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EDE71610 stars poster:stars poster 1 AW

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a guide to THE STARS
A STAR CAN HAVE T WO TYPES OF LIFE CYCLE...
SOLAR STAR
(EG-THE SUN)

WHAT IS A STAR?
A star is basically a globe of hydrogen gas which shines because of nuclear reactions at its heart. These consume hydrogen, converting it to helium and releasing energy in the process. These reactions don't occur in hydrogen gas in normal conditions on Earth as very high pressures and temperatures are needed to start them. The controlling factor in the the life of a star is its mass. Low mass stars, a tenth or so of the mass of the Sun, are very dim and red and last for billions of years. Those of high mass (which could be up to about 100 times that of the Sun ) shine brilliantly with a strong bluish light, but last only a few million years. Stars are categorised based on their temperature. The sequence starts with type 0 stars, which are the most massive, hottest and bluest, and runs through type B and type A, then F and G (yellowish), K (orange), and M (orange-red). Within these classifications there are also giants and dwarfs, but these terms refer merely to the star's diameter compared with others of its type, not to its actual mass. Only about 6 per cent of the stars in the sky are as bright as the Sun or brighter, yet these are the ones that we see and, as a result, the ones that we consider typical. About 70 per cent of the stars in the Galaxy are red dwarfs, like Proxima Centauri, the nearest star of all, which can only be seen with a telescope.

RED GIANT

WHITE DWARF

PLANETARY NEBULA

THE LIFE AND DEATH OF OUR SUN
The Sun, like all stars, is powered by hydrogen. In about 5 billion years, when its fuel runs low, it will expand to become a red giant. It will then have a cooler outside temperature with a bright core. Eventually the outer layers are lost. When the Sun can no longer support itself through nuclear energy, a planetary nebula is left ­ a gas shell surrounding the dying star. When the gas blows away, the core of the old star is left as a white dwarf, an enormously dense star (around the size of the Earth but with the mass of the Sun). Over time the white dwarf will cool down to leave a cold, dark object.

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STELLAR NEBULA MASSIVE STAR

RED SUPERGIANT NEUTRON STAR SUPERNOVA BLACK HOLE

LIGHT YEARS AWAY...
Even if you could travel at the speed of light, it would still take you 13.7 billion years to reach the edge of the known universe. A light year is a measure of distance, not time. It is the total distance that a beam of light, moving in a straight line, travels in one year. As light travels at 186,000 miles per second, a light year is a long way! ...Nearly 6 trillion miles. Knowing the distance that light travels in one year means we can calculate the distance between objects in the universe. Proxima Centauri, the closest star to us, is 4.37 light years away - around 24 trillion miles.

BLACK HOLES
A black hole is a region of space in which gravity is so powerful not even visible light can escape. The term comes from the fact that the absorption of light renders the black hole's interior invisible and indistinguishable from the black space around it. Despite the interior being invisible, a black hole may be detected through interaction with matter that lies in orbit outside an area called the event horizon, for example, tracking stars that orbit around its centre.

GALAXIES
A galaxy is a huge assembly of stars, dust and gas, an example of which is our own Milky Way Galaxy. There are three main types, Elliptical galaxies are round or elliptical systems, showing a gradual decrease in brightness from the centre outwards. Spiral galaxies are flattened, disk-shaped systems in which young stars, dust and gas are concentrated in spiral arms coiling out from a central bulge, the nucleus. Barred spiral galaxies are distinguished by a bright central bar from which the spiral arms emerge.

THE HUBBLE TELESCOPE
The Hubble Space Telescope was launched in 1990. Because it orbits outside the Earth's atmosphere, It can see much further than ground based telescopes and has helped determine the age of the universe, the identity of quasars and the existence of dark energy. Hubble has sent back some stunning images. For example, the Sombrero galaxy. 50,000 light-years across and 28 million light-years from Earth. There are about 800 billion suns in this image.

STAR CLUSTERS
Star clusters may contain anything from several dozen to several hundred stars. Globular clusters are symmetrical systems containing up to a million very old stars in the halo of our Galaxy. They are typically 50 to 150 light years across and at least 20,000 light years away. The best-known open cluster is the Pleiades, or Seven Sisters, which is visible to the naked eye.


EDE71610 stars poster:stars poster 1 AW

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GIANTS OF ASTRONOMY
Astronomy is the oldest of the natural sciences, dating back to antiquity, with its origins in the religious, mythological, and astrological practices of pre-history. Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.
Stone Henge

Early cultures identified celestial objects with gods and spirits. They related these objects and their movements to phenomena such as rain, drought, seasons, and tides. It is generally believed that the first astronomers were priests (such as the Magi), and that their understanding of the heavens was seen as divine, hence astronomy's ancient connection to what is now called astrology. Ancient structures with possibly astronomical alignments (such as Stonehenge) probably fulfilled both astronomical and religious functions. COPERNICUS From the time of the ancient Greeks it was believed that the Sun and all the stars and planets revolved around the earth. To suggest otherwise was against the teachings of the church and could result in a charge of heresy. Born 1473, Nicolaus Copernicus was a Polish astronomer and mathematician who was convinced that the Earth and the planets were in orbit around the Sun. Wisely as it turned out, he waited until near the end of his life to publish his findings. In 1600 one of his followers, Giordano Bruno, was burnt at the stake. In 1601 a young German, Johannes Kepler demonstrated that the planets do not move in perfect circles around the Sun, but moved in elliptical orbits. His laws of planetary motion provided the basis of all subsequent discoveries. THE TELESCOPE In 1608, a Flemish spectacle-maker, Hans Lipperhey applied for a patent for the telescope (which was refused) but news of his invention spread across Europe and the Italian astronomer Galileo Galilei worked out the principles and produced his own eight-power telescope. He gradually improved the power of his telescope, grinding lenses himself, and began observing the heavens. Although not even as powerful as a cheap pair of modern binoculars, he was able to see the satellites of Jupiter, and the phases of Venus. His observations proved that the Earth could Copernicus not possibly be at the centre of the universe. Galileo was not as tactful as Copernicus, and was brought to trial in Rome and condemned as a heretic. He managed to escape the stake by declaring his own discoveries to be false. He was finally pardoned by the Vatican in 1991.

Sir Isaac Newton

According to the well-known story, it was on seeing an apple fall in his orchard at some time during 1665 or 1666 that Newton conceived that the same force governed the motion of the Moon and the apple. He calculated the force needed to hold the Moon in its orbit, as compared with the force pulling an object to the ground. He also calculated the relation between the length of a pendulum and the time of its swing. Newton identified gravitation as the fundamental force controlling the motions of all celestial bodies. Although not really an astronomer, Newton's influence on the study of the universe continues to this day. He was buried in Westminster Abbey in 1727. EDMUND HALLEY Halley was born on 8 November 1656 in London. Along with Robert Hooke, Sir Christopher Wren and Sir Isaac Newton, Halley was trying to develop a mechanical explanation for planetary motion. Although progress had been made, Hooke and Halley were not able to deduce a theoretical orbit that would match the observed planetary motions. However, Newton was already there: the orbit would be an ellipse. In 1705 Halley published 'A Synopsis of the Astronomy of Comets', in which he described the parabolic orbits of 24 comets that had been observed from 1337 to 1698. He showed that the three historic comets of 1531, 1607, and 1682 were so similar in characteristics that they must have been successive returns of the same object - now known as Halley's Comet - and accurately predicted its return in 1758. He never lived to see his prediction come true. In 1720 Halley became astronomer royal at Greenwich, a position which he held until his death on 14 January 1742. The spacecraft Giotto intercepted and photographed the icy nucleus of Halley's Comet in 1986. It will next be visible from Earth in July 2061.
Halley's Comet

THE MILKY WAY
The Milky Way is a member of a collection of more than 50 galaxies called the Local Group. If you were to approach the Milky Way edge-on, the first thing you would notice is a luminous halo made up of gas and stars enveloping the galaxy. The halo is about 100,000 light-years in diameter and 1,000 light-years thick. A light-year is about 6 trillion miles. This halo contains some 170 orbiting star clusters and about a dozen small galaxies. Astronomers Solar System estimate that the Milky Way contains about 100 billion stars. Recently, however, this number was upped by about a billion after the discovery that very old, nearly invisible stars had escaped earlier detections. The Milky Way is believed to contain four major spiral arms, all of which start at the galaxy's centre, plus a number of smaller arms. Our Sun is located on a spur of the Orion Arm. Most of the Milky Way's stars are concentrated in a main disk, which lately has been described as a series of disks, none of which are entirely distinct, but instead overlap one another. The largest is known as the thick disk; this disk is fairly flat and spirals like a slow-spinning hurricane because of our galaxy's rotation. Nestled within the thick disk is an even flatter disk The stars in this thin disk rotate even faster around the thick disk. Further in is yet another disk, known where stars and clouds of gas are moving fastest of of stars, known as the thin disk. the galactic center than those in as the extreme disk, all.

THE GALACTIC BAR As you near the centre of the Milky Way you will feel a greater pull of gravity as you approach the densest and brightest part of our galaxy, a spherical region known as the central bulge. Things are very different here. Most of our galaxy is relatively uncrowded-the nearest star to our Sun, for example, is 4.2 light-years away. But roughly 10 million stars are known to orbit within a light-year of the galaxy's centre. Recent infrared surveys with NASA's Spitzer space telescope confirmed that the Milky Way is not a perfect spiral galaxy but instead sports a long bar of stars within the central bulge. This galactic bar is believed to be made up of about 30 million stars, stretching 27,000 light-years from end to end. It consists mainly of old, red stars, which is one reason it stands out and can be detected. The galactic bar is thought to spin like a propeller inside the Milky Way centre, helping to create our galaxy's unique spiral shape. Observations of other galaxies also suggest that galactic bars play an important role in feeding the colossal black holes believed to lie at the heart of many galaxies, including our own. OUR LOCAL BLACK HOLE The Milky Way's suspected black hole is called `Sagittarius A' and is thought to have between 3.2 and 4 million times the mass of our Sun. It is thought that all of this mass is confined, amazingly, to an area approximately 10 times smaller than Earth's orbit around the Sun. `Sagittarius A' is also probably rotating, making one full revolution about every 11 minutes. We can tell this from the incredible speeds of the stars around it, which move 50 times faster than Earth orbits the Sun. The gravity required to keep these stars in such a fast, tight orbit is calculable, and the tiny area into which it must fit indicates that it has to be a black hole.

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THE STARS

Our Sun is located 26,000 light-years away from the galactic centre on one of the spiral arms. It is a location considered more suitable than others for harboring life, in part because the central region is too chaotic, and in part because the concentration of metals there is too heavy, and it's too light in the galaxy's outer fringes. The Sun makes one complete orbit around the galaxy about once every 225 million years. In contrast, stars near the galactic centre complete a lap in a few million years or less. These stars as a group tend to be younger than the galactic average, most ranging in age from 1 billion to 10 billion years old.

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Poster design by Jonathan Simms

2009 SPACE ODYSSEY

THE STARS

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SIR ISAAC NEW TON Born in 1643, the same year Galileo died, Newton was to become regarded as the greatest scientist of all time. He discovered that light from the Sun is composed of waves of different colours of light which he demonstrated by passing light through a glass prism and producing the first spectrum. He also produced the first reflector telescope, which used a concave mirror to gather light. This gave a much better image because a mirror reflects all colours of light equally. Modern reflector telescopes are vastly larger than Newton's original 1 inch version but follow exactly the same principle.

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Don't miss tomorrow's poster - The Sun

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