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Skywatching

Solar wind a brisk breeze

Whether we have been lucky enough to see a total eclipse of the sun, or have just seen pictures, the most striking thing is the pearly, streamers and loops of the solar corona. 

It is always there, but swamped by the glare of light from the solar disc. However, when that is blocked by the moon, we see the corona extending off far into space. 

Our world lies inside the outer reaches of the solar corona. This part of the sun is hot, over a million degrees Celsius. The "body" of the sun, which provides us with heat and light is cooler, a mere 6,000 Celsius. 

The corona is hard for us to see because it is very rarefied, and also so hot that it does not produce visible light; it glows with x-rays. This raises a fascinating issue. 

If the bottom of the corona is touching the 6000 C sun, and the top of the corona merges with the cold of interplanetary space, how can it be so hot? The laws of thermodynamics say that heat flows from hot objects to cool ones, not the other way round.

Physicist Eugene Parker pointed out something even more intriguing. The solar corona cannot be stable. It has to be flowing out into space at hundreds or even thousands of kilometres a second. 

We now know that Parker was right, and call this continuous outward flow of hot, ionized gas and magnetic fields the solar wind. Usually, it is a "brisk breeze," but sometimes an instability in the solar magnetic fields catapults a huge mass of material off into space at high speed. 

These masses, called "coronal mass ejections" can cause us serious problems on Earth.
 
Almost all stars have their equivalents of the solar wind. Some are merely light breezes, while others emit blasts that extend far into space, blowing bubbles in the gas and dust clouds between the stars, which we can see with our telescopes. 

In some cases, a pair of stars that are close together, orbiting one another, have a sort of wind competition. A shock wave forms where the two winds collide, causing all sorts of interesting high-energy physical processes to happen.

As stars age, most of them swell up into red giants. Since they have the same masses as they had before, they become less dense, and the outer parts of the star, being now much further out, have less gravitational attraction holding them down, so they start to flow out into space as a particularly dense wind. This rate of mass loss can be quite high. 

A typical isolated star, like the sun, will swell up, eject a lot of its mass, leaving its core as a white dwarf star, about the size of the Earth, but so dense a teaspoonful would weigh tonnes. 

If the star is, say, several times the mass of the sun, it goes unstable, eventually blowing up. Things get more interesting if that star has a companion star orbiting it, forming a double-star system.
 
Typically, when a double-star system forms, one sibling grabs more material than the other.  

In a sort of cosmic justice, greedier stars burn brighter and age faster, swelling up into red giants while their less massive siblings are still enjoying their longer maturity. 

In this case, the companion star captures material blowing off the red giant. Eventually, the red giant loses enough material to end up as a white dwarf. 

In the meantime, the star that collected the material is now massive enough to brighten and age faster, so that it eventually swells into a red giant and starts losing material at a faster rate. 

Some of this falls onto the surface of the white dwarf that was originally the greedier star. If this happens fast enough and enough mass is collected, instabilities develop which result in a nuclear fusion explosion, blowing the originally more massive star apart. 

What's left is either a neutron star, a few kilometres in diameter, or a black hole, with the originally less massive star orbiting it. The companion star might survive.

  • Venus lies low in southeast before dawn.
  • After dark Mars is low in the south-southwest. 
  • The moon will be full on the 22nd


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Using radio to see the sky

As you come onto our observatory site, you get a good view of the assortment of antennas that act as the signal collectors for our radio telescopes. 

In front of you, in the distance, near the laboratory and workshop buildings, is the 26-metre dish antenna. On the right lie the four huge troughs of the antennas for the CHIME (Canadian Hydrogen Intensity Mapping Experiment) and on your left, the railway track and seven nine-metre antennas making up our Synthesis Radio Telescope. 

These antennas, spread over a 600 metre east-west line, form a radio camera, making images of the sky as we would see it if we could see radio waves. 

Recording an image takes about 10 days, with the antennas being moved to different positions each day. The result is a radio image with about the same level of detail at radio wavelengths as we get with the unaided human eye, using visible light.
 
So, why do we bother with radio telescopes?

We do it for the same reason as we do astronomy at infrared and other wavelengths. Different wavelength ranges give us completely different views of the universe and what is going on in it.

Electromagnetic waves, which include gamma rays, x-rays, ultraviolet, infrared and radio waves, are distinguished only by their wavelength. Gamma rays have the shortest wavelengths and radio waves the longest. 

All these waves come in little packets called quanta, which cannot be broken into smaller bits. The amount of energy needed to make one quantum of electromagnetic waves depends on its wavelength. 

If the required energy to make quanta with a particular wavelength is not available, those quanta will not be produced. Radio quanta have the lowest energy of all, and can be produced in regions with insufficient energy to produce anything else.
 
A good example is the cold, dark material forming the dark lanes and blobs in the Milky Way.

We can only see this material optically because it is sitting in front of stars and glowing clouds of gas. In fact, there is a lot of that dark, cold stuff out there, and radio telescopes can detect and image it. 

We have found that this material makes up most of the stuff in our and most other galaxies. It is the raw material for making new stars and planets, and its gravity has a large say in what goes on, so we are very interested in mapping and studying it and how it behaves. 

At other radio wavelengths, the gas and dust clouds are transparent and we can see what is happening beyond. There are lots of distant, high-energy objects that give off radio waves as well as light and other waves, which we can only see with radio telescopes because our optical view is blocked by clouds of gas and dust.
 
Probably the biggest asset to radio astronomy is the radio emission produced by cold, cosmic, hydrogen gas. Hydrogen atoms are the simplest in nature; they consist of a single proton with a solitary orbiting electron. 

Occasionally, due to starlight or other causes, the electron flips over, and when it flips back, it gives off a quantum of radio emission, with a wavelength of 21 centimetres.

Since there are a lot of hydrogen atoms out there, this emission can be detected by radio telescopes and the clouds of hydrogen gas imaged. 

This is one of the main purposes of our Synthesis Radio Telescope. Over the last 10 years or so, the hydrogen clouds in our part of the galaxy were mapped as part of the Canadian Galactic Plane Survey. 

The project also included mapping the hot clouds of gas near new-born stars and counting the radio galaxies and quasars lying far beyond our galaxy.

During the last couple of decades, we have found that many of the molecules forming and reacting in those cold, dark, gas and dust clouds have detectable radio signatures, and many of them play roles in the chemistry of life.

  • Venus lies low in the dawn glow. 
  • After dark Mars is low in the south.
  • Saturn is low in the southwest. 
  • The moon will reach first quarter on the 15th.


Redder than red

In the 18th Century, Isaac Newton passed white light through a prism, splitting it into a rainbow of colours.

This happened because the different wavelengths of light, which our eyes and brains interpret as colours, are bent by different amounts when passed through a prism. 

At the beginning of the 19th Century, William Herschel repeated Newton's experiment, but he went a step further. He scanned along the spectrum of rainbow colours with a thermometer.

The bulb of the thermometer was blackened with carbon powder so it would absorb any light falling on it. 

He found, probably as expected, that the various colours all warmed the bulb, but then found something odd. When he held the thermometer beyond the red end of the spectrum, where nothing was visible, it showed a temperature increase. It was being warmed by some sort of invisible light. 

This light became known as "infrared" (below red, or "redder than red"). The experiment showed that what we call radiant heat is actually a longer-wavelength form of light.  

One of the most scientifically useful aspects of this infrared light is that it is emitted by bodies as cold as a degree or so above absolute zero (-273 C). Everything in the universe is warmer than this.

Even the fading glow of the Big Bang — the beginning of the universe almost 14 billion years ago — is mainly infrared.

When we look at an object like the moon with our eyes, or using binoculars or an optical telescope, we see it because it is lit up by the sun. When the moon is not full, we see only part of the half of it that is lit by the sun. 

If we look at the moon using an infrared telescope, or with a radio telescope operating at centimetre or shorter wavelengths, which is essentially long-wavelength infrared telescope, we see the whole lunar disc, because we are seeing energy radiated from the moon itself. 

The moon's average temperature is about -50 C, which is warm enough to glow brightly in the infrared. The sunlit part glows brighter because the surface is a bit warmer.

We can use these measurements as a long-distance thermometer to measure the temperature of astronomical bodies. 

We can follow how the temperature of the moon's surface changes during the lunar "day" and we can measure the temperatures of asteroids, and other objects.

Have you ever noticed that when driving in fog, during the day or night, orange-tinted glasses often help you see things more clearly? This is because the water droplets in mist or fog scatter the light, making it glow and blocking our vision. 

The amount of scattering depends very strongly on the wavelength of the light. Blue light has half the wavelength of red light and is scattered about 16 times more seriously.

By blocking out the more seriously scattered blue light, we can often see better, depending on the size and concentration of the water droplets.

Space is filled with a fog of gas and dust. If we look low in the south on summer evenings, we are looking in the direction of the centre of our galaxy. However, thanks to the dense clouds of gas and dust in that direction, we can only see a small fraction of the way there. 

However, if we observe at infrared wavelengths those clouds of dust and gas are almost transparent, and we can see all the way to our galaxy's centre. In the same way, we can look into the birth clouds of new stars and planets and see the material collapsing into discs and then into new "solar systems."
 
Infrared observations show us things we cannot see with visible light, and are now an important branch of astronomy. However, considering the wide range of conditions and processes taking place out there, even then we don't get the whole story; we need to observe at other wavelengths.

  • Venus lies low in the pre-dawn glow.
  • Mars is low in the southern sky in the evening
  • Saturn low in the southwest.
  • The moon will be new on the 7th.


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Birth of a planet

Although we have a pretty good idea how stars and planets form, we have only now got our first image of a newborn planet.

This image was obtained by astronomers using the Very Large Telescope, in Chile. It shows a star surrounded by a disc of gas and dust, with a gap in the disc, and sitting in that gap is a planet. It is a big one, 5-14 times the mass of Jupiter, the largest planet in our Solar System.

The large size of this planet is probably why it could be detected.

Getting this image was a tremendous challenge. When we point a telescope or binoculars at a star in the sky, we see something twinkling, flashing different colours and dancing around. What we are seeing is beautiful but has little to do with the star.

The huge distances stars are from us mean that even through our biggest telescopes they would appear as mere points of light; at least they would if there were no atmosphere.

Putting telescopes in space bypasses the atmosphere problem, but at the moment the only telescopes we can deploy in space are in the small to moderate size range, with mirrors maybe up to three or four metres. The Very Large Telescope consists of four telescopes, each with a mirror 8.2 metres in diameter. In order to minimize the atmosphere problem, it is located on a high plateau.

This helps, but is not a total cure. To image a star and its planets requires an ability to resolve extremely fine detail, and to see something extremely dim - a planet - very close in the sky to something extremely bright - a star.

Even on that high plateau, the shimmering in the image due to the atmosphere is still enough to wipe out fine details in what we try to observe. If this were the end of the story, then there would have been no point in making this telescope, because the atmosphere would prevent the instrument ever reaching its true imaging potential.

What made the project worthwhile is a technique known as adaptive optics.

This is easy to visualize but technically extremely challenging to actually make happen. If we are looking at a star in the sky, we can predict what its image should look like through our telescope. However, the turbulence in the atmosphere makes it look like something else altogether.

So we add a very flexible mirror to the telescope, which has a lot of computer-controlled actuators on the back of it. The computer then rapidly adjusts the actuators to correct that star image to make it look the way it should, and in the process, the rest of the image is corrected too.

If there is no suitable reference star, we shine a laser into the Earth's upper atmosphere to emulate one. In order to make the planet visible despite being close to its "sun", a blocking disc was used to block out the starlight, rather like using our hands to block out the Sun's glare on a sunny day. The image shows a planet that has swept out a clearing in the disc of material around the path of its orbit. From how far it has got in doing this suggests the planet is no more than about 5 million years old.

Our Earth is about 4.5 billion years old. This very young system supports an idea that astronomers have been discussing for a while, namely that when giant planets form, they slow down and ultimately limit the growth of their star.

They do this by taking up material that would have become part of the star, and then gravitationally interfering with the spiralling in of material the star would otherwise have captured. That young planet is already much bigger than Jupiter and is still growing.

An interesting question is whether it will grab enough more material for it to get promoted from giant planet to red dwarf star. However, we won't find out for at least a million years or so.

Saturn lies low in the southwest after dark and Mars is still conspicuous in the southern sky overnight. The Moon will reach Last Quarter on the 31st and be New on the 7th.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9. Tel (250) 497-2300, Fax (250) 497-2355 E-mail: [email protected]
 



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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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