nightfall man page

nightfall — binary star astronomy


nightfall -h
nightfall -U [more options] [mass_ratio inclination primary_size secondary_size primary_temperature secondary_temperature]
nightfall [-G] [-A] [more options] mass_ratio inclination primary_size secondary_size primary_temperature secondary_temperature
nightfall [-G | -U] [-A] [more options] -C file


nightfall is an interactive astronomy program for fun, education and science. It can generate animated views of eclipsing (or non-eclipsing) binary stars, compute light curves and radial velocity curves, and determine best-fit models for observational data.

In the simplest case, nightfall computes the light curve for a binary star system with some given mass_ratio (mass of secondary star/mass of primary star), inclination (0 = plane-on view, 90 = edge-on view of the orbital plane), stellar sizes primary_size,secondary_size (dimensionless, in the range 0 - 1.3), and stellar temperatures primary_temperature,secondary_temperature (in Kelvin), and writes the light curve to a file NightfallCurve.dat.

nightfall is able to show many non-trivial, and sometimes spectacular, physical effecs  in binary stars, as it uses a detailed physical model rather than simply assuming the stars to be  spherical.

The full documentation  for nightfall is distributed only in DVI and HTML format, as it is quite big,  and  thus  not very well suited to the 'man' page format.  It includes some discussion of binary stars (at a popular science level, hopefully) that you may find helpful in understanding what  the program does.



Print a usage message on standard output and exit successfully.


Run in interactive mode.

-C file

Use binary star parameters from a configuration file instead of giving them on the command line. Sample configuration files are in share/nightfall/cfg.


Plot a graph of the lightcurve after its computation (P|S - zoom on primary/secondary eclipse, 1|2 - plot 1/2 orbits).


Generate an animated view of the binary star.


Visualize the geometry of the binary star system (v - view of the stars, i - image of the potential, c - contour plot  of the potential, a - all of them).


Send plot to a postscript file. If the postscript file exists,  it will be overwritten.


Select the filter/bandpass for which the lightcurve should be plotted. U-K range from ultraviolett to infrared, best match to the human eye is V. u-y are narrow-band filters. 1|2 will select radial velocity curves instead of a light curve.  

-fP/-fS value

By default, nightfall assumes synchroneous  rotation, which means that the stars are rotating with the orbital period, and thus show each other always the same 'side'. With this option, you can set the ratio of stellar rotation frequency to orbital frequency to some value different from one, seperately for the primary P and the secondary S.

-e eccentricity periastron_length

By default, nightfall assumes the orbit to be circular. With this option, you can set the eccentricity of the orbit (0 is circular, maximum is less than 1), and the periastron length, which is the point of closest approach of the two stars in their orbit (0 - 360 degree).

-sP/-sS longitude latitude radius dimfactor

Place a spot on the primary (P) or secondary (S). The spot parameters are the location of the spot (longitude,latitude), its radius, and the dimfactor by which the temperature is reduced (or increased) within the spot area.

-tP/-tM/-tD value

Set the absolute value for the orbital period P (in days), the total mass M (in units of solar masses), and/or the orbital separation D (in solar radii) of the system. Any two of these are independent, the third is then calculated from Kepler's laws (i.e., you should set only two of these).

-I file

Read in observational data from a data file.  Sample data files are in share/nightfall/data.


Diagnostic output (v - verbose, w - warnings, b - status messages).


The definition of primary/secondary is inverse to the usual convention in astronomy.

Obviously, the size of a star in a binary system is limited by the orbital separation of the two stars. Instead of having to calculate the maximum useful stellar size herself, the user simply gives the desired stellar size as a fraction (0.001-1.3) of the maximum polar radius of the star (which is calculated by the program).  In the output file NightfallCurve.dat, you will then find the 'real' size of the star(s).

If no absolute values for total mass and orbital period/separation are given, the program will use some default values (mass = two solar masses, orbital separation = distance earth-sun). In this case, sizes/masses/velocities given in absolute units (e.g. kg, m, m/s, solar masses/radii) are fictuous only - they would be valid only for a system with the assumed default values of total mass and orbital separation.

The newest version of nightfall can be found on, and on the homepage

To subscribe to the nightfall mailing list, send mail to with a body of subscribe nightfall-l.


Rainer Wichmann (

Bug Reports

If you find a bug in nightfall, please send electronic mail to Please include your operating system and its revision, the version of nightfall, what C compiler you used to compile it, and the output from 'configure'.


28 December 1999 Nightfall manual