Stardust and Eternity – 3.4.3

Nebulae: Orion Nebula, Emission Nebulae, Reflection Nebulae

Nebulae (from the Latin nebula, meaning “fog”, “cloud”) are enormous dark or bright clouds composed of gas – mostly hydrogen and helium – and dust – microscopic particles made of various elements, such as silicates, and also complex organic compounds – that occupy the “interstellar” space between the stars of a galaxy. Some of these diffuse celestial objects are easily visible to the naked eye as distinct but foggy patches, while others are only visible through telescopes. Nebulae play an important role as building blocks for stars and planets, so that they are sometimes referred to as “stellar nurseries”.

Diffuse nebulae can be mainly classified as “dark” or “luminous”.

“Dark nebulae” – also referred to as “absorption nebulae” – do not emit or reflect light. They consist of clumpy clouds of gas and dust so dense that they are able to block the visible light of the stars lying behind them from our point of view. Light extinction is mainly caused by sub-micrometer-sized dust grains, which are comparable to the wavelength of visible light. Information on their physical structure can, however, be obtained through observations in the radio and infrared part of the electromagnetic spectrum which, unlike the visible radiation, is not absorbed due to their larger wavelengths. Since the temperature inside dark nebulae is sufficiently low (much below zero) hydrogen molecules (H2) and other different types of molecules can form, such as water (H2O), ammonia (NH4) and even sugars and amino acids. The shape of dark nebulae is usually irregular, with no clearly defined outer boundaries. The largest ones – which contain millions of solar masses and extend for a few hundred light years – are called “giant molecular clouds”; they are also suitable locations for the formation of new stars. Dark nebulae are visible to the naked eye as dark patches against the bright, diffuse background of the Milky Way. The Horsehead Nebula in the constellation of Orion, whose shape is similar to a horse’s head, is one of the most famous dark nebulae.

“Luminous nebulae” are divided into two types: “emission” nebulae or “reflection” nebulae. “Emission nebulae” are not dark because they emit light at visible wavelengths. These interstellar clouds are mainly formed by gases that have been ionized – some of the electrons have been stripped away from their atoms – by the high-energy ultraviolet radiation emitted by nearby massive hot O or B stars born within the cloud itself. The most common types of emission nebulae are the so-called “H II regions”, roughly glowing spherical clouds that mostly consist of ionized hydrogen atoms surrounding a central hot star. These nebulae often look red; this is due to the fact that the strongest visible emission line given off in the ionization process of hydrogen atoms is a line in the red part of the electromagnetic spectrum. The Orion Nebula (M42), one the most famous and brightest emission nebulae in the night sky, is also visible to the naked eye and contains a young open cluster of four stars called the Trapezium. Planetary nebulae also include emission nebulae that consist of the expanding outer layers ejected by a dying “red giant” star, made luminous by the ultraviolet radiation emitted by the exposed hot and luminous stellar core. The Ring Nebula (M57) in the constellation Lyra is a famous example of planetary nebula.

The second type of bright nebulae are the “reflection nebulae”. They consist of clouds of interstellar gas and dust that glow due to scattering or reflection of light from nearby stars, not hot enough to ionise the gas – without which they would appear as dark as absorption nebulae. The spectrum of light scattered by the cloud is the same as that of the illuminating stars, although blue light is scattered more efficiently than red light (it is the same scattering process that generates blue skies and red sunsets), so these nebulae often appear blue when they are filmed. A famous reflection nebula is that of the Pleiades (M45) or “Seven Sisters” in the constellation Taurus, clearly visible to the naked eye in a dark site.

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The Orion Nebula – Image Credit: *NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team*

The Carina Nebula – Image Credit: *NASA, ESA, M. Livio and the Hubble 20th Anniversary Team (STScI)*

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Images

The Orion Nebula (NASA, ESA, M. Robberto)