鹰状星云、欧米茄星云、三裂星云和泻湖星云：四大著名星云

Nebula [ neb-yuh-luh ]: star-forming cloud of gas and dust. Noun, plural neb·u·lae [neb-yuh-lee, -lahy]

星云:由气体和尘埃组成的恒星形成云。名词，复数neb·u·lae [neb-yuh-lee， -lahy]

These four nebulae are known for their breathtaking beauty: the Eagle Nebula (which contains the Pillars of Creation), the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula. In the 1950s, a team of astronomers made rough distance measurements to some of the stars in these nebulae and were able to infer the existence of the Sagittarius Arm. Their work provided some of the first evidence of our galaxy’s spiral structure. In a new study, astronomers have shown that these nebulae are part of a substructure within the arm that is angled differently from the rest of the arm.

这四个星云以其令人叹为观止的美丽而闻名：鹰状星云（包含创世之柱）、欧米茄星云、三裂星云和泻湖星云。20世纪50年代，一组天文学家对这些星云中的一些恒星进行了粗略的距离测量，从而能够推断出人马座臂的存在。他们的工作为我们银河系的螺旋结构提供了首批证据。在一项新的研究中，天文学家已经表明，这些星云是旋臂内一个亚结构的一部分，该亚结构的角度与旋臂的其余部分不同。

A key property of spiral arms is how tightly they wind around a galaxy. This characteristic is measured by the arm’s pitch angle. A circle has a pitch angle of 0 degrees, and as the spiral becomes more open, the pitch angle increases. Most models of the Milky Way suggest that the Sagittarius Arm forms a spiral that has a pitch angle of about 12 degrees, but the protruding structure has a pitch angle of nearly 60 degrees.

旋臂的一个关键特性是它们绕星系旋转的紧密程度。该特性通过旋臂的俯仰角测量。一个圆的俯仰角为0度，随着螺旋变得更开放，俯仰角增大。银河系的大多数模型表明，人马座的旋臂形成一个俯仰角约为12度的螺旋，但突出的结构具有近60度的俯仰角。

Similar structures – sometimes called spurs or feathers – are commonly found jutting out of the arms of other spiral galaxies. For decades scientists have wondered whether our Milky Way’s spiral arms are also dotted with these structures or if they are relatively smooth.

类似的结构——有时被称为刺或羽毛——通常可以在其他螺旋星系的旋臂上发现。几十年来，科学家们一直想知道我们银河系的旋臂上是否也点缀着这些结构，或者它们是否相对平滑。

Image Credit: NASA/JPL-Caltech

图片来源：NASA/JPL加州理工学院