Astronomers Spot a Intriguing ‘5-Star’ Multiple System

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An interesting multiple star discovery turned up in the ongoing hunt for exoplanetary systems.

The discovery was announced by Marcus Lohr of Open University early this month at the National Astronomy Meeting that was held at Venue Cymru in Llandudno, Wales.

The discovery involves as many as five stars in a single stellar system, orbiting in a complex configuration.

The name of the system, 1SWASP J093010.78+533859.5, is a phone number-style designation related to the SuperWASP exoplanet hunting transit survey involved with the discovery. The lengthy numerical designation denotes the system’s position in the sky in right ascension and declination in the constellation Ursa Major.

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The SuperWASP-North array of cameras at La Palma in the Canary Islands. Image credit: The SuperWASP consortium

And what a bizarre system it is. The physical parameters of the group are simply amazing, though not as unique as some media outlets have led readers to believe. What is amazing is the fact that both pairs of binaries in the quadruple group are also eclipsing along our line of sight. Only five other quadruple eclipsing binary systems of this nature are known, to include BV/BW Draconis and V994 Herculis.

The very fact that the orbits of both pairs of stars are in similar inclinations will provide key insights for researchers as to just how this system formed.

The first pair in the system are contact binaries of 0.9 and 0.3 solar masses respectively in a tight embrace revolving about each other in just under six hours. Contact binaries consist of distorted stars whose photospheres are actually touching. A famous example is the eclipsing contact binary Beta Lyrae.








An animation of the orbits of the contact binary pair Beta Lyrae captured using the CHARA interferometer. Image credit: Ming Zhao et al. ApJ 684, L95 

A closer analysis of the discovery revealed another pair of detached stars of 0.8 and 0.7 solar masses orbiting each other about 21 billion kilometres (140 AUs distant) from the first pair. You could plop the orbit of Pluto down between the two binary pairs, with room to spare.

But wait, there’s more. Astronomers use a technique known as spectroscopy to tease out the individual light spectra signatures of close binaries too distant to resolve individually. This method revealed the presence of a fifth star in orbit 2 billion kilometers (13.4 AUs, about 65% the average distance from Uranus to the Sun) around the detached pair.

“This is a truly exotic star system,” Lohr said in a Royal Society press release. “In principle, there’s no reason it couldn’t have planets in orbit around each of the pairs of stars.”

Indeed, ‘night’ would be a rare concept on any planet in a tight orbit around either binary pair. In order for darkness to occur, all five stellar components would have to appear near mutual conjunction, something that would only happen once every orbit for the hypothetical world.

Such a planet is a staple of science fiction, including Tatooine of Star Wars fame (which orbits a relatively boring binary pair), and the multiple star system of the Firefly series. Perhaps the best contender for a fictional quadruple star system is the 12 colonies of the re-imagined Battlestar Galactica series, which exist in a similar double-pair configuration.

How rare is this discovery, really? Multiple systems are more common than solitary stars such as our Sun by a ratio of about 2:1. In fact, it’s been suggested by rare Earth proponents that life arose here on Earth in part because we have a stable orbit around a relatively placid lone star. The solar system’s nearest stellar neighbor Alpha Centauri is a triple star system. The bright star Castor in the constellation of Gemini the Twins is a famous multiple heavyweight with six components in a similar configuration as this month’s discovery. Another familiar quadruple system to backyard observers is the ‘double-double’ Epsilon Lyrae, in which all four components can be split. Mizar and Alcor in the handle of the Big Dipper asterism is another triple-pair, six-star system. Another multiple, Gamma Velorum, may also possess as many as six stars. Nu Scorpii and AR Cassiopeiae are suspected septuple systems, each perhaps containing up to seven stars.

Fun fact: Gamma Velorum is also informally known as ‘Regor,’ a backwards anagram play on Apollo 1 astronaut ‘Roger’ Chaffee’s name. The crew secretly inserted their names into the Apollo star maps during training!

What is the record number of stars in one system? Hierarchy 3 systems such as Castor are contenders. A.A. Tokivinin’s Multiple Star Catalogue lists five components in a hierarchy 4 system in Ophiuchus named Gliese 644AB, with the potential for more.

How many stars are possible in one star system? Certainly, a hierarchy 4 type system could support up the eight stars, though to our knowledge, no example of such a multiple star system has yet been confirmed. Still, it’s a big universe out there, and the cosmos has lots of stars to play with.

A wide-field view of the constellation Ursa Major, with Theta Ursae Majoris selected (inset). image credit; Stellarium
A wide-field view of the constellation Ursa Major, with Theta Ursae Majoris selected (inset). Image credit; Stellarium

And you can see 1SWASP J093010.78+533859.5 for yourself. At 250 light years distant, the +9th magnitude binary is about 1.5 degrees north-northwest of the star Theta Ursa Majoris, and is an tough but not impossible split with a separation of 1.88” between the two primary pairs.

Image credit: Stellarium
Finder chart for 1SWAP J093010.78+533859.5 with a five degree Telrad foV. Image credit: Stellarium

Congrats to the team on this amazing discovery… to paraphrase Haldane, the Universe is proving to be stranger than we can imagine!

Into Oblivion: What If the Earth Had No Moon?

A shattered Luna as depicted in the summer blockbuster Oblivion. (Credit: Universal Pictures).

AVAST gentle reader: mild SPOILER(S) and graphic depictions of shattered satellites ahead!

We recently had a chance to catch Oblivion, the first summer blockbuster of the season. The flick delivers on the fast-paced Sci-Fi action as Tom Cruise saves the planet from an invasion of Tom Cruise clones.

But the movie does pose an interesting astronomical question: what if the Earth had no large moon? In the movie, aliens destroy the Earth’s moon, presumably to throw our planet into chaos. You’d think we’d already be outclassed by the very definition of a species that could accomplish such a feat, but there you go.

Would the elimination of the Moon throw our planet into immediate chaos as depicted in the film? What if we never had a large moon in the first place? And what has our nearest natural neighbor in space done for us lately, anyway?

Earth is unique among rocky or terrestrial planets in that it has a relatively large moon. The Moon ranks 5th in diameter to other solar system satellites. It is 27% the diameter of our planet, but only just a little over 1/80th in terms of mass.

Clearly, the Moon has played a role in the evolution of life on Earth, although how necessary it was isn’t entirely clear. Periodic flooding via tides would have provided an initial impetus to natural selection, driving life to colonize the land. Many creatures such as sea turtles take advantage of the Full Moon as a signal to nest and breed, although life is certainly resilient enough to find alternative methods.

The 2000 book Rare Earth by Peter Ward and Donald Brownlee cites the presence of a large moon as just one of the key ingredients necessary in the story of the evolution of life on Earth. A Moon-less Earth is also just one of the alternative astronomical scenarios cited by Arthur Upgreen in his 2005 book Many Skies.

Save our satellite: A possible target for an alien attack? (Photo by author).
Save our satellite: A possible target for an alien attack? (Photo by author).

Contrary to its depiction on film, the loss of the Moon wouldn’t throw the Earth into immediate chaos, though the long term changes could be catastrophic. For example, no study has ever conclusively linked the Moon to the effective prediction of terrestrial volcanism and earthquakes, though many have tried. (Yes, we know about the 2003 Taiwanese study, which found a VERY weak statistical signal).

All of that angular momentum in the Earth-Moon system would still have to go somewhere. Our Moon is slowly “braking” the rotation of the Earth to the tune of about 1 second roughly every 67,000 years. We also know via bouncing laser beams off of retro-reflectors left by Apollo astronauts that the Moon is receding from us by about 3.8 cm a year. The fragments of the Moon would still retain its angular momentum, even partially shattered state as depicted in the film.

The most familiar effect the Moon has on Earth is its influence on oceanic tides. With the loss of our Moon, the Sun would become the dominant factor in producing tides, albeit a much weaker one.

But the biggest role the Moon plays is in the stabilization of the Earth’s spin axis over long scale periods of time.

Milankovitch cycles play a long term role in fluctuations in climate on the Earth. This is the result of changes in the eccentricity, obliquity and precession of the Earth’s axis and orbit. For example, perihelion, or our closest point to the Sun, currently falls in January in the middle of the northern hemisphere winter in the current epoch. The tilt of the Earth’s axis is the biggest driver of the seasons, and this varies from 22.1° to 24.5° and back (this is known as the change in obliquity) over a span of 41,000 years. We’re currently at a value of 23.4° and decreasing.

But without a large moon to dampen the change in obliquity, much wider and unpredictable swings would occur. For example, the rotational axis of Mars has varied over a span of 13 to 40 degrees over the last 10 to 20 million years. This long-term stability is a prime benefit that we enjoy in having a large moon .

Perhaps some astronomers would even welcome an alien invasion fleet intent on destroying the Moon. Its light polluting influence makes most deep sky imagers pack it in and visit the family on the week surrounding the Full Moon.

But I have but two words in defense of saving our natural satellite: No eclipses.

The diamond ring effect as seen during a 2008 total solar eclipse. (Credit: NASA/Exploratorium).
The diamond ring effect as seen during a 2008 total solar eclipse. (Credit: NASA/Exploratorium).

We currently occupy an envious position in time and space where total solar and lunar eclipses can occur.  In fact, Earth is currently the only planet in our solar system from which you can see the Moon snugly fit in front of the Sun during a total lunar eclipse. It’s 1/400th the size of the Sun, which is also very close to 400 times as distant as the Moon. This situation is almost certainly a rarity in our galaxy; perhaps if alien invaders did show up, we could win ‘em over not by sending a nuclear-armed Tom Cruise after ‘em, but selling them on eclipse tours… Continue reading “Into Oblivion: What If the Earth Had No Moon?”

Nailing Down Goldilocks: What’s “Just Right” for Exo-Earths?

Cresent Earth

For Goldilocks, the porridge had to be not too hot, and not too cold … the right temperature was all she needed.

For an Earth-like planet to harbor life, or multicellular life, certainly temperature is important, but what else is important? And what makes the temperature of an exo-Earth “just right”?

Some recent studies have concluded that answering these questions can be surprisingly difficult, and that some of the answers are surprisingly curious.

Consider the tilt of an exo-Earth’s axis, its obliquity.

In the “Rare Earth” hypothesis, this is a Goldilocks criterion; unless the tilt is kept stable (by a moon like our Moon), and at a “just right” angle, the climates will swing too wildly for multicellular life to form: too many snowball Earths (the whole globe covered in snow and ice with an enhanced albedo effect), or too much risk of a runaway greenhouse.

“We find that planets with small ocean fractions or polar continents can experience very severe seasonal climatic variations,” Columbia University’s David Spiegel writes*, summing up the results of an extensive series of models investigating the effects of obliquity, land/ocean coverage, and rotation on Earth-like planets, “but that these planets also might maintain seasonally and regionally habitable conditions over a larger range of orbital radii than more Earth-like planets.” And the real surprise? “Our results provide indications that the modeled climates are somewhat less prone to dynamical snowball transitions at high obliquity.” In other words, an exo-Earth tilted nearly right over (much like Uranus) may be less likely to suffer snowball Earth events than our, Goldilocks, Earth!

Ultraviolet view of the Sun. Image credit: SOHO

Consider ultra-violet radiation.

“Ultraviolet radiation is a double-edged sword to life. If it is too strong, the terrestrial biological systems will be damaged. And if it is too weak, the synthesis of many biochemical compounds cannot go along,” says Jianpo Guo of China’s Yunnan Observatory** “For the host stars with effective temperatures lower than 4,600 K, the ultraviolet habitable zones are closer than the habitable zones. For the host stars with effective temperatures higher than 7,137 K, the ultraviolet habitable zones are farther than the habitable zones.” This result doesn’t change what we already knew about habitability zones around main sequence stars, but it effectively rules out the possibility of life on planets around post-red giant stars (assuming any could survive their homesun going red giant!)

(Credit: NASA)

Consider the effects of clouds.

Calculations of the habitability zones – the radii of the orbits of an exo-Earth, around its homesun – for main sequence stars usually assume an astronomers’ heaven – permanent clear skies (i.e. no clouds). But Earth has clouds, and clouds most definitely have an effect on average global temperatures! “The albedo effect is only weakly dependent on the incident stellar spectra because the optical properties (especially the scattering albedo) remain almost constant in the wavelength range of the maximum of the incident stellar radiation,” a German team’s recent study*** on the effects of clouds on habitability concludes (they looked at main sequence homesuns of spectral classes F, G, K, and M). This sounds like Gaia is Goldilocks’ friend; however, “The greenhouse effect of the high-level cloud on the other hand depends on the temperatures of the lower atmosphere, which in turn are an indirect consequence of the different types of central stars,” the team concludes (remember that an exo-Earth’s global temperature depends upon both the albedo and greenhouse effects). So, the take-home message? “Planets with Earth-like clouds in their atmospheres can be located closer to the central star or farther away compared to planets with clear sky atmospheres. The change in distance depends on the type of cloud. In general, low-level clouds result in a decrease of distance because of their albedo effect, while the high-level clouds lead to an increase in distance.”

“Just right” is tricky to pin down.

* lead author; Princeton University’s Kristen Manou and Colombia University’s Caleb Scharf are the co-authors (“Habitable Climates: The Influence of Obliquity”, The Astrophysical Journal, Volume 691, Issue 1, pp. 596-610 (2009); arXiv:0807.4180 is the preprint)
** lead author; Fenghui Zhang, Xianfei Zhang, and Zhanwen Han, all also at the Yunnan Observatory, are the co-authors (“Habitable zones and UV habitable zones around host stars”, Astrophysics and Space Science, Volume 325, Number 1, pp. 25-30 (2010))
*** “Clouds in the atmospheres of extrasolar planets. I. Climatic effects of multi-layered clouds for Earth-like planets and implications for habitable zones”, Kitzmann et al., accepted for publication in Astronomy & Astrophysics (2010); arXiv:1002.2927 is the preprint.