Could we see a gamma ray source on the Moon?

If a gamma ray source were placed on the moon and oriented properly, could it be detected on earth?

Gamma rays are one of the highest energy forms of light in the Universe - its wavelength is around a picometer (that’s 10^-12 meters), which is roughly the same order of magnitude as the size of a hydrogen atom. These wavelengths are a thousand times shorter than that of visible light.

The high energies of gamma rays means that this form of light is extremely damaging to life. Ultraviolet (UV) light doesn’t penetrate far into your skin - it can damage the surface layers of your skin, giving you a sunburn, but it won’t damage anything past the first few layers of your skin. (UV light will, however, increase your chances of skin cancer, if you get severe sunburns.) Gamma rays can penetrate much further into your body, and can destroy or alter the DNA within your cells, causing drastic changes to the replication instructions of your cells. This can cause radiation sickness and/or cancer to appear, but with no surface burning of your skin.

Fortunately, our atmosphere is opaque to gamma rays, so the average level of gamma radiation coming towards our planet is blocked out by our atmosphere. Our atmosphere does a wonderful job of protecting us from the beating our cells would otherwise be taking.

On the other hand, the fact that our atmosphere is such an effective wall to this wavelength of light means that we can’t observe any of the gamma rays produced elsewhere in the universe (which is a very interesting field with a lot of science to be done) from the surface of the Earth. In order to look at this light, we have to put our telescopes into space, outside of our atmosphere. At the moment, we have the Fermi Gamma Ray Space Telescope observing the whole sky for gamma ray sources; that telescope produced the map of the sky at the top of the page. The bright line through the middle is all gamma radiation from within our own galaxy. Anything far away from that central region is probably coming from another galaxy. Individual bright points in our galaxy are likely to be coming from the aftermath of a supernova.

The moon, unlike the Earth, doesn’t have an atmosphere to protect it from any kind of battering, which means that there’s a constant flow of high energy light and small particles pounding into the surface of the moon. One of these objects smacking into the surface of the moon is called a cosmic ray (which is a highly accelerated tiny piece of grit - most of them are protons). Cosmic rays are constantly streaming throughout the universe. When they smash into the surface of the moon, they can create gamma radiation as they come to a stop.

This means that the moon itself glows faintly in gamma rays, and this is detectable by our space based telescopes, which can observe gamma rays. (Again, we couldn’t detect this from the surface of the Earth, because our atmosphere gets in the way.) The image below was produced by the Compton Gamma Ray Observatory, which was in orbit around the Earth from 1991 - 2000, when it was instructed to crash itself back into our ocean. The moon, as imaged below, is actually brighter than our sun in gamma rays - our sun, very fortunately, does not produce much in the way of gamma radiation. (If it did, the gamma radiation from such a close source might have destroyed our atmosphere, removing our shelter from this wavelength.)

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Are the pictures with millions of stars & nebulae in them real?

The photos in which millions of stars can be seen alongside several nebulae- are these real or edited? If the former, are they taken from space? Can you see them from the naked eye?

While there are certainly a number of edited space pictures floating around the internet, there are also a lot of real pictures like what you describe. The most famous of these tend to be of the Orion Nebula - it’s a very beautiful nebula (along with being scientifically fascinating), so lots of people - both professional astronomers and amateur astronomers - have taken pictures of it. (The Trifid Nebula is another common target.)

Why are there so many stars in these pictures? Almost all of the gas (and most of the stars) in our galaxy is contained up in a very thin disk - proportionally, the Milky Way is thinner than a razor blade. This means that all these nebulae - which are clouds of glowing gas - are also found within this very thin disk. If we look towards them, we are looking through the disk towards the greatest concentration of stars in the sky. The thousands and thousands of stars captured in these images are just the handful of the stars in our galaxy that happen to be in the same direction as the nebula. The fainter the nebula you’re trying to observe, the more stars you’ll capture.

These nebulae can be imaged either from the ground or from space - it depends on what we need the pictures for. The pretty Hubble images you see are usually not the main goal of pointing Hubble at that patch of sky - scientists are usually concerned with how much light there is of a certain colour in a certain region, and whether or not there is more of a certain colour than a second color - these differences in colour can help us understand the structure of the nebula, and how warm or cool it is, along with a number of other properties. This information can be used to make a colour image of the nebula, but it often isn’t done right away, or at all.

If, however, you’re simply wanting to make a really good-looking picture of a photogenic patch of the sky, you would do that from the ground. There are so many things that only space telescopes can do, that we can’t spend much (or any) time just to get good pictures. But on the ground, your time is less constrained, and a lot of amateur astronomers have their own telescopes and cameras attached to them, so they can just go out and take pictures whenever the sky is dark and clear. In this case, these detailed pictures are taken over a long period of time, to make sure that there’s enough time for the light from all those stars to reach the camera.

Most nebulae are too faint to be seen with the naked eye - but the Orion nebula is a notable exception. If the constellation Orion is up, look for the middle “star” in Orion’s sword - it’s actually the Orion nebula. In a very dark sky, it might even look a bit pink.

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