image-of-the-bean-nebula

The colour out of space. Image credit: NASA, ESA and Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain)

This fluffy pink blob is the star-forming region LHA-120-N 11 in the Large Magellanic Cloud some 170 000 light years from the Sun. N 11 (occasionally called the Bean Nebula) is about 1000 light years across. Take a moment to think just big that is!

What is the source of the delicate pink hue we see here and in so many other nebulae? Hydrogen is the main element making up such clouds. A typical hydrogen atom consists of a nucleus orbited by a much smaller electron just like a tiny solar system (this should not be taken too literally, as an accurate model it is about a century out of date, but it is still a useful visualisation of the situation). There is a certain amount of energy associated with the electron’s orbit and there are a series of possible orbits the electron can be in around the nucleus. Each has its own energy level, and note the electron must be in one of these levels or orbits, no intermediate positions are allowed (this is about as deep as I wish to tread in quantum mechanics). Sometimes an electron will change energy level. It jumps straight from one orbit to another, without moving through the intervening distance. Imagine Venus suddenly disappearing from its solar orbit and instantly reappearing in Mercury’s orbit and you will get the picture. Now too, you will realise where the often misused phrase “quantum leap” came from.

To balance the books, when the electron changes from a high energy level to a low energy level, the difference in energy must be removed. The atom gets rid of it by ejecting a photon, a particle of light, of exactly the right energy. For the most common such leap, the light of this energy has is a characteristic red colour of light. Emitted by hydrogen atoms when their electrons shift energy levels it is known as Hydrogen Alpha and is well-known to astronomers.

This soft pink glow is the by product of squillions and bazillions of electrons jumping around their atoms like hyperactive toddlers. Imagine that!


3 Comments

Susan · September 1, 2016 at 05:55

Would the absorption of wavelengths also contribute to its colour? Similar to why a sunset is pink and the sky is blue?

    admin · September 1, 2016 at 08:35

    Dear Susan, thank you for your question. Many nebulae like the one in the picture are classed as “emission nebulae” as the vast majority of the light we see from them is actually being emitted by the atoms and ions in the gas. One thing I didn’t say in the article was that this emission is triggered by the fierce radiation from young stars in the nebula, and I will add this fact.

    You’re right to say that there will be some absorption of some wavelengths by the nebulae but this isn’t a big factor in determining its colour.

    I hope this helped you.

Wonders of the August Night Sky | Astronotes · February 6, 2014 at 09:52

[…] So in this case, perseverance brings its own reward as this faintly ‘cloudy’ background, running almost vertically up and to the left of our archer is a positive treasure chest of cosmic wonders for you to feast your eyes upon! That said we need only namedrop a few of them as a bit of independent ‘space-hopping’ with a small telescope or pair of binoculars will reveal plenty besides. One absolute gem that should not be overlooked is Messier 8, or M8 for short, also known by the name the Lagoon Nebula. To find a deep space object 4100 light years away from Earth yet bright enough to be seen with the naked eye is a very rare thing. In fact, it makes up exactly half of an exclusive group of nebulae that can be detected without binoculars or a telescope from mid-northern latitudes. Moreover with an apparent magnitude of 6.0, on a clear night if your eyes are focused on the top of the hunter’s bow, you literally cannot manage to overlook the oval-shaped glow a little farther over on the right hand side. That said, with human night vision abilities being limited, the rods and cones in your eyes will only just be able to register the presence of M8 and so paint it in shades of grey. On the other hand a slow-exposure camera with some zoom capabilities will more honestly capture the Lagoon Nebula in its pinkish colours. […]

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