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Starry Skies Above Santa Monica: February 23-29, 2000

Editor’s Note: Former Mirror columnist Mirek Plavec passed away on January 23. We are reprinting one of his old columns in his honor.

Mirek Plavec,

Professor Emeritus of Astronomy, UCLA

Stellar Nursery Called Orion

This is about the best time of the year to admire – rain permitting – the magnificent constellation of Orion. The classical image of a giant, and my image of a pretty girl, agree on the presence of an interesting belt of three equally bright and equally spaced stars of the second magnitude, which splits the imaginary body in two halves. Above the belt shines the reddish supergiant Beteigeuze, below the belt and in the opposite direction dominates the bluish-white supergiant Rigel. The three stars of the belt are a bit fainter, yet their arrangement makes them quite conspicuous. If you have read James Michener’s Hawaii, you may recall that the ancient seafarers from Tahiti navigated their boats by looking for “the three in a row.”

For astronomers, every star in Orion is interesting: these are relatively distant stars, some 1,500 light years away. Since they appear bright to us, they must actually be very luminous – and, as a consequence, their lifetimes are significantly shorter than the life expectancy of the Sun. An approximate calculation is quite simple: A star with a mass of 10 solar masses has 10 times more hydrogen, and hydrogen is the dominant fuel; but the star would be called a “gas-guzzler” by President Carter: it shines as about 10,000 Suns, i.e. consumes the fuel at that rate, and therefore its active life is 1,000 times shorter than that of the Sun. Thus very luminous stars must be fairly massive, but then they age rather fast. Are they replaced by new stars, where and how?

The answer is: look into Orion! There is a big stellar nursery in that constellation, combined with a kindergarten. Look below the belt: under good conditions, you may see three stars arranged vertically – the ancients saw in them the giant’s sword. The middle star may look nebulous to you, and if you look even through a small telescope, you will see a beautiful nebula. This is the Great Orion Nebula, also known simply as M42, from its number in the famous catalog by Messier.

The nebula is a large cloud of glowing gas, and at its heart sits a quadruple star – the famous Trapezium. In a large telescope, this is a fantastic sight. I had the opportunity to see it through the large telescope of the Lick Observatory (above San Jose); the main mirror of the telescope has a diameter of three meters, and thus concentrates plenty of light into the eyepiece. Naturally, astronomers who get the privilege of using that telescope for research purposes are not expected to use it for star-gazing, but I was very lucky: One of the four stars of the Trapezium is actually an eclipsing binary of rather a peculiar character, and I was studying its spectrum and behavior. All around that star was the nebula, shining mostly in greenish light. Photography shows it as predominantly red, but human eye is not very sensitive to red light, so the greenish-orange color dominates. While admiring the glowing gas, I felt lucky that it is not my task to study the structure of that huge gas clouds it is extremely complex, consisting of smaller clouds of different density.

The nebula is at a distance of about 1,500 light years, and measures about 16 light years across. That’s an impressive size! Such a huge cloud of gas does not produce its own light; it needs a star, or better several stars, that would supply the energy. And not every star would do! In fact, only stars hotter than 25,000 degrees on their surface qualify for this job! The gas in the nebula does not just reflect the light of the star; rather, the photons of radiation from the star are absorbed and re-processed by the atoms of the gas. Only very energetic photons can do the job of making the gas cloud shine, namely those that have a wavelength shorter than 912 A. These are far-ultraviolet photons, invisible to our eyes, and actually prevented from hitting us by our photosphere.

The fundamental property of these photons is that they can ionize hydrogen atoms, that is, the only orbiting electron in the hydrogen atom absorbs such a photon and gets so much energy that it escapes the electromagnetic attraction of the nucleus of the atom. So how does all this lead to the radiation of the nebula? First of all, a bound electron in a hydrogen atom typically waits for about two weeks before a sufficiently powerful photon comes. The electron absorbs the photon and escapes from the atom. After that, it wanders through the nebula at a typical speed of some 500 km/s. It encounters many other electrons; because both sides have a negative charge, they repel each other, and change their orbits. The electron also encounters many positively charged protons, which have lost their orbiting electron and are eager to catch a new one. To these protons, our wandering electron is attracted; but because it flies at high speed, it usually escapes. Eventually, typically after some 120 years, our electron happens to pass extremely close to a proton; then the proton’s electromagnetic attraction prevails and the electron is captured.

There are many possible orbits about the proton into which the captured electron can land; the kinetic energy lost as it slows down is radiated away in the form of a photon. No matter into which orbit the captured electron lands, it never stays long, and very soon falls in, closer to the nucleus. In this way, the electron cascades down, until it lands in that permitted orbit that is nearest to the nucleus. There, it will quietly orbit until another powerful photon comes again.

The cascading inwards is extremely fast; typically, it is completed within one millionth of a second. During each intermediate jump, the electron radiates a photon whose energy corresponds to the energy difference between the two orbits. Jumps from an outer level to the second level (counted from the atomic nucleus) produce photons, some of which we perceive as visible light. Thus, jumps from three to two (very frequent) produce red light – this explains the red color of the nebula as seen on photographs. Jumps from four to two emit green light; jumps from five to two send blue light to us, etc. This is why and how the nebula shines!

Stay tuned!

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