What is a sextillian in decimal form and as a power of ten?

A sextillian is a thousand pentallion, which in turn is a thousand quadrillion, which is a thousand billion. That makes it:

Sextillian = 1000 * Pentallion

= 1000 * 1000 * Quadrillion

= 1000 * 1000 * 1000 * Trillion

= 1000 * 1000 * 1000 * 1000 * Billion

= 1000 * 1000 * 1000 * 1000 * 1000 * Million

= 1000 * 1000 * 1000 * 1000 * 1000 * 1000 * Thousand

= (10^3)^6 * Thousand

= 10^21

That makes it 21 zeroes!