The Canvas of Interstellar Space

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Nebulae are one of the most beautiful wonders in the sky, a blend of colours and hues in a painting drawn upon the canvas of interstellar space. As Sherlock Holmes famously once said, ‘You see, but you do not observe.’ Today I will teach you how to not just see nebulae, but how to observe them. There will be many pretty pictures to follow, many of which will be familiar to avid stargazers.

The word nebula is the Latin word for cloud or fog, and rightly so, nebulae are clouds of gas and dust in interstellar space. Scattered within nebulae are many stars, most of which appear to be blue when observed from Earth. This is because blue stars are hotter than other colours of stars and are therefore brighter; this also explains why the spirals of a galaxy are often traced out by blue stars. However, blue stars burn out their fuel more rapidly in order to maintain their brightness, so they tend to be very young. Massive blue stars typically have a lifespan of several millions of years, whereas an orange or red Sun-like star can live for many billions of years.

Pinwheel Galaxy
Blue stars trace out the spirals arms of the Pinwheel Galaxy, located ~21 million light-years away in the constellation of Ursa Major. Image: Hubble Space Telescope, Subaru Telescope, Robert Gendler

Interstellar space is sadly a rather empty place – it is sparsely filled by material collectively called the interstellar medium. The lion’s share of the interstellar medium exists as cold, neutral hydrogen gas, with the remaining proportion existing as helium and other heavier elements ejected by the collapse of stellar cores. This cold gas is easily excited (temporarily increases in energy) when in close proximity to a hot star; the excited hydrogen gas gives nebulae a distinctive red and pink colour.

Rosette Nebula
The excited gas in the Rosette Nebula, located ~5200 light-years away in the constellation of Monoceros, glows pink. Also note the major proportion of bright stars which are blue. Image: Robert Gendler

The third component of nebulae are the dust clouds. Cosmic dust commonly consists of carbonates and silicates, and can entirely block out light from stars despite being as fine as cigarette smoke. This dust is embedded into gas clouds which is why there are patches of darkness within images of nebulae and even galaxies.

Pacman Nebula
The distinct shape of the appropriately named Pacman Nebula, located ~9500 light-years away in the constellation of Cassiopeia, is due to the presence of opaque dust clouds. Image: Rod Pommier

Dust clouds are one of the coldest regions in interstellar space – temperatures can reach as low as a few Kelvin (a few degrees Celsius above absolute zero). The Boomerang Nebula, located ~5000 light-years away in the constellation of Centaurus, is considered to be the coldest place in the universe, measuring at 1 K. Even the average temperature of space is only about 2.7 K due to cosmic microwave background radiation.

The borders of a dust cloud can provide more information about the nebula and the direction of incident radiation. There tends to be a clean-cut border in regions where the dust cloud is warmer and denser, and incoming radiation is seeking to excite the dust. In ‘fluffy’ regions where the border is more gradual, the dust clouds tends to be colder, less dense, and shielded from radiation. Light eats into dust clouds, erodes it, and sculpts it.

Horsehead Nebula
The Horsehead Nebula, located ~1500 light-years away in the constellation of Orion, is a dark nebula (a dust cloud which blocks out light from behind it like a silhouette). Note the border of the cloud is sometimes sharp and sometimes gradual. Image: Ken Crawford

Multiple generations of stars can be found within one nebula. The oldest generations tend to be located in the centre and the newest generations towards the edge. As a result the stars can erode the nebula from the inside out, which is why dark regions are often seen in the centre of nebulae.

But wait! How do we explain the nebulae which aren’t red? Blue nebulae are also a common sight due to the scattering of direct light from stars by dust clouds. The amount of scattering depends on the density of the dust cloud and on the particle size of the dust. This phenomenon, called Rayleigh scattering, is the same reason why the sky blue, because air particles in the atmosphere scatter direct light from the Sun.

Pleiades
The Pleiades, or the Seven Sisters, is a star cluster located ~440 light-years away in the constellation of Taurus. The Pleiades appear blue not only because the stars are blue, but because the blueness is enhanced by scattering. Image: Jim Kaler

Additional colours can arise from different compositions of elements within the nebula. In other cases, the photographs were taken not using optical telescopes but infrared or ultraviolet telescopes for example. The original images would not be coloured and consequently colours would have been synthetically added to represent the degree of intensity of light.

We can now analyse the minor features of nebulae. Pillars exist due to the ‘erosion’ of less dense dust around denser dust by radiation. This leaves a cloud of denser dust, and also less dense dust behind it which has been shielded. Examining pillars is extremely helpful as they point towards the source of the greatest intensity of light (usually stars).

Pillars of Creation
The renowned Pillars of Creation, located ~7000 light-years away within the Eagle Nebula in the constellation of Serpens. Note the clouds of dense dust at the peaks of the pillars.

When the pillars are eroded to such an extent that they become detached from the original dust cloud, they become Bok globules which often take on the appearance of tadpoles. Globules go on to form new generations of stars. Large globules are rarely seen as cosmic wind tends to blow away material in these globules.

Caterpillar Globule
The Caterpillar Globule, located within the Carina Nebula (~6500-10000 light-years away) in the constellation of Carina.

Planetary nebulae are another type of nebula which are misleadingly not actually related to planets. They often appear round in shape and blue or green in colour, giving in them the appearance of planets in the absence of high resolution telescopes. Planetary nebulae are ejected by red giant stars (a late stage in the lifecycle of a star) as they become white dwarves.

Ring Nebula.JPG
The Ring Nebula, located ~2300 light-years away in the constellation of Lyra. Note how it could be easily mistaken for a planet without a high resolution telescope.
Cat's Eye Nebula
An X-ray/optical composite photograph of the Cat’s Eye Nebula, located ~3300 light-years away in the constellation of Draco. Note the synthetically added colours.
Crab Nebula.jpg
The Crab Nebula, located ~6500 light-years away in the constellation of Taurus. It is the remnant of a supernova which was observed by Chinese astronomers in 1054.

I highly recommend perusing the Hubble Heritage Gallery of Images and the Spitzer Space Telescope Images for more stunning images, and the Astronomy Picture of the Day for something to brighten up your morning!

Yanhao

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