Cowardice is something that comes fairly naturally to me, this explains why I always played wing or full-back at schoolboy rugby. For non-rugby fans that means I was generally far away from the action and unlikely to end up at the bottom of a ruck with 16 huge forwards piling on top of me. One or two I wouldn’t mind, but two whole packs could have crushed a skinny little kid like me.
To use heavily technical, scientific terms, stars are big balls of stuff. That stuff is mostly Hydrogen, with some Helium and bits and bobs of other heavier elements. Hydrogen is a bit like I was in my rugger days, small, light and entirely unsafe for use in airships. Take a load of Hydrogen and pile a few rugby players on top of it and it gets compressed and hot. If you kept piling on more and more players it would get hotter. Eventually 16 million billion billion players piling on and forming a massive spherical ruck would lead to the poor Hydrogen getting as hot as three million degrees Celsius. This is a critical temperature as at this point the atoms of Hydrogen are so hot and whizzing about at such huge speeds that they can slam in to each other to form Helium. This releases energy and the Hydrogen has an energy source and can stay hot and continue to fuse into Helium. This is how stars work, the huge amount of material pressing down on the core causes it to get so hot these reactions happen and allow the star to support itself, produce energy and become stable. Luckily rugby union teams are limited to 15 players a side so there are no recorded incidents of skinny full-backs bursting into spontaneous fusion.
So imagine what happens if the star is slightly less massive than 16 million billion billion rugby players (8% of the mass of the Sun or 80 times the mass of Jupiter). The star doesn’t get hot enough to start the Hydrogen fusion reactions and it can’t produce energy or become stable. It radiates away heat and begins to cool. These stars that don’t work are called brown dwarfs.
When stars are burning Hydrogen, the more massive they are, the hotter they are. The surface temperature of the star determines the colour of the star, hot stars blue, cool stars red. It also determines what sort of elements and molecules dominate the star’s spectrum.
While the interior of a star is a hot plasma with atoms split into nuclei and electrons, in the atmospheres of stars atoms can stay together. Lights flies through the atmosphere and comes into contact with the atoms. In this case the atoms can gobble (absorb) the light up. Atoms are picky eaters so they can only gobble up light of particular wavelength and only when the atmosphere is in a narrow temperature range. Each chemical element has a different series of wavelengths it can absorb and each has a separate temperature range where it absorbs most. Hence you can tell how hot a star is from what wavelengths of light the atoms have taken a tiny nibble out of.
Really cool stars can also have molecules in their atmosphere. Unlike lone atoms, molecules are rather gluttonous, taking huge bites out of stellar spectra. The coolest of the original spectral class determined in the early 20th century was “M”, which (like its namesake played by Judy Dench) has huge chunks out its visible spectrum from Titanium Oxide. In the last few decades of the last century cooler objects were found so new spectral classes of L and T have since been added. While L has features from metal hydrides, the main spectral feature of the very cool T spectral class (below 1100C) is huge mouthfuls taken out the spectrum by methane and water. Generally hot stars are blue and cool stars red. While T type objects follow these rules for their spectra in visible light in the infrared the spectral gobbling of methane and water cause them to look blue. Jupiter, while not a brown dwarf and cooler than all known T type objects, also shows methane absorption leading to this stunning infrared image (above). Now to make life difficult, there are stars that are spectral class M or L and brown dwarfs can be anything from M, L, T to even the still theoretical Y class. These spectral classes are an extension of the standard spectral classification system (hot to cool) O,B,A,F,G,K,M (now with L,T). The traditional way of remembering the old ones is “Oh Be A Fine Girl Kiss Me” (trust me that only works for remembering stellar spectral types). I don’t think there is an “official” extension to the mnemonic, but suggestions are welcome in the comments section.
So I’ve told you what a brown dwarf is and what one looks like. However finding them and distinguishing them from stars can be pretty tricky. I’ll tell you how this can be done in the next post.
Kirkpatrick, J., Reid, I., Liebert, J., Cutri, R., Nelson, B., Beichman, C., Dahn, C., Monet, D., Gizis, J., & Skrutskie, M. (1999). Dwarfs Cooler than “M”: The Definition of Spectral Type “L” Using Discoveries from the 2 Micron All‐Sky Survey (2MASS) The Astrophysical Journal, 519 (2), 802-833 DOI: 10.1086/307414
Burgasser, A., Kirkpatrick, J., Brown, M., Reid, I., Burrows, A., Liebert, J., Matthews, K., Gizis, J., Dahn, C., Monet, D., Cutri, R., & Skrutskie, M. (2002). The Spectra of T Dwarfs. I. Near‐Infrared Data and Spectral Classification The Astrophysical Journal, 564 (1), 421-451 DOI: 10.1086/324033