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of years we looked up at this beacon and assumed it was a single star, but in 1862 American astronomer Alvan Graham Clark observed a sister star hidden in the glare of Sirius’s light. It took so long to notice Sirius’s companion because, as the photograph taken by the Hubble Space Telescope (bottom right) reveals, it is so much dimmer than its sibling. Shining faintly in the lower left-hand corner, the small dot of light is an image of the white dwarf star Sirius B. This is one of the larger white dwarf stars discovered by astronomers, with a mass similar to our sun that is packed into a sphere the size of Earth. With no fuel left to burn, white dwarfs like Sirius B glow faintly with the residual heat of their extinguished furnaces. Like most white dwarfs, Sirius B is made primarily of oxygen and carbon (the remnants of helium fusion) packed tightly with a density a million times that of a younger, living star. This is the future of our star; a vision of the Sun’s death. Slowly cooling in the freezing temperatures of deep space, it is estimated that our sun will reach this phase in around 6 billion years’ time. From Earth, if indeed there is an Earth at that time, our sun will shine no brighter than a full moon on a clear night.

Death must come to all stars. One day every light in the night sky will fade and the cosmos will be plunged into eternal night. This is the most profound consequence of the arrow of time; this structured Universe that we inhabit alongside all its wonders – the stars, the planets and the galaxies – cannot last forever. As we move through the age of stars, through the aeons ahead, countless billions of stars will live and die. Eventually, though, there will be only one type of star that will remain to illuminate the Universe in its old age

An artist’s impression of Sirius A and its diminutive companion Sirius B in close-up. They are overlain on a real image of the night sky containing the three stars of the Summer Triangle: Vega, Deneb and Altair. As seen from Sirius, our sun would appear as a moderately bright star in this same area of sky. It is shown here just below right of Sirius A.

NASA

This image from NASA’s Hubble Space Telescope shows the Boomerang Nebula in early 2005 and the two lobes of matter that are being ejected from the star as it dies. The rapid expansion of the planetary nebula around this dying star has made it one of the coldest places found in the Universe so far.

NASA

A Hubble Space Telescope image of the dazzling Sirius A with the faint speck of Sirius B to its lower left. Sirius B is 10,000 times fainter than Sirius itself.

NASA

THE DEATH OF THE SUN

Although relatively young now, the Sun, like every other star in the Universe, must one day die. In around five billion years’ time, the Sun’s stores of hydrogen will run dry and the star will begin its long, dramatic swansong. During this lengthy goodbye, the last dying bursts of extra heat will extend towards us, passing Mercury and Venus on the way and leaving a trail of destruction in its wake. Long after life has disappeared on Earth, the Sun will continue to fill the horizon as it swells in the Red Giant phase until, in about six billion years’ time, our Sun will shed its outer layers of gas and dust into space, exposing its core which will fade into a white dwarf, living on in the heavens as a shadow of its former self.

Nathalie Lees © HarperCollins

THE LAST STARS

The nearest star to our solar system is Proxima Centauri. Although only a mere 4.2 light years away, Proxima Centauri is not visible to the naked eye from Earth and doesn’t even stand out against more distant stars in many of the photographs that have been taken of it. The reason for this is that Proxima Centauri is small, very small when compared to our sun – having just 12 per cent of the Sun’s mass – so to our eyes this star would appear to shine 18,000 times less brightly than our sun.

Proxima Centauri is a red dwarf star – the most common type of star in our universe. Red dwarfs are diminutive and cold, with surface temperatures