Flaring red dwarf stars

The nearest known star to us after the Sun is a red dwarf star called Proxima Centauri.

It is 4.25 light years away, which means its light takes 4.25 years to reach us. This star has about an eighth of the Sun's mass, is about a seventh of the Sun's diameter, and its energy output compared with our star is microscopic.

Its fuel consumption is so low that it should be able to keep shining happily for tens of billions of years. This suggests that Proxima Centauri, along with the countless other red dwarf stars scattered around the universe, could be ideal hosts for inhabited planets.

They can provide a stable environment over a long enough time for life to appear and evolve. It turns out that Proxima Centauri has a potentially inhabitable planet, but on the downside, this star, along with many other red dwarfs, produces large flares.

Stars are big spheres of hot, churning plasma, atoms so hot they have lost some of their electrons. This means plasma is a good conductor of electricity. Fluorescent lights contain a little bit of plasma.

Stars are also threaded by complex systems of strong magnetic fields. The continuous churning leads to the magnetic fields getting crowded, twisted and/or stretched.

These distortions lead to a colossal amount of energy being stored. In most cases this energy gets slowly dissipated again. However, sometimes the tangle of plasma and magnetic fields is too complicated or severe to just relax.

In this situation, the stresses build up until something snaps, and all the stored energy, which in the case of the Sun could add to millions of hydrogen bombs, is released in seconds.

The result is blasts of X-rays, even gamma rays in some cases, beams of high-energy particles, and often a large chunk of material is shot off into space at thousands of kilometres a second. These are called coronal mass ejections, or solar storms.

Flares don't threaten living things on the ground, but can pose a risk to those at high altitudes or in space. On other hand, flares can severely degrade or even destroy the complex power, transportation and communications infrastructure our lives depend on.

It does help that we live about 150 million kilometres away from the Sun. However, imagine living 20 times closer, to a star that produces even bigger flares.

For any star there is a Goldilocks Zone, the distance from the star where a planet would be at the right temperature for liquid water to exist on its surface.

For dimmer stars that zone lies close to the star, and is quite narrow. So the chance of a planet orbiting a dim, red dwarf star lying in the zone is smaller than it would be for a brighter star. However, red dwarf stars are the commonest stars in the universe, so there must be lots of them out there with planets lying in the zone.

Red dwarf stars produce much larger flares than do more massive stars like the Sun, which is why we can detect them from so far away.

The result for a planet in the Goldilocks Zone for Proxima Centauri or other red dwarf stars is that even a solar flare of the size produced by the Sun would hit the planet hundreds of times harder than we get hit.

Imagine a flare tens of times larger than what our Sun produces. The environmental impacts would be huge, let alone the disruption of any technological infrastructure.

It is likely that whether there is liquid water, the radiation and wild environmental instabilities would stop life getting going on any of these worlds.

However, not all red dwarf stars we observe are producing these huge flares. This could mean some of these stars could have inhabited worlds. That is unless this flaring behaviour is something all these stars go through at some points in their lives.

Of course, our arguments are based on life as we know it. There could be other forms of life.

  • Mars is still high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon be New on the 11th.

First flight on Mars

Until we discover otherwise, the first ever known powered flight on Mars has just happened.

The helicopter that went to Mars with the latest rover just had a test flight, going about three metres into the air, hovering and then landing safely. Having an aircraft on Mars is going to have a huge impact on our exploration of the planet.

This is not the equivalent of flying a drone here on Earth. There are two huge, additional challenges.

Mars' atmosphere is much thinner than what our drones fly in on Earth.

With radio signals taking many minutes to get from Mars to Earth and just as long to go the other way, there is no way we can remotely control the vehicle.

Although here on Earth jet aircraft can reach the edge of space, helicopters are confined to low altitudes. It is possible to coax the most advanced helicopters to heights of around eight kilometres, but the highest they can hover is about three kilometres.

The atmospheric pressure on the surface of Mars is about the same as we find here on Earth at an altitude of around 25 kilometres. Our earthly helicopters would not get off the ground on Mars.

The problem is getting enough lift. This is the force that gets an aircraft off the ground and keeps it in the air.

The amount of lift depends on the density of the air, the area of the wing surface and how fast that wing surface is moving. Can we not just "flap our wings" faster? Unfortunately that won't work.

When the ends of the blades of the propeller get close to the speed of sound, they stop producing lift. So the only real solution is to go for as much wing area as possible, so that the propeller blades don't have to move as fast.

The other thing we can do is cut the weight of the aircraft to a bare minimum, and this includes the weight of our larger propeller blades. Luckily there are advanced construction materials available that provide strength while being lightweight.

Thanks to great efforts by NASA, that helicopter weighs in at less than two kilograms, is ready to go. The cameras and other electronics on the helicopter are small and very lightweight.

The first spacecraft that landed on Mars were immobile. They just observed their surroundings, so the fact that signals took many minutes to get between Earth and Mars was not a huge problem. Things changed when the rovers landed on the Red Planet.

The solution to the time-delay problems involved first, driving slowly, which was necessary on rough, unpredictable terrain; the main thing was to make the vehicles smart.

They would have to make all the decisions necessary for driving safely, relying on operators on Earth to merely point out what to drive to.

Robot helicopters bring up a whole range of new issues. They fly faster, so they have to "think" faster. It is true there are few things to hit while flying, but Mars can be a windy place, so conditions can be turbulent.

With nobody around to put things right when something happens at take-off or landing, the aircraft has to be smart enough to identify all the hazards a human pilot has to, and to respond to them.

For example there are frequent "dust devils" on Mars: small tornadoes of whirling dust that extend well up into the atmosphere. It would not be good to fly a fragile, small, light aircraft into one of them.

These first flights can be planned to avoid or minimize these threats, but eventually vehicles carrying out long, autonomous exploratory flights will have to be smart enough to avoid such hazards.

This flying Martian explorer is the first of its kind, but certainly won't be the last. The ability to explore Mars by flying around, studying the atmosphere and maybe landing for a closer look at places of particular interest will revolutionize our knowledge of our neighbour world, and make it all much safer for those first human explorers.

  • Mars is still high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon will reach Last Quarter on the 3rd.

New solar systems

Not that many years ago, we believed we would never be able to detect planets orbiting other stars, or to see planets forming around a newborn star.

Planets would be impossible to see because it would be like trying to spot a firefly next to a searchlight, viewed from a few kilometres away.

If anything, seeing the birth of new stars and planets would be even more difficult, because these births are discreetly hidden in the middle of the huge clouds of gas and dust from which those new stars and planets are forming.

We first detected extra-solar planets by detecting the slight wobbling of a star as its planets orbited around it. Then, we found an easier method: searching for the slight dimming of a star as a planet moves across in front of it.

Today, extra-solar planets are being detected by amateur astronomers by using this method.

Finally, surprisingly, we now have the technology to detect that firefly hovering by the searchlight, or a faint planet orbiting a distant star.

Space is filled with clouds of cold gas and dust. This is the raw material for making new stars and planets. In places where the clouds are thicker and denser, parts of the interiors of the clouds become unstable and collapse.

Our calculations suggested the collapsing material would form a disc, with the core of the disc becoming a new star and the rest of the material forming planets.

However, our optical telescopes could show us nothing, because the fog of cloud material discreetly hid the birth process from view.

We only get to see the new star and planetary system when it starts shining and evaporates and blows away its birth cloud.

The problem is a process called scattering. This happens when light passes through fog. Details just become a blurred glow.

However, scattering is more pronounced at shorter wavelengths. If instead of light, we make our observations at longer wavelengths, such as infrared or radio, we should be able to see through the fog.

Luckily, as the cloud material collapses into a disc, it gets warm, not much, but enough to glow at millimetre wavelengths, which can be observed by means of radio telescopes.

Unfortunately, there is a catch — our damp atmosphere is really good at blocking these wavelengths.

There are two ways to deal with this problem. One is to put our radio telescope in space, above the atmosphere, and above the problem.

However, to make a detailed image, radio telescopes have to be extremely large, certainly far too large to put on top of a rocket.

A second option is to find the highest, driest place we can find, and put our instrument there. This is what has been done. The site is the Atacama Plateau, which straddles Chile and Argentina.

A large radio telescope, one of the largest on Earth, has been built there, about five kilometres above sea level, at a location where rain is almost unknown.

The instrument, known as the Atacama Large Millimetre Array, is an array of more than 60-dish radio telescopes that can emulate an antenna about 16 km across.

It can image to a higher degree of detail at millimetre wavelengths than the Hubble Space Telescope can with visible light.

The resulting images look like golden bull’s eyes: concentric circles of gold and black. Some were golden discs with only one dark ring; others had more. Most bull’s eyes are a bit elliptical, because we're not seeing them face-on.

Initially the cloud forms a disc, then lumps in the disc start growing into planets. As they orbit their young star they sweep up material, each forming a dark ring.

By looking at these bull’s-eye images, we can count the young planets — one per ring, and from the width of the ring get an idea how big each planet has become.

Growth will continue until the disc material is used up or blown away by the star.

  • Mars is high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon will be full on the 26th.


Sirius has a secret

If you look low into the southwest these evenings, you will see the brightest star in the sky after the Sun, Sirius.

This star is prominent in the winter sky, and now, as we get through spring, it, along with the winter constellations, are vanishing behind the Sun.

Sirius has a little secret. It is a double star. The two partners orbit each other every 50 years. The partner is difficult to see because it is very faint. It is a white dwarf star, the gradually cooling off core of a dead star.

If it were not close to a really bright star, and lost in the glare, we would be able to see it with our unaided eyes.

When an average star, like the Sun, starts to run out of fuel, it swells into a red giant and then sneezes off its outer layers. What's left is the core of the star, about the size of the Earth, and so compressed a teaspoon full of its material would weigh tons.

Although very hot, such a small object loses heat quite slowly, taking billions of years to completely cool off.

This is a white dwarf star.

Having almost no fuel for energy production and insufficient mass to compress that fuel enough to trigger energy production, we would expect white dwarf stars to be among the most stable objects in the universe.

There are certainly lots out there, gradually cooling off. However, on occasion they can go off the rails in spectacular fashion.

We know of one way this can happen. It arises in double stars that are orbiting close to one another. Although the two stars were born together, one will inevitably have a larger mass than the other.

A quirk of the way stars work is that although more massive stars have more fuel, they burn it much faster and run out sooner.

At some point, the more massive star starts running out of fuel, while its partner is still happily shining. It swells into a red giant, sneezes off its outer layers and settles down to retirement as a white dwarf star.

Then, eventually, its partner also runs low on fuel and swells into a red giant. When this happens the star has a much weaker gravitational hold on its material and its white dwarf companion starts to grab it and pull it in.

The result is that an increasing mass of the other star's material accumulates on the white dwarf's surface. This contains a lot of unburned fuel, and eventually enough collects to cause a huge explosion that can destroy both stars.

This type of event is known as a Type 1a supernova.

We have observed many of these, in our galaxy and in others. However, recent research has come up with a really bizarre way white dwarf stars can destroy themselves in spectacular fashion.

It involves uranium.

Stars are powered by nuclear fusion, where small atoms, like hydrogen, combine to form larger atoms, such as carbon, oxygen and so on.

These waste products are useful, in that they make it possible to make planets, and living things. Some stars, especially the more massive ones, can produce much larger atoms, such as uranium.

Such large atoms are not stable; they break into smaller atoms, releasing energy as they do so. The break-up, or fission of a uranium atom can trigger the fission of other uranium atoms.

If there are enough uranium atoms around, then the fission process can run away. We call this a chain reaction. If controlled it can be a source of energy. If not, the result is a nuclear explosion.

As molten material cools, the atoms moving around in it tend to join together making crystals. It has been suggested that in a cooling white dwarf, uranium atoms will do this.

However, a point might be reached where the crystal body reaches the critical mass, and a runaway chain reaction takes place.

This small nuclear explosion can act as a detonator for any unburned fuel in the white dwarf, causing a thermonuclear explosion that can blow the star apart.

It seems that for stars, just like people, ageing does not mean predictability.

  • Mars is high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast before dawn.
  • The Moon will reach First Quarter on the 19th.

More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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