In terms of stellar properties,
the fundamental attribute of a star is its mass.
Mass determines everything about a star,
from what type of fusion process can occur to its fate and eventual end point.
Mass is actually the hardest thing to measure directly for a star, for
an isolated star can only be done through a stellar model.
But luckily, many stars are in binary systems, and the binary orbit and
the parameters of the orbit, give the mass of both components.
In nature, the bounds on star mass are about 8% of the Sun's mass at the low end,
to roughly a hundred times the Sun's mass at the high end.
Second fundamental property of a star is its luminosity.
How many ergs per second or
watts that it puts out in a particular wavelength regime.
Or overall, across the energy spectrum, this is called bolometric luminosity.
The luminosity of stars on the main sequence, which is to say,
stars that are fusing hydrogen into helium as the Sun is, has an enormous range.
Going up to about a million times luminosity of the Sun
down to about one 10,000th of the luminosity of the Sun.
So, this is a fundamental attribute of stars.
Their mass ranges only by a factor of a hundred where
the luminosity ranges over a factor of 10 billion.
The other direct observable quantity of most stars is their temperature.
But it's important to remember that the temperature we talk about
is the surface or photosphere temperature,
the cool outer region where radiation travels freely to the observer.
This true action is taking place deep in the stellar
core at a temperature thousands of times higher,
typically millions of Kelvins if hydrogen is fusing into helium.
Stellar photospheres range from cool red stars with temperatures of 3,000 Kelvin or
so, maybe lower,
to extremely hot blue-white stars with temperatures of 50 or 60,000 Kelvin.
Because astronomers are able to take SPECTROstars to decide what they're made
of, they also classify ter, stars in terms of spectral types.
In this categorization, the Sun is a g star.
What is it that determines the range of stellar properties and
why are there bounds at either end in particular in mass?
It turns out that nature puts a limit on the mass of a star,
both at the high and the low end.
So, it's not possible to have stars that are in the mass of a galaxy.
Nor is it possible to have stars that are the mass of the earth.
The lower bound on a star is essentially the size at which the central
temperature never gets to the point where it confuse helium from hydrogen.
This corresponds to about 8% of the mass of the Sun.
Stars around this mass can actually do a feeble amount of deuterium burning,
which is the first of three steps of the proton-proton chain.
Where protons are fused to make a proton neutron combination called deuterium,
or heavy hydrogen.
This releases a rather feeble amount of energy,
substantially higher temperatures are acquired to create helium.
So, while nature may indeed collapse clouds of gas that are 1% of the mass of
the Sun or 0.1% or 5%, those clouds will get warm and hot in their interiors but
never hot enough to switch on like light bulbs and be stars in the sky.
The upper bound on mass is a little less well-determined and requires computer
simulations and theoretical calculations to understand in detail.