DELTA-2 LYR (Delta-2 Lyrae). While Delta Lyrae, which anchors the northeast corner of the little parallelogram that helps makes Lyra (the Harp), has no proper name, it does have two stars that make a widely-spaced naked eye "double," the fainter western one called Delta-1, the more obvious eastern member called Delta-2. Since they are not a true double (as was once suspected), we take them one at a time. While Delta-2, a class M (M4) bright- giant, is measured to be 900 light years away, Delta-1 (a class B2.5 dwarf) is almost 200 light years farther. The errors
A closeup view of Vega (the bright star at top) and its surroundings reveals the duplicity of the famed "double-double star" Epsilon Lyrae at left (west is up in the picture). The left- hand star of the pair is Epsilon-1, the right hand star Epsilon-2. Each of the two are also double. Zeta Lyrae is at the center of the right-hand edge, while the unrelated pair Delta-1 and Delta-2 Lyrae are at bottom right, Delta-2 the brighter.
in the distance measurements would allow the two to be at the same distance. However, they would still be separated by three light years. Given star-separating tides raised by the Galaxy, that is triple what is considered possible for stars to be bound in orbit. Moreover, the two are moving in different directions, so Delta's double is a mere accident of alignment. As an unusually cool (3640 Kelvin) M star, Delta-2 is well worth a look on its own. Accounting for its considerable infrared radiation, the star shines with the light of 6500 Suns and has a computed radius of 200 solar, 0.95 times the radius of the Earth's orbit (the AU). Direct measurement of angular diameter coupled with distance gives an even larger radius of 1.3 AU, which, given the errors of measurement, is rather good agreement. Luminosity and temperature tell of a star that began its life 75 million years ago as a hot class B3 star of 6 solar masses. It is now growing with a dead helium core and becoming an even larger giant. Delta-2 is a "semi-regular variable" that changes its brightness by about 0.2 magnitudes over an ill-defined period. It may be preparing to become a much more obvious pulsating variable like Mira. Separated from Delta-2 by 86 seconds of arc is a faint double star whose 11th-magnitude members are themselves separated by 2.2 seconds of arc. Though these stars were ironically thought NOT to belong to Delta-2, their combined G-subdwarf classification is consistent with being at the same distance as the brighter star, making the odds seem rather good that they indeed belong together. If so, the little pair is separated from the giant by at least 24,000 AU and takes at least 1.3 million years to make an orbit. The two small stars, separated by at least 600 AU, do it much faster, taking at least 10,500 years to make a circuit (the lower limits the result of having only a projected angular separation).
Written by Jim Kaler. Return to STARS.