Nebula is a Latin word meaning “cloud”, but in an astronomical context it refers to any celestial object that appears cloud-like when viewed through a telescope.
Since telescopes were not as powerful as they are today, this term included galaxies such as our neighbor Andromeda, often referred to as the “Andromeda Nebula”.
But with the advantage of modern telescopes, we know that galaxies are not cloud-like at all, but consist of billions of stars. This means that astronomers now reserve the word nebula for real clouds – of gas and dust – that lie inside our own galaxy.
Related: What is the Eagle Nebula?
Where is the fog?
Fogs are often found in the space between stars, known as the interstellar medium. On average, this region contains only about one atom per cubic centimeter. In some places, however, the density may be significantly higher than this høy— high enough to be visible through a telescope.
The result is what we call a fog, and they are among the most spectacular sights in astronomy. In fact, many of the most iconic Hubble telescope images, such as the “Pillars of Creation”, are images of fog.
(opens in new tab)
There are several different types of nebulae, depending on how they are formed and their composition. Most mists are mainly made of gas, which is able to light with its own light, creating the colorful screens we are familiar with.
But other nebulae som— such as the so-called “dark nebula” — are much dustier in their composition, and instead of glowing, this dust has the effect of blocking light from more distant objects outside it.
What are the different types of fogs?
Picture 1 of 5
The relationship between fog and stars
Nebula plays a key role in the life cycle of stars, both at birth and death. Stars are born in dense clumps of gas, dust and other material inside diffuse emission nebulae, also often referred to as “star nurseries”.
Hubble’s Pillars of Creation is in this category, as is the famous one The Orion Nebula Som— as you may have seen yourself through binoculars or a small telescope.
The main force working here is gravitywhich causes the flimsy interstellar medium to condense into a nebula, and the force of gravity which causes lumps inside the nebula to collapse down into stars.
(opens in new tab)
At the other end of a star’s life, we encounter another type of emission nebula. Stars like sun end life as very compact white dwarfs, but when they shrink down in this phase, they release clouds of gas that form a so-called “planetary nebula”. This is a rather misleading name, because such nebulae have nothing to do with planets.
In contrast to diffuse emission nebulae, these have a more clearly defined appearance, usually circular in shape, which reminded William Herschel of a planet when he first observed them in the 1780s.
Not all stars end their days in the relative stillness of a planetary nebula. A star that is much more massive than the sun will eventually explode like a supernova, and debris thrown from that explosion forms another type of nebula called a supernova remnant. The most famous of these is the Crab Nebula, which is all that is left of a spectacular supernova that was observed by Chinese astronomers in 1054.
Seeing fogs in vibrant colors
(opens in new tab)
To capture the spectacular nature of nebulae, telescopes – such as the Hubble Space Telescope and the James Webb Space Telescope (JWST) – use infrared radiation emitted by nebulae to create an image.
The visible light emitted by stars formed in and around a nebula can be blocked by the dense cosmic clouds of gas and dust that make up a nebula. Therefore, researchers need to look at other wavelengths of light emitted from the nebula, such as infrared radiation.
Infrared cameras on board the JWST have facilitated some of the most detailed images of nebulae, such as the southern ring. The southern nebula – also known as NGC 3132 – is about 2,500 light-years away from Earth and home to a dying star in its core.
Using its near-infrared camera (NIRCam) and the mid-infrared instrument (MIRI), JWST has taken one of the most detailed images of the nebula. In the two images above, JWST has trapped layers of gas and dust in the southern ring nebula. Each layer is created by the expulsion of cosmic matter from the central dying star.
For more information on fog, check out “The Hubble Legacy: 30 Years of Discoveries and Image (opens in new tab)“, by Jim Bell and” Planetary Nebulae and How to Observe Them “(Astronomers’ Observation Guides) (opens in new tab)“, by Martin Griffiths.
- Planetene, “Nebula Facts – A Guide To Nebulae (opens in new tab)“, opened July 2022.
- The universe today, “Fog: What are they and where do they come from? (opens in new tab)“, opened July 2022.
- University of Utah, “Nebula (opens in new tab)“, opened July 2022.
- Australia Telescope National Facility, “Star Formation (opens in new tab)“, July 2022.
- Cosmos, “The Emission Mist (opens in new tab)“, opened July 2022.
- James Miller, “What are nebulae and how are they formed? (opens in new tab)“, Astronomy Trek, December 2014.
- Andy Briggs, “What a Supernova (opens in new tab)“, EarthSky, November 2020.