The Sun and the Galaxy

The Sun and the Galaxy

Sun, a Moving Star

As the telescope grew in size and power, time-honoured beliefs in astronomy were swept aside. A whole new world of experience and discovery lay open to the astronomer who could construct telescopes superior to those used by Galileo Galilei. In 1655 Christian Huygens discovered Titan, the brightest member of Saturn’s large family of satellites. Four years later he announced that Saturn was accompanied by a “ring, thin, plane, nowhere attached, and inclined to the ecliptic”. In 1671 Jean D. Cassini came across Iapetus, another of Saturn`s satellites, and by 1684 had discovered three more. But the most exciting addition to the solar system came in 1761 when William Herschel, using a home-made telescope, discovered the seventh planet, Uranus.

Evidence that the old notion of fixed stars was incorrect was provided by Edmund Halley in 1718. A comparison of old and new catalogues of star positions revealed that three bright stars, Sirius, Arcturus, and Aldebaran, were moving in relation to their neighbours. Herschel and other astronomers made similar studies and concluded that the sun and its family of planets were moving through space. Copernicus had supposed that the sun was at the centre of the universe, but that centre turned out to be a moving one.

Herschel’s 40-foot-long reflecting telescope erected at Slough, England, in 1787. Engraving: Leisure Hour, November 2, 1867, page 729

The Milky Way System or Galaxy

With the aid of large reflecting telescopes which he made himself, Herschel found that the sun is one star in a system of many thousand million stars. At one time he thought that the system called the Milky Way System or Galaxy contained all the stars of the Universe. At another he suspected that there were other similar systems, but that they were so distant as to defy resolution into stars by even his largest telescopes.

We now know that the sun is a star in a vast rotating complex of stars, gas and dust, similar in shape to a pin-wheel. The complex is so enormous that the distance across it expressed in miles would give an absurdly great number. Astronomers therefore use a much larger unit, the light year. This is the distance the light travels in a year. Light travels at a speed of 186, 283 miles a second, so in a year it covers a distance of about 6,000, 000, 000, 000 miles. If it could travel across a diameter of the Galaxy, the journey would take about 100,000 years.

Hence we say that the diameter of the Galaxy is approximately 100,000 light-years. To obtain a mental picture of this, imagine that the entire solar system could be reduced to the size of a dime. The sun would then be a microscopic speck but the Galaxy would measure some 1,000 across.

Herschel thought that the sun was near the center of the Galaxy but we now know that it is about two-thirds of the way from the center to the edge. The sun and neighbouring stars. The sun and neighbouring stars travel round the centre at a speed of about 160 miles a second, but the distances involved in their flight are so immense that they take at least 200,000,000 years for one complete journey.