Mapping the Stars
Celestial cartography, mapping the heavens, is not only naming and charting the position of the stars, planets and other celestial objects in relation to the earth and each other but also looking for patterns, the constellations, to help us find our way around the heavens and hopefully reveal answers to questions about the universe, and deepen our understanding about the world in which we live. 95% of the universe is made of things we can’t see or don’t understand but observations of the skies and stars was always possible. The changing constellations in the night sky was observed but it was not understood how the sun, moon, and stars moved across the sky. It was believed that the rising sun was moving across the sky when in fact, the sun is a stationary star and it is the movement of the earth that creates the illusion of the sun moving across the sky. It was Nicolas Copernicus who first published that the earth turns once daily on its own axis and orbits the sun in 'Six Books Concerning the Revolutions of the Heavenly Orbs' in 1543. Contested by the Church, his theory took over a century to be accepted.
Almost every culture has made a study of the stars. Before the advent of the 24-hour day and the formal construction of time, people woke when the sun rose and went to bed when the moon and stars came out. Night and day were viewed as opposing realms. The cyclic motion of the sun, moon, and stars was used to tell the passage of hours of the day or night and the earliest astronomers were the shepherds who watched the heavens for signs of the changing seasons. If you look at a certain point in the sky, facing in the same direction, the stars and constellations that are visible to you will change dependent on the time of day and the time of year. Before calendars were used, it was possible to choose the best time of year for harvesting or sowing as you could determine the time of year by how bright the constellations appeared. The visible stars followed a similar course, appearing to rotate in permanent groups. The fixed patterns of stars in the sky changed as the seasons changed, and the same cycle was observed year on year. It is important to recognise the fixed patterns of the brightest stars in a constellation because when you are observing the sky you are observing thousands of stars visible in our night sky at any one time. When we look up at the stars, we are looking at a past event. The Sun is the closest star to us at approximately 93 million miles away. The light from the Sun takes about 8.3 minutes to reach us which means that we always see the Sun as it was about 8.3 minutes ago. A light year is a unit of astronomical distance equivalent to the distance that light travels in one year which is nearly 6,000,000,000,000 (Six trillion million) miles.
Ancient philosophers believed that the stars were connected to a higher power and that their behaviour had a direct effect on life on earth. The stars were imagined as myths, Gods, deity’s, or characters, that could be angered or appeased. 4,000 years ago the phenomenon of a solar eclipse was an unforeseen event, and with no understanding as to what it was, a startling event to witness. It was believed by the Chinese that it was a celestial dragon eating the sun, by the Inca in South America that it was a sign that the sun God Inti was angered, and in West Africa that human anger and fighting had spread to the sun and the moon. It was believed that if the movements of the stars could be mapped, it would be possible to understand the Gods, their functions, and their movements and to decode messages from the heavens.
The earliest recorded star list was created by the Babylonians around 1000 BC. It contains around 71 stars and constellations with their associated deity, along with their rising and setting times. It also catalogues the constellations that follow the path of the ecliptic. The ecliptic is the apparent path of the Sun in the Earth’s sky that the sun takes over the course of one year; from another view point, the projection on the celestial sphere of the orbit of the Earth around the Sun. At certain points over the year the sun appears in each of the 13 constellations that fall along the ecliptic line and it is these constellations that astronomers recognise as the zodiacal constellations. As most of the constellations through which the Sun passes represent animals, the ancient Greeks called its zone the “circle of little animals”. Astrologers usually use 12 of these constellations as signs of the zodiac, omitting Ophiuchus, and unlike astronomy, astrology is not based on scientific fact.
The positions of all objects in space are measured according to specific celestial coordinates. Every object is assigned two numbers that fix its location. The starry (celestial) and earthly (terrestrial) share the same coordinates, such as the North and South Pole and an equator. The coordinate lines on a star map are called the right ascension (RA) and declination (DEC) and are the equivalent of longitude and latitude on earth. Astronomers navigate the skies by imagining the stars as a giant celestial sphere around the Earth. This sphere is usually divided into two halves – north and south. Observers who live in the Northern Hemisphere can see all the constellations of the Northern Celestial Hemisphere at some time during the year, and those in the Southern Hemisphere the Southern Celestial Hemisphere. Both can see some constellations in the opposite hemisphere dependent on how close to the equator that they live.
Astronomers have found that stars come in many different sizes and masses, as well as colours, temperatures and brightness’s. Stars have always been characterised by two visible parameters – brightness and colour. 21st century Astronomer’s define a star’s brightness in terms of magnitude – how bight the star appears from Earth, and absolute magnitude or luminosity – how bright the star appears at a standard distance of 32.6 light years. In 150 BC Greek astronomer Hipparchus was the first to make a catalogue of stars according to their brightness on a magnitude scale of 1 – 6, with 1 being the brightest, but the apparent brightness of a star depends on the location of the observer and distance from the star. Stars that are closer to earth but fainter, could appear brighter than far more luminous ones far away. Absolute magnitude measures the luminosity of the star – the power of the star – the amount of energy (light) that star admits from its surface, recorded from an equal distance away without any extinction of the stars light by interstellar dust along the line of sight of the observer, for example. To the naked eye they generally appear white, and are mostly represented in fiction as yellow, but they really range from red, orange through true white to blue. Colour is caused by the temperature of the surface of the star. Spectral Types indicate the colour & temperature of a star. Astronomers Ejnar Hertzsprung in Denmark and Henry Russel in the USA devised the HR diagram in the early 1900’s demonstrating the colours of heat and spectral type labelled with the letters. Blue/Green stars are very hot up to 50,000 degrees and are usually young stars which are very active. They live very active short lives and die young usually in a massive explosion called a Super Nova. Yellow stars are in their mid-life and are normal average stars. Orange or red stars are approaching the end of their lives. Astronomers have classified them on the following spectrum:
Green/White Giant very hot active stars Type W & O 36000 + degrees C
Blue Very hot Helium stars Type B 28600 + degrees C
White Large hot stars Type A 10700 + degrees C
Yellow/White Stars larger & brighter than the sun Type F 7500 + degrees C
Yellow Like our sun Type G 6000 + degrees C
Orange Cooler stars Type K 4800 + degrees C
Orange/Red Old dying stars Type M 3400 + degrees C
Red Cool carbon stars Type N & S 2500 + degrees C
In 1603 it was Johann Bayer, a German celestial cartographer, who first used the Greek alphabet for labelling the brightness of stars in a constellation in star cartography. In his star atlas ‘Uranometria’ every star in a constellation was given a letter from the Greek alphabet depending on its apparent brightness from the Earth, or its magnitude. There are 24 letters from Alpha to Omega, with alpha being the brightest. The size of the star illustrated on the A.K.Johnston maps published here is also indicitive of its magnitude based on a scale of 1- 5. The largest illustrated star is the brightest with a magnitude of 1.
Just as all living things, stars are born, live, and die but we see the same stars in our sky today that ancient astronomers saw thousands of years ago and the same stars will still be seen thousands of years from now. This is because stars live for hundreds of millions of years, even tens of billions of years, and over the course of a few thousand years the stars appear not to change at all. Star birth is a process immerging over hundreds of thousands of years. Afterwards stars may shine steadily for several billion years. As they die, they shed much of their matter into space. This finds its way into the molecular clouds that will in time birth a new clutch of stars. Stars like our sun burn for around 9 to 10 billion years. The sun is about halfway through its life so will burn for about 5 billion more years. The precession of the Earth, a gradual shift in the Earth's axis of rotation on a cycle of approximately 25,772 years caused by the gravitational forces of the moon and the sun, means that our perception of north gradually shifts over a 26,000 year period. This precession will steadily cause a shift visible enough that the North Celestial Pole, an area covering a small circle in the sky directly over the North Pole where the heavens appear to turn, will see a succession of stars located closest to the North Pole. From the 4th - 2nd millennium BC it was Thuban, in the constellation Draco, that was the North Star. In the 1960's it was recognised that ventilation shafts constructed within the Pyramids 4,500 years ago were built in alignment with Thuban. The shafts are not straight so it is not possible to view the stars through the shafts but they align with the area of sky where Thuban would have shone brightly. Our current North Star is Polaris, in constellation Ursa Minor. It is possible to map and predict these changes and 12,000 years from now Vega, in constellation Lyre, will be the North Star, and in 26,000 years it will be back to Polaris once again.
Depending on where you are in the world, the area of sky and the stars that you can see will be different. Various cultures have differing ideas of boundaries and constellation names, and it was only 100 years ago in 1922 that astronomers in the International Astronomical Union agreed on a universal set of 88 official constellations. The official 88 constellations depict 42 animals, 29 inanimate objects, and 17 human or mythological characters. A crow, a river, a sea serpent, and a unicorn are all officially recognised.
Observing the sky with the naked eye can only get you so far. Light from buildings and streetlamps light up the sky causing light pollution, and natural light pollution from the moon, makes the fainter constellations unsee able to the backyard astronomer. Apart from Andromeda, stars outside of our Milky Way are simply not visible because they are too far away. The invention of the telescope greatly advanced our understanding on the universe. Italian Galileo Galilei built his telescope in 1609 and is widely considered to be the father of "observational astronomy". With his telescope we discovered that the Milky Way is not a cloud but a galaxy of as many as 200 billion stars. In the late 17th century Isaac Newton built bigger and more sophisticated telescopes able to reveal never seen nebula, and the planet Uranus in 1781. The space between the stars is not an empty space. Often clouds of interstellar matter (wisps of gas or tiny solid particles of dust) can be seen and these are called nebulas. The Orion nebula is one of the brightest nebulae in the Milky Way and can easily be seen with binoculars. The nebula measures about 16 light years across and lies about 1,500 light years away. Edwin Hubble discovered in the 1920’s that the universe contains a huge number of external galaxies beyond our Milky Way. Edwin’s namesake The Hubble Space telescope sent into orbit in 1990, was the first telescope to have an unobstructed view of the universe, far above the light pollution and rain clouds of our atmosphere.
The coming of radio astronomy meant that mapping the invisible became possible. The first evidence of radio radiation coming from outer space was collected by American scientist Karl Jansky in 1931. Building on Jansky’s discovery, Grote Reber began to map the radio emissions coming from the Milky Way in 1936. From the late 1950’s Satellites like IRAS (Infrared), Cobe (Microwave), and Chandra (X-Ray) meant it became possible to study the heavens at wavelengths blocked by Earth’s atmosphere and in the 1960’s astronomers began listening for radio signals broadcast by alien civilisations.
