Sunday, 15 January 2012

Gamma Velorum - multiple star

Gamma Velorum is a superb multiple star - all four components visible in a 60mm refractor - though star D at 9.1mg needs good seeing. Colour contrasts are not large, but primary star A, also called Gamma Velorum 2 has a green or pale yellow tint.  Its spectral type is O7.5e+WR8. The spectrum has been viewed often and photographed (below). It's possible the blue and yellow emission bands combine with strong violet and green in the continuum  to give it a green or violet tint (?) Secondary B (type B) is very blue to my eyes, and C and D seem white (both type A)
Gamma Vel is the brightest of the rare type O stars - none are close to Earth (thank heavens!) and the second brightest is nearby Zeta Puppis. Both are sited within the Gum Nebula, a recent (~5000y BP) and huge supernova remnant. The whole region is rich in stellar exotica. 
(This is a sketch - photos of this object are usually much overexposed)

The Spectrum of Gamma Velorum

Gamma Velorum - the brightest Wolf-Rayet star in the sky is sited at RA 08h09m Dec -47:20, and shines brilliantly in constellation Vela, close to the second brightest type O star Zeta Puppis. This (1998 3M1000) photo is to show what the Baader Spectroscope records. The visual view is much nicer! The star is multiple, A is the WR type, but fainter B has recorded too - & their spectra overlap.The stars were drifted in RA to widen the prime spectrum - tricky with film. The brilliant emission bands are arrowed, wavelengths in Angstroms. The faint orange line 5876A, easy visually, has barely recorded - film's response was far from linear.The UV enhancement at rhs is partly from the WR star A and secondary star B's overlapping spectrum. The bright blue and lemon-yellow lines (all carbon) have recorded well. Problem: the Airy disc of the star broadens the lines - a slit-type spectroscope would solve this, but brings more problems of design and control. Ideas welcome!

Monday, 9 January 2012

Spectrum of Wolf-Rayet Star

The Baader Planetarium spectroscope would not normally show detail in the spectrum of any star as faint as ~7mgv - but there are exceptions. This is one!  Years back I had seen the faint blue carbon line in the spectrum of star WR79 in rich galactic cluster NGC6231, close to Zeta Sco. Recently, after removing the cylindrical lens that broadens the spectrum - producing a prime spectrum - I had sucess seeing emission lines in the spectrum of Eta Carina (see post below)

The same setup, when used on N6231 produced this view. It was spectacular! This star is a Wolf-Rayet type WC6, of 6.7mgv. The bands are almost identical to the brightest WR star Gamma Velorum. The blue line is a blend of two carbon lines, as is the lemon-yellow line. The fainter orange line is due to helium. Kaler J. tells us the WR stars are helium burners - having used up their hydrogen through fusion and mass-loss - exposing a helium shell.  Carbon is a product of helium fusion - hence these gaudy bands.  A nice example of stellar physics in a rare star type.  No cameras used for this sketch - but software helped to create the colours.

The other cluster stars are all type O and B - and showed no details in my setup. There are several WR stars in the southern summer sky. I hope to sketch them soon.