unclebobmartin on Nostr: Out in the vast interstellar emptiness float huge clouds of dust and gas -- mostly ...

Out in the vast interstellar emptiness float huge clouds of dust and gas -- mostly neutral hydrogen, H2 molecules that are loosely bound together by their diffuse gravity. The masses of such clouds are sufficient to create hundreds of stars like our Sun; but the thermal energy of their temperature, and their weak but non-zero angular momentum, are sufficient to prevent their gravity from collapsing them. They are stable structures that will last hundreds of millions, if not billions of years, if not disturbed.

But disturbance can come in the form of a shock wave from a distant supernova or stellar explosion. That shock wave can compress portions of the cloud into densities with sufficient gravitational potential to enter a runaway collapse.

As the collapse proceeds it can stall if the collapsing fragment has too much angular momentum. The cloud, now spinning much faster because of the collapse will often assume a dumbbell shape and the two lobes will separate. With most of the original angular momentum having been shed into their mutual orbit, the two lobes are free to continue their collapse into a binary star system.

Most of these new stars are small, and will collapse into an object roughly the size of Jupiter with little or no internal energy. We call them brown dwarfs. They have been heated by their collapse to glow in the infra red, but will gradually cool.

Some clouds are larger and will collapse to the point where fusion reactions will begin in their cores. First it is the deuterium that fuses, generating quite a bit of heat. But deuterium is rare and that fuel is rapidly exhausted. If the new star is massive enough it may begin to fuse regular hydrogen into helium. Many of these objects are one tenth the mass of the Sun and glow in the red, and near infra-red. We call them red dwarfs. They burn slowly and will last for tens of billions of years.

Some have about the mass of our Sun. They are comparatively rare, but will shine brightly in the visible spectrum, fusing Hydrogen to Helium, and then eventually Helium into Carbon. In the final stages of their live they will become red giants, and will end their lives by repeatedly ejecting their outer shrouds of hydrogen, until only the white hot carbon core remains. These are White Dwarf stars.

But as noted earlier, stars often form in binary pairs. And sometimes the two partners are as massive, or even more massive than the Sun.

The first to die will spread into a Red Giant and disgorge massive amounts of material into the space around it as it pulsates through its death throes. Sometimes that material reaches the partner star, adding slightly to its mass, and reducing the distance between them through friction.

When the Red Giant finally become a White Dwarf, it may be close enough to the partner star to gradually steal material from it -- especially if the partner enters the Red Giant stage.

Matter, mostly hydrogen, builds up on the hot surface of the White dwarf. The White Dwarf may have half the mass of the Sun, and be only a few thousand miles in diameter, roughly the size of the Earth or Mars. Thus the gravitational potential at the surface is huge, and the infalling hydrogen is strongly compressed and heated.

This process continues until the hydrogen accumulating on the surface of the White Dwarf reaches the temperatures and pressures sufficient to ignite hydrogen fusion. This results in a massive thermonuclear explosion called a nova. The explosion is strong enough to blow all the accreted material off the White dwarf and re-expose the carbon core to begin the process again.

From our point of view the binary pair will brighten by many orders of magnitude every few decades. We may just see one of these this Summer or Fall in the constellation of the Northern Crown (Corona Borealis)