Roaming the vast expanse of the universe, between the smallest stars and the most massive planets, exists a peculiar class of celestial objects known as brown dwarfs. Often referred to as "failed stars," these objects, while more massive than gas giants like Jupiter, fall short of the mass of the smallest stars. Despite their diminutive size, brown dwarfs are far from rare. Recent discoveries by astronomers suggest that there could be up to 100 billion of these faintly glowing bodies scattered throughout the Milky Way. In fact, their number might be almost on par with the stellar population of our galaxy, which ranges from 100 to 400 billion.

But what exactly prevents these brown dwarfs from becoming full-fledged stars? The answer lies in their insufficient mass to spark the steady nuclear fusion of hydrogen, a fundamental process that defines stars. Both stars and brown dwarfs originate from massive clouds of gas and dust that collapse to form "protostars." These protostars continue to accumulate material until they reach a mass that ignites hydrogen burning, fusing hydrogen atoms to create helium. However, brown dwarfs, lacking the necessary mass, fail to achieve this stable fusion over the long term.

The Enigma of Brown Dwarfs: The Universe’s "Failed Stars"

Between the smallest of stars and the most massive of planets, the universe is filled with a unique celestial class of objects known as brown dwarfs, or "failed stars." These objects are more massive than gas giants like Jupiter but less so than the smallest stars.

A Universe Full of Failed Stars

Recent astronomical findings suggest that there could be as many as 100 billion of these dimly glowing entities scattered throughout the Milky Way. This puts them at almost the same prevalence as stars, considering the Milky Way’s estimated stellar population, which ranges from 100 billion to 400 billion.

Why Are Brown Dwarfs "Failed"?

The simple answer to why brown dwarfs fail to become stars lies in their mass. They lack the necessary mass to trigger the steady nuclear fusion of hydrogen. Both stars and brown dwarfs are born from massive clouds of gas and dust collapsing to form "protostars". These protostars continue to gather material until they amass enough to start hydrogen fusion, fusing hydrogen atoms to create helium. As Nolan Grieves, a postdoctoral researcher in the Department of Astronomy at the University of Geneva, explains, low mass stars have stable hydrogen fusion, lasting for trillions of years. In contrast, high mass brown dwarfs do not achieve stable fusion over the long term.

More Than Just Massive Planets

However, this doesn’t mean brown dwarfs are incapable of burning hydrogen. Some brown dwarfs do get hot enough to start hydrogen fusion, but they cannot maintain a balance between nuclear burning in their core and photon losses at their surface, resulting in their core temperature eventually falling below the hydrogen burning limit. Thus, they can’t be classified as stars, but neither can they be easily categorized as very massive planets.

The line dividing brown dwarfs and gas giant planets is somewhere between 10 to 14 times the mass of Jupiter. Bodies of this size can trigger the steady nuclear burning of deuterium, a "heavy" form of hydrogen, which gives brown dwarfs their faint glow. Grieves explains, "The major difference between brown dwarfs and planets is their mass and the occurrence of deuterium burning."

However, the dividing line is blurry, as other characteristics beyond mass could result in the burning of deuterium. In the future, we might redefine the difference between planets and brown dwarfs based on their creation process. For now, they continue to exist on the edge of "failure."

Final Thoughts

The existence of brown dwarfs adds another layer of complexity to our understanding of the universe. They pose a challenge to our traditional categorizations of celestial bodies and prompt us to reassess our definitions of stars and planets. This complex interplay between mass, nuclear fusion, and creation processes underscores the versatility and diversity of the universe’s celestial bodies. The future of astronomical research could reveal even more about these intriguing "failed stars," deepening our understanding of the mysteries of our universe.