A surface temperature of about 25% is the ideal temperature to achieve success as a star. A new study has found that this temperature is what separates red dwarf stars (which shine long-term) from brown dwarfs, or failed stars.
It can be difficult to tell the difference between brown and red dwarfs because they look almost identical when young: red and dim. Only red dwarfs have enough mass to be able to sustain the same kind of nuclear reactions as stars like the sun. Brown dwarfs, on the other hand, glow red due to their birth heat, but then their nuclear activity ceases, which causes them to cool down and fade. Dino Hsu, Adam Burgasser and colleagues at the University of California San Diego have now identified the line that separates the two types of astrophysicists by studying how they move through space.
A star’s birth is a circular orbit around the Milky Way center. However, stars are subject to gravitational tugs from spiral arms, giant gas clouds and other stars over time. These perturbations cause the star’s orbits around the galactic centre to become more elliptical. The orbital paths of stars may reveal their age.
Red dwarfs tend to be older than expected, with their projected lifetimes being much longer than the age of the universe. However, because brown dwarfs age and fade, those that remain warm are still young. Red dwarfs should therefore follow more elliptical orbits around our galaxy than young brown dwarfs.
Hsu’s new study analyzed 172 brown and red dwarfs of various spectral types. The classifications were based on their spectra, which correlate with their surface temperatures. The researchers found that a sharp break in stellar motions separates warmer objects, which on average have more elliptical orbits and are older, from cooler ones, which on average have more circular orbits and are younger. The break occurs at L4 to L6 spectral types, which corresponds to a surface temperature between 1200deg and 1400deg Celsius (1.500 to 1,700 Kelvins), the team reports July 5, at arXiv.org.
The dim suns above this temperature are composed of both young brown dwarfs and long-lived red dwarfs. Hsu states that below this temperature it is all brown dwarfs. These stars are likely to die. This study will be published in the Astrophysical journal Supplement Series.
Although this new method of detecting the temperature boundary of red and brown dwarfs looks intriguing, it is not yet proven, according to Trent Dupuy, an University of Edinburgh astronomer who was not involved in the research. He says, “It’s exactly where you would expect.” Dupuy suggests that additional brown and red dwarfs be observed in order to confirm the discovery.
Hsu also agrees that we need to have a larger sample. Expanding the sample is both simple and difficult. Red dwarfs outnumber all other stellar types combined, and brown dwarfs are common. However, brown and red dwarfs are very faint. It is difficult to measure their Doppler shifts which show how fast they move towards or away from Earth. This motion is crucial for calculating the orbital path of a star around the galaxy.
