The Little Stars that Couldn’t
In the cosmic cast of characters, there are shining stars and an assortment of planets orbiting some of those stars like bit players. And there are very faint objects of intermediate size, brown dwarfs, that have been stepping out of the wings with increasing and mystifying frequency.
Brown dwarfs were once called failed stars—more massive than planets but without enough heft to ignite hydrogen fusion and shine under their own power. In recent years, astronomers have learned that they are among the most complex objects in the sky: Pressure has crushed their interiors into super-dense states astronomers call “degenerate” while their cool atmospheres may harbor clouds of iron and silicon. They could hold the keys to understanding why solar systems form the way they do a and serve as clocks for determining ages throughout the galaxy.
Astronomers in 2015 reported on new telescope observations providing the first strong evidence that most, if not all, brown dwarfs appeared to have formed like stars but lacked sufficient mass to ignite and sustain the thermonuclear reactions that make larger stars shine. The evidence even suggested that some brown dwarfs, which presumably can range in size from 10 to 70 times the mass of Jupiter, might have planets of their own.
An international team of astronomers reached that conclusion from an examination of 100 young brown dwarfs in the Orion Nebula. This was the largest population of brown dwarfs ever studied in one region. It is a region where stars are being born in abundance. Because the brown dwarfs there are also young, less than one million years old, they are still relatively hot and easier to detect than older brown dwarfs, which dim with age. It was thus possible to find clues that they had originated like stars, as had been theorized but never established.
Apparently, more than 60 percent of the brown dwarfs were surrounded by vast disks of dust and gas. Each disk showed up as an excess of infrared light extending from the brown dwarfs themselves, but the light did not match the colors of known brown dwarfs.
Similar disks have been found around all young stars and are assumed to be the remnants of the rotating cloud of gas that collapsed to form the stars. “This means that the origin of brown dwarfs is very similar to the origin of stars,” Dr. Lada said. “They are more similar in nature to stars than to planets.”