The Interstellar Medium (ISM) Part I: Interstellar Dust

How the ISM Manifests Itself The ISM is in two principal forms: dust (multi-atom solid particles) and gas (free-floating atoms and relatively simple molecules) Each component can be studied because it absorbs light or emits light. We begin with a study of the dust.

The Dust is Surprisingly Plentiful Dust grains are tiny, and far apart – only about 1000 in every cubic kilometer of space. But space itself is a near vacuum, and the actual amount of dust is quite a high percentage of the total material. Indeed, if you were to compress a huge volume of interstellar space to the density of the Earth’s atmosphere, it would be so ‘smoggy’ that you would not be able to see your hand at the end of your arm! - yet we can see many hundreds of light-years out into space. The transparency is attributable to the fact that the ISM is so thinly spread out.

By Comparison: A Clean Toronto Rain regularly sweeps dust and smog out of Toronto’s air. But in the ISM, there is no comparable mechanism.

Don’t be Misled by the Name! It’s nothing like household ‘dust.’

Consider Dust in Absorption What absorbs light? On Earth, we may have fog (water droplets), smoke, smog, or dust (sand) in the air. What are the effects? Does this help our understanding? Is the material in the ISM like any of these?

Fog (i.e. Water Droplets) The droplets scatter light from remote objects, making them look dim. Does not usually alter colours. The fog is gray.

Dust (Sand) Can be very thick, but quickly settles out when the wind dies.

Smog Gives a blue haze It dims, and it alters the colours.

Cigarette Smoke Blue haze!

Questions to Address Why are there different effects (on colour, in particular)? Does this help us understand the interstellar dust? What are the ‘dust’ particles in the ISM most like? What are they likely to be made of?

Dust Dims the Stars The dust grains absorb some of the light, but also simply scatter some of it – that is, bounce it off in some new direction. For both reasons, less of the original light reaches us. On average, stars which lie a few thousand light years away (that is, within our own galaxy) will look only half as bright as they would if there was no dust! This makes the stars look farther away than they truly are, so we will overestimate their distances if we ignore these effects.

Sometimes It’s Much Worse! In places like the ‘Horsehead Nebula’, the density of dust is very high. The dust particles can completely block the visible light. We can’t see through at all!

How About The Effects on Colour? As noted, the dust dims the stars by absorbing some light (which vanishes), but also by scattering some of the light into random directions, so it never reaches our telescopes. But these effects are not equal at all wavelengths, so the colours of the stars are also affected.

How the Colours Change In the visible part of the spectrum, the dust particles scatter short-wavelength (blue) light more strongly than the longer wavelengths (red). Consequently, most of the red light gets through; but some of the blue light does not. So the distant stars look misleadingly red.

Even Longer Wavelengths Get Through Quite Readily

Big Grains, However, Don’t Affect Colours! In a Fog: the droplets of water are much bigger than the wavelength of light. They dim the light without any strong effect on the colour balance. (So remote traffic lights are fainter, but still look green!)

Very Small Particles Affect Colours a Lot! Hence the intensely blue skies of Earth!

Scattering by Tiny Molecules (smaller than dust!) The molecules of air (CO2, O2, N2) are much smaller than the wavelength of visible light. They scatter blue light about 16x as strongly as red light. Hence the blue sky!

The Grains in the ISM Are Not That Effective Blue light is scattered only about twice (2x) as effectively as red light. [For example, if five red photons in every thousand are scattered to the side, then ten blue photons will be – twice as many.] This tells us that the dust grains in the ISM are bigger than simple molecules, smaller than water droplets, and in fact comparable in size to the wavelength of light.

The Result: Interstellar Reddening Since a greater fraction of the red photons than blue ones make it through the murk to reach your eye, the star looks redder than it really is. You might think that it’s a cool star. But the spectrum (the pattern of absorption lines) reveals the truth.

What is the ISM Dust Made Of? Problem: we cannot collect any ISM samples directly! (In the solar system, we’ve sent out space probes to collect some small bits of interplanetary dust – not necessarily the same!) How do learn about the grains in the ISM?

An Interplanetary Sample (that is, from within the Solar System)

Simple Expectations The dust is surely not made of very rare elements like protactinium, europium, yttrium, scandium,... It is presumably made of the more common elements! But how can we find out for sure?

Clue # 1: Polarization In general, we don’t expect the light from stars to be polarized. (The photons are emitted from many independent parts of the star; the incoming ‘waves’ of light should be independent and randomly oriented.) But the overall light from stars is sometimes a bit polarized. This is puzzling.

Polarization – a Reminder

Clue #2: A Correlation The stars that are ‘reddened’ the most tend to show stronger polarization. Since the dust grains produce the reddening, they are presumably also responsible for the polarization. But so what?

Two Inferences The dust grains cannot be spheres. There has to be some ‘preferred direction’ to them if they are going to have any effect at all on the polarization of light passing through. The dust grains have to be aligned in some way. If they are all randomly oriented, then there will be no net effect.

What Lines Up the Dust Grains? Remember that the individual tiny grains are separated by many hundreds of meters at least, and not directly interacting! Can you think of small, elongated objects in North America which do not interact directly but which are aligned – and why this happens? (For example, the ones in Toronto are parallel to the ones in Halifax.)

Hint:

Magnetic Effects The magnetic field between the stars is about one millionth of the strength of the Earth’s field. (We can determine this independently.) This lines up the dust grains to some degree. Consequently, the grains must have some magnetic properties.

Clue #3: The ‘Reddening Curve’ Dust absorbs (‘extinguishes’) more light at shorter wavelengths -- that is, to the right in this figure -- than it does at long wavelengths. But the pronounced ‘bump’ in the spectrum suggests that the dust grains may contain silicates (as in Earth rocks) that absorb light particularly well at the wavelength shown.

Overall, There is No Conclusive Answer There is no ‘complete’ answer (and indeed there may be grains of various kinds). The best models include: iron-rich materials? graphite? (carbon) silicates? (‘sand’) a mix of these? probably often surrounded by a frozen mantle of common ices (water, methane, ammonia, CO2, etc)

Here’s An Artist’s Impression The grain is tiny in size -- roughly the same as the wavelength of visible light, a few hundred nanometres.

Dust in Emission - a much shorter story! The dust gets warmed up for two reasons: it absorbs starlight, and it also gets ‘bumped into’ by atoms in the ISM. As a result, it glows in the infrared part of the spectrum.

Consider the ‘Sombrero Galaxy’ In visible light, we see the stars. The dust in the flattened plane of the galaxy obscures our view to an extent. In infrared light, the warm dust glows and shows up as a ring.

Infrared Emission Allows us to identify particularly dense regions, full of gas and dust, that are possible hotbeds of star formation!

The Milky Way Seen at Various Wavelengths