Sky & Telescope - December 2022 - 33

NASA / ESA / G. BACON (STSCI)
DOG AND PUP SHOW This illustration shows the star Sirius A (the dominant star at left) and its white-dwarf companion, Sirius B (pictured at center
right) - sometimes called the " Pup. " Although some sources list Sirius B as the first white dwarf discovered, that distinction actually goes to 40
Eridani B. In backyard telescopes, Sirius B is a challenging target, while 40 Eridani B is relatively easy to spot.
Luyten, who had earned his doctorate under Hertzsprung the
year before, coined the term " white dwarf. " Although Luyten
went on to discover many additional white dwarfs, their
numbers grew so slowly that by 1940 The Sky (a forerunner of
this magazine) reported that only 25 were known.
The Modern Era
Today astronomers have cataloged many thousands of white
dwarfs. As it turns out, the stars come in a range of colors,
from blue to red. That's because white dwarfs cool and fade
with age - blue ones are the hottest and youngest, whereas
red ones are the coolest and oldest. Both Sirius B and 40
Eridani B are as hot as blue main-sequence stars of spectral
type B. Sirius B's surface temperature is 25,000 kelvin and 40
Eridani B is also hot, with a surface at 17,000K - nearly three
times the Sun's temperature of 5800K.
In 2017 Brian D. Mason (U.S. Naval Observatory) and
his colleagues observed 40 Eridani B and C with the same
26-inch refractor Asaph Hall had earlier employed. This work
revealed that the orbital period of the B and C components is
about 230 years, a bit shorter than indicated by older observations.
The shorter period means that the two stars must be
more massive than previously thought to have enough gravity
to dance around each other faster than originally believed.
Furthermore, the newly calculated masses resolved a longstanding
puzzle regarding 40 Eridani B.
When a star possesses extreme surface gravity, photons
lose energy as they're emitted from the star. This stretches
the photons' wavelengths toward the red part of the spectrum,
a process known as gravitational redshift.
Gravitational redshift helps astronomers determine a
white dwarf's mass: The more massive a white dwarf, the
stronger its surface gravity (both because the greater mass
exerts more force and because gravity squeezes the star,
reducing its diameter). The gravitational redshift of 40 Eridani
B is about 26 km per second - far greater than the Sun's
gravitational redshift of only 0.6 km per second. The old
orbital period for the pair of stars implied a mass too low and
a size too large to produce so great a redshift for 40 Eridani B.
However, the new orbital measurements yield a white dwarf
mass that's about 57% that of the Sun, which agrees well
with the 53% figure derived from the gravitational redshift.
This new mass estimate also means that 40 Eridani B is
quite average for its class, because the typical white dwarf
is about 60% as massive as the Sun. In contrast, Sirius B is
unusually heavy - estimates place it at one solar mass.
Compared with Sirius B, 40 Eridani B is much easier to
see. Although it's a magnitude fainter, the Eridanus white
dwarf doesn't have to compete with the brilliance of a star
like Sirius. In his monumental Celestial Handbook, Robert
Burnham, Jr., notes that 40 Eridani B is " the only white
dwarf star which can honestly be called an easy object for
the small telescope. "
A century ago this easy-to-see star made astronomical history
as the first white dwarf ever found - a star that foretells
the destiny of our own Sun.
¢ KEN CROSWELL has long been interested in the nearest
stars. He earned his PhD at Harvard University and is the author
of The Alchemy of the Heavens and Magnificent Universe.
sk yand tele scope .o r g * DECEMBER 2022 33

https://skyandtelescope.org/

Sky & Telescope - December 2022

Table of Contents for the Digital Edition of Sky & Telescope - December 2022