Laser Stars

Observe Laser Stars

( from your own backyard )

Amateur Spectroscopy

Observe the spectrum of naturally occurring thermonuclear fusion powered lasers from your own backyard, with the naked eye !

Lasers Observed 130 Years Ago !

The brightest quasars in the sky at magnitude 13 can barely be seen in an amateur telescope, most quasars are hundreds of times fainter. Fortunately many Wolf-Rayet stars also have unusual emission line intensities due to laser action, especially the early or hot spectral types. At about magnitude 8 the brightest Wolf-Rayet stars are hundreds of times brighter than the brightest quasar and are easily observed with a small telescope. Place an inexpensive piece of high dispersion holographic diffraction grating between your eye and the eyepiece and you can make out very bright laser emission lines in the spectra of many Wolf-Rayet stars. The first three Wolf Rayet stars were discovered in this way. The spectra of V1042, MR103 and MR100 was visually observed by C.J.Wolf and G.Rayet in 1867, before the systematic use of photographic plates. It was the first known instance of a laser being observed almost a century before the first optical lasers were built.

Stars near gamma Cygnus, Cygnus Constellation
The first image from E.Barnard's 'Atlas of Dark Nebula' is a 3 by 3 degree region of sky within the small green rectangle in the constellation Cygnus near the OB1 region. The red dots are Wolf-Rayets stars, from top to bottom : MR102, MR103, V1042, MR100 and MR99. This region lies on the galactic equator where the star density is extremely high.

Star chart from 'Uranometria', Tirion et al. (1987)

List of Wolf-Rayet Stars

NGC 6888 Nebula

NGC 6888

  The stellar wind and strong UV radiation from the Wolf Rayet star MR 102 (WN6 type) created this nebula which is also visible on the chart. The emission spectra shows evidence of helium and nitrogen enrichment relative to the interstellar medium. (Hydrogen alpha photo courtesy Yvan Dutil observatoire du Mont Mégantic; Other pictures by Wallis and Provin, and by Fletcher Astrophoto Gallery, there is a drawing by Jere Kahanpää,   see also here)

Step by Step Instructions

  1. Obtain a piece of holographic film diffraction grating, preferably of high dispersion. Learning Technologies sells them in small plastic sheets of 23 centimeters for about $5 U.S. (plastic replica gratings have less dispersion and much lower efficiency than holographic gratings.)

  2. Hold a piece of the grating between your eye and the eyepiece, or tape it to the eyepiece. Examine a bright star, notice how you can still see the star through the grating. Determine where the rainbow of the dispersed starlight falls. Rotate the orientation of the grating in such a way that the star and its spectrum lie horizontaly. (it is easier for the eye to move right and left rather than up and down)

  3. Using the above star chart, star hop to one of the Wolf-Rayets in the list. If you own a computer assisted telescope, enter its coordinates.

  4. It is easy to pick out the Wolf-Rayet in the field of view because its spectra contains two or more prominent dots corresponding to the bright emission lines. The spectra of ordinary stars are featureless streaks, although they may contain harder to see absorption line features or dark notches.

  5. Compare what you see with the digitized spectra below. Under very low illuminations the human eye doesn't see colors very well. The spectra of fainter stars may look somewhat greyish. You may have trouble seeing faint red colors, which restricts the spectral wavelength range you can observe.

  6. There are further suggestions available to improve the spectra visibility, mostly having to do with atmospheric conditions and shielding from light pollution.

Wolf Rayet Spectra
Optical spectra of four Wolf-Rayet stars as a function of wavelength in Angstrom units, from top to bottom : V1042, MR103, MR107 and MR84 (marked with a * symbol in the table). Spectra plotted in grey as they appear to the eye under very low illumination conditions. The first two spectra were observed by Wolf and Rayet back in 1867.

Visual Spectra

  The visual appearance of diffraction grating spectra of a cluster of ordinary stars as seen through the telescope's eyepiece. In this graphical simulation, we made the approximation that all the stars have the same brightness and spectral type, with absorption lines blurred by excessive atmospheric turbulence. Without a cylindrical lens each spectra would appear as a streak instead.

Digitized Spectra

The broad Wolf-Rayet emission lines didn't correspond to those observed in other stellar spectra. (A similar situation occurs in quasar spectra) These anomalous lines were originally thought to be molecular bands, because pressure broadening was not an acceptable mechanism. Later Edlen (1932) correctly identified them with lab spectra of highly ionized carbon, nitrogen and oxygen in extremely hot stellar atmospheres between 50000 and 100000 degree K. Doppler broadening within strong stellar winds contributes to line width. It was later discovered that not all of the broadening can be attributed solely to this effect. Laser action is now thought to contribute significantly to these exceptionally broad emission lines.

The prominent emission lines get broader and stronger in hotter stars from WC8 through WC7 and WC5, indicating increasing laser action in earlier spectral types. The strongest emission line at 4650 angstroms is a blend of two carbon+3 transitions; 6g-5f at 4658 and 6f-5d 4646. From extensive computer simulations of Wolf-Rayet stellar atmospheres, these two transitions have a stronger population inversion when the plasma cools more rapidly. These figures do not give a good indication of line strenght because high intensities saturate to white. If the equivalent line strenght is plotted on a graph, the prominent emission lines are clearly stronger in WC5 stars, corresponding to hotter spectral types. (digitized spectra from Torres et al., 1987).

NGC 2359

  Vigorous stellar winds near the very hot Wolf-Rayet star HD 56925 (WN4) produces nebulosity with visible shock fronts. At magnitude 11.74 it is too faint for visual spectra observations, however digitized spectra are available from Torres-Dodgen et al. (1988). (Photo courtesy Anglo-Australian Obs.)

Observational evidence suggests that hotter atmospheres have stronger stellar winds favoring rapid cooling by expansion. Stronger winds eject more material enhancing the laser gain by gas-contact cooling with a dense colder envellope of accumulating stellar 'ashes' from previous ejections. This enormous mass loss created the circumstellar nebula in NGC2359 above.

Subluminous Wolf-Rayets and Quasars

Although most Wolf-Rayet stars are bright and massive with absolute luminosity from -4 to -6, about ten percent are the subluminous central stars of planetary nebula with absolute magnitude from -2 to +4. They are hundreds of times fainter than the average Wolf-Rayet. Quasars are close cousins to these miniature subluminous Wolf-Rayets and some even share common emission lines. Quasars are an extremely high excitation version of the shell-stars, a sub-type of the Be/Ae emission line stars. For more information on shell stars consult The Be star Newsletter.

As final interesting note, the quasar Cygnus-A lies just beyond the northern edge of the above star chart, near 20 hours of right ascension and about the same declination as the bright star gamma Cygnus in the upper left corner of the chart. The very high star density (Cyg OB1) in this region of the galactic plane is strong evidence against the standard interpretation that this object is extragalactic.

REFERENCES

  1. Color plots of the optical emission line spectra of the elements.
  2. References on Wolf-Rayet spectra
  3. More on Wolf-Rayet stars
  4. Amateur CCD Spectroscopy - Christian Buil, took spectra of quasar 3C 273
  5. Quasar spectra at Worcester Park Observatory - UK
  6. Gauvin,M.: 1995, CCD Astronomy, Fall, p.8. 'Amateur Spectroscopy'
  7. Univ. College London Wolf-Rayet research group
  8. Wolf,C.J., Rayet,G.: 1967, Comptes Rendus Acad.Sci., 65, 292.
  9. Tirion,W., Rappaport,B., Lovi,G. : 1987, Uranomertia, Vol I, p.119, Willmann-Bell,Inc. (can be obtained from Sky Publishing Corp.) 1.[100].... III/85 Sixth Catalogue of Galactic Wolf-Rayet Stars (van der Hucht+ 1981) 2.[60].... III/143 Spectrophotometry of Wolf-Rayet Stars (Torres-Dodgen+ 1988) 3.[50].... III/136 Optical Spectrophotometry Wolf-Rayet C and O Stars (Torres+ 1987)
  10. van der Hucht,K.A., et al. : 1981, Space Sci.Rev., 28, 227. The Sixth Catalog of Galactic Wolf-Rayet Stars
  11. Burnham,R.: 1978, Burnham's Celestial Handbook, p. 80, 782, 1383. Dover Publications. p.1724 Chart for MR 64 and MR 65 in NGC 6231 . p.2034 Extensive background on Gamma Velorum the brightest Wolf-Rayet in the sky.
  12. Cappa De Nicolau, C.E., Niemela, V.S., Dubner, G.M., Arnal, E.M.: 1988, AJ, 96, 1671, The H I bubble around the Wolf-Rayet star HD 156385 and its environs
  13. Dubner, G.M., Niemela, V.S., Purton, C.R.: 1990, AJ, 99, 857, A stellar wind blown bubble associated with the Wolf-Rayet star HD 197406
  14. Marston, A. P., Meaburn, J.: 1988, MNRAS, 235, 391, The Wolf-Rayet nebula NGC 6888 as a pressure driven bubble
  15. Miller,G.J., Chu,Y.: 1993, ApJS, 85, 137, A new survey of nebulae around Galactic Wolf-Rayet stars in the northern sky
  16. Niemela, V. S., Cappa De Nicolau, C. E.: 1991, AJ, 101, 572, Search for H I bubbles around Wolf-Rayet stars between l = 302 deg and 312 deg
  17. Treffers, R.R., Chu,Y.: 1982, ApJ, 254, 569, Galactic ring nebulae associated with Wolf-Rayet stars. V - The stellar wind-blown bubbles
  18. Cappa,C.E., Dubner,G.M., Rogers,C. et al.: 1996, AJ,112, 1104, The interaction of NGC 6888 and HD 192163 with the interstellar medium
  19. Barnard,E.E.: 1927, A Photographic Atlas of Selected Regions of the Milky Way, Carnegie Institution, (523.113 B259)
  20. Holographic diffration grating sheets, $5.00 from
    Learning Technologies, Inc.
    59 Walden Street
    Cambridge, MA 02140, (tel: 1-800-537-9703)
 

LASER SAFETY

Observing laser stars is perfectly safe and will not cause eye damage, even with the most powerful telescopes. This is true of astrophysical lasers ONLY, most earth based lasers can cause severe eye damage if proper eye protection safety precautions aren't taken.

 

For more information on the hazards of laser radiation, consult the following :

Amateur Astronomy Home Page.