Explorers of the Universe – The Story of Caroline and William Herschel
Caroline Herschel Caroline Lucretia Herschel was born in Hanover, Germany, in 1750. She was the 8 th of 10 children, but only 6 survived to adulthood. Her father was a musician, who tried to give Caroline a basic education; her mother was illiterate, and tried to keep Caroline from getting an education so that Caroline would have no option but to stay home and help her mother with housework. For example, Caroline’s mother prevented her from learning French, which would have enabled her to get a job as a governess. Caroline learned how to read and write, but didn’t learn arithmetic. Caroline’s father died in 1767, when Caroline was 17. She remained at home with her mother and her abusive brother Jacob. In her diary she compared herself to Cinderella….
William Herschel When Caroline was 22, “Prince Charming” arrived in the form of her brother William (Friedrich Wilhelm Herschel), who had been living in England for 15 years. He had fled to England when he was 19 to avoid army service in the Seven Years’ War, and was now making a good living as a musician. He told their mother that he needed Caroline to come to England with him as his housekeeper. He gave their mother some money to hire a servant to take Caroline’s place, and brought Caroline to England with him. There he put her in charge of his household, and gave her lessons in English, arithmetic, and singing. Before long, Caroline was working as a singer. In 1778 she was a soloist in Handel’s “Messiah” and other operas, and was offered a job as a singer in Birmingham. But she decided to stay with William instead.
William’s Telescopes William loved to read; he read books about music, then books about harmonic theory and math. One day in 1773 he bought a book about astronomy. This was about to change his life… and Caroline’s, too. Inspired by the book, William built several telescopes, and began to observe the skies as a hobby. (In his lifetime he built over 1000 telescopes. Some are still on display in museums today.) In 1781, using a 7-ft long reflector with a 6” mirror, William noticed an object that he thought was a comet; but it was disk-shaped rather than fuzzy, had no tail, and seemed to move much slower than a comet usually does. William sent the information on the object’s position to Charles Messier in France, who shared it with other astronomers.
A New Planet Calculation of the object’s orbit showed it to be a planet beyond Saturn, at a distance greater than 16 astronomic units – the first planet to be discovered since ancient times. (Jupiter is 5 a.u. from the sun, Saturn is 10.) William wanted to name it “George” after England’s King George III, but astronomers suggested instead to name the new planet after a Greek mythological figure. The new planet was named Uranus. King George offered William a permanent pension of 200 pounds a year as an astronomer, for which he (and Caroline) needed to move to a town near Windsor where they could show the sky to the king’s guests through their telescopes. The pension was less than William could earn as a musician, but enabled him to do astronomy full time.
A Very Large Telescope In 1783 William built a 20-ft long reflector with an 18.7” mirror. At that time, mirrors were made of a shiny alloy, “speculum” which tended to tarnish; he needed to make 4 mirrors for this scope so that one was always available while the others were being polished. In order to enable him to observe without interruption, William would stand outside near the telescope, with dark-adapted eyes, while Caroline would sit inside near an open window with a candle and notebook, writing down the observations that William called out to her. Sometimes it was so cold that the ink froze in her pen.
Charting the Sky The 20-ft telescope was very large and heavy, and hard to move. William would start his observing each evening by aiming the scope at a specific height; and instead of moving the telescope, he would observe as the sky moved past it (= constant declination). Caroline would note the time when each object was seen, and could calculate each object’s right ascension. The following night, William would aim the telescope a little higher or lower and in this way he was able to scan the whole sky. William and Caroline found over 2000 deep-sky objects (“nebulae”) with the 20-ft scope. William would note the position of each object relative to known stars, but the available catalogs did not list stars in an orderly way. Johann Bayer’s “Uranometria” (1603) and the British Catalog by John Flamsteed (1725) listed stars by constellation rather than by coordinates.
Caroline’s Work To make it easier for William to identify the location of the objects he discovered, Caroline prepared an index of the stars in Flamsteed’s atlas, in order of RA (right ascension) and in zones of 5 degrees declination, regardless of constellation. For example - starting from the north pole, declination 0 to 5 degrees; then 5 to 10; etc. In the course of observing, William and Caroline found a large number of stars that were diagrammed in the wrong place. But Flamsteed’s observing notes were in a separate volume with no index. During the next two years, Caroline prepared a cross-index of the stars, corrected errors, and found over 500 stars that had been observed by Flamsteed but not included in the atlas.
Caroline’s Telescope William also gave Caroline a small telescope, similar to the one with which he had discovered Uranus. With this telescope Caroline discovered 8 comets and over a dozen “nebulae” between 1783 and 1797. These included: Spiral Galaxy NGC 253 Elliptical Galaxy M110 Open clusters NGC 189, 225, 659, 752, 2360, 6633, 6819, 6866, 7789, and M48 Open cluster with emission nebula NGC 7380 Once Caroline rode 30 miles on horseback, at night, to the Royal Greenwich Observatory, in order to report a comet that she had discovered. She wanted to be sure that there would be no delay in its confirmation.
Later Years In 1788, William got married. His wife, Mary Pitt, was a young and wealthy widow. William and Mary had a son, John. Caroline moved out to a rented apartment, and William offered Caroline an annuity. However, she did not want to remain dependent on him. Instead she asked him to request a pension for her from the king. The king approved a permanent pension of 50 pounds per year for Caroline as William’s assistant – the first woman to actually get a salary for astronomy! In 1789 William also built a larger telescope (40 ft long, with a 48” mirror) funded by the king, and discovered two moons of Saturn (Mimas and Enceladus); but the scope was not really useful for science due to its large size and weight, and constant tarnishing of its mirror. It was better for showing off sky objects to the king’s friends.
More Discoveries In 1800, William discovered infrared radiation by splitting sunlight with a prism and measuring the temperature of each color. The temperature increased going from violet to red, so he measured the temperature of the “light-less” area just past the red and found it had a higher temperature than any of the colors. At the time, William did not realize that he had discovered another wavelength of light; he thought that the sun’s rays included “light rays” and “heat rays”. William and Caroline measured the rotation rate of Saturn by timing the motion of faint markings on its surface. They also measured the positions of the components of over 800 double stars, and in 1803 William concluded that the components of many binaries were actually revolving around each other. .
And More Theories William drew interesting conclusions from his observations of planetary nebulae. Although clearly these stationary objects were not planets or even comets, William had given them this name because many of them were shaped like small disks, resembling the appearance of Uranus and other planets. Since the invention of the telescope, it had been assumed that all “nebulae” were unfocused star clusters. But William noticed that in most planetary nebulae, a single star is visible right at the center. He correctly reasoned that because this star is so perfectly centered, it must be part of the nebula rather than a foreground star; and the rest of the nebula must be made of a “shining fluid” because it is too faint compared to the central star to actually be made of unfocused stars. William also tracked sunspots, and incorrectly theorized that maybe the sun was cooler inside than at the surface, and that the sunspots were “windows” to the sun’s cooler interior. .
Final Years and Honors In 1822, William died, and Caroline decided to return to Hanover. There Caroline prepared a catalog of all the nebulae that William had discovered. She and William had originally recorded them in order of discovery, and their position was noted in relation to the guide stars. Caroline arranged them in zones of declination and in order of RA, as she had done earlier with the guide stars. This was of great assistance to William’s son John in rechecking William’s discoveries, and to astronomers everywhere – it was afterwards the beginning of John Herschel’s “General Catalog” and J.L.E. Dreyer’s “New General Catalog” (the NGC) prepared at the end of the 19 th century. As a woman, Caroline was not eligible for membership in the British Royal Astronomical Society, but in 1828 she received a medal from the Royal Society for her work, and later received honorary membership. She also received a gold medal from the king of Prussia, and astronomers and royalty came to see her in Hanover. Caroline died in 1848, at the age of 97. A crater on the moon is named for her, as well as an asteroid “Lucretia” in her honor. .
Open Cluster M48 (NGC 2548) – T1 hm Constellation: Hydra Distance 2000 LY Size 30 LY Age 300 million years Caroline observed this cluster with her small telescope in March 1783. Although it was previously observed by Charles Messier in 1771, he mislabeled its position; therefore Caroline did not know that this cluster had already been observed. .
Open Cluster M48 (NGC 2548) – T2 wf Constellation: Hydra Distance 2000 LY Size 30 LY Age 300 million years Caroline observed this cluster with her small telescope in March 1783. .
Satellite Galaxy M110 (NGC 205) – T1 hm Constellation: Andromeda Distance 2.2 million LY Size 10,000 LY Caroline discovered this small elliptical galaxy, a satellite of the Andromeda Galaxy M31, in August 1783. It had been recorded by Messier in his drawing of M31 in 1773, but he did not recognize it as a separate galaxy .
Satellite Galaxy M110 (NGC 205) – T1 wf Constellation: Andromeda Distance 2.2 million LY Size 10,000 LY In this wide-field picture, M110 is visible at the center; M31 and its other major satellite M32 are visible at its left. .
Silver Coin Galaxy (NGC 253) – T1 hm Constellation: Sculptor Distance 10 million LY Size 72,000 LY This barred spiral galaxy was discovered by Caroline Herschel in September 1783 (quite a feat because it never gets higher than 12 degrees above the horizon in England).
Silver Coin Galaxy (NGC 253) – T2 wf Constellation: Sculptor Distance 10 million LY Size 72,000 LY This barred spiral galaxy was discovered by Caroline Herschel in September 1783.
Silver Coin Galaxy (NGC 253) – T1 wf Constellation: Sculptor Distance 10 million LY Size 72,000 LY NGC 253 is at the center of this wide-field picture. The fuzzy object at the bottom left is globular cluster NGC 288, discovered by William Herschel in 1785 NGC 288 is about 30,000 LY away and about 100 LY in diameter.
Golf Ball Cluster (NGC 752) – T2 wf Constellation: Andromeda Distance 1000 LY Size 15 LY Age 1 billion years This open cluster was discovered by Caroline Herschel in September 1783. It had been observed earlier by Giovanni Hodierna in 1654, but he never published his notes, and they were discovered only recently.
Golf Ball Cluster (NGC 752) – T1 wf Constellation: Andromeda Distance 1000 LY Size 15 LY Age 1 billion years This open cluster was discovered by Caroline Herschel in September 1783.
Open Cluster NGC 7789 – T1 hm Constellation: Cassiopeia Distance 8000 LY Size 30 LY Age 2 billion years This open cluster was discovered by Caroline Herschel in October 1783.
Open Cluster NGC 7789 – T1 wf Constellation: Cassiopeia Distance 8000 LY Size 30 LY Age 2 billion years This open cluster was discovered by Caroline Herschel in October 1783.
Open Cluster NGC 659 – T1 hm Constellation: Cassiopeia Distance 6000 LY Size 10 LY Age 40 million years This open cluster was discovered by Caroline Herschel in September 1783. It has at least 7 variable stars.
Open Cluster NGC 659 – T1 wf Constellation: Cassiopeia Distance 6000 LY Size 10 LY Age 40 million years NGC 659 can be seen faintly at the center of this wide-field view. Open cluster M103 (distance 9000 LY) is at the right, NGC 663 (6000 LY) is above center, and NGC 654 (7000 LY) is near top center. NGC 663 and 654 were discovered by William Herschel in 1787.
Open Cluster NGC 7380 – T1 hm Constellation: Cepheus Distance 7000 LY Size 25 LY Age 10 million years This open cluster was discovered in 1787 by Caroline Herschel. It is embedded in the faint emissions nebula Sh 2-142 (size about 60 LY), which is the remains of the nebula that formed the cluster. The nebula can be seen faintly in this picture.
Open Cluster NGC 7380 – T2 wf Constellation: Cepheus Distance 7000 LY Size 25 LY Age 10 million years This open cluster was discovered in 1787 by Caroline Herschel. The surrounding emissions nebula can be seen in this wide-field view.
Open Cluster NGC 7380 – T1 wf Constellation: Cepheus Distance 7000 LY Size 25 LY Age 10 million years The cluster and surrounding emissions nebula can be seen at the center of this wide-field view. The bright star at the right is variable star delta Cephei – prototype of a category of variable stars that have been used as “standard candles” to measure distances to galaxies.
Explorers of the Universe – A Slooh Production by Kochava Yerushalmit