15 April, 1998
The ability to separate two closely-spaced stars is a function of the aperture of the telescope that's observing them. An optical interferometer array is equivalent to a single telescope with an aperture the size of the array. Optical long-baseline interferometers are now being built which extend over an area several hundreds of meters across. One of them, the Navy Prototype Optical Interferometer (NPOI), has looked at one of the hottest, bluest, most massive, and intrinsically brightest stars in the sky, Zeta Orionis A, and has found a companion just 0.04 arcseconds* away from it! While the primary shines at magnitude 2, the secondary is only 2 magnitudes fainter and would be easily visible to the naked eye if it weren't so close to its brilliant neighbor. NPOI has already detected relative motion of the two stars, most likely due to their being gravitationaly bound with an orbital period of a few years.
Zeta Orionis is well-known to amateur astronomers as Al Nitak, the eastern star of the three which make up the "Belt" of Orion. In amateur telescopes it appears as an optical double star, with a 4th magnitude companion, Zeta Orionis B, lying 2.3 arcseconds to the southeast of Zeta Orionis A. Zeta Orionis A's primary component is a supergiant star of spectral type O9.5, which also shows peculiar emission lines. The European Space Agency's astrometry satellite Hipparcos has measured Zeta Orionis A's distance from Earth to be 815 light years, giving it an absolute visual magnitude of -5, somewhat under-luminous for stars of this class.
Type O stars are the hottest stars in the spectral sequence, and the supergiants show strong stellar winds that produce optical spectral emission lines, thermal radio, and X-ray emission. How these stars produce high-energy X-rays is still subject to intense research, since they lack significant magnetic fields and are not hot enough (despite their very high surface temperatures). Shock waves created in the turbulent stellar wind flow are one ingredient of current theories.
NPOI is routinely observing double stars as well as single giant and supergiant stars near Flagstaff, Arizona. It is a joint project of the U. S. Naval Observatory and the Naval Research Laboratory in Washington, DC, in association with the Lowell Observatory in Flagstaff. It is currently being extended to its final diameter of 437 meters, which will enable it to observe double stars with separations several hundred times smaller than that of Zeta Orionis A.
* An arcsecond is a unit of angular measure on the plane of the sky. One arcsecond is 1/3600 of a degree, about the size of a penny as seen at a distance of 2 kilometers.
PHOTO CAPTION: http://www.usno.navy.mil/zetorinews.html
The image below is a stack of three image-maps of Zeta Orionis, each spaced by about 2 weeks, aligned and color-coded in red, green, and blue. At the position of the primary, where all three colors, i.e. images coincide, the result is a white colored image. At the position of the secondary, a band of colors appears, indicating motion of the secondary with respect to the primary. In the image, north is up, and the separation between the two stars is about 43 milliarcseconds.
Credit: Dr. Christian A. Hummel, USNO - NPOI