L and T and OBAFGKM and Oh My!
As regular readers might have noticed, I’ve recently discovered the ‘Mass Effect’ series of games (yes, I know, late to the party, as usual!). I’ve recently started on Mass Effect 2 – I think I’m now about halfway through it, as best I can tell. So far I can honestly say that the entire thing is made of pure awesome, from an interesting storyline to well-drawn and entertaining characters. Also, it’s quite refreshing to come across a game series where I don’t have to simply turn off the science-nitpicking part of my brain, as the series is actually rather well-researched. (We’ll take the plot-devicium oxide that is Element Zero as a given for the time being, of course.)
Anyway, imagine my surprise when I discovered that one of the first missions in ME2 involves a star system that contains a brown dwarf! Obviously this got my attention, as I have more than a passing interest in these failed stars…
In fact the object in question is called Urdak and in-game it looks like this:
Image credit: via the Mass Effect Wiki.
My first observation is that we don’t think brown dwarfs would actually look brown, despite the name. Perhaps they’d actually look a very deep, sooty red – one suggestion, actually, is that some would actually appear magenta to the human eye! However, this isn’t really an objection as such, as we don’t have any resolved images of brown dwarf discs, so it will be a long time (if ever) before we actually know for a fact what one looks like close-up.
Now, I have some knowledge regarding brown dwarfs, so I couldn’t help but feel terribly clever and terribly pleased with myself because for a moment, I thought I’d spotted a research error.
It turns out, though, that I was prematurely-smug. (Serves me right, to be honest!)
So what had I thought I’d seen?
Well, I couldn’t help but notice that Urdak is described as being a class L brown dwarf; this is fine, as there is a spectral class L. But it’s also described as having a surface temperature of 1,300 degrees. My first thought here was ‘Oh-oh! Too cool for an L!’ But then I realised something. 1,300 degrees C, not 1300 K! Of course to convert any temperature from Kelvin to Celsius you have to add 273.16, to account for the freezing point of liquid water. (The Kelvin scale has its zero-point at the temperature where thermal particle motions cease – ‘Absolute Zero’ – whereas Celsius uses the freezing point of water at 1 bar of atmospheric pressure. However the interval on both scales is the same – 1 K is as ‘big’ as 1 degree Celsius.) And 1,573 degrees C is fine.
Oh my. They got it right. They actually got it right! I didn’t actually have a nerd-gasm at that point, but I think it was a close-run thing.
Now, I imagine a lot of readers will be scartching their heads and wondering why the dotty postrgrad is getting fussed over 1,574 versus 1,300. And that’s a fair point – let me tell you why!
Essentially, it’s all down to spectral typing.
Astronomy is, at base, an observational and categorical science. We started dividing objects into groups and categories long before we had a clear idea of what the physical theory behind those categories was. That might sound backwards, but actually it wasn’t – by analysing and sorting everything, it made apparent various patterns and behaviours that in turn pointed people in the correct directions toward the underlying physics.
The thing we can directly observe for any astronomical object, be it star, planet, galaxy or brown dwarf, is its light. Now, the light emitted by an object is rather important, as it will be affected by the structural and chemical properties of that object. Coloured glass will filter the sunlight that shines through it; so too will the gases in the Sun’s atmosphere. So, if we can identify the wavelengths and regions over which certain chemicals alter an object’s light, then in principle we can identify the presence of those chemicals in an object’s atmosphere. This is the very basic idea of spectroscopy; it lets us study the chemical composition and the structure of objects that are so far away that we’ll probably never visit them!
Now, in astronomy, you’ll frequently encounter stars being described as a ‘G2’ or a ‘B5’ or an ‘M3’ and so on. This is the Morgan-Keenan series of stellar classification; stars are grouped into types according to features that we see in their spectra. This is the letter part of the classification. The types themselves are split into subtypes, generally running from 0-9. The biggest input to the classification is actually from an object’s temperature; as temperature falls, the amount of energy emitted declines and an object’s peak wavelength shifts redwards; the sequence of letters runs OBAFGKM.
Originally, when it was first set up, the series was thought to correspond to the stellar life expectancy, and was arranged alphabetically. However, it later turned out that the classifications weren’t in anything like a sensible sequence, and what they actually reflected was surface temperature, not the star’s position in its life-cycle. However, the letter-designations hung on; this is why the sequence is ordered in such a strange way!
Type O corresponds to the very hottest and brightest stars; O-types are ferocious monsters thousands of times brighter than our Sun. They’re so hot and bright that their spectra actually peak in the ultraviolet(!). However, they’re also very rare and extremely short-lived – these monsters manage a few million years at most before they explode in supernovae. They’re bright because their high masses lead to high pressures at the star’s core, in turn meaning that these stars are burning through their nuclear fuel at truly prodigious rates.
B-types are bluish but cooler and (somewhat) more sedate than type O stars. They’re still much hotter, brighter and shorter-lived than our Sun, though. Rigel is an example of a B-type star.
Moving onwards, Sirius is an example of a type A star; our Sun is classified as type G2, meaning that it’s toward the hotter/brighter end of the G spectral type. G-type stars have life-expectancies measured in several billions of years – rather longer than type O, luckily for us! G-type stars also die more gently than O-types, eventually expanding into red giants rather than detonating in vast supernovae.
Anyway, the last clearly-stellar spectral type is M. M dwarfs, also known as ‘red’ dwarfs, are by far the most common type of star. Up to 80% of all stars, in fact, are M dwarfs. M dwarfs are smaller and cooler than the Sun; their surface temperatures are in the region of 3,000 degrees C versus about 5,500 for the Sun. M dwarfs are also much fainter; an exceptionally bright M dwarf might have as much as 5% the luminosity of the Sun. Usually they have rather less. But M dwarfs are much more frugal; their projected life-expectancies run into the trillions of years. These things will be shining long after our Sun is gone.
So where do the Ls and the Ts come into this?
Well, when you get to spectral class M, a new category of object starts creeping in: brown dwarfs. Brown dwarfs are so-called ‘failed’ stars; they’re what happens when the star formation process doesn’t quite get it right. Sustained nuclear fusion requires high temperatures and pressures at a star’s core, and below a certain mass there just isn’t enough of either. For a solar-like chemical composition, this mass is about 7.5% that of our Sun.
So, brown dwarfs don’t really shine through fusion. Some of them can burn dueterium for a while; however there’s not much deuterium around in the Universe, and this gets exhausted within a short time. After this, brown dwarfs shine by radiating away their store of internal heat; in the jargon of physics they convert gravitational potential energy into electromagnetic radiation through contraction. Essentially, to quote Burrows et al. (2001), they ‘cool like a rock’!
Initially brown dwarfs have a lot of internal heat, so they glow (relatively) brightly. These newly-formed, red-hot objects are warm enough that they present more or less as M dwarfs. However they don’t stay this hot for long; after a few million years, the brown dwarf will have cooled into the L spectral type. Presumably this is also Urdak’s life story!
But due to their much lower temperatures, L and T dwarfs have spectra that are very distinct from those of normal stars. This is why they have their own spectral classes.
So, what does an L dwarf’s spectrum look like? Well, here’s an example:
Note that this plot shows the spectrum in the near-infrared – it starts at a wavelength of about 0.8 microns. (By contrast, the human eye can see between about 0.4 – 0.75 microns.) You can also see, just by looking at this spectrum, that it’s not a smooth-ish curve, like an actual star’s would be. See those big bites? Those are produced by steam – by water vapour. Water vapour is a powerful absorber in the near-infrared (hence its role in the terrestrial greenhouse effect). And because the L dwarf is relatively cool (for a glowing thing out in space), the temperature is low enough that its atmosphere contains a lot of water vapour. H2O is a big, chunky molecule, and isn’t stable at very high temperatures. You dn’t see steam bands in the spectrum of the Sun!
But what about the T dwarfs? Well, this is where the 1573 versus 1300 issue would become important for Urdak’s class L status – because, you see, at about 1400 K, something odd happens. That thing is called methane.
To illustrate, here’s the spectrum of a T5 brown dwarf:
Methane, or CH4, is a big, chunky molecule. Like all big chunky molecules, it isn’t stable at high temperatures. There are a few L dwarfs that show hints of it, here and there, but it only becomes a significant spectral issue at temperatures below about 1400 K. But when methane absorption kicks in, it kicks in with a vengeance. You can see it in the plot above – the T dwarf’s light has basically been eaten entirely away in those deep bites. Also, the absorption causes light to be re-emitted at shorter wavelengths than one might otherwise expect; you can see this happening in the form of the giant spike around 1.25 microns in that plot.
Spectroscopically, T dwarfs are weird. There’s nothing else in the sky that’s quite like them.
Basically that utterly weird-looking structure – three spikes and a bump, between 1 and 2.5 microns – *is* the T spectral class. The T type is basically defined by the presence of methane absorption, which happens below 1400 K. So, if Urdak was actually at 1300 K – rather than degrees Celsius! – it should be showing methane absorption and thus would count as T dwarf, not an L!
But, it turns out they’d though of that first. So score one for the game-design team!