Our Sun is what is called a yellow dwarf star. This means that it is, as it’s name suggests, yellow (it’s not really) and comparatively small, (welllll yes and no). Actually it appears blindingly white when seen without the filter of an atmosphere and it radiates most of it’s energy in the green area of the spectrum, but that’s another story. As for “dwarf”, well it is a lot smaller than many stars, but a lot bigger than others, such as Red Dwarfs, which are the most abundant stars. This article will look at something even smaller and which could be said to have something of an identity crisis, the “Brown Dwarf”.
To paraphrase the old Superman thing, one could say “Is it a star? Is it a planet? No it’s Brown Dwarf Star!” Ok, bad jokes aside, there is a great deal of doubt about whether to call them stars or planets. Let’s start with the size thing. Next to the Earth, Brown Dwarves are big, I mean really big, even bigger than Jupiter, but compared to a star like our Sun, they are tiny, puny little excuses for stars, in fact they are frequently referred to as “failed stars”. Here’s a comparison.
But, as we often hear elsewhere, size isn’t everything, and this is also true to a certain extent when it comes to stars. With stars it’s all about the mass of the object, and of course it’s radius which gives you it’s density, it’s no good having a huge mass if it is all too far apart to do anything. The minimum mass for a star is usually considered to be 0.08 times the mass of the Sun, anything lower and it will not be able to create the conditions required to fuse Hydrogen which is what being a star is basically all about. Brown Dwarf stars contract until electron degeneracy pressure stops further contraction and there is not enough mass to overcome this. They do, however, create heat, by the compression of the gas they are made of and this makes them different from gas giant planets. Gas giants like Jupiter and Saturn have a “surface” temperature of only about -100 C. Brown dwarfs have a surface temperature of up to 2000 C although many are much, much colder. In fact NASA has recently discovered the coolest Brown Dwarf so far, only 7.2 light years away, http://www.nasa.gov/jpl/wise/spitzer-coldest-brown-dwarf-20140425/#.VIRtj1Dd9OA . It is close enough that it’s movement through the Galaxy, it’s so called proper motion, can be easily seen-
This object, called WISE J085510.83-071442.5 was discovered using Infrared data from the WISE and Spitzer telescopes and would appear to have a temperature about the same as the North Pole here on Earth, so it’s quite chilly to say the least, and others have been found with a temperature of around 20 to 30 C, so basically a comfortable room temperature. Here’s a little illustration comparing different types of stars and their temperatures:
http://en.wikipedia.org/wiki/Teide_1 The temperatures are shown in Kelvin so to convert to Centigrade just deduct 273.15.
So why do we call them stars at all? They are colder than stars, much smaller and don’t fuse Hydrogen like stars, so are they planets? The closest planets are the gas giants like Jupiter and Saturn, so aren’t Brown Dwarfs really just larger Gas Giant planets? Theoretical estimates of the minimum mass of an object formed by the process that forms stars predict that no star can have a mass less than about 0.08 Solar masses, but it also seems that Brown Dwarf stars are formed in the same way as “real” stars, by the accretion of gas in a gas cloud collapsing under it’s own gravity, whereas Gas Giant planets are formed by the accretion of a rocky core which then rapidly gathers up huge amounts of gas from it’s orbit.
So they are formed in the same way as stars, but are closer to Gas Giant planets in terms of size and mass than they are to even Red Dwarf stars. One interesting result of their low temperature is that they are comparatively difficult to find. Most stars are bright enough to be seen with the naked eye because they radiate strongly in the visible range of the spectrum. Brown Dwarf stars, because of their lower surface temperature, radiate most of their energy in the Infrared range-
Brown Dwarf stars have also been theorized to be a giant planet or double star component that has been thrown out of their original star system by powerful tidal forces, either from another star in the same system or a star that passed close by. Others have their own planetary systems, MOA-2007-BLG-192L, is known to host a small world of approximately 3.3 times the mass of Earth and there are other, unconfirmed, candidates. It is even possible that the planets of Brown Dwarf stars could be home to life if tidal forces warmed their interiors in the same way as we observe on Europa.
So there you have it, the Brown Dwarf, somewhere between a Gas Giant planet and a very small star, with characteristics of both and it seems like there are plenty of them, some even right in our cosmic backyard. On some we have detected clouds of water ice and on others it can even rain iron. Are they failed stars or planets with inflated egos? I guess the jury is still out on that one but one thing is certain, Brown Dwarfs are cool in all senses of the word.