 放大圖片
現代天文學家使用多種不同的手段測量宇宙的大小，不過，在20世紀初，銀河系就是整個當時已知的宇宙，這個情況一直維持至20年代初期。1912年，在哈佛大學天文台工作的亨麗愛塔．絲韻．勒維特在天文台的星野底片中發現了「造父變星」（一種光度會出現周期性變化的恆星）變光幅度與變光周期的關係，也即是天文學的「周光關係」。1925年，著名天文學家哈勃藉「周光關係」，計算出仙女座M31大星雲其實是銀河系以外的天體，哈勃不單發現了宇宙的基本結構—星系，他更把宇宙的半徑向外擴張至一百萬光年。可惜勒維特並未能分享這份光榮，她於1921年因癌症與世長辭。上圖為一億光年以外，也是現時所知，最遙遠造父變星所處的星系。
The woman who expanded the universe
In the early 20th century the Milky Way, a structure about 100 thousand light years across by modern standard, was the entire universe. The scene was destined to change, soon. In 1912 Henrietta Swan Leavitt, while plotting the brightness of stars from the Harvard photographic plates, discovered the Period-Luminosity Relationship of the Cepheid variable stars. The P-L Relationship describes that the brighter a Cepheid star, the longer is its period of light variation. In 1925 Edwin Hubble applied the P-L Relationship to the ‘spiral nebula’ M31 and shattered the Milky Way boundary. The radius of the universe was extended to 1 million light years. It was a pity that Leavitt was not able to share the moment of glory. She died of cancer in 1921. ■甄枝強 資深天文愛好者
|
放大圖片
