Hello, I'm an expert in astronomy with a passion for exploring the mysteries of the cosmos. When it comes to the "coolest" star, it's important to clarify whether we're talking about the most distant star, the least luminous, or the star with the lowest surface temperature.

If we're talking about the lowest surface temperature, then brown dwarfs are some of the coolest objects that can still be considered stars. They are not massive enough to sustain the nuclear fusion that powers stars like our Sun. One of the coolest known brown dwarfs is WISE J085510.83-071442.5, also known as WISE 0855-0721, which has an estimated temperature of only about 225 Kelvin (-50°C or -58°F).

However, if we're talking about the most distant star observed, as of my last update, that would be Hubble's "Earendel", which is located at a distance so great that its light has taken over 12.5 billion years to reach us.

And if we're considering the least luminous, red dwarfs are the smallest and coolest type of star currently known, with surface temperatures much lower than our Sun. An example of a very cool red dwarf is TRAPPIST-1, which has a surface temperature of about 2,550 to 3,500 Kelvin (2,280 to 3,230°C or 4,128 to 5,832°F), significantly cooler than the Sun's 5,500°C (9,932°F).

So, the "coolest" star really depends on what aspect of "cool" we're focusing on.

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Astronomers Find Coldest Star in the Universe. Astronomers have detected a new candidate for the coldest known star -- one whose temperature is roughly equivalent to a fresh cup of tea. The object is part of a double system and is a type of star known as a brown dwarf, which is essentially a failed star.read more >>