In the early 19th century, as people began to understand the combustion of coal, people were scratching their heads about how the Sun worked. Coal was one of the most energy rich fuels and it was assumed the Sun must be fueled by something like that - finding oxygen somewhere to allow combustion. The thorny little problem was that the Sun would run out of fuel in one or two thousand years - an age in the ballpark of what biblical creationism calculated, but a number that didn't square with several million minimum age that the emerging science of geology required.
The German astrophysicist Hermann von Helmholtz theorized the Sun might be contracting at a slow rate and, the increasing pressure inside would increase the temperature and provide the necessary energy for the Sun to shine. The shrinkage turned out to be about 130 feet per year, too small to measure, but as the real source of solar power - nuclear fusion - had not been discovered, it had to do for several decades.
I'll let you in on a curious bit of science to impress others with at cocktail parties. It turns out its power production density - the power produced by a volume of the stellar core where fusion occurs - is much lower than most people imagine. A quick back of the envelope calculation suggests 275 watts/m3. To put this in context a human's metabolism requires about 1,400 watts/m3
the Sun's power production density is closer to the metabolism of a lizard than that of you or me - let alone fossil fuel power plants or nuclear reactors.
It turns out the fusion reactions in the Sun proceed at a very slow rate - slow enough that the Sun can last about ten billion years. It makes up for this slow rate by being enormous - the core is about one hundred thousand Earth volumes.
Much of what I do these days involves applied physics or trying to puzzle out the intersection of the social and technical worlds - both delightful areas that respond well to a physics background. But astronomy is an old love that is impossible to leave behind and I get involved in a bit of research out of passion rather than occupation. For something over a year I've been thinking about mechanisms for the formation of brown dwarfs and a regular reader suggested I should write a bit.
You probably remember the mnemonic for stellar classification - oh be a fine girl kiss me...1 O,B,F, G, K and M are labels that describe the temperature of the stellar surface. The important thing to remember is O is really hot and M is fairly cool. Other parts of the classification system include the luminosity of the star and a bit about its evolution - technically our Sun is a G2V star.2
Once a protostar grows to about 75 times the mass of Jupiter there is enough matter to fuse helium into hydrogen - the slow but steady process our Sun currently uses. About forty years ago it was postulated that very small and cold "stars" could exist. These "brown dwarfs" would be more massive than planets, but less than conventional stars. If a protostar is about 13 times the mass of Jupiter there is enough heat and pressure to allow deuterium to fuse. This process proceeds much more quickly and the little brown dwarf quickly runs out of fuel to burn. All it can do is radiate its residual heat into space getting colder, and colder and colder... The larger ones just take more time. At about 65 times the mass of Jupiter a brown dwarf can participate in another type of fusion - this process involves lithium into helium and doesn't last very long and produces an object with somewhat different chacteristics than its smaller brothers.3
The surface temperatures of a brown dwarf is so low that infrared telescopes are required - an area of enormous progress in orbiting and earth bound telescopes in the past twenty years. Thousands have been discovered and new categories have emerged expanding the classification system .. now, ranging from the hottest to the coldest, it is O, B, F, G,K,M, L, T, and Y.
My interest is in the coldest of the cold - the Y class brown dwarfs. These have surface temperatures less than 440° F.4 The record low temperature is held by a body with the strikingly unpoetic name WISEP J1828 + 2650 - its surface temperature is just 80° F - as I write it is 78° F in Basking Ridge - almost exactly the same. So cold no visible light is produced - just a little infrared radiation - and less per kilogram than your body produces (you are brighter!).
At these low temperatures it is possible for liquid water to exist - a bit colder and you there would be ice, but 1828 + 2650 is close to the current limits of detection. What I've been working on are possible water cycles in the atmosphere. A slightly warmer brown dwarf might have rather complex weather patterns where there water vapor rises in the atmosphere and condenses into liquid water where it rains to lower levels and vaporizes again. Other substances in the atmosphere - lithium and ammonia for example - interact with the process and the rotation of the dwarf adds to the fun. It gets really interesting when paired with a real star and there is significant energy flowing striking its surface. You get storm patterns - cyclones, spots and bands - just like Jupiter and Saturn and some similarities to the Earth.
and on a quiet morning walk before dawn your mind wanders and you wonder what type of life might evolve in that atmosphere
Every question you ask begets at least dozen more and you begin to curate the communal ignorance on the subject and learn from others who are doing the same thing.5
A few weeks ago I was talking with Juliette about how wonderful it is to be a tiny part of this Universe .. a 70 kg bag of mostly oxygen, carbon, hydrogen, nitrogen, calcium and phosphorous that has evolved to the point where it is - that I am - alive and able to ask questions about the Universe.
In a few very special places our Universe is self-aware.
It is a beautiful thing to realize that all of me save the hydrogen was forced in the cores of stars that exploded.6 Atoms through my body - throughout your body - are from several stars. Stars that exploded allowing life and us to eventually form. Such a powerful and delightful thought.
Ray Bradbury died earlier in the week. I read a lot of his stories as a teenager and recommend them to any teenager. This video seems to be a fitting memorial - he was asked to give a speech at JPL the evening before a spacecraft was to go into orbit around the planet. Enjoy!
Carl Sagan can be seem similing here - the first time I saw this clip was in a talk of his about a year before he died. Both of them would have approved of this cartoon. Phil Plait is an astronomer I know a bit. He was asked to speak at a high school science fair, but tore up his talk when he watched the TV new just before going to the fair. The reporter was spouting pseudoscience given "equal time" .. a bizarre concept as Nature is - well - Nature, and our crazy ideas often are wrong. Only those that describe Nature are acceptable to a scientist. He took out a notepad an wrote a short talk later sending it around and posting it to his blog. Then a few days ago an artist posted an illustrated version - click to embiggen. You can order a copy at zenpencils if you like.
1 Sadly that's all that most people remember as memorization was stressed more than learning the science in most classes. There are also a number of more objectionable mnemonics.
2 It is amazing what can be gleaned from a tiny bit of starlight. This is directly from astrophysics, which emerged as a science around 1840. It was started by amateur, rather than professional astronomers as conventional astronomy had become associated with commerce (navigation) and had not tried the thing that was obvious to the physicist amateurs - namely to look at the spectrum of the starlight to deduce something much deeper. This was driven by the emerging technology of photography and an explosion of telescope construction - largely in the United States.
3 This sweeps a lot under the carpet - these processes are multi-step and figuring them out was a wonderful accomplishment of nuclear physics. To make fusion work on Earth so you get more power out of a reactor than you put in, it is necessary to run at extremely high temperatures and pressures - much higher than in the Sun's core. This is an incredible challenge and has been 30 to 50 years off since people first starting trying in the late 1940s. My view is we have a working fusion reactor that is only about five light minutes away - complete with shielding and it runs without any work on our part. All we have to do is catch and convert the radiation to useful work - a comparatively trivial technology by comparison.
4 Astrophysicists use a the absolute Kelvin temperature scale - Y class corresponds to 500° K
5 In past posts I've talked quite a bit on the importance of expanding ignorance as a scientific driver. Information is a byproduct, but the expansion of information is much smaller and far less interesting. I've been thinking about how to describe this without the baggage of the modern usage of ignorance ... I'm not there yet, but in science we talk about a communal ignorance - what the community finds new and unanswered - rather than common willful ignorance - the ugly ignorance that is the product of dogmatic systems.
6 The hydrogen came from the original formation - the Big Bang. Only about 15 pounds of me is hydrogen - the remaining 135 is the stuff of stars.
no recipe this time - out of time
The great lizard photo is from www.wildretina.com