Magnitude is the measure of the brightness of a celestial object. It applies to all objects, stars, planets, galaxies, supernovae … Astronomers of antiquity called stars of the first magnitude (1) those that were brighter and that were already seen at sunset. And successively, stars of magnitude 2, 3, 4 … even the dimmest ones, magnitude 6, which could only be seen in total darkness. So, in theory, magnitude 6 is the smallest measure of brightness that could be seen with the naked eye. But you have to keep a few more things in mind. The magnitude is logarithmic, and that means that from 1 to 2 does not increase the same as from 2 to 3, and so on … the more magnitude it becomes, much weaker. That magnitude 6 would correspond, for example, to the planet Uranus. But today, Uranus cannot be seen with the naked eye in places with light pollution.
To give you an idea of what can actually be observed in the sky with the current light pollution conditions, if you go to the Big Dipper, which everyone knows, and which is that constellation that is shaped like a dipper, and you start to look through the handle of the saucepan, locate the second star that is not binary (double) but looks as if it were. Today, in a place with a sky without excessive light pollution, people with good eyesight can see the binary of the second star of the mango. And that star is of magnitude 3.5. There are also people with very good eyesight who can discern, for example, the nebulosity of the Andromeda galaxy which is also attached to Ursa Major which also has a magnitude of 3.5.
The magnitude depends on the flux (amount of light) that reaches us from an object. And that flow also depends on how far away you are. A very bright object that is very far away will have a higher magnitude (we will see it less bright) than a less bright object that is closer and that will have a lower magnitude. An example is the Moon that does not have its own light but as it is very close and reflects the light of the Sun we see it bright. If we went far away, to Mars or Saturn, we would see it much weaker even though its brightness would be the same. Magnitude therefore is a relative measure.
It is very likely that the largest object that has been detected with the naked eye from Earth was a supernova. There are several types of supernovae. One of the best known is that of the giant stars (from 2-3 solar masses) that when they reach the end of their life explode. A series of physical processes take place that make the way in which these stars die is with an explosion. That explosion is very bright at first and then fades. When supernovae have been seen, in 1987 one was seen in the Large Magellanic Cloud, they are seen only in their first phase, then they stop being seen. And it also has to occur in a nearby area of the sky. Those that occur in distant galaxies are not seen from Earth.
Supernovae are very important and some help us to determine distances to other galaxies. Supernovae always shine the same, that is, when they explode we know their maximum brightness, we know their absolute magnitude. So it’s a matter of scaling the distance. If, for example, you see a supernova explode in the Andromeda galaxy, since you know its absolute magnitude, you can calculate the distance.
Elena Manjavacas is a doctor in astrophysics and researcher at the Space Telescope Science Institute (Baltimore, United States)
Question sent via email by Jaume Moreno Roca
Coordination and writing: Victoria Toro
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