As you probably know already, a star is a luminous sphere of plasma held together by its own gravity. The Sun is a pretty spectacular example of a star – and it’s also the closest one to our planet, Earth. If you don’t live in a pollution-infested city, you’ll most likely have seen stars at night. We’re able to see a number of stars from Earth with our eyes and nothing else; they appear as fixed luminous points in the sky because they’re so far away.
Right, now we’ve got the basics out of the way, it’s time to turn to the nitty gritty of the most important things you’ll need to know about stars before your exams. For starters, when a star is forming, the most important factor in determining the type of star it will become is its mass. This is because a star begins life as a protostar, which is when a cloud of dust and hydrogen collapse under their own gravity, which heats the core. For low masses, there isn’t enough heat to star nuclear fusion, and so the star becomes a brown dwarf. Otherwise, the star will join the main sequence and begin to form.
And if – for some reason – you’re asked to calculate the brightness of a star, the best way to do this is using this equation: b=L/(4*pi*R^2). If you’re as lost as I am as to why this is, it’s because where L is the luminosity of the star (total amount of energy it radiates per second) R is the distance away. This equation follows the inverse square law. Brightness has units of watts per square metre.
If you need to explain how absolute magnitude is defined, you would simply say that where M is the apparent magnitude, M is the distance of the star from the Earth in parsecs. From here, the distance to nearby stars can be found using trigonometric parallax, which relies on the fact the Earth orbits the Sun.
Another thing you should know about stars is that they can be black bodies. What on earth is a black body, you ask? In short, an object that absorbs all incident from electromagnetic radiation in accordance with Planck’s Law. Riveting, I know… But anyway, moving on! It’s important to note that in order for the body to remain at a constant temperature, after absorption it must then re-emit the radiation in order to remain at thermal equilibrium.
When working under the assumption that a star is a black body, Stefan’s Law comes into play. Sometimes referred to as the Stefan-Boltzmann law, it describes the power radiated from a black body, in this case a star. Where sigma is the Stefan-Boltzmann constant, A is the surface area and T is the temperature. If the object were not a black body, it would be necessary to multiply the terms on the right by the emissivity, epsilon (which is always less than one for so-called grey bodies).