ABC of Astronomy - B Is for Bok Globule

ABC of Astronomy - B Is for Bok Globule
Eagle Nebula with dark patches called Bok globules where star formation is occurring. [Photo: ESA / Hubble & NASA]

Why are Bart Bok, Johann Bayer and Francis Baily remembered? They were all well known in their lifetimes, but their lasting fame is due to each one having something astronomical named for him.

B is for Bok globule
Dark nebulae are so dense with dust that they're often opaque to visible light. They show up against a luminous background, looking rather like holes in space. Some of them are small, less than a light year across. (In astronomy that's small.) And some of these are rounded. They were first noticed by Bart Bok (1906-1983), a Dutch-American astronomer who was an authority on the Milky Way and a great popularizer of astronomy. He called them globules and thought they might be small star-forming nebulae.

Even in the 1940s when Bok was researching the globules, some astronomers had already suggested that new stars could form in nebulae. Bok suggested that the dark globules might well “represent the evolutionary stage just preceding the formation of a star.” However most of what we know about star formation is from observations made in the last several decades by space telescopes, especially in infrared and radio wavelengths. Therefore it was only after Bok's death that observation confirmed his hypothesis that baby stars were developing in what are now called Bok globules.

The globules are useful to astronomers. Stars commonly form in large nebulae which are difficult to study. Not only are they far away, but the conditions are quite complex in such massive clouds. Yet there are many Bok globules that are comparatively close to us, and since they are each nurturing only a few stars, the processes are simplified.

B is for Bayer designations
German lawyer Johann Bayer (1572-1625) was widely known as a celestial cartographer. His 1603 star atlas Uranometria Omnium Asterismorum went to several editions.

It seems odd now, but in Bayer's day individual stars rarely had names. Ptolemy's 2nd century Almagest was about constellations. Stars were listed within each constellation along with the latitude, longitude and brightness. But Ptolemy identified the stars by describing their positions within the constellation. For example, Mirfak, a second magnitude star in Perseus, is the bright star on the right side.

Interestingly, many of the common Arabic star names in use today are derived from an Arabic description of the star's position in a constellation. For example, the name Deneb is derived from tail. Besides naming the brightest star in Cygnus, Deneb is also found in the names of several other stars.

Bayer's innovation was a system that gave each star a designation. He used a Greek letter and the genitive (possessive) Latin form of its constellation. For example, the Bayer designation of Deneb is α Cygni. People often write this out, e.g., Alpha Cygni.

The astronomical telescope hadn't been invented yet, so only stars visible to the unaided eye were involved. Furthermore the number of stars was limited to those forming part of the figure of the constellation, and not a whole area of sky. Even with these restrictions you can see the how limited the Greek alphabet was. Some constellations had more stars than the alphabet had letters. In that case, when Bayer ran out of Greek letters, he used Roman ones.

At first glance, it looks like Bayer listed the stars in order of brightness, because the alpha star is so often the brightest. However, it's impossible to order all the stars in a constellation using only the unaided eye. Bayer usually divided the stars into the six classes of the brightness scale developed by Greek astronomer Hipparchus (190-120 BCE).

When Bayer listed the stars by brightness class, how did he order them within a class? It varied. It might be according to their positions in the constellation. Sometimes it was the order in which they rose in the east, or even according to their historical interest. And quite frankly, some of them have no obvious logic. However having a system was useful, and Bayer designations are still in common use.

B is for Baily's beads
Francis Baily (1774-1844) was a very successful businessman, as well as one of the founders of the British Royal Astronomical Society, of which he was four times the president. His contributions to astronomy were numerous, but his name is attached to Baily's beads. If you've seen a total solar eclipse you may have seen this phenomenon yourself.

Just before and just after totality little blobs of sunlight, like shiny beads, appear at the edge. If the Moon were a perfectly smooth sphere, as had been assumed in the Middle Ages, this wouldn't occur. However, like Earth, the Moon has topography. As the Moon is just about to hide the Sun from view, sunlight blocked by lunar mountains gets through the valleys between them. [Photo: Fred Espenak - Mr. Eclipse - via EarthSky]

In fact, Francis Baily wasn't the first person to notice the beady blobs of light, or the first to record it. The first account that we know of was from Edmond Halley's (1656-1732) observation of the 1715 eclipse, but there are several subsequent sightings. However Baily was very much associated with the beads because of his extensive description of them following the 1836 solar eclipse. He also encouraged others to look for them at the next eclipse, and generated wide interest and enthusiasm for solar eclipses. So although Baily didn't discover the beads, nor was he the first to explain their cause, he did get them a lot attention.

You Should Also Read:
Naming Stars
Edmond Halley

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