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Largest ever yellow star is 1300 times bigger than sun

A monster version of our sun has been found, the largest known member of the family of yellow stars to which our sun belongs.

The whopper sun emits light in similar wavelengths as our sun but its diameter is over 1300 times larger. That means it would engulf all the planets between Mercury and Jupiter if placed at the centre of our solar system. The star’s size also means it is touching its smaller, companion star (see diagram, below).

(Source: ESO)

Dubbed HR 5171 A, the star is located in the constellation Centaurus around 12,000 light years from Earth. It was already known to be a hypergiant, the largest class of stars, but its exact size hadn’t been well studied.

Now a team led by Oliver Chesneau of the Côte d’Azur Observatory in Nice, France, has taken a closer look with the Very Large Telescope in Atacama, Chile. They report that it is twice as large as expected.

It still isn’t the largest star we know about – that crown goes to UY Scuti, which is around 1700 times larger than our sun. But UY Scuti is in a different stage of stellar evolution and so belongs to a different family of stars called red stars.

Peanut sun

Both red and yellow stars can be hypergiants, but yellow hypergiants were previously thought to be at most 700 times the size of the sun. The new measurement of HR 5171 A shows they can be much bigger. HR 5171 A is 50 per cent larger than the red hypergiant Betelgeuse, which is located in the constellation Orion and is the ninth brightest star in the night sky.

Another surprise for Chesneau’s team was the discovery that HR 5171 A has a little brother. Previous observations suggested the star varied in brightness. Now the team has shown that this is due to a companion star that is around one third its size.

The two stars orbit each other, forming a binary system. However, though their centres are separated by more than the distance between our sun and Saturn, HR 5171 A is so large that the two are touching, forming a continuous peanut-shaped structure. Guess this star system ain’t big enough for two.

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Solar Mini-Max

Years ago, in 2008 and 2009 an eerie quiet descended on the sun.  Sunspot counts dropped to historically-low levels and solar flares ceased altogether.  As the longest and deepest solar minimum in a century unfolded, bored solar physicists wondered when “Solar Max” would ever return.

They can stop wondering. “It’s back,” says Dean Pesnell of the Goddard Space Flight Center.  “Solar Max has arrived.”


A new ScienceCast video examines the curious Solar Max of 2014.  Play it

Pesnell is a leading member of the NOAA/NASA Solar Cycle Prediction Panel, a blue-ribbon group of solar physicists who meet from time to time to forecast future solar cycles.  It’s not as easy as it sounds. Although textbooks call it the “11-year solar cycle,” the actual cycle can take anywhere from 9 to 14 years to complete.  Some Solar Maxes are strong, others weak, and, sometimes, as happened for nearly 70 years in the 17th century, the solar cycle can vanish altogether. 

Pesnell points to a number of factors that signal Solar Max conditions in 2014: “The sun’s magnetic field has flipped; we are starting to see the development of long coronal holes; and, oh yes, sunspot counts are cresting.”

Another panelist, Doug Bieseker of the NOAA Space Weather Prediction Center, agrees with Pesnell: “Solar Maximum is here …. Finally.” According to an analysis Bieseker presented at NOAA’s Space Weather Workshop in April, the sunspot number for Solar Cycle 24 is near its peak right now.

They agree on another point, too:  It is not very impressive. 

“This solar cycle continues to rank among the weakest on record,” comments Ron Turner of Analytic Services, Inc. who serves as a Senior Science Advisor to NASA’s Innovative Advanced Concepts program.  To illustrate the point, he plotted the smoothed sunspot number of Cycle 24 vs. the previous 23 cycles since 1755. “In the historical record, there are only a few Solar Maxima weaker than this one.”

As a result, many researchers have started calling the ongoing peak a “Mini-Max.”


This plot prepared by Ron Turner of Analytic Services, Inc., shows the smoothed sunspot number of Cycle 24 (red) vs. the previous 23 cycles since 1755.  Larger image

Pesnell believes that “Solar Cycle 24, such as it is, will probably start fading by 2015.” Ironically, that is when some of the bigger flares and magnetic storms could occur.  Biesecker has analyzed historical records of solar activity and he finds that most large events such as strong flares and significant geomagnetic storms typically occur in the declining phase of solar cycles—even weak ones.

Indeed, this “Mini-Max” has already unleashed one of the strongest storms in recorded history.  On July 23, 2012, a plasma cloud or “CME” rocketed away from the sun as fast as 3000 km/s, more than four times faster than a typical eruption. The storm tore through Earth orbit, but fortunately Earth wasn’t there. Instead it hit NASA’s STEREO-A spacecraft, which recorded the event for analysis.  Researchers now believe the eruption was as significant as the iconic Carrington Event of 1859—a solar storm that set telegraph offices on fire and sparked Northern Lights as far south as Hawaii. If the 2012 “superstorm” had hit Earth, the damage to power grids and satellites would have been significant.

It all adds up to one thing: “We’re not out of the woods yet,” says Pesnell.  Even a “Mini-Max” can stir up major space weather—and there’s more to come as the cycle declines.

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Discovered: A Cold, Close Neighbor of the Sun

NASA’s Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope have discovered what appears to be the coldest “brown dwarf” known — a dim, star-like body that  surprisingly is as frosty as Earth’s North Pole. Named “WISE J085510.83-071442.5,” the brown dwarf appears to be 7.2 light-years away, earning it the title for fourth closest system to our sun.

“It’s very exciting to discover a new neighbor of our solar system that is so close,” said Kevin Luhman, an astronomer at Pennsylvania State University’s Center for Exoplanets and Habitable Worlds, University Park.


This animation shows the brown dwarf WISE J085510.83-071442.5 moving across the sky. It was first seen in two infrared images taken six months apart in 2010 by NASA’s Wide-field Infrared Survey Explorer (WISE). Two additional images of the object were taken with NASA’s Spitzer Space Telescope in 2013 and 2014. All four images were used to measure the distance to the object — 7.2 light-years — using the parallax effect.  Movie

Brown dwarfs start their lives like stars, as collapsing balls of gas, but they lack the mass to burn nuclear fuel and radiate starlight. The newfound coldest brown dwarf is named WISE J085510.83-071442.5. It has a chilly temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius). Previous record holders for coldest brown dwarfs, also found by WISE and Spitzer, were about room temperature.

“It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the sun’s nearest neighbors,” added Michael Werner, the project scientist for Spitzer at NASA’s Jet Propulsion Laboratory. “This exciting new result demonstrates the power of exploring the universe using new tools, such as the infrared eyes of WISE and Spitzer.”

WISE was able to spot the rare object because it surveyed the entire sky twice in infrared light, observing some areas up to three times. Cool objects like brown dwarfs can be invisible when viewed by visible-light telescopes, but their thermal glow — even if feeble — stands out in infrared light. In addition, the closer a body, the more it appears to move in images taken months apart. Airplanes are a good example of this effect: a closer, low-flying plane will appear to fly overhead more rapidly than a high-flying one.

“This object appeared to move really fast in the WISE data,” said Luhman. “That told us it was something special.”

After noticing the fast motion of WISE J085510.83-071442.5, Luhman spent time analyzing additional images taken with Spitzer and the Gemini South telescope on Cerro Pachon in Chile. Spitzer’s infrared observations helped determine the frosty temperature of the brown dwarf. Combined detections from WISE and Spitzer, taken from different positions around the sun, revealed the object’s parallax, and thus its distance. The closest system to Earth, a trio of stars, is Alpha Centauri, at about 4 light-years away. WISE J085510.83-071442.5 is only a few light years farther than that.


This diagram pinpoints star systems closest to the sun. The year when the distance to each system was determined is listed after the system’s name.  More

WISE J085510.83-071442.5 appears to be 3 to 10 times the mass of Jupiter. With such a low mass, it could be a gas giant similar to Jupiter that was ejected from its star system. But scientists estimate it is probably a brown dwarf rather than a planet since brown dwarfs are known to be fairly common. If so, it is one of the least massive brown dwarfs known.

In March of 2013, Luhman’s analysis of the images from WISE uncovered a pair of much warmer brown dwarfs at a distance of 6.5 light years, making that system the third closest to the sun. His search for rapidly moving bodies also demonstrated that the outer solar system probably does not contain a large, undiscovered planet, which has been referred to as “Planet X” or “Nemesis.”

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Astronomers Discover Our Sun’s “Sibling”

Recent studies have found that 80-90% of stars are born in groups of 100 or more, most likely including the sun. After a 100 million years or so these stellar siblings drift apart, losing each other in the crowd. Now however, University of Texas astronomers have identified a star they believe came from the same birth cloud as the sun.
For lead author Dr Ivan Ramirez the motivation is the same as for any orphan. “We want to know where we were born. If we can figure out in what part of the galaxy the sun formed, we can constrain conditions on the early solar system. That could help us understand why we are here.”
Lost family can be disappointing, but in this case we stand to gain a fair inheritance, if only in knowledge. At 15% more massive than the sun it is the sort of place that might be a suitable candidate for life and at 110 light years-away it’s not that distant, by galactic standards. The only disappointment is it’s boring name, HD 162826, and that you need binoculars to pick it out in the constellation Hercules, not far in the sky from Vega.
Ramirez investigated the chemical composition of 30 previously proposed siblings, looking for the same concentrations of rare earth elements. In the Astrophysical Journal he reveals two passed the test, and one of these turned out to have an orbit around the center of the galaxy that ruled it out. As a relatively nearby sunlike star HD 162826 has been a target for planet hunters for 15 years. They’ve ruled out the presence of hot Jupiters, and think it also unlikely a large planet lurks at larger distances. However the observations have not been sensitive enough to pick up Earth-sized planets if they exist.
The discovery provides a starting point for more ambitious projects. 
Ramirez hopes to use the findings to develop  better ways to find many more family members. The Gaia observatory, launched last year, will provide a catalog of 1% of the stars our galaxy, mapping their motions and positions in three dimensions. “The number of stars that we can study will increase by a factor of 10,000,” says Ramirez. Having this data and processing it effectively are two different things however, and Ramirez hopes his work will find a way through the maze. “You can concentrate on certain key chemical elements that are going to be very useful,” he says. Barium and yttrium stand out as the elements whose concentration is both easy to measure and most useful in establishing kinship.
With a large enough family tree, Ramirez believes he will be able to identify the place in the galaxy where this scattered sample of stars began, as well as something about the cloud from which we came.
A far more speculative undertaking is to study HD 162826 for planets planets in the habitable zone. The two stars must have been quite close together in the period when large asteroids and planet sized objects were crashing into each other, blasting bits off in the process. It’s not impossible a few bits of Earth got knocked from the solar system and ended up in neighboring systems. It’s more of a stretch that these bits could have transferred life either from the Earth to HD 162826, or in the other direction, seeding life here, but Ramirez still considers “solar siblings key candidates in the search for extraterrestrial life.”
Ramirez was previously involved in the identification of the best “solar twin” then found. The term solar twin refers not to stars born close to the sun, but ones that most closely resemble our star in mass and chemical composition.