Binary stars are more common than we thought

Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A. Weiss et al. (Submillimetre); NASA/CXC/CfA/R. Kraft et al. (X-ray)1 / 16

High-mass stars are rarely solitary. This is what Bochum's astronomers found out at the Ruhr-Universität's (RUB's) observatory in Chile.

For several years, they observed 800 celestial objects that are up to one hundred times heavier than our sun.

More than 90 per cent have turned out to be multiple systems. These data support the theory that heavy stars are already formed as twins.

University observatory as key to success
Even with the world's largest telescopes, binary stars cannot generally be distinguished as two discrete points.

To prove their existence nevertheless, the team headed by Prof Dr Rolf Chini from the RUB Institute of Astronomy used a trick.

They watched the celestial bodies over a period of many weeks and months and detected that their spectra and their brightness oscillated.

Regular brightness variations occur if two or more stars pass each other again and again.

These long-term measurements were possible only because the Ruhr-Universität operates its own observatory in the best place for astronomical observations worldwide: the Atacama Desert in Chile.

Twins that weigh the same
The statistical analysis of the data revealed that stars in multiple systems usually have a partner with the same mass.

According to Rolf Chini, this is no coincidence: "Why should a star of 50 solar masses capture, of all stars, a partner of likewise 50 solar masses in its surroundings?"

"It would be much easier to attract a star of only one solar mass. Surely, the stars' formation process is what provides the explanation."

The celestial objects originate from gas and dust clouds which then become dense. In the final stage, the cloud apparently splits into two parts of similar size.

Rolf Chini, today Head of the university observatory, spent the first money he's ever earned on a telescope. 

Credit: RUBIN, photo: Nelle

You can find the complete article about the research conducted by Prof Dr Rolf Chini's team at the RUB Institute of Astronomy in the online magazine RUBIN.

LAMOST: Nearest bright 'hypervelocity star' found: Speeding at 1 million mph

This is an astrophysicist-artist's conception of a hypervelocity star speeding away from the visible part of a spiral galaxy like our Milky Way and into the invisible halo of mysterious "dark matter" that surrounds the galaxy's visible portions. 

University of Utah researcher Zheng Zheng and colleagues in the US and China discovered the closest bright hypervelocity star yet found. 

Credit: Ben Bromley, University of Utah.

A University of Utah-led team discovered a "hypervelocity star" that is the closest, second-brightest and among the largest of 20 found so far.

Speeding at more than 1 million mph, the star may provide clues about the supermassive black hole at the center of our Milky Way and the halo of mysterious "dark matter" surrounding the galaxy, astronomers say.

Zheng Zheng

"The hypervelocity star tells us a lot about our galaxy, especially its center and the dark matter halo," says Zheng Zheng, an assistant professor of physics and astronomy and lead author of the study published recently in Astrophysical Journal Letters by a team of U.S. and Chinese astronomers.

"We can't see the dark matter halo, but its gravity acts on the star," Zheng says.

"We gain insight from the star's trajectory and velocity, which are affected by gravity from different parts of our galaxy."

In the past decade, astronomers have found about 20 of these odd stars.

Hypervelocity star appear to be remaining pairs of binary stars that once orbited each other and got too close to the supermassive black hole at the galaxy's center.

Intense gravity from the black hole – which has the mass of 4 million stars like our sun – captures one star so it orbits the hole closely, and slingshots the other on a trajectory headed beyond the galaxy.

Zheng and his colleagues discovered the new hypervelocity star while conducting other research into stars with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, (LAMOST), also called the Guo Shou Jing Telescope is located at the Xinglong Observing Station of the National Astronomical Observatories of China, about 110 miles northeast of Beijing.

LAMOST boasts a 13.1-foot-wide aperture and houses 4,000 optical fibers, which capture "spectra" or light-wavelength readings from as many as 4,000 stars at once. A star's spectrum reveals information about its velocity, temperature, luminosity and size.

LAMOST's main purpose is to study the distribution of stars in the Milky Way, and thus the galaxy's structure.

The new hypervelocity star, named LAMOST-HVS1, stood out because its speed is almost three times the usual star's 500,000-mph pace through space: 1.4 million mph relative to our solar system.

Its speed is about 1.1 million mph relative to the speed of the center of the Milky Way.

Despite being the closest hypervelocity star, it nonetheless is 249 quadrillion miles from Earth. (In U.S. usage, a quadrillion is 1,000,000,000,000,000 miles or 10 to the 15th power, or 1 million billion).

"If you're looking at a herd of cows, and one starts going 60 mph, that's telling you something important," says Ben Bromley, a University of Utah physics and astronomy professor who was not involved with Zheng's study.

"You may not know at first what that is but for hypervelocity stars, one of their mysteries is where they come from – and the massive black hole in our galaxy is implicated."

Background perspective

Our solar system is roughly 26,000 light years or 153 quadrillion miles from the centre of the galaxy, more than halfway out from the centre of the visible disk.

By comparison, the new hypervelocity star is about 62,000 light years or 364 quadrillion miles from the galactic centre, beyond as well as above the galaxy's visible disk.

It is about 42,400 light years from Earth, or about 249 quadrillion miles away.

As far as that is, the star has a magnitude of about 13, or 630 times fainter than stars that barely can be seen with the naked eye, it nevertheless "is the nearest, second-brightest, and one of the three most massive hypervelocity stars discovered so far," Zheng says.

It is nine times more massive than our sun, which makes it very similar to another hypervelocity star known as HE 0437-5439, discovered in 2005, and both are smaller than HD 271791, which was discovered in 2008 and is 11 times more massive than the sun.

As seen from Earth, only HD 271791 is brighter than LAMOST-HVS1, Zheng says.

The newly discovered hypervelocity star also outshines our own sun: It is four times hotter and about 3,400 times brighter (if viewed from the same distance), but compared with our 4.6-billion-year-old sun, the newly discovered LAMOST–HVS1 is a youngster born only 32 million years ago, based on its speed and position, Zheng says.

Runaway binary stars

A Hubble image of the binary stars Sirius A and B. Sirius B, the very faint companion, is a white dwarf, an evolved star that has burned its nuclear fuel.

There is one known binary star pair consisting of two white dwarfs that is also a "runaway" star, moving very rapidly through the galaxy. 

A new study concludes that this runaway star was probably ejected from a dense stellar cluster. 

Credit: NASA/Hubble

CfA astronomers made a remarkable and fortuitous discovery in 2005: an extremely fast moving star, clocked going over three million kilometers an hour.

It appears to have been ejected from the vicinity of the galactic center's supermassive black hole around 80 million years ago by powerful gravitational effects as it swung past the black hole.

Racing outward from the galaxy, the star lends added credibility to the picture of a massive black hole at the galactic center, and to calculations of how black holes might interact with their stellar environments.

Other hypervelocity stars and less fast-moving runaway stars have also been found. Most of them have been accelerated by one of the two other gravitational mechanisms: ejection from a dense cluster of stars as random motions bring it into a slingshot-like orbit, or ejection from a supernova binary system after the supernovae explodes and frees it from its orbit.

A binary star is a pair of stars that orbit each other, and many (perhaps most) stars are members of binary systems.

So far, there have been no hypervelocity binary stars discovered. They have been predicted, however, with at least one theory proposing that the discovery of a hypervelocity binary pair might indicate that the nuclear black hole is itself a binary pair.

More information: Kilic, M. et al. The Runaway Binary LP 400/22 is Leaving the Galaxy, MNRAS, 434, 3582, 2013.

UKIRT Discovers Unusual Binary Stars with Close Orbit

Scientists have discovered four unusual pairs of stars that orbit each other in less than four hours, according to a new study.

Scientists claim that the orbital period is quite strange because till now they have never found a binary star with such close orbital period.

According to the scientists, more than half of the stars in our milky way are part of a binary system and orbit each other at a certain period, which is more than 5 hours.

They believe that if binary stars orbit very close to each other, they will quickly emerge into one huge star.

The unusual binary stars were discovered while monitoring the brightness of thousands of stars, including red dwarfs by using the United Kingdom Infrared Telescope (UKIRT) in Hawaii.

"To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the 5 hour cut-off found for Sun-like stars, something previously thought to be impossible", said Bas Nefs, scientist at the Leiden Observatory, Netherlands, in a statement.

"It means that we have to rethink how these close-in binaries form and evolve," Nefs added.

The study suggested several reasons behind the short orbital period of these stars. One of the reasons is that since stars shrink in size in their lifetime, there is a possibility that their period could also have shrunk; otherwise the stars would have emerged quite long ago.

However, scientists are not sure as to how these orbits could have shrunk so much.

Another possibility is that the magnetic field lines radiating from the cool star companions get twisted and deformed as they orbit each other, generating the extra activity through stellar wind, explosive flaring and star spots.

Powerful magnetic activity could apply the brakes to these spinning stars, slowing them down so that they move closer together.

"Without UKIRT's superb sensitivity, it wouldn't have been possible to find these extraordinary pairs of red dwarfs", said David Pinfield, researcher at the University of Hertfordshire.

"The active nature of these stars and their apparently powerful magnetic fields has profound implications for the environments around red dwarfs throughout our Galaxy," he added.