A nebula the size of Earth would weigh only a few kilograms
Nebulae look like solid glowing clouds in photographs, yet most are thinner than the best vacuum any laboratory on Earth can make. A patch of nebula as big as our planet would hold just a few kilograms of gas and dust. Spread over hundreds of light-years, that wisp is enough to build stars.
The word is Latin for cloud or fog, and for centuries it covered any fuzzy smudge in the sky. Ptolemy listed five nebulous-looking stars around 150 AD, and in 964 the Persian astronomer al-Sufi described a little cloud where the Andromeda Galaxy sits. The Orion Nebula, the brightest of all and twice the Moon's apparent width, is visible without a telescope, yet early astronomers overlooked it; Peiresc found it with a telescope in 1610, and Huygens, studying it in detail in 1659, thought he was the first.
Catalogues grew fast. Halley listed six in 1715, Charles Messier compiled 103, and William and Caroline Herschel published three catalogues between 1786 and 1802 totalling some 2,500 objects. William long assumed they were all star clusters too distant to resolve, until in 1790 he found a star wrapped in genuine haze. From 1864 William Huggins split the light of about 70 nebulae and found roughly a third glowed like hot gas. In the 1920s it became clear that many so-called nebulae, Andromeda included, were entire galaxies far beyond our own.
Today the term is narrower. Emission nebulae shine because hot young stars ionise their hydrogen; reflection nebulae merely bounce starlight; dark nebulae show up only as black shapes against brighter background. All glow strongly in infrared from their dust. Many are stellar nurseries, where a giant cloud collapses under its own gravity and new massive stars light up the leftover gas, as in the Eagle Nebula's Pillars of Creation.
Others are made by dying stars. A star like the Sun will eventually shed its outer layers as a planetary nebula, lit by the white dwarf left behind. Heavier stars end in supernovae: the Crab Nebula in Taurus is debris from an explosion Chinese and Arabic observers recorded in 1054, with a neutron star at its heart.
Source: Nebula
