Double Sunsets May be Common, But Twin-Star Setups Still Mysterious
Credit: Graduate University for Advanced Studie

The Earth may orbit around a single star, but most stars like our sun are binaries ? two stars orbiting each other as a pair. In fact there are many three-star triple systems, even going up perhaps as high as seven-star ? or septuplet ? systems.

Although astronomers once thought these systems might not easily support planets, worlds with multiple sunsets might actually prove common.

And now, powerful telescopes are beginning to resolve these systems to reveal how they work.

The alien suns

Our sun is a so-called class G star, but it is oddly solitary when compared to other stars in the same class.

"One-third are single, while two-thirds are multiples," explained astronomer Charles Lada at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Still, while most systems with stars like our sun are multiple, most systems in the universe are not, Lada explained. Class G stars only make up roughly 7 percent of all stars we see. When it comes to systems in general, "about 70 percent in our galaxy are single, while the rest are multiple," he said.

The overwhelming majority of stars in our galaxy are class M stars, ones roughly one-tenth to one-half our sun's mass. "If there are 400 billion stars in the Milky Way galaxy, then about 325 billion of those are M stars," Lada said. "They have a very low binary or multiple frequency ? maybe only about 15 to 25 percent of such systems."

For many years, scientists thought most stars were multiples. "It's only in the last 15 years or so that we've been able to identify and study these low-mass stars, which are very faint," Lada said. On the other hand, when it comes to massive stars, "say four to 10 times the mass of our sun, they're almost always in multiples," Lada said.

Of course, while most systems only have one star, since binary and multiple systems do after all have more than one star, "you can say that roughly half of all stars are in binary or multiple systems," Lada clarified.

How binary stars are born

These reason high-mass stars are often in multiples while low-mass ones are not might be due to differences in how they form.

"Low-mass stars tend to form in very dense, cold, dark clouds," Lada said. "They are born when gravity wins out, causing these 'cores' to contract further, usually forming single systems. On the other hand, massive cores tend to be turbulent, with gas within generally moving at higher velocities. Those are very susceptible to fragmentation, and so can form multiple stars or binaries."

Curiously, the nearby Taurus-Auriga star-forming region "does have a very high fraction of binary stars, even for G stars, and maybe even extending to M stars," Lada said. "Because of this, there's the thought that maybe most stars form as multiples, but these systems are then broken apart." However, he noted that all calculations attempting to simulate this happening have not born fruit so far.

Where the suns set twice (or more)

Although worlds with fantastic multiple sunsets might seem the province of sci-fi flicks like "Star Wars" and "Avatar," alien worlds orbiting binary star systems are science fact. In fact, "Avatar" is set on an alien moon around a fictional planet orbiting the real binary star system Alpha Centauri.

For instance, scientists have detected a planet estimated at 1.76 times as massive as Jupiter orbiting the primary star of the Gamma Cephei binary system roughly 45 light years away. When the stars in binary systems are close together, planets have been spotted in orbits that take them around both ? such is the case with PSR B1620-26 b, which orbits a pulsar and white dwarf located roughly 12,400 light-years away and is the oldest known alien planet at roughly 12.7 billion years old.

Many unanswered questions remain about how planets are born and develop in binary and multiple systems. Now scientists are imaging the disks of gas and dust around these systems that might eventually give rise to planets.

Scientists in Japan captured the first direct images of a young binary system using the Subaru Telescope. The system, SR24, is located in the constellation Ophiuchus, 520 light years away.

In a binary system, two stars orbit around a common center of mass. The brighter or more massive star is referred to as the primary star and the fainter or less massive one is called the secondary star. The circumprimary disk encircles the primary, while the circumsecondary disk rings the secondary, and a third circumbinary disk may surround both.

The researchers in Japan were first to capture high-resolution, near-infrared images of both circumprimary and circumsecondary disks around a young stellar object. They also saw bridges extending from one disk to another, which supercomputer simulations suggest emerged from the collision of gas rotating around the stars. A long spiral arm of gas and dust was also seen extending out from the circumprimary disk, likely to the circumbinary one, which might be feeding the circumprimary disk matter and fuel its growth.

"In such widely spaced binaries, you could have planets around the two," Lada said.