哈勃望远镜拍摄的苍白旋涡星系NGC 4921
影像来源与版权:NASA, ESA, Hubble; Processing & Copyright: Kem Cook (LLNL) & Leo Shatz

说明:旋涡星系NGC 4921距离我们有多远?知道这一点非常重要。虽然它目前的估计距离约为3亿光年远,但是一次更加精确的测定再加上其已知的退行速度,能够帮助人类更好地校准整个可见宇宙的扩张速度。为了实现这个目标,哈勃太空望远镜拍摄了数张照片,以帮助确定造父变星的关键性恒星距离指标。由于NGC 4921是后发星系团的成员,因此精确测定它的距离也能更好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和表面亮度低,壮丽的旋涡星系NGC 4921被非正式地称为苍白星系。在这幅特征影像中,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,最近形成的蓝色星团,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。
翻译:北京天文馆

Anemic Spiral NGC 4921 from Hubble
Image Credit: NASA, ESA, Hubble; Processing & Copyright: Kem Cook (LLNL) & Leo Shatz

Explanation: How far away is spiral galaxy NGC 4921? It’s surpringly important to know. Although presently estimated to be about 300 million light years distant, a more precise determination could be coupled with its known recession speed to help humanity better calibrate the expansion rate of the entire visible universe. Toward this goal, several images were taken by the Hubble Space Telescope in order to help identify key stellar distance markers known as Cepheid variable stars. Since NGC 4921 is a member of the Coma Cluster of Galaxies, refining its distance would also allow a better distance determination to one of the largest nearby clusters in the local universe. The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the featured image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.
Astronomy Picture of the Day 05/15/2019