Molecular cloud

Space has big clouds of gas.

These clouds are very thick. They are like a nursery for stars.

Inside the clouds, gas and dust clump together. These clumps can turn into stars. This happens when the clumps pull inward.

Most of the gas is a type of hydrogen. It is hard to see. Scientists look for other things to find them. They look for a gas called carbon monoxide. It helps them find the clouds.

These clouds do not last a long time. They change or go away in 10 million years. They are always being made and broken.

Space is full of these busy places. They help make the stars we see.

Space is filled with giant clouds of gas and dust. These are called molecular clouds.

They are very dense parts of space. Many people call them stellar nurseries. This is because they are places where new stars are born.

Inside these clouds, the gas is mostly molecular hydrogen. This is a gas made of two hydrogen atoms joined together. It is hard to see directly. To find it, scientists look for carbon monoxide. This is a different gas that is easier to spot.

Clouds have different parts inside them. Some parts are called clumps. These are larger groups of gas and dust. Other parts are called cores. Cores are much smaller and very thick. If gravity pulls these parts inward, they collapse. This collapse starts the birth of a star.

These clouds do not stay the same forever. They are short-lived. They usually change or break apart in 10 million years. New clouds are always forming as old ones fade away. This keeps the cycle of star birth going in our galaxy.

Molecular clouds are huge, dense regions in space. They are often called stellar nurseries. This name fits because new stars are born inside them.

These clouds make up about 50% of all gas in a galaxy. They are much denser than other parts of space. Most of this gas is in a molecular state.

Inside these clouds, the gas is mostly molecular hydrogen. This is made of two hydrogen atoms joined together. It is hard to see because its structure is very symmetrical. To find it, astronomers look for carbon monoxide instead. This molecule is much easier to spot.

Inside a cloud, the structure is irregular and looks like long threads. The cloud has different levels of thickness. Larger areas called clumps form the main structure. These clumps can be about 1 parsec in size. Inside clumps, there are even smaller, thicker spots called cores.

These cores are ten times smaller than clumps. If gravity is strong enough, these cores will collapse. This collapse is the very first step of making a star. Dust inside the cloud helps by shielding the gas. This shielding protects the molecules from bright ultraviolet light.

Finding these clouds took many years of hard work. In 1951, two groups found radio signals from hydrogen. Ewen and Purcell found the 21-cm line in March. Muller and Oort found it in May of that same year.

Later, astronomers began looking for other molecules. In 1963, Alan Barrett and Sander Weinred found OH at MIT. In 1965, Harold Weaver found more OH in the Orion Nebula.

By 1968, scientists found ammonia. In 1969, they found formaldehyde and molecular hydrogen. These steps led to the first real molecular cloud discovery in 1970.

In 1970, Arno Penzias, Keith Jefferts, and Robert Wilson made history. They found carbon monoxide in the Omega Nebula.

They also found a giant cloud called Sagittarius B2. This cloud is 390 light years from the center of our galaxy. This was the first time a molecular cloud was truly identified. Since then, scientists have found over 10,000 molecular clouds.

Most of these clouds live in the spiral arms of the Milky Way. The Sun is about 8.5 kiloparsecs from the galactic center. These clouds are found in a thin layer in the galaxy. This layer is only 50 to 75 parsecs thick.

Molecular clouds are short-lived structures. They usually last about 10 million years. After that time, they are destroyed or change completely.

We know this because of the age of young stars. Most stars in these regions are 10 to 20 million years old. This matches the life of the clouds. As old clouds fade, new ones are always being built. In the Milky Way, about 150 solar masses of gas enter clouds every year. This cycle keeps our galaxy active and full of new stars.

Molecular clouds are massive, dense regions of interstellar gas and dust. They are often called stellar nurseries because they provide the raw materials for new stars to form. These clouds make up about 50% of the total interstellar gas in a galaxy. While they occupy less than one percent of the interstellar medium's volume, they are the densest parts of it.

Most of the gas within these structures exists in a molecular state. This state allows for complex chemistry and the eventual collapse of matter into stars.

The internal structure of a molecular cloud is irregular and filamentary. It is not a uniform blob but a collection of varying densities. Large substructures called clumps form the main body of the cloud. These clumps are typically about 1 parsec in size. Inside these clumps, there are even denser regions called cores.

Cores are about ten times smaller than clumps. When gravity becomes strong enough, these cores undergo a gravitational collapse. This collapse is the primary mechanism that begins the process of star formation.

Chemistry plays a vital role in how these clouds function. The most abundant gas is molecular hydrogen, known as H2. However, H2 is very difficult to detect directly because its symmetrical shape results in weak rotational and vibrational modes. To find these clouds, astronomers use carbon monoxide (CO) as a tracer. CO is much easier to observe because of its asymmetrical structure and rotational energy.

Cosmic dust also plays a key role by providing shielding. This dust protects the internal molecular gas from being broken apart by ultraviolet radiation. Without this shielding, ultraviolet photons would cause dissociation, turning molecular gas back into atomic gas.

The history of discovering these clouds is tied to the rise of radio astronomy. In 1951, astronomers began detecting radio emissions from neutral hydrogen. Ewen and Purcell reported the 21-cm line in March of that year. Later that May, Muller and Oort reported the same detection at the Kootwijk Observatory.

This 21-cm line occurs when a proton and electron in a hydrogen atom flip their spin states. This flip releases excess energy as a radio signal at 1420.405 MHz. This discovery allowed scientists to map the spiral arms of the Milky Way for the first time.

Following the detection of hydrogen, astronomers began searching for other interstellar molecules. In 1963, Alan Barrett and Sander Weinred found OH emissions at MIT. By 1968, researchers detected ammonia (NH3) in interstellar space. In 1969, scientists identified formaldehyde and molecular hydrogen.

The final breakthrough came in 1970. Arno Penzias, Keith Jefferts, and Robert Wilson identified CO in the Omega Nebula. They also found the giant molecular cloud Sagittarius B2, located 390 light years from the galactic center.

This was the first official detection of a molecular cloud.

Molecular clouds have a specific distribution within the Milky Way. They are mostly found in a ring between the Sun and the galactic center. The Sun is located about 8.5 kiloparsecs from the center. These clouds also align with the galaxy's spiral arms.

Because they reside in the spiral arms, they must form and dissipate quickly. They inhabit a narrow midplane of the galactic disc. This layer has a scale height of only 50 to 75 parsecs. This is much thinner than the layers of warm atomic or ionized gas.

These clouds are short-lived structures in cosmic terms. They typically exist for about 10 million years before being destroyed or changing. We know this because the young stars found inside them are usually 10 to 20 million years old.

Once stars like OB stars reach 10 million years of age, the surrounding cloud material is often dispersed. The clouds are constantly being assembled and destroyed through a continuous cycle. In the Milky Way, approximately 150 solar masses of gas are assembled into molecular clouds every year. This cycle ensures that star formation remains a constant process in our galaxy.