Eye Astronomy #10: The Spring Triangle
by Dale E. Lehman
No matter how much you know about astronomy, there’s always more to learn. Sometimes you learn things that were literally right in front of your eyes for decades. This is the story of one of my recent discoveries.
There’s a well-known asterism called the Summer Triangle. Some asterisms are parts of constellations, like the Big Dipper in Ursa Major. Others cross constellations. The Summer Triangle is one of them, consisting of three stars in three different constellations: Vega in Lyra (the lyre), Altair in Aquila (the eagle), and Deneb in Cygnus (the swan).
The Summer Triangle is one of the first asterisms most amateur astronomers learn to spot. It’s bright and obvious, which makes it a great tool for finding nearby stars and constellations. But I didn’t realize until just this month that there’s also a Spring Triangle, and astoundingly, it’s very much like its summer counterpart. It should have been obvious!
The stars of the Spring Triangle are Arcturus in Boötes (the herdsman), Spica in Virgo (the virgin), and Regulus in Leo (the lion). The similarities between the two triangles are striking. For one thing, both are roughly isosceles triangles, although the Spring Triangle is bigger. For another, the stars are about the same brightnesses. Here they are, paring the two brightest, the two in the middle, and the two least bright:
· Vega is magnitude 0.00; Arcturus is magnitude -0.04
· Altair is magnitude 0.77; Spica is magnitude 1.04
· Deneb is magnitude 1.25; Regulus is magnitude 1.35
These small differences in magnitude aren’t easily noticed. Even experienced observers would be hard-pressed to distinguish between Altair and Spica, the pair with the biggest brightness difference at just a quarter magnitude.
I’ll grant you, there is one striking difference between the Summer and Spring Triangles. Most of these stars appear white to the eye, all but Arcturus, which is distinctly orange or orange-yellow. It lends some color to the vernal asterism, whereas the summer version is pure white.
Here are a few other interesting similarities (and differences).
Vega and Arcturus aren’t just nearly identical in brightness, they’re the two brightest stars north of the celestial equator. Yes, Sirius, Canopus, and Rigil Kentaurus (Alpha Centuari), are all brighter, but they’re southern stars. If you stood at the north pole, none of them would ever rise. Vega’s magnitude is defined as zero. All other stars’ brightnesses are based on it. Arcturus is just a teeny bit brighter, which is why its magnitude is a negative number. Why are Vega and Arcturus so bright? In part because they’re so close to us: 25 and 37 light years away, respectively.
Most of these triangle stars are, in fact, close. Altair is just 17 light years away, Regulus is 79. Spica is a bit farther out at 250, but that’s still close in astronomical terms. The outlier is Deneb. Unusually for stars on the “brightest” list, Deneb so far away that we don’t even know its distance with certainty. Estimates range from 1,400 to 2,600 light years. If it’s at the high end of that range, it would be the most distant star with an magnitude brighter than 2.5. To be so bright when it’s so far away, Deneb must be awesome, a blue supergiant about as big as Earth’s orbit with a luminosity between 55,000 and 196,000 times that of our sun!
Each of the triangles contains a fast-spinning star. Altair in the Summer Triangle is a bit bigger than our sun, but whereas the sun rotates once on its axis every 24.5 days, Altair spins once every 7.8 hours. That means its rotation speed is 178 miles per second! Rapid rotation flattens the star at its poles and bulges it out at the equator, yielding a shape called an oblate spheroid. Regulus in the Spring Triangle is also a rapid rotator, spinning once in 15.9 hours. Although its rotation period is longer, Regulus has twice the mass of Altair and a larger radius, so its rotational speed is higher, 198 miles per second. It, too, is oblate. Those are mind-boggling speeds. In fact, neither star could spin much faster without being torn apart!
One the biggest differences between the triangles is invisible to the naked eye. All Summer Triangle stars are single stars, whereas two of the Spring Triangle stars are multiples. Spica is a double star. Its companions are so close that they orbit each other in four days and can’t be split visually, not even with the largest telescopes. (Astronomers use spectroscopy to detect the components of such stars.) Regulus is a quadruple, two stars of which are visible in binoculars. A larger telescope is required to split the two fainter components.
Next time you are out on a clear spring night, have a look at the Spring Triangle and think about these amazing facts. In a couple of months, you’ll be able to see the Summer Triangle, too. Compare them. See if you can spot any differences I missed.
I’ll be doing the same.
