“Will the orbit of these [brown dwarfs] over a long period of time, eventually become smaller and smaller from the loss of energy through gravitational waves? Will they then eventually end up merging? If so, what happens in a [brown dwarf] merger? Will they merge to become an actual star that goes through fusion? Or is it something else entirely?”
For every star that’s out there in the Universe, for every object that ignited hydrogen fusion in its core, there are many other objects out there that never got massive enough to do so. The largest failures, those that gathered between 13 and 80 Jupiter masses’ worth of material, are known as brown dwarfs. They achieve deuterium fusion in their core, but never cross that critical threshold to become true stars. Many brown dwarfs, just like many stars, though, come in binary systems. If you were to add up all the mass in some of these systems, it would, in fact, be enough to create a star out of. The closest brown dwarf binary to us, Luhman 16, has exactly the right conditions that it could create a star if a merger ever took place.
“Why can suns grow to… many different sizes? That is, ranging from somewhat larger [than] Jupiter up to suns exceeding Jupiter’s orbit?”
“Bigger mass makes a bigger star,” you might be inclined to say. The smallest stars in size should be small because they have the least amount of material in them, while the largest ones of all are the largest because they’ve got the most material to make stars out of. And that’s a tempting explanation, but it doesn’t account for either the smallest stars or the largest ones in the Universe. As it turns out, neutron stars and white dwarfs are almost all larger in mass than our own Sun is, and yet the Sun is hundreds or even many thousands of times larger than they are. The most massive star known is only 30 times the physical size of our Sun, while the largest star of all is nearly 2,000 times our Sun’s size. As it turns out, there’s much, much more at play than mass alone.
“Perhaps, as many suggested, this was evidence of an alien megastructure being constructed?
But another astrophysical scenario could explain it: a recently devoured planet.
Gases would dim the star overall, while outbursts and flares create irregular flux dips.”
Earlier this decade, the Kepler mission became the most successful planet-finding endeavor of all time, turning up thousands of new worlds by measuring the transit data of some 150,000 stars. When planets passed in front of their parent star, they blocked a tiny fraction of their light, leaving behind an imprint of a periodic dimming signal. But one star dims differently from all the others. KIC 8462852, known as Tabby’s star, has irregular dips of up to 20% in brightness, equivalent to ten times the effect of all the Solar System’s planets combined. What could be causing this? While a few astronomers have proposed alien megastructures, another, simpler explanation might explain it all: a recently devoured planet.
“I’m sorry to disappoint you, but there aren’t any black dwarfs around today. The Universe is simply far too young for it. In fact, the coolest white dwarfs have, to the best of our estimates, lost less than 0.2% of their total heat since the very first ones were created in this Universe. For a white dwarf created at 20,000 K, that means its temperature is still at least 19,960 K, telling us we’ve got a terribly long way to go, if we’re waiting for a true dark star.”
Stars live for a variety of ages, from just a million or two years for some to tens of trillions of years for others. But even after a star has run out of its fuel and died, its stellar corpse continues to shine on. Neutron stars and white dwarfs are both extremely massive, but very small in volume compared to a star. As a result, they cool very slowly, so slow that a single one has not yet gone dark in all the Universe. So how long will it take, and who will get there first: neutron stars or white dwarfs? Believe it or not, there’s still enough uncertainty about how neutron stars cool, mostly due to uncertainties in neutrino physics, that we think we know the answer to be white dwarfs – and 10^14 or 10^15 years – but we’re not entirely sure!
“Above a certain mass, the atoms inside large planets will begin to compress so severely that adding more mass will actually shrink your planet.
This happens in our Solar System, explaining why Jupiter is three times Saturn’s mass, but only 20% physically larger.
But many solar systems have planets made out of much lighter elements, without large, rocky cores inside.”
You might think that Jupiter is the largest planet in the Solar System because it’s the most massive, but that’s not quite right. If you kept adding mass to Saturn, it would get larger in size, but if you kept adding mass to Jupiter, it would shrink! For a given set of elements that your planet is made out of, there’s a maximum size it can reach, that’s somewhere in between the mass of Saturn and Jupiter in general. Our Solar System is on the dense side of things, meaning that we’ve discovered a large number of exoplanets out there that are approximately twice the physical size of Jupiter without becoming brown dwarfs or hydrogen-fusing stars. For worlds like WASP-17b, where we’ve measured both the radius and mass, we find that they’re only about half the mass of Jupiter, despite being double the size.