我们如何猜出银河系的样子的？

好像我们看到的也只是过去的宇宙的模样，所以最好用一个以时间为轴的立体图像描述我们所处的宇宙，况且宇宙中有这么多大的星体，更具广义相对论，光线到我们这里不知道已经被弯曲过多少次了，所以很难从银河系内知道银河系的确切模样，有点身在庐山中的味道！

不确切地说是根据观察河外星系的形状和记录银河系中恒星位置推测出来的。最早是恒星计数方法，现在主要通过旋臂示踪天体，例如21cm氢发射区（最主要）。

另外，银河系并不具有“蝶形外观”，据说是“碟子形的”，但是现在有新的证据表明银河系是棒旋星系，而非先前所认为的旋涡星系。

给段英文，你能看懂就看。
Because we live inside the Milky Way, it is very hard for us to determine its shape and size. It is clearly planar, but estimating it's exact shape and our position inside it was not done until the early 20th century.
Not only do we have a bad angle, but we also have to deal with dust in the plane of the galaxy. This dust obscures our view of most of the Milky Way.

The Milky Way's shape was first determined through studying globular clusters
Globular clusters are spherical clusters of up to 106 stars (usually Pop II), typically outside the plane of the galaxy.
The distance to globular clusters is measured with Cepheid and RR Lyrae variable stars.