Our summer almanac 16.07.2010 – 22.07.2010

Regulus (α Leo / α Leonis / Alpha Leonis) is the brightest star in the constellation Leo and one of the brightest stars in the nighttime sky, and lies approximately 77.5 light years from Earth. Regulus is a multiple star system composed of four stars which are organised into two pairs.

Rēgulus is Latin for ‘prince’ or ‘little king’. The Greek variant Basiliscus is also used. It is known as Qalb Al Asad, from the Arabic قلب لأسد or Qalb[u] Al-´asad, meaning ‘the heart of the lion’. This phrase is sometimes approximated as Kabelaced and translates into Latin as Cor Leōnis. It is known in Chinese as 轩辕十四, the Fourteenth Star of Xuanyuan, the Yellow Emperor. In Hindu astronomy, Regulus corresponds to the Nakshatra Magha.
Persian astrologers around 3000 BC knew Regulus as Venant, one of the four ‘royal stars’. It was one of the fifteen Behenian stars known to medieval astrologers, associated with granite, mugwort, and a kabbalistic symbol.

Summer Stars
Looking in the night sky this week it is worth trying to find some of the bright stars of summer. In the north-northeastern part of the sky you may find the constellation Cygnus, the Swan. The brightest star in this star pattern is called Deneb and is one of the largest known super-giant stars. Deneb is 25 times as big and 60,000 times brighter than our Sun.

Look out for the bright star Vega in the constellation Lyra in the eastern sky. This star pattern can be found looking northeast at about 22.30h. It is to be found almost overhead late on July evenings. Vega has a brilliant bluish colour, and that together with its brightness makes it easy to recognize. Vega is only 25 light years away and is the fifth brightest star in the night sky. Vega is a brilliant blue-white star, two and a half times the size of our Sun, and twice as hot.

Only a few years after the invention of photography in the 1830’s the star Vega became the first star ever to be photographed through a telescope. American astronomers in the Harvard Observatory in Massachusetts USA made the photograph using the 15 inch refractor.

The Sky at Night
Have you ever wondered why the sky is dark at night? If space went on to infinity, and it was filled with stars, then everywhere you looked in the night sky there would be a star! If the night sky was filled with starlight then there would be no darkness, only light! A nineteenth-century German astronomer called Heinrich Olbers suggested that the darkness we see at night is due to the fact that we look out through gaps between stars. In what became known as the ‘Olbers’ paradox ‘, he suggested in 1823, incorrectly, that there must be an edge to the Universe, and that beyond that edge there were no stars, only dark empty space.

In February 1848, a brilliant amateur scientist, Edgar Allen Poe, better known today as the author of tales of ghoulish, gothic horror, gave a lecture setting out the solution to Olbers’ paradox. Even though his solution fits so well with current ideas about the Universe, his ideas were not taken up seriously in his own time. What Poe quite ingeniously realized was that by looking further out into space we are looking further back in time, to that time in the story of the Universe before stars were born.

Olbers’ edge in space turns out to be an edge in time. The darkness of the night sky means that the Universe was born at a definite moment in time.

According to Edward R Harrison, the first to conceive of anything like the paradox was Thomas Digges, who was also the first to exposit the Copernican system in English and may have been the first to postulate an infinite universe with infinitely many stars. Kepler also posed the problem in 1610, and the paradox took its mature form in the 18th century work of Halley and Cheseaux.

Although the paradox is commonly attributed to the German amateur astronomer Heinrich Wilhelm Olbers, who described it in 1823, but Harrison shows convincingly that Olbers was far from the first to pose the problem, nor was his thinking about it particularly valuable. Harrison argues that the first to set out a satisfactory resolution of the paradox was Lord Kelvin, in a little known 1901 paper, and that Edgar Allan Poe’s essay Eureka (1848) curiously anticipated some qualitative aspects of Kelvin’s argument:

Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.

Looking towards the centre of our galaxy

The night sky at this time of year is full of stars of our galaxy the Milky Way. This is because on summer nights our view of space is directed towards the centre of our galaxy.

Space exploration in our part of the galaxy
In our last post we mentioned how European spacecraft Rosetta raced past the Lutetia asteroid at 15 km/s completing the flyby in just a minute. But the cameras and other instruments had been working for hours and in some cases days beforehand, and will continue afterwards. Shortly after closest approach, Rosetta began transmitting data to Earth for processing.

Lutetia has been a mystery for many years. Ground telescopes have shown that it presents confusing characteristics. In some respects it resembles a ‘C-type’ asteroid, a primitive body left over from the formation of the Solar System. In others, it looks like an ‘M-type’. These have been associated with iron meteorites, are usually reddish and thought to be fragments of the cores of much larger objects.

The new images and the data from Rosetta’s other instruments will help to decide but not tonight. Compositional information is needed for that.

This project has even bigger ambitions though. ESA’s Rosetta spacecraft will be the first to undertake the long-term exploration of a comet at close quarters. It comprises a large orbiter, which is designed to operate for a decade at large distances from the Sun, and a small lander. Each of these carries a large complement of scientific experiments designed to complete the most detailed study of a comet ever attempted.

After entering orbit around Comet 67P/Churyumov-Gerasimenko in 2014, the spacecraft will release a small lander onto the icy nucleus, then spend the next two years orbiting the comet as it heads towards the Sun. On the way to Comet Churyumov-Gerasimenko, Rosetta will receive gravity assists from Earth and Mars, and will fly past main belt asteroids.

Why ‘Rosetta’?
The European Space Agency’s unprecedented mission of cometary exploration is named after the famous ‘Rosetta Stone’. This slab of volcanic basalt – now in the British Museum in London – was the key to unravelling the civilisation of ancient Egypt.

French soldiers discovered the unique Stone in 1799, as they prepared to demolish a wall near the village of Rashid (Rosetta) in Egypt’s Nile delta. The carved inscriptions on the Stone included hieroglyphics – the written language of ancient Egypt – and Greek, which was readily understood. After the French surrender in 1801, the 762-kilogram stone was handed over to the British, and so now everyone can see it in the British Museum in London.

By comparing the inscriptions on the stone, historians were able to begin deciphering the mysterious carved figures. Most of the pioneering work was carried out by the English physician and physicist Thomas Young, and the French scholar Jean François Champollion. As a result of their breakthroughs, scholars were at last able to piece together the history of a long-lost culture.

Just as the Rosetta Stone provided the key to an ancient civilisation, so ESA’s Rosetta spacecraft will unlock the mysteries of the oldest building blocks of our Solar System – the comets. As the worthy successor of Champollion and Young, Rosetta will allow scientists to look back 4600 million years to an epoch when no planets existed and only a vast swarm of asteroids and comets surrounded the Sun.

Tisha B’Av (Jewish )
Tuesday 20 July is a solemn day that commemorates a series of tragedies that have befallen the Jewish people over the years, many of which have coincidentally happened on this day.

Discovery of the most massive star ever is announced 21 July
R136a1 is a blue hypergiant star, currently on record as the most massive star known, at an estimated 265 solar masses.

The star also holds the record for the most luminous at 8,700,000 times the luminosity of the Sun. It is a member of R136, a super star cluster near the center of the 30 Doradus complex (also known as the Tarantula Nebula), in the Large Magellanic Cloud. The mass of the star was determined by astronomers at the University of Sheffield.

Left to right: a red dwarf, the Sun, a blue dwarf, and R136a1. R136a1 is not the largest known star in terms of volume; this distinction belongs to VY Canis Majoris.


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