HST emission-line images of five of the arguably closest planetary nebulae have shown that there is a progression of characteristics of their knots. This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-à-vis ionization shadowing. The one object that does not fit into this sequence is NGC 2392, which is the most complex nebula in our sample. In this case the inner part of the nebula is composed of a series of loops of material, some being ionization bounded, which cover only a small fraction of the area illuminated by the star. This complex structure may be what gives rise to the large variations in electron temperature inferred from low spatial resolution observations. Cometary-form knots are seen in the outer part of this object, with these objects closely resembling those found in the largest nebula in our sample, NGC 7293.
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机译：可以说是最近的五个行星状星云的HST发射线图像表明，其结的特征在不断发展。这种进展始于暗切向结构，显示与中心恒星和主电离前沿附近的位置不对齐。在最大星云的行进结束时，这些结位于整个电离区的大部分区域，在那里它们在面向中心恒星的一侧被光电离，并伴有沿径向排列良好的长尾巴。如果这些结在主电离前沿附近或外部形成，并获得足够高的密度，导致结在被莱曼连续谱（Lyc）辐射场完全照亮时仅被部分电离，那么这种特性的改变是可以预期的。它们的膨胀速度必须低于星状壳主体的膨胀速度。尽管尚不清楚辐射压力相对于电离阴影作用在尘埃成分上的相对作用，但它们的形式通过暴露于恒星的辐射场而改变。 NGC 2392是不适合此序列的一个物体，它是我们样本中最复杂的星云。在这种情况下，星云的内部由一系列物质环组成，其中一些物质被电离限制，仅覆盖恒星照射区域的一小部分。这种复杂的结构可能会导致由低空间分辨率观测结果推断出的电子温度的巨大变化。在该天体的外部可以看到彗星状的结，这些天体与我们的样本NGC 7293中最大的星云中的天体非常相似。
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