What Makes up the Interstellar Medium?

Starburst GalaxyThe universe is so vast that even the spaces between Earth and the nearest celestial objects to us is on such a grand scale compared to what we understand on a practical daily level. Anyone could be forgiven for imagining that these great spaces are empty – after all, we do call it the ‘vacuum of space’. This could not be further from the truth. Examine what lies between the stars – the interstellar medium – and you will see a rich fabric of space connected to the stars and, and indeed, all of life.

What is the Interstellar Medium?

The interstellar medium is the matter and radiation which exists between the stars in a galaxy, including dust, gas and cosmic rays. It does not only fill interstellar space, but also blends smoothly into the surrounding intergalactic medium. When we talk about the energy which also occupies this space in the form of electromagnetic radiation, we can refer to the interstellar radiation field.

Hydrogen is by far the most abundant element in the interstellar medium, followed by helium. Other elements that make up the medium include amounts of carbon, oxygen and nitrogen (these are trace amounts when compared to the amount of hydrogen present).

The interstellar medium has different phases to it which depend on the state of the matter (whether it is atomic, ionic or molecular), and which also depends on the temperature and density of the matter.

Visible-Wavelength Observations of the Interstellar Medium


Orion NebulaNebulae are clouds of gas and dust that fill interstellar space. We can see a few nebulae from Earth with just our naked eye, though they look like nothing more than milky, fuzzy patches. If you look south of the belt of one of the most famous constellations, Orion, you can see such a fuzzy region. It is called M2, or the Orion Nebula, and it is one of the brightest nebulae to view from Earth. Nebulae come in types, with some even being star-forming regions. They are vast, extending for many light years across space.

Emission Nebulae

Emmission NebulaAn emission nebula is produced when very hot stars excite the low density gas of the nebula. The nebula is made mainly of hydrogen gas which is ionized by the light of nearby stars. An ion is an atom which has lost or gained one or more electrons, so it no longer has a neutral charge. Ionization is one of the ways in which radiation, like X-rays, transfers energy to matter. The hydrogen atoms in emission nebulae have lost their electron, and astronomers denote these atoms as HII. For this reason, emission nebulae are also called HII regions. The stars exciting an emission nebula must be at a minimum of 25 000 °K to be hot enough for ultraviolet radiation to ionize the cloud.

In an HII region, the ionized nuclei and free electrons are mixed. When one of the nuclei of the ionized atom captures an electron, the electron falls down the atomic energy levels, a process which emits a photon at a specific wavelength. Scientists can study the resulting spectra, and this is how astronomers know that emission nebulae have compositions of mainly hydrogen. Emission nebula have a density of roughly 100 to 1000 atoms per cubic centimetre.

Reflection Nebulae

Reflection NebulaReflection nebulae are produced when starlight scatters off the surrounding dust of a nebula. This is much the same principle as when light scatters off of the particles in our atmosphere and causes the sky to look blue. In fact, emission nebulae appear blue for this very same reason as well. This is because short wavelengths, like blue, scatter more easily than long wavelengths like red. The blue light, having been scattered more efficiently, enters our eyes from all directions and makes the sky (and nebula) appear blue. The blue colour of emission nebulae shows that the dust particles of the cloud must be quite small for the blue photons to be scattered more effectively.

The spectrum of a reflection nebula is, consequentially, the reflected absorption spectrum of the starlight which scatters off the dust. Of course, the nebula has its own gases present, but the heat of the reflected starlight is simply not hot enough to emit photons.

Dark Nebulae

Dark NebulaThe third kind of general nebulae are produced when dense clouds of gas and dust are silhouetted against a backdrop of bright stars and bright nebulae. They are called dark nebulae for this reason. These dense clouds obscure the view of the distant background stars, forming what looks like hole and rifts in the starlit Milky Way.

Dark nebulae are important because they prove that there must be breezes and currents pushing through the interstellar medium. Even though dark nebulae are usually round, they can come in all kinds of twisted and distorted shapes, even though there are no nearby stars to ionize the gas or force the dark nebulae into these kinds of shapes.

Interstellar Dust

Interstellar DustThe dust that exists in the spaces between the stars of a galaxy is called interstellar dust. Its presence is made abundantly obvious by dark nebulae, but it is not always concentrated into large clumps like these. Astronomers estimate that interstellar dust makes up around 1% of the mass of the interstellar medium.

One of the simple but ingenious ways that scientists are sure of the presence of interstellar dust is a phenomenon known as interstellar extinction. The interstellar dust makes the distant stars appear fainter than they would had space been completely transparent. A star which lies 1000 parsecs (3261.564 light years) away from Earth would be 2 magnitudes brighter than it actually appears if space was indeed perfectly empty. For every 1000 parsecs, interstellar dust diminishes the magnitude by 2, so a star 2000 parsecs away would appear 4 magnitudes dimmer due to interstellar extinction.

There is also another way scientists know space is filled with low density gas: interstellar reddening. Interstellar dust does not only effect the magnitude of stars, but their colour too. Based on temperature, an O star should be blue, but many appear redder than they should. The change in colour is due to the interstellar dust particles scattering light.

Interstellar Absorption Lines

The spectra of the distant stars are a great resource for further analysing the interstellar medium. When we study a star’s spectrum, we see spectral lines which are produced by its own gases, but we can also see sharp spectral lines caused by the gases of the interstellar medium. They are called interstellar absorption lines, and reveal a lot about the make-up of the interstellar medium. There are several different methods to this. Astronomers look at the spectral lines’ ionization, width, and components.

To give an example of how one of these methods work – analysing ionization – we can look at the spectrum of a very hot O star. We would not expect to see the spectral lines of CaII (once-ionized calcium), because this ion simply cannot exist in the extreme temperatures of this type of star. However, CaII spectral lines are indeed present, and so this ion must be produced in the interstellar medium and not in the star itself.

HI Clouds

Interstellar medium is filled with low density gas, though in some parts of the medium the clouds of gas and dust do have a slightly higher density, and can be classified into different varieties based on their visual wavelengths.

Some clouds have neutral interstellar absorption lines which show that the gas within the cloud is neutral (it is not ionized). They are called HI clouds for this reason. These regions of gas are slightly denser than other clouds, with a density of ten to a few hundred atoms per cubic centimetre. HI clouds are generally 50 – 100 parsecs (163 – 326.156 light years) across, contain only a few solar masses, and have a temperature of approximately 100° K.

On the surface they simply look like spherical blobs, but deeper observations of these clouds show that they twisted into long filaments, and tangled into distorted shapes, proving once more that the interstellar medium is far from static.

Intercloud Medium

The space between the HI clouds is called the intercloud medium. Unlike the HI clouds which are cool, they are quite hot with a temperature of a few degrees Kelvin. Whereas the HI clouds are also slightly denser, the intercloud medium has a very low density of only about 0.1 atoms/cm3. Surprisingly enough, these regions consist of ionized hydrogen.

You might be wondering how this is possible if this type of interstellar medium is nowhere near enough to hot stars to be able to be ionized. Ultraviolet photons from distant stars occasionally zap through the interstellar medium. It is not very common, but when it does happen an atom will become ionized by absorbing this photon and thus losing its electron. In a dense gas, the atom would quickly find and capture another electron and once again be neutral. However, such a low density region means that the atoms are isolated and have to wait a very long time for an electron to happen past. This is how the gas stays ionized.

Molecular Clouds

Molecular CloudsMolecular clouds are some of the densest regions of the interstellar medium, dense enough for molecules to be able to form. Some clouds contain nearly 100 different types of molecules and can be quite complex. How they are formed is not yet fully understood.

Hydrogen is by far the most abundant atom in these types of clouds too, but it is very challenging to detect hydrogen molecules using radio methods. That makes other molecules which are good emitters of radio energy very important for finding and mapping out molecular clouds. Carbon monoxide (CO) is a poisonous gas here on Earth, but is one such emitter of radio energy, and so is a very helpful molecule for a molecular cloud to have. These molecules which are good radiators of energy also help lower the temperature and keep the cloud cool.

Molecules are quite fragile, and ultraviolet photons have a high enough energy to be able to separate the bonds keeping the molecules together. These molecules could not exist unless there was a dense cocoon protecting them, with sufficient dust to absorb and scatter these high energy photons so that the molecules stay intact.

Molecular clouds have to get very big, and then they are referred to as giant molecular clouds. These vast, massive regions of gas are less dense than the gas of stars. These clouds contain anywhere from 100 to a million solar masses. Despite their great mass, they are relatively cool, only reaching a couple of degrees Kelvin. Giant molecular clouds are potential stellar nurseries.

Gravity can cause some regions of the cloud to start contracting until the density and the temperature are higher. It is not a spontaneous event solely reliant upon gravity, and other factors have to be met for the conditions to be conducive to star formation. The speeding shock waves from a supernovae event could trigger the gas into contracting, and even collisions with other giant clouds can get the ball in motion.

Once contractions are triggered, gravity works so that the atoms all start falling together toward the centre of the cloud. The atoms gain momentum as gravity forces them to the centre, but this in itself is not enough for the gas to heat sufficiently. The random motion of the atoms colliding into one another at very high speeds is enough to produce thermal energy, and the temperature of the gas starts to rise. The gravitational energy of the atoms falling and colliding is converted to thermal energy as they grow hot and the gas is heated.

The dense clumps this process forms within the giant molecular cloud can loosely be called protostars. A protostar must be hot enough to radiate infrared light, but isn’t yet hot enough for nuclear reactions to start taking place. At the centre, protostars have a higher density than the clouds that cocoon them. Such an object is very large and very bright too, although the surrounding dust of the cloud will obscure its luminosity for the most part. As they contract from the inside out, more and more mass will collect at the centre. The protostar also rotates and flattens the surrounding cloud into a disc. Eventually, the stellar winds and radiation of the growing protostar will drive remaining gas away, until it is finally revealed from its dusty nursery.

Infrared Radiation, X-rays and Ultraviolet

In 1983, the Infrared Astronomy Satellite mapped the sky and found that the galaxy is full of infrared radiation from stellar dust. Interstellar dust makes up only a small percent of the interstellar medium, but when you take a closer look, it is easy to understand how astronomers can detect the dust in infrared. Remember that giant molecular clouds can be as massive as 1 million solar masses.

Let’s assume we have a giant molecular cloud of only 100 000 solar masses, and 1% of that is dust. We would have 1000 solar masses of dust which, gathered in a single region, would be 10 times the diameter of the Sun, with a surface area 100 times greater than the Sun’s.  Such a tremendous surface area would generate a large amount of radiation.

X-rays and ultraviolet tell us even more about the interstellar medium. How could it be possible to detect X-rays when the interstellar medium is so cold (a temperature of around 100°K or –143°C)? There are some parts of the interstellar medium with very high temperatures where X-rays are generated. The gas in these regions is called coronal gas because it is as hot as the as corona of the Sun (106 °K) and can reach even higher temperatures than this. Naturally, the coronal gas in the interstellar medium is totally unrelated to the corona of the sun.

Despite its extremely high temperature, coronal gas has a very low density, of only 0.0004 to 0.0003 particles per cubic centimetre. You would have to comb through hundreds or even thousands of cubic centimetres before stumbling upon a single ionized atom or free electron. Several scenarios may explain the origins of coronal gas. Coronal gas could be the result of very hot gas pouring outward as it is ejected from a violent supernova explosion. Although these events are quite rare, a large amount of matter is ejected from such an epic blast. Gas flowing out from young hot stars may also add to the coronal mass. Astronomers estimate that only around 20% of space is filled with isolated regions of this interstellar medium.

As we have seen, the interstellar medium is far from being a static vacuum. The interstellar medium is an intricate and busy place, closely linked to the stars. Certain regions of the interstellar medium are the birth place of stars, and stars throughout their life cycle add to the medium, or stir it up so that further star formation takes place. It is a continuous cycle with one feeding into the other.

Best Nebula Filters

Start viewing deep sky nebula in finer detail by choosing a filter from these top picks.

Baader Planetarium OIII Nebula Filter 1.25”

The Baader Planetarium Filter adds a high contrast to your views of emission nebulae as well as planetary nebulae, separating light and dark features across the entire field of view. The filter provides a more detailed view of a variety of deep sky objects including the magnificent “Pillars of Creation”, or M6 in the Eagle Nebula. The OIII filter is not only effective at blocking out longer wavelengths, but short ones too, perfectly isolating the two doubly-ionized oxygen lines, keeping the correct colour for your views. The filter is coated for extra quality. The price does tend to be higher, but this Baader filter is a solid design.

Astromania 1.25” Narrowband Nebula Red Filter

The Astromania red filter is custom designed to be well-suited to telescopes across the board. It works wonderfully at enhancing nebulae without losing too much of the surrounding stars in the field of view. This adds depth to the nebula, instead of it looking 2-dimensional against an unnaturally black background. The filter has a high transmission of light for greater sharpness and contrast, while still blocking out light pollution and loner wavelengths. The filter is made from good material and has anti-reflective coating. It is not only great for deep sky objects, but can be used to enhance certain planets as well. The Astromania is a top choice for a good filter at an affordable price.

Orion 5581 1.25” Oxygen III Nebula Eyepiece Filter

Orion’s 1.25” OIII filter is ideal if you have a telescope of 8” aperture or bigger. The filter offers nearly 90% transmission of the 2 oxygen III spectral lines at 496 and 501 nanometres. It adds beautiful contrast and detail to deep sky objects, and is made with high end glass for crisp optics. Orion’s filter adds sharpness and dimension to several deep sky objects including the Swan, Dumbbell, Helix and Eagle Nebula. It even makes it easy to view challenging reflection nebulae. The price is on the higher end, but it is an investment worth making.

Baader Planetarium UHC – S Nebula Filter 2” FUHC – 2

For those with a telescope that has a 2” eyepiece barrel, the Baader Planetarium’s 2” filter is a choice threaded option. The price is a little steeper, but it completely delivers on quality and promise. The filter takes emission nebulae and instantly transforms them into detailed images without compromising too much on the brightness of the nebulae and the views and views of neighbouring stars. This superior filter has 97% transmission across the board, while dramatically reducing light pollution. It works just as well in smaller telescopes as it does in larger aperture instruments.

Celestron 93623 Narrowband Oxygen III 1.25” Filter  

Celestron offers a filter made of good quality glass and anti-reflective coating, perfect for reducing glare and ghosting, as well as blocking out light pollution. It is an affordably priced accessory, ideal for viewing the most popular nebulae and deep sky objects. It is best used with telescopes of 6 to 8” and bigger.

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