The universe is an astoundingly vast and complex expanse filled with a multitude of wonders, many of which spark our curiosity and drive us to seek understanding. Among these marvels, stars hold a particularly special fascination due to their immense variety and the vital roles they play in the cosmos. While the term "star" typically conjures images of dazzlingly bright celestial bodies like our Sun, the stellar family encompasses a broad spectrum of objects that can differ dramatically in size, temperature, and luminosity. Two such interesting members of this family are the smallest and coldest stars. These diminutive and frigid stars may not shine as brilliantly as their larger counterparts, but they possess an allure deeply rooted in their extreme and intriguing characteristics.

The smallest stars belong to a class known as red dwarfs. These stars, also referred to as M-dwarfs, are relatively faint and cool compared to stars like our Sun, which is classified as a G-type main-sequence star. The defining feature of red dwarfs is their low mass. They hold a particularly significant place in the stellar hierarchy because they represent the lower end of the mass spectrum for stars capable of sustaining hydrogen fusion in their cores. It is this ability to undergo fusion that differentiates them from brown dwarfs, which are often termed "failed stars" due to their insufficient mass to sustain hydrogen fusion.

Red dwarfs are typically much smaller than the Sun, with diameters and masses that can be as low as one-tenth of those of our star. Their surface temperatures range between two thousand five hundred to three thousand five hundred Kelvin, far cooler than the Sun's surface temperature, which stands at around five thousand seven hundred seventy-eight Kelvin. Despite their relatively modest energy outputs, red dwarfs make up the majority of the stellar population in the Milky Way galaxy. They are extraordinarily long-lived due to their efficient fuel consumption and slow fusion rates. Red dwarfs can persist for tens to hundreds of billions of years, far exceeding the lifespan of more massive stars.

The characteristics of the smallest red dwarfs present an intriguing scenario for planetary formation and the potential for extraterrestrial life. Their long lifespans allow for a stable and enduring environment around which planets can orbit. Additionally, the habitable zones of red dwarfs, where conditions might be right for liquid water to exist, are located much closer to the star itself. This proximity poses both tantalizing possibilities and substantial challenges. While it increases the chances of detecting exoplanets within these habitable zones, it also subjects those planets to intense stellar activity, including flares and radiation, which could impact the potential for life.

One exceptionally captivating example of the smallest class of red dwarfs is the star known as EBLM J0555-57Ab. This star is part of a binary system and, incredibly, is only slightly larger than Saturn. Being barely massive enough to sustain hydrogen fusion, EBLM J0555-57Ab represents the lower size limit for stars in general. Its discovery was noteworthy because it underscores the fine line between stars and brown dwarfs. EBLM J0555-57Ab’s minimal size showcases the breadth of stellar diversity and pushes the boundaries of our understanding of star formation and classification.

On the colder end of the stellar spectrum lies another category of fascinating objects: the brown dwarfs. Although they are not classified as true stars, brown dwarfs deserve mention due to their unique properties that blur the traditional lines between stars and planets. Brown dwarfs possess masses that range between thirteen and eighty times that of Jupiter, which places them below the threshold needed for sustained hydrogen fusion. However, they can undergo limited fusion of deuterium and sometimes lithium in their cores, producing faint amounts of heat and light.

Among the brown dwarfs, the coldest known specimens are those that fit into the Y-dwarf classification. Y-dwarfs are so cool and dim that they can be challenging to detect using traditional astronomical methods. Their surface temperatures are comparable to those of the outer planets in our solar system, with some Y-dwarfs having temperatures as low as two hundred fifty Kelvin. This extreme coolness means that Y-dwarfs emit most of their radiation in the infrared spectrum, making them effectively invisible to optical telescopes. It is through sophisticated infrared observatories, such as the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE), that astronomers have been able to pinpoint these elusive objects.

One of the most remarkable Y-dwarfs discovered is WISE J085510.83-071442.5, often abbreviated as WISE 0855-0714. This brown dwarf is located relatively close to our solar system, at a distance of approximately seven point two light-years. Its temperature is estimated to be between two hundred twenty-five and two hundred sixty Kelvin, even lower than the freezing point of water on Earth. The discovery of such a cold object is immensely significant because it challenges and expands our understanding of the thermal properties and atmospheres of substellar objects. It also reinforces the notion that the galaxy is teeming with a wide variety of celestial bodies that occupy the gray areas between traditional classifications.

The existence of these smallest and coldest stars and star-like objects provokes profound questions about the processes of stellar and planetary formation, as well as the criteria we use to define these objects. The study of red dwarfs, especially those at the minimal mass threshold like EBLM J0555-57Ab, helps us refine our models of star formation and understand the lower mass limits at which hydrogen fusion can occur. Meanwhile, the exploration of Y-dwarfs like WISE 0855-0714 invites us to delve deeper into the characteristics of ultra-cool atmospheres and the complex interplay of gravitational and thermal dynamics in brown dwarfs.

Furthermore, these objects may have implications for the search for extraterrestrial life. Given the abundance and longevity of red dwarfs, along with their extensive habitable zones, they present numerous opportunities for planets orbiting within these zones to develop and sustain life. Although the intense stellar activity of red dwarfs may pose obstacles, it is conceivable that life, if it exists, could adapt to or be protected from such harsh conditions by various natural mechanisms. As for brown dwarfs, particularly the coldest among them, their study can offer insights into the characteristics of rogue planets and free-floating planetary-mass objects, broadening the scope of our quest to discover life beyond our solar system.

The smallest and coldest stars and star-like entities are vital pieces of the cosmic puzzle. They embody the incredible diversity of the universe's stellar population and challenge our perceptions of what a star can be. These objects may be diminutive in size and low in temperature and luminosity, but they are colossal in their capacity to inform and inspire. As we continue to probe the depths of space with ever more sophisticated technologies, these small and cold denizens of the cosmos will undoubtedly yield more secrets, deepening our understanding of the universe and our place within it.