When looking at the night sky, it’s clear that there is a multitude of stars. But invisible to the naked eye are stars that never reached their full potential — brown dwarfs doomed to burn out completely, unable to sustain nuclear fusion the way stars like our sun do.Dr. Michael Cushing, UT assistant professor of astronomy, is mentioned in the July issue of Sky and Telescope magazine because of his study of brown dwarfs. He was the lead author of a paper published in the Astrophysical Journal in December on the discovery of Y dwarfs — an entirely new class of brown dwarfs.
“To have Sky and Telescope, which is a very popular amateur astronomy magazine, discuss our work is pretty exciting,” Cushing said. “I was pretty happy when the article came out.”
Cushing was a part of a team of astronomers who discovered the Y dwarfs, which are the coldest type of brown dwarfs known. Cushing helped find six using data from NASA’s Wide-field Infrared Survey Explorer (WISE). One of these stars burned at only around 80 degrees Fahrenheit, the temperature of a summer day on Earth.
WISE was launched into Earth’s orbit in December 2009, and it snapped images every 11 seconds using four different infrared wavelengths to map the entire sky. After WISE retired in February 2011, scientists like Cushing used the data to search for brown dwarfs.
While analyzing the WISE data, astronomers looked at the ratios of the intensity of light at two different wavelengths to find candidates that had similar ratios to previously studied cold brown dwarfs. When they had a list of candidates, they then used other telescopes to confirm the candidates are bona fide brown dwarfs and then studied them further.
Astronomers used the Hubble Space Telescope, Spitzer Space Telescope and the Keck Observatory in Hawaii to see the light spectra given off by these candidates and infer whether they contained high levels of methane gas and steam — a characteristic common among cold brown dwarfs.
The discovery and analysis of brown dwarfs are important because, by looking at these failed stars and how they form, it is easier to understand far away planets that orbit other stars in the galaxy, Cushing said. Brown dwarfs share features of both low-mass stars and gas giant planets like Jupiter, earning them the nickname of “misfit stars,” being in some ways half planet and half star.
In order to even discover an exoplanet, astronomers must analyze the glare of its parent star and differentiate between that light and the light of the planet. Although nearly as faint, brown dwarfs are easier to study because they typically live in isolation.
“It’s much easier to study a faint brown dwarf — even if you have to use the Hubble Space Telescope — to make sure that you understand how very cold atmospheres work,” Cushing said.
Another goal of the astronomers, according to Cushing, is to obtain a complete census of all brown dwarfs in the solar neighborhood, which is the region of space around the sun out to a distance of roughly 20 light-years.
The data obtained by WISE is still being analyzed and likely will be for years to come. As Cushing told Sky and Telescope: “Right now we have 1,600 candidates in our list, and we’re not done searching the WISE data.”
In fact, Cushing said that his team has just received 125 orbits of the Hubble Space Telescope, which will be useful in their search for brown dwarfs.