Dust in Galaxies

David A Williams, Cesare Cecchi-Pestellini

• Publisher: Royal Society of Chemistry
• Year: 2019
• ISBN:
  9781788019255
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

Without interstellar dust, the Universe as we see it today would not exist. Yet at first we considered this vital ingredient merely an irritating fog that prevented a clear view of the stars and nebulae in the Milky Way and other galaxies. We now know that interstellar dust has essential roles in the physics and chemistry of the formation of stars and planetary systems, the creation of the building blocks of life, and in the movement of those molecules to new planets. This is the story in this book.

After introducing the materials this interstellar dust is made of, the authors explain the range of sizes and shapes of the dust grains in the Milky Way galaxy and the life cycle of dust, starting from the origins of dust grains in stellar explosions through to their turbulent destruction. Later on we see the variety of processes in interstellar space involving dust and the events there that cause the dust to change in ways that astronomers and astrobiologists can use to indirectly observe those events.

This book is written for a general audience, concentrating on ideas rather than detailed mathematics and chemical formulae, and is the first time interstellar dust has been discussed at an accessible level.

CHAPTER 1

Interstellar Dust in Galaxies

We present the idea that – without dust – the Universe as we know it would not exist. Dust in the Milky Way galaxy – and almost all other galaxies – enables the formation of stars and planets. Dust drives a chemistry that provides molecules of such complexity that they are the building blocks of the molecules of life. Dust transported these molecules to planets. Without dust, life would not have evolved on planet Earth and human beings would not have evolved here.

1.1 Dirt is Good

We live in a dirty galaxy. It's one of billions of galaxies in the Universe. Almost all of them are dirty. Our galaxy, the Milky Way, is dirty because it contains a lot of dust, very roughly about a billion solar masses of dust. That's a lot of dust, because a solar mass (i.e., the mass of the Sun) is huge – about two million billion trillion tonnes. This dust is found to be mixed with a gas that occupies the space between the stars. In total, this interstellar gas is a hundred times more massive than the dust, and is almost entirely hydrogen. The gas and dust together are called the interstellar medium, which makes up about one tenth of the total mass (i.e., stars plus gas and dust) of the Milky Way. Astronomers tend to measure masses in terms of solar masses, because the units of tonnes or kilogrammes that we use on Earth seem inappropriate for extra-terrestrial space. Using the mass of the Sun as the unit of mass, then the entire mass of the Milky Way galaxy, including stars, gas and dust is roughly one trillion (or 10¹²) solar masses; the mass of the interstellar medium (gas and dust) of the Milky Way is about one hundred billion (or 10¹¹) solar masses; and the mass of the dust in the Milky Way is approximately one billion (or 10⁹) solar masses.

In all galaxies, the interstellar medium is the source of matter – mainly hydrogen – from which new stars are formed. The Milky Way is thought to be forming new stars at an average rate of about one star per year. Interstellar space is also the place into which dying stars eject their debris, a lot of which is dust. So, the interstellar medium of a galaxy tends to get dirtier and dirtier (more and more dusty) as the galaxy becomes older. New stars replace older stars that have used up their fuel (hydrogen) and which eventually die; for example, our Sun is middle-aged and will begin to run out of fuel, go through some exciting death throes, and die in about five billion years (long enough so that we don't need to worry). As we shall see in later chapters, interstellar dust plays crucial roles in the processes of creating new stars from interstellar gas, new planets in orbit around these stars, and new comets and meteorites traversing these planetary systems. Some of these planets may be suitable for life, and dust plays an important part in creating molecules that may be involved in biochemistry and the initiation of life itself on new planets (or, some say, even in space). Studies related to extra-terrestrial life are very new topics of research, and are called astrobiology.

We'll argue in this book that interstellar dust in the Milky Way and in many other galaxies in the Universe is an essential ingredient in the recipe for forming stars and planets, and in the provision of molecules that are necessary for life to begin. We use the word “essential” because – from the biased point of view of human beings on planet Earth – we simply wouldn't exist were it not for the roles of dust. A “clean” Universe is not a place where life as we know it could evolve. It is good to be dirty!

In this book, we shall describe how we know that dust is present in the interstellar medium of the Milky Way (Chapters 1, 2, and 3), and what we know of the “life cycle” of dust from its formation in stars to its destruction in space (Chapters 4 and 5). We shall investigate the physical and chemical properties of dust that allow it to play characteristic roles in the interstellar medium, and we shall explore these roles in Chapters 6–10. We shall come to the conclusion that a galaxy without dust is not a place in which life would be expected to exist. Indeed, the existence of life appears to depend on the presence of dust.

In the remainder of this introductory chapter, we shall set the scene by describing the structure of the Milky Way and its interstellar medium. Although we shall talk frequently about dust in the Milky Way, our discussion will apply generally to almost all galaxies. We'll review a little of the history of the discovery of dust in the Milky Way, and we'll remind ourselves that while we must consider that dust in space is a good thing, dust here on Earth can be rather bad for us.

1.2 The Milky Way and Other Galaxies

The Milky Way is very big. Imagine a disc of diameter 100 000 light years and thickness 1000 light years. A light year is the distance travelled in one year at the speed of light, which is about 300 000 km s⁻¹, and so one light year is roughly ten thousand billion kilometres, or 10¹³ km. This imagined disc encompasses such a very large volume that the even the huge amount of interstellar matter in the Milky Way has a very low density indeed. In fact, the average number density of the interstellar gas is about one hydrogen atom per cubic centimetre. That average number density – one atom per cubic centimetre – is very low indeed compared to the number density of molecules (oxygen and nitrogen) in the air that we breathe in Earth's atmosphere. The average number density of dust grains in the Milky Way is very much lower than the gas density. On average, there is about one large dust grain in a cube of interstellar space of side 100 metres (m). This is incredibly clean, absolutely immaculate by terrestrial standards! But there's a lot of space in the galaxy, and so there's a lot of dust, too.

However, the interstellar medium in the Milky Way and other galaxies – the gas plus dust – isn't distributed smoothly, but is very clumpy. The density may be many thousands of times larger than the average in some small locations, while large regions may have densities that are very much lower than the average (we'll return to this in the following section). Dust and gas are generally well mixed within galaxies, so that where the gas is denser, it contains proportionally more dust.

We can't step outside our Milky Way to view its shape and size, but we can look at other galaxies, some of which we think are similar to the Milky Way. Figure 1.1 (left) shows an optical image of the Andromeda galaxy, taken in visible light (i.e., light to which our eyes respond). Andromeda is a spiral galaxy believed to be similar to the Milky Way. However, the Andromeda galaxy is thought to contain about twice as many stars and be about twice as massive as the Milky Way. Andromeda is the nearest major galaxy to the Milky Way, and is about 2.5 million light years away. It is approaching the Milky Way at about 110 km s⁻¹ and will eventually collide with it. That would be a collision worth watching, but we will have to wait for some billions of years before it happens.

Figure 1.1 The Andromeda galaxy, (left) optical image (Reproduced from https://commons.wikimedia.org/w/index.php?curid=12654493 under the terms of the CC BY 2.0 license, https://creativecommons.org/licenses/by/2.0/deed.en) and (right) infrared image (credit: ESA/Herschel). In the optical image, the spiral structure of the galaxy is traced in the white light of bright stars that are too far from Earth to be resolved. In some places, the starlight is obscured by clouds of dusty gas, and these clouds appear dark in the image. However, when dust absorbs starlight it is heated and then it radiates in the infrared. The right image shows the Andromeda galaxy in emitted infrared radiation at a wavelength of 24 micrometres (or 10⁻⁶ metres, or µm). The dark region in the optical image coincides with the bright regions of the infrared image.

The optical image, Figure 1.1 (left), clearly shows the spiral structure of the Andromeda galaxy. The dark ‘lanes’ trace denser regions of the interstellar medium, where dust mixed with the interstellar gas absorbs so much of the starlight that these regions appear dark. The energy of the starlight that is absorbed by the dust makes the dust warm enough for it to radiate in the infrared part of the spectrum (see Box 1.1 if you would like more information about the spectrum of radiation). Figure 1.1 (right) shows an image of the Andromeda galaxy taken not in visible light but in this infrared radiation. By comparing the two figures, we can see that this emitted infrared radiation arises in the places where the optical starlight is absorbed; i.e., the infrared radiation is coming from warm dust.

Box 1.1 Basic ideas of radiation

Light is part of the electromagnetic spectrum. It is a waveform in combined electric and magnetic fields (called the electromagnetic field) that travels at a speed c of about 300 000 km s⁻¹. We generally use the word “light” to refer to radiation to which our eyes respond. When a beam of white light (say, from the Sun) is passed through a glass prism, the beam is split into the familiar rainbow of visible colours between red and violet. These different colours of light that our eyes detect are in fact simply different wavelengths of electromagnetic waves. Red light has wavelengths of about 700 nm (1 nm or “nanometre” is one billionth of a metre) and violet light has wavelengths of about 400 nm. Human eyes evolved naturally to detect light as the familiar spectrum of red, orange, yellow, green, blue, indigo, and violet between these limits, but not outside these limits because sunlight is most intense between the limits of these wavelengths. It's amusing to consider that if the Sun were hotter, human eyes would have evolved a sensitivity at wavelengths shorter than violet (i.e., the ultraviolet) and if the Sun had been cooler we would have eyes sensitive to wavelengths longer than red (the infrared).

The electromagnetic spectrum extends far beyond red wavelengths to much longer wavelengths, and to much shorter wavelengths than those of violet. These radiations are just as real as the light we can see, but our eyes don't respond to wavelengths outside the visible range. However, we can build instruments that detect or emit non-visible radiation. At wavelengths of increasing size beyond visible red light there is infrared radiation, microwave radiation and radio radiation. Wavelengths shorter than violet light of decreasing size correspond to ultraviolet (UV) radiation, X-rays, and γ-rays.

If we could observe one electromagnetic wave, we would see the waveform passing us by at speed c, and we could count the number of wave peaks (each separated by one wavelength, λ) passing in one second. This number is called the frequency, ν. Obviously, the speed of the wave is equal to the number of wave peaks, each separated by one wavelength, passing by in one second, so the speed c is simply the product of the wavelength, λ, and the frequency, ν.

For example, taking the speed of light to be 300 000 k s⁻¹, ultraviolet radiation with a wavelength λ of 300 nanometres (or 10⁻⁹ metres, or nm) has a frequency ν of about 1 million billion per second, or in more convenient notation, 10¹⁵ Hertz.

Radiation packs an energy punch (called a photon) that is proportional to frequency ν. Thus, radio waves, which have relatively long waves and relatively low frequencies, have low-energy photons, while X-rays – with short wavelengths and high frequencies – have high-energy photons.

Figure 1.2 illustrates the electromagnetic spectrum, showing the very small visible region of the entire spectrum.

Figure 1.2 The electromagnetic spectrum. The figure shows the electromagnetic spectrum, ranging from γ-rays at the shortest wavelengths to radio waves at the longest wavelengths. The visible region, ranging from violet to red, is a tiny part of the entire spectrum.

The Milky Way and Andromeda are both examples of spiral galaxies, but galaxies come in many shapes and sizes (some examples are shown in Figure 1.3). Some have relatively little interstellar gas and dust, and their period of active star formation is therefore drawing to a close since the reservoir of matter for new stars – the interstellar gas – is nearly empty. Unlike spirals, such galaxies may show a smooth near-ellipsoidal structure, and are called elliptical galaxies. Other galaxies have large reservoirs of gas and dust, and consequently they may be forming stars at a high rate, turning interstellar matter into stars; these are called starburst galaxies. Galaxies do not always show the elegant symmetries possessed by spirals or ellipticals, and some have quite messy shapes; such galaxies are called irregular galaxies. Galaxies in the process of formation are called proto-galaxies, and probably form from the accumulation of smaller objects. All galaxies rotate, and every part of every galaxy is spinning to a greater or lesser extent. The familiar spin of the Earth that defines our day length, and its orbit around the Sun...