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Dust in galaxies

Essentially we all live on a very large interstellar dust grain because the majority of the constituents making up the Earth and the other planets originally came from space. However, interstellar grains are more than a million million times smaller than the Earth and the other planets. So, what do we really know about dust?

What is dust?

In astronomy “dust” refers to the small solid particles to be found in space between the stars, it is often called cosmic dust, interstellar dust or stardust. Figure 1 shows the dust and gas filaments in the galaxy NGC 4013, which is like our own spiral galaxy, the Milky Way, but seen edge-on. The largest of the cosmic dust particles are only about one tenth of the diameter of a human hair.

How do we know that dust is out there?

We know that dust is out there because it stops us seeing! Just like smoke dims your view of what lies behind the barbecue on summer’s day, so the dust in space dims our view of the stars and galaxies that lie behind the clouds of dust and gas that fill the spaces between the stars (Fig. 1). Our view is dimmed by this dust in two ways, firstly, the dust absorbs the starlight (Figs. 2 and 3) and that’s why you can’t see behind the smoke. This is called extinction or reddening (Fig. 3). Secondly, it scatters the starlight out of our line of view (Fig. 4) and that’s why you can see the barbecue smoke from wherever you look and because it is scattered light it looks greyish. In general scattered light is bluer that the illuminating source (Fig. 4).

We also see dust grain in meteorites and comets that must have existed before the solar system was formed and must therefore have been present in the cloud of gas and dust that formed the Sun and the planets, this cloud is called the solar nebula. These “pre- solar” grains have a very special composition and so we know that they cannot have formed in the solar nebula but must have formed around distant stars (see “Where does dust come from?”).

Fig. 1 - The dark filaments in this galaxy are due to absorption by clouds of dust and gas. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Fig. 2 - Absorption by dust in dark clouds. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Fig. 4 - Starlight scattered by dust in the reflection nebula NGC 1999 in the constellation of Orion. Note how the scattered light is bluer than the starlight. Credit: NASA/ESA and the Hubble Heritage Team (STScI).

Fig. 3 - Absorption by dust in dark clouds. Note that in the left image the stars seen through the dark cloud of the Horsehead Nebula are redder due to extinction (absorption and scattering by dust). The right image shows the nebula at infrared wavelengths where the extinction is less and the stars shine through the cloud more brightly. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

What is dust made of?

The dust in galaxies is basically made of tiny specks soot and sand. The sandy part is not quite like beach sand but has a chemical composition close to that of the crystalline silicate minerals known as olivine and pyroxene. These minerals are normally crystalline but in space the grains of these minerals are mostly amorphous, which means that they do not show nice regular crystal structures. The grains are amorphous, in part, because of their small sizes but mostly because the interstellar medium is a tough place to be and out there they are irradiation with fast atoms and ions in shock waves (see “Where does dust come from?” and “How old is dust?”).

The sooty part of cosmic dust is made mostly of carbon and hydrogen atoms in the form of amorphous hydrocarbon grains, which are a sort of mix of bits of solid carbon structures including things that look a bit like polyethylene (plastic bags!), diamond (tiaras!) and polyunsaturated hydrocarbon chains (margarine!). These sorts of structures are all mixed together and really don’t look anything like any one of the individual bits. These hydrocarbon grains probably also include some oxygen and nitrogen atoms as a part of the structure. We also know from the dust grains that are found in meteorites, interplanetary dust particles (IDPs) and comets (Fig. 5), which formed before the solar system (called pre- solar dust), that there are also grains of nano-diamonds, carbides of silicon and titanium, graphite, oxides and very rarely silicon nitride (Fig. 6).

Fig. 5 - An interplanetary dust particle (IDP), typical of asteroid and comet dust.

Fig. 6 - Some example of pre-solar grains extracted from meteorites: nano-diamond, SiC and graphite (from left to right).

Where does dust come from?

New dust is formed around stars, and for this reason it is often called stardust, in particular it is formed in the shells of matter around stars that are at the end of their lives. This includes stars like our Sun that live for billions years before expanding to form red giant stars. This is the fate of our Sun but it will be a few billion years before the Earth is swallowed up by the expanding Sun when it becomes a red giant in it’s old age. Just before the star reaches the red giant stage it dims as the expanding matter from the star cools and condenses into molecules and tiny dust grains (Fig. 7). We can recognise these grains by their chemical signatures revealed by the way in which they absorb particular wavelengths of light coming from the star (their spectra). We also recognise them in the grains extracted from meteorites and comets by their chemical signatures because they have characteristic isotopic compositions. This simply means that atoms of the same element (carbon silicon, oxygen, nitrogen, etc.) can have different masses (these are called the isotopes of an element) and these are often very characteristic of the type of a particular type of star. Hence, we can often tell from what sort of star a dust grain originated.

Dust is also formed around stars that are at least eight times more massive than the Sun. In this case the stars become unstable as they age and after a few million years they will explode as spectacular supernovae. However, in the case of supernovae, even though they form dust, they are also very efficient at destroying it as the blast wave from the massive explosion interacts with the dust and gas in the surrounding interstellar medium and destroys the dust grains. It also seems that dust must be formed in the interstellar medium itself when the clouds of dust and gas condense and form molecules that stick onto the dust surfaces and therefore cause them to get bigger and heavier as they grow by the sticking of gas molecules such as carbon monoxide and water (this is called accretion) but also as the grains themselves stick together to form aggregates (this is called coagulation). This happens in the densest clouds in space and occurs en route to cloud collapse and star formation in the same way as the solar nebula formed the solar system.

Fig. 7 - Concentric shells of dust formed around the carbon star IRC+10216 (Leao et al. 2006, A&A, 455, 187). Reproduced with permission of ESO.

How old is dust?

Dust is both young and old in space, as per the human population it has a very wide range of ages! It can be very young because we see it forming today around giant stars and in the exploded remnants of supernovae. The dust that sits between the stars, today’s interstellar dust, is probably hundreds of millions of years old but it is constantly being destroyed and re-formed there, so some of it is very old and some of it is practically brand new. Nevertheless, the fact that we see pre-solar dust in the solar system, in meteorites and comets, means that this dust must be much older than the age of the solar system, which is about 4.6 billion years old. Hence, some dust is protected from the ravages of the interstellar medium by incorporation into comets, asteroids minor planets and planets. However, most of this dust has lost all memory of where it came from because it has been eroded, melted and vaporised en route to comet, asteroid and planet formation.

Is dust the same everywhere?

Space is a very nasty environment for dust. If you think that getting sunburn on the beach in summer is bad then you should try the same thing without a protective atmosphere, never mind not being able to breathe! Out in space the dust and gas is very cold; very cold here means about -250◦ celsius, i.e., 250 degrees of frost! Space is at the same time very hot but in this case we are talking about the intensity of the radiation field from all of the stars out there. For comparison the Sun is a comparatively cool 5500◦ celsius but many stars are way hotter than this and can have surface temperatures as high as 30,000◦ to 40,000◦ celsius. So, in space the dust, especially the less resistant hydrocarbon dust is at the same time both cold (-250◦ celsius) and burned to a frazzle by the intense starlight.

Is dust there dust beyond the galaxies?

Yes, there is also dust in the close proximity of and between galaxies, the extragalactic medium. This dust most likely comes from the galaxies themselves being pushed out into the extragalactic regions by galactic winds produced during irregular star formation episodes (starbursts). However, the dust in the extragalactic medium lives in an even hotter environment than within galaxies and the gas there can have temperatures of more than a million degrees. Even though this environment is very hot it is not very dense but it is a deathly environment for dust, especially nano-particles (dust with sizes of about one millionth of a millimetre) which are instantly zapped into oblivion by collisions with the hot electrons from the ionised gas (a gas where every atom has lost at least one electron to the gas).