Understanding and Observing Double Stars

This last club meeting we had a unique opportunity to be the recipients of a lecture by Joe DalSanto. Joe is an very experienced double star observer who also shared with us a excellent overview of some of the technical aspects of Solar science. If you missed it, this was one of our best educational sessions we have had recently. Having browsed through his “A Field Guide to Double Star Observing” I would recommend picking up a copy (see end of article for ordering information.)

The most interesting to me was discussing the spectral classification of stars. While this initially may sound bland, understanding the stellar spectrum makes observing stars very interesting. How many stars have you looked at without understanding their characteristics? Well, let’s get started at understanding them.

Stars are divided into 7 classes designated by the letters O, B, A, F, G, K, and M. The hottest stars (O and B) are blue-white in color, while the coolest (M) are red.¹ I’ve always assumed that Betelgeuse (M2 classified) in Orion was very hot but, bright blue-white Rigel is much hotter, (its classed as A1 V, a main-sequence blue-white star). I just think it is so ‘cool’ to understand what I am looking at! Anyway, each of the letter classes has subdivisions indicated by numerals 0 through 9. Thus, a B0 is the hottest B-type star, B5 is halfway between types B and A, and B9 is only slightly hotter than type A. A simple method to remember these classification in regard to how hot star has been developed . The boys at Harvard adopted a sequence that runs from hottest to coolest like this; B, A, F, G, K, M. Astronomy students have been taught to remember this sequence as the mnemonic "Be A Fine Girl, Kiss Me". In the male dominated and perhaps lonely days of astronomy that was acceptable - even for Harvard. You can always substitute "Guy" for "Girl" as it suits you.²


Double Star Drawings by the Belmont Society, http://www.belmontnc.4dw.net/index.htm

The diagram below wonderfully depicts what is known as the main sequence as well as showing where some of the familiar stars are as well.

Diagram used with permission courtesy of Dr. Jamie Love:
http://www.synapses.co.uk/astro/hrdiag.html

Notice again Betelgeuse and Rigel. Rigel falls in at about an B8 classification which shows that it is Blue, very hot and very bright. Betelgeuse is classified as a M1 which shows that it has a very low surface temperature and yet is very bright. Our own sun can be found in the main sequence (the band where most of the stars fall into.) Some day Betelgeuse may become a White Dwarf after it goes supernova. That’s nearly the opposite on the chart where it is today. Comparing it to our sun, you can deduce that it’s luminosity is nearly 60,000 times brighter than our own sun.

H-R Diagram different types of stars are
in temperature and luminosity.

I hope this article is helpful in giving you insight into double star observing.

If you would like to order a copy of Joe DalSanto’s, “A Guide to Double Star Observing” use the preceding link.

- Andy Weeks

1. Encyclopedia.com— http://www.encyclopedia.com/html/s1/spectral.asp

2. Color Designation and the Hertzsprung-Russell Diagram- http://www.synapses.co.uk/astro/hrdiag.html