53 AQR (53 Aquarii). At sixth magnitude (5.6), barely visible to the naked eye in a dark sky, lying in a dim region of southwestern Aquarius to the east of Deneb Algedi and Nashira in northeastern Capricornus, 53 Aquarii (named with but a Flamsteed number) hardly makes an impact on the its surroundings. Only when we look more closely does the faint one begin to fascinate, as do all solar type stars, since we can then get the chance to see ourselves at a great distance. Or not such a great distance, since our 53 Aqr lies only 66 light years away, similar to the distances of the stars of the Big Dipper. Solar-type yes, but with a difference, since 53 Aquarii is double. And not JUST double, but a binary that consists of a PAIR of sunlike class G stars, or "solar analogues" as they are known in the trade. (The best is probably 18 Scorpii.) The brighter (magnitude 6.3), 53 Aqr A, is listed in the standard catalogue as a G1 dwarf, the fainter (6.6) as a very solar G2 dwarf (the same as the Sun), though later listings give them both as G2. Well separated at 13 seconds of arc in the 1800s, Smyth and Chambers call them "pale white" and say "a neat double star...This is a beautiful object." Their orbital motion has now taken them to just over a second of arc apart.
53 Aqr From two centuries of data, the motion of 53 Aquarii B is plotted around 53 Aqr A as if the latter were at the orbital focus and stationary. The scale is in seconds of arc and the dot-dash line is the orbital major axis. A tilt of 44 degrees distorts the orbital path from what it would look like were it face-on. In reality, both stars go around a common center of mass almost exactly between them. Over a period of 3500 years, the highly elliptical path takes the stars from as far as 575 AU apart to as close as 30. They will be closest (at the short end of the major axis) in 2023. From the average separation of 300 AU and the period we find the sum of the stellar masses to be 2.25 times that of the Sun, or 1.13 solar masses for each, close to that derived from theory. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
An elliptical orbit fitted to what data we have tell of a long 3500 year period at an average separation of 300 Astronomical Units. But the average does not mean that much, as a high eccentricity takes them as far as 575 AU and as close as 30 (which will happen in the year 2023). From Kepler's Laws, we derive a system mass of 2.25 times that of the Sun, given the inevitable errors in a partial orbit very consistent with their spectral classes. Physically, the two are very similar. 53 Aqr A and B have respective temperatures of 5830 and 5790 Kelvin (compared with the Sun's 5780), luminosities of 1.07 and 0.98 times solar, and radii of 1.02 and 0.99 solar. The masses of both are very close to that of the Sun, again consistent with the sum derived from the orbital parameters. Even the ages come in about the same (as expected for a binary, the two together since birth), at 5.5 billion years, just a bit older than our 4.5 billion-year-old Sun. One difference with the Sun quite stands out, however. The Sun rotates (at the equator) with a period of 25 days. Both of 53's stars spin notably faster, taking less than 6 days to make their turns. If the rotational equators are inclined through the same angle as the orbit (44 degrees to the plane of the sky (which seems unlikely), the periods are down to 4 days. Such rapid rotations would make the stars more magnetically active than the Sun. There are no known planets. Imagine one, though (double stars known to possess them, as in the prime example of 16 Cygni, which consists of two solar-type stars as well). At their closest, a resident of an Earthlike planet would see two suns, the more distance about 6 magnitudes fainter than the one it is orbiting. At times there would be no significant "night" at all. At greatest separation, though, the "other Sun" would not be much brighter than a few full Moons. Imagine the mythologies. Imagine too actually being able to visit another star, the two close enough to allow space travel between them.
Written by Jim Kaler 10/9/09. Return to STARS.