They discovered that star formation is pretty well non-existent until the surface density reaches a certain threshold in parts of the cloud.
他们发现恒星只有在表面密度达到一定的阀值时才会较好的形成。
2
But if the region has a strange shape or if a density is not homogeneous, if parts of it are heavier then you will get whatever the weighted average will be.
但如果这区域的形状很特殊，或者密度不均匀，也就是某些部分比较重，那就需要求加权平均值。
3
Electroplating: use the principle of electrolysis, the metal and alloy parts deposition in the surface, form even, density, good adhesion of the metal layer process.