Happy Birthday Carl Sagan

Carl Sagan was a towering figure in science. He was born on November 9, 1934, in Brooklyn, New York, and died following a long battle with cancer on December 20, 1996, in Seattle, Washington. In between, in just 62 years, he reshaped public understanding of physics, astronomy, and space exploration. More than this, he was a leader in exploration and discovery, involved in many of the scientific teams behind truly ground-breaking space missions in the 1960s and 1970s, including the Apollo moon landings and the Viking missions to Mars.

I first started to encounter Carl Sagan as a popularizer of science while an undergraduate at University of Guelph, and wrote a couple of columns mentioning him for The Ontarion, the university’s newspaper, in the early 1980s. I briefly thought of pursuing grad work on exobiology but ended up continuing along with evolutionary ecology instead, at Guelph for a few more years. His influences are still all around us.

Visit the Carl Sagan Portal to experience a little of this amazing gentleman’s life and contributions: http://www.carlsagan.com/

Betelgeuse: A Supergiant To Love

In the evening hours of late winter and early spring in the northern mid-latitudes like Ontario, the constellation Orion is a very familiar friend. The brightest star in Orion, Betelgeuse, is itself endlessly fascinating.

If you can picture the constellation then you can find Betelgeuse right away. It’s the orange-shaded bright star at the “right shoulder” of Orion – or on the left as we see the asterism. The other stars in Orion don’t have a noticeable colour most of the time, but Betelgeuse is decidedly reddish-orange.

Consellation Oroion rising over a surbab street in Burlington, Ontario, on the evening of 2013 March 26. Betelgeuse, the brightest star in Orion, is in the middle of the frame and about 1/8th of the way down from the top.

Constellation Orion rising over a suburban street in Burlington, Ontario, at about 9:45 PM on the evening of 2013 March 26. Betelgeuse, the brightest star in Orion, is in the middle of the frame and about 1/8th of the way down from the top.

Betelgeuse has been known as an interesting star since antiquity, but what astronomers have learned in the past 20 or more years make it all the more fascinating. For one thing, we don’t know how far away it is too much in the way of accuracy. Betelgeuse is relatively close to earth – somewhere between 400 and 700 light years away, or only about half as far as the Great Nebula in Orion, which we see with our naked eyes as the third “star” in the sword hanging from Orion’s belt. The lack of accuracy is no indication of lack of trying. For stars of this distance, astronomers often use a triangulation method called parallax to work out distances. Betelgeuse is hard to pin down this way because it is not in fact a “point” of light in the sky. The star is so big and so close that it actually has been photographed as a disk by the Hubble Space Telescope in 1995 (Gilliland & Dupree. 1996). It has a complex outer envelope that is changing its size and shape, and makes the parallax method no better than about 1 part in 5 for accuracy. The star is about 640 light years away, but that’s plus & minus 140 light years!

The size of this star is also staggering. Its diameter is approximately the same as the diameter of the orbit of Saturn in our own solar system. It’s also shining about 100,000 times as bright as our own sun. It’s likely a relatively young star compared to our own sun, and some time in the near future (in astronomical terms) it will likely explode as a supernova.

Recent scientific papers on Betelgeuse have gathered together more observations of the star itself and have tried to interpret various areas that look brighter to us as either bright patches on a darker background, or possibly  as bright areas areas showing up through overlaying dark features.

This star is also moving quickly toward a linear “wall” of material that is part of the local stellar environment. Betelgeuse has a shell of glowing material thought to be part of the material blown off of the surface of the star in the past. This shell will hit the wall in about 5,000 years, followed by the star itself about 12,000 years later (Decin et al. 2012). Don’t wait up for it.


Decin et al. 2012. The enigmatic nature of the circumstellar envelope and bow shock surrounding Betelgeuse as revealed by Herschel. I. Evidence of clumps, multiple arcs, and a linear bar-like structure. Astronomy and Astrophysics 548, A113 (http://www.aanda.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/aa/full_html/2012/12/aa19792-12/aa19792-12.html).

Gilliland & Dupree. 1996. HST imaging of Betelgeuse. Stellar surface structure: proceedings of the 176th Symposium of the International Astronomical Union, held in Vienna, Austria, October 9-13, 1995. Edited by Klaus G. Strassmeier and Jeffrey L. Linsky. International Astronomical Union. Symposium no. 176, Kluwer Academic Publishers, Dordrecht, p.165 (http://adsabs.harvard.edu/full/1996IAUS..176..165G)

Copyright © 2013 David Allan Galbraith



Star Stuff? Try “Big Bang Stuff!”

One of the (deservedly) frequently quoted observations by my hero Carl Sagan is that we are all star-stuff. The chemical elements in our bodies – and everything we see around us on Planet Earth – were forged in exploding stars billions of years ago. This is a profound realization. It seems to me that doesn’t go far enough, however.

I started thinking about the origins of the elements in our bodies, and made a connection I haven’t seen elaborated before. To explain myself, I have to explain the origin of the universe first.

The 98 elements that occur in nature are divided up by astronomers into two groups: hydrogen and helium, and “metals:” all the other stuff. Hydrogen and helium were the products of the evolution of matter following the big bang. The “metals” were subsequently produced in a process dubbed nucleosynthesis: nuclear fusion taking place within stars (the process was worked out over half a century ago; the landmark paper is E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle. 1957. Synthesis of the Elements in Stars, Rev. Mod. Phys. 29: 547). The proportions of these things are considered very important. The ration of hydrogen to helium in the observable universe is one of the hallmark tests for cosmology and models of the origins of the universe. Different models predict different ratios, and only the natural ration of about 76% hydrogen to 24% helium gets to decide which models fly.

The other stuff is used to characterize stars, with a measure called metallicity – the proportion of the stuff of the star that is not hydrogen or helium. For example, the metallicity of the sun is approximately 1.8% by weight. Put the other way, the sun is 98.2% hydrogen+helium by weight. This quantity is very helpful to astronomers as it’s a measure of the age of stars, among other things. The older the star, the higher the expected metallicity, as the metals are added by the very process of fusion. Looked at one way, it’s stellar pollution.

This started me thinking about human metallicity. There’s a nice summary on Wikipedia on the elemental composition of the human body (http://en.wikipedia.org/wiki/Composition_of_the_human_body). Here are the top ten elements and the percentage of the body, by weight and atomic proportion, that they represent:

  • Oxygen – 65% by weight but 24% by atomic proportion
  • Carbon – 18% by weight but 12% by atomic proportion
  • Hydrogen – 10% by weight but 63% by atomic proportion (!!)
  • Nitrogen – 3% by weight but 0.58% by atomic proportion
  • Calcium – 1.4% by weight but 0.24% by atomic proportion
  • Phosphorus – 0.78% by weight but 0.14% by atomic proportion
  • Potassium – 0.25% by weight but 0.033% by atomic proportion
  • Sulfur – 0.25% by weight but 0.038% by atomic proportion
  • Sodium – 0.15% by weight but 0.037% by atomic proportion
  • Chlorine – 0.15% by weight but 0.024% by atomic proportion

Ok, so what, I hear you say. Well, look at #3 in this list – hydrogen. Ten percent of our body mass is hydrogen, in chemical compounds like water, sugars, and all sorts of other things. However, two facts about hydrogen are important. First, it’s the lightest element there is, so 10% by weight is a big number by atoms. Second, hydrogen was not made by nucleosynthesis. It was made by the Big Bang itself – and sixty-three percent of the atoms in our bodies are hydrogen.

If we shift our attention away from overall proportions by mass and re-list things by number of atoms, we see a different picture of our own composition. Yes, we are star-stuff – but 63% of the atoms in our bodies have their origins in the Big Bang itself. These humble hydrogen atoms that are the majority population in our bodies – and are the most abundant stuff in the visible universe –  went through stars that exploded, but they came from the Big Bang. In a real sense, so did we.

© 2012, David Allan Galbraith