BILLIONS and billions of stars fill our galaxy. Many burn bright, destined to become supernovae, while others are dim burnouts. They come alone and in pairs; with or without planetary companions. We have searched the far reaches of the universe in the hope of understanding the stars, but ultimately everything we know is based on our sole reference point, the sun. Yet our home star remains plenty mysterious.
“It’s expected that it’s understood, because it’s right there, it’s so close and dominant in the sky,” says from Trinity College, Dublin, Ireland. “How are we going to understand any other aspect of space if we can’t get to grips with the nearest star?”
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While we may have to go back to square one, there are things we do know about our sun. It is made of plasma – gas that has been ionised, or highly charged. It fuses hydrogen in its core. It blasts us with radiation and, crucially, its life-giving light. As stars go, it is roughly middle-aged, having been around for 4.6 billion years. And it probably has 5 or so billion more to go before it swells into a red giant that consumes Mercury, Venus and Earth. Yet strange solar phenomena abound and here are some of the strangest.
It rains on the sun
We know the sun affects weather on Earth and in space, but it has its own dramatic weather phenomena, too.
“People have this image of a giant ball of gas that’s on fire, and everything is streaming away from it at thousands of kilometres per second,” says Scullion. In fact, the sun’s plasma can fall back to the surface as rain.
Though this so-called coronal rain was predicted about 40 years ago, we couldn’t see or study it until our telescopes became powerful enough to spot it happening. It works a bit like the water cycle on Earth – where vapour warms, rises, forms clouds, cools enough to condense into a liquid and falls back to the ground as precipitation. The big difference is that the plasma doesn’t change from gas to liquid, it simply cools enough to fall back down to the solar surface.
This all happens very quickly and on a gargantuan scale, with “droplets” the size of countries plunging from heights of 63,000 kilometres – about one-sixth the distance from Earth to the moon. “You basically generate something the size of Ireland in 10 minutes, and drop it out of the sky at a rate of 200,000 kilometres an hour,” Scullion says.
Solar tornadoes also form in a familiar fashion. Swirling solar plasma creates a vortex, which causes magnetic fields to twist and spiral around into a super-tornado that reaches from the surface into the upper atmosphere. Here they transfer energy and help to heat it, or so scientists believe.
It has long-lost siblings
The sun may be on its lonesome now – its closest neighbour is 4.2 light years away – but that wasn’t always the case. Once upon a time it had close family. After their birth in the same cloud of dust and gas that formed our solar system, these solar siblings scattered hundreds of light years apart in the Milky Way (see diagram). In May 2014, : a star called HD 162826.
“It looks like the sun, but a little bit bluer,” says Ivan Ramirez at the University of Texas at Austin, . It’s also warmer than the sun and 15 per cent more massive. The star is about 110 light years away, and you can see it with the aid of a pair of binoculars in the left arm of the constellation Hercules.
To find its family ties, Ramirez’s team combed through galactic archaeology studies, which model the motions of the Milky Way. These predictions laid out where sibling stars would be now if they had formed in the same place as the sun. Though they spread out in different directions, their positions still give away their birthplace, Ramirez says.
He narrowed down the search area to 30 stars, and then looked at them closely to find a family resemblance. Only HD 162826 had a similar chemical make-up to the sun. A separate team led by at the University of Rochester in New York also studied the star and , as would be expected for two stars born together. Even more tantalising, HD 162826 is already in a catalogue of stars that might harbour planets.
Locating solar siblings could tell astronomers more about the birth of our solar system, including what conditions were like when the sun and planets formed. But beyond scientific curiosity, Ramirez just wanted to find a member of the sun’s nuclear family. “It’s a cool thing to do,” he says.
He plans to keep looking for more of our sun’s lost littermates. Most are probably red dwarf stars, which are the most common stars in the galaxy. They are smaller and cooler than the sun, so they are much harder to find. But the Gaia telescope, launched in 2013, may help locate more solar siblings as it will observe a billion stars to make the first 3D map of the Milky Way.
It has a freaky calendar
Our planet’s calendar is well known: it takes 24 hours to spin once on its axis – a day – and 365 days to travel around the sun – a year. Yet the sun’s schedule is nothing like ours. Different parts of the sun spin at different rates. So while a day at the equator lasts 25 days, regions close to the poles take a few days longer to make a complete rotation. This uneven spin leads to distortion in the sun’s magnetic field, which has knock-on effects. As the equator spins, it drags the magnetic field that connects the sun’s poles, says at ’s Goddard Space Flight Center in Greenbelt, Maryland. This results in another strange calendar phenomenon: solar maxima and minima.
As the sun’s magnetic field gets wound up by the spin “it starts to build tension and pressure, much like when you twist a rubber band and it knots up”, Young says. Something has to give, so the magnetic fields snap and release energy in the form of heat, either as solar flares or furious clouds of energy called coronal mass ejections (CMEs).
This cycle, from magnetic twisting to energy releasing, happens over roughly 11 Earth years – giving the sun its own calendar. During what’s called a solar minimum, flares are few and so are dark patches called sunspots that appear on the sun’s surface due to intense magnetic fields.
In solar maxima, more sunspots burst over the surface where they spew more flares and CMEs. Torrents of charged particles also stream through gaps in the sun’s atmosphere and across the entire solar system. This can affect us, causing blackouts on Earth and damaging satellites. But each solar cycle varies, and we don’t understand why, which makes them and their effects unpredictable.
The current cycle is unusually calm and has been one of the weakest since records began in 1755. This is in spite of some major solar storms, together with a colossal solar flare in 2012, which would have packed some punch had it hit Earth.
Predictions just a couple of years ago suggested this cycle would be a scorcher, which shows just how little we understand solar cycles, says , a solar physicist at Stanford University in California. “It’s like predicting the stock market. Past performance is no guarantee,” he says.
Also roughly every eleven years, the sun undergoes yet another calendar change: its magnetic field reverses. North becomes south, and vice versa. Earth does this, too, but only every 300,000 years or so (we are long overdue one). The sun’s polarity last reversed in 2013, though the flip took scientists many months of analysis to confirm.
“Why is it 11 and 22 years and not 15 and 30? We don’t know the answer to that yet,” Young says. “When you think about it, it’s such a short amount of time, given that the sun has been around for 4.6 billion years.”
It breathes
As the sun follows its 11-year solar cycle, it changes, altering its output of solar wind, X-rays, ultraviolet and visible light. This has the knock-on effect of changing the size of the huge magnetic bubble of charged particles, called the , that the sun blows around itself to way out beyond Pluto.
These changes affect everything from Earth’s climate to the Voyager 1 spacecraft, which finally entered interstellar space in 2013.
The sun provides nearly all the energy that drives Earth’s climate – 2500 times as much as all other sources combined, according to , a solar physicist at the University of Colorado’s Laboratory for Atmospheric and Space Physics. In past epochs, solar cycles were partly responsible for warm periods and mini ice ages. Low solar activity drives cold winters in northern Europe and the US, and mild winters over southern Europe – although global warming means globally averaged temperatures are on the rise.
We now understand what’s going on a little better thanks to a , launched by NASA in 2003. TIM keeps tabs on the spectrum of energy the sun emits, and detects subtle changes in energy output so scientists can distinguish between human causes of climate change and purely natural causes we can’t control.
Changes in the sun’s output affect much more than just our climate, however. During a solar minimum, the solar wind streams from the poles at a much faster speed, so there’s more pressure pushing against material from interstellar space. During solar maxima, the sun’s magnetic fields are more knotted up and not as much wind escapes, so the heliosphere contracts. “There’s sort of an 11-year breathing,” says Hoeksema.
The solar wind has been 20 to 40 per cent weaker than expected this cycle, he says. This shallower breath is one reason why Voyager 1 left the heliosphere earlier than scientists expected.
It defies thermodynamics
Solar tornadoes are bizarre enough on their own, but they might help explain one of the sun’s weirdest characteristics: its atmosphere is hotter than its surface. At 5700 kelvin the sun’s surface is scarcely cold, but it is frigid compared to the corona. The highest part of the sun’s atmosphere, more than 1 million kilometres above the surface, can reach temperatures of several million kelvin (see diagram, below).
Generally, an object cools as it moves away from a heat source; a marshmallow will toast faster when it’s closer to a campfire flame than further away. But the sun’s atmosphere does the opposite. Energy must be flowing into the corona, heating it up – but no one knows where this energy comes from. “We don’t fully understand the physics of what’s going on,” Scullion says.
Computer visualisations might paint a clearer picture of this process – and quite artistically, too. In one simulation, NASA Goddard astrophysicist added colour to data coming in from ’s Solar Dynamics Observatory (SDO), which observed the sun’s coronal plasma in 10 different wavelengths that each correspond to a temperature. The result is a swirling movie reminiscent of a Van Gogh painting (). But Viall’s visualisation suggested the atmospheric plasma was cooling, not heating. This may be because the heating is happening faster than SDO can detect.
Much of the energy that heats the corona appears to come from the so-called transition region – the area between the sun’s corona and the next atmospheric layer down. Tornadoes, rain, magnetic braids, plasma jets and strange phenomena called “spicules” are all thought to play a role in this heating process, bringing energy from the lower regions of the sun and depositing it higher up. But no one knows exactly how. ’s has been observing this region since 2013, and physicists like Scullion try to simulate these energy exchanges using models in the hope that they will yield clues that scientists can look for on the real thing.
It’s hard to get there
To truly understand all these solar conundrums, we need to get as close to the sun as possible. That’s not as simple as flying straight there, as the operators of two new spacecraft that will fly closer to the sun than ever before are finding.
is a European Space Agency mission launching in 2018, aiming to fly within 45 million kilometres of the sun (see diagram, above). It will photograph the sun’s poles for the first time, which should help scientists understand how the sun generates its magnetic field, and may even give insights into why its magnetic polarity flips so frequently. By getting a close-up view, the probe will also be able to sniff the pristine solar wind, before it has reached Earth. The main goal is understanding how the sun interacts with the environment around it, says , a physicist at Imperial College London and the principal investigator on the Solar Orbiter’s magnetometer. “The basic physics is understood, but a lot of the detail is not,” he says.
’s mission is set to launch shortly before the ESA’s mission and come even closer, just 6 million kilometres from the sun’s surface. To get there, it will approach in a looping, circuitous route, like a matador approaching a wary bull. The slow approach is partly for safety’s sake: as the probe gets closer, scientists can carefully monitor any threats from radiation or heat and adjust the approach if anything goes awry.
Solar Probe Plus will lap round Venus seven times to put it on the right trajectory and also to build up speed and momentum to slingshot closer to the sun – at its closest approach, it’ll zip past the sun at 200 kilometres per second.
Shielding a spacecraft from solar radiation is one of the most important jobs in spaceflight, but it’s even harder when you are sidling up to the source. The technology to do it hasn’t existed until now, Horbury says. Both craft will have beefy heat shields to protect their sensitive instruments from searing temperatures.
Both spacecraft will try to answer questions, including how the atmosphere is heated and how the sun generates its wind. But they will still be far from answering everything there is to know about our star, says Young. “The problem is that you don’t know what you don’t know,” he says.
This article appeared in print under the headline “Strangest star”
Article amended on 23 September 2014
When this article was first published, it confused the sun’s 22-year magnetic cycle with its 11-year magnetic flip interval.
