Stars may appear static, but they're on the move. Put these two speed demons on your observing list this summer. When you return in a year or two, you'll be pleasantly surprised.
Last week we visited with mover-and-shaker star Arcturus in Boötes. Despite its great speed, it requires a minimum of a couple decades for us to see the orange giant shift against the more distant background stars in a telescope.
While that might make an excellent very-long-term observing project, most of us would prefer something a smidge more immediate. Fortunately, there are two stars visible this season to accommodate our wishes. The first, Barnard's Star, is a 9.5-magnitude red dwarf in Ophiuchus, just 6 light-years from Earth. That makes it the second closest star to Earth after the Alpha Centauri system.
Discovered by American astronomer E. E. Barnard in 1916, it scoots faster across the sky than any other star in the heavens. Moving at a rate of 10.3″ per year, Barnard's Star covers a quarter degree, or half a full Moon diameter, in a human lifetime. (Note: Barnard's Star proper motion has been revised by Gaia to 10.4″).
That's plenty fast enough for anyone with a 4-inch or larger telescope to detect its northward movement in a year or two. Barnard's rises high enough for a clear view around 10-10:30 p.m. local time. To find it, first locate 66 Ophiuchi (mag. 4.8), then use a detailed map to star hop ~1° to the northwest, where you'll arrive at the dwarf. You can either photograph the star or pull out a pencil and make a sketch of its current position. Next June, when you return to the field, sketch it again.
Compare Barnard's position to the magnitude 11.9 and 11.5 stars or watch for it to form a straight line with a pair of 12th-magnitude stars to its northwest between now and 2020.
Our second featured star, 61 Cygni, is one of summer's best and brightest double stars — a gorgeous pair of orange-red dwarfs (magnitudes 5.2 and 6.0) located 11.4 light-years away in Cygnus.
61 Cygni has an equally colorful history. In 1792, Italian astronomer Giuseppe Piazzi observed the star and noticed it had moved against the starry background to the tune of more than 3′ from the position noted in observations made 40 years earlier. When word of this fleet star got out, astronomers nicknamed it "Piazzi's Flying Star." Nine years later, Piazzi would discover Ceres, the first asteroid and current focus of NASA's Dawn Mission.
By the 1830s, star positions could be measured with enough precision that astronomers began competing to be the first to measure a star's annual parallax or displacement against the distant background stars due to Earth's revolution around the Sun. Friedrich Bessel chose 61 Cygni, assuming it to be nearby due to its large proper motion of 5″ per year. Bingo! Working at his limit, he saw the star shift by a mere 0.314″, yielding a distance of 10.3 light-years, close to the current value of 11.4 light-years.
61 Cygni A, the brighter of the pair, happens to lie only 15″ from a 10.7 magnitude star this summer. Each year its distance from the star increases by an additional 5″ With high magnification you might be able to see it slide northeast in as little as year. The maps above will help you locate the star and track its motion over the next 9 years.
There are several other stars with high proper motion — Groombridge 1830 in Ursa Major (7″ per year), Kapteyn's Star in Pictor (8.7″ per year), and Lacaille 9352 in Piscis Austrinus (6.9″ a year). We'll pay them all a visit in future installments.
Seeing a star move in one's lifetime allows us to peer into the inner workings of the Milky Way galaxy. We experience firsthand the illusory nature of the "static" sky. All is in flux as stars stream about the galactic core. Today's constellations will eventually slip away to be replaced by new ones created by distant descendants.
Looking for 61 Cygni? The Sky & Telescope Pocket Sky Atlas can help you with that!