How do you separate a true star from the stellar wannabes of the Universe? After a decade of collecting data, astronomer Trent Dupuy thinks he finally has the answer.
With so many objects known to sit in that weird middle ground between giant planets and tiny stars, scientists have struggled to boil it down to a simple answer. What Dupuy boils it down to is mass.
“Mass is the single most important property of stars because it dictates how their lives will proceed,” Dupuy, from the University of Texas at Austin, explained at the American Astronomical Society’s summer meeting earlier this month.
We benefit from that here on Earth, as our Sun is in the stellar goldilocks zone – its mass is just right to sustain nuclear fusion within its core for billions of years. This has provided the conditions for life to develop and evolve on our planet.
But not everything in the galaxy is so nice and stable. More massive stars burn through their nuclear fuel quicker, dye young, and go out with a violent bang in the form of a supernova.
Less massive objects, like brown dwarfs, are like stellar runts, possessing more mass than a planet, yet not enough mass to be a fully fledged star.
Often referred to as failed stars, they’re ubiquitous throughout the Universe, but their exceedingly dim glow makes these objects difficult to study.
First proposed to exist 50 years ago, these enigmatic objects help bridge the gap between stars and planets, but it wasn’t until more recently that astronomers began to study them in great detail.
Stars like the Sun shine as a result of nuclear reactions that constantly converts the supply of hydrogen in their cores into helium.
These same reactions determine how bright a star shines – the hotter the core, the more intense the reaction and subsequently the brighter the star’s surface will be. As expected, less massive stars are dimmer due to cooler centres, which produce slower reactions.
Don’t let the name fool you – brown dwarfs aren’t always brown. These stellar wannabes are actually red when they form, then turn to black as they slowly fizzle out over trillions of years.
That’s because despite outweighing even the largest of planets, brown dwarfs have so little mass that their centres aren’t hot enough to sustain nuclear reactions.
In the 1960s, astronomers theorised that there must be a mass limit for fusion.
Previous studies of stellar evolution have suggested that the boundary between red dwarfs (the smallest stars) and brown dwarfs was around 75 Jupiter masses (or roughly 7-8 percent of the Sun).
But until now, his measurement was never directly confirmed.
Dupuy and Michael Lui of the University of Hawaii spent the past 10 years studying 31 binary pairs of brown dwarfs with the help of the most powerful telescopes on Earth – the Keck Observatory and the Canada-France-Hawaii Telescope, as well as some input from Hubble.
By analysing a decade’s worth of imagery, Dupuy and Liu have created the first large sample study of brown dwarfs masses.
According to Dupuy, an object must weigh the equivalent of 70 Jupiters in order to spark nuclear fusion and become a star, which is slightly less than previously suggested.
The duo also determined there’s a temperature cut-off, with any object cooler than 1,600 Kelvin (approximately 1,315 Celsius and 2,400 degrees Fahrenheit) classified as a brown dwarf.
The study will help astronomers better understand the conditions under which stars form and evolve – or in the case of brown dwarfs, fail.
It could also provide new insight into planetary formation as the success or failure of star formation directly impacts the star systems they could potentially produce.
The research will be published in an upcoming edition of The Astrophysical Journal Supplement, and a pre-print is available here.
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Mysterious objects called brown dwarfs are sometimes called “failed stars.”
They are too small to fuse hydrogen in their cores, the way most stars do, but also too large to be classified as planets.
But a new study in the journal Nature suggests they succeed in creating powerful auroral displays, similar to the kind seen around the magnetic poles on Earth.
“This is a whole new manifestation of magnetic activity for that kind of object,” said Leon Harding, a technologist at NASA’s Jet Propulsion Laboratory, Pasadena, California, and co-author on the study.
On Earth, auroras are created when charged particles from the solar wind enter our planet’s magnetosphere, a region where Earth’s magnetic field accelerates and sends them toward the poles.
There, they collide with atoms of gas in the atmosphere, resulting in a brilliant display of colors in the sky.
“As the electrons spiral down toward the atmosphere, they produce radio emissions, and then when they hit the atmosphere, they excite hydrogen in a process that occurs at Earth and other planets,” said Gregg Hallinan, assistant professor of astronomy at the California Institute of Technology in Pasadena, who led the team.
“We now know that this kind of auroral behavior is extending all the way from planets up to brown dwarfs.”
Brown dwarfs are generally cool, dim objects, but their auroras are about a million times more powerful than auroras on Earth, and if we could somehow see them, they’d be about a million times brighter, Hallinan said.
Additionally, while green is the dominant color of earthly auroras, a vivid red color would stand out in a brown dwarf’s aurora because of the higher hydrogen content of the object’s atmosphere.
The foundation for this discovery began in the early 2000s, when astronomers began finding radio emissions from brown dwarfs.
This was surprising because brown dwarfs do not generate large flares and charged-particle emissions the way the sun and other kinds of stars do. The cause of these radio emissions was a big question.
Harding, working as part of Hallinan’s group while pursuing his doctoral studies, found that there was also periodic variability in the optical wavelength of light coming from brown dwarfs that pulse at radio frequencies.
He published these findings in the Astrophysical Journal.
Harding built an instrument called an optical high-speed photometer, which looks for changes in the light intensity of celestial objects, to examine this phenomenon.
In this new study, researchers examined brown dwarf LSRJ1835+3259, located about 20 light-years from Earth.
Scientists studied it using some of the world’s most powerful telescopes the National Radio Astronomy Observatory’s Very Large Array, Socorro, New Mexico, and the W.M. Keck Observatory’s telescopes in Hawaii in addition to the Hale Telescope at the Palomar Observatory in California.
Given that there’s no stellar wind to create an aurora on a brown dwarf, researchers are unsure what is generating it on LSRJ1835+3259.
An orbiting planet moving through the magnetosphere of the brown dwarf could be generating a current, but scientists will have to map the aurora to figure out its source.
The discovery reported in the July 30, 2015 issue of Nature could help scientists better understand how brown dwarfs generate magnetic fields.
Additionally, brown dwarfs will help scientists study exoplanets, planets outside our solar system, as the atmosphere of cool brown dwarfs is similar to what astronomers expect to find at many exoplanets.
“It’s challenging to study the atmosphere of an exoplanet because there’s often a much brighter star nearby, whose light muddles observations. But we can look at the atmosphere of a brown dwarf without this difficulty,” Hallinan said.
Hallinan also hopes to measure the magnetic field of exoplanets using the newly built Owens Valley Long Wavelength Array, funded by Caltech, JPL, NASA and the National Science Foundation.
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