters, have a loose or open structure. They contain anything from a few dozen to several hundred stars which travel together through space at speeds of many miles a second and which presumably had a common origin. Two conspicuous examples in Northern skies are the Hyades (the V-shaped group that forms the face of Taurus, the Bull) and the Pleiades, popularly called the Seven Sisters. The latter is particularly striking when seen through binoculars or a small telescope, for its brightest stars are huddled together in a space no larger than the apparent area of the full moon. The cluster also contains several hundred fainter stars, but owing to its distance, 400 light-years, most of these can be seen only in large telescopes.

NGC 700. North America Nebula.Image in public domain

Globular Clusters

The other kind of cluster takes the form of a hall-like swarm of at least several thousand stars, packed densely toward the center, and spread more thinly in the outer regions. In small telescopes these globular clusters look insignificant, but it is because they are so far away. The brightest, Omega Centauri, lies in Southern skies at a distance of about 22,000 light-years. To the unaided eye it looks like a hazy star, but in reality it consists of several hundred thousand stars spread over at least 200 light-years of space.

The brightest globular cluster in Northern skies is Messier 13 in Hercules, discovered by Edmond Halley in 1716. “This is but a little patch,” Halley wrote, “but it shows itself to the naked eye when the sky is serene and the moon absent.” Later, in the 18th century, William Herschel observed it through large reflecting telescopes and estimated that 14,000 were “cribb’d, cabined and confined “in Halley’s “little patch”. Modern studies have let astronomers to raise their number to at least 50, 000, and to suggest that it may surpass one million. The estimated distance is about 34,000 light-years.

Crab Nebula. Messier I, the Crab Nebula in Taurus. Image in public domain

Hercules Cluster. Messier 13, a globular star cluster in Hercules. Image: Bareket.astro.com

The Pleiades. Image in public domain

Stars

Stars

Star Distances

If we represented the Sun by a globe two feet in diameter, the Earth could be shown by a pea on a circumference of a circle 215 feet in radius. On this scale a large pin’s head about one and a half miles from the sun globe would serve for Pluto, but Proxima Centauri , the star nearest to the Sun, would be 10,000 miles away.

Models of this kind help us to appreciate the immensity of interstellar space compared with the extent of the Solar system. They also emphasize the need for a unit of distance, like the light-year, equivalent to about six million million miles.

Proxima Centauri, a faint star in southern skies, is 4,3 light-years away. The Sun, on the other hand, is just over eight light-minutes away, so a rough comparison between the distance of Proxima Centauri and that of the Sun is one of 4,3 years with eight minutes.

Stars, photograph in public domain

Bright Sirius, another near neighbour of the Sun, is 8,7 light-years away, but Canopus, second brightest star in the night sky, is 300 light-years away.

At least 20 stars, most of them faint objects, lie within a radius of 11,5 light-years from the Sun, while 17 of the brightest stars lie within a radius of 400 light-years. Space, it seems, is remarkably empty of stars, so empty as to make the solar system appear overcrowded by comparison.

Differences is BrightnessAlthough the stars are at different distances from us, this fact alone does not account for their differences in apparent brightness. They also differ in intrinsic brightness, or luminosity. Some stars are many thousand times more luminous than the Sun, others are over a thousand times less luminous. The immense range arises largely from differences in surface area and surface temperature. Thus, if two stars are equal in size but different in surface temperature, the more luminous star will be the one with the greater surface temperature. Conversely, If they both have the same surface temperature, the more luminous star will be the larger of the two.

Temperature and Colour

The surface temperature of a star also determines its colour. Reddish stars, like Betelgeuse and Antares look red because they are fairly cool. The temperature of their surface is of the order of 3,500 degrees centigrade, or below that of the filament of an electric light bulb. The sun with a surface temperature of nearly 6,000 degrees centigrade, looks yellow. Rigel and Regulus, two blue-white stars, have surface temperatures of about 20,000 degrees.

Despite their red colour, Betelgeuse and Antares are among the 20 brightest stars in the sky. This is because of their immense size. Betelgeuse, with an average diameter of about 300 million miles, could easily contain the orbit of Mars, while Antares is only slightly smaller. Yet their great distances from us (650 light-years and 400 light-years respectively) cause them to look like points even in the largest telescopes.

Double Stars

Telescopes reveal that many stars which appear to be single objects to the standard eye consist of two, three or even more stars. Of course, two stars will look close together if they happen to lie in almost the same direction, but one of them may be hundreds of light-years beyond the other. Two stars that actually are close together are said to form a binary system. The components, tied together by the invisible bonds of gravitation, revolve about each other. Sirius, Alpha Centauri, Procyon, and Spica are binaries, although Alpha Centauri is more strictly a triple system since it includes a third member in the form of Proxima Centauri. At least, one third of the brighter stars consist of two or more suns.

Variable Stars

Many stars change in brightness and are therefore called variables. In some cases the star itself pulsates, that is, it varies in diameter (and hence in surface area) and also in surface temperature. Quiet often, as with Delta Cephei the light changes are remarkably regular, but they can be quite irregular, as in the caseof Betelgeuse. Rhythmic light changes are also characteristic of certain binaries. If the orbits of the component stars are presented edgewise or nearly edgewise, one star will partially eclipsed by the other. A well-known example of this type of variable, known as an eclipsing binary, is Algol, or Beta Persei.

Erupting Stars

Also among the variables are stars that undergo one or more sudden increases in brightness. The surges are probably due to great flare-like eruptions, which in extreme cases cause a star to shed practically all of its outer regions. In the latter event, the star, termed a nova, increases in brightness by a factor of 5,000 to 100,000 times within two or three days and then gradually fades to return to its former insignificant state. But these changes are mild compared with those associated with supernovae, or stars like the one seen by Tycho Brahe in 1572. These stars undergo titanic explosions in which practically all their material is ejected into space. At maximum brightness they can reach the luminosity of 100 million stars.

In some respects stars are like people. In general they have a common form, structure, and composition. They have various individual characteristics and traits, yet can be sorted into a relatively small number of different times. They tend to form associations: some go through life singly, others in pairs, and others in closely knit groups and clusters.

Mars

Mars

The planet Mars is named after the Roman god of war because of its red colour. It orbits the sun once in nearly two years at an average distance of about 142 million miles. It is brightest and most easily observed when it is in opposition, or in a direction opposite to that of the sun, but even then it can come no closer than about 35 million miles. The large distance, coupled with the planet’s comparatively small diameter of 4,200 miles, makes all but the coarser surface markings difficult to detect with earth-based telescopes.

During the Mars night and especially in the polar regions, the ground temperature falls well below the freezing point of water.

Mars. Photo by NASA in public domain

The red color of Mars is thought to be due to the existence of large deserts of orange-red dust. Any oxygen the planet once had in its atmosphere is now probably imprisoned in the surface rocks in the form of iron oxides. Carbon dioxide and water vapour have been detected in the Martian atmosphere, but the amount of water vapour is so small that it is were all turned into water and would cover the planet to a depth of only three-thousandths of an inch. White, high-altitude clouds, presumably composed of ice crystals, occasionally drift over the surface or appear beyond the planet’s edge or limb. A yellow haze, thought to be swirling dust clouds, sometimes obscures large areas for days or even weeks.

Mars has two white polar caps and various permanent dark areas. The caps change considerably in size with the Martian seasons, being largest in winter and smallest in summer. The dark areas also change with the seasons: a few change hardly at all, but the rest tend to darken as the polar cap in their particular hemisphere gets smaller. The effect is as if a wave of darkening were moving from the polar cap toward the equator.

A Parched World

The gravitational pull of Mars is much less than that of the Earth: an astronaut who weighed 140 pounds of the Earth would weigh only 56 pound on Mars. Consequently the Martian atmosphere is thin – so thin, perhaps, as to rule out all known forms of life. We have good reasons for thinking that the general climate on Mars would be similar to that on a dry, cold desert some 11 miles above the Earth’s surface.

The combined length of day and night on Mars is slightly longer than on the Earth. But during the day, at the planet’s equator, the temperature reaches a high of only 30 degrees centigrade.

Canals on Mars?

These changes encouraged the idea that Mars possessed some form of vegetation partly nourished by flows of moist air from the shrinking polar caps. One astronomer, Percival Lowell, suggested that the moisture took the form of water which moved towards the equator through artificial waterways. He made an intensive study of Mars during its close approach or opposition of 1894-1895, and concluded that the certain dusky streaks, first seen in 1877 by the Italian astronomer Giovanni Schiaparelli, were canals designed and built by intelligent Martians.

Although he drew them as forming a highly geometrical network of straight lines, he did not claim to see the canals themselves, but merely the vegetation growing on their banks.

Lowell’s conclusions have no foundation in the light of modern observations, but the precise nature of the dark areas still remains a mystery. One modern view, based largely on radar studies, is that they are high plateaux, comparatively free from dust. Another is that they are areas where alternate freezing and thawing on the ground has produced a highly porous surface. The polar caps certainly cannot produce large quantities of free water, for the atmospheric pressure on Mars is so low that ice and snow would not melt but sublime, or pass directly from the solid to the vapour state. The caps are undoubtedly quit thin, but whether they are deposits of snow, hoar frost, solid carbon dioxide, or a combination of all three, is still an open question.

Mariner IV. Photo by NASA in public domain

Mariner IV

If canals bordered by fringes of vegetation exist on Mars they should have shown up on the close-up photographs taken in July, 1965, by Mariner IV. As this little planetary probe passed within 9,000 miles of Mars its camera photographed sections of a long, narrow strip of the planet’s surface. Much to everyone’s surprise, the photographs showed dozens of ring mountains similar in appearance to some of the craters on the moon. Nearly 12 per cent of the well-defined rings have central peaks, but several others are ill-defined, as if almost eroded away by dust-laden winds. Studies of the photographs have led some experts to interpret certain linear features as mountain ridges similar to those found on the beds of oceans on the Earth. But any oceans Mars might have possessed dried up long ago.

Mars has two tiny satellites, discovered by the American astronomer Asaph Hall in 1877 and named Phobos and Deimos. Strangely enough, the existence of these flyweight moons was mentioned by Jonathan Swift in his Gulliver’s Travels some 150 years before Hall’s discovery. Phobos, 10 miles in diameter, flies around Mars in seven hours, 39 minutes at a distance of 5,800 miles from the planet’s center. It therefore revolves roughly three times faster than Mars rotates and consequently travels across the Martian sky in a west-to-east direction. Deimos, five miles across, has a period of revolution of 30 hours, 18 minutes and is 14,600 miles from the planet’s center. Since the period is only slighter longer than the Martian day. Deimos moves slowly across the Martian sky in an east-to-west direction.