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But physically speaking, what is it the electromagnetic radiation produced by stars?

You know much about the waves, because we are talking about them all the time. Light, wifi, microwave, telephone, radio, X-rays, radioactivity and sound.

All these elements are waves. Only the sound is a bit different. Indeed, it is only a mechanical wave, a vibration of the air. We are talking about different frequencies too, however, the sound will not be able to move in a vacuum, and therefore not in the universe. It is a terrestrial phenomenon, which is widely transformed into the surrounding environment (eg water).
The sound is traveling slower than other waves. You know that, because of the famous wall of sound, or because of the thunder comes after the appearance of lightning.

All other waves travel at the same speed, that is to say what is known as the speed of light. They are all “real” waves, they are not mechanical and they propagate in a vacuum, and therefore in the universe.
These electromagnetic waves are composed of photons, and each photon carry a dose of energy depending on its frequency (or have a frequency that depends of the dose of energy it carries). Regarding the formation of this energy, this event is explained as follows: it seems that in an atom when an electron approaches the nucleus (when it changes its orbit), it emits photons. Depending on the place where it is located in the atom, and depending on the place where it moves, the dose of energy, and thus the frequency of the photon, is different. I would not go further in the description related to quantum physics, for several reasons, including that sometimes these researchers are lacking modesty and common sense compared to the scholars from antiquity. Quantum physicists are perhaps too often in religion, while the scholars from antiquity, who mixed science, common sense, logic and philosophy was in physical explanation.

« The truth is so obscure in these times and falsehood so established, that unless we are loving the truth, we can not recognize it. » B. Pascal

But back to electromagnetic radiation…

The frequencies made it easy to understand what make waves penetrate more or less the matter, or interact with it. For example, light (visible frequencies) interacts with the surface matter. That is why it is said that the light is more or less absorbed it. The frequency of the light is changed (because partially or totally absorbed) by the matter, and that is why a sheet of red paper appears to us as « red ». The photons have lost energy in the red leaf. Conversely, they lose very little in the snow, for example, which reflects light very well (and therefore absorbs almost no energy from the photons).
White skin absorbs little light (little energy from the photons), while dark skin absorbs more. The frequency of the photon (but not its speed!) changes by interacting with the matter. That is why we use lead shielding against X rays and gamma rays: lead absorbs these frequencies which have very much energy, lead thus reduces the frequency or photon by absorbing its energy. Your skin, for example, unlike lead, is penetrated and traversed by the X-rays or gamma rays, and also all the atoms of your body, and that is why they will be destroyed by them (cancer, etc.).. Your body is not a barrier for these frequencies.

Note: The laser is not light with a specific frequency (this is why there are lasers of all colors, so different frequencies), but just a light whose photons have been artificially and with optical effects pushed next to each other, in close rows. Thus they will disperse less in the air than conventional light, and their effect will be much more accurate. This is also why they can be dangerous for the eye, for example, because it will receive in a specific point an abnormally high light quantity.

http://en.wikipedia.org/wiki/Photon
http://en.wikipedia.org/wiki/Electromagnetic_radiation
http://en.wikipedia.org/wiki/Light

Electromagnetic waves are all elements of the same nature, traveling at the same speed (in vacuum), with a speed known as “the speed of light”, that is to say about 300 000 000 m / s. In the air or in the water, the light will be highly dispersed, as it will reflect continuously. However, you do not lose light, it is simply redirected. It’s the same for other waves. The main reason that explains the constant speed of the wave, or photon, is its lack of mass. The photon mass is not a mass object, and thus it is insensitive to acceleration or deceleration. You can not “throw” the light, or you can try, but it will not go faster because of that. You can not accelerate or decelerate a photon. This means of course that the photon is insensitive to gravity.

Of course, I can already hear some voices saying “But Einstein have said that the photon falls into the curvature of space-time! “. Yes, well I challenge you not to believe everything Einstein have said just because someone have said to you that he was very clever.
You, too, are intelligent, and we will see more about this point a bit further.

http://en.wikipedia.org/wiki/Gravitation
http://en.wikipedia.org/wiki/Spacetime

So, according to Einstein, the photon would have a constant speed, but would be affected by the gravitational force. This apparent paradox is supposedly solved by the curvature of the space-time. The curvature of the space-time, I will dwell on it later, is however a picture of gravitation.

According to Einstein, the photon is slowed by the gravitational force, even if it has no mass. It is very strange.

The supposedly proof of that is the redshift observed near the sun and white dwarf stars, but… It may simply be due, in addition to the movement of the sun and the star, to a reabsorption of the photon energy by the surrounding atoms, as on Earth…

The reason why a photon of light emitted from the Earth follow a slightly curved path is certainly only due to reflection phenomena in the air, not to gravity. This reflection is also used for radio waves (originally: http://fr.wikipedia.org/wiki/Propagation_des_ondes_radio).

Such assumptions as black holes and wormholes (which we will discuss later) have resulted from this Einstein’s idea of a light « eaten » by the gravitationnal force.