The following table shows how a difference in magnitudes translates to a difference in brightness. If you multiply it by itself five times you get one hundred. Now, this may seem a strange number to use but there is a good reason for it. With the stellar magnitude scale however we do not use a simple number like two to multiply each step instead we use approximately 2.512. For example 1, 2, 4, 8, 16, 32, 64 is a progression where each number is twice the previous one. A geometric progression is one where each number is the previous one multiplied by a fixed amount. The magnitude scale is what is known as a geometric progression and this can often be confusing, particularly for beginners. If you wish you can skip this and go straight to the next section. This shaded section delves a little more deeply into the mathematics of the magnitude scale.
The planets out to Uranus are all visible to the naked eye at some time although Uranus will require a clear, dark sky free from light pollution and a keen eye. The table of Solar System objects below shows the magnitudes of our nearest neighbours when they are at their brightest. Because of this, the apparent brightness and hence magnitudes of the planets varies as viewed from Earth. It is not just stars which can be allocated magnitude values for brightness, planets too, and other bodies which either emit or reflect light, including comets, asteroids, moons, and more distant entities including nebulae, star clusters and galaxies.Īs the planets in our solar system orbit the Sun their distances from both the Sun and the Earth change. Those from 1.5 to 2.5 are second magnitude, 2.5 to 3.5 third magnitude and so on. Generally, stars with magnitudes down to 1.5 are considered to be of the first magnitude. The table below, extracted from data in the BAA Handbook, lists these 21 stars: Star There are 21 first magnitude stars, 15 of which are visible from at least some part of the UK. A few stars and some solar system objects can appear brighter than magnitude 0.0 and are given negative magnitudes.Īt the other end of the scale, fainter and fainter objects have higher and higher magnitudes and the limit is continually being pushed back with the building of larger telescopes and more sensitive detectors. The scale continues beyond the six magnitudes visible to the naked eye to fainter stars with seventh, eighth, ninth magnitudes and so on. Both are first magnitude stars, but Rigel appears brighter than Procyon. So for example the bright star Rigel, in the constellation of Orion has a magnitude of 0.18, while the star Procyon, not far away in Canis Minor has a magnitude of 0.40. We retain the concept of six magnitudes but add precision by using continuous numerical values. This is obviously quite a coarse measure and in the 19th century it was refined into the numerical system we use today.
The key point to remember is that the larger the numerical magnitude the fainter the star. Those of the second magnitude were fainter and so on down to the faintest stars visible to the naked eye which he called sixth magnitude. The brightest ones he called stars of the first magnitude. He divided the stars into the six groups that we now call magnitudes (strictly the correct term is apparent magnitude but magnitude is generally used). One of the first people to attempt to categorise the brightness of stars was the Greek astronomer Hipparchus in the second century BC. Finally there is a discussion of how stars can have different brightness in different colours. The concepts of apparent and absolute magnitude are introduced which allow meaningful comparisons of the stars’ true brightness to be made. This tutorial introduces the stellar magnitude scale which is used to describe the brightness of the stars and a method is described which predicts the faintest stars any given telescope will show. Having a system to describe a star’s brightness is useful for many reasons in astronomy, including the scientific study of variable stars and describing the brightness of a new object in the sky such as a nova, supernova or comet. The larger the aperture of the telescope or binoculars the fainter the stars you can see. Scanning the sky with a pair of binoculars or a telescope will bring many fainter stars into view. Some are bright while others are at the limit of naked eye visibility and everything in between. (Image courtesy Andrew Paterson).Looking at the stars on a dark, clear night one of the most obvious features is that they are of different brightness (Figure 1). The stars come in many different levels of apparent brightness.