Sky & Telescope - June 2022 - 33

meant he had a shorter interval of time over which to detect
stellar movements, it also meant he used only the best data.
In this way, he detected the proper motion of Arcturus.
Galactic halo
From 1584, when Danish astronomer Tycho Brahe observed
the star, to 1738, when Cassini himself did, Arcturus had
moved south by 5 arcminutes. That's close to the correct figure,
which Verbunt and van der Sluys put at 6.4 arcminutes.
In contrast, Cassini claimed he couldn't detect the proper
motion of Sirius, which, as we know today, is smaller than
Arcturus's.
Why Arcturus?
What makes Arcturus special? Why does this beautiful
orange giant boast the largest and easiest-to-detect proper
motion of all the bright stars visible from France, making it
the " star " of the proper motion story?
The answer, we now know, is because Arcturus is unique.
Of all the stars first magnitude and brighter, Arcturus is the
only one that doesn't belong to the thin disk. As its name
implies, the thin disk population consists of stars that lie
close to the Milky Way's midplane. Most are within 1,000
light-years of this 120,000-light-year-diameter plane. The
thin disk includes young stars like Betelgeuse, older (but
still young) stars like Sirius, middle-aged stars like the Sun,
and somewhat older stars like Alpha Centauri. These suns
revolve rapidly around the Milky Way's center on fairly
circular orbits and on similar paths and therefore have fairly
small velocities relative to one another. As a result, their
velocities with respect to the Sun are small, leading to modest
proper motions.
In contrast, Arcturus is an old, somewhat metal-poor
star that doesn't belong to the thin disk population. Instead,
its orbit around the Milky Way's center is highly elliptical.
Right now, Arcturus is much farther from the galaxy's center
than usual. As a result, the star is moving slowly around
the galactic center, just as Halley's Comet is slowest when
farthest from the Sun. Arcturus therefore has a large velocity
relative to us and a large proper motion, even though it's
37 light-years from the Sun - more than four times farther
than Sirius.
Nevertheless, contrary to a misconception that has propagated
widely, Arcturus never leaves the Milky Way's thin
disk. Arcturus has only a modest velocity perpendicular
to the galactic plane: The star is moving upward at just 4
kilometers (2½ miles) per second. That's even smaller
than the Sun's vertical velocity, which is 7 km per
second, and the Sun never rises above or dives below
the galactic disk.
Velocities in 3D
Jacques Cassini's discovery of proper motion
in 1738 opened the way to determining
three-dimensional velocities of stars
through space. But proper motion alone
doesn't tell you a star's true speed across
Thick disk
Sun
Bulge
Galactic center
Thin disk
EDGE-ON VIEW The Milky Way's thin disk population includes the
Sun and most of its stellar neighbors, which revolve fast around the
galactic center on fairly circular orbits.
our line of sight. To calculate that number, which is called
the tangential velocity, astronomers also have to measure the
star's distance. In Cassini's day, no one had ever done that -
but proper motion played a key role in the first success.
Astronomers had long been trying to measure the distances
to stars via stellar parallax - the tiny shift that occurs
in a star's apparent position because we view the star from
slightly different vantage points as Earth circles the Sun. Aristotle
had cited the failure to detect stellar parallax as evidence
that Earth doesn't really orbit the Sun. In fact, the farther a
star, the smaller its parallax, and even the closest suns are so
distant that their parallaxes are tiny, making them difficult to
measure. Efforts to find stellar parallax targeted bright stars,
because these were presumed to be the nearest and therefore
should have easy-to-detect parallaxes.
But Friedrich Wilhelm Bessel at Königsberg Observatory in
Prussia took a radically different approach, betting on a dark
horse named 61 Cygni. At 5th magnitude, 61 Cygni is barely
visible to the naked eye. He chose this obscure star because
Italian astronomer Giuseppe Piazzi had found that it had a
Arcturus
Radial velocity
(toward Sun)
5 km/sec
Total space velocity
(relative to Sun)
122 km/sec
Tangential velocity
(across our line of sight)
122 km/sec
Sun
FLIGHT PATH Arcturus has a large tangential velocity
(orange) but only a small radial velocity (purple). Both
motions together give the star's total space velocity
(green) relative to us.
sk yand tele scope .o r g * JUNE 2022 33
MILKY WAY THIN DISK DIAGRAM: GREGG DINDERMAN / S&T; ARCTURUS
VELOCITY DIAGRAM: GREGG DINDERMAN / S&T, SOURCE: KEN CROSWELL
http://www.skyandtelescope.com
Sky & Telescope - June 2022

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