A variable star is any star that varies in brightness. David Fabricius (1564-1617) was the first to discover a variable star, Mira, in 1596. In 1782 John Goodricke (1764-86) and his neighbour, Edward Pigott (1753-1825), began to observe bright variable stars. He found the variation of Beta Persei (Algol) was regular and suggested that light from the distant sun is periodically blocked (occultation) by a dark body orbiting it. He went on to observe other such eclipsing binaries. Of the three other short-period variables he discovered, Beta Lyrae consists of a pair of companion stars, while Eta Aquilae and Delta Cephei are individual stars (now known as Cepheids) that are intrinsically variable, i.e. their luminosity actually changes (old stars, oscillating between two states).
The Magellanic Clouds are two star formations seen from the southern hemisphere, which appear as large, misty smudges of light that could be mistaken for thin veil-like clouds. They are named after Ferdinand Magellan (c.1480-1521), who described them during his voyage around the world. (1519-22).
In 1834 William Herschel’s son, John Frederick William Herschel (1792-1871) went to Cape Town in southern Africa to map the southern hemisphere skies. He identified large numbers of star clusters and nebulae, including dozens in the Magellanic Clouds. He commented that ‘in no other portion of the heavens are so many nebulous and stellar masses thronged together in an equally small space’. Cleveland Abbe (1838-1916), director of the Cincinnati Observatory (est.1842), realised that this must imply that the Clouds must be distant separate systems similar to the Milky Way. He went further by suggesting that this implied that the smaller and fainter nebulae were separate galaxies (1867).
Today, the Magellanic Clouds are considered to be satellite galaxies of the Milky Way with the Small Magellanic Cloud (SMC) lying ≈190,000 light years away. When Henrietta Swan Leavitt (1868-1921) examined them the Clouds were thus close enough for individual stars to be identified and studied, though not close enough for their distances to be measured by direct parallax.
Leavitt worked with Edward Charles Pickering (1846-1919) at Harvard College Observatory (est.1839), Massachusetts, measuring the brightness of stars on photographic plates. By 1908 she had found 1777 variables in the SMC, and for sixteen of these she had determined the periods. She noted that the longer the periods, the brighter the stars. As the SMC is so distant that its stars can be regarded as being at the same distance from Earth; their apparent magnitudes therefore correspond with absolute magnitudes (luminosity). Four years later the sixteen had become twenty-five and Leavitt had found a mathematical relationship between period and apparent magnitude. If she discovered a cepheid variable star somewhere else in the sky she could measure its period and from the period-luminosity relationship and thus determine how bright that star would be if it were in the SMC and therefore how its distance compared with that of the stars in the SMC. As with Kepler’s third law and with Herschel’s siriometers, Leavitt was able to compare distances but did not have absolute distance measurements.
Leave a Reply