What effect does foreground dust have on the appearance of background stars? choose all that apply.

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Let's now consider the dust. Photoexcitation (by absorption of photons) and collisional excitation (by atoms/molecules) occur in the atoms and molecules that constitute the surface of a dust grain. Much of this energy is shared throughout the grain, raising its temperature until thermal radiation from the grain balances the energy absorbed. An alternative fate for an incident photon is to be scattered (Figure 15), a process that is very efficient at certain wavelengths. Figure 20 illustrates what is seen by observers when a cloud of interstellar dust is in the line of sight to the star and when it is out of the line of sight. The typical size of the interstellar dust grains means that they scatter short wavelengths most efficiently. This means that relatively more blue light is removed from the star's spectrum after passing through the cloud and it therefore appears redder when viewed from behind the cloud (position b in Figure 20). This process is called interstellar reddening. If the cloud is observed from out of the line of sight to a star then the dust cloud can appear as a faint blue glow from the scattered starlight.

Figure 20 The effect of interstellar dust on radiation from a star. (a) is the spectrum emitted from the star. Spectrum (b) is seen by an observer looking at the star through the dust cloud, while spectrum (c) is seen by an observer looking at the dust cloud against a star-less background.

The combined effects of absorption and scattering (extinction) by interstellar dust are shown in Figure 21. Note how the extinction increases strongly through visible wavelengths, and on into the UV. Note also how broad the spectral features are, which makes it difficult to determine the composition of the dust from such spectral studies. Not much more about composition is revealed by the emission spectrum of the dust, which is a broad smooth thermal spectrum, depending on the dust temperature, the particle size, and only weakly on its composition. At 20 K, the dust emission lies right across the far-IR and microwave parts of the spectrum.

Figure 21 Extinction by interstellar dust: the top curve is the sum of the other two. The extinction is measured in magnitudes per unit distance (usually per kiloparsec).

We have seen that absorption of starlight by interstellar dust can cause stars to appear fainter than they should and therefore cause us to overestimate their distance or underestimate their luminosity. In addition, interstellar reddening can cause stars to appear redder than they should. Since colours, as measured by the colour index, are often used to infer temperature, the temperature can also be underestimated. If plotted on the H-R diagram a star will appear in the wrong place if the effects of interstellar absorption and reddening are not accounted for.

A star like our Sun is located in a star cluster at a known (large) distance and is subject to significant interstellar extinction. If its absolute visual magnitude mV is derived from its apparent visual magnitude mv using Equation C and its temperature determined from its observed colour index, B - V, what will be the effect on its position in the H-R diagram (Figure 5)? Explain how its true position can be determined if its spectrum is observed.

Equation C does not take account of interstellar extinction, A, as in Equation D:

The derived absolute visual magnitude will therefore be too faint (M numerically too large). Since interstellar dust also causes reddening, the B - V colour will be redder and therefore the derived temperature will be too low. Examination of the axes of the H-R diagram in Figure 5 shows that the star will appear below and to the right of its correct position.

If a spectrum is observed, the temperature can be derived from its spectral type (based on the strengths of certain spectral lines) and therefore not affected by interstellar reddening. Its luminosity can also be inferred directly from its spectrum and hence its true position on the H-R diagram can be determined.

I've heard it said that without interstellar dust to obscure starlight that the night sky would be ablaze with light. Is this true?

By Stuart Gary

Dust obscures a lot of light coming from stars, particularly more distant ones. It also gives a glow to such strange things as nova nebulae. I've heard it said that without interstellar dust to obscure starlight that the night sky would be ablaze with light. Is this true?
—Barry.

When the early 19th century astronomer William Herschel tried to gauge how many stars there were in the sky, he counted the stars he could see in the field of his telescope in each direction.

He counted far more stars when he looked in the plane of the Milky Way than when he looked away from it, so he correctly deduced that the Sun was embedded in a flattened disc of stars.

But, based on the observation that there was roughly the same number of stars in all directions of the Milky Way, he wrongly deduced that the Sun must be near or at the centre of the galaxy.

Herschel didn't know about all the dust between stars, which meant he could only see stars nearest to Earth, says Professor Fred Watson from the Australian Astronomical Observatory.

"Had the dust not been there obscuring his view, Herschel would have seen a blaze of stars in the direction of Sagittarius, which we now know to be the direction of the galactic centre."

Dust reduces our view of distant objects particularly in the plane of the galaxy, says Watson.

"The galaxy is a flattened disc of stars, gas and dust, in which our Sun is embedded. So whenever we look in the disc of the galaxy we are always looking through the thickness of disc and hence through the greatest amount of dust," he says.

"The centre of the galaxy is about 25,000 light-years away. If the dust wasn't there you would see most of the 400 billion or so stars in the Milky Way rather than just a few million."

"Had Herschel known this, he would have realised that the Sun is a long way from the centre of the galaxy."

It wasn't until the beginning of the 20th century that astronomers realised interstellar space is full of dust.

"American astronomer Harlow Shapley was studying the distribution of tight balls of thousands of stars called globular clusters which sit above and below the plane of the galactic disc. He realised that they were all predominantly in the one part of the sky towards Sagittarius," says Watson.

"By measuring their distance, Shapley figured out these globular clusters were indicating that the galactic centre was a long way from the Sun. Combined with the work of Swiss astronomer Robert Trumpler, Shapley realised the Sun was nowhere near the centre of the galaxy."

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Dust clouds put on a show

Located in the constellation Orion, the Witch Head is a reflection nebula association with the bright star Rigel. (Source: NASA/GFC)

Dust makes the light shining through it appear redder, as blue light is scattered in a phenomenon known as Rayleigh scattering where light is scattered by particles much smaller than the wavelength of light itself.

"But when the dust is too thick to penetrate with visible light, such as towards the centre of the galaxy, you can use infrared light and radio telescopes to penetrate the dust."

Dust clouds can be seen in the night sky as dark patches between stars.

"It's what causes those dark lanes we see among the bands of stars in the Milky Way. The best known in the southern skies is the Coalsack, a blob of dust which sits right next to the Southern Cross."

The Aboriginal constellation the ''Emu in the Sky' features the Coalsack at its head and stretches left into Scorpius.

"These clouds of dust blocking background stars are called dark nebulae," says Watson.

The Horsehead Nebula and the dark part of the Lagoon Nebula are other examples of dark nebulae.

Dust particles can also create beautiful reflection nebulae, which often appear in white-blue colours.

"That's caused by dust being illuminated by a bright star. Just like wood smoke looks blue when you illuminate it, Rayleigh scattering causes blue light to be preferentially scattered so it looks blue to our eyes."

The Witch Head Nebula and the ghostly Barnard's Merope Nebula IC-139 are both examples of reflection nebulae.

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It's a gas, gas, gas...

The Crab Nebula is a supernova remnant in the constellation of Taurus. (Source: NASA/JPL-Caltech/R.Gehrz (Univ. Minn))

Dust is not the only substance that can obscure light from distant stars. Larger gas molecules excited by ultra-violet light from nearby stars appear as spectacular emission nebulae.

"Nebulae coloured pink or red are caused by hydrogen excitation, if it's oxygen they tend to be green."

Most emission nebulae are red because of all the hydrogen in interstellar gas and its relatively low energy ionisation, but helium, oxygen, nitrogen and other elements can also be ionised if there's enough energy resulting in green and blue nebulae.

Two of the brightest are the Orion Nebula, the bright fuzzy patch in Orion's sword, and Eta Carina which contains a massive gas shrouded hyper-giant star, says Watson.

While there are several other types of nebulae, one of the most spectacular are supernova remnants created by the explosion of giant stars at the end of their lives. The best known examples include the Crab Nebula, Cassiopeia A, and Supernova 1987A.

These are clouds of gas, dust and plasma comprising ejected stellar material expanding from the explosion as well as interstellar material swept up along the way. The clouds are ionised both by the explosion and by subsequent shockwaves.

Other stars don't go out with a bang, they just puff off their outer layers (like the Cat's Eye, Helix and Ring Nebulae) and die slowly. Known as planetary nebulae, they are ionised by energy from the exposed stellar core.

Astronomer, Professor Fred Watson from the Australian Astronomical Observatory was interviewed by Stuart Gary.

Published 28 July 2011