Archive for the ‘Astronomy’ Category

Gravity Wave Detector#2

July 15, 2017

GEO600 One arm

GEO600

After presenting a paper at the European Society of Precision Engineering and Nanotechnology (EUSPEN) in Hannover back in May, I was offered the chance to visit a Gravity Wave Detector. Wow! I jumped at the opportunity!

The visiting delegation were driven in a three-minibus convoy for about 30 minutes, ending up in the middle of a field of cabbages.

After artfully turning around and re-tracing our steps, we found a long, straight, gated track running off the cabbage-field track.

Near the gate was a shed, and alongside the road ran some corrugated sheet covering what looked like a drainage ditch.

These were the only clues that we were approaching one of the most sensitive devices that human beings have ever built: the GEO600 gravity-wave detector(Wikipedia or GEO600 home page)

Even as we drove down the road, the device in ‘the ditch’ was looking for length changes in the 600 metre road of less than one thousandth the diameter of a single proton.

Nothing about how to achieve such sensitivity is obvious. And as my previous article made clear, there have been many false steps along the way.

But even the phenomenal sensitivity of this detector turns out be not quite good enough to detect the gravity waves from colliding black holes.

In order to detect recent events GEO600 would have to have been between 3 and 10 times more sensitive.

The measuring principle

The GEO600 device as it appears above ground is illustrated in the drone movie above.

It consists of a series of huts and an underground laboratory at the intersection of two 600 metre long ‘arms’.

In the central laboratory, a powerful (30 watt) laser shines light of a single wavelength onto a beam-splitter: a piece of glass with a thin metal coating.

The beam-splitter reflects half the light and transmits the other other half, creating two beams which travel at 90° to each other along the two arms of the device.

At the end of the arms, a mirror reflects the light back to the beam-splitter and onto a light detector where the beams re-combine.

Aside from the laser, all the optical components are suspended from anti-vibration mountings inside vacuum tubes about 50 cm in diameter.

When set up optimally, the light traversing the two arms interferes destructively, giving almost zero light signal at the detector.

But a motion of one mirror by half of a wavelength of light (~0.0005 millimetres), will result in a signal going from nearly zero watts (when there is destructive interference) to roughly 30 watts (when there is constructive interference).

So this device – which is called a Michelson Interferometer – senses tiny differences in the path of light in the two arms. These differences might be due to the motion of one of the mirrors, or due to light in one arm being delayed with respect to light in the other arm.

Sensitivity

The basic sensitivity to motion can be calculated (roughly) as follows.

Shifting one mirror by one half a wavelength (roughly 0.0005 millimetres) results in an optical signal increasing from near zero to roughly 30 watts, a sensitivity of around 60,000 watts per millimetre.

Modern silicon detectors can detect perhaps a pico-watt (10-12 watt) of light.

So the device can detect a motion of just

10-12 watts ÷ 60000 watts per millimetre

or roughly 2 x 10-17 mm which is 10-20 metres. Or one hundred thousandth the diameter of a proton!

If the beam paths are each 600 metres long then the ability to detect displacements is equivalent to a fractional strain of roughly 10-23 in one beam path over the other.

So GEO600 could, in principle, detect a change in length of one arm compared to the other by a fraction:

0.000 000 000 000 000 000 000 01

There are lots of reasons why this sensitivity is not fully realised, but that is the basic operating principle of the interferometer.

The ‘trick’ is isolation

The scientists running the experiment think that a gravity wave passing through the detector will cause tiny, fluctuating changes in the length of one arm of GEO600 compared with the other arm.

The changes they expect are tiny which is why they made GEO600 so sensitive.

But in the same way that a super-sensitive microphone in a noisy room would just makes the noise appear louder, so GEO600 is useless unless it can be isolated from noise and vibrations.

So the ‘trick’ is to place this extraordinarily sensitive ‘microphone’ into an extraordinarily ‘quiet’ environment. This is very difficult.

If one sits in a quiet room, one can slowly become aware of all kinds of noises which were previously present, but of which one was unaware:

  • the sound of the flow of blood in our ears:
  • the sound of the house ‘creaking’
  • other ‘hums’ of indeterminate origin.

Similarly GEO600, can ‘hear’ previously unimaginably ‘quiet’ sounds:

  • the ground vibrations of Atlantic waves crashing on the shores of Europe:
  • the atom-by-atom ‘creeping’ of the suspension holding the mirrors

Results

So during an experiment, the components of GEO600 sit in a vacuum and the mirrors and optical components are suspended from silica (glass) fibres, which are themselves suspended from the end of a spring-on-a-spring-on-a-spring!

In the photograph below, the stainless steel vacuum vessels containing the key components can be seen in the underground ‘hub’ at the intersection of the two arms.

GEO600 Beam Splitter

They are as isolated from the ‘local’ environment as possible.

The output of the detector – the brightness of the light on the detector is shown live on one of the many screens in the control ‘hut’.

GEO 600 Control Centre

But instead of a graph of ‘brightness versus time, the signal is shown as a graph of the frequencies of vibration detected by the silicon detector.

Results

The picture below shows a graph of the strain – the difference in length of the two arms – detected at different frequencies.

[Please note the graph is what scientists call ‘logarithmic’. This means that a given distance on either axis corresponds to a constant multiplier. So the each group of horizontal lines corresponds to a change in strain by a factor 10, and the maximum strain shown on the vertical 10,000 times larger than the smallest strain shown.]

Sensitivity Curve

The picture above shows two traces, which both have three key features:

  • The blue curve showed the signal being detected as we watched. The red curve was the best performance of the detector. So the detector was performing close to its optimal performance.
  • Both curves are large at low frequencies, have a minimum close to 600 Hz, and then rise slowly. This is the background noise of the detector. Ideally they would like this to be about 10 times lower, particularly at low frequencies.
  • Close to the minimum is a large cluster of spikes: these are the natural frequencies of vibration of the mirror suspensions and the other optical components.
  • There are lots of spikes caused by specific noise sources in the environment.

If a gravity wave passed by…

…it would appear as a sudden spike at a particular frequency, and this frequency would then increase, and finally the spike would disappear.

It would be over in less than a second.

And how could they tell it was a gravity wave and not just random noise? Well that’s the second trick: gravity wave detectors hunt in pairs.

The signal from this detector is analysed alongside signals from other gravity wave detectors located thousands of kilometres away.

If the signal came from a gravity wave, then they would expect to see a similar signal in the second detector either just before or just afterwards – within a ‘time window’ consistent with a wave travelling at the speed of light.

Reflections

Because powerful lasers were in use, visitors were obliged to wear laser google!

Because powerful lasers were in use, visitors were obliged to wear laser goggles!

This was the second gravity wave detector I have seen that has never detected a gravity wave.

But I have seen this in the new era where we now know these waves exist.

People have been actively searching for these waves for roughly 50 years and I am filled with admiration for the nobility of the researchers who spent their careers fruitlessly searching and failing to find gravity waves.

But the collective effect of these decades of ‘failure’ is a collective success: we now know how to the ‘listen’ to the Universe in a new way which will probably revolutionise how we look at the Universe in the coming centuries.

A 12-minute Documentary

The days of our lives

September 30, 2014
MIchael de Podesta is 20,000 days old today.

Michael de Podesta is 20,000 days old today.

Today, 30th September 2014 is a special day: I am 54 years, 9 months and 2 days old!

Special? Yes, because today I am 20,000 days old!

It is natural to mark the passage of time since our birth.

Traditionally we do this by counting the number of times the Earth has orbited the Sun (years) since the day we were born.

We then celebrate when this number reaches a multiple of 10, the number of fingers and thumbs on our hands.

Is it any less arbitrary to count the number of rotations of the Earth about its axis (days) since we were born?

And then celebrate when this number reaches a multiple of 10. Or 100. Or 1000, Or in my advanced case, 10,000.

I don’t expect to see my 30,000th day on Earth (15th February 2042), but I am looking forward to 15th May 2023.

If I am alive, then sometime during that day I will pass my 2 billionth second on Earth! Wow!

On reflection, I realise it is arbitrary to pick any particular unit for counting our age – we should just follow the local convention.

But make sure that we celebrate every second of every minute of every hour of every day of every year. They all pass so quickly.

Happy Whatever 🙂

=================

You can plan the dates for your own celebrations using this excellent web site.

The hours of our days

September 8, 2014
The chart shows the number of hours between sunrise and sunset in London for each day of the year. In the spring and autumn day length changes by more than 3.5 minutes each day - or 25 minutes each week.

The chart shows the number of hours between sunrise and sunset in London for each day of the year. In the spring and autumn day length changes by more than 3.5 minutes each day – or 25 minutes each week.

It is around this time of year that I begin to feel the nights closing in, encroaching on my evenings and generally making me feel that the summer is slipping away.

The change in the number of hours of daylight in the UK is dramatic, falling from a peak of over 16.5 hours to less than 8 hours. If it wasn’t so familiar we would be amazed.

At this time of year ‘day length’, the difference between sunrise (when the Sun just pops over the horizon) and sunset (when it just disappears from view) changes by more than 3.5 minutes each day – or 25 minutes each week.

But of course it does not get dark immediately after the the sun sets. Even when it is below the horizon the sun illuminates the sky which scatters light downwards giving rise to ‘twilight’.

Google poetically informs me that twilight is:

the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun’s rays from the atmosphere.

Wikipedia tells me there are three definitions of twilight based on how far the Sun is below the horizon.

  • Civil‘ twilight is defined by the time that the Sun is less than 6º below the horizon.
    • Wikipedia describes this as “the limit at which twilight illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished
  • Nautical‘ twilight is defined by the time that the Sun is less than 12º below the horizon
    • Wikipedia describes this as “when there is a visible horizon (at sea) for reference“.
  • Astronomical‘ twilight is defined by the time that the Sun is less than 18º below the horizon.
    • Wikipedia describes this as “when the dimmest stars ever visible to the naked eye become visible”.

The chart below shows the times of Sunrise, Sunset and the three twilight times compiled from data on this wonderful web site.

Graph showing the time of sunrise (red line) and sunset (blue line) for each day of the year. Also shown are times of 'civil'. 'nautical' and 'astronomical' twilight.

Graph showing the time of sunrise (red line) and sunset (blue line) for each day of the year. Also shown are times of ‘civil’. ‘nautical’ and ‘astronomical’ twilight.

This chart tells us what we already knew, that in the winter, not only is the day length shorter, but so is twilight.

In the chart above I have removed the anomaly of British Summer Time and so all the times shown are Greenwich Mean Time.

If we leave in this anomaly, then (looking only at sunset and sunrise) the effect of changing to British Summer Time can be seen below.

The time of sunrise (red line) and sunset (blue line) for each day of the year. The solid line shows the 'clock' time in the UK and has a jump when we switch to British Summer Time. The coloured dotted lines shows the 'summer time' in Greenwich Mean Time.

The time of sunrise (red line) and sunset (blue line) for each day of the year. The solid line shows the ‘clock’ time in the UK and has a jump when we switch to British Summer Time. The coloured dotted lines shows the ‘summer time’ in Greenwich Mean Time. The Black dotted lines 8:30 a.m. and 4:00 p.m. typical times for going to and coming from schools.

This shows that when we switch the clocks, instead of giving ourselves extra hours of daylight early in the morning, we choose to give ourselves an extra hour of daylight during summer evenings.

I think this makes sense. Alternatively, we could all just choose to get up a little earlier? Mmmmm.

Sun Spotted in Teddington

January 9, 2014
A picture of the Sun taken on 9th January 2014 from Teddington UK

A picture of the Sun taken on 9th January 2014 from Teddington UK. Click for more detail. Image is courtesy of Peter Woolliams 2014.

The Sun – as you know well – shines night and day, but direct visibility of the Sun has been in short supply in Teddington these last two weeks.

But at break this morning I caught two of NPL’s astronomical gurus sipping lemon tea and enjoying the sunshine streaming through the windows. I sat with them and it felt so good.

We chatted about telescopes and things, and I must have said something amazing because later in the day Peter Woolliams sent me an e-mail with the picture above.

“Michael, inspired by you (and the unusual presence of the sun) I dashed back home at lunchtime to grab the first bit of solar image data for a few months, just caught the sun before the clouds rolled in… and the netbook battery expired…Rapid processing, see attached….”

I was astonished. First of all at the beauty of the image and second at the rapidity with which Peter had got to work. Another colleague joined in a discussion and she sent a me a link to the astonishing Helioviewer site.

At Helioviewer you can look at satellite images of the Sun for any particular moment in the last few years and create your own movies such as the one below. The movie below shows 6 hours of images (not much happens) but it is astounding nonetheless – especially when you notice the scale image of the Earth in the lower left corner.

And so I found myself wondering which image to be more moved by: the breathtaking Helioviewer with its movies and whizzy interface, or Peter’s astonishing image – surely the most astounding image produced in Teddington today.

Between the two I would vote for Peter’s, but the winning image of the day is one I can’t share with you: it exists only in my mind.

Inspired by Peter and Andrea Sella I looked out the skylight at Jupiter. Using first binoculars, then a small telescope, and then finally the telescope I bought for Maxwell I saw Jupiter’s disc and its 4 Galilean moons perfectly arranged in a line.

Seeing the image of Jupiter and its satellites which had astonished Galileo, and provided crucial evidence for Newton’s theory of Universal Gravitation, I felt in touch not only with vastness of the universe, not only with my family who I dragged upstairs to see it, but with history too.

==========================================

How did Peter create the image above?

“Images taken during my lunchbreak (just before the sun vanished behind cloud where it has been for the past few months!). 80mm William Optics refractor with Lunt B600 CaK filter onto a DMK41 mono camera. 600frame stacks processed in Autostakkert, post processed in Registax6 (wavelets and gamma stretch), Microsoft ICE (to stitch the disk together from 2 images) and GIMP to colorize. AR1944 is very complex and generated an X class flare yesterday. The sun is very low in the sky so higher magnification images were impossible, it’s nice to see the sun putting on a good show given all the warnings of it being past Solar Maximum!”

Telescope for Christmas?

January 3, 2014
Did you buy a telescope like this for Christmas? Well please don't expect to ever see images like the ones shown.

Did you buy a telescope like this for Christmas? Well please don’t expect to ever see images like the ones shown.

A few years ago I bought a telescope for my son: Sorry Maxwell, it wasn’t Santa Claus.

I bought a decent model, a Meade ETX-80, which had a mount that could automatically track stars and conveniently packed away into a special back-pack. It cost about £300.

It was easily the most expensive piece of optics I had ever bought, costing even more than my spectacles. But despite all my knowledge, and all the reviews I read, I really didn’t have any idea what I would be able to see.

Looking at terrestrial targets – distant chimney pots and the like – the telescope was astounding. It was easy to see insects on bricks 100 metres away. But it was all much harder trying to find an astronomical target.

With help from an expert colleague, Maxwell and I managed to attach a web-cam and after many hours we got some nice pictures of the moon. We were really pleased.

But viewing ‘deep space’ objects such as the galaxies shown in the advertisement was impossible. Actually it was possible to just about detect and get a sense of these galaxies. But it was impossible to see these object in anything resembling the detail shown.

It was not really a matter of magnification: it is a matter of faintness. The telescopes just don’t capture enough light, and our eyes are not sensitive enough.

I received the advert on an e-mail from a well-known camera shop, and I imagined such telescopes being bought for children who would then be disappointed. So I just thought I would mention it.

Seeing astronomical objects with your own eyes through a telescope is a transformatively positive experience: I remember buying a £40 telescope from pedlars in Greece a few years ago and watching the moons of Jupiter changing from one evening to the next: I was astounded. And the fact that the light wave which reached my eyes was the same light wave that had left Jupiter a few hours previously was part of the wonder. Seeing it on a screen would not have been the same.

But failing to see astronomical astronomical objects can have a similarly powerful negative experience: confirming one’s anxieties about the difficulty of ‘science’.

My top tip is to buy an astronomy magazine and attend a local astronomy club. In my experience you will find the people a bit ‘odd’: but they will be delighted to let you see what their years of experience have achieved.

Update: For my Birthday, I was given a book on the history of telescopes – An Acre of Glass by J B Zirker. To be honest, it’s not a very well-written book, but I do find the subject interesting and it has many fascinating details which a better-written book might have left out.

The book makes it clear that the last 50 years has seen astonishing progress in astronomical imaging, and this is likely to continue for many decades to come.

But I fear the perfect images of the cosmos which can now be produced routinely have spoiled us. They are beautiful and mysterious, to be sure. But in the same way that an unobtainable super-model can make one’s actual partner seem ordinary, somehow the perfect images which can never be seen directly, make the glimpses of distant galaxies viewed from Teddington seem less astonishing than they should be.

Cosmic rays surprise us again

April 7, 2013
The Alpha Magnetic Spectrometer being tested at CERN by being exposed to a beam of positrons.

The Alpha Magnetic Spectrometer (AMS-02)being tested at CERN by being exposed to a beam of positrons. (Picture from Wikipedia)

[Text and figures updated on April 9th 2013 due to insight from Ryan Nichol: Thanks]

The team running the Alpha Magnetic Spectrometer (AMS-02) have produced their first set of results. And as expected, they are full of surprises.

AMS-02 is an awesomely complex device – too power-hungry, heavy and complex to be placed on its own space platform, it was attached to the International Space Station 18 months ago on the last space shuttle mission. I wrote about this here.

It has with 650 separate microprocessors, 1118 temperature sensors and 298 active thermostatically-controlled heaters. It is basically a general-purpose particle detector like those found at CERN, and represents the culmination of nearly one hundred years of ‘fishing for particles’ in the high atmosphere.

  • First we flew balloons and found that ‘radiation levels’ increased as we went higher.
  • Then we discovered a ‘zoo’ of particles not yet observed on Earth – positrons, muons, pions, and anti-protons.
  • Then we discovered that ‘cosmic rays’ were not ‘rays’ but particles. And we realised that at the Earth’s surface we only observed the debris of collisions of ‘cosmic ray’ particles with the atoms in the upper atmosphere.

Where did these primary cosmic ray particle from?  What physical process accelerated them? Why did they have the range of energies that we observed? What were they? Protons? Electrons? Positrons? We just didn’t know. The AMS-02 was sent up to answer these questions.

I have found much of the comment on the results incomprehensible (BBC Example) with the discussion being exclusively focussed on ‘dark matter’.  So I thought I would try to summarise the results as I see them based on reading the original paper.

Over the last 18 months (roughly 50 million seconds) AMS-02 has observed 25 billion ‘events’  (roughly 600 per second). However, the results they report concern only a tiny fraction of these events – around 6.8 million observations of positrons or electrons believed to be ‘primary’ – coming straight from outer space.

  • They found that – as is usual for cosmic rays – there were fewer and fewer particles with high energies (Figure 1 below)
  • Looking at just the electrons and positrons (i.e. ignoring the protons and other particles they observed) there were only about 10% the number of positrons compared with electrons, but that the exact fraction changed with energy (See Figure 2 below)
  • They found that there were no ‘special’ energies – the spectrum was smooth.
  • They observed that the particles came uniformly from all directions  – the distribution was uniform with variations of greater 4% very unlikely.
  • The electron and positron fluxes followed nearly the same ‘power law’ i.e. the number of particles observed with a given energy changes in nearly the same way – indicating that they probably have the same source.

They conclude very modestly that the detailed observation of this positron ‘spectrum’ demonstrates…

“…the existence of new physical phenomena, whether from a particle physics of astrophysical origin.”

I like this experiment because it represents a new way to observe the Universe – and our observations of the Universe have always surprised us. Observations have the power to puncture the vast bubbles of speculation and fantasy that constitute much of cosmology. I am sure that over the 20 year lifetime of this experiment, AMS-02 will surprise us again and again.

Figures

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

Figure 1: Graph of the number of positron events observed as a function of energy in billions of electron volts (GeV). Notice that there only roughly 100 events in teh highest energy category.

AMS Figure 6

Figure 2: Graph of the fraction of positrons compared with electrons as a function of energy in billions of electron volts (GeV). The ‘error’ bars show the uncertainty in the fraction due to the small number of events detected.

My big problem with astronomy

February 22, 2012
Pretty Galaxy

A Pretty Galaxy - the subject of erudite speculation by astronomers and mindless reporting by hacks. The circle marks the apparent location of a black hole called HLX-1. Don't believe the colours - the picture is 'data' and not a photograph. The galaxy - which is inferred to be spiral in shape even though we see it edge on - is called ESO 243-49

Recent stories in Wired and The Register illustrate perfectly everything I hate about popular astronomy. First of all, you can see these are both routine hacks by comparing them with the press release.

Don’t get me wrong: I am filled with admiration for astronomers: their instruments are astounding; the maths and physics of observing is inspiring; and of course the Universe is just breathtakingly beautiful. What irritates the pants off me is the ridiculous desire to ‘explain’ what they observe. What we end up with is a pretty picture and a fantastical, unverifiable ‘sciency’ tale. Frankly we would be better of with just the pretty picture and good old fashioned ‘fairy’ tale.

To explain what I mean I have reproduced extracts from the ‘Wired’ article below in blue with what the article should (IMHO) have said.

Wired: The Hubble space telescope has spotted a supermassive black hole floating on the outskirts of a large galaxy.
Actual: Scientists looking at data from the Hubble Space Telescope have inferred the existence of a black hole near a large galaxy.(How?)

Wired: The location is odd because black holes of this size generally form in the centers of galaxies, not at their edges. This suggests the black hole is the lone survivor of a now-disintegrated dwarf galaxy.
Actual: The location is odd because evidence indicates black holes of this size are generally  found near the centers of galaxies, not at their edges. Scientists don’t understand this.

Wired:The black hole — named HLX-1 — is 20,000 times more massive than the sun, and is situated 290 million light-years away at the edge of the spiral galaxy ESO 243-49.
Actual: The black hole — named HLX-1 — is estimated to be 20,000 times more massive than the Sun (how?), and is estimated to be 290 million light-years away at the edge of the spiral galaxy ESO 243-49

Wired: Hubble detected a great deal of energetic blue light coming from the black hole’s accretion disk — a massive collection of gas and dust that spirals into the black hole’s maw, generating x-rays. But scientists studying Hubble’s data also noticed the presence of cooler, red light, which shouldn’t have been there.
Actual: Hubble detected blue light  and red light. They inferred that the blue light came from an accretion disk. But they couldn’t understand the red light.

Wired: Astronomers suspect the red light indicates the existence of a cluster of young stars, roughly 200 million years old, orbiting around the black hole. These stars, in turn, are the key to explaining the chaotic history of the supermassive black hole.
Actual: Astronomers could explain the red light if there were young stars orbiting around the black hole. They even thought up a story about these unobserved stars that might actually exist.

Wired: HLX-1 was likely formed at the center of a dwarf galaxy that once orbited ESO 243-49. But in this dog-eat-dog universe of ours, large galaxies often swallow up their smaller brethren. When the dwarf galaxy came too close to ESO 243-49, the larger galaxy plucked away most of its stars, leaving behind the exposed central black hole. The force of the galaxies’ collision would have also triggered the formation of new stars, explaining the presence of a young stellar cluster around the black hole. The cluster’s age, 200 million years, gives a good estimate of when the merger occurred. HLX-1 may now be following the same fate as its parent galaxy, slowly getting sucked into ESO 243-49. But researchers don’t know the details of the black hole’s orbit, so it could also possibly form a stable orbit around the larger galaxy, circling as the isolated reminder of a vanished dwarf.
Actual: HLX-1 was likely formed when a space dragon called PTMD-X1 laid an egg, which grew into a blue headed X-ray dragon. Astronomers speculate that the dragon’s mother died when it was just 200 million years old  causing the youngster to cry tears which then turned into stars through a process astronomers call tear-star -formification. The blue colour of the stars shows the dragon was sad and astronomers hope that it is happier now and has made friends.

One small ringtone for man…

September 26, 2011
NASA Logo

NASA Logo

After having visited NASA last week, I noticed that it is now possible to download ‘sounds of NASA‘ as ring tones, including some of the most famous sound bites. For example

The NASA home page has lots of other links you may enjoy too. Strangely, they didn’t have ‘May the Force be with you’.

Why is the Sun hot?

July 11, 2011
An image of the surface of the Sun.

An image of the surface of the Sun. We know the Sun is hot, but why? Image from the NASA Solar Dynamics Observatory.

Some questions are so obvious that it seems barely necessary to ask them. But in fact the reason the Sun is hot is not because of the nuclear fusion taking place at its heart. Or at least not only for that reason. The main reason the Sun is hot is because it is an excellent thermal insulator.

What? Yes, that is what I said. Although the Sun produces a phenomenal amount of energy, it is also phenomenally large and the energy has to travel stupendous distances from the core to the surface to escape which insulates the core, allowing a tremendous build up of temperature.

I learned this while reading Ben Craven’s superbly browsable web pages wherein he compares the Sun with another heat-generating object: a human body. He shows that per unit mass, a human body generates more than 6000 times more energy than the Sun! But in our bodies the heat can be quickly lost because the surface of our bodies is not so far from our warm ‘core’. I have taken the liberty of summarising Ben’s more detailed calculation:

Mass of the Sun. Close to 2 x  1030 kg (from wiki).

Total power output of Sun.  3.87 × 1026 watts (from wiki).

Mass of a person. 80 kg (I should diet I know)

Power output of a person. A typical adult eats 2000  kcal a day, or about 8.6 × 106 joules. Divided by the number of seconds in the day (=60 x 60 x 24 = 86400), this gives an average power of  about 100 W.

Power output per kilogram. If we divide the total power output by the mass in each case we get

  • 0.00019 W kg-1 for the Sun and
  • 1.25 W kg-1  for a human being.

Now I quote this here because I was so shocked by the result. Seeing the images of the broiling mass of gas on the Sun’s surface, it seemed obvious that reason for the Sun’s high surface temperature was the phenomenal power released in the Sun’s core. And it is. But it is not until one makes some calculations that one realises that the physics is a little more accessible than one previously thought.

Or as the rapper Fizzy Willow puts it:

Don’t give me no suss or saz,
Or I will get numerical on your ass.

Close-UPDATED

June 27, 2011
Asteroid Trajectory

Asteroid Trajectory projected for the object known as 2011 MD. It will miss the Earth by 7600 miles. Click for Larger version.

The LA Times reports that a ball of rock between 5 metres and 20 metres in diameter will flash past the Earth today, missing us by just 7600 miles. This is just under one Earth diameter away, and in cosmic terms it counts as ‘close’.

Wikipedia tells me that similar objects hit the Earth roughly once a year and make a really big bang, like a ‘small’ atomic explosion. Larger objects can create explosions equivalent to large nuclear devices and the Tunguska Event in June 1908 provides direct evidence for this. Wikipedia tells me that  a Tunguska-sized event is likely roughly once every 300 years.

I just find it amazing that we all get by in this cosmos of terrible chance. It makes me want to be nicer to people tomorrow. I hope you feel the same.

UPDATE: The Daily Mail reports that the UK is near the top of league of dangerous places for an asteroid to crash. Just thought you would like to know.


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