A nebula is a truly wondrous thing to behold. Named after the Latin word for “cloud”, nebulae are not only massive clouds of dust, hydrogen and helium gas, and plasma; they are also often “stellar nurseries” – i.e. the place where stars are born. And for centuries, distant galaxies were often mistaken for these massive clouds.
Alas, such descriptions barely scratch the surface of what nebulae are and what there significance is. Between their formation process, their role in stellar and planetary formation, and their diversity, nebulae have provided humanity with endless intrigue and discovery.
For some time now, scientists and astronomers have been aware that outer space is not really a total vacuum. In fact, it is made up of gas and dust particles known collectively as the Interstellar Medium (ISM). Approximately 99% of the ISM is composed of gas, while about 75% of its mass takes the form of hydrogen and the remaining 25% as helium.
The interstellar gas consists partly of neutral atoms and molecules, as well as charged particles (aka. plasma), such as ions and electrons. This gas is extremely dilute, with an average density of about 1 atom per cubic centimeter. In contrast, Earth’s atmosphere has a density of approximately 30 quintillion molecules per cubic centimeter (3.0 x 1019 per cm³) at sea level.
Even though the interstellar gas is very dispersed, the amount of matter adds up over the vast distances between the stars. And eventually, and with enough gravitational attraction between clouds, this matter can coalesce and collapse to forms stars and planetary systems.
Nebula Formation:
In essence, a nebula is formed when portions of the interstellar medium undergo gravitational collapse. Mutual gravitational attraction causes matter to clump together, forming regions of greater and greater density. From this, stars may form in the center of the collapsing material, who’s ultraviolet ionizing radiation causes the surrounding gas to become visible at optical wavelengths.
Most nebulae are vast in size, measuring up to 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. In fact, a nebular cloud that was similar in size to Earth would only so much material that its mass would be only a few kilograms.
Nebula Classification:
Stellar objects that can be called Nebula come in four major classes. Most fall into the category of Diffuse Nebulae, which means they have no well-defined boundaries. These can be subdivided into two further categories based on their behavior with visible light – “Emission Nebulae” and “Reflection Nebulae”.
Emission Nebulae are those that emit spectral line radiation from ionized gas, and are often called HII regions because they are largely composed of ionized hydrogen. In contrast, Reflection Nebulae do not emit significant amounts of visible light, but are still luminous because they reflect the light from nearby stars.
There are also what is known as Dark Nebulae, opaque clouds that do not emit visible radiation and are not illuminated by stars, but block light from luminous objects behind them. Much like Emission and Reflection Nebulae, Dark Nebulae are sources of infrared emissions, chiefly due to the presence of dust within them.
Some nebulae are formed as the result of supernova explosions, and are hence classified as a Supernova Remnant Nebulae. In this case, short-lived stars experience implosion in their cores and blow off their external layers. This explosion leaves behind a “remnant” in the form of a compact object – i.e. a neutron star – and a cloud of gas and dust that is ionized by the energy of the explosion.
Other nebulae may form as Planetary Nebulae, which involves a low-mass star entering the final stage of its life. In this scenario, stars enter their Red Giant phase, slowly losing their outer layers due to helium flashes in their interior. When the star has lost enough material, its temperature increases and the UV radiation it emits ionizes the surrounding material it has thrown off.
This class also contains the subclass known as Protoplanetary Nebulae (PPN), which applies to astronomical objects that are experiencing a short-lived episode in a star’s evolution. This is the rapid phase that takes place between the Late Asymptotic Giant Branch (LAGB) and the following Planetary Nebula (PN) phase.
During the Asymptotic Giant Branch (AGB) phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase, where it is energized by a central star, causing it to emit strong infrared radiation and become a reflection nebula. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which it is hot enough to ionize the surrounding gas.
History of Nebula Observation:
Many nebulous objects were noticed in the night sky by astronomers during Classical Antiquity and the Middle Ages. The first recorded observation took place in 150 CE, when Ptolemy noted the presence of five stars in Almagast that appeared nebulous in his book. He also noted a region of luminosity between the constellations Ursa Major and Leo that was not associated with any observable star.
In his Book of Fixed Stars, written in 964 CE, Persian astronomer Abd al-Rahman al-Sufi made the first observation of an actual nebula. According to al-Sufi’s observations, “a little cloud” was apparent in a portion of the night sky where the Andromeda Galaxy is now known to be located. He also cataloged other nebulous objects, such as the Omicron Velorum and Brocchi’s Cluster.
On July 4th, 1054, the supernova that created the Crab Nebula (SN 1054,) was visible to astronomers on Earth, and recorded observations that were made by both Arabic and Chinese astronomers have been identified. While anecdotal evidence exists that other civilizations viewed the supernova, no records have been uncovered.
By the 17th century, improvements in telescopes led to the first confirmed observations of nebulae. This began in 1610, when French astronomer Nicolas-Claude Fabri de Peiresc made the first recorded observation of the Orion Nebula. In 1618, Swiss astronomer Johann Baptist Cysat also observed the nebula; and by 1659, Christiaan Huygens made the first detailed study of it.
By the 18th century, the number of observed nebulae began to increase and astronomers began to compile lists. In 1715, Edmund Halley published a list of six nebulae – M11, M13, M22, M31, M42, and the Omega Centauri globular cluster (NGC 5139) – in his “An account of several nebulae or lucid spots like clouds, lately discovered among the fixt stars by help of the telescope.”
In 1746, French astronomer Jean-Philippe de Cheseaux compiled a list of 20 nebulae, included eight that were not previously known. Between 1751 and 53, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were previously unknown. And in 1781, Charles Messier compiled his catalog of 103 “nebulae” (now called Messier objects), though some were galaxies and comets.
The number of observed and cataloged nebulae greatly expanded thanks to the efforts of William Herschel and his sister, Caroline. In 1786, the two published their Catalogue of One Thousand New Nebulae and Clusters of Stars, which was followed up in 1786 and 1802 by a second and third catalog. At the time, Herschel believed that these nebulae were merely unresolved clusters of stars, a belief he would amend in 1790 when he observed a true nebula surrounding a distant star.
Beginning in 1864, English astronomer William Huggins began to differentiate nebulae based on their spectra. Roughly one-third of them had the emission spectrum of a gas (i.e. Emission Nebulae) while the rest showed a continuous spectrum, consistent with a mass of stars (i.e. Planetary Nebulae).
In 1912, American astronomer Vesto Slipher added the subcategory of Reflection Nebulae after observing how a nebula surrounding a star matched the spectra of the Pleiades open cluster. By 1922, and as part of the “Great Debate” about the nature of spiral nebulae and the size of the universe, it had become clear that many of the previously observed nebulae were in fact distant spiral galaxies.
In that same year, Edwin Hubble announced that nearly all nebulae are associated with stars and that their illumination comes from star light. Since that time, the number of true nebulae (as opposed to star clusters and distant galaxies) has grown considerably, and their classification has been refined thanks to improvements in observational equipment and spectroscopy.
In short, nebulae are not just the starting points of stellar evolution, but can also be the end point. And between all the star systems that fill our galaxy and our universe, nebulous clouds and masses are sure to be found, just waiting to give birth to the next generation of stars!
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