Hello and welcome to astronomy with Mr. Gehrin. This is part two of my two-part video on modern telescopes. In part one we discussed modern digital techniques and how to use modern technology to view the entire electromagnetic spectrum. We talked about the effects of the atmosphere on astronomy and finally how radio telescopes work. Today we'll look at how we can use spectral lines to analyse the chemical composition of stars and consider what we can learn using different parts of the electromagnetic spectrum. Stars release electromagnetic radiation across the entire spectrum. This image shows just the visible spectrum but all other wavelengths are produced. However, the elements in a star absorb some specific parts of the spectrum. In the second image we can see four black bands where that particular colour has been absorbed. These bands are called spectral lines or absorption lines and we can study them using a device called a spectrometer, usually attached to a telescope. We can study the spectral lines of elements and chemicals in labs on earth and compare them to the spectral lines in a star to find out what elements the star contains. Helium was first identified in 1868 when unknown spectral lines were seen in sunlight during an eclipse. The new element was named Helium after Helios, the Greek god of the sun. We'll discuss these lines more in a later video on starlight. But now let's look at some of the elements in the sun. These absorption lines indicate oxygen. These lines are mostly due to absorption by the earth's atmosphere but there is some oxygen in the sun. The sun is mostly hydrogen, indicated by these two lines. The sun is about 98% hydrogen and helium. The so-called sodium doublet responsible for the yellow orange colour of many street lights shows that sodium is also present. At 0.2% of the sun is iron and we can also see evidence of magnesium and calcium. We can use spectroscopy at any wavelength, not just visible light and we can see the evidence for most elements in the sun. All elements except hydrogen and helium were created in long dead stars. And amusingly, astrophysicists call all these elements, everything except hydrogen and helium, metals. Now let's look at some of the astronomical objects we can study using non-visible light. We'll start with radio waves, arguably the most important non-visible part of the spectrum. Interstellar dust clouds block visible light but not radio waves. If they did, rain would stop TV and phone signals from getting through. This means we can use radio astronomy to see through dust clouds. We've studied and mapped most of the structure of the Milky Way, apart from the region beyond the dense galactic core. And we've studied proto-planetary disks where stars and planets are forming to learn more about how they form. We can also study black holes. Black holes rotate and absorb nearby matter. This causes them to emit jets of matter. For some supermassive black holes at the centres of galaxies, these jets are extremely bright across the whole electromagnetic spectrum and they are detected and studied by radio telescopes. One of the most important type of galaxies studied this way are quasars, discovered by radio telescopes in the 1950s. We'll talk more about quasars in a future video. X-rays are very high energy short wavelength radiation. Smaller black holes absorb nearby matter, just like the supermassive black holes we just discussed, only on a smaller scale. Matter from nearby dust clouds or from a binary partner star falls into the black hole, forming an accretion disk. The disk gets very hot and releases X-rays, which we can study using space-based X-ray telescopes. This artist's impression shows a black hole absorbing matter from its binary partner. Inch-ray radiation is emitted by warm objects. The hotter the object, the more radiation is emitted. We can use this to study proto-stars. As a cloud of gas and dust contracts due to gravity, a star may form at the centre. The cloud obscures the star's light, but the gas and dust heat up, and we can study the warm cloud in infrared wavelengths, usually with space-based telescopes such as Hubble. Dust and molecular clouds, also called stellar nurseries, are huge clouds of mostly hydrogen and helium at an early stage of star formation. At this stage there is little visible light, and the cloud is quite cool and thin. We can use infrared astronomy to see through the cloud and map the warmer regions where stars may be forming. Here we can see the enormous Karina nebula, a huge region of star formation and other exciting events. And infrared radiation is used to locate hotspots on moons, such as Jupiter's moon IO. This shows us volcanoes and other interesting geological features, and this image helped NASA identify four previously unknown volcanoes. Gamma rays are the most energetic part of the spectrum. A rare event called a gamma ray burst is thought to be produced at the moment when a massive star collapses into a neutron star or a black hole. These were first detected by accident, by classified spy satellites, designed to detect covert nuclear weapons. Now they're studied by dedicated space-based gamma ray telescopes. And finally for today ultraviolet. The chromosphere and corona of a star are very hot and release a lot of ultraviolet radiation. We can study this radiation to see the structure of the chromosphere and corona. And we can use ultraviolet spectroscopy to determine the chemical composition of the outer parts of a star. The solar dynamics observatory constantly monitors our sun, and watches for disturbances in its atmosphere like solar flares, as you can see here. That concludes our two part look at modern telescopes. We'll discuss many of these objects in more detail in another video. Thank you for watching. Goodbye and have an excellent day.