 Have you ever heard about neutron stars or do you know what will happen when two neutron stars collide with each other or have you ever heard about an astrophysical concept called as superliminal motion? If you want to know about them then this is the right place for you. We are going to see about them in this video. See recently scientists observed the merger of two neutron stars to form a black hole. This phenomenon was observed using laser interferometer gravitational wave in short called as LIGO observators and other telescopes that measured visual and electromagnetic signals. The observations made by all the equipment coincided. Here what is a neutron star? See to understand what a neutron star is we have to know what is a massive star and we have to understand its life cycle. See a star can be called a massive star when it is at least 8 times more massive than our sun. See a star maintain its stability through a fine balance between its own gravity which holds it together and the outward pressure from ongoing thermonuclear fusion processes taking place at its core. You can look at this image to understand better. Here the thermonuclear fusion process is one in which two hydrogen atoms merge to create a helium and release massive amounts of energy. But what happens when the hydrogen in the star's core starts decreasing? See once a star's core run out of hydrogen the state of equilibrium which is said will exist in the star is lost. The inward force of gravity overpowers the outward force of thermonuclear fusion. This mismatch results in the collusion of the star's core. Once the core of the star begins to collapse the plasma around the core begins to expand. As the shell of plasma increase in size it loses more heat due to this its temperature starts decreasing. As the temperature decreases the color of the star changes from bright yellow to red. In this phase the star is called a redgaint. See a normal size star turns into a redgaint and the massive star turns into supermassive redgaint or supergaint. As I already said that is a mismatch between gravity and thermonuclear fusion right? As gravity takes over the density of the supergain's core keeps on increasing. This increase in pressure and density due to gravity finally result in a huge explosion. This explosion is a supernova. You can see that in this GIF. See after a core collapse supernova all that reminds is a dense core. When massive star undergoes supernova explosions they either turn into a black hole or an ultra dense neutron star. Now coming back to the question what is a neutron star? See basically a neutron star is the collapsed core of a massive star. Neutron stars are the smallest and the densest star known to exist. These neutron stars are so small that they radius is only around 10 to 20 kilometer. Compare this to our Sun whose radius is 6,96,340 kilometer. But even though neutron stars are very small they weigh 2 to 3 times more than our Sun. That is why they are called the densest stars of our universe. The next thing is a black hole. I hope most of you are aware of what a black hole is. Especially Nolan Hetz who witnessed the marvel that is interstellar. See for people who are not familiar with it let me explain. A black hole is an astronomical object whose gravity is so strong that even light cannot escape from it. Now having understand the basics look at this image. See in this image you can see two neutron stars colliding with each other resulting in the form of a black hole. If you notice here at the time of merger light gravitational ripple in space time and an unusual jet of matter is ejected. The light was observed using telescopes that measured visual and electromagnetic signals and the gravitational ripple were measured using LIGO. As I already said only after both the observations matched this astrophysical event was confirmed. Another interesting thing that was noticed was the speed of the jet of matter that was ejected was observed to be travelling at speeds faster than the speed of light. Now this is not supposed to happen right? Because we know that no object in the universe can travel at speeds faster than the speed of light. Now this is an issue. But right now after further research the physicists have found that it is just the apparent speed of the jet of matter that was travelling at speeds higher than the speed of light. But its actual speed was lesser than the speed of light. This is due to the phenomenon of astrophysics called superliminal motion. Now before explaining to you what is the superliminal motion I have a question. Have you wondered how astrophysicists measure the distance of stellar objects? See when I was a kid I used to think they must use a light beam and find the time taken for the light beam to go and come back and then using this they might find the distance. But this is not correct because astrophysicists have the distance of stellar objects that are millions of light years away from us. So what do they actually use? Now the answer to this is parallax. See they find the distance using parallax. Now it works like this. Take out your hand, close your right eye and place your extended thumb over a distant object. Now switch eyes so that your left is closed and your right is open. Your thumb will appear to shift slightly against the background. By measuring this small change and knowing the distance between your eye you can calculate the distance to your thumb. That's trigonometry. Now when it comes to measuring distances to other stars there are no two eyes that could do the trick. Instead the orbit of earth around the sun provides the base line for these calculations. Look at this image. This is the actual position of the star and these two are the apparent position of the star based on the position of the earth. By observing the angle subtended by the apparent positions of the star and as we already know the distance between the sun and earth we can find the distance using the simple formula. We know tan P is opposite by a distance right? There opposite is the distance between the sun and the earth which we already know. We also know the angle P. Using this information we can find the adjacent which is the distance between the sun and the star. So using this simple calculation only we are measuring the distance of stellar objects. But there is one small issue here. The light rays from stellar objects are distorted by our earth's atmosphere before they can reach our land based telescopes. To address this in 2013 the European Space Agency launched the GAIA spacecraft that is global astrometric interferometer for astrophysics GAIA spacecraft. Since this is placed outside India's atmosphere the light that it absorbs is not distorted hence its accuracy and precision is higher. GAIA not only measures the distance of stars and other stellar objects it also has a radial velocity spectrometer on board. This instrument measures how fast stars move towards or away from GAIA. So now how is the speed of stellar objects are measured? See it is measured using a phenomenon called the Doppler effect. To understand this phenomena consider an ambulance speeding towards you. It's sirens scream at a high pitch when it approaches you. The pitch suddenly drops when the vehicle rushes by and races away. This is due to the phenomenon of Doppler effect. Similarly if a star is moving towards us its light waves gets scrunched up to shorter or bluer wavelengths producing a blue shift. If a star is moving away its light waves get stretched out to longer redder wavelengths producing a red shift. And the faster the sun the greater this shift ok. Having understood how we find the space and distance of stellar objects let us see about the superliminal motion. Say an object which is light years away from earth is moving towards earth. If we want to find the speed of the object what we will do we will find the time taken for the object to move from point A to point B. If we find the distance between point A and point B and divide it by time taken we can find its speed right. See using this method we can find only the apparent speed of stellar objects. This is because if the stellar object is moving at very high speed towards us the time taken for light to reach earth from point A is longer than time taken for light to reach earth from point B. Due to this the time taken that is observed on earth appears to be lower than the actual time taken. It is due to this only some objects like the jet of particles ejected from neutron star collusions appear to move at speeds more than the speed of light even though in reality they move at speeds lesser than this speed of light ok. This is mainly happening when the object travels at speeds closer to the speed of light and they are moving in earth's direction. So this is the phenomenon of superliminal motion and due to this sometimes objects in the universe may appear to move at the speeds greater than the speed of light but in reality they move at speed closer to the speed of light. So I hope you all came to know about a new astrophysical phenomena called superliminal motion. To know more science related articles you can subscribe to Shankar Ayes Academy YouTube channel. Now thank you for listening.