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Skywatching

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.


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How dry is Mars?

With Mars now the most intensively explored planet in the Solar System after ours, it is not surprising that we are continually having to review our ideas and preconceptions.

The Red Planet continues to challenge our imaginations.

The mainstream idea is that originally Mars had lots of water, but being a smaller world, with weaker gravity, and losing its magnetic field early on, its atmosphere and water got lost to space, leaving a cold, dry desert world.

Now, it looks as though we could be seriously wrong in our assumption that Mars is now a dried-up world.

Current estimates are that around three billion years ago, when life was first getting started in the Earth's oceans and possibly on Mars too, there was enough water on Mars to submerge the whole planet to a depth of between 100 and 1000 metres.

By comparison, if our Earth were a smooth ball, the existing oceans would cover it to a depth of well over 2000 metres. Mars once had a lot of water; by about a billion years ago, it had disappeared.

There are three places the Martian oceans could have disappeared to.

  • One is, as we know, the loss of water to space.
  • A second is the presence of lots of ice hidden underground, and possibly underground or under-ice briny lakes.
  • A third option is that water got taken up and combined with various minerals. This water is chemically tied up and can remain sequestered for a long time.

At some point in our high-school science career most of us have heated copper sulphate.

As we warmed it, those nice blue crystals turned into white powder and water came off as steam. Before we applied the heat the crystals were perfectly dry.

The water molecules were locked up inside the crystals as part of the chemical. There are many minerals that similarly lock up water.

The first rocks on Mars would have been volcanic. However, Mars, like Earth, was then a wet world, with rain and other weather.

On Earth, rocks are continually attacked and broken down, a process we call weathering. The situation on Mars would have been the same. In the process, minerals in the rock become new minerals that contain water, such as clays.

From observations done from orbit and on the Martian surface, it looks as though there is a lot of water tied up in hydrated minerals.

Apart from our interest in this information in helping us understand the history of the planet in the Solar System most like ours, it also is important regarding our plans to have long-term manned bases, or even colonies on Mars.

The red surface of Mars indicates iron oxides, which contain oxygen. In addition, we can use solar-generated electricity to liberate oxygen from water.

Given that current space technology means Mars is always many months away, the more self-sufficient our bases are for the key needs of energy, water and oxygen, the less dependent they will be upon Earthly support, and the more secure they will be in the long term.

Moreover, the more water there is, the better the long-term prospects of terraforming the planet.

Before messing with the Martian environment, we need to know whether there are living things on the planet. The last thing we would want to do is render their world uninhabitable to them.

The presence of water tied up in minerals rather than just frozen solid offers easier prospects for Martians to make a living, because water is an important component in extracting useful chemicals from their surroundings, maybe, as is the case on Earth, with the help of the Sun.

It is true that the solar ultraviolet radiation level on the surface of Mars is dangerous to us, but that radiation has a lot of energy in it, and there is no reason Martians, if any, will be like us.

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


Story of a solar storm

Over days, stresses had been building up in a big magnetic loop extending upward from the surface of the Sun.

The solar wind, flowing outward at hundreds of kilometres a second, was dragging it upward, stretching it. In addition, the movement of the foot points of the loop and the encroachment of other growing loops further increased the strain.

To start the story properly, we need to step back a bit. Many articles and books describe the Sun and other stars as balls of hot gas. Stars are far more complicated than that.

It's obvious, a ball of hot gas would be a fuzzy blob. Instead. we see the Sun has something like a surface, with sunspots and many other complex structures. This is all due to an additional ingredient, magnetic fields.

In our every-day lives, magnetic fields are invisible. We know they are there because they can pull themselves hard onto ferrous surfaces, like refrigerator doors, keeping shopping lists in place. However, this is because we live in a world of low temperatures.

At temperatures of thousands or millions of degrees, the atoms making up the air break up. At the temperatures we live in, atoms consist of a nucleus surrounded by a cloud of orbiting electrons.

At solar temperatures, the nuclei of the atoms cannot hang onto all their electrons and some, or even all of them wander off on their own. The gas has become ionized; it is now a plasma.

Unlike un-ionized air, plasmas conduct electricity, which means they stick to magnetic fields, and vice versa, producing something, a "magnetoplasma,” which is like rubber or elastic. You can stretch it, twist it and make things out of it, like huge magnetic loops.

When the Sun formed some 4.5 billion years ago, it collected not only a huge amount of gas and dust, the raw material we use to make stars, it also dragged in a lot of cosmic magnetic fields too.

These mix with the churning plasma inside the Sun, generate electric currents, which in turn generate even stronger magnetic fields. These sit inside the Sun like huge ropes and tangles of magnetoplasma.

Sometimes, these get dragged up to the surface and erupt into space as huge magnetic loops, loaded with plasma.

Finally, the stresses in the loop become too much. Electric currents are generated that are too much for the plasma to carry, the magnetic fields snap close to the anchor points and the loop catapults off into space at speeds of up to thousands of kilometres a second.

This is a coronal mass ejection, or more popularly, a solar storm. We see it briefly against the bright surface of the Sun, and get bursts of radio waves from the snapping of the magnetic fields.

Then, that great blob of magnetic fields and plasma, far larger than the Earth, moves into interplanetary space and vanishes. From where it launched from the solar surface, we can estimate whether it is coming in our direction.

When a coronal mass ejection slams into the Earth's magnetic field, it can produce huge magnetic storms, causing power outages, infrastructural damage, high altitude radiation hazards, and on the plus side, auroral displays.

The effect the coronal mass ejection has on us depends on the orientation of its magnetic fields when it hits the Earth's magnetic fields. One orientation produces only minor effects. The opposite orientation produces serious consequences.

However, we cannot deduce this orientation when it is shot off the Sun. Then, for most of its 36-48 hour trip to the Earth, it is invisible.

We next learn about it when it hits solar monitoring satellites lying 1.5 million kilometres sunward of the Earth. Then we learn of the magnitude of the threat, with only15 minutes warning. This last-minute warning is a problem, and is getting a lot of attention, understandably.

  • Mars is high in the southwest after dark.
  • Jupiter and Saturn lie low in the southeast just before dawn.
  • The Moon will reach Last Quarter on the 4th.


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Wandering black holes

There is a really large black hole at the centre of our galaxy, the Milky Way.

It has a mass millions of times that of the Sun. It pulls in anything that comes too close, and the radiation produced as it sucks in material would be a hazard for any life-bearing worlds in the neighbourhood.

Fortunately for us, it lies some 25,000 light years away from us, and is not showing any signs of moving from its central location.

It seems that most large galaxies host one of these supermassive black holes (SMBHs) at their centres. They were probably formed at the same time as their host galaxies, although we don't know that for sure, and have probably grown a bit over time from engulfing material from their surroundings.

Black holes are probably the most bizarre objects in nature. They are the result of matter becoming extremely compressed.

This can happen by making a big enough lump of matter so that its own gravity can compress its core to the point where it becomes a black hole, or more usually, when a big star explodes, and shock waves compress the core.

We don't think the black holes at the cores of galaxies formed in this way. However, a black hole forms when, due to a combination of the amount of matter it contains and the degree of compression, its gravity becomes intense enough to close off a bubble of space-time.

Nothing can get out except gravity, which pulls stuff in on a one-way trip. No light can get out, so the object is completely black. They are completely invisible.

The only way we can detect them is how they interact with their surroundings. Their gravity tugs at nearby stars, and they are surrounded by a huge accretion disc, made up of the disintegrated remains of anything: stars, planets or other bodies, which got too close.

As the material spirals in, it becomes extremely hot, millions of degrees in some cases. This produces X-rays, radio waves and other electromagnetic waves, which we can detect.

We can detect SMBHs in distant galaxies because the huge mass of the black holes in their centres distorts the way their host galaxies rotate.

An SMBH is usually the biggest concentration of mass in a galaxy. It sits still because there is nothing around strong or massive enough to move it. Basically the only known object that would fit the bill is another SMBH.

This agrees with our observation that all the SMBH's in galaxies are sitting happily at their centres. Now we have an exception.

That exception lies in the distant spiral galaxy J0437+2456, which is located some 228 million light-years away.

The galaxy is odd because it seems to be being generally churned up, rather than showing a stable structure like other galaxies, including ours.

However, the oddest thing is that its central black hole is moving at a speed different from the galaxy in which it is located. It is wandering around.

Whether the black hole is moving away from its galaxy or the other way round, we are still posed with the question as to how this could arise. Whatever happened is probably uncommon, since we have observed a very large number of galaxies and this wanderer is the only we have found.

Of course, now we know what we are looking for, more might turn up.

We know that even back in the youth of the universe, when galaxies we see were newly formed, they seem to have had large black holes in their centres. So SMBHs are probably bizarre but normal things to find in the cores of galaxies.

Despite this recent discovery of a wandering SMBH, almost all of them stay home. So the SMBH in our galaxy poses no threat to us, yet.

  • Mars is high in the southwest after dark.
  • You might catch Saturn, Jupiter and maybe, with luck Mercury, low in the southeast just before dawn.
  • The Moon will be full on the 28th.


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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