Stars form when clouds of interstellar gas made from hydrogen molecules cool and collapse under the influence of gravity. This primordial soup is referred to by astronomers as a nebula. The colder the cloud the less resistant the materials are to gravitational collapse.
If the materials accrete into a sufficient mass they will give rise to a protostar. From this point it will start to collapse, and the internal temperature and pressure will rise until the point of ignition when fusion begins, and the star will shine. In protostars of less than 0.08 of Solar mass the pressure and temperature at the core is insufficient to initiate nuclear fusion and they become brown dwarfs and are generally found to be between 13 to 80 times the mass of Jupiter.
During their lifetime stars pass through various stages, with their behaviour and the timing of life events depending on their mass. This is the single most important factor in determining the birth, career path and the manner of death of all stars. The majority of them, about 90% of all stars, will go onto the “Main Sequence”. This refers to an important feature of the Hertzsprung-Russell Diagram, which is described in detail in a separate entry.
Our Sun is a very good example of a main sequence star where it has already resided in a stable state for five billion years, converting hydrogen to helium and producing photons in the form of both visible and invisible light, x-rays, gamma rays and neutrinos. It will be another five billion years before the sun leaves the main sequence to begin more complex fusion processes, creating heavier elements such as oxygen and carbon. However, only the most massive stars are able to progress further up the periodic table until they eventually form iron in their core, and the process halts.
The luminosity of a star is its intrinsic brightness at visible wavelengths and is measured as the total energy it radiates per second. It is an important measure for determining the size of a star, its composition and its distance from Earth.
This brightness gives the apparent magnitude of a star, without taking distance into account. Two stars appearing in our night sky with the same luminosity will unlikely be at the same distance. Indeed, their distances from Earth will often vary by millions of light years. Sirius is visually the brightest star in our night sky but is not the brightest in absolute terms.
Once a star’s distance is known its absolute magnitude can be calculated. This will give its intrinsic brightness from which its true luminosity can be determined. For the stars that are of similar chemical composition their luminosities will indicate their mass. How we determine their composition is by measuring their spectral lines. This is explained further on.
Note: If you come across the term “bolometric luminosity” this refers to the luminosity of a star across all wavelengths, that is, both visible and non-visible light.
The scientific measure of luminosity is in joules per second, equivalent to watts (one unit of power). For comparison a star might be rated in terms of “solar luminosity”. The Sun has a total power output of 3.846 x 1026 watts, and this is called “one solar luminosity”. Absolute magnitude, apparent magnitude and distance are interrelated parameters: if you know two of them, you can work out the third.
All these features are explained in more detail elsewhere.
Magnitude, luminosity and brightness
A scale of brightness was first introduced by Hipparchus in 125 BC. In terms of this, brightness is measured as an inverse magnitude: the larger the number the dimmer the star. For example, a magnitude of 1 is about 100 times brighter than a magnitude of 6—which happens to be the faintest star visible to the naked eye. It is for this reason that very bright stars have a negative value!
The brightness of stars to our view is affected by their distance from Earth. Brightness is inversely proportional to the square of the distance. Put simply, if you double the distance, the brightness reduces to one quarter; if the distance is halved then the brightness increases fourfold. Whatever the case this does not indicate the intrinsic or real brightness of a star. Indeed, a star close to Earth might appear brighter than a more distant star; yet that distant star could in reality be many times brighter.
|Some of the brightest stars (in order of visual brightness)|
|Name||Apparent magnitude (m)||Absolute magnitude (M)||Distance (in light years)|
|Alpha Centauri A||-0.26||+4.38||4.37|
|As a comparison|
|*This is the best guess for the distance to Deneb. I have seen a figure suggested of as high as 7 400, testifying to the difficulty of accurately measuring extreme distances. Absolute magnitude (M) is not an exact science and several authoritative sources disagree on some of the numbers. Apparent magnitude (m) is the brightness you discern with the naked eye.|
Astronomers have devised Absolute Magnitude as a way to compare all stars as if they were at the same distance from Earth. The standard distance used is 10 parsecs (equivalent to 32.6 light years). So the absolute magnitude of a star is equal to its visual, or apparent magnitude it would have if it were at the standard distance of 32.6 light years. In calculations and so on you will see absolute magnitude denoted with “M” and apparent magnitude by “m”.
Let’s look at an example. The apparent (visual) magnitude of Sirius is -1.46. Its parallax is 0.376”. By algebraic calculation its absolute magnitude is +1.42. Sirius is 8.67 light years away, or 2.66 parsecs. The calculation for our sun gives it an absolute magnitude of 4.83, far dimmer than Sirius. This puts Sirius at more than 20 times brighter than the sun. Canopus, with an absolute magnitude of -8.5, is 218 900 times brighter than the sun.
By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Wednesday, 3rd March 2021