Visibility of comets
The visibility of comets depends on several factors:
- of the brightness under which it appears from the earth ( freedom of view ). She in turn depends
- from the distance to earth,
- from the distance to the sun ,
- and on the composition of the comet's nucleus and the processes of outgassing when approaching the sun
- from astrometric visibility . She depends
- the position of the orbit plane to the earth's orbit ( ecliptic ),
- and the location of the observer .
Apparent brightness
Most comets run on very elongated elliptical orbits around the sun and only become visible when they approach the sun within around 2 AU - on the one hand because they then also come closer to earth and on the other hand because the sublimation of ice begins, which the core with a glowing shell ( coma ) surrounds it. Depending on the size of the core (a few kilometers on average), which consists of various frozen gases as well as loose rock and dust particles , the comet is visible to the naked eye or is reserved for telescopic or photographic observation.
A few weeks after the discovery of a new comet and a preliminary orbit determination , certain prognoses for future phenomenology can be made. They extrapolate the observed brightness as a function of the radius vector r (distance from the sun) and the distance d from the earth . However, the increasing brightness when approaching the sun is fraught with great uncertainty. It is mostly modeled with the function r −n , whereby the exponent n can be between 3 and 5 depending on the composition of the comet's nucleus. In the case of a great distance from the sun (r> 4 AU, no self-luminosity yet ), however, it is 2. The effect of the distance from the earth can be shown more easily. It is approximately d -2 , see the article surface brightness .
Nevertheless - due to the different composition - the development of brightness and comet's tail can only be approximately predicted. They often remain far below the expected brightness, but there are also isolated outbreaks of brightness of 2 to 5 magnitudes , for example with comet 29P / Schwassmann-Wachmann 1 . Nevertheless, among the 20 to 30 comets visible annually, there are rarely really impressive phenomena, on average only about 10 per century.
These so-called large comets can develop up to the brightness of Venus (−4 mag) as they approach the sun and earth and then even be visible in the daytime sky . However, they only become spectacular when a longer tail develops (see also the earlier fear of comets ). However, it only makes a significant contribution to the overall brightness when the comet moves between the sun and earth.
One such case was the comet Tebbutt (1861). The enormous brightness and breadth of its dust tail in the backlight resulted from the large surface of the billions of dust particles, while in the plasma tail even every atom and molecule contributed to the luminosity. Some comets in the last few decades had similar, but weaker effects, such as the sun streaker Comet West in 1975 , 1996 C / 1996 B2 (Hyakutake) and C / 1995 O1 (Hale-Bopp) and 2006/07 C / 2006 P1 (McNaught) .
Location of the railway
While the absolute brightness depends primarily on the distance to the sun and its minimum ( perihelion ), the distance to the earth is decisive for the visible appearance. Only when the comet passes through on "our" side of the solar system can it reach or exceed the brightest stars in terms of conspicuity and develop a tail longer than 10 °.
As for the location affects the observer, is crucial in what area of the declination , ie in which the apparent distance from the earth's equator lies the path section with the greatest brightness. The steeper the plane of the orbit is, the more different the local conditions for observation can be.
So was z. For example, Comet McNaught , discovered in August 2006, is more visible in the southern hemisphere , but for a few weeks also visible in Europe with free eyes and even accessible for daytime observation with telescopes . After its perihelion, however, it dived south and the splendid tail development was reserved for observers in Australia and South Africa.
The opposite case occurred in 1861 with the above mentioned comet Tebbutt . Largely unnoticed, it developed in the southern sky and only appeared in the north as it approached its perihelion - with a tail across half the sky. That the earlier fear of comets struck again with this sudden phenomenon - it coincided with the outbreak of the American Civil War , may already be incomprehensible in our time. Nevertheless, many still felt in 1910 when Comet Halley a kind of gloom and doom , because the Earth recently passed through the gas tail, which small amounts of the highly toxic cyanogen contained.
literature
- K. Wurm: Die Kometen (= Understandable Science. Volume 53). Springer, Berlin / Göttingen 1954.
- John C. Brandt, Robert D. Chapman: Introduction to Comets. University Press, Cambridge 2004.
- H. Zimmermann, A. Weigert: Lexicon of Astronomy. Spectrum Akademischer Verlag, Heidelberg / Berlin 1999, chapter comets , pp. 171–180.