The Perseid meteor shower

Meteor showers are probably the easiest astronomical events to see, and the Perseids is one of the most spectacular. This year the shower will be in our skies from 17 July to 24 August, it will peak on 12-13 August. So how do you go about watching the show?

Perseid meteor shower

Credit: Fred Bruenjes

How to watch a meteor shower

The first thing you need to work out is where to look. The place where the meteorites originate from is called the radiant and for the Perseids this is found, as the name would suggest, in the constellation of Perseus. It’s near the ‘W’ shaped constellation of Cassiopeia if that’s a more recognisable landmark. At this time of year you can see both Perseus and Cassiopeia in north to north east of the sky, but as meteors appear over a wide area of the sky there’s no need to locate either constellation exactly. Now you’re ready to settle in for a night of meteor watching.

Perseid Sky Chart

Credit: Sky and Telescope

The great thing about meteor showers is that they are best viewed with equipment no more advanced than your eyes. You don’t even need particularly dark skies. I’ve seen them while lying down in the middle of a city (in my back garden I should add, not the middle of the road which is a terrible idea – too many street lights). All that you need is a place you can get away from direct sources of light, such as streetlamps and the moon. Luckily the Moon is a few days off new, as the light from our nearest celestial neighbour can sometimes put a dampener on things.

Once you’ve found your spot I suggest making yourself comfortable – you’re going to be there for a while. Find a place to sit down, but make sure you have some decent insulation, it can take your eyes up to half an hour to adapt and it gets cold much faster than you think. Soon enough you’ll start seeing those few flecks of light as they streak across the sky. Try and keep count of how many you see and how fast they come.

How many Perseids can I expect to see?

You may have heard a number being bandied about in the press claiming that the rate of meteors is around 100 per hour. That’s not exactly true. That number is what is called the zenithal hourly rate (ZHR). This is the number of meteors you would see if conditions were perfect, if the radiant was directly overhead at a dark sky site and if you could see the whole sky at once. Unless you are a fish (which I doubt if you are reading this) or have some very strange glasses on you cannot see the whole sky at once. For most people observing from an urban setting you’re more likely to see a third to a half of this rate. Still that will mean you’re going to see one shooting star every minute or so, which isn’t exactly shabby.

What are meteors?

So now you’ve seen a couple of shooting stars, what are they? Dust and debris from the Solar System hit our atmosphere all the time, about 100 tons every day. Most of this is dust and rocks the size of gravel that burn up * before they reach the ground without much ceremony. Some burn with a light bright enough to be seen from the ground and it’s these we see as shooting stars. However much of this is just stuff that’s floating around our planet or in its orbital path and hits our atmosphere pretty randomly.

Meteor showers a bit different in that they happen regularly. In most cases it’s caused by the Earth passing through the trail of debris left behind by a comet. In the case of the Perseids this was the comet Swift Tuttle which passes through the Earth’s orbit every 133 years, the last time in 1865. As the Earth’s orbit shifts slightly every year we haven’t managed to completely clear a path through the debris field and so we are left with the glorious sight that is the Perseids meteor shower.

So I hope that helps you some, and I hope you manage to get out there and see a few shooting stars for yourselves!


*: Technically the glowing isn’t caused by the meteor ‘burning’ but because it’s travelling so fast. The friction between it and the air causes the air to super heat and it’s this air that stars glowing. Of course this then melts the meteor, and there’s probably some oxidisation going on somewhere so you could say the rock is burning, but it’s not the flaming rock that you can see streaking across the sky.

From Russia with Rocks

On 15th February 2013 an explosion happened in the air above Chelyabinsk, Russia blowing out windows and doors for miles around. It was not, as many locals thought, a bomb going off but the shock wave created when a huge asteroid hit the earth’s atmosphere. News carriers around the world quickly picked up on the story, inviting experts to come and talk about the meteorite. Experts like me! I was contacted asking whether or not I’d like to go to Russia to film a documentary on the meteor. I, of course, said yes. After several manic days of trying to get visas and cold weather gear I was off.

It was great to be in the place where the meteor actually struck down.  It’s the first time something like this has happened and it’s been caught on camera. By looking at the footage scientists were quickly able to work out what direction the asteroid came from and how fast it was going. The best estimates put it somewhere around the 40,000mph mark.

Meteoroids are small particles of space rock, ranging from dust grains all the way up to kilometre sized asteroids. Meteoroids hit the Earth’s atmosphere all the time, around 50,000 tonnes of them a year. When this happens they, they become known as meteors. Most are small and burn up in the atmosphere, meaning they’re only dust when they reach the ground. By using stations across the globe designed to listen out for nuclear bombs, astronomers managed to work out how much energy the meteor created when it exploded, putting the size of the meteor at about 10,000 tonnes. Meteors of this size are thought to hit the planet once every 100 years or so. While it’s sad that so many people got hurt, it was really fortunate to have the event caught on so many cameras.

On the hunt

A quick explanation to camera before heading out into the snow fields to look for my rocks!

The day after we arrived we headed out into the countryside to ground zero: the spot directly underneath where the asteroid exploded. The fact that the area is covered in snow was actually a huge help to us meteorite hunters. I had to look for tiny holes in the snow, made by the meteorite as they showered from above. When you found a hole you had to dig around it, clearing a ring. Then I had to sift through the snow in the middle and hopefully find a meteorite. A lot of the holes were from mice or bits of tree but eventually me and the team got lucky and we found one! It was about the size of a jelly bean, covered in a black crust and felt really heavy.

Me and my Meteorite

Me with a 1kg meteorite found by the Urals Federal University.

We took the fragment to Urals Fedral University, where they were looking at the fragments already found. The discovered that the meteor was actually a rather average meteor, with about 7-10% iron, which was why the rock felt so heavy. I even got to look at a piece in a travelling electron microscope.

Despite the devastation that this meteor caused it was actually quite small, probably only 15m across, around the size of a house. The one that killed the dinosaurs was about 10km in size. Luckily those only happen every few million years and we’d probably see it coming. Projects like Pan-STARRS regularly scan the sky and can find nearly all asteroids that would risk wiping out all life on the planet. It hasn’t found any that are on a collision course with Earth, so there’s no need to worry. We’re not going the way of the dinosaurs anytime soon.

Meteor Strike: Fireball from Space is currently available on 4OD. If you haven’t seen it already, you might want to give it a look.

The Harvard Computers

The birth of the photographic plate was one of the most revolutionary moments in the history of astronomy. Before, an astronomer would have to spend long hours in the middle of the night, sitting in the cold at an often precariously placed eyepiece. Taking observations often took many hours and careful notation in less than ideal conditions.

The invention of the photograph had two different ramifications for astronomy. Firstly you could leave the plate there for hours, exposed to all the light of the night sky for as long as you could keep it steady. The human eye refreshes every 10 to 12 times a second, and so we can only see the light that falls on the eye in this time. With a plate you can leave it for 30 seconds, 5 minutes or all night if you can track what you’re looking at across the sky, gathering all the light falling on it in this time. This increase in light collection means that you can observe objects that are much dimmer than what can be seen with the human eye.

Secondly, once you have your photo taken and developed you can then take it away and look at it in the comfort of your office or study, in the middle of the day while sitting by the fire, which was much more cosy.

Williamina Fleming

Williamina Fleming

However there was a problem. The young male astronomers, usually PhD students and the like, didn’t think that sitting inside all day looking at photos was real astronomy. At Harvard College Observatory the men working under director Edward Charles Pickering moaned so much and did such a bad job that in 1881 he fired the lot of them, allegedly claiming that ‘his maid could do a better job’. True to his words, Prof. Pickering hired his maid, Williamina Fleming.

However, it transpired that Mrs Fleming was not just any ordinary maid. She was a highly intelligent and educated woman fallen on hard times after her husband abandoned her just when she was ready to give birth to their son. Initially she did simple tasks; clerical duties, copying and ‘computing’ i.e. maths [1]. However over time she began to take on more scientific and complicated tasks, such as looking at and analysing the spectrum produced by shining a star’s light through a prism.

Pickering quickly realised that not only was hiring women considerably cheaper than their male counterparts, they actually did a much better. For the price of one male astronomer Pickering could hire a dozen women, and they were soon known by the unflattering title of Pickering’s Harem, or the Harvard Computers. Together these women catalogued every single star observed in the sky, carefully measuring their colour, temperature, spectra and many other important properties. This was a feat that many thought impossible but after decades of hard work they completed the task. One of the group’s leaders, Annie Jump Cannon, cataloged and categorized over 350,000 stars in her lifetime, finding more stars in just four years than every male astronomer in history to the point put together [2].

The Harvard Computers

The Harvard Computers taken on 13th May 1913 . Back row, left to right: Margaret Harwood, Mollie O’Reilly, Prof. Pickering, Edith Gill, Annie Jump Cannon, Evelyn Leland, Florence Cushman, Marion Whyte, Grace Brooks. Front row: Arville Walker, Johanna Mackie, Alta Carpenter, Mabel Gill, Ida Woods. For more information look here.

The women were often paid less than the secretaries employed by the university and working in such a career was often seen as an admission of spinsterhood. All these women took this job for the love of astronomy and understanding. Many of these women went on to publish astronomical papers in their own right and many were the first women to be granted into the ranks of institutions such as the Royal Astronomical Society and the American Astronomical Society, and many are remembered on the moon having craters named after them. Henrietta Swan Leavitt, was even considered for a Nobel Prize in Physics for her work on Cephid variables that allowed astronomers to measure the enormous distances of the universe. Unfortunately she died before she could be officially nominated [3].

The work of the Harvard Computers was a huge leap forward not just in terms of astronomy but also for women in science. Though they themselves remain relatively unknown their work is still used by countless astronomers, both professionally and amateur, and will be for years to come.

[1] – Women of Science: Righting the Record by Gabriele Kass-Simon

[2] – Ladies of the Laboratory 2 by Lewis D. Eigen

[3] – Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How To Measure The Universe by George Johnson

Did you know…?

The Square Kilometer Array (SKA), a radio telescope being built in both Australia and South Africa and expected to actually cover 5 square kilometres of ground, will produce enough raw data to fill 15 million 64GB iPods a day. That’s equivalent to every piece of information sent and received over the entire internet. Twice.

SKA antenna

An artists impression of what the SKA antenna will look like. Thousands of these 12m diameter dishes, split between the two sites, will cover the square kilometre.

But it’s alright. You don’t have to rush out and panic buy iPods. Most of that information doesn’t tell us anything and gets thrown out straight away. Working out what to lose and what to keep, however, is one of the most challenging aspects of any project as big as the SKA.

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.