Classic Stellar Observation
Stars are often mostly observed by amateur astronomers in aggregates, -- from constellations, associations and asterisms to globular and open clusters, and down to multiple and double systems. And of course as ”stepping stones” and ”faint fuzzy” galaxies, when star hopping to DSO.
But stellar physics and evolution are also interesting topics ”per se”, and with simple amateur spectroscopes it is possible -- even with small telesciopes and from suburban environments -- to take the ”fingerprints” of the brightest stars and study their astronomical features such as chemical composition, temperature, movements and more.
This early morning I am out under the night sky at 01:30 local time (UT+2) in nordic (Danish) nautical twilight (SQM 16.3, NELM 4.2m), and I can just see the main outline of the summer constellations (Cygnus, Lyra, Aquila), but not much more. DSO and even low power rich field observing is pretty much out of the question tonight.
Never the less, I have my classic Vixen FL-80S/640mm refractor out this early morning, together with a classic Zeiss ocular spectroscope :
The Zeiss star spectroscope screws right onto the top of a standard CZJ eyepiece. It consists of a Amici dispersion prism followed by a cylindrical lens for widening the spectrum. The prism dispersion is non-uniform, so that the short (violet end) wavelengths are more widely spread out. This device works best at good seeing and at a magnification of 5-10 x Dcm (ie for my 8cm scope : 40-80x. The magnification of the CZJ O-16 is @ 68x (using a 1.7xGPC in front of my diagonal) , which gives a small but nice bright spectrum of Vega. At
-0.6m +0.03m Vega is the second brightest star in the N. hemisphere (after blazing Arcturus at: -0.0m -0.05 m) and with my 3” Vixen refractor I should be able to use the EP spectroscope on stars down to 2-3m. (With a 8” scope it should be useable down to ~5m). --
note: [Thanks to cincosauces for providing the correct stellar magnitudes for Vega & Arcturus!]
I start out by viewing Vega with the CZJ eyepiece spectroscope; You have to defocus a little inwards, to broaden the spectrum enough to be able to see the lines. The spectrum of Vega is dominated by two absorption lines: one strong where green fades into blue (between 480-490nm), plus a weaker absorption line in the far end of the blue (between 430-440nm). The ultra far violet and red parts of the spectrum (below ~420nm and above ~670nm) can not be seen in the CZJ okular-spectroscope :
The two absorption lines I am observing in Vega match the hydrogen lines : H-beta (486nm) and H-gamma (434nm) of the Balmer Series, ie. photons absorbed in the stellar atmosphere by Hydrogen atom electrons being ionized (”kicked”) from the 2. quantum state (”shell”) to the 4. (beta) rsp. 5. (gamma) state :
The prominent Balmer series Hydrogen absorption lines are characteristic of massive (>1.3x Msolar) and hot (core: TC>18M K) blue/white stars of spectral type Secchi-I (corresponding to type A-F of the Yerkes/MKK system). These stars fuse hydrogen protons to helium nuclei through the CNO-cycle (carbon-> nitrogen->oxygen), which results in a convection zone around the core, that distributes the 'ash' from the fusion below an overlying atmosphere in radiative equilibrium. The photosphere shows strong H- absorption lines, as seen in for example: B: Rigel, Bellatrix -- A: Vega, Deneb, Altair – F: Polaris Procyon.
I now change to my 100li/mm transmission difraction grating spectroscope, for a closer analysis of the Vega spectrum.
(to be continued...)
Edited by AllanDystrup, 03 July 2017 - 10:44 AM.