First Precise Distance to Shaula

By Ken Croswell

June 8, 2006

Image of Scorpius by Bill and Sally Fletcher. Used by permission.

Shaula, the second brightest star in Scorpius, is only half as far from Earth as the Hipparcos satellite indicated, say astronomers in Australia, America, and Europe. New interferometric observations show that Shaula's Hipparcos distance is off by several hundred light-years.

Shaula, or Lambda Scorpii, has an apparent magnitude of 1.62, making it the twenty-fifth brightest star in the night sky. It is triple, consisting of two blue B-type stars that orbit each other every 1,053 days, plus a much fainter T Tauri star--a precursor of a Sunlike star--that circles the brighter B-type star every 6 days. In the 1990s, the Hipparcos satellite measured a parallax that placed Shaula about 700 light-years from Earth.

But that distance is wrong, says a team led by William Tango and John Davis of the University of Sydney in Australia. They observed Shaula with the Sydney University Stellar Interferometer. This instrument combines the light of small telescopes in northern New South Wales, Australia, to yield sharp views of stars.

Using the interferometer, Tango and his colleagues saw the two main stars of Shaula separately. The brighter, Shaula A, is spectral type B1.5, and the fainter, Shaula B, is spectral type B2. These spectral types correspond to temperatures of 25,000 and 21,000 Kelvin--much hotter than the Sun's temperature of 5,770 K.

Tango and his colleagues find that Shaula A shines 0.66 ± 0.10 magnitudes brighter than Shaula B, implying that the former has 1.32 times more mass. The orbital period of the two stars is known and reveals their total mass: the more mass they have, the faster they revolve around each other, and the shorter is their period. Knowing both the total mass and the mass ratio, the astronomers deduce that Shaula A has about 10 times the mass of the Sun, while Shaula B has about 8 solar masses. (See data table below.)

Furthermore, the period and mass indicate that the two stars are on average 5.5 AU apart--slightly greater than the distance from the Sun to Jupiter. (One AU, or astronomical unit, is the mean Sun-Earth distance, about 93 million miles.) The angular separation the astronomers observe between Shaula A and B is 49.3 milliarcseconds. (One milliarcsecond is 1/3600000 degree.) Comparing the angular separation, in milliarcseconds, with the actual separation, in AU, yields Shaula's distance.

Shaula turns out to be only 365 ± 16 light-years from Earth. In contrast, the Hipparcos distance for the star was 703 ± 137 light-years. The new distance, accurate to 4 percent, is the best ever measured for the star.

"These people have done a really technically impressive piece of work," says B. Cameron Reed of Alma College in Michigan, who maintains a database of about 18,800 Galactic OB stars. "The apparent separation between the two B stars is equivalent to the angular size of a dime held at a distance of forty-seven miles." Reed adds that obtaining good mass estimates for O- and B-type stars is notoriously difficult. "These folks have done a tremendous thing," he says. "They've tightened up the mass estimates for Shaula to an error of about 10 percent."

Reed also notes that the new distance implies an absolute magnitude for Shaula A and B that's right in line with what's expected for stars of their spectral type. In contrast, the old Hipparcos distance implied that the stars were too luminous. Says Reed, "It makes me wonder how many other systems like this are out there, where a discrepancy is lurking because of an incorrect distance."

Why did Hipparcos fail to measure Shaula's distance correctly? Tango and his colleagues suspect the orbital motion of Shaula A and B contaminated the Hipparcos parallax.

The same problem caused Hipparcos to measure the wrong distance for Beta Centauri, the eleventh brightest star in the night. New work indicates that Beta Centauri is about as far from Earth as Shaula is, but Hipparcos also gave that star a much greater distance.

Shaula's home, Scorpius the Scorpion, is a prominent constellation. From the northern hemisphere, Scorpius appears in the southern sky on summer evenings. Its brightest star, Antares, is a red supergiant that marks the Scorpion's heart. Shaula is near the end of the Scorpion's tail; its name means "the sting."

According to legend, Scorpius stung and killed Orion the Hunter after he bragged about killing animals. So when Scorpius rises, Orion sets. Orion is the only constellation besides Scorpius that has a bright red supergiant. The two stars--Antares and Betelgeuse--lie in opposite directions from the Sun, just as New York and San Francisco lie in opposite directions from Kansas City.

Tango and his colleagues will publish their work in Monthly Notices of the Royal Astronomical Society.

Ken Croswell is an astronomer and the author of Magnificent Universe and Ten Worlds. His book for beginning stargazers, See the Stars: Your First Guide to the Night Sky, features Scorpius for the month of July.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.

"On the basis of its striking design and photographs, this handsome, large-format volume is well worthy of praise. And astronomer Croswell's concise yet conversational, information-packed text wins it sky-high accolades in the narrative sphere as well."--Publishers Weekly, starred review. See all reviews of Ten Worlds here.

"Finally! An astronomy guide that the reader can actually follow without being a rocket scientist!" See all reviews of See the Stars here.

Shaula

	A	a	B
Constellation	Scorpius	Scorpius	Scorpius
Distance from Earth (light-years)	365 ± 16	365 ± 16	365 ± 16
Spectral Type/Luminosity Class	B1.5 IV	T Tauri star	B2 V
Apparent Magnitude	2.09 ± 0.10	----	2.75 ± 0.10
Absolute Magnitude	-3.16 ± 0.10	----	-2.50 ± 0.10
Mass (Sun = 1)	10.4 ± 1.3	1.8 ± 0.2	8.1 ± 1.0
Visible Light Output (Sun = 1)	1,600 ± 160	-----	900 ± 90
Temperature	25,000 ± 1,000 K	-----	21,000 ± 1,000 K

Shaula A and a orbit each other every 5.9525 days; Shaula B orbits Shaula A and a every 1,053 days.

KenCroswell.com

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