The Astrophysics of Nebulae Colours

The centre of the Eagle Nebulae probably was one of the very first images of the Hubble Space Telescope I saw. I was starting my Physics degree at the University in Spain then, and I was so inspired by this image that I printed it in colour and put it on my bedroom wall

You’ve probably seen the amazing image of the Eagle Nebula (M 16), and in particular – a section of it that is known as the ‘Pillars of Creation’.

These magnificent finger-like structures were photographed by the Hubble Telescope in 1995 and have since been the target of amateur and professional astronomers and several orbiting spacecraft.

The objects themselves are enormous clouds (about four light-years in length) of molecular hydrogen and dust, that are being eroded and photo-evaporated by ultraviolet light from nearby massive hot stars. Intriguingly, the top of the pillars are regions where new stars and new solar systems are forming.

It is these processes that are taking place through photo-evaporation and ionisation of the different elements cause a range of different colours that we observe in beautiful images like that of Hubble.

"The 1995 Hubble image of M 16 revealed that the pillars have some small bumps, sometimes almost completely detached from them. Astrophysicists named these structures the "EGGs" of the Eagle, being EGG the abbreviation of "evaporating gaseous globules". These EGGs are really small, only 100 AU in size, but they have dense gas and dust that shield its centre of the powerful ultraviolet radiation of nearby massive stars. Astronomers think that these structures are predecessors of protostars. Hubble only newborn stars on a couple of then, but using infrared light ESO's VLT data have shown that at least 15% of EGGs show star-formation activity,” said Ángel. 

To bring out the colour that we see in images like the Pillars of Creation and the Eagle Nebula, astronomers use a number of different filters that focus on particular wavelengths of light, which itself is representative of each element and the processes taking place.

Ángel outlines these processes in his video and in particular references the difference between the nomenclature of describing these elements/processes. For example, when describing the light signature of Oxygen III, there is a key difference in writing it as OIII vs. [OIII] – with the brackets signifying a different astrophysical process.

 "We are talking about features that have a completely different origin. The Physics behind the creation of the [O III] and O III features is different. It is like if you were talking about distances and you give the units in mass," he said.

"Recombination lines in the O++ ion, abbreviated O III, can be observed on Earth, and we see them sometimes in stars as absorption features."

"However, collisionally excited lines of the O++ ion, abbreviated [O III], with the brackets, can only appear under very low densities (10-1000 particles per centimetre cube), as those found in nebulae. They are the consequence of the collisions of free electrons with the O++ ion, that create energy levels that cannot be seen in standard conditions," continued Ángel. 

"[O III] can only be seen as emission. That is the same for all emission lines we observe of metallic elements (not hydrogen or helium) in nebulae: [S II], [O II], [N II], [O I], [S III], [Ne III], [Fe II], [Ar III] (for naming some few of them)."

"[O III] 5007 A is one of the brightest (sometimes, the brightest, as it can overpass H-alpha in some cases) emission line in nebulae and galaxies, and that is why we developed special narrow-band filters isolating that feature for getting images of only the nebular emission."

The video is a great introduction into the intersection between astrophotography and astrophysics – describing how the beautiful images we see are the results of different physical processes in the distant Universe, and how we can share this knowledge through the correct classification of labels used when describing images, filters, and objects. 

"I'm always inspired by the amazing colourful images of nebulae and galaxies that amateur astronomers are obtaining. But sometimes I think they are not able to exploit fully what they see on those outstanding images. That is why I prepared this video (the first in a series combining Astrophysics and amateur Astronomy), to let them and the general public understand that the colours have a physical meaning." said Ángel.

Dr Ángel López-Sánchez is an astronomer and science communicator at the Australian Astronomical Optics and the Department of Physics and Astronomy at Macquarie University (Sydney, Australia). He is a recognised expert in the study of how the gas is converted into stars in nearby galaxies and how this affects galaxy evolution.

He leads the "HI KOALA IFS Dwarf galaxy Survey" (Hi-KIDS) program, that uses the instrument KOALA at the 3.9m Anglo-Australian Telescope (AAT) to obtain integral-field spectroscopy (IFS) data of a sample of 80 gas-rich nearby dwarf galaxies. He also provides support for visiting astronomers to the AAT.

Ángel is also an active member in large spectroscopic galaxy surveys and upcoming optical and radio galaxy surveys. He is a globally-recognised science communicator, with visibility in Spanish and Australian printed, broadcast, and social media (one of the Top-100 most-followed astrophysicists in Twitter). His stunning astronomy timelapse videos have received 1/4 million views on YouTube and have been used by ABC, BBC, TVE (Spain), US TV channels, and in Science Museums worldwide. He is also a passionate amateur astronomer that uses his own equipment for capturing the beauty of the Cosmos.

Ángel also appears as one part of The Skyentists – which are featured on a regular basis on our website. Follow Ángel on Twitter or subscribe to his YouTube channel.

This video is syndicated with permissions.