Astronomy Picture of the Day——哈勃望远镜拍摄的苍白旋涡星系NGC 4921
Anemic Spiral NGC 4921 from Hubble
Image Credit: NASA, ESA, Hubble; Processing& Copyright: Kem Cook (LLNL) & Leo Shatz
Explanation: How far away is spiral galaxyNGC 4921? It's surprisingly important to know. Although presently estimated tobe about 300 million light years distant, a more precise determination could becoupled with its known recession speed to help humanity better calibrate the expansionrate of the entire visible universe. Toward this goal, several images weretaken by the Hubble Space Telescope in order to help identify key stellardistance markers known as Cepheid variable stars. Since NGC 4921 is a member ofthe Coma Cluster of Galaxies, refining its distance would also allow a betterdistance determination to one of the largest nearby clusters in the localuniverse. The magnificent spiral NGC 4921 has been informally dubbed anemicbecause of its low rate of star formation and low surface brightness. Visiblein the featured image are, from the center, a bright nucleus, a bright centralbar, a prominent ring of dark dust, blue clusters of recently formed stars,several smaller companion galaxies, unrelated galaxies in the far distantuniverse, and unrelated stars in our Milky Way Galaxy.
哈勃望远镜拍摄的苍白旋涡星系NGC 4921
旋涡星系NGC 4921离我们有多远？知道这一点非常重要。虽然现在估计这个距离有3亿光年，但是对它的一次更精确的测量联同它的已知退行速度，能够帮助人类校准整个可见宇宙的扩张速度。为实现这个目标，哈勃拍摄了数张照片，以帮助确定造父变星的关键性距离指标。因为NGC 4921是后发座星系的一员，因此精确测量它的距离也能更精确地确定近域宇宙中最大邻近星系团之一的天体距离。因其恒星形成速度慢且表面亮度低，壮丽的旋涡星系NGC 4921被民间称为苍白星系。在这幅特征图像中，从中央向外依次是：明亮的星系核，明亮的中央棒，显著的暗尘环，形成不久的蓝色星团，数个较小的伴星系，遥远宇宙中不相关的星系以及我们银河系中不相关的恒星。
参考文献：
https://apod.nasa.gov/apod/image/1905/NGC4921_HubbleShatz_4046.jpg
近期文章：
Astronomy Picture of the Day——哈勃拍摄的年轻星团特朗普勒14