Sky & Telescope - May 2024 - 38

Stellar History
Wolf 359's large proper motion was due to proximity or to
high speed, astronomers needed to determine its distance.
The Distance to Wolf 359
It wasn't until the 1920s, however, that astronomers were
able to measure Wolf 359's parallax, the tiny annual shift that
occurs in a star's apparent position when we view it from different
vantage points as our planet circles the Sun. The larger
the parallax is, the closer the star (see S&T: Dec. 2023, p. 74).
To detect the dim star's parallax, Adriaan van Maanen at
Mount Wilson Observatory in California used what was then
the world's largest telescope, the 100-inch Hooker Telescope.
He recorded 18 photographic plates between April 1922 and
January 1927. The verdict was clear: The star had such a large
parallax that " With the exception of the system of α Centauri
and of Barnard's star, it is therefore the nearest star known at
present, " van Maanen and his colleagues wrote. The parallax
measurements indicated Wolf 359 was just 8.1 light-years
from Earth. That number is close to the modern value of 7.86
light-years obtained with the Gaia spacecraft.
Nevertheless, despite the star's proximity, it appears very
faint. If we viewed the Sun from the same distance, it would
look brighter than Polaris, the North Star. Thus, Wolf 359
must emit very little light. To understand just how little, van
Maanen's colleague, Julius Brown, measured the star's precise
apparent magnitude. Coupled with its distance, this revealed
Wolf 359's absolute magnitude, the apparent magnitude the
star would have if we viewed it from a standard distance of 10
parsecs (32.6 light-years). The resulting number was close to
the modern value of the star's absolute magnitude, 16.6, compared
with 4.83 for the Sun.
No stars that feeble were then known. The previous
record-holder was Proxima Centauri, whose absolute brightness
is about a magnitude brighter, so " it seems that Wolf 359
is by far the faintest star, intrinsically, known at present, "
van Maanen and his colleagues wrote. Indeed, they entitled
their article " A Star of Extremely Low Luminosity. "
The article also reports that in April 1927, Mount
Wilson astronomer Milton Humason used the 100-inch
telescope to obtain the dim star's spectral type: M6. Like
Proxima Centauri and Barnard's Star, Wolf 359 was therefore
a red dwarf - the most common type of star, outnumbering
all others put together. Yet not a single one is visible
to the naked eye.
We now know that red dwarfs generate energy as the Sun
does. Their cores use nuclear reactions to convert hydrogen,
the lightest element, into helium, the second-lightest element.
The difference between bright, yellow G-type mainsequence
stars like the Sun and a red dwarf is simple: mass.
Red dwarfs were simply born less massive. As a result, they
SOUTHERN SUNS The A and B components of the triple Alpha (α) Centauri system blend together to appear as one brilliant star, at left. Unlike
Wolf 359, they each emit roughly as much light as the Sun. Alpha Centauri A and B orbit each other every 80 years. Nearly 100 times farther away
than Alpha Centauri is the unrelated star Beta (β) Centauri, at right.
38 MAY 2024 * SKY & TELESCOPE
GARY SERONIK

Sky & Telescope - May 2024

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