# Stars/Greens

## Sun

The upper rim is green while the lower one is red, as the Sun sets behind the Golden Gate Bridge. Credit: Brocken Inaglory.
The green rim and flashes of a setting Sun are imaged. Credit: Brocken Inaglory.

As an astronomical object sets or rises in relation to the horizon, the light it emits travels through Earth's atmosphere, which works as a prism separating the light into different colors. The color of the upper rim of an astronomical object could go from green to blue to violet depending on the decrease in concentration of pollutants, as they spread throughout an increasing volume of atmosphere.[1] The lower rim of an astronomical object is always red.
A green rim is very thin, and is difficult or impossible to see with the naked eye. In usual conditions, a green rim of an astronomical object gets fainter, when an astronomical object is very low above the horizon because of atmospheric reddening,[2] but sometimes the conditions are right to see a green rim just above the horizon.

A number of emission lines have been detected in solar limb faculae.[3]

## Limb brightness

"An excess brightness [at or near the "edge" of the Sun] can be expected to have a pronounced color dependence, whereas a geometrical oblateness cannot depend on color."[4]

There is "a time varying, excess equatorial brightness"[5]

Def. "the radial distance q from the Sun's center such that the following finite Fourier transform is zero:

${\displaystyle F(G;q,a)=\int _{-1/2}^{+1/2}G(q+a\sin \pi s)\cos(2\pi s)ds,}$

where s is a dummy variable, G is the observed solar intensity as a function of the radius, and the parameter a determines the extent of the solar limb used"[5] is called the solar edge.

"When F(G; q, a) = 0, the a dependence of q can be used to choose different points as the edge."[5]

"The wavelength dependence of the facular excess brightness ... is ... normalized to unity for the [intensity of transmission of] the green filter [(525±70 nm)]. ... An unweighted least-squares fit to the five points [(five filter transmission intensities)] gives

R(λ) = -0.0568 ± 0.0634 + (0.582 ± 0.038)λ-1"[4].

"[T]he blue contrast lies about 1 σ above the λ-1 curve. ... [I]f real, [this] may be the result of an increasing opacity in the blue and an increasing ΔT/T in the upper layers of the photosphere. An increasing opacity in the blue may be due to line haze since the blue filter has a 78 nm width."[4]

## Alpha Centauri

This is a wide-field view of the sky around the bright G2V star Alpha Centauri A, created from photographic images forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2.

Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous [151.9% the luminosity of the Sun] than the Sun. It is a solar-like main sequence star with a similar yellowish color,[6] [surface temperature of 5790 K][7] whose stellar classification is spectral type G2 V.[8] From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger.[7] The projected rotational velocity ( v·sin i ) of this star is 2.7 ± 0.7 km·s−1, resulting in an estimated rotational period of 22 days,[9] which gives it a slightly faster rotational period than the Sun's 25 days.

## Capella B

This is an optical image of Capella B. Credit: Aladin at SIMBAD.

Capella B, the image at right, has a surface temperature of approximately 5700 K, a radius of approximately 9 solar radii, a mass of approximately 2.6 solar masses, and a luminosity, again measured over all wavelengths, approximately 78 times that of the Sun.[10]

Capella B is spectral type G0III star and an X-ray source from the catalog [FS2003] 0255 by ROSAT. Its surface temperature has an uncertainty of 100 K. It is part of a binary star with Capella A a G8III. From SIMBAD, the orbital period is 104.0217 d with an eccentricity is 0.001 and inclination of 137.2° to the line of sight.

Although the binary star Capella is not an eclipsing binary, it is a RS Canum Venaticorum variable. These are close binary stars[11] having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity. Systems can exhibit variations on timescales of years due to variation in the spot surface coverage fraction, as well as periodic variations which are, in general, close to the orbital period of the binary system. Typical brightness fluctuation is around 0.2 magnitudes.

## Alpha Scorpii B

Antares A; an LC type variable red giant star, and Antares B, a class B2.5V blue main sequence star, make up a binary star system in the Scorpius constellation. Credit: Sephirohq.{{free media}}

"Buried within the wind is a fifth magnitude (5.5) hot class B (B2.5) companion star (only 3 seconds of arc away) that hides within Antares' bright glare. The two are separated by roughly 550 AU and take perhaps 2500 years to orbit each other. The companion hollows out a small ionized region within the wind, and although blue-white, has the reputation of appearing green as a result of a contrast effect with its brilliant reddish mate."[12]

## Beta Librae

"Another mystery concerns the fact that this white star has so often been described as "greenish" or "pale emerald". Olcott refers to it as "the only naked-eye star that is green in color" while T.W.Webb refers to its "beautiful pale green hue". Star colors are strangely elusive, of course, and there are many such discrepancies in the guidebooks, but modern observers generally agree that the only stars which definitely appear green are the close companions to red stars such as Antares itself."[13]

"Zubeneschamali [Beta Librae] is a hot "main sequence" (hydrogen fusing) star with a surface temperature of close to 12,000 Kelvin, double that of the Sun. While such stars are normally considered blue-white in color, Zubeneschamali has long had a reputation of being the only naked eye star that oddly appears GREEN to the human eye."[14]

## Galaxies

This is a visual astronomy image of NGC 5728 from the Hubble Space Telescope. Credit: Fabian RRRR.
This is a Hubble Space Telescope image of the green continuum in the nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.
This is a Hubble Space Telescope image of the green [O III] emission line in the active galactic nuclear region of NGC 5728. The observation was made on September 4, 1992, using the Planetary Camera with appropriate filters. Credit: A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, & G. K. Miley/NASA-Hubble.

"[H]igh-resolution (0.1") observations of the Seyfert 2 galaxy NGC 5728 with the Hubble Space Telescope",[15] in the images at right, show the full color of the galactic nucleus and a green continuum image of the active galactic nucleus.

At left is a Hubble Space Telescope image in the light of the green [O III] emission line in the active galactic nuclear region of NGC 5728. "The emission-line images reveal a spectacular biconical structure with overall extent [of] 1.8 kpc. The two cones share a common axis and apex. The cones' axis coincides to within ≃ 3° with that of the one-sided nuclear radio continuum emission but is not aligned with the rotation axis of the galaxy disk."[15]

These "ionization cones" are "conical regions of high-excitation emission-line gas extending from an active nucleus. ... The generally accepted interpretation is that partially collimated ionizing radiation shines out from the nucleus and ionizes gas in its vicinity. ... two exposures were taken through the F492M filter (4906 Å/364 Å) to cover the [O III] λλ4959, 5007 emission. ... The high-excitation gas can be traced to ≃ 8.5" (1.6 kpc) from the apex in the SE cone, but only to 1.5" (270 pc) in the NW one."[15]

## References

1. Dispersive refraction by webexhibits.org.
2. Green and red rims by Andy Young.
3. G. Stellmacher and E. Wiehr (August 1991). "Geometric line elevation in solar limb faculae". Astronomy and Astrophysics 248 (1): 227-31.
4. G. A. Chapman & T. E. McGuire (October 15, 1977). "The wavelength dependence of the facular excess brightness". The Astrophysical Journal 217 (10): 657-60. doi:10.1086/155611.
5. H. A. Hill & R. T. Stebbins (September 1, 1975). "The intrinsic visual oblateness of the sun". The Astrophysical Journal 200 (9): 471-7, 477-83. doi:10.1086/153813.
6. The Colour of Stars. Australia Telescope, Outreach and Education. Commonwealth Scientific and Industrial Research Organisation. December 21, 2004. Retrieved 2012-01-16.
7. Kervella, Pierre; Thevenin, Frederic (March 15, 2003). A Family Portrait of the Alpha Centauri System. ESO. Retrieved 2008-06-06.
8. Research Consortium on Nearby Stars (2007-09-17). The One Hundred Nearest Star Systems. RECONS. Georgia State University. Retrieved 2007-11-06.
9. Bazot, M.; et al. (2007). "Asteroseismology of α Centauri A. Evidence of rotational splitting". Astronomy and Astrophysics 470 (1): 295–302. doi:10.1051/0004-6361:20065694.
10. C. A. Hummel et al. (May 1994). "Very high precision orbit of Capella by long baseline interferometry". The Astronomical Journal 107 (5): 1859–67. doi:10.1086/116995.
11. Berdyugina 2.4 RS CVn stars
12. Jim Kaler (26 June 2009). ANTARES (Alpha Scorpii). Urbana-Champaign, Illinois USA: University of Illinois. Retrieved 2017-09-15.
13. Robert Burnham (1978). Burnham's celestial handbook: an observer's guide to the universe beyond the solar system. 2. Dover, New York. p. 1105. ISBN 9780486235684. Retrieved 26 August 2017.
14. James B. Kaler (14 July 2006). ZUBENESCHAMALI (Beta Librae). Urbana-Champaign, Illinois USA: University of Illinois. Retrieved 2017-09-15.
15. A. S. Wilson, J. A. Braatz, T. M. Heckman, J. H. Krolik, and G. K. Miley (December 20, 1993). "The Ionization Cones in the Seyfert Galaxy NGC 5728". The Astrophysical Journal Letters 419 (12): L61-4. doi:10.1086/187137.