The humble brown dwarf

Author: Du Lian

Audit│Huang Jian

Editor: Zhao Jingyuan

There is a special class of inconspicuous celestial bodies in the universe, the mass of which ranges between the heaviest gas giant planets and the lightest stars, about 13 to 80 times the mass of Jupiter. Due to their low mass, these celestial bodies are not enough to undergo hydrogen-helium nuclear fusion (deuterium, tritium and lithium nuclear fusion can occur, but hydrogen-protium fusion cannot be initiated), so they cannot be counted as true stars, and such substellar celestial bodies are called failed stars. In 1975, American astronomer Jill Tarter proposed to call such celestial bodies brown dwarfs, which has now become the recognized name for such celestial bodies. It is worth noting that the type of celestial bodies below this mass range and that do not orbit any celestial body are called sub-brown dwarfs, sometimes also called wandering planets.

Brown dwarf artistic concept map (Image source: NASA)

The process of discovering brown dwarfs was very long and tortuous. Because of their low brightness and difficulty in observation, brown dwarfs were only a type of celestial body that existed in theory for a long time, and no one could answer whether such celestial bodies really existed. In 1963, American astronomer Shiv S. Kumar first theoretically predicted the existence of brown dwarfs. Subsequently, for more than 30 years, many astronomers conducted countless searches, but found nothing. Brown dwarfs seemed to be playing hide-and-seek with astronomers, hidden deep in the universe. The turning point came in 1995. At the Cold Stars, Stellar Systems and the Sun Symposium held at Cambridge University in the UK, a group of astronomers announced that they had discovered the brown dwarf Gliese 229B, whose spectrum contained methane and lithium, surface temperature below 1000 K, and mass 20 to 50 times that of Jupiter. These characteristics indicated that it was not a star. Combined with the various characteristics of Gliese 229B, this object was considered to be one of the earliest discovered brown dwarfs (another group of astronomers discovered another brown dwarf, Taide 1, almost at the same time). The relevant papers were published in Nature and Science in late November and early December of the same year. 1995 seemed to be the year that opened the door to the hiding places of brown dwarfs. In the following years, with the advancement of sky survey facilities, many telescopes around the world began to search for brown dwarfs, and more brown dwarfs were discovered one after another. People gradually realized that this type of celestial body might not be rare.

Palomar Mountain Observatory and Hubble Space Telescope images of GL229B (Image courtesy of NASA)

The spectral types of common stars are O, B, A, F, G, K, and M. After discovering stars and brown dwarfs with lower temperatures, astronomers expanded the spectral types to the low temperature region and added three spectral types: L, T, and Y. Among the expanded spectral types, some late-type M, most L, all T, and all discovered Y-type dwarfs are brown dwarfs. The surface temperatures of M-type and L-type brown dwarfs are relatively high, and their spectra are close to those of M-type stars; the surface temperatures of T-type brown dwarfs are relatively moderate, and their spectra are similar to those of giant planets; the surface temperatures of Y-type brown dwarfs are relatively low, and they are the coldest known spectral types of brown dwarfs.

Comparison chart of stars, red dwarfs, brown dwarfs, and planets (picture from Wikipedia)

The discovery of brown dwarfs has made people realize the complexity of the universe, and the study of brown dwarfs helps us understand the evolution of the universe, star formation, etc. However, the formation mechanism, internal structure and physical properties of brown dwarfs are still full of mysteries, and scientists need to invest more energy in research.

References

(1) Linda Schweitzer, A Century of Cosmic Exploration: How Astronomers Changed Our Understanding, translated by Qu Yan, China Radio, Film and Television Publishing House, May 2022

(2) Zhang Changxi, Searching for Brown Dwarfs, Astronomy Enthusiasts, Issue 7, 2021

(3) Grady Robbins et al., CWISE J105512.11+544328.3: A Nearby Y Dwarf Spectroscopically Confirmed with Keck/NIRES, The Astrophysical Journal, 2023.11

(4) SK Leggett et al., The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres, The Astrophysical Journal, 2023.12

(5) What is a brown dwarf?, NASA Star Child, https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question62.html

(6) Wang Youfen, Shao Zhengyi. Observational characteristics and search for brown dwarfs[J]. Progress in Astronomy, 2013(1):17-36.

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