The Coolest Stars in the Universe

Source: NASA

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.

Source: NASA

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.

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3 responses to “The Coolest Stars in the Universe

  • drgrundstrom

    Cool topic Vineet!
    Brown dwarfs really are quite interesting things – lots of numerical modelers want to work on them, but they’re rather complicated 😉 It’s also really interesting that they fuse lots of other things in their cores – many people wouldn’t actually call them “stars” but many would…
    Another thing to make sure everyone realizes is that it’s the SURFACE of the brown dwarf we’re seeing – they’re hotter inside.
    My hypothesis on the coolness of some of them (especially that 80F one!) is that they’re really old and have no companion to help warm them thus they just keep cooling off. Thanks for the topic!

  • alliemikels

    This was a really interesting topic, and I was amazed by the relatively cool temperatures you cited (even if they’re of the surfaces). It’s interesting how the conditions in which a star forms seals its fate. I wonder…what percentage of “stars” turn out to be brown dwarfs? I also wonder if there are any star-like objects out there that we have yet to discover.

  • dae3212

    Very interesting. It’s fascinating that the brown dwarfs or “failed stars” are so relatively cool for almost being a star. The fact that one can be 80 degrees on its surface is amazing. Most of my summer back home is spent in weather constantly above 80 degrees. But like Dr. Grundstrom’s comment, it does make sense that they would continue to cool without a companion star to heat them. Look at Jupiter for instance. Although it’s 15-75 smaller than the brown dwarfs you talk about, It can be considered a failed star. Jupiter doesn’t have a defined surface, but at the upper edge of the cloud cover, it is thought to be about -150 degrees C. Overall, a very interesting topic and a good post.

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