There is a maximum height of a uniform vertical column made of any material that can support

Question:

There is a maximum height of a uniform vertical column made of any material that can support itself without buckling, and it is independent of the cross-sectional area (why?). Calculate this height for
(a) Steel (density = mss/volume = 7.8 X 103 kg/m3). And
(b) Granite (Density = 2.7 X 103 kg/m3).