The Truth About Dust

When we astronomers talk about dust we don’t mean the type that you spend your life endlessly hoovering up (that’s mostly good old fashioned dirt). We mean cosmic dust, the stuff that is made in the space between stars and covers whole galaxies in huge clouds. It’s the stuff that gives birth to stars and planets. Every thing you ever touched or tasted or loved was once part of a massive cloud of dust floating out in space.

But what actually is this dust? I’ll be honest. We are not 100% sure but we’ve got a pretty good idea. The problem is we can’t just go out into space and grab a handful to look at in the lab. Voyager 1, the furthest man made object from the Earth, is only just leaving our solar system and it took 35 years to get there and won’t be coming back. Going on a jolly to pick up some dust isn’t looking likely. We can try and look at the dust that is in the solar system, and spacecraft like Ulysses and Cassini are doing just that, but it’s still not the same as going out and looking at all the lovely dust inbetween stars.

Sometimes we get lucky and we get hit by a meteorite. If you crack open a meteorite you might just get some stardust (and that is the technical term). It’s quite tricky to get out without destroying it, but we’ve managed to find that cosmic dust is mostly made up of stuff like graphite, silica carbide, aluminium oxide and other such fun things. When a star dies, either by going supernova or just wasting away to a white dwarf, it throws all the elements it made in its life out into the universe. All these atoms form something called the interstellar medium (astronomers call it the ISM, because we love our acronyms!). In the ISM the atoms come together to form dust grains, fractions of a millimeter long. How exactly they come together is still something we’re trying to work out.

These grains all hang out together in huge clouds that go right the way across entire galaxies. The problem is these clouds aren’t transparent, and this can cause a lot of issues if you’re an astronomer. Plonk a big cloud of dust in front of a star and it will soak up all the light from that star and block it from view. However, as it soaks up all that light the dust cloud is also soaking up all of the heat. Everything that has a temperature emits thermal radiation and you can see these clouds if you use giant, very fancy, very expensive heat cameras. With these cameras we can reclaim a lot of lost information as nearly half of all the light that shines from stars gets soaked up by dust clouds. If we didn’t look at the heat images of these clouds, all that information would be lost.

Andromeda Galaxy

The Andromeda galaxy is the closest galaxy that resembles the Milky Way and so has long fascinated astronomers. The top image shows the visible image, so if your eyes were giant telescopes this is what you would see. Throughout you can see dark bands, where dust lanes are blocking out the star light. The lower images is in the infrared, so shows the heat pattern, with the bands of dust glowing brightly. If you look carefully you can see that the dark patches in the optical match up with the bright lanes in the infrared. (Visible: Kitt Peak National Observatory, Infrared: Spitzer, Image: HubbleSite)

This dust isn’t just acting like a giant rain cloud blocking out the starlight. It’s really important to the growth and life cycle of stars and galaxies. Dust is made from dead stars but it’s in these huge clouds, or nebula, that new stars get born. Our own sun was born from the remains of other stars that died millions of years before, as were all the planets including the Earth and everything on it. As Carl Sagan said ‘we are all made of star stuff’. The next time you’re doing the hoovering spare a thought for the dust, because once that dust was cosmic and lived in the space between the stars.

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