Nebula | Research & Encyclopedia Articles

Nebula

In deep space, there are numerous glowing patches of light which astronomers called nebulae, the Latin word for clouds. Many of these objects are just that: glowing clouds of gas. Many others are not, but with only the naked eye to guide them, early astronomers could not distinguish any difference, so every glowing patch of light was called a nebula.

One of the earliest-discovered nebulae was located in the constellation Andromeda. It had been known as far back as 905 a.d. and was called the "Little Cloud." The first telescopic observation is credited to Simon Mayr (1573-1624) in 1611 or 1612. Later Lord Rosse (1800-1867) used his huge telescope to observe many nebula, many of which showed an odd spiral structure.

It was not until Edwin Powell Hubble analyzed Cepheid variable stars in 1923 in the Andromeda nebula that its true nature was revealed; it was a gigantic spiral galaxy hundreds of thousands of light-years away. The numerous other "spiral nebulae" were soon identified as galaxies.

Beginning in the 1860s, William Huggins had used spectroscopy to analyze the light of many nebulae, and he proved many of them were truly gaseous objects located in our own Galaxy. These true nebulae take many different forms.

The so-called planetary nebulae are produced by stars of comparable mass to the Sun as they evolve past the red giant stage of stellar evolution. Such stars eject their outer atmospheres when they run out of fuel in their cores, and the "ring" is just this shell of expanding hydrogen gas. The expanding cloud was dubbed a planetary nebula by early observers because it looks like the glowing disc of a planet through a telescope. One of the most famous planetary nebulae is the Ring nebula in the constellation Lyra. It was discovered by French astronomer Antoine Darquier in 1779. Charles Messier (1730-1817) found it later that same year and added it to his famous list.

An emission nebula is produced when the atoms in gas near a hot star are ionized by the star's energetic ultraviolet radiation. When the free electrons recombine with ionized atoms, light is emitted. A classic example of this is the Orion Nebula, discovered by Nicholas Peiresc (1580-1637) in 1611. Christiaan Huygens gave it widespread attention in 1656, and William Herschel began his observing career with it. This also was the first nebula to be successfully photographed, by Henry Draper (1837-1882), in 1880. The Orion nebula is a stellar nursery; the cloud, rich in hydrogen gas, is collapsing together to form new stars.

A reflection nebula occurs where light from a star is reflected from dust in the area surrounding the star. This is often the case for young stars, where dust from the remnant cloud that formed them is still present. The nebulae near the stars in the Pleiades cluster in Taurus are excellent examples. They were discovered by W. Temple in 1859 and first photographed by Paul Henry (1848-1905) and Prosper Henry (1849-1903) in 1885. Vesto Melvin Slipher discovered the spectrum of the nebulosity was identical to that of the nearby stars in 1912.

The Crab nebula, also in Taurus, is the remnant of a supernova explosion. The supernova itself was seen on July 4, 1054, by Native Indian and Chinese observers. A Chinese text called the nova a "guest star." An English amateur astronomer, John Bevis, is credited with the discovery of the nebula in 1731. Later, in 1758, it was found independently by Messier, who made it item number one on his list (M1). The discovery of the expansion of the nebula was made by C. O. Lampland in 1921, which led to the theory of its forming from the supernova of 1054.

In 1967 Jocelyn Bell and Antony Hewish discovered pulsars. The following year a pulsar ( neutron star) was detected at the core of the Crab, substantiating the supernova theory.

Immanuel Kant (1724-1804) and Pierre Laplace independently considered a nebula as the source of the solar system. The "nebular hypothesis" suggested the planets had coalesced from condensing gasses that had been left behind as the Sun contracted.

In addition to the visible nebulae, there are dark nebulae throughout the interstellar medium. Composed mostly of dust, a dark nebula is identified by the starlight which it blocks, thereby creating a dark patch in the sky. A dark rift in the stars of the Milky Way galaxy in the constellation Cygnus is caused by distant clouds of dust. The "Coal Sack", in the southern hemisphere constellation Crux, was noticed by Portuguese sailors in the sixteenth century. While dark nebula can not be directly observed by visual means, they glow brightly in infrared wavelengths.