The Not so Hot Stars

We usually associate stars with ludicrously high temperatures, indeed the surface of the Sun is around 5780 Kelvin (of course there are many stars with much higher temperatures, and the cores of all stars are several million degrees Kelvin).

There are objects out there that are very similar to stars but have much cooler temperatures. These are the Brown dwarfs, sometimes termed failed stars they are more properly known as a class of sub-stellar object. That is a group of objects that are below the mass limit (about 0.08) to sustain hydrogen fusion – the primary energy source of stars.

Brown dwarfs occupy the lower right hand corner of the Hertzsprung–Russell diagram as shown below (with all objects falling below the magnitude +15 line being Brown dwarfs): -

The Hertzsprung-Russell Diagram Original Source Credit: Kaler, James B. Modifications: Peter Clark

There are three sub-classes of Brown dwarfs, each identified as a distinct spectral class (and no there is no easy way to remember the three): -

• L
• T
• Y

Though it should be noted that the hottest Brown dwarfs fall into the spectral class M along with the majority of Red dwarfs.

Spectral Class L

An L Class Dwarf Credit: R. Hurt/NASA

This group contains high mass, hot Brown dwarfs and the coolest Red dwarfs that are the faintest and least massive of all true stars. L class dwarfs are very dark red in colour and their emission peak lies within the infra-red region of the electromagnetic spectrum. As such they are very faint and exceptionally difficult to detect at optical wavelengths even when using a telescope as powerful as Hubble.

Their spectra show the presence of atomic alkali metals such as Sodium and Potassium (these ionise at low temperatures further demonstrating the low temperatures) and metal hydrides – a metal ion bound to a negative hydrogen ion for example Magnesium hydride – MgH. The defining spectral feature of the class is the presence of deep absorptions due to  Titanium(III) oxide (TiO) and Vanadium (III) oxide (VO).

Due to the nature of supergiant stars, L class giants and supergiants cannot form in isolation or under normal conditions experienced by evolving stars. In rare cases they may be produced through stellar collisions such as V838 Monocerotis (assuming the outburst event was indeed a stellar merger).

Spectral Class T

Class T contains only Brown dwarfs, no star regardless of how small or unusual the conditions can occupy this spectral class. Such Brown dwarfs have surface temperatures of between 700 and 1300 kelvin which is in and around the temperature of a wood fire.

They are even dimmer than their L class counterparts and similarly have peak emission in the infra-red, emitting virtually no visible light. Though they would appear to glow dimly with a purplemagenta tinge due to their chemical composition that absorbs much of the little green light they emit (Brown dwarfs that aren’t brown – go figure!).

T class Brown dwarfs are sometimes reffered to as Methane (CH₄) dwarfs as their spectra are dominated by absorptions related to Methane.

An artist's impression of a T class Brown Dwarf Credit: R. Hurt/NASA

Spectrum for Gliese 229B Credit: Oppenheimer et al.

Spectral  Class Y

These represent the coolest Brown dwarfs. Their surfaces have temperatures below 600 kelvin, and they should have detectable differences in their spectra compared to class T dwarfs. had I been writing this at the same time last month that would be all their was to say on this class but that was then this is now.

NASA’s Wide Infrared Survey Explorer – WISE – has thus far detected 100 new brown dwarfs, 6 of these fall into the Y spectral class and are located within 40 light years of our own sun.

One of these in particular in especially interesting.

An Artist's Impression of a Spectral Class Y Dwarf Credit: NASA/JPL-Caltech

WISE 1828+2650 is the coolest brown dwarf ever detected at just 298 Kelvin – 25 degrees Celsius - which is ten degrees below human body temperature; it is in the direction of the constellation Lyra.

WISE 1828+2650 Credit: NASA/JPL-Caltech/UCLA

The tiny unassuming green speck at the centre of this image is WISE 1828+2650. It may look insignificant in this image but if this same area was viewed in visible rather than infra-red that green blob would be completely undetectable.

Scientists have barely scratched the surface of the WISE data which covers one and a half sweeps of the sky captured between January 2010 and February 2011 before the spacecraft depleted its cryogenic coolant and was placed in indefinite  hibernation. The team of astronomers working on the data are very excited by the fact that there could be many more such dwarfs hidden in the data, perhaps some even cooler.

You can read more on this new discovery here.

This post is part of the Young Astronomers Databank Project.