Star

Stars are extremely large, extremely luminous bodies of gas at great distances. They are the most obvious features found in the universe. They are principally composed of hydrogen that is undergoing nuclear fusion to become helium. Our Sun is the nearest star to Earth, at a distance averaging 93 million miles. The Earth orbits the Sun in a period of approximately 365.25 days, and this defines the year. The diameter of the Sun, which is a typical star, is about 870,000 miles and its power output is about 10²⁶ watts. The temperature inside the Sun is estimated to be in excess on ten million degrees, and this is hot enough for nuclear reactions to occur.

The Magnitude scale of brightness

The Greeks devised a rough and ready brightness scale for stars. The brightest stars were designated 'stars of the first magnitude', less bright stars were designated 'stars of the second magnitude', etc., down to stars of the sixth magnitude, which were barely visible to the naked eye. The magnitude scale is still in use, but modern telescopes and photometers have made it very much more exact and rigorous. A star of magnitude 1.0 is now 2.51188643 times as bright as a magnitude 2.0 star, which is 2.51188643 times as bright as a magnitude 3.0 star, and so on. This number has been chosen so that a magnitude 6.0 star is exactly 100 times as bright as a magnitude 1.0 star, i.e. 2.51188643 = 10^0.4.

The brightness of a star as seen from Earth depends on two things, how luminous it really is, and how far away it is. Stars vary greatly in their luminosity and their distance from Earth. The apparent brightness of a star varies according to the inverse-square law. Consider two stars that are equal actual brightness, but one is at a distance of 100 light-years and the other is at a distance of 200 light-years. The farther star is twice as far away, but it will not appear to be half as bright, it will be only one quarter as bright. More generally, if one star is x times further away than the other it will appear to be x² times less bright.

Variable stars

Some stars vary in brightness and are known as variable stars. The star Algol in the constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the period. The most famous of the variable stars is delta Cepheus, the first-found member of the Cepheid group of variable stars. In 1908 Henrietta Swan Leavitt noticed that the variable stars in the Magellenic Clouds (two nearby galaxies in the Local Group) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the Milky Way) had been discovered, but it was not possible to measure the distances to them. It was soon realised that the variable stars in the Magellenic Cloud were of the Cepheid type. Since Cepheid variables also occur in our galaxy it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in globular clusters (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.

Research into the age of stars

A team of astronomers from the University of Texas at Austin McDonald Observatory recently estimated the age of one of the oldest stars in the Milky Way, HE 1523-0901, at 13.2 billion years. They used radioactive decay dating techniques. This star is close to the estimated age of the universe, 13.7 billion years.

The team leader, Dr. Anna Frebel, said that it was hard to pin down the age of a star, but that it can be inferred that chemically primitive stars must be very old, since they would have been born before many later generations of stars had provided chemical enrichment to the galaxy. A very few old stars contain huge amounts of some chemical elements, including radioactive thorium and uranium, and astronomers can use known rates of decay to deduce the ages of the stars.

Frebel's team measured the uranium in HE1523-0901 using the UVES spectrograph on the Kueyen Telescope. This is one of four that together comprise the Very Larte Telescope based in Chile. She explained that although uranium had been discovered in two other stars, HE 1523-0901 also contains thorium. Uranium has a half-life of 4.5 billion years while thorium has a half-life of 14 billion years. However, HE 1523-0901 also contains other elements that can be anchored to the radioactive elements - europium, osmium and iridium. This combination provided Frebel with six 'cosmic clocks' that could be used to check each other.

The team hope to gain new experimental data and important clues about the hypothesized creation and evolution of the chemical elements shortly after the Big Bang.

Other References

The Observer's Book of Astronomy, by Patrick Moore. Published by Frederick Warne and Co. 1967.

The Cosmological Distance Ladder, by Michael Rowan-Robinson. Published by Freeman. 1985.