(Today’s article is an excerpt of the book The Armchair Astronomer, Volume 1. It’s available from Cosmic Pursuits in multiple formats at this link).
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When 18th-century composer William Herschel grew restless with his life as a professional musician, he turned to the new science of astronomy for inspiration and challenge. Like most new stargazers, Herschel began by reading popular works of astronomy, learning the names of the stars and constellations, and inspecting the heavens with telescopes made with small glass lenses that collected the feeble light from distant stars .
Herschel, frustrated by the poor view afforded by even the finest of these small telescopes, learned to build his own instruments. Based on a design by Isaac Newton, Herschel’s telescopes collected starlight using large mirrors that were handmade from polished metal disks. He eventually made hundreds of telescopes, some of which were sold to the kings and upper class of Europe. In his most ambitious attempt, he tried to make a three-foot-wide metal mirror using a cast of hardened horse dung. The cast leaked hot molten metal onto the floor of his workshop, causing flagstones and dung to explode and ricochet off the walls and ceiling.
But Herschel’s passion and effort paid off. He became a supremely patient and skilled observer, and his detailed knowledge of the stars enabled him to discover the planet Uranus. This achievement gained him fame and freedom when he was awarded a fellowship in the Royal Society and a rich stipend from King George III.
With the help of his sister Caroline, Herschel discovered thousands of objects beyond our solar system that no one had seen before. He found many of the faint and diffuse objects he saw with his powerful telescopes resolved into stars. But he also found that some objects remained cloudy and unresolved. These objects that defied resolution were called nebulae— Latin for “clouds”.
Herschel was fascinated by nebulae. He speculated that some, such as the famous Andromeda Nebula, would resolve into individual stars when seen through more powerful telescopes. But he also speculated that some nebulous objects like the Orion Nebula were not made of stars, but of a mysterious “shining fluid”. He believed the Orion Nebula was the “chaotic material of future suns”.
Amazingly, Herschel’s speculations were correct. The big telescopes of the early 20th century resolved the Andromeda Nebula into individual stars, and we now know Andromeda is an immense galaxy like our own Milky Way. And in the late 19th century, the amateur astronomer William Huggins attached a spectroscope to his telescope and found that the Orion Nebula and other nebulae were made not of a mysterious heavenly fluid, but of radiant clouds of hydrogen, helium, and oxygen gases, the same type of gases found on Earth.
The mechanism by which the gas in a nebula is set aglow is what distinguishes one type from another. When vast clouds of hydrogen and helium gas in the arms of our Milky Way galaxy collapse and form pockets of bright new stars, the light from these stars energizes the atoms in the clouds of remaining gas. When the atoms relax again to a state of lower energy, they settle accounts by emitting light at characteristic colors of red, green, and blue. We see this light as a colorful emission nebula, one of the most visually appealing sights in the heavens. The Orion Nebula, which is just barely visible to the unaided eye as a fuzzy patch in the constellation Orion, is the most famous and nearest example of a region where new stars are being born. So is the patch of nebulosity known as the Heart Nebula seen at the top of this page.
Many clouds of newborn stars also contain fine grains of interstellar dust akin to soot from diesel exhaust. The dust reflects the blue-white light from new stars, and we see this light as a reflection nebula. In the foreground of some emission and reflection nebulae, we can see the dark, inky fingers of cold gas and dust that block out background light. These are dark nebulae. As you will see in the images in this book, many regions of newly formed stars contain all three types of nebulae.
Dying stars also produce a characteristic type of nebula. As they run out of fuel, small and mid-sized stars with a mass similar to our Sun eject the outer layers of their atmospheres into a thin expanding envelope. The hot core of the star excites the rarified gas to emit light, and the resulting luminescent shell is called a planetary nebula, a name coined by Herschel because the round and pale disks of light resemble the small disk of a planet seen in a telescope. Planetary nebulae are short-lived, so there are comparatively few in the night sky, but their shape and structure can be enormously complex and beautiful. Astronomers can gain knowledge of the end states of Sun-like stars by studying planetary nebulae. This knowledge helps them understand how our own Sun will die in several billion years.
When larger stars run out of fuel, they do not go gently. They explode in enormously violent events called supernovae during which a dying star produces as much energy in a few weeks as our Sun will produce in its entire 10-billion-year lifetime. The expanding gas from the supernova produces a shock wave as it slams into the cold gas in the space between the stars of the Milky Way. This sets the interstellar gas aglow and produces a shell-like nebula called a supernova remnant. The Veil Nebula in the constellation Cygnus is an example of this type of object.
In this book you will tour examples of some of the brightest of these types of nebulae visible in the night sky. Unless otherwise noted, the images of nebulae in this book were made with small telescopes that are the modern versions of those used by Herschel, along with specialized electronic cameras and processing techniques. All these tools are within the reach of the dedicated backyard astronomer, and they now enable the creation of images superior to those made by professional astronomers with much larger telescopes less than a generation ago.
The color and detail in these images are not accessible visually to stargazers, even with large telescopes. The human eye is not sensitive enough. But these images are a testament to the intrinsic beauty and complexity of all types of nebulae and to the skill and dedication of the astrophotographer.
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