Shallow Sky Object - October 2021

Original article appears in GuideStar October, 2021.

The Garnet Star

by Bill Pellerin, previously published

Object: The Garnet Star (SAO 33693; Mu Cep)
Class: Star
Magnitude: 4.08 (variable 3.4 to 5.1)
R.A.: 21 h, 43 m, 30 s
Dec: 58 degrees, 46 minutes, 48 seconds
Distance: 5,260 LY
Constellation: Cepheus
Optics needed: Naked eye, binoculars or small telescope


Why this object is interesting.
This bright (but variable) orange or reddish star is easy to
spot and is high in the sky at this time of the year. It transits
at 9:13 p.m. on October 20. The star sits on the edge of
IC1396...
The star is very large at 2.4 billion miles across. This is
larger than the orbit of Saturn and 1650 times the diameter
of our Sun. The period of variability is 730 days.
It was Sir William Hershel (1738-1822)
who gave it the name the 'Garnet Star'.
One problem with this description, of
course, is that garnet rock show up
with different colors. Which one did he
mean? There are even reports of the star
appearing to be purple to some observers,
although the star is designated as a 'M'
star. This is how he described the star in
the transactions of the Royal Astronomical
Society:
A very considerable star, not marked by Flamstead, will be found
near the head of Cepheus. It is of a very fine deep garnet colour,
such as the periodical star o ceti was formerly, and a most
beautiful object, especially if we look for some time at a white
star before we turn our telescope to it, such as a cephei, which is
near at hand. (from aavso.org web site)
The Garnet Star's history is typical for a star of this size.
The star burns (fuses) hydrogen into helium early in its life,
releasing energy in the form of light in the process. When
much of the hydrogen has been used up the star expands
to its current red supergiant phase and it is now believed
to be fusing helium into carbon. Fusion continues through
several phases, and the star ends its life as a supernova.
In a supernova, the star's core collapses, then rebounds. A
massive shock wave moves through what's left of the star's
materials and we see a supernova.
The star currently varies in brightness by 1.5 magnitudes
and this variability indicates some instability in the star and
tells us quite a bit about what's going on internally with the
Shallow Sky Object of the Month
The Garnet Star
The location of the Garnet Star
from TheSky v6
star. Stars such as Mu Cep (the Bayer designation) are
called 'Small Amplitude Red Variables'.
There are other red variables that you can enjoy, one
of which has been written about in this series -- Hind's
Crimson Star. This one's in Lepus, just south of Orion,
so look for this one later in the year.
Other examples of SARV's are:
Betelgeuse - Alpha Ori. Most of us don't think of this
star as a variable, but it is.
R Lyr - north of the main part of the constellation
W Cyg

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