Planetary nebula classification
Planetary nebulae are classified according to a scheme developed by Vorontsov-Velyaminov. This classification takes the shape of a Roman numeral, from I to VI, followed to a letter:
Out of 200 billion stars, about 1500 planetary nebulae are exist in our galaxy are known to exist.
I: Stellar image (like a star);
II: Regular disc:
III: Irregular disc:
IV: Annular structure
V: Irregular form between a planetary nebula and diffuse nebula;
VI: Abnormal form without a regular structure (like an S or an 8, etc).
Some more complex object shapes can be characterized by two classes, the second being written between brackets.
Other deep sky objects' classification: