Ask Ethan: Can under-stars still become stars?

The closest to Earth system with a brown dwarf, Luman 16 , generally contains enough mass to form a red dwarf star, if it were to gather all that is there, together. The question whether it will ever happen in our Universe is quite interesting.

In the night sky, stars are perfectly visible, in any direction from us, wherever we look. But for every star that has gathered enough mass to start nuclear fusion in its center, burning hydrogen, converting it into helium, and converting matter into energy through E = mc ² , there are many other objects that have not reached this. Most of the lumps of mass that begin to form in nebulae never grow large enough to become a star - instead they become fragmented gas clouds, asteroids, rocky worlds, gas giants or brown dwarfs. Brown dwarfs are the "under-stars" of the Universe, which have gathered enough mass to trigger the reactions of synthesis of rare isotopes, but not enough to become true stars. But many brown dwarfs exist in pairs, which is why our reader asked the following question:

Will the orbit of these brown dwarfs become smaller with time due to the loss of energy on gravitational waves? Will they merge in the end? If so, what happens after that? Will they become a real star performing the synthesis? Or something completely different?

In astronomy, as in life, just because you have failed something the first time does not mean that you will never succeed. Let's start with the stars that could.


Illustration of a giant planet orbiting a star-red dwarf. The difference between a planet, an understar, and a true star is only in the mass

In order to ignite nuclear fusion in the center of the star, and force the hydrogen nuclei to react, it is necessary to reach temperatures of about 4,000,000 K. The gas in the interstellar space from which stars are formed is quite cold — only a few tens of degrees above absolute zero. But then gravity connects and causes a cloud of gas to collapse. At this time, the atoms inside are gaining speed, collide with each other and warm up. If there were few atoms, they would radiate this heat into interstellar space, sending streams of light to travel through the entire galaxy. But if you gather together a lot of atoms, they do not release this light, because of what the insides of the gas cloud begin to warm up.


Orion constellation, along with a huge complex of molecular clouds and the brightest stars. Many new stars are formed due to the collapse of gas, which does not allow heat to escape from the place of star formation.

If something small is formed, asteroid, Earth, or even Jupiter, it can warm up to thousands or even tens of thousands of degrees in the core - but it will still be very far from the synthesis temperature. But having reached a certain critical mass — about thirteen masses of Jupiter — you will receive temperatures of about 1,000,000 K. This is not enough for helium synthesis from hydrogen, but this is the critical temperature for a particular reaction: deuterium synthesis. In the order of 0.002% of hydrogen in the Universe, the nucleus contains not just a proton, but a proton associated with a neutron, that is, a deuteron. At temperatures of a million degrees, deuteron and proton are able to synthesize helium-3 (a not very common isotope of helium), and this reaction occurs with the release of energy.

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The proton-proton chain reaction, responsible for much of the Sun’s energy, is an example of nuclear fusion. In the synthesis of deuterium, only the ² H (deuterium) + ¹ H (proton) -> ³ He (helium-3) reaction can occur.

It is important! This energy output, especially in the protostar phase, emits high energy radiation that resists internal gravitational collapse and prevents the center from heating up too much, which could raise temperatures to 4,000,000 K. This gives extra time - tens of thousands of years and more - for that to gather even more mass. After all, as soon as the star begins the synthesis of pure hydrogen (protons), the energy output becomes so large that the star no longer grows - therefore the early stages of development are critical. If it were not for the participation of deuterium in the synthesis, the largest stars would exceed the Sun by a maximum of three times, rather than a hundred times, like those that we now have nearby.


Composite image of the first exoplanet ever observed directly (red) and its maternal brown dwarf. A true star would be physically larger and its mass would be greater than that of this brown dwarf.

To get to the core temperature of 4 000 000 K and become a true star, you need to gain at least 7.5% of the solar mass: about 1.5 × 10 ²⁹ kg. To become a brown dwarf and start the synthesis using deuterium, you need from 2.5 × 10 ²⁸ kg to 1.5 × 10 ²⁹ kg. And just as often as double stars, in space, there are double brown dwarfs.


The two brown dwarfs that make up the Luman 16 system, and they can eventually merge together to create a star.

In fact, the brown dwarf closest to us, the Luman 16 system, is a dual system. It is also known that giant planets move around other brown dwarfs in orbits. In the case of Luhmann 16, the following masses of two brown dwarfs were identified:

1. The main one is from 8.0 × 10 ²⁸ to 1.0 × 10 ²⁹ kg.
2. Secondary - from 6.0 × 10 ²⁸ to 1.0 × 10 ²⁹ kg.

In other words, there is an excellent chance that if these two sub-stars orbiting around each other at a distance of about three times the distance from the Sun to the Earth join, they will form a real star. Any addition of mass that carries an understar across the mass line necessary to start burning hydrogen will turn it into a star.


The two brown dwarfs that make up Luman 16 were photographed 12 times by the Hubble telescope, and we determined their motion and relative orbits over a period of several years.

Intuition does not fail our reader: yes, masses revolving around each other emit gravitational waves, and this radiation will cause the orbits to decrease. But for such masses and distances, this reduction will take somewhere around ^10,200 years, which is much, much longer than the lifetime of the Universe. It is even longer than the life of any star, or even a galaxy, or even a central black hole in a galaxy. If you are going to wait until the gravitational waves turn this pair of brown dwarfs into a star, you will have to wait for a disappointingly long time.


The scenario of falling on each other in a spiral due to gravitational waves for two as well separated brown dwarfs as these will take a very long time. But collisions are very likely. Just as the red stars in a collision produce individual blue stars, the collisions of brown dwarfs can produce stars — red dwarfs.

Periodically objects in space collide. The fact that stars, under-stars, roving planets and everything else moves in the galaxy, mainly under the influence of gravity, means that there is a finite probability that two objects will accidentally collide. It is much better than expecting a decrease in orbit due to gravitational waves, except in particularly extreme cases. On a time scale of the order of 10 ¹⁸ years, “only” is 100 million times larger than the current age of the Universe, brown dwarfs will accidentally encounter either other brown dwarfs or star corpses, and generate a new life in an understar. According to our estimates, this fate awaits about 1% of all brown dwarfs.


The solar atmosphere is not limited to the photosphere or even the corona, but extends millions of kilometers in space, even under the conditions of the absence of flares and emissions

But even if you can’t wait for gravitational radiation to work, and you’re not lucky to encounter another brown dwarf in the interstellar space, you will still have a chance to unite. Usually we imagine that the stars have a certain space that they occupy in space, a certain volume. We also imagine, for example, the atmosphere of the Earth: as if it has a clear ending, a border, between what we consider to be the atmosphere and space. This is stupid! In fact, atoms and particles stretch for millions of kilometers, and flares from stars are more than the radius of the Earth's orbit. Recently it was discovered that brown dwarfs also emit flares - just as a satellite in a low orbit will eventually fall back to Earth, so the friction exerted on each other by brown dwarfs will eventually attract them to each other. For Luhmann 16, this will not work, but if the distance between the two under-stars were comparable to the distance from the Sun to Mercury, and not to the distance from the Sun to Ceres, this effect could work.


Luigi Bedin's long-term study of observing Luman’s under-stars 16 showed us how their position and movement change over time, with a cyclic sequence resulting from the annual motion of the Earth.

So what happens after a merger or collision? Such events are rare, and take away more time than the current age of the Universe. By that time, even the brown dwarf will consume all of its deuterium, and its corpse will cool to a temperature of just a few degrees above absolute zero on the surface. But the energy of a collision or fusion should be enough to create such pressure and heat in the core, which can still - if the threshold of the critical mass is crossed - to launch nuclear fusion. The star will have a small mass, red and live for a very long time, more than 10 trillion years. When the understar finally lights up, it is likely to become the only star glowing in the galaxy during its lifetime; such events will be rare and far apart in time. However, the type of this star will be interesting in itself.


When the two brown dwarfs in the distant future still merge, they are likely to become the only light in the night sky, since all the other stars will already burn out. The resulting red dwarf will become the main source of light remaining in the Universe by then.

It will burn its fuel so slowly that helium-4, which is obtained in this case — the result of a fusion reaction involving hydrogen in the core — will, as a result of convection, leave the core, which will allow additional hydrogen to participate in the synthesis. Convection will be so effective that 100% of hydrogen can burn in the star to the end, resulting in a solid mass of helium atoms. Mass will not be enough to burn this helium, so the stellar remains will shrink to a type of star that is not yet in our Universe: a white helium dwarf. It will take about a quadrillion years to make this white dwarf cool and stop glowing, at which time other galaxy’s brown dwarfs will collide and ignite. By the time the understar finally succeeds and goes through its entire life cycle, becoming a black dwarf, another understar will wait for its opportunity.


A comparison of the colors and sizes of the white dwarf (left), the Earth reflecting sunlight (in the middle) and the black dwarf (right). When white dwarfs finally emit the remnants of their energy, all of them will eventually become black dwarfs

If you could become immortal, you could, in principle, travel from one understar to another, getting energy from the last rare successes of the Universe. Most of the under-stars will remain in this state forever, but some luck will be smiled will burn much later than all other light sources go out. As Winston Churchill's famous phrase says: "Success is not final, failures are not fatal, only courage to continue is important." Perhaps this is true even for the stars, and even more than for us.

Ethan Siegel - astrophysicist, popularizer of science, blog Starts With A Bang! He wrote the books Beyond The Galaxy , and Treknologiya: Star Trek Science [ Treknology ].

FAQ: if the universe is expanding, why aren't we expanding ; why the age of the universe does not coincide with the radius of the observed part of it .