galaxy

The red shift causes lines in the spectra of galaxies to be shifted towards the red end of the spectrum. More distant galaxies have greater red shifts than closer galaxies. The red shift indicates that distant galaxies are moving apart rapidly, as the universe expands.

Grouping of millions or billions of stars, held together by gravity. There are billions of galaxies in the universe. There are different types, including spiral, barred spiral, and elliptical galaxies. Our own galaxy, the Milky Way, is about 100,000 light years across (a light year is the distance light travels in a year, about 9.5 trillion km/6 trillion mi; 1 trillion = 10¹²), and contains at least 100 billion stars.

The galaxies are moving away from our own in all directions. The universe is thus expanding in all directions. The evidence for this comes from examining light from the galaxies by splitting the light into a spectrum. A feature known as the red shift appears, where the light is shifted towards the red end of the spectrum due to an increase in wavelength caused by the galaxies' recession.

For a prominent galaxy in the constellation Virgo, which is about 38 million light years away, the red shift indicates the galaxy is moving away from us at 1,200 km/730 mi per second, while another galaxy in the constellation Hydra, which is about 2,000 million light years away, is moving away from us at 56,000 km/35,000 mi per second.

Spiral galaxies, such as the Milky Way, are flattened in shape, with a central bulge of old stars surrounded by a disc of younger stars, arranged in spiral arms like a Catherine wheel.

Barred spirals are spiral galaxies that have a straight bar of stars across their centre, from the ends of which the spiral arms emerge. The arms of spiral galaxies contain gas and dust from which new stars are still forming.

Elliptical galaxies contain old stars and very little gas. They include the most massive galaxies known, containing a trillion stars. At least some elliptical galaxies are thought to be formed by mergers between spiral galaxies. There are also irregular galaxies. Most galaxies occur in clusters, containing anything from a few to thousands of members.

The Milky Way is a member of a small cluster, the Local Group. The Sun lies in one of its spiral arms, about 25,000 light years from the centre.

Galaxies vary in size, structure, and luminosity, and, like stars, are found alone, in pairs, or in clusters. As these systems are very remote, they appear in telescopes as hazy, nebulous objects and were first described as nebulae (see nebula). Later, when their remoteness was understood, they were known as ‘island universes’ or ‘extragalactic nebulae’.

Only two, the Magellanic Clouds, are easily visible to the naked eye. The next brightest, the Andromeda galaxy, is just visible. About 35 of the brightest galaxies appear in the list compiled by the French astronomer Charles Messier and several thousand in the New General Catalogue. Billions can be photographed with modern telescopes.

Distances About 20 galaxies are known to be within 2.5 million light years of the Sun, and several thousand within 50 million light years. The distances of those closer than 10 million light years can be estimated from the brightness of individual Cepheid variables if such stars can be identified. Up to about 100 million light years away the magnitudes of supergiants, and of novae or supernovae at maximum, can be used to determine distance. Still greater distances have been estimated by comparing the apparent magnitude of a galaxy with its estimated absolute magnitude. The greatest distances of all are found by measuring the red shift (that is the lengthening of the wavelengths of light from an object as it moves away), and assuming the truth of the red shift–distance relation, one of the essential dogmas of modern cosmology. Once the distance is known, it becomes possible to estimate the masses of some of the nearer galaxies. It is also possible to estimate the masses of clusters of galaxies, but the masses so found have been larger than would be expected from the sum of the masses of the visible individual galaxies. This discrepancy, sometimes known as the problem of the missing mass or ‘dark matter’, has not been explained.

Types of galaxy Normal galaxies were classified by the US astronomer Edwin Hubble into three basic types: spiral, elliptical, and irregular. Spiral galaxies, of which our own Galaxy is a typical example, consist of a nucleus, a disc containing the spiral arms, and a halo. Spiral galaxies are classified according to the appearance of their arms. Sa spirals have a large nuclear bulge and tightly coiled spiral arms, while Sc spirals have a small nucleus and arms less tightly wound. Sb spirals are intermediate between Sa and Sc. Barred spirals are classified SBa to SBc. Elliptical galaxies are something like huge globular clusters, with no spiral arms. They are divided into eight subgroups, E0–E7, the E0s appearing spherical and the E7s the most elongated. Irregular galaxies have a chaotic appearance and show no symmetry. Irregulars are very much less frequent than spirals and ellipticals; they also tend to be smaller but brighter in proportion to their mass.

Clusters Clusters of galaxies can be roughly classified as regular or irregular. Regular clusters have spherical symmetry, central concentration, and usually at least 1,000 members brighter than absolute magnitude −16. One of the nearest examples is in Corona Borealis. Irregular clusters are made up of loose groups of small clusters. Unlike the regulars, which consist almost entirely of ellipticals, the irregular clusters contain all types of galaxies. They vary greatly in content and may contain more than a thousand galaxies, as in the cluster in Virgo, or only 20 or so, as in the Local Group to which our Galaxy belongs. The Local Group contains about 30 members within a region 3 million light years across, including 2 large Sb spirals (our own and the Andromeda galaxy), 1 smaller Sc spiral, 14 ellipticals of which 10 are dwarfs, and 4 irregulars, of which 2 are the Magellanic Clouds. It has been suggested that our Local Group is only a subsection of a ‘Local Super Cluster’. This may be centred in or near the Virgo cluster and is about 100 million light years in diameter and 25 million light years thick.

Recent discoveries In 2003 astronomers identified the nearest galaxy to the Milky Way. The Canis Major dwarf galaxy is situated 88,000 light years from us, half the distance of the previously known closest galaxy, The Large Magellanic Cloud. It was discovered using infrared data and had been unnoticed previously as its position was masked by dense dust clouds.

Also in 2003 astronomers discovered the first ‘dark galaxy’, using the Arecibo radio telescope in Puerto Rico. The cloud, called HVC 127-41-330, is situated 2 million light years from Earth and is rotating too fast to be able to hold itself together unless there is some unseen force preventing it from falling apart. As the cloud contains no obvious gravitational sources such as stars, it is thought to be over 80% dark matter. In 2005 astronomers using the Lovell telescope at the Jodrell Bank Observatory, Manchester, UK, discovered a second dark galaxy.

In 2006 astronomers discovered the earliest massive galaxy, HUDF-JD2, at a distance where theories indicated galaxies of this size had not previously formed.

Galaxy

The star system to which the Sun and most of the naked-eye objects in the night sky belong. Study of the Galaxy is difficult because the Earth is inside it, and numerous dust clouds block views of most of the system.

Its name is derived from the Greek word for milk. The Galaxy is sometimes referred to as ‘the Milky Way system’ or simply as ‘the Milky Way’; this is misleading as this really refers to the band of faint stars seen on the celestial sphere.

Composition Our picture of the Galaxy is still far from precise. The greater part of its mass and most of its bright stars are thought to be concentrated within a flattish circular disc about 100,000 light years in diameter and 2,000 light years thick; at its centre this bulges out into the nucleus, a flattened spheroid about 6,000 light years in diameter and 3,000 light years thick. Surrounding the disc is a spherical halo at least 100,000 light years in diameter. Two spiral arms expand outwards from the edges of the rotating central disc. At about 10,000 light years from the centre these arms merge into a tighter pattern and from there outwards spiral features appear at intervals of about 6,000 light years. These features are not smooth but are broken into clouds about 2,000 light years in diameter and a few hundred light years thick, themselves consisting of clouds each 50–100 light years across and containing enough hydrogen to make thousands of stars. Both radio and infrared observations indicate great activity within the nucleus, with an extremely strong concentrated source known as Sagittarius A at its centre. The activity arises from a black hole with a mass several million times that of the Sun.

There are probably about 200 globular clusters scattered through the Galaxy. It was the distribution of these clusters, which are relatively unobscured by the dust that blots out distant objects, that gave Harlow Shapley his ideas about the extent of the Galaxy and the position of its centre. The spiral arms are composed predominantly of concentrations in the interstellar matter. Since such concentrations are the birthplaces of stars, the arms are marked out by large numbers of bright, bluish young stars. It is near one of these concentrations that the Sun is situated, about 26,000 light years from the centre of the Galaxy.

Distribution The distribution of the stars in the Galaxy is not uniform. They are strongly concentrated towards the centre and towards the central plane of the disc, but the degree of this concentration varies with the age of the stars. Old stars show less concentration than young ones while the very youngest are strongly concentrated in the spiral arms. This difference in distribution between young and old objects was first noticed by Walter Baade in the Andromeda galaxy before the effect of stellar evolution was fully understood. The difference in age is also apparent in their velocities and chemical composition. Young objects tend to be rich in metal, and to move round the galactic centre in nearly circular orbits, while older objects tend to be poor in metal, and to move in elliptical orbits.

Rotation The rotation of the interstellar matter is controlled by gravitation, by its own viscosity, and by magnetic fields. The rotation of the stars is controlled by gravitation. Their periods of rotation increase from the centre outwards, the observation of which first led to the deduction that the Galaxy as a whole was rotating. Observations of extragalactic objects indicate that the velocity in the neighbourhood of the Sun is about 250 kps, corresponding to a cosmic year (the time of one complete revolution) of 200 million years.

Evolution The present view on stellar evolution is that the Galaxy developed from a huge, chaotic, whirling, turbulent cloud of hydrogen and helium, collapsing under its own gravitation to become the organized, smoothly rotating flattened system of today. From time to time stars condensed out, retaining the motion of the part of the cloud in which they were formed. The oldest globular clusters suggest that the first stars were formed at least 10,000 million years ago. The more massive of them evolved rapidly, exploded as supernovae and enriched the surrounding interstellar matter with heavier elements. Succeeding generations of stars were born in an increasingly flattened system, and contained an increasing proportion of heavier elements.