RHO PEG (Rho Pegasi). Less than a degree north of the border with Pisces, northwest of the constellation's famed Circlet, lies a rather special star, Rho
Pegasi (in Pegasus, Perseus's steed that he rode to rescue Andromeda). A fifth magnitude (4.90)
class A (A1) dwarf, after
allowance for some ultraviolet
light from a surface heated to 9586 Kelvin, the star shines with the
light of 92 Suns, from which (with temperature)
we derive a radius 3.5 times the solar value. It's exactly the radius
derived from the estimated angular diameter that is used as a
calibrator to determine stellar dimensions. Spinning rather quickly,
with a projected equatorial velocity of 102 kilometers per second, Rho
Peg rotates with a period of under 1.7 days, which is nothing unusual
for its class. The high speed is consistent with a lack of spectral
peculiarities that would suggest odd abundance anomalies caused by
separation of elements that would take place in the atmosphere of a
quiet, barely-rotating, star. Theory applied to luminosity and
temperature tell of a star that carries a mass 2.7 times that of the
Sun and that is perhaps three-fourths or so of the way through its
hydrogen-fusing lifetime. With a speed of 40 kilometers per second
relative to the Sun, it's moving some 2.5 times faster than average,
but that is nothing terribly unusual either. So what makes it so
special? It's not companionship, as in the case for nearby Sigma
Pegasi, which lies just a degree to the northwest, as Rho is decidedly
single. By astronomical definition, Rho Pegasi is pure white.
Standard stellar magnitudes are as seen in the visual realm of the
spectrum. Appropriate to the yellow part of the spectrum, where the
eye is most sensitive, they are formally known as "V" magnitudes. The
use of photography in the nineteenth century defined a "photographic
magnitude" as would be seen by eyes sensitive to blue light (to which
early emulsions were sensitive). In the electronic era, they became
known as "blue" or "B" magnitudes. Moving deeper into the spectrum
we can also define "ultraviolet," or "U," magnitudes. The long-used
UBV system of magnitudes has been supplemented, if not replaced, by
vast numbers of other systems that define not just ultraviolet but red
and infrared magnitudes. Rho Pegasi is one of the few stars for which
all three magnitudes, U, B, and V, are the same. That is, the magnitude
differences (B-V and U-B), which are the defined as the "colors" of
the star, are all zero. Formal star colors are often used as proxies
for spectral class and are of immense importance in calculating the
amount of obscuring dust that might
lie in the line of sight, since as stars are dimmed they are also
proportionately reddened. The trick is to find the proportion, the
search for which has the reputation of driving astronomers mad.
Written byJim Kaler 10/23/15.
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