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Skywatching

Cosmic 'bubbles' are just some of what fills 'empty' space

Blowing cosmic bubbles

Our Solar System is currently moving through a cosmic bubble.

This void in space is about 1,000 light years wide. (A light year is the distance light travels in a year—just under 10 trillion kilometres). And we are close to the middle.

Its existence is nothing to do with us. We just happen to be moving through it. Blowing this bubble started around 14 million years ago. It is still growing.

Space is not empty. Along with stars and planets, there is a huge amount of gas and dust. The material is the raw material from which new stars and worlds are made and it is also where the remains of dead stars and planets end up.

What's in it and what it is doing tells us a lot about the past and gives us an idea of what might be coming.

Stars form from the collapse of a cosmic gas and dust cloud. The star’s birth happens in the densest parts of the clouds, where we cannot see with our optical telescopes. However, over the last decade or two, infrared and radio telescopes have been developed which can see into the clouds and image the forming star and planetary system. However, newly born stars do not stay hidden.

When the star turns on and starts to shine, its heat and its version of the solar wind blows all more finely-grained cloud material away. When the bubble bursts out of the cloud, we get to see the newborn star with our optical telescopes.

Like other stars, the Sun is continuously blowing out a wind of material in all directions. This "solar wind" blows out through the Solar System, pushing back the dust and cloud in interstellar space, forming our own local bubble.

The two Voyager spacecraft, launched in 1977 to explore the outer Solar System, have now left our solar wind bubble and are now out there inside that big, 1,000-light-year bubble. All stars have winds, but some stars surround themselves with bigger bubbles.

The very biggest bubbles are formed at the ends of the lives of giant stars. When these stars run short on fuel, they collapse and then explode, sending debris in all directions at thousands of kilometres a second. This meets the cosmic dust cloud, compressing it and pushing it outwards, and generating huge shock waves.

We find the bubble-shaped remains of these "supernova" explosions all over the sky. They are often hard to see or invisible through optical telescopes but they stand out well at radio wavelengths.

Some time ago, our observatory conducted a massive sky survey, known as the "Canadian Galactic Plane Survey". A mural of the survey's radio image of the sky (the sky as it would look if we could see radio waves) is on the wall in the foyer.

There are many supernova bubbles. There are also many regions where new stars are being born.

The bubble we are moving through is believed to be the result of multiple supernova explosions. This is not hugely unusual.

Stars are often born in groups, or clusters. In some cases the siblings disperse and go their own way. In other cases they stick together. Since they were all born at the same time, they age together, so we might expect that at some point there would be multiple explosions as ageing stars run out of fuel together.

The bubbles formed by exploding old stars can trigger the birth of new stars. When the shock front of an expanding bubble hits dense clouds, it destabilizes them, causing some of them to collapse, forming new stars and planets.

We see this happening on the skin of our bubble. One generation of stars begets the next.

It was not that long ago when the space between the stars was called "empty". Now we know it’s nothing like empty, and what is going on there is one of the most fascinating things in nature.

•••

• Venus and Mars lie low in the dawn twilight.

• Jupiter is low in the southwest after sunset.

• The Moon will be New Jan. 31.



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Cosmics explosions: From solar flares to the Big Bang

Cosmic explosions

There are places in the universe where colossal amounts of energy are stored.

Most of the time, it is slowly released non-catastrophically, but on some occasions that is not possible and it is released explosively. Examples range from solar flares to the birth of the universe: the Big Bang.

Solar flares are the biggest explosions that occur in our Solar System. They are the result of magnetic loops, loaded with plasma, becoming so stretched, twisted or compressed the only way to relax them is in an explosion.

The loops we see in the solar atmosphere are firmly anchored at the "surface,” the photosphere. The stress buildup is due to the loop's anchor points moving around and new loops coming up and crowding against it.

When we stress a plasma with a magnetic field in it, electric currents start to flow. When the currents get too big for the plasma to carry, the loop can become unstable and explode. The energy of tens of millions of hydrogen bombs is released at the same time, producing pulses of X-rays, beams of high-energy particles, and chunks of solar material ejected at thousands of kilometres per second.

Moving up the energy scale, we come to the nova (plural is novae). For centuries astronomers noticed the appearance in the sky of new stars, where nothing was visible before, so they called them new stars—novae. A new star would shine for a while, maybe varying in brightness and then fade from view. The most accepted explanation for these involves two stars, maybe born together, where one is more massive than the other. They orbit around each other fairly closely.

The larger star shines brighter and burns its fuel so quickly that it starts to run out while its partner is still shining happily. The ageing star expands into a red giant, then shots off its outer layers and ends up as a white dwarf, around the size of the Earth, with no fuel left and so compressed that a teaspoonful of its material weighs in at a few tonnes.

All remains quiet until the other star starts to age and expands into a red giant. As it gets bigger and bigger, its gravitational hold over its outer layers gets weaker and weaker, eventually to the point where the other star starts to pull in that material, building up a huge accumulation on its surface.

Eventually this accumulation, which still contains a lot of hydrogen fuel, goes “critical" and explodes. This produces the nova we see in the sky. The pair of stars may be wrecked by the explosion. If they survive, the whole buildup process may start again, building up to another explosion, years or decades later.

The next one up the energy ladder is the supernova. In this case, an ageing giant star runs low on fuel, collapses and then explodes, for days or weeks shining brighter than all the billions of other stars in its galaxy combined.

From here we enter the regime of the truly high-energy universe, the realm of colliding neutron stars (the collapsed cores of really massive stars) or black holes. The amounts of energy released here can be so large the events can be seen from almost anywhere in the observable universe.

Until now, we have looked at things ending with a bang. However, the biggest known bang marked the beginning of everything, the beginning of the universe.

The Big Bang is the least understood explosion of the lot.

All of the explosions discussed earlier can be interpreted in terms of the physics knowledge we have. However, the conditions during the first few millionths of a second of the Big Bang were so extreme they are beyond our comprehension.

This may well remain the case, but what happened after those first few microseconds will keep researchers busy for a long time to come.

•••

• Mars lies low in the dawn twilight, with Venus hiding lower in the glow.

• Jupiter is low in the southwest after sunset.

• The Moon will reach its last quarter on Jan. 25.



The search for rogue planets in outer space

Rogue planets

Imagine a world upon which the Sun has not shone for millions or billions of years, or maybe ever.

It is wandering the spaces between the stars and the only light is distant starlight. It is cold, a few degrees above absolute zero: the temperature at which everything stops. Most gases are frozen solid. There could be a thin atmosphere of helium, the gas with the lowest boiling point. Any oxygen or water, key life-giving substances on our world would be permanently frozen rock minerals on this one.

If life were possible at all on such a planet, it would not be life as we know it. So far, telescopic surveys have found more than a hundred of these "rogue planets". There could be a lot more of them, because they are very faint and hard to spot against a black sky.

This raises a big question. If planets form along with stars, how can they come to be wandering the galaxy alone?

Not that long ago, the only planetary system we had to study was ours: the Solar System. Moving outward from the Sun, we first encounter four rocky worlds: Mercury, Venus, Earth and Mars. Then there is a band of rubble—the asteroids—that never got to form a planet.

Next we encounter four large worlds that are mostly gas, and called, logically, "gas giants”—Jupiter, Saturn, Uranus and Neptune. Beyond them there is a belt of thousands or millions of lumps of stuff left over from the birth of the Solar System.

We had a theory that explained this beautifully. A cloud of cosmic gas and dust collapsed, forming a rotating disc. A randomly moving cloud will have an average movement in some direction, and a degree of rotation, so collapsing into rotating discs is expected, and our telescopes are coming up with numerous examples.

The middle of the disc collapses to form a star, much of the rest collapses to form planets, leaving a belt of leftover stuff surrounding it.

All planets started off with the same recipe. However, the closest planets had a lot of their gases driven off by the heat of the newborn star. The outer planets were far enough away to hang onto their gas.

The asteroid belt came about because the gravitational tugging by the giant planet Jupiter stopped the material coalescing into a planet. It is all rather neat, and makes great sense.

Then we started observing systems of planets orbiting other stars. It turned out that most of them are unlike our system. There are no signs of the nice rocky to gas giant progression we see in our Solar System. There are gas giants orbiting closer to their stars than should be the case, and rocky planets scattered among the gas giants.

Now it looks as though once the disc has collapsed to form a star and planets, there is a period of instability with planets changing orbits in all sorts of strange ways. During this planetary billiards phase, close encounters between two planets could make one move in closer to its star, and the other thrown out of its system.

Another way we could get a rogue planet is through a planetary system having another star pass close by. The gravitational perturbations due to this stellar visitor could send planets - hopefully uninhabited ones - flying off in all directions.

We can estimate the mass of an object in space by measuring the gravitational effects it has on nearby bodies. However, rogue planets are, as far as we can see, completely isolated, and anyway, getting any sort of measurement of a faint dot against a black background is very hard.

The upper limit is around thirteen times the mass of Jupiter. Anything more massive will become a star.

It is hard to tell when we will get a close look at a rogue planet. Over the last few years two bodies we might describe as rogue asteroids passed through our Solar System.

It would be wonderful to have a close look at one, and better still to get a rock or soil sample from one.

•••

• Mars lies low in the dawn twilight.

• Jupiter is low in the southwest after sunset.

• The Moon will be full on Jan. 17.





Where to look for life in the Solar System

Looking for other life

One of the main science objectives of the James Webb Space Telescope Project is to search for life on planets orbiting other stars.

Even with this powerful telescope, those extra-solar planets will reveal themselves either as tiny, faint dots lying very close to the stars they orbit, or as minute dimmings as they pass in front of their stars, blocking a tiny fraction of the stars' light.

Taking this into account, how can we make a meaningful search for signs of life on those worlds?

The answer to this question dates back to Sir Isaac Newton, who found that passing light through a prism isolated all the different colour components of the light. His discovery evolved into the modern science of spectroscopy. It makes it possible to look at a distant star and see what it is made of, what its temperature happens to be, and whether it is moving towards or away from us.

Matter is made up of atoms. The atoms of different elements are made up of different numbers of protons, neutrons and electrons. This gives them a distinct light signature. Two or more atoms can combine to form molecules. For example, a molecule of carbon dioxide consists of one carbon atom and two oxygen atoms. A molecule of water is made up of two hydrogen atoms and one oxygen atom.

An oxygen molecule consists of two oxygen atoms joined together. Molecules too have their own unique light signatures.

Stars are generally too hot for molecules to exist for very long. Planets are cooler, so molecules can exist in their atmospheres and on their surfaces. When we observe the light from a star, we see the signatures of atoms.

When one of a star's planets passes in front of it, some of the starlight passes through the atmosphere of the planet—if it has one—and in doing so picks up the signatures of molecules in the planet's atmosphere. So, as that planet moves in front of the star, we see the signatures of molecules, and when the planet moves out of the way, we see only atoms. This gives us a pretty reliable way of finding out what the planet's atmosphere is like.

The trick for searching for life is to look for the signatures of gas and chemical molecules in the planet's atmosphere that should not be there. For example, we can search for oxygen molecules.

Oxygen is a very highly reactive material. It would very rapidly disappear from a planet's atmosphere by reacting with iron and other materials in rocks and soils.

The reason there is so much of it in our atmosphere is that living things are making it, in quantities large enough to replace any loss.

Another reactive gas that would be an indicator of life on a world would be chlorine. This gas, deadly to us, would disappear rapidly if not continually topped up.

The question is what stars to point the telescope at in our search for life. A start would be stars like the Sun. We know that star supports life.

The Sun and the planets of the Solar System formed some 4.5 billion years ago. Living creatures appeared about 3.8 billion years ago. If we assume that on average, this is the amount of time necessary for life to appear on a planet and evolve into fairly advanced forms, than stars with shorter lives, which includes the blue and white ones, will not qualify. Maybe red dwarf stars would be ideal candidates. They might be dim, so their planets would have to lie close to them, but they are so stingy in their use of fuel that they are able to shine steadily for tens of billions of years.

Worlds orbiting these stars could have a long time to develop life and have it evolve, and even restart if the living creatures on those worlds wreck their environments, wiping themselves out. Our Sun will start running out of fuel in about three billion years, so if we mess up, there won't be time for a restart.

•••

• Mars lies low in the dawn twilight.

• After sunset, Venus lies close to the southwest horizon, with Saturn to its left and then Jupiter.

• The Moon will reach first quarter Jan 9.



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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