It took thousands of years to learn that stars are distant suns
On a dark night you can see about 4,000 stars, every one of them inside our own galaxy, out of somewhere between ten sextillion and a septillion in the observable universe. For most of history they were thought to be fixed lights on an unchanging sphere. Proving otherwise took Chinese supernova records, parallax and a 1925 doctoral thesis.
A star is a ball of plasma held together by its own gravity. It forms when a cloud of mostly hydrogen and helium collapses, then shines for most of its life by fusing hydrogen into helium in its core. How much mass it starts with largely decides how it ends: as a white dwarf, a neutron star or, for the heaviest, a black hole. Along the way stars forge nearly every natural element heavier than lithium and scatter them back into space, where they seed the next generation. The English word itself goes back to an ancient root probably tied to burning, the same source as ash.
Cataloguing began early. An Egyptian star chart is dated to 1534 BC, Babylonian catalogues followed later that millennium, and Hipparchus listed 1,020 stars in the 2nd century BC. Chinese observers recorded a supernova in 185 AD, and the blast of 1006, the brightest stellar event on record, was noted in Egypt and China. Medieval Islamic astronomers named many stars we still use; al-Sufi's Book of Fixed Stars of 964 even described the Andromeda Galaxy.
The idea of an unchanging sky cracked slowly. Tycho Brahe spotted new stars, Giordano Bruno argued in 1584 that stars were suns possibly orbited by worlds, and Edmond Halley showed that some had shifted since ancient times. In the 1780s William Herschel counted stars in 600 directions and found them piling up toward the Milky Way's core. Friedrich Bessel finally measured a stellar distance in 1838: 61 Cygni, 11.4 light-years away.
Spectra then revealed what stars are made of. Angelo Secchi began sorting stars by type in 1865, and Annie J. Cannon refined the modern scheme in the early 1900s. In 1925 Cecilia Payne-Gaposchkin proposed that stars consist mainly of hydrogen and helium. Today gravitational lensing has let astronomers pick out a single star, nicknamed Icarus, nine billion light-years away.
Source: Star
