Planetary nebula

A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding glowing shell of ionized gas ejected from old red giant stars late in their lives. The word "nebula" is Latin for mist or cloud and the term "planetary nebula" is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects appeared to him to resemble the rounded shapes of planets. Herschel's name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds. Eventually, after most of the red giant's atmosphere is dissipated, the exposed hot, luminous core emits ultraviolet radiation to ionize the ejected outer layers of the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, appearing as a bright coloured planetary nebula at several discrete visible wavelengths.

Nebula

A nebula (Latin for "cloud"; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others.

Most nebulae are of vast size, even hundreds of light years in diameter. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created in an Earthen environment – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets and other planetary system objects.

Ring Nebula

The Ring Nebula (also catalogued as Messier 57, M57 or NGC 6720) is a planetary nebula in the northern constellation of Lyra. Such objects are formed when a shell of ionized gas is expelled into the surrounding interstellar medium by a red giant star, which was passing through the last stage in its evolution before becoming a white dwarf.

History

This nebula was discovered by the French astronomer Antoine Darquier de Pellepoix in January 1779, who reported that it was "...as large as Jupiter and resembles a planet which is fading". Later the same month, fellow French astronomer Charles Messier independently found the same nebula while searching for comets. It was then entered into his catalogue as the 57th object. Messier and German-born astronomer William Herschel speculated that the nebula was formed by multiple faint stars that were unresolvable with his telescope.

In 1800, German Count Friedrich von Hahn announced that he had discovered the faint central star at the heart of the nebula a few years earlier. He also noted that the interior of the ring had undergone changes, and said he could no longer find the central star. In 1864, English amateur astronomer William Huggins examined the spectra of multiple nebulae, discovering that some of these objects, including M57, displayed the spectra of bright emission lines characteristic of fluorescing glowing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but were nebulosities. The nebula was first photographed by the Hungarian astronomer Eugene von Gothard in 1886.

Space observatory

A space observatory is any instrument (such as a telescope) in outer space that is used for observation of distant planets, galaxies and other outer space objects. The first such space observatory was the Hubble Space Telescope launched in 1990. Space observatories avoid many of the problems of ground observatories, such as light pollution and the filtering and distortion of electromagnetic radiation (scintillation).

This category is distinct from other observatories located in space that are pointed toward Earth for the purpose of reconnaissance and other types of information gathering.

History

In 1946, American theoretical astrophysicist Lyman Spitzer was the first to conceive the idea of a telescope in outer space, a decade before the Soviet Union launched the first satellite, Sputnik 1.

Spitzer's proposal called for a large telescope that would not be hindered by Earth's atmosphere. After lobbying in the 1960s and 70s for such a system to be built, Spitzer's vision ultimately materialized into the Hubble Space Telescope, which was launched on April 24, 1990 by the Space Shuttle Discovery (STS-31).

Hubble Space Telescope

The Hubble Space Telescope (HST) is a space telescope that was launched into low Earth orbit in 1990, and remains in operation. With a 2.4-meter (7.9 ft) mirror, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared spectra. The telescope is named after the astronomer Edwin Hubble.

Hubble's orbit outside the distortion of Earth's atmosphere allows it to take extremely high-resolution images with negligible background light. Hubble has recorded some of the most detailed visible-light images ever, allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as accurately determining the rate of expansion of the universe.

Although not the first space telescope, Hubble is one of the largest and most versatile, and is well known as both a vital research tool and a public relations boon for astronomy. The HST was built by the United States space agency NASA, with contributions from the European Space Agency, and is operated by the Space Telescope Science Institute. The HST is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.