What Are Light Echoes, and Why Do They Matter?

When I was a child, I sometimes played basketball on a school courtyard next to a brick wall. By bouncing the ball, I would notice that its sound repeated a fraction of a second later from the direction of the wall. It seemed a little different, but it was clearly the same noise that the balloon made when it struck the Blacktop, just delayed by a wink.

I had discovered echoes. Nerdy Kid that I was, I thought that the sound of the ball was going to the wall, bouncing then seeing me towards me. Later, I would learn that if you know the speed of this sound (about 1,200 kilometers per hour) and the length of the delay, you can calculate the distance to the wall.

Of course, nature understood it a little earlier than me; Many animal species use this fact to map their environment using echolocation.

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Astronomers can also do it, but we do not use sound echoes. We use light echo.

Like sound, light moves at a finished speed. It’s very fast, but on the study of huge astronomers, it’s actually rather slow. The echoes of light that we see in the sky can take years, even centuries to reach us.

What East A light echo? Imagine instead of bouncing a basketball, there is a star in space that suddenly and quickly clears, as when a massive star explodes at the end of his life to create a supernova. The Flash of Light develops in a sphere, moving far from the explosion site at around 300,000 kilometers per second. It’s a billion kilometers per hour!

At any time, the flash of light will define a spherical shell at a certain distance around the explosion, like the thin wall of a bubble. After an hour, for example, the light shell is a billion km from the site. Anyone at this distance on the shell will see the start of the event at the same time. If you are more distant than that at that time, you will not see the explosion because the light has not yet reached you.

The echo comes into play when we adjust this idealized scenario to take into account the complexities of the real world, such as the probability of material surrounding the light source. Imagine, for example, that there is a thin gas shell around a supernova which is, for example, a light year in the department and that we are witnessing the explosion of much further, like thousands of light years (security first; did not want to be too close to an explosive star). The supernova explodes, releasing a wave of expanding light. A year after the explosion, this light strikes all the gases in the shell enveloping simultaneously. But our point of view from afar means We do not see the whole gas shell light up at the same time.

Instead, the part of the shell that we see first illuminated is its closest point to us, directly on a line with the supernova. This is because, after the sparkling shell was on, the light of this place had the least distance to go to reach us in space, so it happens first.

Then, we see an apparent ring of light seeming to expand from this initial point while the light of the supernova crosses parts of the sparkling shell which are slightly more distant from us. We then witness a surprising spectacle: the expanding ring becomes increasingly large until it reaches the maximum size of the shell, its diameter, then begins to shrink. As it moves on the other side of the spherical shell of our line of view, the light echo gradually illuminates smaller sounds until it’s a point, then POOF! Let’s go.

Even this more complicated scenario is rather unrealistic. More likely, a supernova occurs inside a galaxy loaded with many clouds of gas and dispersed dust. As the wave of light develops, it will illuminate these clouds, creating more ornate echoes of light which can be many light years.

The geometry of the way in which a light echo was quantified for the first time by the French astronomer Paul Couderc in 1939 – something that I referred to my own doctorate. Work analyzing how supernova 1987a illuminated its surrounding gas. What COUDERC noted was that an observer on one side sees the echo expand as a thin paraboloid shell – a geometry in the shape of a gobele or cutting, with the observer looking towards the opening and the light source centered on the apex. At any time, everything that is on this shell will be considered illustrated by a distant observer.

Keep in mind, however, that we look in the axis of this shell, which has a circular transverse section. This means that the material we see illustrated will form a circle on the sky Whatever the real three -dimensional distribution! All the clouds of dust on this shell will light exactly at the same time, even if they are largely separated in space. What we see of the earth is a circle in the sky that expands over time – or even circles if the gas is lit and takes some time to fade (in general, once a gas cloud is affected by, for example, ultraviolet light, it re -specifies this light with lower wavelengths during the weeks or months).

And this exact phenomenon has been seen! The supernova SN2016ADJ exploded in the Galaxy Centaurus A, creating an echo of expansion circular light which was captured by Hubble (and transformed into an incredible animation by community scientist Judy Schmidt).

https://www.youtube.com/watch?v=nwuvxtiu0is

In addition to being simply a cool effect, these light echoes can tell us about the environment around a supernova; Massive stars explode young, before they can get out of the cloud of gas and dust where they were born. The light echo illuminates this material, giving us an overview of its conditions and even of the three -dimensional structure when the star was formed.

This was demonstrated in a ridiculously dramatic way when the star V838 Monocertis underwent a huge explosion seen in 2002. Hubble images taken over time showed that dust around him developed and changed quickly, but it was an illusion: it was the light echo Extend through the stationary dust, illuminating different materials when he swept. The animation of this is as bizarre and supernatural as everything I have ever seen.

https://www.youtube.com/watch?v=fpwrog5bpk

Remember that this material is not physically expanding! It is just lit by the light flash of the explosion. The scientists analyzing these data came to the rather surprising conclusion than the V838 Monocerotis event was caused by two stars in collision and fusion, exploding a fierce light impulse which illuminated the surrounding material. A careful measure of the expanding light echo was used to determine the distance from the V838 against us, around 20,000 light years.

Light echoes are particular phenomena that only seem to be at first nothing more than curiosity – until you start to look at mathematics and physics, then they become an important tool to probe space. I am fascinated to see how nature hands us these gifts which help us to explore the universe around us, releasing data that we can examine to better understand the cosmos in which we live – and, at the same time, nourish our feeling of wonder and fear.

My thanks to the astronomer Kirsten Banks for reminding me SN2016ADJ.

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