Astronomers Spot Unusual Five-Star System

Two binary systems and a fifth stellar wheel make for a rare, and so far unique, set of stars.

Multiple systems — when two or more stars orbit a center of gravity between them — are not unusual. In fact, almost all bright, massive stars have companions, as do about half of Sun-like stars. Some famous examples include Sirius, the brightest star in the Northern sky and its faint  white dwarf companion, and the triple Alpha Centauri system that includes the closest star to Earth, Proxima Centauri.

But systems containing more than three stars are rare. So Marcus Lohr (The Open University, UK) and colleagues were understandably excited in 2013 when they discovered a quadruple star system (1SWASP J093010.78+533859.5) in Ursa Major. It contains two eclipsing binaries, where both systems are angled edge-on to Earth so that during every orbit, one star passes in front of, and is eclipsed by, its partner.

It was a rare find, but this year the team upped the ante when they discovered a fifth star hiding in the mix. The five-star system is around 9 to 10 billion years old.

This diagram depicts the quintuple star system to scale, using Neptune’s orbit around the Sun for reference. (The binary orbits are not included since they are too small to show on this image.) The box at the top shows what a contact binary might look like. These two stars orbit close enough to touch and share their mass. The box at the bottom shows a detached binary. The orbital periods are 6 hours and 31 hours, respectively. Lohr and colleagues think the fifth star is located to the right of the separated binary, though this is not confirmed. Marcus Lohr

This diagram depicts the quintuple star system to scale, using Neptune’s orbit around the Sun for reference. (The binary orbits are not included since they are too small to show on this image.) The box at the top shows what a contact binary might look like. These two stars orbit close enough to touch and share their mass. The box at the bottom shows a detached binary. The orbital periods are 6 hours and 31 hours, respectively. Lohr and colleagues think the fifth star is located to the right of the separated binary, though this is not confirmed.
Marcus Lohr

Commenting in a press release, Lohr related the discovery to what for now remains science fiction: “This is a truly exotic star system . . . Any inhabitants [of this system’s exoplanets] would have a sky that would put the makers of Star Wars to shame — there could sometimes be no fewer than five Suns of different brightnesses lighting up the landscape. Days would have dramatically varying light levels as the different stars were eclipsed. They would though miss out on night for a large part of their ‘year’, only experiencing darkness (and a night sky) when the stars were on the same side of their world.”

(Star Wars isn’t the only one to take on multiple stars: Isaac Asimov wrote a pretty bleak tale on how alien people living under six suns might react to rare total darkness in his short story, “Nightfall.”)

The two binaries’ similar inclination angles hint that the quintuplets formed when a single protostellar disk fell into pieces. The authors don’t speculate further, but write, “This bright, close, and highly unusual star system would doubtless repay further investigation.”

Reference:

Marcus Lohr et al. “The doubly eclipsing quintuple low-mass star system 1SWASP J093010.78+533859.5.” Astronomy & Astrophysics. June 2015.

CATEGORIES
News, Stellar Science
Anne McGovern

About Anne McGovern

Anne is the 2015 summer Editorial Intern at Sky & Telescope and a graduate student in Science and Medical Writing at Johns Hopkins University. She is sustained by science and literature, and loves to travel the world.

11 thoughts on “Astronomers Spot Unusual Five-Star System

  1. Roger

    Of course, Mizar and Castor have long been known to be 6-star systems. Both systems are within 100 light years of Earth, which can give us an idea of how common such 6-star systems are. (I’ll leave the math to others.)

    1. MaiaMaia

      “Two binary systems and a fifth stellar wheel make for a rare, and so far unique, set of stars.”
      If 6-star systems are common, why is the article subtitled like this?

      1. Monica YoungMonica Young

        The subtitle refers to the fact that this is the first doubly eclipsing binary system with a fifth star, which is so far unique. Five-star (and six-star) systems aren’t common to my knowledge, though there are certainly examples of both.

          1. MaiaMaia

            …I did not THINK, they were common, I meant to write. ( No editing function on this site!)

  2. Ian-Cooper

    I’m surprised no one has mentioned that the TV show Firefly is set in a system with five stars.

  3. michael-spencer

    Just consider the scale drawing. Look at the contact binary: the two stars together are about the diameter of the Sun, but because of their distorted shape their apparent surface area is much less. Their perceived brightness, then, at about the radius of Venus’ orbit, is not too different from the perceived brightness of our Sun from here.

    So let’s assume that we’re on an Earth-type planet in orbit around the contact binary, say 50 million miles out. How bright will the other three stars appear to be? Well they are at best considerably further from us than Neptune is from the Sun, and they are all a bit less than the Sun in diameter. So, not unbearably bright, then. Bright enough to cast shadows? Probably. Bright enough for sunbathing? No chance. Bright enough to make perpetual daylight? You’re kidding.

    1. Monica YoungMonica Young

      Hi Michael,
      You inspired me to do the math :). So let’s see how bright the detached binary stars are as seen from the contact binary. The brighter of the detached binary stars has a radius of 0.84 solar masses and a corresponding effective temperature of 5185 K (these numbers are from the paper). That gives it a visual absolute magnitude of 6.1, according to standard spectral type definition. According to the diagram, the detached binary is orbiting the common binary at a distance of 2.5 Neptune orbits, or 0.00073 pc. Then use the standard equation, m = M + 5 log10 ( d / 10 pc), where m is apparent magnitude, M is absolute magnitude, and d is distance in pc. Fill in the numbers, and you have an apparent magnitude of -14.5. For comparison, apparent visual magnitude of our Sun is -26 and same for the Moon is -6. So if you’re an alien on a world orbiting the contact system, you might not get a good tan from the detached binary stars, but you’d certainly see their light as being significant in your sky.

      (If I’ve done the math/measuring wrong, let me know! It’s the end of a long Monday…)

  4. Antonio Mario

    Anne/Monica,

    Thanks for the article and the additional comments. Just to nitpick a little (TGIF, after all), Sirius has actually a *Southern* declination, about -17 degrees. It is, in fact, the brightest star in the whole sky.

    Thank you.

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