A common envelope is a shared cloud of gas that temporarily surrounds two stars, causing them to get much closer together.

Key idea: Drag from the common envelope gas causes the two stars to spiral closer together, leading to either the gas being ejected or the stars merging.

During this phase, the two stars inside the common envelope experience a kind of friction from the gas. This friction is called a drag force. Because of this drag, the stars lose energy. When orbiting objects lose energy, they move closer together.

This process continues until one of two things happens: Either the gas cloud gets completely blown away, leaving the two stars much closer than before, or the stars get so close that they crash into each other and become one single star. This whole common envelope phase is very quick compared to the entire life of a star.

“The common envelope phase is very quick compared to the entire life of a star.”

Key idea: Common envelope evolution explains how very close binary systems, often involving compact objects, are formed.

When the common envelope is ejected, it can leave behind some very interesting pairs of stars. These pairs often include a compact object, which is a very dense remnant of a star that has died. Examples include cataclysmic variables, X-ray binaries, or even two white dwarfs or two neutron stars orbiting very closely.

These systems are special because their stars are incredibly close to each other. If they had not gone through a common envelope phase, they would be much farther apart. The common envelope evolution explains how they ended up so close. These tightly packed systems are important because they can create powerful gravitational waves or even explode as Type Ia supernovae.

A common mistake is to confuse a common envelope with a contact binary. In a common envelope, the gas cloud does not rotate with the stars. In a contact binary, the stars actually touch and share material, and the shared material rotates along with the stars, almost like a single, peanut shaped object.

Key idea: Common envelopes form when one star expands rapidly or when mass transfer between stars becomes unstable, causing a shared gas cloud to engulf both.

A common envelope forms when something makes the two stars in a binary system get much closer very quickly, or when one of the stars suddenly grows very large. Imagine one star, called the 'donor star', starts to expand. As it expands, it can begin to spill its outer layers of gas onto its companion star. This is called mass transfer.

As the donor star loses mass, the two stars get even closer. This makes the donor star expand even more and spill even more gas. This creates a fast, out of control process. If the receiving star cannot take all this new gas, the extra gas forms a giant cloud that surrounds both stars, creating the common envelope.

“If the receiving star cannot take all this new gas, the extra gas forms a giant cloud that surrounds both stars.”

Key idea: Inside the common envelope, the stars spiral closer due to drag, and this lost orbital energy heats and eventually expels the gas cloud, or leads to a merger.

Once the common envelope forms, it contains two main things: the dense core of the original donor star and the companion star. These two objects continue to orbit each other within the gas cloud. However, as mentioned before, the drag forces from the gas make them lose energy.

This energy loss causes the two objects to spiral inward, getting closer and closer. This inward spiral is called a spiral-in. The energy that the stars lose is not just wasted; it heats up and expands the surrounding gas envelope. The common envelope phase ends when either the heated gas is completely pushed away into space, or the two objects inside merge together because they have run out of energy to push the envelope away.

Key idea: Common envelope events are hard to observe directly but are thought to appear as luminous red novae, showing a specific pattern of brightening and fading.

It is very hard to directly see a common envelope event happening. Scientists mostly figure out that they must have happened by looking at unusual binary star systems that cannot be explained any other way. However, we do have some ideas about what a common envelope event would look like.

Such an event should be brighter than a typical nova (a small stellar explosion) but not as bright as a supernova (a huge stellar explosion). The outer part of the common envelope would be relatively cool, around 5,000 Kelvin, and would glow red. But because the cloud would be so large, it would shine very brightly, like a red supergiant star.

An event would start with a sudden increase in brightness, then stay bright for a few months as the hydrogen gas in the envelope cools and recombines. After that, the brightness would quickly fade. Several events called luminous red novae (LRNe) or intermediate luminosity red transients (ILRTs) have been observed that fit this description. These events have relatively slow expansion speeds and release a lot of energy.