Neutron is commonly used to initiate a fission chain reaction.
The formula of density is given by
Density = Mass ÷ Volume
We have:
Mass = 1.989 × 10³⁰ kg
Volume =

=

km³
Density =

=1.13×10¹⁸ kg/km³
Converting 1.13 × 10¹⁸ kg/km³ to g/cm³
1.13 × 10¹⁸ kg = 1.13 × 10¹⁸ × 10³ = 1.13 × 10²¹ grams
1 km³ = 1 × 10⁶ cm³
(1.13 × 10²¹) ÷ 10⁶ = 1.13 × 10¹⁵ gr/cm³
Answer: Density 1.13 × 10¹⁵ gr/cm³