CHI HER (Chi Herculis). Sunlike stars are always a fascination, as
we wonder about what planets they might have, and whether or not
someone might be looking back at us. Here's one that does not get
much press, fifth magnitude Chi Herculis, which lies well off Hercules' main pattern in the far northwestern part of the constellation
between Bootes and Draco. Indeed, it's so far west that it became Flamsteed's "number 1," and is
also thus called 1 Herculis. By chance Chi Her is located not far
from a fainter red variable star called X
Herculis, the similarity of lower case Greek "chi" with Roman
"X" sometimes causing confusion between the two. Chi Her is a
class F (F8) dwarf 52 light years away with a temperature of 5840
Kelvin, just a bit warmer than the Sun but
notably lower than the 6100 Kelvin expected for such a class,
probably the result of a low iron (and metal) content only about a
third solar, which can confuse classification. Chi Her shines with
the light of about three Suns, its radius 1.7 times solar, both the
result of a bit higher mass (about 15 percent more than that of the
Sun) and an age of more than eight billion years. One study even
suggests a mass nearly identical to that of the Sun. Either way,
the star is near the end of its hydrogen-fusing life. Chi thus
illustrates something of what will happen to our own star as the
Sun's internal hydrogen fuel supply diminishes and it swells and
brightens over the next billions of years. While Chi's rotation
speed is measured at zero kilometers per second, there is ample
evidence for magnetic activity, suggesting that the star is indeed
rotating, and that its pole is pointed nearly at us such that
rotation is not sensible. Is anyone there to watch? Apparently
not. Chi Her reveals no evidence for planets
(though if viewed pole-on, there would be no back-and-forth shifts to be observed as a
result of a planet's gravitational pull). More to the point, there
is also no evidence for any debris disk akin to our asteroid belt.
The low metal content also argues against planets, since most
planet-holding stars are metal-rich compared to the Sun. The age,
metal deficiency, and a relatively high velocity of 80 kilometers
per second relative to the Sun (about five times average) suggest
that the star is a visitor from another part of the Galaxy, and is just passing
through the local neighborhood.
Written by Jim Kaler 8/08/08. Return to STARS.