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| The brightest object in the entire universe |
The sky we see every night is full of thousands of stars. But in reality, what we see with our naked eyes is only a small part of the universe. In fact, the universe is so big that most distant objects are not visible to us. However, some objects are so powerful that despite their immense distance from us, we are still able to detect them (albeit only using telescopes). I'm talking about quasars or "quasi-stellar radio sources". Do you want them to know? Quasars were discovered in the 1950s, and it is interesting to look back at the history of their first observations. In the 1950s, astronomers began looking at the sky using radio telescopes for the first time. Radio telescopes, unlike ordinary telescopes, detect radio waves instead of visible light. When astronomers first began to "see" the sky with these instruments, they were surprised to find that several objects emitted large amounts of radio waves, but almost no visible light.
In fact, most of these sources of radio waves did not correspond to any known visible objects. The first quasar discovered was 3C 273, an odd name that refers to the 273rd object in the Third Cambridge Catalog of Radio Sources (3C). It was discovered to account for a "lunar eclipse," a phenomenon that occurs when the moon passes between an object and Earth, hiding the object from our view. In 1959, a team of astronomers at the University of Cambridge detected a radio source in the sky (3C 273), but they could not find an optical counterpart. Three years later, in 1962, John Bolton and his Caltech radio astronomy group used the Parkes radio telescope to realize a series of sky observations during which the Moon was passing in front of the radio source discovered three years earlier. Thanks to these lunar missions, Bolton and his team were able to calculate the location of the source with precision. And more importantly, they were able to associate it with a visible counterpart, an obscure stellar object.
Later new quasars were discovered. Usually, their position in the sky coincides with very faint objects like very distant stars: that's why they were called quasars, a contraction of "quasi-stellar" (star-like) and "radio source". However, the chemical composition of these objects, as observed by their spectral lines, was very different from that of any known star. Also, the amount of radiation emitted by them was too high to be a normal star. Observations of these quasars continue throughout the year. By observing their radiation spectrum, scientists have found that these objects are moving very fast and away from Earth. This can be estimated thanks to a phenomenon known as "redshift". What is it? Every object in the sky emits radiation at a wide range of frequencies - and even in the visible part of the spectrum, which consists of different colors: violet, blue, green, yellow, orange and red.
The color our eyes perceive depends on the frequency of the light we observe: violet corresponds to the highest frequency, while red corresponds to the lowest. The light emitted by an object in the sky contains a mixture of different frequencies, with a specific color peak. If we look at the sky, in fact, we notice that some stars appear red, while others appear blue. However, when the object we are looking at moves relative to Earth, the frequency of light emitted by the object appears different to our eyes. In particular, if the object is moving towards us, the frequency of its light increases (and therefore, it becomes bluer: this is known as the "blue shift"). Instead, if the object moves away from us, the frequency of its light decreases (and therefore, it becomes redder: this is known as the "red shift"). So, when scientists observed the radiation spectrum of these quasars, they noticed a large redshift, which means these objects are moving away from us.
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| Quasar |
Not only that: they also learned that these objects were extremely far away from us. This information can also be estimated by the amount of redshift. In fact, in 1931, the famous astronomer Edwin Hubble observed hundreds of galaxies and discovered that their speed is proportional to their distance from Earth. In other words, the higher the redshift of the light from those galaxies, the faster they were moving away from us and the farther away they were. This result is known as Hubble's Law, one of the most important results in astronomy. Applying Hubble's Law, scientists in the 1960s discovered that the quasars they were observing must be very far from Earth: some of them are 10-20 billion light years away from us! This immediately places the quasar among the most distant known objects. Now that we've learned a bit of the history of quasar observations, we might wonder: What exactly is a quasar? Before learning more about Quasar.
The main problem in observing these quasars was to explain how they could produce such enormous amounts of energy: indeed, some of these quasars were visible despite their great distance from Earth. So, scientists had no idea what these quasars were. The explanation was proposed by Edwin Salpeter and Yakov Zeldovich in 1964. According to them, quasars were supermassive black holes surrounded by a large amount of matter. But wait... we actually know that light (and any other form of radiation) cannot escape a black hole: so, how can these objects produce such enormous amounts of light and energy? This is because the quasar's energy is actually generated outside the black hole. A supermassive black hole is surrounded by a large amount of matter in rapid rotation around it and collapsing towards it.
Due to the immense gravitational force exerted by the black hole, this matter undergoes very strong friction and as a result, it emits a lot of energy as radiation in various forms (such as radio waves). This rotating material takes the shape of a disk and is known as an accretion disk. The amount of energy produced by this process is enormous: when this matter falls towards a black hole, 6% to 32% of its mass is converted into energy! By comparison, nuclear reactions inside our Sun convert less than 1% of the mass into energy. In fact, quasars are among the most powerful and powerful objects in the universe. As a comparison, a quasar can emit an amount of energy equal to a thousand times the amount of energy emitted by the entire Milky Way, our galaxy. However, due to their great distance from us, quasars are too faint to be observed from Earth with ordinary telescopes. However, if we could place one of these quasars at the same distance from Earth as the Sun, it would be a trillion times brighter than our star.
It is interesting to note that despite their extreme brightness, the size of a typical quasar is relatively small. In fact, quasars are typically a few light-years in diameter, and some of them can be as small as our solar system. To give you an idea, the Milky Way is about 200,000 light years in diameter. Quasars are part of a larger family of objects known as active galactic nuclei (AGN), which include those galactic cores with anomalous emissions of electromagnetic radiation, explained by the accretion disk mechanism. AGN emit not only radio waves and visible light, but also other radiation such as X-rays and gamma rays. An interesting phenomenon sometimes associated with quasars is the presence of relativistic jets. Although their origin is not entirely clear, it is believed that they arise from the interaction between the black hole's magnetic field and the accretion disk. Under certain circumstances, this interaction creates jets of particles traveling at nearly the speed of light.
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| Another Quasar |
These jets erupt from both poles of the black hole, so they appear as two beams of highly energetic particles traveling in opposite directions. They are extremely luminous (sometimes more so than quasars) and can extend for millions of light years. These quasars with relativistic jets are also called blazars. You may wonder why these quasars are so far away from us? Are there any quasars near us? This is an interesting question, and the answer has to do with galaxy evolution. It is believed that quasars were more common in the early universe. This is because quasars are very active objects, so they require a lot of material to form. For example, collisions between two galaxies can produce quasars: when this happens, the black holes at the center of the two galaxies can fuse, creating a supermassive black hole that can give rise to a quasar.
The early universe was much smaller in size and denser than it is now, so extreme events like collisions between two galaxies were much more likely: hence, the formation of quasars was much more frequent than it is now. Due to their nature, quasars "burn" a lot of energy very quickly, as matter falls into the black hole. This means that the accretion disk does not last forever, and by now, most quasars have "absorbed" all the available energy. We also need to remember that there is a relationship between the distance of a celestial body and its age. In fact, light emitted by an object takes some time to reach us: the farther away the object is, the longer it takes for the light to reach us. Because the universe is in constant expansion, most quasars are now very far away, even 10 or 20 billion light years away, which means light takes 10 or 20 billion years to reach us! The implication is that we are actually observing quasars 10 or 20 billion years ago: it is like "looking into the past".
Presumably, these quasars have now exhausted their energy and are no longer as bright. This is why most of us don't see quasars near us. It is also possible that some of the galaxies we observe today were actually quasars in the past. In fact, the galaxies we observe today have massive black holes at their centers. So, it is possible that in their early age, the matter very close to the black hole went through the phenomenon we described earlier (accretion disk), but in the sense that we no longer observe it, quasars could be although young galactic cores: indeed, scientists Believes that the Milky Way itself was a quasar in its early ages. However, this does not mean that quasars can no longer form: on the contrary! The Milky Way itself, in fact, could possibly host a new quasar in the future… In fact, the Milky Way and the Andromeda Galaxy are currently moving towards each other, and it is predicted that they will collide in about 4.5 billion years. We have to remember that both galaxies have a supermassive black hole at their center.
Therefore, when this massive collision occurs, these two black holes will merge and interact in a cataclysmic event. The matter surrounding these two giants will undergo extreme frictional forces, creating a quasar or even a blazar. So, in the end, this is the fate of our galaxy! Interesting, isn't it? In a way, we can say that quasars represent our past, but also our future...
Do you find quasars cool? Is there anything else you want to know about them? Let me know in the comments below..