Comparing the brightness of the stars in The Plough.

The Bayer Designation. Johann Bayer published his Uranometria star map in 1603. He assigned Greek letters to individual stars within each constellation. Many people today assume that these letters were assigned according to the brightness of the stars. In many constellations this is true, but at the time magnitudes of stars could not be very accurately measured. His lettering assignment was quite often done in a less regimented manner. It could be done by the position of the stars within the constellation, or using the stars positions in right ascension. Today we still use these to identify the brightest stars and many people still cling to the belief that alpha designates the brightest star, beta the second brightest and so on. Let’s go on to disprove this myth using the familiar asterism of the Plough.

Here are the stars in The Plough listed according to their Bayer letters:

Table 5.2 The Seven Main Stars of The Plough. Star Name Bayer Letter Dubhe Alpha () Merak Beta () Phad Gamma () Megrez Delta () Alioth Epsilon () Mizar Zeta () Alkaid Eta ()

You would assume from this table (had Bayer assigned the Greek letter according to brightness) that Dubhe would be the brightest star, Merak the second brightest and so on straight down the table. Is that indeed the case? Next time it’s a clear night go and have a look for yourself.

A Map of the Plough.

Print off the map above and try and estimate the stars relative brightness. Assign a Greek letter, or label the stars 1, 2 3 etc in order of brightness and mark this on your chart by each star. Label what you estimate to be the brightest star as Alpha, the second brightest Beta and so on right the way through all seven stars. How does your new Bayer designation compare to how they are labeled on your star maps?

You will have noticed that Megrez, located where the handle meets the bowl, is definitely the faintest star of the seven. If the Bayer letter is to be believed, then it would be the 4th brightest star. But it isn’t.

So we have now disproved that the Bayer letter always indicates the star brightness in a constellation. Bayer actually assigned his Greek letters in Ursa Major according to their right ascension, not brightness.

The exercise you have just done is also useful in looking the accuracy of your perception of brightness. Look up the measured magnitudes of the Ploughs Stars in table 5.3 below. Did you get all seven stars listed in their correct order of brightness?

Table 5.3 Star Name Magnitude Dubhe 1.81 Merak 2.34 Phad 2.41 Megrez 3.32 Alioth 1.76 Mizar 2.23 Alkaid 1.85

It might also be useful to repeat this same exercise on a night when conditions are different. e.g. a slightly misty night, or a night when the Moon is near to full to see if you get different results. Or how about putting the exercise to a group of people to try the exercise individually and collating those results to see how accurately these estimations become when more data is collected in this way? This will certainly show the power of collecting data from a number of observers.

Dave Eagle

Eagleseye Observatory. Higham Ferrers, UK www.eagleseye.me.uk [email protected]