Protoplanetary nebula

A star changes as it grows old.

It lets out big clouds of gas. This is a very fast change. The star gets much hotter during this time.

Strong winds blow from the star. These winds shape the gas into new forms. The gas can look like jets or knots.

Soon the star becomes very hot. It sends out bright light. This light makes the gas glow.

This makes a new shape in space. It is called a planetary nebula. These shapes are very beautiful.

Stars go through many stages in their lives. One stage is called a protoplanetary nebula. This is a very short time for a star.

This happens to stars with some mass. The star loses its outer shell of gas. As the gas leaves, the star gets hotter. It is too cool to make the gas glow yet. Instead, it acts like a reflection nebula. This means it reflects light.

The star sends out fast winds. These winds hit the old gas. This shapes the gas into new forms. The gas might look like jets or knots. These shapes help decide what the star will look like later.

Soon, the star reaches 30,000 K. That is a very high temperature. The star now gives off ultraviolet radiation. This light hits the gas. It makes the gas glow. We call this a planetary nebula. This change must happen in less than 10,000 years. If it takes too long, the gas becomes too thin. Then, no nebula will form.

A protoplanetary nebula is a special part of a star's life. It is a very short period of time. This happens as a star changes very quickly. It occurs between two other stages. First, the star is in the late asymptotic giant branch phase. Later, it becomes a planetary nebula. This phase is important for stars with medium mass. These stars are between 1 and 8 times the mass of our Sun. They emit strong infrared radiation during this time.

This stage works through a series of changes. First, the star loses much of its outer gas layer. This is called the hydrogen envelope. As the star loses this mass, it gets hotter. The temperature starts at around 5,000 K. The star then sends out fast, narrow winds. These winds hit the older, slower gas around the star. This process shapes the gas into new forms. These shapes can look like jets or knots.

Scientists have studied these objects for a long time. The name can be a bit confusing for astronomers. People sometimes use the same name for different things in space. To fix this, researchers Sahai, Sánchez Contreras, and Morris suggested a new name. In 2005, they proposed using "preplanetary nebula." This helps avoid confusion with protoplanetary disks. Early astronomers used the term "planetary nebula" because they saw something like planets. They thought the gas looked like Neptune or Uranus.

There are many specific facts about this change. The star must reach 30,000 K to finish this stage. At this heat, it makes ultraviolet radiation. This light makes the surrounding gas glow. This change must happen in less than 10,000 years. If it takes longer, the gas becomes too thin. This thin gas is called a "lazy planetary nebula." The gas density must stay above 100 particles per cubic centimeter.

We can think of this like a sculptor working with clay. The star is like the artist. The gas is like the soft clay. The fast winds are like the artist's hands. These winds push and shape the gas into beautiful patterns. Some scientists think gravity and disks help shape these jets. They study how energy moves to create these fast winds. This helps us understand how stars change over time.

A protoplanetary nebula, or PPN, is a short-lived stage in the life of an intermediate-mass star. These stars have a mass between 1 and 8 times that of our Sun. This phase occurs during a period of rapid evolution. It happens between the late asymptotic giant branch (LAGB) phase and the planetary nebula (PN) phase. During this time, the object emits strong infrared radiation. It also acts as a type of reflection nebula. Understanding this phase helps astronomers track how stars change as they age.

The process begins during the LAGB phase. At this stage, the star loses mass from its hydrogen envelope. The mass of this envelope drops to about 10^-2 solar masses for a core of 0.60 solar masses. As mass loss continues, the envelope reaches about 10^-3 solar masses. This disruption makes further significant mass loss unlikely. The star begins to move toward the blue side of the Hertzsprung–Russell diagram. The PPN phase officially starts when the star's effective temperature reaches 5,000 K.

As the PPN phase progresses, the central star's temperature continues to rise. This rise happens because of mass loss caused by hydrogen shell burning. During this stage, the star is still too cool to ionize the surrounding gas. This gas is a slow-moving circumstellar shell ejected during the earlier AGB phase. However, the star does drive high-velocity, collimated winds. These winds strike and shape the slower shell. They also entrain the slow-moving ejecta to create a fast molecular wind.

These winds change the physical shape of the nebula. The envelope shape shifts from being roughly spherically symmetric to being axially symmetric. This means it develops a specific axis. The resulting shapes can be bipolar, which means they have two lobes. They can also appear as knotty jets or bow shocks. These shocks are similar to Herbig–Haro objects. Observations from 1998 to 2001 show that this phase shapes the final look of the planetary nebula.

The name "protoplanetary nebula" can be confusing for scientists. It is sometimes used to describe protoplanetary disks, which are unrelated. The term "planetary nebula" was originally chosen by early astronomers. They thought the gas looked like planets such as Uranus or Neptune. To prevent confusion, Sahai, Sánchez Contreras, and Morris suggested the term "preplanetary nebula" in 2005. Some scientists also call these objects "post-AGB stars." However, that category includes stars that never ionize their ejected matter.

The PPN phase ends when the star reaches a temperature of 30,000 K. At this heat, the star produces enough ultraviolet radiation to ionize the gas. Once the gas is ionized, it becomes an emission nebula known as a planetary nebula. This transition must happen quickly. It must take place in less than 10,000 years. If the transition is too slow, the gas density falls too low. It must stay above 100 particles per cubic centimeter. If it falls below this, a "lazy planetary nebula" is formed.

Scientists use different models to explain how these fast winds work. An older model suggested radiatively-driven winds. However, research by Bujarrabal in 2001 showed this model could not explain certain observations. The winds had more momentum and energy than the model allowed. Because of this, theorists investigated accretion disk scenarios. These models suggest that binary interactions create an accretion disk. This disk can then use magneto-centrifugal launching to turn gravitational energy into kinetic energy. This process helps explain the highly collimated jets seen in many systems.