Monthly Archives: August 2010

Look up and see the stars tonight…

Tomorrow morning, I’ll be heading away from the bright lights of London towards the not-so-bright lights of my home town in the North West of England. By complete chance, my trip away from the city coincides with a yearly event that requires a clear sky and as little light pollution as possible to be fully appreciated. Other than finding the right conditions, all you need to do to witness this event is look up.

A Perseid meteor from 8th August this year. Image: Tamas Ladanyi

The Perseid meteor shower was first observed two thousand years ago, and is visible every year from around the middle July to the end of August. At the peak of the shower, there should be 60 or so shooting stars every hour – meaning anyone looking to the sky can expect to see around one a minute, depending on location and a few other factors that can affect visibility. This year, the peak of the shower is tonight at around 0100 GMT.

The meteor shower originates from the comet Swift-Tuttle, which was discovered in 1862 and has a solid nucleus that’s nearly 17 miles across. Unusually, the comet is locked into an orbital resonance with Jupiter, meaning that for every 11 times Jupiter completes an orbit of the Sun, Swift-Tuttle will go round only once. It was last seen in 1992, but we see its debris every year in the form of the Perseids.

Small particles in the comet’s tail spread out along its whole orbit, forming something known as a meteoroid* stream. When the Earth passes through this stream we get a meteor shower. As the particles enter the atmosphere, they travel extremely fast (around 20km/s) causing the air in front of them to compress. The compressed air heats up, and both it and the meteor can reach temperatures of just over 1500C. At temperatures this high, the meteor doesn’t last long – it burns up in the atmosphere creating shooting stars that we can see. Technically, the fast streak of light we see is called the meteor’s trail, and the remnant after the trail has passed is known as the train.

All of the meteors in a shower appear to come from the same point in the sky, a spot called the radiant. This happens because all the meteors are travelling parallel to each other (the same effect causes train tracks to appear to converge in the distance). The meteors in the Perseid shower all appear to be coming from the direction of the constellation Perseus, and this is how the shower got its name.

The best time to see the Perseids, and all other meteor showers, is in the last few hours before the Sun comes up in the morning. As the Earth rotates, the side turning towards the Sun is able to catch more meteoroids, upping the number of meteors in the sky.

This year astronomers are expecting a more spectacular light show than usual. The peak of the shower is coming only two days after a new moon, so there will only be a little moonlight around to spoil the view. Even in urban areas the number of meteors visible per hour could reach between 10 and 20.

Wherever you are, don’t forget to look up.

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* Before entering our atmosphere, the particles are known as meteoroids. When they’re travelling through the atmosphere they are meteors, and if one managed to make it to the ground intact it would be called a meteorite (a Perseid meteor is very unlikely to reach Earth, as the biggest ones are only around the size of a pea).

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For details on how best to see the Perseids where you are, take a look here and here.

Post title stolen from Patrick Wolf.

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Where have all these auroras come from?

Aurora borealis above Bear Lake in Alaska

You’ve probably noticed all the stories floating around recently about the Sun’s increase in activity and auroras being visible in places that they usually aren’t. It’s all been pretty exciting. Especially if, like me, you’ve always wanted to see the northern lights and there was a (very small, but still non-zero) chance of the phenomenon being visible in your home town.

In light of this (no pun intended), I decided a blog post about the science behind auroras was in order…

What exactly is happening with the Sun at the moment?

The Sun goes through cycles, each lasting around 11 years. During this cycle, its magnetic field increases and then decreases again. The magnetic field of the Sun is the source of its “activity” – a term which describes solar phenomena like sunspots, faculae and prominences. Activity can also come in the form of coronal mass ejections (CMEs). These are huge bubbles of material with diameters a few times that of the Sun(!), that explode into space, releasing billions of tons of plasma.

A couple of years ago the Sun’s activity was at an exceptionally low and long-lasting minimum, but since then it’s been increasing and we’re heading for a maximum in 2013. This means lots more activity is on the horizon: near a solar minimum we get around one CME a week, near a maximum this increases to two or three per day.

The coronal mass ejection that occurred on 1st August 2010, causing the recent spell of auroras.

What has this got to do with the northern lights?

The northern lights (aka aurora borealis) are an amazing display of green and sometimes red light seen near to the magnetic north pole, and they’re caused by CMEs. Their southern equivalent occurs near the south pole, and is known as aurora australis.

After a CME erupts from the Sun, it can interact with the solar wind and cause huge interplanetary shock waves that go on to reach the Earth. When particles from the solar wind get to Earth, they are channelled down our planet’s magnetic field lines and end up accelerating towards the magnetic north and south poles. These particles then interact with atoms and molecules in our atmosphere and excite them, causing them to release photons. It is these photons that make up the light we see in the sky during an aurora.

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For for information about the CME pictured above, and a video, see here.

Also, this BBC News article has a good illustration showing the solar wind’s interaction with the Earth’s magnetic field.

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Images: Top, U.S. Air Force photo by Senior Airman Joshua Strang. Bottom, NASA.

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New limit on neutrino mass… from cosmology, not particle physics

Physicists at University College London have found a new upper limit on the mass of a neutrino – one of the tightest constraints yet from either particle physics or cosmology.

Neutrinos are elementary particles that travel close to the speed of light, but are very difficult to detect because they are not electrically charged. In fact, in the time it takes you to read this sentence, thousands of billions of neutrinos will have passed through your body – and you won’t have felt a thing.

According to the Standard Model of particle physics, neutrinos should be massless – just like the photons that make up light – but in reality they do have a very small mass. What the Standard Model failed to take into account is the fact that neutrinos undergo something known as oscillations, or mixing.

Neutrinos come in three “flavours”: electron neutrinos, mu neutrinos and tau neutrinos (as well as a corresponding antiparticle for each). When a neutrino is created, for example in the Sun during nuclear fusion, it has a specific flavour. Over time the neutrino can change flavour, but only if its mass is non-zero. All of the neutrinos created in the Sun are electron neutrinos. However, by the time they reach the Earth, we detect equal amounts of each flavour of neutrino; this is how we know that neutrinos must have mass.

Sudbury Neutrino Observatory: underground detector of solar neutrinos. Image: NASA

Shaun Thomas and colleagues at UCL have now found an upper limit on that mass. It’s the tightest constraint ever placed on the neutrino mass, and comes from cosmology rather than particle physics. Thomas et al took redshift data from over 700,000 galaxies using the Sloan Digital Sky Survey and combined this with results from Wilkinson Microwave Anisotropy Probe, which measured the radiation left over from the Big Bang. They were then able to compare density distributions from the early and more recent Universe.

A Universe where neutrinos are massless would look a bit different to our own. Having massive neutrinos means that the total mass in the Universe is higher than it otherwise would be, and the evolution of the Universe is affected. Larger neutrinos suppress the growth of galaxies more than smaller ones; the more mass neutrinos have, the fewer galaxies will be able to form.

From this relationship, Thomas et al were able to provide the first limit on neutrino mass to come from a galaxy redshift survey, and the tightest constraint yet. The new limit found will complement future neutrino experiments such as KATRIN, and provides hope that the next generation of galaxy surveys may be able to put even tighter constraints on the mass.

Reference:
Thomas, S., Abdalla, F., & Lahav, O. (2010). Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey Physical Review Letters, 105 (3) DOI: 10.1103/PhysRevLett.105.031301

PDF available here.

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