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Skywatching

The connection between microwave ovens and astronomy

Microwaves and astronomy

It is very likely that when you are heating up some food in a microwave oven, you never thought you were using a descendent of the instrument that led to the birth of radio astronomy in Canada. However, that is the case.

The device making your microwave oven possible is the resonant cavity magnetron.

It is small but generates large amounts of radio energy with wavelengths of a few centimetres, just what is needed to warm up yesterday's pizza. Today, magnetrons are small, energy efficient and cheap. However, in the 1940s, they were a carefully guarded military secret that changed the course of the Second World War.

At the start of the war, radar was an important component of defence systems, being used to detect distant ships and aircraft in daylight, fog and during the night. Radar sends out short, intense pulses of radio energy. They reflect off distant metallic objects and can be detected. The time interval between the transmission of a pulse and when the echo is received gives the range of the target and the direction the antenna is pointing at the time the echo is received. However, to determine that direction accurately requires the antennas to be as large as possible compared with the radio wavelength used.

That was the big problem. Technologies in use at the time could not produce the required high radio powers at wavelengths, much less than a metre or so. That was not a problem for fixed, ground-based radar systems. It was a problem for mobile ground systems and ship-borne radars. For aircraft it was a very big issue. Radar-equipped night fighter aircraft looked like hedgehogs, covered with antennas that were big enough to give at least a hint of the direction in which the target lay.

In 1942, Britain was struggling to defend itself, and did not have the resources to develop weapon systems based upon the latest developments. It was decided the British military secrets would be passed to the United States and Canada, where the expertise and resources were available to develop those much needed military aids. One of those was the resonant cavity magnetron, developed by Randall and Boot at Birmingham University. This device could develop huge levels of radio energy at very short wavelengths, down to a centimetre or so. This was just what was needed for the implementation of practical radar systems.

Some magnetrons were sent to the National Research Council, in Ottawa, which led to that organization becoming a major centre of radar development. Magnetrons made possible radar systems using dishes as small as 60 centimetres, which could fit in the nose of an aircraft, and mobile radars with dishes with diameters between one and two metres. The magnetrons from Britain operated at wavelengths of around 10 centimetres, so that wavelength was used for radar development.

When the war ended, the National Research Council found itself with lots of radar components and systems that were no longer needed. Arthur Covington and his colleagues, who worked on wartime radar development, heard about the discovery of cosmic radio waves and used those radar components to build Canada's first radio telescope. They based their design on the receiving portion of an unused radar. Because the magnetron in the transmitter operated at a wavelength of 10.7 centimetres, so did the receiver, which meant it became the operating wavelength of that radio telescope.

Among other things, that led to the beginning of a solar radio monitoring programme, which continues to the present day, giving the world the F10.7 index of solar activity.

Enjoy that pizza.

•••

• Jupiter lies low in the dawn glow, with Mars and then Saturn higher and further to the west.

• The Moon will reach its last quarter on June 28.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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Little green men sending messages or just radio waves in space?

The sounds of space

If you ever see a radio astronomer wearing headphones, he or she is probably listening to music while overseeing the computer controlling the observations, not what the radio telescope is receiving.

The reason is almost all cosmic radio sources emit a steady signal that may vary in intensity over months, years, centuries or millennia. What you would hear in the headphones is a steady hiss. The radio astronomer is measuring the properties of the signal responsible for the hiss, and in many cases imaging its source.

For much of the history of radio astronomy, there were only two cosmic radio sources worth listening to—the Sun and the planet Jupiter.

Solar flares produce bursts of radio emission that sound like storms on the seashore. Those bursts, called "noise storms", can be picked up by backyard radio telescopes and enjoyed by amateurs. Jupiter's radio bursts can be picked up by means of short wave radios.

The distant radio sources of interest to professional radio astronomers would just produce a steady hiss in the headphones; not worth listening to apart from when making sure the system is working, or identifying interference.

At least, that was the situation until 1967. That was when Jocelyn Bell, a student at Cambridge University, was making observations for a rather unusual project. Just as turbulence in our inhomogeneous atmosphere makes stars twinkle, turbulence and inhomogeneities in the solar wind can make some cosmic radio sources twinkle.

That required the radio telescope she was using to be designed to observe radio emissions that were fluctuating over seconds or less. Around Cambridge, a lot of manmade interference was inevitable, but there was one signal that struck her as odd—sharp pulses of signal recurring every 1.3 seconds, precisely.

Her first thought was this signal was just a bit more manmade interference. However, over a few days it became clear the signal was coming from space. Not entirely humorously, the source was referred to as LGM-1, standing for “Little Green Men 1.”

Some suggested, maybe light-heartedly it could be a lighthouse for interstellar travellers. Many more of these sources have been discovered since , and have revealed themselves to be rotating neutron stars emitting beams of radio waves. We see a pulse every time a beam points in our direction.

They are not evidence of “little green men,” however the discovery awoke us to the possibility of rapidly changing radio emissions from deep space.

Pulsars continue sending their pulses for long lengths of time, so they are comparatively easy to search for. However, in 2007 a radio telescope was pointing in just the right direction to catch a much more elusive event, a “Fast Radio Burst” (FRB).

These last just milliseconds, and if the radio telescope is not pointing in that direction at the time, they are missed. Things changed with the advent of radio telescopes that can observe huge chunks of sky at a time, such as the CHIME radio telescope at our observatory, near Penticton and the ASKAP radio telescope in Australia.

Now, thousands of FRBs have been observed, with a few of them repeating. At this point, we have no idea what they are. To be detectable over huge distances the energies involved must be enormous. Neutron stars are a possibility. Little green men are not seen as a serious possibility…yet.

The ASKAP radio telescope has come up with something even weirder. Long, slow pulses that repeat over an hour or so, with some of the pulses being strong, others, weak, and sometimes missing altogether. It could be some sort of data transmission by those little green men.

But the most widely supported idea is that we are looking at a rather odd sort of neutron star.

•••

• Jupiter lies low in the dawn glow, Mars a bit higher and to the west and Saturn a bit higher and further west.

• The summer solstice will be on the June 20.

• The Moon will be full on June 21.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



Measuring the universe through distance and speed

How big is the universe?

At this point, we don't know how big the universe is. However, a question we can address is how big is the chunk of universe we can see.

That is basically a measurement problem. It is not an easy one, but one we have been making good progress with over the last century or so. It comes down to two sets of measurements: distance and speed.

For nearby stars in our galaxy, we can determine how far away they are by using the classical method of triangulation, as used by surveyors for centuries, if not longer. Unfortunately, for anything lying outside our galaxy, that method is useless.

Luckily, there are variable stars (such as cepheids), where measuring the time-scale of the variability enables us to calculate how bright the star would look at a standard distance. Then, comparing this with how bright the star looks enables us to determine how far away that star is. Spotting these stars in distant galaxies enables us to estimate how far away they are.

Then there is a type of supernova explosion, which always liberates more or less the same amount of light. These are produced by two stars orbiting closely around one another. One ages first and becomes a white dwarf. This then snacks on material from the other star. When a critical mass of this is captured and accumulates on the white dwarf's surface, it explodes, producing a "standard flash". If we spot these in distant galaxies and measure how bright they look, we can calculate the distance. Even further away, there are galaxies that have more-or-less standard brightnesses, extending our "cosmic ruler" out to even greater distances.

Measuring the speed with which distant galaxies are being carried away from us by the expansion of the universe is comparatively easy. The elements making up stars and other structures in the universe have specific light signatures. If the source of the light is receding from us, then the colours compared with the light emitted by those elements in the laboratory are reddened by an amount dependent on the speed the source is receding from us. So we can look at distant galaxies and get their distances and the speeds with which they are being carried away by the expansion of the universe. These data indicate that at some point in the past everything in the universe was together in one dense lump. From the speeds and distances we can estimate when that lump started to expand, in an event now known as the Big Bang. It happened about 13.8 billion years ago.

Since cosmic distances can be huge, light from distant objects can take up to billions of years to get to us. We see a galaxy a billion light years away as it was when the light we are seeing set out on its journey to us, that is, a billion years ago.

Over the time the light was travelling to us, that galaxy has been moving further and further away, carried by the expansion of the universe. The James Webb Space Telescope has detected galaxies that existed 300 million years after the Big Bang.

That means, the light just observed is some 13.5 billion years old, from a distance of 13.5 billion light years. Since then those galaxies have continued to recede from us. So how far away are they now? This depends on the rate of expansion of the universe and how that expansion changes with time. The best estimate is that today, due to its expansion over the last 13.8 billion years, the edge of the observable universe now lies some 46.5 billion light years away.

That 46.5-billion-light-year distance refers to the “Observable Universe,” where the most distant things we can see in the sky lie today. We have no way of knowing what, if anything, lies further away than that. Of course, to see what the most distant galaxies look like today, in 2024, we are going to have to wait a long time for their light to reach us.

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Just before dawn, look for Mars low in the dawn glow and Saturn a bit higher and further west. The Moon will reach its first quarter on June 13.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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The life and death of stars

Star death, star birth

In the year 1054, observers around the world noticed a new star in the sky.

Over days, it brightened until it was visible during the day and then, during the ensuing months, it faded to invisibility. Today, telescopes reveal an expanding cloud of gas. In the core of that cloud is a strong radio source and a pulsar—the remains of the star—compressed down to a diameter of a few kilometres, rotating rapidly and radiating beams of radio waves and X-rays.

That expanding cloud is now known as the Crab Nebula. It lies some 6,500 light years away. The event the ancient astronomers observed was a supernova, the collapse and explosion of a dying giant star.

In the years after the Second World War, many countries built radio telescopes. They showed bright radio emissions from the Milky Way, and a number of bright sources. The Crab Nebula was one of them. Back then, most radio telescopes were huge dishes, like the 75-metre diameter dish at Jodrell Bank in the U.K. Those telescopes could determine the direction the signals were coming from, and by scanning them to and fro, make radio maps of the sky. However, those big dishes weren't big enough to see structural details in those sources.

Today things have changed. Although it is still technically very difficult to make large, single-dish radio telescopes, we can make far bigger radio telescopes out of large numbers of small, relatively cheap antennas. Thanks to modern signal processing and imaging techniques, we can make radio images of large areas of the sky with better detail than the human eye or even many optical telescopes can achieve.

What those technical developments are showing us is stunning. We now have "radio eyes", and can see how the sky would look if we could see radio waves instead of light. Where our optical telescopes would show black space, stars and maybe a wisp or two of cloud, the radio view shows huge, complex clouds and other structures.

There are particularly dense clouds where new planetary systems are forming. Most of this material has been around since the beginning of the universe. Scattered in these clouds we see the remains of numerous exploded stars. Some of those remains, such as the Crab Nebula, have complex shapes. However many are more or less spherical bubbles, voids with glowing skins. When the star exploded, it blew away the surrounding cloud material, which piled up against the surrounding cloud, forming the glowing shell. Sometimes we see a pulsar or some other remnant of the dead star in the middle.

Supernovas are endings, but they are also beginnings. That expanding shell of material ejected by the exploding star contains the waste products from a lifetime of energy production, which includes all the elements from the lightest to iron. In the explosion all the heavier elements were produced.

This material goes out and mixes with the clouds, providing the raw materials for planets and people. The explosions also cause something else. They can destabilize clouds, making them collapse to form new stars and planets.

Those clouds exist in an environment of magnetic fields, radiation pressure from stars and the interplay of weak gravitational fields. As long as these more or less cancel out, the clouds remain, slowly changing shape over time, maybe for millions of years. However, if that balance is disturbed by causing a little bit of a cloud to increase in density, that concentration starts to gravitationally pull in other material, leading to a local collapse and the formation of new stars and planetary systems.

An ideal agent to make these density enhancements and local compressions of the cloud material is the supernova explosion. Without them the universe would be very different.

•••

• Just before dawn, look for Mars low in the dawn glow, and Saturn a bit higher and further to the west.

• The Moon will be new on June 6.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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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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