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Skywatching

Exploring a neutron star

If we could be transported to the surface of a neutron star, we would be destroyed down to the atomic level or beyond before we even knew we'd arrived.

The environment is so hostile that nothing familiar to us could survive there. Pulsars, cosmic radio sources discovered in the 1960s, are believed to be rapidly rotating neutron stars.

We are familiar with the idea that atoms consist of a nucleus, containing a collection of protons and neutrons, which is surrounded by a cloud of electrons.

Compared with the diameter of the atom, the nucleus and electrons are tiny. Atoms are almost entirely empty space.

However, despite their apparent emptiness, atoms are highly resistant to compression, which is why we can walk on the ground without falling through, and can pick things up and interact with our environment.

However, in the explosion at the end of the life of a giant star, the shock waves can be intense enough to compress the star's core so hard the atoms themselves collapse. The electrons are jammed into the nucleus where they meld with the protons to make neutrons.

The result is that the core of the star becomes a lump of neutrons, occupying vastly less space. This is a possible fate for stars around 1.4 times the mass of the Sun, or larger.

The compression that occurs when the atoms collapse takes us from a star maybe two million kilometres in diameter down to a ball of neutrons 20 km or so across.

If one could bring a teaspoonful of neutron star material to Earth, it would weigh about a billion tonnes. On a neutron star, we would weigh about 100 billion times what we weigh on Earth.

The temperature would thousands of degrees Celsius, and the air we'd be breathing would include vaporized metals.

Stars contain a large amount of magnetism. When they are compressed into neutron stars the magnetic fields are compressed, too. The result is that on the surface the magnetic fields are billions of times stronger than the strongest laboratory magnetic fields we can make on Earth.

This means that if we were wandering around on the surface of a neutron star and not being crushed or roasted, we would find something really odd. There is an easy direction to walk and a hard one. Just as beads slide easily along a string, we would find it easy to walk back and forth along the magnetic field.

However, walking at right angles would mean crossing the magnetic field, which would be really hard. The Earth's magnetic field is so weak by comparison we don't notice this effect.

When a spinning star collapses to a smaller diameter, the rotation speeds up, so our neutron star could be rotating many times a second.

The intense magnetic field reaches far out into space and gets connected to nearby clouds of material that are not rotating as quickly, so the magnetic fields get wound up, snap, and release energy, producing intense X-rays, other radiation, and radio emissions.

Assuming we are surviving the radiation too, it would be worth looking at the ground we are walking on. It would be glowing, because it is hot, but also it would not be like the ground here on Earth.

It would be fibrous, like a doormat, or grainy, like wood. This is because the lines of magnetic force extend into the ground, forcing the material to coagulate along the direction of the magnetic field rather than across it.

Gardening would be no fun because the magnetic fields would make it extremely hard to turn the soil over. It is unlikely we will ever even get close to a neutron star, or even try to.

These are objects we will continue to observe from a safe distance and comprehend through physics.

About 2,000 neutron stars have been discovered scattered around our galaxy, the Milky Way.

  • Mars is now lost in the sunset glow.
  • Jupiter and Saturn rise around midnight and will lie low in the south at dawn.
  • The Moon will be Full on the 24th.


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Dust in the works

Here on Earth, if we find our car covered with dust we brush or wash it off. If the vehicle is a robot explorer on the planet Mars, the issue becomes enormously more complicated.

Those of us living in dusty environments, or who have worked in them, have experienced how much of a problem sand and dust can be, and it's not just cosmetic.

Sand in the works is a bad thing. It clogs things up and when it gets between moving surfaces, such as wheel bearings, it sticks in the lubricant and turns smooth bearings into grinding machines.

In the desert, the situation is even worse, because the sand is fine, dry and is always blowing around.

The first astronauts to land on the Moon ran into problems with dust. Over billions of years of vacuum and wide temperature changes, it had become very fine, and very dry.

The result is that any disturbance got it electro-statically charged, so it stuck to space suits and equipment and got into everything. Their space suits were white to reflect the Sun's heat. The dust was dark grey and coating the suits degraded their effectiveness at doing this.

The dust got into the machinery of the vehicles and other devices taken to the Moon to help the astronauts explore. When the astronauts got back into the lunar lander, the dust came off the suits and got everywhere. It smelled of burned gunpowder.

Human astronauts can brush it off. It's harder for robot explorers.

Mars has a far denser atmosphere than the Moon, but it is still less than one percent of the density of our world's atmosphere at sea level, and moreover it is very, very dry.

There is, however, enough atmosphere to provide winds strong enough to blow the dust around. In fact there is so much dust in the Martian atmosphere the sky does not look blue; it looks pink.

There are dust devils and sandstorms, sometimes big enough to hide large areas of the planet's surface. Moreover, continually being blown around makes the particles get finer and finer, and, in the dryness, they become electro-statically charged.

The result is that, as in the case of lunar dust, the Martian dust sticks to everything, and the blowing around of the dust by the winds ensures it gets everywhere. The selfies the Mars rovers send back show them getting more and more dusty.

Most of the rovers so far have been powered by solar cells, which convert sunlight into electricity. The solar cell arrays are flat panels that are almost horizontal, so that during the Martian days (called "sols") they are always catching the sunlight.

So they are almost ideal dust catchers. However, wind can blow dust off as well as drop it on, and occasionally positioning the rover so that the wind blows off most of the dust brings the electrical output back up.

If the spacecraft is an immobile lander the situation is more difficult. Over time one might expect the dust cover to sort of average out, with wind blowing as much off as it blows on.

However, this does not rule out occasional severe dust coverings where the weather is uncooperative, which is the situation NASA's Insight Mars Lander is faced with at the moment.

Luckily NASA came come up with an ingenious solution. The operators of the rover commanded it to scoop up some gritty soil from the ground beneath the rover.

Then, when the wind was blowing hard enough in the right direction, the grit was slowly released so the wind blew it over the surface of the solar panels, scouring off a lot of the dust and immediately bringing up their electrical power output.

Engineers are working on better ways to deal with dust accumulations on critical parts of the equipment. The solution I like best is to have an astronaut around who can go outside with a brush.

While there on the Red Planet, he or she could do a few other things too, such as explore, and exploit any new opportunities on the spot.

  • Jupiter and Saturn lie low in the southeast before dawn.
  • Mars is getting lost in the sunset glow.
  • The Moon will reach First Quarter on the 17th.


At the core of the Milky Way

Isaac Asimov's Foundation trilogy is one of the iconic works of science fiction.

The book, published in 1953, is based upon our vision of what our galaxy was like at that time, and describes the decline and fall of a galactic empire.

The empire was ruled and administered from the planet Trantor, located at the centre of the galaxy. This location made administrative sense because at the time, there was no known scientific reason not to.

On summer evenings, if we look south you will be looking toward the centre of our galaxy, which lies some 30,000 light years away. However, thanks to great clouds of dust and stars, we can only see a few thousand light years.

Back in the 1950s, large optical telescopes on the ground could not do much better; the centre of the Milky Way was hidden from view.

Radio astronomy was a young science in those days, but radio telescopes had found a strong source of radio waves located at the Galactic Centre.

It was called Sagittarius A because it was the brightest known radio source in the constellation of Sagittarius. However, nobody knew what the source might actually be.

Back then, we believed the universe was on the whole a fairly quiet place, which is the impression we get when lying under a dark, clear sky. Stars grew old and some blew up, but basically things changed only slowly.

Today, that illusion has gone. Now, in addition to optical telescopes on the ground, we have telescopes in space, which can see ultraviolet, X-rays and other high-energy radiation that does not reach the ground.

Radio telescopes have improved immensely. In the past, we could measure the characteristics of radio waves coming from a small patch of sky.

Today, we can make radio images, in some cases better than anything our eyes can give us or we can obtain using an optical telescope.

This has changed the whole picture. One of the big surprises the New Astronomy has given us is that the centre of the Milky Way is not a tranquil place at all.

X-ray and infrared telescopes orbiting the Earth can see through the dust clouds to the core of our galaxy, and modern radio telescopes can image Sagittarius A, not just detect its presence. It turns out that the energy driving that radio source is a black hole, with a mass four million times that of the Sun.

There are stars orbiting close to the hole and in the process of falling into it. There is dust and other material being sucked in and its disappearance from sight is marked by strong X-ray emissions.

It seems that most large spiral galaxies like ours have a large black hole in their cores. There does not seem to have been enough time since the beginning of the universe for the black holes to have been formed through the death of giant stars and the merging together of the resulting small black holes.

At the moment, it looks as though it is more likely these huge black holes were formed when the galaxies formed, and when two or more galaxies merged, their black holes spiralled in on each other and eventually merged too.

Back in the 1950s, the physics of the time predicted the possibility of black holes. However, the overall opinion was that there was more likely a problem with the physics.

It took quite an accumulation of evidence before the scientific community accepted that no matter how bizarre they might seem to us, there are such things.

Today, we are used to the idea that the universe has many more weird things to show us.

The high-radiation environment at the core of our galaxy, together with the general instability, would make Trantor uninhabitable. It might not have even had the time to form.

This is why, decades later, when Asimov wrote sequels to his Foundation books, he relocated Trantor to somewhere safer.

  • Jupiter and Saturn lie low in the southeast before dawn.
  • Mars is low in the sunset glow.
  • The Moon will be New on 10th.




Auroras on other worlds

Two of the most striking astronomical images taken in recent years show the aurora.

One was taken from Earth orbit and shows a ring of aurora like a crown around the North Magnetic Pole.

The other shows a similar ring around one of the magnetic poles of the planet Jupiter.

We know auroras happen on Saturn, Uranus and Neptune. No doubt there are other planets scattered around the universe that experience the same spectacles. The recipe for auroral displays includes three main ingredients:

  • Planet with an atmosphere
  • Strong magnetic field
  • Nearby star.

To have a magnetic field, a planet needs a hot core with flows of hot, electrically conductive material, like molten iron and nickel.

When planets form, they pick up a bit of magnetism from the cosmic material that made them. These interact with the flows of hot core material, forming a natural dynamo.

Huge electrical currents are generated which in turn produce strong magnetic fields that extend beyond the planet, far out into space.

Mars is a smaller world than ours. Its core cooled faster and the flows stopped. The result is that the Red Planet has just little patches of remnant magnetic fields and consequently no auroras.

Mercury doesn’t have an atmosphere. Venus has only a very weak magnetic field.

The Sun appears in the sky as an extremely bright disc. The disc is a layer called the photosphere, because that is where the light and heat are radiated into space. Its temperature is about 6,000 degrees Celsius.

Above the photosphere is a layer called the chromosphere, and above that the corona, which extends far into space, beyond the Earth.

The corona is very hot, about a million degrees. This is odd, because something between the 6,000-degree photosphere and the cold of space should have a temperature lower than 6,000 degrees, certainly not a million degrees.

One theory we have for this is that the huge turbulence at the photosphere sends sound waves upward and, as they move into more and more rarefied material, they get bigger until they dissipate as heat.

The high temperature makes the solar atmosphere unstable, so that it expands outward as a high-speed wind.

This wind, of million-degree plasma and magnetic fields, flows at hundreds to thousands of kilometres a second, ranging from a "breeze" to a "gale,” with an occasional "storm.”

In pre-space-age books the Earth's magnetic field was depicted as a sort of doughnut shape. Then we learned about the solar wind, which blows it into a teardrop, tapering to a point facing away from the Sun.

Gales and storms in the solar wind create instabilities in the Earth's magnetic field that accelerate electrons. These follow the magnetic field down into the Polar Regions, where they collide with atoms of nitrogen and oxygen in the air, making them glow.

Pulses and instabilities in the electron streams create the changing curtains, rays and flickering blobs of the aurora.

The occurrence of aurora on planets out as far as Neptune indicates the solar wind is still blowing strongly even that far out in the Solar System.

Now, thanks to the Voyager spacecraft, we know the solar wind extends out well beyond the planets, until it finally collides with the interstellar plasma, marking the edge of the Solar System.

On our world, the aurora has attracted artists and scientists, and has accumulated a wealth of myths from many cultures living at high latitudes.

Since inhabited planets need a star, a magnetic field to prevent the stellar wind scraping away the atmosphere, and an atmosphere to breathe, it is likely there is an accumulation of auroral myths on those distant worlds too.

Will we ever find out?

  • Jupiter and Saturn lie low in the southeast before dawn.
  • Mars is now slowly sinking into the sunset glow.
  • The Moon will reach Last Quarter on the 2nd and be New on 10th.


More Skywatching articles

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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]



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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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