Editing Astronomical object (section)

==Shape{{anchor|Shapes}}==
{{further|Spherical Earth#Cause}}
{{see also|Equatorial bulge|Hydrostatic equilibrium#Planetary geology}}

[[File:Eros, Vesta and Ceres size comparison.jpg|thumb|Composite image showing the round dwarf planet [[Ceres (dwarf planet)|Ceres]]; the slightly smaller, mostly round [[4 Vesta|Vesta]]; and the much smaller, much lumpier [[433 Eros|Eros]]]]

The [[IAU definition of planet|IAU definitions of planet]] and [[dwarf planet]] require that a Sun-orbiting astronomical body has undergone the rounding process to reach a roughly spherical shape, an achievement known as [[hydrostatic equilibrium]]. The same [[spheroidal]] shape can be seen on smaller rocky planets like [[Mars]] to [[gas giant]]s like [[Jupiter]].

Any natural Sun-orbiting body that has not reached hydrostatic equilibrium is classified by the IAU as a [[small Solar System body]] (SSSB). These come in many non-spherical shapes which are lumpy masses accreted haphazardly by in-falling dust and rock; not enough mass falls in to generate the heat needed to complete the rounding. Some SSSBs are just collections of relatively small rocks that are weakly held next to each other by gravity but are not actually fused into a single big [[bedrock]]. Some larger SSSBs are nearly round but have not reached hydrostatic equilibrium. The small Solar System body [[4 Vesta]] is large enough to have undergone at least partial planetary differentiation.

Stars like the Sun are also spheroidal due to gravity's effects on their [[plasma (physics)|plasma]], which is a free-flowing [[fluid]]. Ongoing [[stellar fusion]] is a much greater source of heat for stars compared to the initial heat released during their formation.