Orion Arm

The Orion Arm is a major structure within the Milky Way galaxy.

It is also known as the Orion–Cygnus Arm or the Local Arm. This galactic feature encompasses our entire Solar System, including the Earth. Scientists often refer to it as the fifth feature of the Milky Way. It serves as our cosmic neighborhood within the larger spiral galaxy. Understanding this arm helps us map our specific location in space.

This arm is located between two other large galactic structures. It sits between the Carina–Sagittarius Arm and the Perseus Arm. The Carina–Sagittarius Arm projects toward the Galactic Center. The Perseus Arm forms the main outer-most arm of our galaxy. Our Solar System is positioned near the inner rim of the Orion Arm. We are located about halfway along the arm's total length. We also reside in a relative cavity called the Local Bubble. This bubble exists within the arm's interstellar medium.

The arm works through a process involving spiral density waves. These waves move through the galaxy and compress interstellar gas. This compression triggers the formation of new stars. The Orion Arm contains many high-mass star-forming regions, or HMSFRs. These regions are areas where large, bright stars are born. Researchers have observed that star formation lags behind gas compression. This happens because the density wave lasts longer than star formation timescales. These timescales are estimated between 10^7 and 10^8 years. This process creates a visible offset between gas and stars.

For a long time, astronomers held different views of this structure. They previously identified it as the Local Spur or Orion Spur. A spur is a minor structure between larger arms. However, evidence presented in 2013 changed this scientific view. The BeSSeL Survey, or Bar and Spiral Structure Legacy Survey, provided new data. They analyzed the parallax and proper motion of many interstellar masers. These masers are found in high-mass star-forming regions. The survey looked at more than 30 methanol and water masers. This research suggests the arm is more than just a minor spur.

Recent measurements show the Orion Arm is quite large. It has a length of approximately 20,000 light-years. The width of the arm is about 3,000 light-years. It wraps around less than a quarter of the Milky Way. The arm has a pitch angle between 10.1° and 11.6°. Scientists used the Gaia DR2 mission to map stellar density. They studied stars that are about 1 billion years old. These stars are more evolved than the gas in star-forming regions. This helped researchers compare gas distribution to stellar density.

The arm is named after the prominent Orion constellation. This constellation is very bright in the winter Northern Hemisphere. It is also visible in the summer Southern Hemisphere. Many famous celestial objects reside within this arm. You can find the bright stars Betelgeuse and Rigel here. The three stars of Orion's Belt are also part of it. The Orion Nebula is one of the most well-known objects. Many Messier objects are also located in this region. These include the Pleiades and the Ring Nebula.

Many diverse objects populate the Orion Arm's vast space. It contains the Butterfly Cluster, also known as M6. The Ptolemy Cluster, or M7, is another resident. You can find many open clusters like M23, M25, and M29. The Dumbbell Nebula, or M27, is located here as well. Other notable objects include the Beehive Cluster, M44. The Owl Nebula, M97, is also found in this arm. These objects help scientists understand the composition of the arm. They show how gas and stars are organized together. This complex system connects our local space to the wider galaxy.