Alcor is the faint star beside Mizar in the handle of the Plough, an old test of good eyesight and probably a true member of Mizar’s system.
At a glance
| type | white star (class A) |
|---|---|
| distance from the Sun | 84.7 light-years (26 pc) |
| its light left it | 84.7 years ago |
| luminosity | 15 × the Sun |
| radius | 1.8 × the Sun |
| surface temperature | 8,200 K |
Read off the piece’s own catalogue, the same values it draws with.
About Alcor
Look at the bend of the Plough’s handle and Mizar has a faint neighbour, Alcor. Seeing the two apart with the naked eye is an old test of eyesight, often attributed to Arabic tradition, though for most people with good sight under a dark sky it is not hard.
For a long time it was uncertain whether Alcor and Mizar were physically connected or just in the same direction. Their distances and motions through space are very close, and they are now generally thought to be bound, though so far apart that one orbit would take an immense time.
Alcor is a white A-type star, hotter and more luminous than the Sun, and it has a faint red dwarf companion of its own.
What you see in the piece
From Earth (HUMAN EYE) Alcor sits just beside Mizar; flown to, the gap between them turns out to be vast compared with the tight pairs inside Mizar.
Measured or modelled
Whether Alcor is gravitationally bound to Mizar is inferred from their similar distances and motions; the evidence favours it but the orbit itself has never been observed.
Questions
Are Mizar and Alcor a binary star?
Probably. They lie at nearly the same distance and move together through space, so most astronomers count them as one wide system, though an orbit has never been followed.
How far away is Alcor?
Alcor is 84.7 light-years (26 pc) from the Sun.