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Do you believe in black holes ...?

4/11/2013

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As you may know, I do a bit of teaching on short courses for the University of Southampton. These are mostly on the topic of Space System Engineering, and often delivered to technical staff at ‘ESA Estec’, which is the European Space Agency’s technical HQ located on the North Sea coast of the Netherlands.  On a recent course, one of the delegates was a gentleman who works at the European Space Astronomy Centre (ESAC) near Madrid, and we got talking about his research – and I have to say, what he told me ‘blew me away’!

His work involves observation of the region of sky in the constellation of Sagittarius where the centre of our own galaxy, the ‘Milky Way’, is located, and in particular a feature referred to as ‘Sagittarius A’.  When I was growing up, ‘Sag A’ was just an interesting and active radio source, but now – thanks to the work done by the ESAC team (and others around the world) – it has taken on special significance.  Part of the Sag A region contains an object labelled Sag A* which is believed to be a supermassive black hole, with a mass about 4 million times the mass of our Sun!

It is becoming widely accepted that such huge black holes lurk at the centre of most spiral galaxies, and Sag A* just happens to be the one closest to us, allowing an opportunity for detailed study of these extraordinary objects.  When I say‘closest’, however, observation of Sag A* still represents a challenge as it is at a significant distance of 26,000 light years away from us.  In order to study the object, the team at ESAC use methods with an angular resolution of the order amazingly of micro-arcseconds, where a micro-arcsecond is about 0.000 000 000 3 of a degree.  Using this extraordinary accuracy, the team were able to determine the orbits of stars around Sag A* (see picture), and found some astonishing results.  Firstly, that the mass of the object they were orbiting could be estimated (from the orbits) to be around 4,300,000 solar masses. And secondly, that the closest approach of one of the stars (labelled S2 in the picture) to the mystery object is only around 6 ¼ light hours (about the same size as the diameter of the orbit of Uranus around the Sun).  Putting these results together, the inescapable conclusion is that the only way so much mass can be contained within such a small volume is that Sag A* is a black hole!

The actual size of such a black hole can be estimated theoretically by calculating something called the Schwarzschild radius, which turns out to be about 11.8 million km for Sag A* (about 20% of the orbit radius of Mercury around the Sun).  The Schwarzschild radius corresponds to the position of the boundary of the black hole, the so-called ‘event horizon’.  This is the radius from the centre of the black hole where the escape velocity equals the speed of light.  The escape velocity inside this surface exceeds the speed of light, so no information about events going on inside the event horizon can be seen from the outside. All massive objects have an escape velocity – for example, spacecraft leaving Earth orbit will need a speed of about 11 km/sec to escape the Earth’s gravitational influence.  The defining attribute of any black hole is that it has a surrounding surface, called the event horizon, on which the escape velocity equals the speed of light.

So why have we only recently realised that such an extraordinary object resides at the centre of our galaxy?  Well
firstly, the observational technology to observe such an object has emerged only relatively recently.  More importantly, the Sag A* black hole appears to be in ‘dynamical equilibrium’ with its surroundings, so that it is currently relatively inactive – in other words, it is not tearing apart and swallowing stars and solar systems whole as it may well have done in the past.  You can get a clue about the object’s past behaviour by looking at other galaxies at great distance, and therefore as they were billions of years in the past.  The majority of these very young galaxies are observed to have extremely compact and very active sources of radiation at their centres – an era when the galactic centre black holes were not so quiescent as Sag A* is today.  Just as well, given that we are only 26,000 light years away!   

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Image of the location of the Sag A* black hole at the centre of our galaxy

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Schematic of stars in orbit around the Sag A* black hole, from which the mass and compact volume has been inferred.

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Planck Space Telescope sees 'first light'.

4/4/2013

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The European Space Agency (ESA) released the results recently from the analysis of 15 months of data gathered by the Planck Space Telescope (PST), which is an ESA programme costing around 600 million euros.  

The PST was launched in May 2009 to take up station at the Earth/Sun L2 point, approximately 1 ½ million kilometres from Earth (see picture).  This is an ideal ‘orbit’ for this type of mission with the Sun and Earth obscuring only a very small area of sky.  Like the COBE and WMAP satellites before it, the PST was designed to measure the Cosmic Background Radiation (CBR) which is the remnants of the light emitted by the Big Bang.  To unwrap this a little, currently scientists have adopted what they call the ‘standard model’ of the origin of our Universe.  Basically this comprises a Big Bang event at time-zero, when space, time, and all the energy and matter we now observe in the Universe were created spontaneously.   This was followed by a period of ‘inflation’ in which the resulting embryonic universe expanded at a very rapid rate.  Clearly at this stage the Universe comprised a very hot and dense ‘fireball’, which was opaque to the transmission of light.  However, about 380,000 years after the initial event, matter and radiation ‘decoupled’ and the light we now see as the CBR was free to propagate throughout the Universe.  Because of the huge degree of expansion of the Universe since, the wavelength of this 'first light' has been stretched so that we now see it as long wavelength microwave radiation with a very low temperature – about 2.7 degrees above absolute zero (absolute zero on the Celsius scale is around -273 degrees).  Although this story of the origin of the Universe may seem fanciful to you (sometimes I find it hard to believe myself), nevertheless the very precise measurements of Planck has served to increase confidence that indeed the standard model is correct!

What the Planck Space Telescope has produced (see picture) is a map of the sky showing very minute variations in the temperature of the background radiation.  These fluctuations in the ancient light produced 380,000 years after time-zero reflect variations in density that existed at that time. And it is these variations that seeded the formation of the first stars and galaxies.  The detailed analysis of the Planck data has provided refined estimates of the large-scale characteristics of the Universe, which can be summarised as follows.

Age of the Universe: 13.82 billion years – a little older than previously thought.
Fraction of  ‘normal matter’ (matter composed of particles such as protons, neutrons, electrons, etc.): 4.9 %.
Fraction of dark matter (matter of unknown composition which we can’t see, but we know it has to be there because the dynamics of galaxies, for example, cannot be explained by just taking account of the visible matter): 26.8 %.
Fraction of dark energy (the mysterious agent that is causing the expansion of the Universe to accelerate):  68.3%.
Hubble’s constant: 67.15 km/sec per megaparsec (where a megaparsec is about 3.2 million light years). 
This parameter quantifies the expansion rate of the Universe.  It means that galaxies 3.2 million light years away from us will appear to be receding, due to the universal expansion, at a speed of 67.15 km/sec.  Galaxies at twice this distance will be moving away at twice this speed, and so on.

Of course, not everything that the Planck Space Telescope is telling us is straightforward.   There are various mysterious anomalies in the data, and the hope is that these will lead to ‘new physics’ or to a modified understanding of the origins of our amazing Universe.   It is also quite humbling to realise that we have a reasonable understanding of the composition of only about 5% of the Universe, and currently we really don’t have a inkling about the other 95%! (See related post about the ESA Euclid project).


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The Planck Space Telescope, stationed at the Earth-Sun L2 Lagrange point

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The tiny variations in the cosmic background radaition revealed by the Planck Space Telescope

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    Graham Swinerd - I hope to use this page to highlight current major events in space and spacececraft.

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