Yes, there are stars between galaxies, a population of unbound stars that wander the cosmic void. These wandering suns represent a significant, though faint, component of the universe’s total light. They are stars that have been ejected from their home galaxies and now drift freely, bound only to the collective gravitational pull of a larger galaxy cluster.
The Intergalactic Medium
The environment where these stars reside is called the Intergalactic Medium (IGM). This vast expanse is primarily composed of extremely diffuse gas, mostly ionized hydrogen and helium, which are remnants from the early universe. The density of this material is extraordinarily low, averaging only about one atom per cubic meter across the immense distances of space.
The IGM also contains the bulk of the universe’s dark matter, which provides the gravitational scaffolding for large-scale structures. The gas within the IGM is often very hot, reaching millions of degrees Kelvin, especially within massive galaxy clusters.
Defining Rogue Stars
The stars that drift between galaxies are often referred to as “rogue stars,” though astronomers study them as a collective phenomenon known as Intracluster Light (ICL). These stars are not gravitationally bound to any single galaxy; instead, they are tethered only to the immense gravitational potential of the entire galaxy cluster. The ICL appears as a diffuse glow that permeates the central regions of galaxy clusters.
This population is characterized by its extreme faintness and wide, dispersed distribution. The ICL is generally made up of older stars, with their color and chemical makeup indicating they were shed from their parent galaxies long ago. In some massive clusters, these unbound stars can account for up to 20% or more of the total light emitted by the cluster.
The Origin Story
The existence of these stars is a direct consequence of galactic interactions, primarily occurring within galaxy clusters. The most significant mechanism for their creation is a process called tidal stripping. This occurs when smaller galaxies fall into the deep gravitational well of a massive galaxy cluster.
As the smaller galaxies orbit, the immense differential gravitational forces exerted by the cluster’s core pull stars away from the edges of the infalling galaxies. These forces strip the stars out into long, faint streams called tidal tails before they eventually disperse throughout the cluster. The stars that form the ICL are remnants of repeated galactic mergers and close gravitational encounters. This process has been building up the population of intergalactic stars over billions of years.
Detecting the Faint Light
Observing the faint light from rogue stars is a challenge for astronomers due to its extremely low surface brightness. The ICL is so dispersed that its light is easily lost against the brightness of the night sky, even with powerful telescopes. Researchers must use highly sensitive instruments and employ extremely long exposure times to gather enough photons.
A common technique involves combining and statistically averaging data from hundreds of galaxy clusters to boost the signal over the noise. This allows astronomers to map the ICL as a diffuse, extended structure, rather than resolving individual pinpricks of light. Since the distribution of the ICL closely follows the overall gravitational potential of the cluster, this faint light is a powerful tool for tracing the distribution of invisible dark matter.