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.

How far is the far side of the moon?

As I’m sure many of you are aware the same side of the Moon always faces out towards us, which is why it looks the same every night. This is because the Moon is ‘tidally locked’ and the time it takes to turn on its axis (29.5 days) is the same as it takes to go round the Earth.

It wasn’t always like this though and it didn’t just happen by chance. Since the Moon was created the Earth has been pulling on it and over time this pulling slowed down the Moon’s turning until it got to the state it is in now.

Libration of the moon

This animation shows how the lunar surface appears to wobble over a few nights, giving us a little glimpse of the dark side of the Moon.

You might be thinking that we’ll only ever see half of the Moon with our own eyes, seeing as how holidays to the Moon aren’t looking likely any time soon, but that’s not quite true because of something called¬†libration. The Moon orbits around the Earth in a slightly eccentric orbit, meaning it goes in an ellipse rather than a circle. When the Moon is closer the Earth pulls on it more and it spins faster. When the Moon is further away it turns slower. Over the 29.5 days it takes the Moon to go around the Earth it will rotate once but this slowing down and speeding up means that what we see in the sky wobbles a bit. If you look at the night by night animation on the right you can see what I mean.

If you look at the Moon every night from new Moon to new Moon you’ll actually get to see 59% of the Moon’s surface. The other 41% isn’t completely dark to us though. We’ve sent enough missions around the Moon that we’ve got some pretty good images of it. Personally I prefer our side. Apparently a man lives in it, though I’ve never managed to see the bloke myself…

The far side of the moon.

The far side of the moon, imaged by NASA’s Lunar Recon Orbiter

Why is the sun green?

What colour is the sun? Most people will probably say yellow if countless childhood drawings are anything to go by. If you actually look (don’t! You’ll hurt your eyes) you’ll see that it looks white. But is it actually?

The sun emits something called black body radiation. Everything that has a temperature puts out light, or rather electromagnetic radiation. What colour this radiation is depends upon the temperature of the object. Let’s look at some curves.

Black body radiation curve at different temperatures.

Black body radiation curve at different temperatures.

These are black body radiation curve. They show how bright the light at a certain colour, or wavelength, is for objects at different temperatures (given in Kelvin, which is the temperature in Celcius + 273 degrees). The rainbow shows the wavelengths that are visible to the human eye.

Notice that the peak shifts sideways depending on what temperature the object is. At 3000-4000K the curve only has it’s tail in the visible? You can still see this, but it’s not hugely bright and will probably look a little on the red side. At 5000K you’ll really see it growing, and it will look whiter as the curve goes across the whole of the rainbow, and if you add all the colours of the rainbow together you get white. By 6000K we have something that really looks white without a hint of red, the peak landing slap bang in the middle of the spectrum. If you put even hotter curves on here then you’d see that they were almost entirely in the blue, so these would look blue. Keep going at it will get soooo hot that most of it’s radiation is in the UV and will look quite dim indeed.

The sun is at about 5578K, which means it’s peak is at a wavelength of 502nm, which means that the sun is, infact, green! Of course, this is only the peak of its emission. It’s still glowing all the way across the spectrum which is why it looks white but maybe next time your adding a sun to your drawing you’ll reach for the green and not the yellow paint.