Coolest Brown Dwarf Spotted by Earth-bound Telescopes


Astronomers have found the coldest known star — a brown dwarf in a double system about as hot as a cup of tea. The discovery blurs the line between small cold stars and large hot planets. The star, CFBDSIR 1458+10B, is the dimmer member of the binary system, about 75 light-years from Earth.

Lead study author Michael Liu, from the University of Hawaii’s Institute for Astronomy, said finding ever-cooler stars “has been one of the big themes of this field since it’s existed in the last 15 years.” Brown dwarfs are essentially failed stars; they lack enough mass for gravity to trigger the nuclear reactions that make stars shine. Liu said while the idea of a brown dwarf is many decades old, they were first confirmed in 1995, the same year the first gas giants were detected around other stars.

“Residing at the extremes of low mass, luminosity and temperature, brown dwarfs serve as laboratories for understanding gas-giant extrasolar planets as well as the faint end of the star formation process,” write the authors in the new paper, in the Astrophysical Journal. “The coolest known brown dwarfs, the T dwarfs, have temperatures (~600–1400 K) … that are more akin to Jupiter than any star.”

Liu said cool brown dwarfs are exciting to find partly because they make great proxies for studying the mysteries of water cloud formation in the atmospheres of gas giants. Such clouds are believed to form when temperatures dip below 400 to 450 K.

“We probably will never get as detailed spectra from gas giants around other stars,” he said, “because the planets are gravitationally bound to their stars. It’s very  hard to isolate the light from gas giant.” But brown dwarfs more often occur in isolation.

Three different telescopes were used to study the system: the ESO’s Very Large Telescope (VLT) in Chile, the Keck II Telescope in Hawaii and the Canada–France–Hawaii Telescope, also in Hawaii. The VLT was used to show that the composite object was very cool by brown dwarf standards.

“We were very excited to see that this object had such a low temperature, but we couldn’t have guessed that it would turn out to be a double system and have an even more interesting, even colder component,” said Philippe Delorme of the Institut de planétologie et d’astrophysique de Grenoble, a co-author of the paper.

CFBDSIR 1458+10 is the name of the binary system. The two components are known as CFBDSIR 1458+10A and CFBDSIR 1458+10B, with the latter the fainter and cooler of the two. They seem to be orbiting each other at a separation of about three times the distance between the Earth and the Sun in a period of about 30 years.

The dimmer of the two dwarfs has now been found to have a temperature of about 100 degrees Celsius, or about 370 K — the boiling point of water, and not much different from the temperature inside a sauna. By comparison the temperature of the surface of the Sun is about 5500 degrees Celsius.

The hunt for cool objects is a very active astronomical hot topic. The Spitzer Space Telescope has recently identified two other very faint objects as other possible contenders for the coolest known brown dwarfs, although their temperatures have not been measured so precisely. Future observations will better determine how these objects compare to CFBDSIR 1458+10B. Liu and his colleagues are planning to observe CFBDSIR 1458+10B again to better determine its properties and to begin mapping the binary’s orbit, which, after about a decade of monitoring, should allow astronomers to determine the binary’s mass.

Source: ESO press release and a brief interview with lead author Michael Liu. See also the paper by Liu et al., “CFBDSIR J1458+1013B: A Very Cold (>T10) Brown Dwarf in a Binary System.

9 Replies to “Coolest Brown Dwarf Spotted by Earth-bound Telescopes”

  1. i don’t want to dishonor artists and their impressions, but are the brown dwarfs really glowing red and pink?

  2. I think someone’s getting their fahrenheit and celsius mixed up. 100 degrees fahrenheit in a sauna is good and hot, but it ain’t gonna boil no water! Consider the implications, “Honey, make sure the kids only stay in there until their skin peels off!”.
    40 celsius is sauna, 100 Fahrenheit is sauna, 100 celsius is this star, 212 fahrenheit is boiling water.
    Does this star shine at all? Was it pictures of its partner in infrared that lead us to number 2?

    1. Peter,
      I am not sure what temperatures you are referring to but the temperatures referred to in the article were niether Farenheit nor Celcius, but were instead Kelvin. The Kelvin scale has the same incremental value as Celcius but the Kelvin scale begins at absolute zero (approximately -273 Celcius) instead of the freezing point of water, making 0 degrees Celius approximately 273 Kelvin.
      As far as the emitting light question is concerned, all celestial bodies that sustain sustain some form of fire or experience nuclear fussion (aka stars) produce some measureable amount of light. You will not see a brown dwarf with naked eye, but they do produce some amount of light. Some planets, particularly gas giants, produce, instead of just reflect, light and heat which helps to blur the definition of a stars and gas giants.

    2. Uhhhh…100 degrees fahrenheit in a sauna is NOT good and hot…I’m no scientist, but I do know that a “good and hot” sauna is somewhere around 165-185 degrees. If you’re not into the sauna thing, then a 100-degree “steambath” might be for you.

    3. Under many circumstances, temperatures approaching and exceeding 100 °C (212 °F) would be completely intolerable. Saunas overcome this problem by controlling the humidity. The hottest Finnish saunas have relatively low humidity levels in which steam is generated by pouring water on the hot stones. This allows air temperatures that could boil water to be tolerated and even enjoyed for longer periods of time. Steam baths, such as the hammam, where the humidity approaches 100%, will be set to a much lower temperature of around 40 °C (104 °F) to compensate. The “wet heat” would cause scalding if the temperature were set much higher.

  3. Is this an isolated brown dwarf pair, or are they part of a greater star system? Either answer is amazing.

  4. Hi Peter,
    I double checked with lead author Michael Liu, and he assures me the story text is correct on the temperatures: “the object is about 100 Celsius (200 F),” he wrote in an email. “I’m not an expert on saunas, but 100 F sounds pretty cold for a sauna to me, so I think the 200 F sounds about right.”

  5. I’d bet the artist had IR imagery in mind with the drawing. How else would you represent warm, dark globes of gas alone in interstellar space? 🙂 The article DOES say 100 Celsius, by the way. And a sauna probably wouldn’t be a good idea either. The atmospheres are probably hydrogen, methane and the like. Crushing gravity too, no doubt – even a brown dwarf.
    It is a fascinating discovery. A whole system of potentially very cold planets, satellites, etc. Not likely that there’s a Goldilocks zone around a cup of hot tea!

  6. Come on, temp wise, a person can easily take 212 F in a sauna, so 100 C is no problem. Heat content of air is nothing compared to water, therefore, 212 F in air is okay, but sitting in 212 F water is rather unpleasant.

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