Stars are really, really, hot. This is something you should know by now. Our own sun has a surface temperature of about 9,900 degrees Fahrenheit, and, it gets hotter the closer you get to the core. Of the multitude of stars in the universe, some are bigger than the sun, others are smaller, some are hotter, others are cooler. Yet, as far as we’re concerned, they’re all still really hot compared to the temperatures we face here on Earth… right?
Not all of them. Basically, there’s this group of celestial objects called brown dwarfs. “Normal” stars, like our sun, release energy through hydrogen fusion. Scientists think that a brown dwarf begins its life like any other star, collapsing under its own weight into a dense ball of gas (NASA, 2011). Yet, brown dwarfs lack the mass required “to fuse atoms at their cores and thus don’t burn with the fires that keep stars like our sun shining steadily for billions of years.” (Calvin & Perrotto, 2011) For this reason, they are often called “failed stars.” These objects have masses ranging between 15 to 75 times that of Jupiter. Some are capable of fusing deuterium, while others fuse lithium. They’re… odd. To say the least.
Because they don’t derive energy from hydrogen fusion, these stars are much cooler than their relatives. In March of last year, researchers at the University of Hawaii found a brown dwarf with a surface temperature of 206 degrees Fahrenheit. That’s about the same temperature as a freshly brewed cup of coffee. Just a few month’s later, NASA’s Wide-field Infrared Survey Explorer (WISE) discovered another cold brown dwarf (circled in the picture below). Its temperature was determined to be about 80 degrees Fahrenheit. For now, this is the coolest star ever discovered.
Finding brown dwarfs is a tricky exercise. Because they don’t conduct fusion reactions like normal stars, you can’t look for them by relying solely on visible light. These stars are so cool they emit infrared light, and even with the proper equipment they can still be difficult to see. What makes these objects particularly fascinating is that, for the most part, we envision space to be vast swaths of nothingness. Sure, there are an innumerable number of stars, planets, and other objects in the universe, but the distances between objects in space are extraordinary. Brown dwarfs suggest that the universe is slightly more crowded than we originally thought.
So, there you have it. Stars which are cooler than a pot of boiling water, or even a hot summer day here on Earth. The universe is weird like that.
For more reading, see: