gamma rays are the highest energy form of light that exists in the universe and they're emitted by some of the most extreme objects out there however to get clear images using telescopes we need to focus a lot of light onto a sensor and gamma rays are almost impossible to focus in any practical way so how on earth do gamma-ray telescopes actually work [Music] the methods used by radio microwave visible and uv telescopes are simply impossible to use for gamma ray telescopes these gamma rays are the shortest wavelength highest energy light in the electromagnetic spectrum and photons of gamma ray light pass straight through the materials that make up mirrors this makes it almost impossible to focus gamma rays using the usual methods even x-rays are easier to focus than gamma rays despite the fact that they too would pass straight through mirrors if they impacted them straight on x-ray telescopes get around this by using pairs of mirrors that are almost perpendicular to the incoming light and these mirrors gently redirect the x-rays as they graze the mirrors this allows the x-rays to be focused but they must pass down a long tube in order to have this chance to get focused and again this method is completely impractical for gamma ray telescopes this is because the mirrors that the gamma rays would graze off of would need to be at an even shallower angle than they are for x-rays and hence the tube they travel down would need to be much much longer than for x-ray telescopes the earth's atmosphere also absorbs most gamma rays and hence the best place for a gamma-ray telescope is in space but it will be practically impossible to launch such a long telescope into space so how do gamma-ray telescopes get around all of this and focus their light well they don't focusing gamma rays is so hard that we simply don't do it most of the time and instead we use other clever ways to build useful images and work out where the light has come from let's take nasa's most recent gamma-ray space telescope fermi as an example and look at how it makes detections and how it produces useful images i should still note though that some other gamma-ray telescopes can use similar but different techniques and i'll leave some links in the description if you want to read more about those the fermi telescope launched in june 2008 and it has a detector the size of a small car and it has two observing instruments on board one is optimized for finding gamma-ray bursts which are sudden and often short-lived but usually very bright so this instrument is always on the lookout for those bright flashes of gamma rays the other instrument is the lat or the large area telescope and this is effectively the same kind of detector as those inside particle accelerators like the lhc except this one is in orbit around the earth this detector is made up of thin sheets of metal and when these super high energy gamma rays hit the metal sheets the photon is converted into an electron positron pair these are charged particles and they carry on traveling through the instrument which is then filled with layers of silicon microstrip detectors as the electrically charged electrons and positrons pass through these detectors they create a very small electric current wherever they hit using the data from many layers of the instrument we can then build up the path that these particles have traveled and the end point for these particles is a detector that tells us the total energy of the particles these paths then tell us where on the sky that gamma-ray photon must have come from and hence it eventually lets us build a picture of the gamma-ray emitting objects in space this method is very effective at getting around the limitation of detecting such high energy light and it lets us open up a whole new and exciting window to observe the universe however it does still have some downsides the main one of these is low resolution since these gamma-ray telescopes can't focus the incoming light to produce the extremely sharp images we're used to in visible or infrared telescopes we're left with images that are much fuzzier and they lack the crystal clear resolution of lower energy wavelengths this is made clear here when we look at the crab nebula in visible light we can see so much detail and structure here and when we look at the same object in gamma rays while we do see there's significant gamma-ray emission from the object the entire image is barely more than a few pixels how about this object can you guess what it is as seen in gamma-ray telescopes from fermi's predecessor the 1990s compton gamma-ray observatory it's the moon although it's lacking some of the detail we can see even with our own eyes while the lat on fermi has a field of view of about 20 of the sky at any time and it can image the entire sky in around three hours its resolution is only a few arc minutes compared to something like the hubble space telescope which has a resolution of 0.04 arc seconds this is a huge difference even the x-ray telescope chandra has a much better resolution at half an arc second despite this low resolution gamma rays are emitted by some fascinating objects and studying them lets us understand these objects in new ways for example fermi discovered two enormous bubbles of gamma-ray emission around the milky way galaxy now named the fermi bubbles their origin is still under debate in this all-sky gamma-ray map taken by fermi we see the milky way galaxy in the center and many bright sources outside the galaxy too this one here is a blazer a supermassive black hole that's eating so much matter that this material is heating up and emitting a huge jet of radiation this jet is pointed pretty much directly at us and it's very bright in gamma rays in gamma rays we also see very high energy cosmic rays supernovae and supernovae remnants pulsars and even the afterglow of a neutron star merger back in 2017 showing the importance of gamma-ray astronomy as part of the first ever multi-messenger that's gravitational waves and electromagnetic detection to be discovered it's absolutely mind-boggling to think of the extreme nature of these events and the incredible energy of the gamma rays produced often over billions of times more energetic than the visible light we see with our own eyes many of the sources of gamma rays we've seen are still mysterious and completely unknown so there's still a lot of science to do in this high energy field do you have any questions about gamma rays or other types of telescopes let me know in the comments below if you do and subscribe to the channel on your way out if you enjoyed this until next time stay safety i'll see you soon bye [Music] you