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The sun is obviously the brightest source of light in the sky.
Right? Right?
Well now, it depends on what you mean by "light". Usually – and familiarly, and even to an astronomer if they are not too pedantic – when you simply say "light," you mean "visible light," the kind of light we see. But from blue to red, we find a very small slice of a much wider spectrum of light, literally called the electromagnetic spectrum. It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. What we consider colors are actually different wavelengths (or energies) of light, and it goes far beyond what our eyes are sensitive to.
This is a big reason why we build telescopes. We can equip them with sensors sensitive to these different types of light and tell us in turn what the Universe looks like at these wavelengths, revealing otherwise invisible objects and energy sources.
In most of these wavelengths, the sun dominates. That's because it's close to us! Many things are brighter, give off more energy, but they are so far away that the sun dominates.
The Moon is even closer to us than the Sun, but we see it because it reflects The light of the Sun is transmitted to us, so that the electromagnetic spectrum is much less luminous (in visible light, the Sun is about 200,000 times brighter than the Moon).
But if you look in the gamma rays, it changes.
Gamma rays are at the upper end of the spectral scale, but they cover a wide range of energies. The lower limit is not well defined, but corresponds to about 2 million times the energy of light we see (at these energies, astronomers and physicists like to use an energy unit called electron-volt (or eV), the visible light has an energy of about half a eV and the gamma rays start around one million EV (also called mega-electron-volt or MeV)).
Gamma rays do not enter our atmosphere, so we launch space telescopes to see them coming from cosmic objects. The Fermi gamma ray space telescope is a very sophisticated observatory, able to see these high-energy photons – it can even tag them according to the reception of each photon and the amount of energy it contains . Fermi has observed much of the sky and sometimes the Moon is in his field of vision. Some astronomers went into the database and examined all the gamma rays seen from the Moon above 30 MeV and found that the Moon was brighter at these energies than the Sun!
Gamma rays are produced only by very high energy events, however, like exploding stars and black holes engulfing matter, and places where magnetic fields are extremely powerful. The moon is basically a big rock in space, without a magnetic field, and certainly not enough energy to produce gamma rays. So where do they come from?
Cosmic rays! These are subatomic particles crossing the space at a speed close to that of light, accelerated by many of the same phenomena that produce gamma rays (an important source is subatomic particles slamming around the shockwaves d & rsquo; A star or a gas exploding from galaxy nuclei, they bounce, gain incredible energy and spin at a speed close to light). They travel across the galaxy and then slam on the surface of the moon. It deposits so much energy that the nuclei of the atoms on the surface of the Moon are energized and relieve this excitation by emitting gamma rays.
That's what Fermi detects. In the images above, Fermi does not have the necessary resolution to visualize the characteristics of the Moon. He therefore sees only a faint glow. In short exposures, it is difficult to see the shape of the moon, but by adding more and more observations of gamma rays (essentially creating a longer exposure afterwards), they become clearer.
It's great. And it's not just for smiles; a sensitive gamma ray detector orbiting the moon can be used to map the elements on the surface; different elements emit gamma rays at different energies, so they can be distinguished in this way. Lunar Prospector, which had been circling the moon for about a year, was equipped with such a device.
By the way, if you look at much higher energy gamma rays, such as over a billion eV (GeV), the Sun again dominates the Moon. The source is the same: extremely energetic cosmic rays striking the solar atmosphere. The sun has a decent magnetic field that protects it from certain cosmic rays, but those with really the higher energy passes through – that is why we see much higher energy gamma rays from the Sun.
From Earth, at least, the Sun tends to win contests like this.
The gamma rays are incredible. They can blow up the stars. They create antimatter. And, in some cases, they can make our big gray satellite shine positively.
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