Astronomers can measure it by way of luminosity which is the power of a star or the amount of energy (light) the star admits from its surface. they also measure the brightness of a start as if it were to appear 32.6 light years from Earth
Steam is made from boiling things and water vapor is from boiling water.
To measure length scientists may use rulers, meter sticks, etc. and to measure mass they may use a balance.
