William Herschel, the astronomer who discovered Uranus, was born in Hanover, Germany in 1738. At the age of 14 Herschel joined the Hanover Military Band where he played the oboe alongside his father and brother. An accomplished musician, he also learnt to play the violin, harpsicord, guitar and organ.
In July 1757 while serving in the army,William took part in the disastrous battle of Hastenbeck. In this battle the Hanoverians were defeated by the French.
Unwilling to live under French military occupation, William travelled to London where he arrived a penniless refugee. He quickly learnt English and supported himself both by playing as a freelance musician in orchestras and by giving music lessons.
He moved to the north of England in 1761 where he spent many long hours traveling by horse between music venues in different northern towns. It was at this time that he developed his lifelong fascination with gazing up at the moon and stars in the night sky.
In 1766 he was appointed organist at the Octagon Chapel in Bath, south-west England.
When not working as a musician he spent as much time as he could studying the night sky and reading up on astronomy.
It is said that he frequently interrupted evening music lessons to take students outside to view the moon and stars.
It was in Bath that he started writing an astronomical observation journal. Among his first entries were notes about an eclipse of the moon and the transit of Venus across the face of the sun.
Herschel soon became frustrated with his inability to obtain a reasonably priced high quality telescope. He had discovered that traditional ‘refractor’ telescopes, developed by Galileo Galilei some 150 years previously, were often not up to the job of viewing stars with any clarity.
Before we discuss the disadvantages of refractor telescopes we need to understand how they work.
1) Refractor telescopes bend light
Light is ‘refracted’ (or bent) after it passes through a convex lens. Convex lenses are shaped in such a way that that all rays of light passing through a lens bend and then converge to a single point called ‘the focal point’. It is at the ‘focal point’ that an image is formed.
2) Refractor telescopes magnify light
The image is then magnified as it passes through a concave eyepiece and is now ready to be viewed by the observer.
1) Chronic aberration
A major problem with refractor telescopes is that the curve of the magnifying lens acts like a prism….
……breaking up light into the colors of the rainbow near the edges of the lens. This often makes viewing stars through the eyepiece extremely difficult.
This phenomenon is called ‘chronic aberration’.
2) Restricted field of vision and weak magnification
In Herschel’s time refractor telescopes often had restricted fields of vision and inadequate powers of magnification.
As a consequence of these problems, most eighteenth century British astronomers paid little attention to stellar astronomy.
An exception was made if studying the stars helped improve navigation at sea.
Even Nevil Maskelyne, the Astronomer Royal, largely ignored stellar observations. He mostly observed lunar eclipses, the movement of planets around the sun and passing comets.
In the same way that maritime explorers seek to discover new lands, Herschel wished to explore unmapped oceans of stars that existed far beyond our own solar system.
Were there any telescopes better than refractor telescopes that would enable him to fulfilll his dreams of making new discoveries in deepest space?
Herschel began to think of the possibilities of using Newtonian ‘reflector’ telescopes instead of Galilean ‘refractor’ telescopes.
Reflector telescopes work by collecting light in a curved (parabolic) mirror. The light is then reflected back up the tube and focused onto a secondary mirror.
After bouncing downwards off the secondary mirror the light converges onto a ‘focal point’.The image is magnified as it passes through the convex lens of the eyepiece where it is then viewed by the astronomer.
The main component of a reflector telescope is a dish shaped concave mirror. Herschel could not find a concave mirror (called a ‘speculum’) large enough to fit the reflector telescope that he intended making himself. So he took the decision to make his own ‘speculum’.
Making a six inch diameter metal mirror with a precise parabolic curve was by no means an easy task. After experimenting with different proportions of copper and tin Herschel finally hit on the right mixture of metals which allowed him to cast a concave mirror of the right size and dimensions.
Herschel soon found that his metal mirrors were unusable if they were not sufficiently well polished shortly after being cast. He needed to polish his precious metal mirrors for hours on end if they were to acquire and retain the necessary reflectiveness. If Herschel abandoned polishing for only a few seconds the ‘speculum’ would be rendered useless for stargazing.
Herschel’s assistant, his sister Caroline, recorded that on more than one occasion she would feed William so he did not have to stop to polishing his precious specula. Marathon polishing sessions lasting sixteen hours or more were by no means unheard of!
By 1774 Herschel had successfully assembled his first five foot long reflector telescope incorporating his home made ‘speculum’. A furniture maker made a beautiful octagonal case for his new telescope.
Herschel had created an instrument with amazing light collecting power which magnified the light from stars with incredible clarity. With his deeply held passion for astronomy and equipped with this new telescope Herschel was ready to explore deepest space….
In the 18th century it was a commonly held belief that the North (Polaris) Star, at a distance of 323 light years from the earth, was a single very bright star.
One of the early discoveries that Herschel made with his powerful new telescope was something that very few astronomers of the time suspected.
The Polaris (or ‘North’) star, which for thousands of years had shown sailors the direction of true north, was not one star but two stars located very close together. Herschel proved that the Polaris star was not a ‘single star but a ‘double star’- Polaris A and Polaris B.
We now know that Polaris is not a double but a multiple star system composed of three stars. The third star, Polaris Ab, can be found very close to Polaris A.
Herschel went on to compile three astronomical catalogs containing 800 previously unknown double and multiple star systems. In 1802 he announced his hypothesis, which later proved correct, that many of these double stars were in fact ‘binary star systems’ which orbited each other under mutual gravitational attraction.
This diagram shows two stars orbiting each other around an imaginary point we call the ‘center of mass’.
18th century astronomers referred to clouds of gas and dust floating around the known universe as ‘nebulae’.
No one knew the distance, composition or origins of these ‘nebulae’. They were commonly believed to have been remnants of cosmic gas and dust left over after the Christian deity had finished creating the universe. They were of little significance.
18th century astronomers also believed that all nebulae resided within just one galaxy- our very own Milky Way Galaxy. This belief was hardly surprising since the entire universe was thought to comprise just the Milky Way.
So what did nebulae look like? 18th century drawings of nebulae appear crude and unsophisticated compared with 21st century images captured by space telescopes. This is a sketch drawn in 1771 of the Orion nebula located in the Milky Way Galaxy only 1344 light years away. Compare this 1771 sketch…..
…with this 2006 image taken by the Hubble Space Telescope.
From the back garden of his house in New King Street, Bath, Herschel discovered and recorded 900 nebulae that were previously unknown to 18th century science. These are some of his original drawings.
In addition to discovering new nebulae, Herschel proposed a radical new hypothesis about their composition. Far from being just clouds of cosmic dust and gas floating around the Milky Way, he hypothesized that many nebulae were in fact enormous clusters of stars mixed up with clouds of cosmic dust and gas.
Herschel also proposed that these enormous clusters of stars and clouds of dust and gas were located throughout the universe far beyond the Milky Way. At the time Herschel was not in a position to prove this hypothesis since he could not measure the distance of far off galaxies from the earth.
Conclusive proof had to wait until 1923 with the arrival of the astronomer Edwin Hubble.
Herschel called these clusters of stars ‘island universes’ or (rather confusingly) ‘nebulae’. Today we refer to Herschel’s ‘island universes’ as ‘galaxies’.
Herschel proposed that galaxies were continuously active, forming new stars out of condensed gas in a process of continuous creation. In these ‘laboratories of the universe’ new stars were being born at the same time as old stars were dying.
Herschel implied that the universe was not static and fixed, as was commonly believed at the time, but was continuously changing and evolving.
One of the 900 galaxies discovered by Herschel in 1788 is a spiral galaxy NGC 2683, the so called ‘UFO Galaxy’. It is 20 million light years away from earth.
We now define a galaxy as a cluster of millions (or even billions) of stars, gas and cosmic dust which are held together by the force of gravitational attraction.
Galaxies themselves are constantly on the move, either approaching or moving further away from their neighboring galaxies. For example some galaxies in the ‘Hercules’ cluster (450 million light years away from the earth) are moving closer together while others are moving further apart.
We now know that there are ten main types of galaxy. Our own Milky Way is type ‘Sbc’ in the ‘Hubble classification’ of different types of galaxy.
The Milky Way is located in a cluster of galaxies called the ‘Local Group.’ The center of gravity of this cluster of galaxies lies someway between the Milky Way and Andromeda galaxies. The whole cluster that comprises the ‘Local Group’ is 10 million light years in diameter and includes 54 galaxies in total.
In 1785 Herschel conceived of the Milky Way as being a ‘flat disk’ full of stars, with our sun at the center of that disk. He drew a picture of what he thought the Milky Way looked like.
Herschel was not far wrong. Today we know that the Milky Way is a huge cluster of four hundred billion stars in which most of the stars are indeed concentrated in a flat spiral shaped disk, with a bulge in the middle.
The Milky Way is far larger than Herschel could ever have imagined. From its outermost regions it takes 75 000 light years for light to reach us in our solar system.
In May 1776 Herschel announced that he had seen forests of trees growing on the moon and that the lunar craters were the towns of the ‘Lunarians’ living on the moon. These are his original notes reporting these ‘facts’.
In view of Herschel’s pronouncements about life on the moon, who was going to believe him when he announced in 1781 that he had found a new planet- the first discovery of a new planet for centuries!!