LIFE OF THE STARS
HOW FAR ARE THE STARS?
Until 1838, Astronomers had little idea of the true size of the Universe. But in that year, Friedrich Bessel used a technique called the parallax method to make the first successful measurement of the distance to a star. Modern astronomers have many different ways of the distance to a star. Modern astronomers have many different ways of working out how far away an object is, but they all depend ultimately on the parallax method. Our knowledge of stellar distances was further revolutionized by the Hipparcos survey satellite, which used parallax to pinpoint many thousands of stars.
The European Space Agency launched its satellite Hipparcos in 1989. Located far above the disturbing effects of the Earths atmosphere, Hipparcos spent three and a half years measuring star positions. Its precision was so great that it could have picked out an astronaut standing on the Moon. From the data sent back, scientist calculated the parallaxes of 118,000 stars as faint as magnitude 12.5. Astronomers now have accurate distances to stars up to 500 light years from the Sun.
When the Moon is at its farthest from the Earth, it is not big enough in the sky to cover the Sun completely during an eclipse. Instead of a total eclipse, an annular eclipse occurs. When the Moon is exactly in front of the Sun, a bright ring of the Suns photosphere is still visible around the edge of the Moon, like a ring of fire. Annular eclipses can last for more than 12 minutes.
For a few seconds just before or after totality, the disappearing or emerging Sun shine between the mountains at the edge of the Moons disc. Sometimes this produces a brilliant spot of light, the diamond ring. At other times, an arc of bright points like a string of pearls is visible. These are known as Bailys Beads.