Here is a short science lesson for you all today.

From our small rock, we have a grandstand seat to explore our local galactic neighbourhood. Our nearest star, the Sun, is 93 million miles away, but each night when this star disappears from view, thousands more fill the night sky. In the most privileged places on Earth, up to 10.000 stars can be seen with the naked eye, and all of them are part of the galaxy we call home.

A galaxy is a massive collection of stars, gas and dust, bound together by gravity. It is a place where stars live and die, where the life cycles of our universe are played out on a gargantuan scale. Scientists think there are around 100 billion galaxies in the observable universe, each containing many millions of stars. The smallest galaxies, known as dwarf galaxies, have as few as ten million stars!! The biggest, the giants, have been estimated to contain in the region of 100 trillion. It is now widely accepted that galaxies also contain much more than just the matter we can see using our telescopes. They are thought to have giant halos of dark matter, a new form of matter unlike anything we have discovered on Earth and interacts only weakly with normal matter. Despite this, its gravitational effect dominates the behaviour of galaxies today and most likely dominated the formation of the galaxies in the early Universe. This is because scientists now think that around 95% of the mass of galaxies such as our own Milky Way is made up of dark matter. In some sense this makes the luminous stars, planets, gas and dust an after-thought, although because it is highly unlikely that the dark matter can form into complex and beautiful structures, like stars, planets and people, one might ultimately claim that it`s rather less interesting. The search for the nature of dark matter is one of the great challenges for twenty-first century physics.

The word `galaxy` comes from the Greek word Galaxias, meaning milky circle. It was first used to describe the galaxy that dominates our night skies, even though the Greeks could have had no concept of its true scale. Watching the core of our galaxy rise in the night sky is one of natures greatest spectacles, although regrettably the light of our cities has robbed us of this majestic nightly display. For many people it looks like the rising of storm clouds on the horizon, but as the Earth turns nightly towards the centre of our galaxy, the hazy band of light reveals itself as clouds of stars-billions of them! stretching thousands of light years inwards towards the galactic centre. In Greek mythology this ethereal light was described as the spilt milk from the breast of Zeus`s wife, Hera, creating a faint band across the night sky. This story is the origin of the modern name for our galaxy – The Milky Way. The name entered the English language not from a scientist, but from the pen of the medieval poet, Geoffrey Chaucer: `See yonder, lo, the Galaxy, which men clepeth The Milky Wey, For hit is whyt.`

The Milky Way contains somewhere between 200 and 400 billion stars, depending on the number of faint dwarf stars that are difficult for us to detect. The majority of stars lie in a disc around a hundred thousand light years in diameter and, on average, around a thousand light years thick. These vast distances are very difficult to visualise. A distance of 100.000 light years means that light itself, traveling at 186.000 miles per second, would take 100.000 years to make a journey across our galaxy. Or, to put it another way , the distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours-that`s one sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy!

At the centre of our galaxy, and possibly every galaxy in the Universe, there is believed to be a super-massive black hole. Astronomars believe this because of precise measurements of the orbit of a star known as S2. This star orbits around the intense source of radio waves known as Sagittarius A* that sits at the galactic centre. S2`s orbital period is just over 15 years, which makes it the fastest known orbiting object, reaching speeds of up to 2 per cent of the speed of light. If the precise orbital path of an object is known, the mass of the thing it is orbiting around can be calculated, and the mass of Sagittarius A* is enormous, at 4.1 million times the mass of our Sun. Since the star S2 has the closest approach to the object of only seventeen light hours, it is known that Sagittarius A* must be smaller than this, otherwise S2 would literally bump into it. The only known way of cramming 4.1 million times the mass of the Sun into a space less than 17 light hours across is as a black hole! which is why astronomers are so confident that a giant black hole sits at the centre of The Milky Way. These observations have recently been confirmed and refined by studying a further 27 stars, known as the S-stars, all with orbits taking them very close to Sagittarius A*.

Beyond the S stars is a melting pot of celestial activity, filled with all sorts of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our Sun in size, these stars burn brightly and are consequently very short lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova. It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic centre, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.

In 2007, scientists using the VLT telescope at the Paranal Observatory in Chile were able to observe a star in the Galactic Halo that is thought to be the oldest object in the Milky Way. HE 1523-0901 is a star in the last stages of its life; known as a red giant, it is a vast structure far bigger than our Sun, but much cooler at its surface. HE 1523-0901 is interesting because astronomers have been able to measure the precise quantities of five radioactive elements – uranium, thorium, europium, osmium and iridium – in the star. Using a technique very similar to carbon dating (a method archaeologists use to measure the age of organic material on Earth), astronomers have been able to get a precise age for this ancient star. Radioactive dating is an extremely precise and reliable technique when there are multiple `radiactive clocks` all ticking away at once. This is why the detection of the five radioactive elements in the light from the HE 1523-0901 was so important. This dying star turns out to be 13.2 billion years old – that`s almost as old as the Universe itself, which began just over 13.7 billion years ago. The radioactive elements in this star would have been created in the death throes of the first generation of stars, which ended their lives in supernova explosions in the first half a billion years of the life of the Universe.

G xx

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