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Skywatching

Planets, stars and things between

Creation of celestial bodies

If you want to make a comet, asteroid, planet, star or many other bodies, the recipe is the same. There is just one ingredient: the cosmic clouds of gas and dust.

The procedure is the same, one just gathers some of the material into a lump. What you end up with is entirely determined by how big a lump you are working with. The mass of the lump determines two critical quantities, the pressure and temperature in the core of that body.

The temperature at the core of the Earth is around 5,200 C and the pressure some 3.6 million times the atmospheric pressure at the surface. The pressure comes from the weight of the overlying rock. The internal heat comes from two sources—the energy released by the impacts of incoming objects when the Earth formed, some 4.5 billion years ago, and from the decay of radioactive elements present in the cosmic material.

For a planet the size of ours, the heat escapes very slowly. For smaller worlds, the process is faster.

Imagine that somewhere in a great cloud of cosmic gas and dust, a couple of grains wander into one another, and thanks to static electricity or something else, they stick. The resulting grain is larger, and presents a bigger target for other particles to hit, so it has a higher chance of picking up more particles. Even in these clouds, the density of material is very low, so collisions are rare, but there is lots and lots of time.

As the grain grows it picks up samples of all the chemicals making up the cloud, including hydrogen and other volatiles. Eventually it graduates from being a grain to a lump, and after more time it gets massive enough for a new force to take over in holding the lump together and increasing its growth rate by pulling in more and more of the surrounding material: gravity.

The impacting of new material on the growing lump makes it hot, melting it so that when it gets big enough and its gravity strong enough, it pulls itself into a sphere, maybe a thousand kilometres in diameter. It is now a largish asteroid. Continuing impacts produce more heat. Of course the formation process can stop any time, leaving the object to cool off, and eventually to solidify throughout. However, in our case the growth continues. When it reaches a diameter of several thousand kilometres, it has graduated as a planet.

If our new planet is near enough to a star, the heat from the star will evaporate and drive off most of the gas and other volatile materials, so that we end up with a rocky planet, like Mercury, Venus, Earth or Mars.

On the other hand, if the planet manages to hang onto its gas and volatiles, it can grow into a gas giant planet, like Jupiter, Saturn, Uranus and Neptune. During their formation these planets collected a huge amount of internal heat, so even today, their cores are extremely hot.

Now things get really interesting. If our planet collects material to the point where it exceeds about 20 times the mass of Jupiter, the core pressure and temperature become high enough for some elements, such as deuterium and lithium to undergo nuclear fusion, producing energy. It is no longer a planet and is not yet a star, which obtains energy through hydrogen fusion.

Objects like this one, a not-quite-graduated star, are known as brown dwarfs. These objects show some aspects of star behaviour, such as flaring. Astronomers are very interested in them. If the material keeps coming, and our star reaches 100 or more Jupiter masses of material, we have a new star.

It is amazing what can be done with one recipe, one ingredient, and just changing the amount.

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• Saturn rises soon after sunset, followed a couple of hours later by Jupiter. After another two hours or so, Mars creeps into view, followed, just as the sky starts to brighten for dawn, by Venus.

• The Moon will be full on the 11.

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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James Webb Space Telescope showing how soon galaxies began

Seeking the first galaxies

One of the objectives for the James Webb Space Telescope is to find out how soon after the Big Bang galaxies began to form, and when stars started manufacturing the elements needed for making planets and living things.

The JWST is only now coming into action, but already it is giving us some strong hints. This sort of study is possible because looking out to greater and greater distances takes us back in time. Even though light travels extremely quickly, cosmic distances are so huge that the light from stars and galaxies can take from years to billions of years to get to us, depending on how far away they are.

Expressing such distances in kilometres leads to extremely large numbers, which are hard to visualize. We often express these distances in "light years", which is the distance light travels in a year. It we look at a galaxy located a billion light years away, we are seeing it as it was when that light set out on its way to us, a billion years ago.

By looking at more and more distant objects, we see the universe as it was further back in its history. Telescopes are time machines. Recent observations using the JWST show us galaxies like ours existed a mere 600 million years after the Big Bang, just under 14 billion years ago.

The oldest and most distant thing we can see is the cosmic microwave background, or CMB. This dates back to about 380,000 years after the Big Bang, the beginning of the universe, some 13.8 billion years ago.

The CMB is basically an almost uniform glow over the whole sky, emitted when the universe had expanded and cooled enough for light to travel through. However, when that glow is mapped precisely, we see small temperature variations in it. These mark the clumping of material, collapsing under its own gravity, on its way to becoming the first galaxies.

When the universe became transparent, there were no stars to light it. It was dark. We often refer to the time period between the CMB and the first stars as the "Cosmic Dark Age".

We would really like to know when that age ended and the first stars and galaxies formed. We are doing this by using our ever-improving instruments to work our way outward from Earth and back in time, until we stop seeing galaxies, or hopefully, see the first galaxies and stars actually forming.

Of course, when we see one of those distant, faint galaxies, we need to know how far away it is, and thence how far back in time we are looking. Directly measuring such huge distances is extremely difficult. Fortunately, there is a simple, pretty reliable indirect method. We use the expansion of the universe.

This has been precisely measured over decades of work. The speed a distant galaxy is being carried away from us by the expansion of the universe is directly related to how far away it is. A galaxy twice as far from us as another will be receding twice as fast.

The relationship between speed of recession and distance has been extensively measured and has come to be known as Hubble's Constant.

Measuring the speed of recession of a galaxy is fairly easy, so we apply Hubble's Constant and we have a very good idea of that galaxy's distance. So the search is for fainter and fainter galaxies that are moving away from us at ever-higher speeds, placing them at ever-greater distances. This requires ever better telescopes, like the JWST.

The JWST has only just started operating, so its discovery of normal-looking galaxies in existence a mere 600 million years after the Big Bang is encouraging. Their formation must have got under way very quickly. At this point we still don't know how quickly, because we have still not reached far enough back in time.

Watch this space.

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• Before dawn, Saturn lies low in the south, with bright Jupiter to the left, then Mars and finally Venus, lying low in the dawn glow.

• The Moon will reach its first quarter on July 5.

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



Observatories in space

The James Webb Space Telescope (JWST) is almost at its parking place, 1.5 million kilometres from Earth.

It is in full operation, and sending back beautiful images.

Its 6.5-metre mirror, made up of 18 hexagonal segments, is fully deployed, with those segments all positioned to within millionths of a metre (the spacecraft had to be folded up to fit into the launcher).

The mirror is bigger than the one on the Hubble Space Telescope (HST); it can collect around seven times more light and discern finer detail. It will complement rather than supersede the HST.

Rather than observing visible light, which is the purpose of the HST, the JWST will observe mainly at infrared wavelengths. This makes it better for looking at the formation of stars and planets, and for exploring the remotest regions of the universe.

Putting telescopes in space is a lot of work and is expensive, and nobody will be around if something goes wrong during set-up.

The problems with the HST were addressable because its low orbit made it reachable using the space shuttle. We currently have no easy means to get a service engineer to the James Webb Space Telescope.

Ground-based telescopes can be made larger and the instruments on the telescopes can be easily changed to keep up with evolving scientific needs. If something breaks it can be fixed. So, considering the enormous additional expenses and challenges that go with putting telescopes in space, why do it?

If you look at a star through a telescope, on most nights you will see a blob, dancing around and flashing different colours, whereas you should be seeing a coloured dot. The moon and planets may look as though they are at the bottom of a stream.

The cause of this light show is the curse of ground-based astronomy, atmospheric turbulence. Sometimes, for short periods of time, the atmosphere settles down and we see fine details, but most of the time that turbulence is there.

When we add the time lost due to cloudy nights, our ground-based telescopes can only be used effectively for part of the possible observing time.

Putting telescopes on mountains or dry, high plateaux, above some of the atmosphere and some of the weather, helps a bit. However there is no substitute to lifting the telescope above the atmosphere, into the vacuum of space.

Putting a telescope into space adds years of lead-time to the project. If we cannot easily change or add instruments, we have to launch it with a set of instrument packages that will keep it scientifically valuable for decades. If critical systems cannot be fixed, they must have backups that can be switched in.

There must be as much operational flexibility as possible, in order to accommodate new experiments that could not be foreseen before the telescope was launched. In addition, everything has to work productively for a lifetime of decades. The instrument has to survive the vacuum, temperature extremes and radiation environment of space.

Finally, this highly complex instrument must fold up small to fit into the launch vehicle, survive the shaking and acceleration of being launched, and then, when in orbit, unfold, deploy the instruments, power up, and go to work.

Not many years ago, we would never have thought we would be able to probe the era in the early history of our universe where first structures started to form, or to look at the composition of the atmospheres of planets orbiting other stars.

Now, if we find a planet with significant amounts of oxygen and water vapour in its atmosphere we will be pretty certain we are not alone.

At a more human level, the images starting to come back from the JSWT underline what we have seen in the thousands obtained by the HST; our universe is a beautiful place.

  • Before dawn, Saturn lies low in the south, with bright Jupiter to the left, then Mars, and finally Venus, lying low in the dawn glow.
  • The Moon will be New on the 28th.

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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Radio astronomy has its roots in early telecommunications

The birth of radio astronomy

Back in the 1930s, radio communication was opening up the world.

For the first time it was becoming technically possible for a person in any country to communicate with anyone else on the planet. Companies were investing hugely in services to provide publicly available communications.

However, experience showed shortwave radio (using wavelengths between around 100 and 10 metres) was vulnerable to interference and occasional radio blackouts. Nobody was going to invest in, or use, an unreliable communications network. So companies started researching what affected their communications and to identify possible work-arounds.

One of the companies working on this new communications frontier was Bell, in New Jersey. To help it design the new system, it gave one of its engineers, Karl Jansky, the job of identifying sources of interference that could affect the new services. To do this, he built an antenna that was mounted on wheels from a Model T Ford car, so it could rotate, scanning the horizon to see where the various forms of interference were coming from.

He attached this antenna to a then state-of-the art radio receiver and started work.

Over the following months he identified the radio static from thunderstorms and other natural phenomena and the countless forms of manmade interference. Intriguingly, he found when this interference was absent, he could hear a steady hiss, which went away if he disconnected the antenna.

He scanned with the antenna and found the direction in which the hiss was strongest and found, to his surprise, during the day the interference peak moved from east to west. After months of work he concluded the culprit was the Milky Way, with the strongest signals coming from the direction of the constellation Sagittarius.

Jansky had discovered cosmic radio waves, that the Milky Way is a radio source and the strongest emissions come from the direction of the centre of our galaxy.

One would expect such a discovery would rock the astronomical community because, at the time, all astronomical observations were being made using optical telescopes. However, the attitude of the astronomers was that radio engineers should stick to playing with their radios and stop pretending to be scientists. Fortunately, Jansky's discovery attracted the attention of a radio amateur named Grote Reber.

Reber had successfully talked to radio amateurs in all the continents of the world and was looking for a new challenge. Jansky's discovery fitted the bill, and in his backyard in Wheaton, Illinois, Reber made the world's first radio telescope—a dish antenna almost 10 metres in diameter, made of wood with a reflective surface of galvanized steel plate.

After trying one receiver design after another, covering a wide range of wavelengths, he finally detected the emissions Jansky reported. However, Reber had a great advantage, his antenna was designed to nod up and down, so he went on to make the first radio map of the sky, clearly showing radio emissions from the Milky Way and that they were brightest in the direction of the centre of our galaxy.

Surprisingly, the astronomical community gave Reber the same response Jansky received.

Ironically, it was the Second World War that changed everything. Scientists and engineers were thrown together to develop new devices for the war effort, and in the process a mutual trust developed and the earlier radio astronomy results, and some new accidental discoveries made during the hostilities, established the new science.

When the war was over, the new technologies were used to make radio telescopes and radio astronomy was off and running.

•••

• Before dawn, Saturn lies low in the south, with bright Jupiter to the left, then Mars, and finally

Venus, lying low in the dawn glow.

• The Moon will reach its last quarter on July 20.

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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