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07 August 2020

The Strangest (and Most Dangerous) Celestial Bodies

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The contemplation of a starry sky is a relaxing experience. Thousands of bright spots convey stillness and serenity, forming constellations that appear to move slowly and smoothly throughout the night. But looks can be deceiving. Beneath that tranquil blanket of stars hide astronomical phenomena of unimaginable rarity and violence.

White hole

Black holes are undoubtedly one of the most popular objects in astronomy. Much less well known are white holes, which have the opposite properties. If nothing can get out of a black hole, the opposite is true for a white hole —nothing can get in— and it would require faster-than-light speed to penetrate the event horizon line, which is as close as we can get. Perhaps the word hole is not the most appropriate term to define these objects, since they do the opposite of what we intuit a hole does.

Kruskal–Szekeres diagram, illustrating the mathematical interpretation of relativity that predicts the theoretical existence of black holes (II) and white holes (IV). Credit: Dr Greg/Wikimedia Commons

We have no confirmation that one has ever been detected, and no candidate location for a white hole has even been identified in the sky. They do not arise from observations of the sky but from Einstein’s mathematical equations, which in his day served to predict the existence of black holes —something that took years and years to be confirmed by observation (and much more time to be actually ‘seen’). These may be merely mathematical objects, theoretically possible but not existing, or even more difficult to detect than black holes or perhaps undetectable. 

Another even stranger explanation put forward by theoretical physicist Lee Smolin is the idea that the Big Bang at the beginning of our universe is itself a white hole. Many of the parameters match. It’s an absolute expansion, it’s not possible to get inside the object, and while black holes refer to something going into the future, white holes are about something coming from the past. It could even be that each white hole in our universe generates another new Big Bang in a different universe, at least mathematically.

Quásar

In the late 1950s, strange and mysterious radio signals began to be detected in the sky, which were nothing like those coming from the stars. These waves received on Earth covered the entire spectrum of radiation (visible, radio, infrared, X-rays, gamma rays) and with an unprecedented intensity (thousands of times the brightness of an entire galaxy). Nothing like it had ever been seen before. Astronomers thought that the source of all this varied range of signals would be a new type of astronomical object, called a quasi-stellar radio source (hence the acronym quasar). 

This artist’s concept illustrates a quasar drenched in water vapor. Credit: NASA/ESA

At the time, these newly observed quasars were thought to be white holes. But it didn’t take long for a better theory to be found, which had to do with the supermassive black holes that form at the centre of most galaxies, weighting between one hundred million and one billion times that of our Sun. Black holes cannot shine by themselves, but they can be detected because of what is happening around them, just before the line beyond which nothing can escape. In supermassive black holes, a huge amount of material from the interstellar environment falls towards the object, forming a spiral similar to that made by water draining from a bathtub. The force of gravity is enormous. This material accelerates in the fall to extreme velocities and from the collisions between the particles of matter emerge jets of plasma travelling at super high speed —something like this: if we squeeze an orange with such force that its skin breaks, juice will be ejected. 

These ejections are aligned with the enormous magnetic fields of the black hole, which makes them very directional light sources, not shining in all directions but only in a narrow arc of just one degree, similar to the headlights of a car or a laser beam. Precisely because of their enormous brightness and because they concentrate the light so much, they are the most distant objects that can be observed. And as in astronomy the further we observe, the more we go back in time, that means they are located in galaxies formed shortly after the Big Bang at the beginning of our universe.

Kilonova

At the end of their lives, not all stars end up as black holes. Many, which don’t have enough mass to generate a black hole, paradoxically die in a much more violent and spectacular way. At a certain point, when the star has run out of fuel, it begins to compress, to the point that the nuclear forces of the particles inside it are able to stop the contraction. At that stage, new nuclear reactions begin, ending with a cascade of neutrinos that literally blows up the star; this is what is known as a supernova. 

At the centre of the explosion is a stellar corpse: a neutron star. The material from this star is extremely dense (a tea cup full of this matter would weigh as much on Earth as the whole of Mount Everest). Neutron stars can be found alone or in double systems, with one star orbiting around another. The waltz lasts billions of years, but over time the stars come closer and closer together. When they finally collide, both their extreme density and their velocity (one-third the speed of light) produce the largest known explosion in the universe: a kilonova. 

Credits: NASA’s Goddard Space Flight Center/CI Lab Credit: NASA’s Goddard Space Flight Center/CI Lab

Two jets of gamma rays emerge from the explosion, and if one were to hit the Earth directly it would completely wipe out the exposed hemisphere —fortunately, this is very unlikely to happen. The process generates about a thousand times more energy than a supernova. The force of its explosion is so strong that we detect them on Earth as gravitational waves; in the same way that the water in a pond forms waves when you throw a pebble, these events are such that they generate a very similar disturbance in space-time itself, the fabric that makes up reality.

Thorne-Żytkow Object

In a binary system where two stars have a considerable difference in mass, it may occur that one of the stars quickly exhausts its fuel and ends up exploding as a supernova, leaving behind a neutron star corpse. If the other star is less massive and therefore evolves more slowly, at an earlier stage of compression it may become a red giant. During this process a sort of stellar cannibalism can occur in which the giant star devours the much smaller neutron star. 

This would not be a collision like in the case of a kilonova, since red giants are low-density stars and the neutron star could easily orbit inside the giant until it gets close to the core, by losing speed due to friction with the gaseous material of the red giant. This object, that was theorised in the 1970s by Kip Thorne and Anna Zytkow (a promising candidate to fit this theory has recently been discovered) is the result of interstellar cannibalism. Something similar will happen to the Earth, when in about five billion years the Sun enters the red giant phase and expands to engulf Mercury, Venus and our planet. 

The giant variable star HV 2112, located in the Magellanic Cloud, is the first candidate to be a Thorne-Żytkow object. Credit: Center de Données astronomiques de Strasbourg

No llegaría a ser una colisión como en el caso de las kilonovas, pues las gigantes rojas son estrellas poco densas y la estrella de neutrones podría orbitar dentro de la gigante sin problemas hasta llegar a estar cerca del núcleo —al perder velocidad, debido al rozamiento con el material gaseoso de la gigante roja. Este objeto, que fue teorizado en los años 70 por Kip Thorne y Anna Żytkow (recientemente se ha descubierto un candidato prometedor a cumplir con esa teoría) es fruto de un canibalismo interestelar. Algo parecido le pasará a la Tierra, cuando dentro de unos 5.000 millones de años el Sol entre en fase de gigante roja y se expanda hasta engullir a Mercurio, Venus y a nuestro planeta. 

 

Borja Tosar para OpenMind

@borjatosar

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