Stars: Colour, composition and mass

We on Earth perceive the stars covering the night sky as small shining spots of light. But in fact, they are huge balls of extremely hot gases, which produce incredible amounts of energy. This energy radiates from the stars as light so bright that we can see it from very very far away. Look up! You can see for yourself that some stars shine much stronger than others.

Does this mean the brighter stars radiate more light? Hm, let’s see... Look at a light bulb in your room and a street lamp at the end of your street. The street lamp surely emits more light but still the little light bulb seems brighter. This is because the light bulb is much closer to you.

If we were talking about stars instead, we would call it APPARENT MAGNITUDE because it describes how bright they appear to us on Earth. The Sun, for example, seems to shine much brighter than the North Star - Polaris. The Sun’s apparent magnitude is greater. But Polaris emits more than two thousand times more energy than the Sun! The amount of energy a star actually emits is the star’s LUMINOSITY.

So, how bright a star looks from Earth - its apparent magnitude - depends on how much energy it radiates - its luminosity - and also its distance from the Earth. Luminosity is related to temperature too - hotter stars radiate more energy, so they are brighter. But how do we know how hot a star is? If you light a match, the flame closest to the match where the temperature is the highest will be blue but further away as it cools down it will glow orange or red. The same applies to stars - the light they emit is different colours - from blue and white, to yellow, orange and red.

The colour we see depends on their surface temperature. Sometimes you can spot the differences just by looking at them! However, scientists like astronomers use special instruments to analyze the light emitted by stars more carefully. One such instrument, a spectrograph, works a little bit like a prism. It splits light into a pattern of different colours and lines - a spectrum.

All elements also have their own characteristic spectra. By comparing these characteristic patterns with the spectrum of light emitted by a star, scientists can identify what elements the star is made of - its COMPOSITION. The first scientist to understand the composition of the Sun and other stars was Cecilia Payne. Thanks to this method and Cecilia Payne’s findings we know that the majority of stars are composed mostly of hydrogen and helium, and a fraction of other elements. Methods like spectroscopy help us find out a lot about stars, but mass of stars other than the Sun remained a mystery for a long time.

The mystery was solved when astronomers discovered that sometimes two stars orbit around each other, forming BINARY SYSTEMS. By studying the time it takes for the stars to complete one orbit, and the size of the orbit, the astronomers could finally calculate the mass of each of these stars. They decided to compare them to the Sun and in this way solar mass became a standard measurement unit to express the mass of other stars. Some stars can be less than 0.1 [zero point one] solar masses while the most massive star known in the universe is more than 265 times heavier than the Sun! At first glance, all stars might seem identical, but now you know how different they can be.

And who knows how much more there is to discover…?