Sky & Telescope - May 2024 - 40

Stellar History
Orbit
of Sun
Center of the
Milky Way
all. Instead, they thought it might be what was then known
as a black dwarf and is now called a brown dwarf - a star
born with so little mass that it can't sustain a hydrogento-helium
nuclear reaction. If so, then VB 10 was glowing
simply because gravity squeezes its gas and makes it hot
enough to shine. In truth, as we now know by measuring
the masses of other dim suns, the star is almost certainly a
red dwarf. Astronomers didn't spot the first genuine brown
dwarfs until the 1990s.
After supplanting Wolf 359, Van Biesbroeck's Star reigned
Orbit of
Wolf 359
30
15
(1,000 light-years)
PASSING STARS Although the Sun and Wolf 359 are currently close
neighbors, the two stars usually inhabit somewhat different zones of the
galaxy. As shown in these orbits computed by astronomer Adrian PriceWhelan,
the Sun (yellow path) normally resides farther from the galactic
center than Wolf 359 does (red path). Right now, though, Wolf 359 is
about 2 light-years farther from the galactic center than the Sun is.
worked at Yerkes Observatory - began searching for dim stars
" in order to extend our knowledge toward the lowest luminosity
stars. " He used the new 82-inch telescope at McDonald
Observatory in Texas to take photographic plates of faint
nearby stars in a quest to find even fainter stars that are
orbiting them (S&T: June 1974, p. 370). Wolf 1055 was one of
Van Biesbroeck's targets.
Comparing photographic plates from November 1940 and
October 1943 " revealed the presence of a very faint companion
to that star, sharing its motion, " he reported. That meant
the two stars were traveling through space together. However,
by Van Biesbroeck's estimate, they were at least 434 astronomical
units apart - 11 times greater than the mean distance
between the Sun and Pluto - so they must take many
thousands of years to revolve around each other.
More importantly, Van Biesbroeck noted that his star
was a new stellar underdog: " three magnitudes fainter than
the lower limit known up to now, which was held by Wolf
359 . . . " Modern measurements show that Van Biesbroeck's
Star (also known as VB 10) is brighter than its discoverer
had thought. Its absolute magnitude is 18.6, two magnitudes
dimmer than Wolf 359. Van Biesbroeck's Star takes 37 years
to emit as much visible light as the Sun does in an hour.
If put in place of the Sun, the star would look only slightly
brighter than the full Moon. Noon would therefore resemble
a moonlit night, Earth's oceans would freeze, and all terrestrial
life would perish.
Van Biesbroeck's Star emits so little light that during the
1960s some astronomers suspected it was not a red dwarf at
40 MAY 2024 * SKY & TELESCOPE
Wolf 359's Long Future
Wolf 359 is only a temporary neighbor of our Sun. Although
both stars belong to the same thin-disk population, they follow
different orbits within the Milky Way.
The Sun is currently moving toward the galactic center
and is presently closer to the heart of the Milky Way than it
usually is. In the same way that a comet accelerates as it nears
the Sun, our star is moving faster than average, at around 250
kilometers per second (560,000 miles per hour) around the
galactic center. In contrast, Wolf 359 is moving away from
the galactic center and is farther from it than usual, so the
star currently revolves more slowly than the Sun. Furthermore,
the two stars have opposite vertical velocities. The Sun
is moving upward through the Milky Way's disk at 7 km/s, so
each year we climb higher by a distance equal to that between
the Sun and Mars. In contrast, Wolf 359 is moving downward
through the galactic disk at 7 km/s.
As a result of their different trajectories, the Sun and
Wolf 359 are like ships passing in the night. Look again in
a million years and Wolf 359 will be nearly 200 light-years
distant. In a billion years the two stars may be on opposite
sides of the galaxy.
Wolf 359 is certainly destined for a very long life.
Although born with much less fuel than the Sun, Wolf 359
consumes it so slowly that the little star will still be glowing
a trillion years hence. And who knows, in the far future
Wolf 359 may swing by our Sun again, which by then will
be a fading white dwarf star so dim that even this feeble
red dwarf will outshine it, a sign that the meek have finally
inherited the Milky Way.
¢ KEN CROSWELL, author of The Alchemy of the Heavens
and The Lives of Stars, is especially fond of the nearest stars
and galaxies. He wrote the feature stories about the Milky
Way and Andromeda galaxies in the August and December
2023 issues of this magazine.
15
30
as the low-luminosity champ for four decades. In the early
1980s, however, astronomers discovered two stars that were
even less luminous: RG 0050-2722 in Sculptor, reported in
1981, and LHS 2924 in Boötes, whose distance astronomers
measured in 1983. Both stars have absolute magnitudes
around 19.5. That's about 15 magnitudes fainter than the
Sun's absolute magnitude, which equates to a visual light
output barely one-millionth that of the Sun.
ADRIAN PRICE-WHELAN (HTTP://GALA.ADRIAN.PW AND FLATIRON INSTITUTE)
HTTP://GALA.ADRIAN.PW
Sky & Telescope - May 2024

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