北河三

北河三，即雙子座β星（β Geminorum，縮寫為β Gem），是一顆位於雙子座，距離太陽約34光年，已經演化呈現橙色色調的巨星。它是最接近（英语：list of nearest bright stars）太陽的巨星。 自1943年迄今，這顆恆星的光譜是其它分類的依據標準之一。在2006年，發現它有一顆行星，被命名為北河三b（β Gem b），並已確認它的軌道。 依巴谷天文測量衛星以視差測得它與太陽的距離大約是33.78光年（10.36秒差距） 。 北河三的視星等為1.14等，在亮星表中，它比同星座的北河二（雙子座α）還要明亮。北河三和太陽相比，質量約為太陽的2倍，直徑約為太陽的9倍。北河三原本是一顆A型主序星，但已經耗盡了核心的氫燃料，並演化成為光譜類型為K0III的巨星 。這顆恆星外面殼層的有效溫度大約是7003466600000000000♠4666 K，位於K型恆星呈現橙色色調的範圍內 。北河三的投影轉速是2.8 km•s−1。除了氫和氦之外的其他元素豐度，也就是金屬的豐度，估計範圍從太陽的85%至155%不等，還沒有正確的數值（需要注意的是天文學家所說的金屬，是指氫和氦之外的所有其他元素）。 軌道上的倫琴衛星（ROSAT）探測到微弱的X射線輻射，這是北河三有低水準磁活動的證據。來自這顆恆星的X射線輻射大約是1027 erg s−1，大約與來自太陽的相等。已經證實北河三的表面磁場強度低於1高斯，這是在恆星上發現的最微弱磁場之一。磁場的存在表明，北河三曾經是一顆具有更強磁場的Ap星。恆星的徑向速度有小振幅的變化，但不是光度學上的變化 。 自1993年起，科學家就測量到徑向速度的振盪，因而懷疑北河三有行星環繞著。在2006年6月16日證實並宣布它有一顆行星。計算出北河三b的質量至少是木星的2.3倍，環繞北河三的周期大約是590日。 畢宿五 北河二 科幻小說中的北河三（英语：Stars and planetary systems in fiction#Pollux (Beta Geminorum)） 資料 van Leeuwen, F., Validation of the new Hipparcos reduction, Astronomy and Astrophysics, November 2007, 474 (2): 653–664, Bibcode:2007A&A...474..653V, arXiv:0708.1752 , doi:10.1051/0004-6361:20078357  Ducati, J. 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