I'm a physics (and electronics and astronomy, etc.) enthusiast. As I learn and research topics, I notice that many SI units are often expressed using a variety of prefixes, such as in electronics where we use microvolts, millivolts, volts, kilovolts, and sometimes megavolts. In computer storage, the prefixes kilo, mega, giga, and tera are very familiar.
In physics, however, for large quantities of mass, I usually just see kilograms used with scientific notation:
$$2\times10^6 kg$$
This could also be expressed as 2 gigagrams, but I've never heard anyone use that particular unit (which might be why it sounds silly). I understand that it is impractical to use a prefix for something as large as the mass of the sun, $2\times10^{30}$ kg, but wouldn't it be more appropriate to use grams, as in $2\times10^{33}$g?
Is this simply out of convention, or is there a more logical reason?
Best Answer
It's a weird quirk of the SI system that the base unit of mass is the kilogram, not the gram. So you'll see a lot of things expressed in kilograms.
Of course, scientists in a given field tend to standardize on certain choices of units without any regard to the SI recommendations. And this makes sense; the units you use should be the ones that make your values most understandable for the intended audience. SI is only intended as a fallback to enable unambiguous communication between groups that don't otherwise have a shared convention (especially between experimentalists and theorists). So sometimes you'll see quantities expressed in grams or tons or solar masses or whatever because that is the standard in the context you're looking at.