Interstellar medium

Space is not empty.

It is full of gas and dust. This stuff sits between the stars. Most of the gas is hydrogen. Some is helium too. Tiny bits of dust are there.

This gas can get very hot. It can also stay very cold. Stars grow inside the thickest gas. When stars die, they send more gas out. This helps new stars form.

Two probes have even flown into this space.

Space between the stars is not empty.

This space is filled with the interstellar medium. It is made of gas and dust. Most of the gas is hydrogen. Some is helium. There are also small bits of carbon and oxygen.

The gas can be in many different forms. It can be very cold and thick. It can also be very hot and thin. In hot areas, the gas is a plasma. This means the atoms have a special charge.

This gas and dust are very important for stars. New stars form in the thickest parts of the gas. When big stars die, they explode. These explosions send new gas and energy into space. This helps make more stars later. Two spacecraft have even traveled into this space. Voyager 1 reached it in 2012. Voyager 2 reached it in 2018. They help us study these distant clouds.

The space between stars is not empty. It is filled with the interstellar medium, or ISM. This is a mix of gas and tiny dust particles.

It also contains radiation and cosmic rays. The ISM is very important for a galaxy. It acts as a bridge between stars and the galaxy itself. Stars are born inside the thickest parts of this medium. When stars die, they send energy and matter back into it. This cycle helps decide how long a galaxy can keep making stars.

This medium works in different stages called phases. These phases change based on how hot or thick the gas is.

Most of the ISM is gas, while only 1% is dust. In thick molecular clouds, there are 1 trillion molecules in every cubic meter. In very hot and thin areas, there might be only 100 ions per cubic meter. To understand this, think of the air on Earth. Air at sea level has about 1025 molecules in a cubic meter. Even a high-quality vacuum on Earth has 10 quadrillion molecules per cubic meter. The ISM is much thinner than even the best laboratory vacuums.

Scientists have studied how these phases work for a long time. They use a three-phase model to explain what they see. This model includes cold clouds, warm gas, and very hot gas.

The hot gas is often heated by massive explosions called supernovae. These explosions create blast waves that move through the gas. This heating keeps the gas at very high temperatures. Astronomers have spent decades learning how these different parts balance each other. They look at how pressure and magnetic fields keep the ISM moving.

Most of the matter in the ISM is very simple. About 70% of its mass is hydrogen. Another 28% is helium.

The rest is made of heavier elements like carbon, oxygen, and nitrogen. Astronomers sometimes call these heavier elements "metals." Most of the hydrogen and helium came from the very beginning of the universe. The heavier elements were made later inside of stars. This process is called stellar nucleosynthesis. It is how the ISM becomes enriched with new materials over time.

We have even sent machines to touch this space.

Voyager 1 was the first human-made object to reach the ISM. It arrived there on August 25, 2012. Its twin, Voyager 2, entered the medium on November 5, 2018. These spacecraft help us study the dust and plasma directly. They are still working to send data back to Earth. We expect these missions to continue studying the ISM until at least 2025. Learning about this medium helps us understand our own place in the Milky Way.

The interstellar medium, often called the ISM, is the matter and radiation that exists in the space between star systems within a galaxy.

It is not an empty void, but a complex mixture of gas and dust. This gas exists in several forms: ionic, atomic, and molecular. The ISM also contains cosmic rays and an interstellar radiation field made of electromagnetic radiation. This medium is vital to the life of a galaxy. It serves as an intermediate link between the scale of individual stars and the scale of the entire galaxy. Stars form within the densest regions of the ISM. In turn, stars return matter and energy to the ISM through stellar winds, planetary nebulae, and supernovae.

To understand the ISM, we must look at its various phases. These phases are distinguished by their temperature and density. The matter is mostly gas, making up 99% of the ISM by mass, while dust accounts for only 1%. The gas is primarily hydrogen, followed by helium. A small amount of heavier elements, such as carbon, oxygen, and nitrogen, are also present. Astronomers call these heavier elements "metals." Most hydrogen and helium resulted from primordial nucleosynthesis. However, the heavier metals come from stellar nucleosynthesis, which occurs during stellar evolution.

Scientists use a three-phase model to explain how these different parts behave. This model includes a cold, dense phase consisting of neutral and molecular hydrogen clouds. There is also a warm intercloud phase made of rarefied neutral and ionized gas. The third phase is a dynamic, very hot phase. This hot gas is often heated by shock waves from supernovae. These phases exist in a rough thermal pressure equilibrium. This means that if one region has excess pressure, it will expand and cool. If a region has low pressure, it will be compressed and heated.

Density varies wildly across these different regions. In dense molecular clouds, number densities can reach 10^12 molecules per cubic meter. This is 1 trillion molecules in a single cubic meter. In contrast, hot and diffuse regions may have a density as low as 100 ions per cubic meter. To put this in perspective, air at sea level has about 10^25 molecules per cubic meter. Even a high-quality laboratory vacuum contains about 10^16 molecules per cubic meter. Because the density is so low, the ISM behaves like a plasma. This means it responds to electromagnetic radiation rather than acting as non-interacting particles.

Specific processes drive the changes between these phases. For example, massive OB stars produce high-energy photons. These photons can ionize neutral hydrogen atoms. This creates a dynamic equilibrium between ionization and recombination. This process sets up the boundary between the Warm Ionized Medium and the Warm Neutral Medium. In very dense regions, stars create "H II regions." These are areas where gas is highly ionized and at a much higher pressure than the average ISM. This overpressure causes the ionized gas to expand rapidly in a process called a Champagne flow.

When massive stars reach the end of their lives, they explode as supernovae. These explosions create massive blast waves. These waves heat the surrounding gas to the coronal phase, also known as the Hot Ionized Medium. This phase can reach temperatures between 1,000,000 and 10,000,000 Kelvin. This gas is so thin that collisions between particles are rare. Because of this, it loses very little energy through radiation. This allows the temperature to remain high for hundreds of millions of years.

Humanity has even sent tools to explore this space directly. Voyager 1 became the first artificial object from Earth to reach the ISM on August 25, 2012.

Its twin, Voyager 2, entered the medium on November 5, 2018. These spacecraft allow us to study interstellar plasma and dust firsthand. The missions are expected to continue providing data until at least 2025. By studying these probes, we gain a better understanding of the environment that surrounds our own solar system.