2025年1月14日
North Star: Polaris and Surrounding Dust
Image Credit & Copyright: Davide Coverta
Explanation: Why is Polaris called the North Star? First, Polaris is the nearest bright star toward the north spin axis of the Earth. Therefore, as the Earth turns, stars appear to revolve around Polaris, but Polaris itself always stays in the same northerly direction — making it the North Star. Since no bright star is near the south spin axis of the Earth, there is currently no bright South Star. Thousands of years ago, Earth’s spin axis pointed in a slightly different direction so that Vega was the North Star. Although Polaris is not the brightest star on the sky, it is easily located because it is nearly aligned with two stars in the cup of the Big Dipper. Polaris is near the center of the five-degree wide featured image, a digital composite of hundreds of exposures that brings out faint gas and dust of the Integrated Flux Nebula (IFN) all over the frame. The surface of Cepheid Polaris slowly pulsates, causing the famous star to change its brightness by a few percent over the course of a few days.
Today: Zoom APOD Lecture hosted by the Amateur Astronomers of Association of New York
Tomorrow’s picture: north nebula
指北星: 北极星及其周围的尘埃
影像提供与版权: Davide Coverta
说明: 为何北极星有指北星之称？首先，目前北极星是北天离地球自转轴最近的亮星。因此，随着地球自转，北天的恒星也看似在旋绕北极星，而北极星本身则恒定不动，故为指北星。不过，在南天的地球自转轴指向附近，目前没有亮星，故无南极星。数千年前的地轴指向与现在稍有不同，在那时织女星曾是指北星。虽然北极星并不是夜空中最明亮的恒星，但要找到它并不难，因为它与北斗七星斗瓢的两颗星几乎连成一直线。上面这幅数位组合了数百张照片、以突显共照耀星云（IFN）的昏暗云气和尘埃之5度宽主题影像里，北极星位于中心附近。分类为造父变星的北极星，在为期数天期间，这颗著名恒星的表面会缓缓脉动胀缩，造成数个百分点的亮度变化。
明日的图片: north nebula
