Apparent and Absolute Magnitude in Context of Celestial Bodies

Originally, the apparent magnitude scale was divided into only 6 divisions from 1-6 (brightest to dimmest), mostly including distant stars and planets. However, with the inclusion of objects such as the sun (-26.8 M) and the moon (-12.6 M), the scale was extended to even negative values.

When you look towards the night sky, why is it that some stars appear brighter than the others, and why do planets such as Jupiter, Saturn, and Venus appear to be brighter than most stars? Such questions bounced around since man took an interest in astronomy.

Brightness is a fundamental property of any celestial body that is studied to determine the size and chemical make-up of an object, besides finding out other facts such as its age and the distance of the object from earth and other celestial bodies.

Magnitude is the unit used to describe the degree of brightness of a celestial object. However, the measurement is not in the same scale as a kilogram or a mile. Each magnitude is 2½ times brighter than the previous one, and larger magnitudes are denoted to dimmer objects.

Very bright objects such as the sun and the moon are on the far extreme of negative values. However, magnitudes are also divided into two types, apparent magnitude and absolute magnitude. Let us look at each of them in detail, and see how they help us to gain new knowledge and insight into our huge and fascinating universe.

What is it?: Like any source of light, the light from celestial bodies gets dimmer the further away they are from Earth. Apparent magnitude is a measure of how bright a star appears to us on earth, without the interference of the atmosphere, and explains why stars and planets appear very dim or very bright to us.

However, since this magnitude is based on the variable distance of the stars, apparent magnitude only tells us how bright a celestial body appears and not how bright it really is.

What does it tell us about a star?: Brighter objects have a lower apparent magnitude. Sirius is the brightest star in the visible color spectrum, while Betelgeuse is the brightest in the near-infrared spectrum.

Another important thing to note while studying the apparent magnitude is the color of the stars. For this reason, the UVB system is used to measure the ultraviolet, blue, and visible spectrum of light emitted by a celestial body.

Using these parameters we can say that there a few factors that determine a star's apparent magnitude like its distance from earth. For example, the apparent magnitude of Jupiter changes from 1.6 to -2.6 depending on where it's in its orbit. Other factors are the wavelength of the light emitted by the object, the temperature of the body, its size and age.

The scale of apparent magnitude is based on Pogson's Ratio, which means that the scale is logarithmic. For example, if the difference of apparent magnitude between two stars is 1.76, one star would be approximately 5 times as bright as the other.

What is it?: Apparent magnitude helps astronomers to compare stars, and is popularly used in star charts and maps, as reference guides. On the other hand, absolute magnitudes help astronomers know which stars/celestial bodies are intrinsically the brightest.

This is the measure of the brightness from a body, if it were at a precise distance of 10 parsecs from Earth. One parsec is equal to 3.26 light years, which means 10 parsecs is 32.6 light years from Earth.

Luminosity: Another important method of telling the intrinsic brightness of an object, is to find out its luminosity, i.e., see how much energy and light it emits in a fixed time frame. This is usually measured in Watts. For example, the sun's luminosity is 400 trillion trillion watts.

It is important to note that a difference of 5 on the absolute magnitude, between two objects is equal to a difference of a hundred times on the luminosity scale. The absolute magnitude depends on factors like mass, luminosity, and temperature of the object.

If we know the values of the apparent and absolute magnitudes of an object, we can find how far it is from Earth, using a simple formula.

Distance to Earth= 10 x (Apparent magnitude - Absolute magnitude +5/ 5).
It is a simple formula, that can be used to find the distance of any luminous object from Earth.

Similarly, if we know the distance between earth and the celestial object, along with its apparent magnitude, we can use a simple formula to find its absolute magnitude.

5 x (log10 distance) -[5 +Apparent magnitude] = - absolute magnitude.

If you use these formulas to obtain the absolute magnitude and apparent magnitude of Sirius, the brightest star visible to the human eye, after the sun, you will find that the values are 1.4 and -1.46 respectively.

Similarly, if you use these formulas, you will notice, that for bodies which are closer than 10 parsecs from Earth, the absolute magnitude would be lesser in brightness than the apparent magnitude. The opposite would be true for bodies which are located more than 10 parsecs from Earth. For a celestial object that is located exactly 10 parsecs from Earth, the absolute and apparent magnitudes would be equal, such as Vega.