This NASA/ESA Hubble Space Telescope image features the blue dwarf galaxy Markarian 178 (Mrk 178) against a backdrop of distant galaxies in all shapes and sizes. Some of these distant galaxies even shine through the diffuse edges of Mrk 178.
ESA/Hubble & NASA, F. Annibali, S. Hong
This NASA/ESA Hubble Space Telescope image features a glittering blue dwarf galaxy called Markarian 178 (Mrk 178). The galaxy, which is substantially smaller than our own Milky Way, lies 13 million light-years away in the constellation Ursa Major (the Great Bear).
Mrk 178 is one of more than 1,500 Markarian galaxies. These galaxies get their name from the Armenian astrophysicist Benjamin Markarian, who compiled a list of galaxies that were surprisingly bright in ultraviolet light.
While the bulk of the galaxy is blue due to an abundance of young, hot stars with little dust shrouding them, Mrk 178 gets a red hue from a collection of rare massive Wolf–Rayet stars. These stars are concentrated in the brightest, reddish region near the galaxy’s edge. Wolf–Rayet stars cast off their atmospheres through powerful winds, and the bright emission lines from their hot stellar winds are etched upon the galaxy’s spectrum. Both ionized hydrogen and oxygen lines are particularly strong and appear as a red color in this photo.
Massive stars enter the Wolf–Rayet phase of their evolution just before they collapse into black holes or neutron stars. Because Wolf–Rayet stars last for only a few million years, researchers know that something must have triggered a recent burst of star formation in Mrk 178. At first glance, it’s not clear what could be the cause — Mrk 178 doesn’t seem to have any close galactic neighbors that may have stirred up its gas to form new stars. Instead, researchers suspect that a gas cloud crashed into Mrk 178, or that the intergalactic medium disturbed its gas as the galaxy moved through space. Either disturbance could light up this tiny galaxy with a ripple of bright new stars.
Image credit: ESA/Hubble & NASA, F. Annibali, S. Hong
这张NASA/ESA哈勃太空望远镜拍摄的图像展示了蓝色矮星系马卡良178（Markarian178，简称Mrk178），背景中分布着形态各异、大小不一的遥远星系。其中一些遥远星系甚至透过Mrk178弥散的边缘清晰可见。
图片来源: ESA/Hubble & NASA, F. Annibali, S. Hong
这张NASA/ESA哈勃太空望远镜图像呈现了一座闪耀的蓝色矮星系——马卡良178（Mrk178）。该星系的体量明显小于我们的银河系，距离地球约1300万光年，位于大熊座。
Mrk178是1500多个马卡良星系之一。这类星系得名于亚美尼亚天体物理学家本杰明·马卡良，他编制了一份在紫外波段异常明亮的星系目录。
由于星系中富含年轻、炽热且几乎未被尘埃遮蔽的恒星，Mrk178整体呈现蓝色；但在其边缘最明亮、略显红色的区域，分布着一群罕见而大质量的沃尔夫–拉叶星，使星系局部呈现红色调。这类恒星通过强烈的恒星风不断抛射自身大气，其炽热风流产生的明亮发射线清晰地刻画在星系的光谱中。电离氢和氧的发射线尤为强烈，在这张图像中表现为红色。
大质量恒星在坍缩成黑洞或中子星之前，会短暂进入沃尔夫–拉叶阶段。由于这一阶段仅持续数百万年，研究人员由此判断，Mrk178中必然发生过一次近期的恒星形成爆发。乍一看，其成因并不明显，因为Mrk178附近并不存在能够搅动其气体、触发恒星形成的近邻星系。研究人员推测，可能是一团气体云撞击了Mrk178，或该星系在穿行宇宙空间时受到星系际介质的扰动。这两种扰动中的任何一种，都可能点燃这座微小星系，激起一圈明亮的新生恒星。
图片来源: ESA/Hubble & NASA, F. Annibali, S. Hong
