Interstellar bubbles and their origin

In brief:

• Interstellar bubbles are spherical structures resulting from the dynamic interaction between stellar winds, supernova explosions, and the ambient interstellar gas.
• The origin of bubbles mainly relies on the combined action of powerful stellar winds from massive stars and cataclysmic phenomena such as supernovae.
• The interstellar medium in which these bubbles form consists of a low-density gas but rich in carbon molecules and subject to magnetic fields that influence their development.
• The bubbles influence the structure of the interstellar medium, promoting the formation of molecular clouds and regulating local stellar evolution.
• Recent studies and observations using instruments like ALMA provide a deep understanding of the physical mechanisms behind the formation and expansion of interstellar bubbles.

Between diffuse matter and stellar explosions, interstellar bubbles trace the dynamics of a cosmos in perpetual change. These hollow nebulae, sometimes visible in X-rays or in the radio domain, emerge as witnesses of the winds projected by massive stars or debris from ancient supernovae. The complex interaction between these stellar winds and the interstellar gas generates cavities where pressure, temperature, and density evolve radically.

The interstellar medium, although weakly dense – sometimes with barely a few hundred particles per cubic centimeter – presents a rich chemistry where molecular hydrogen and unsaturated carbon molecules dominate, providing a unique framework for the formation of bubbles. These structures then influence not only the local morphology of matter but also the birth of new stars. Their origin, evolution, and impact constitute a fascinating subject blending astrophysics, astrochemistry, and plasma dynamics.

The physical mechanisms behind interstellar bubbles

Interstellar bubbles mainly form under the combined action of stellar winds and supernova explosions, two powerful phenomena capable of profoundly altering the interstellar medium. Stellar winds are streams of charged particles expelled at high speed by massive stars, particularly Wolf-Rayet or O-type stars. These winds blow away the surrounding gas and dust, sculpting spherical cavities in the interstellar gas with sizes that can reach several tens of parsecs.

When the star reaches the end of its life, it releases immense energy through the supernova explosion. This phenomenon creates a highly energetic shock wave that pushes the surrounding gas, enlarging or generating new bubbles. The pressure exerted by the shock interacts with the gas and dust, forming dense shells around these cavities. This process can be described through the laws of fluid mechanics and magnetohydrodynamics, as the magnetic fields present in the interstellar gas also play a crucial role in the morphology and dynamics of the bubbles.

The classic model of the formation of an interstellar bubble involves several stages:

1. Initial rapid expansion phase where the stellar wind forms a hollow shell around the star.
2. Interaction with the denser ambient medium, which slows down and compresses the shell.
3. Eventual local amplification of magnetic fields, shaping the bubble’s form and influencing gas dynamics.
4. Final phase of gradual dispersion, when internal energy decreases and the bubble merges with the rest of the interstellar medium.

The key parameters that determine the properties of the bubbles are the speed and density of the stellar winds, the density of the surrounding interstellar medium, and the intensity of the magnetic fields. For example, a Wolf-Rayet star generates extremely fast winds (several thousand km/s), resulting in large and hot bubbles that are sometimes observable in X-ray radiation. In contrast, weaker winds create smaller and colder structures.

Chemical composition and characteristics of the interstellar medium favoring bubble formation

The interstellar medium constitutes the essential environment for the genesis of bubbles. It is composed of 99.999% hydrogen and helium, the two lightest elements in the universe, with tiny traces of heavier elements such as carbon, nitrogen, oxygen, silicon, and sulfur. This chemical composition, while highly diluted, is fundamental to the dynamics and chemistry of interstellar bubbles. Dihydrogen (H2) is the most abundant molecule, but it is the rare and complex unsaturated carbon molecules that characterize interstellar chemistry and influence the temperature and density of the gas.

Spectroscopic observation reveals a diversity of molecules, including cyanopolyyne, ammonia, and polycyclic aromatic hydrocarbons (PAHs), which develop in the cold and opaque regions of the interstellar medium. These molecules play an indirect but crucial role in bubble formation by affecting the cooling processes of the gas and the chemical interactions within the cloud.

Parameter	Typical Value	Impact on bubble formation
Average gas density	~ 100 particles/cm³ (diffuse medium) to thousands/cm³ (dense clouds)	Density conditions the resistance to bubble expansion and the compression of shells.
Temperature	—160°C for the diffuse medium, up to several hundred °C in shock regions	Influences internal pressure and thus the expansion dynamics of the bubbles.
Magnetic field	On the order of microgauss to a few tens of microgauss	Controls the shape, stabilizes, or directs the expansion of bubbles.
Chemical composition	99.999% hydrogen + helium; traces of heavy elements and carbon molecules	Ensures the molecular structure of the gas and the formation of compounds influencing thermal processes.

Due to this composition, bubble formation is intimately linked to the stellar cycle. Massive stars gradually enrich the medium with winds laden with heavy elements. This locally increases “metallicity,” a key factor in the formation of molecular clouds. These clouds, in regions of low radiation or behind the protective shell of the bubbles, can in turn initiate the birth of new stars.

Impacts of supernovae and stellar winds on the dynamics of interstellar bubbles

The emergence and growth of interstellar bubbles result directly from the violent phenomena that punctuate the short but intense life of massive stars. The initial stellar winds constantly blow a stream of charged particles, eroding the surrounding gas and forming a hollow structure. Upon the star’s death, the supernova imposes itself as a colossal ejector of energy and matter, massively increasing pressure on the shell and causing rapid expansion or even destabilization of the bubble.

The shock waves from the supernova heat the gas to millions of degrees and generate intense radiation, particularly in the X-wavelengths. This energy transforms the bubble into a hotspot of extreme physical phenomena, where ionized matter interacts with the magnetic field and neutral gas. Such interactions sometimes generate filament structures or nested multiple bubbles.

Interestingly, these phases can also restart the formation of molecular clouds by compressing the peripheral gas, thus serving as an indirect engine of local stellar evolution. This mechanism recycles the material enriched with heavy elements, favoring the emergence of a new cycle of stars-gas life.

The combined effects of winds and supernovae also create an environment conducive to local amplification of magnetic fields. These fields, despite their relatively low intensity, orient the shape of the bubbles and can limit their expansion in certain directions, resulting in non-spherical shapes observed in specific cases.

Role of magnetic fields and interstellar gas interactions in the morphology of bubbles

The magnetic fields present in the interstellar medium play a fundamental role in the shape and dynamics of bubbles. These fields, although weak on a terrestrial scale (a few microgauss), are sufficiently powerful in the vacuum of space to influence the trajectories of charged particles and the structuring of gas. Through their magnetic pressure, they act as a net that channels or limits the expansion of bubbles, often in preferential directions related to the topology of the field.

The interstellar gas, while very low in density, exhibits remarkable complexity due to its local variations in temperature, density, and chemical composition. This gas interacts with stellar winds and shock waves from supernovae, creating a dynamics full of hydrodynamic and magnetohydrodynamic instabilities.

In some cases, these interactions form complex structures of several nested bubbles or fragmented shells that contribute to the formation of molecular clouds, dense and cold regions where interstellar chemistry develops, and where the next generation of stars may emerge. The role of magnetic fields in stabilizing these clouds is now better understood thanks to advanced numerical simulations and high-resolution observations by instruments like ALMA.

The phenomena of gas-magnetic field interaction can also explain intriguing observations, such as asymmetric bubbles or even ring-shaped or thin filament structures, testifying to a dynamic complexity often underestimated but essential for a comprehensive understanding of the interstellar medium.

• Interstellar bubbles primarily form through the combined action of stellar winds and supernova explosions.
• The chemical composition of the medium, dominated by hydrogen and unsaturated carbon molecules, influences their evolution.
• Magnetic fields orient the morphology of bubbles and interact with the interstellar gas.
• Shock waves promote the condensation of molecular clouds around the bubbles.
• These phenomena actively contribute to stellar evolution and the distribution of matter in the galaxy.

What is an interstellar bubble?

An interstellar bubble is a spherical cavity carved out in the interstellar medium, formed by the combined action of stellar winds and supernova explosions, where the pressure and temperature of the gas are modified.

How do stellar winds contribute to bubble formation?

Stellar winds are rapid streams of particles expelled by massive stars that erode the surrounding gas, progressively forming a hollow bubble.

What role do magnetic fields play in the morphology of bubbles?

Magnetic fields influence the shape of bubbles by orienting and limiting their expansion, sometimes creating asymmetric or filamentary structures.

What relationship exists between interstellar bubbles and star formation?

Bubbles compressing the surrounding gas favor the formation of dense molecular clouds, necessary conditions for the birth of new stars.

Why do we primarily observe carbon molecules in the interstellar medium?

Carbon is a key element in cosmic chemistry, capable of forming complex and stable molecules even under extreme conditions of the interstellar medium, unlike other heavier or less abundant elements.