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Skywatching

A long and winding road

Life here on Earth depends on our supply of heat and light from the sun. A question with a suprising answer is how long does the energy produced in the core of the sun take to reach us?

Let's see. The sun is about 150 million kilometres away, and light and heat travel at almost 300,000 kilometres a second. So it would take about 8.3 minutes. That is indeed how long that light and heat takes to reach us from the surface of the sun, but to get to us from the sun's core, we need to add almost a million years to that answer!
 
The sun has a diameter of about 1.5 million kilometres. The energy is produced in the core, which has a diameter of around 300,000 kilometres. In that part of the sun, the temperature is around 15 million degrees Celsius and the pressure about 250 billion times the pressure at the Earth's surface. Under these conditions atoms come apart and rearrange themselves. The most abundant element in the sun is hydrogen, and under these extreme pressures and temperatures, four hydrogen atoms combine to form an atom of helium, releasing a lot of energy. This nuclear fusion reaction is by far the source of the sun's energy output. Around four million tonnes of the hydrogen is annihilated and turned into energy every second. The rest is turned into helium. The energy is released in the form of photons, pulses of electromagnetic waves. Gamma rays, X-rays, ultraviolet, visible light, infrared and radio waves are all forms of electromagnetic waves, and come in the form of streams of photons.

In high densities in the sun's core, almost immediately after an energy photon is produced, it collides with a particle and gets absorbed. Then it gets reradiated in some random direction. This process of absorption and reradiation goes on over and over again. Try this thought experiment. You are in the middle of a field and you want to leave the area. However, after each step or two you pick a new, totally random direction, take a couple more steps, pick a new random direction and so on. How long do you think it would take for you to get off that field? This sort of moving around is known as a "random walk." This is what those photons in the sun are doing. The part where those photons are random walking around is known as the radiative zone. As the lucky photons work their way outwards, the density drops, so they get to move further before being bounced into some new direction. Eventually, after bouncing around for around a million years on average, they find themselves around 70% of the way to the solar surface. This is an important place because at that level the density has fallen to the point where collisions become unimportant, and another transport process takes over, convection.

Here on Earth, we know that hot air rises. This is because it is warmer than its surroundings and less dense, so it floats upwards. As it rises, it cools. However, as long as it is warmer than its surroundings it will continue to rise. This condition exists in the outer 30% of the sun; from this point the energy is carried upwards as flows of hot, rising gas. This arrives at the surface, the photosphere, a layer a few hundred kilometres thick, which we generally think of as the solar "surface." Here that gas radiates its energy into space, cools, and sinks down again, to the base of the convection zone, where it heats, rises, and repeats the process. The result is a pattern of rising and forming currents, forming convection cells, just like what we see in a pan of heating oil when about to make some french fries. Once radiated into space, those important energy photons are on the last leg of their trip to us: 150 million kilometres in 8.3 minutes.

  • Venus is hard to see, low in the sunset glow.
  • Look for Mars low in the southeast before dawn, and, to its right, Jupiter and Saturn, close together.
  • The moon will reach first quarter on the 29th.




A surprise in the attic

An international team of astronomers may have discovered the closest black hole to us so far.

A thousand light years away might sound like a huge distance, but in cosmic terms it is in our backyard. The object has about four times the sun's mass, and was probably once a star itself.

Massive stars can become black holes; less massive ones, neutron stars; and stars like the sun just end their lives as white dwarfs.

This object lies in the direction of the constellation of Telescopium, "The Telescope," and is only visible from the Southern Hemisphere. That is an odd name for a constellation. Our northern sky is filled up with mythical beasts and heroes from Greek, Arab and other ancient cultures. Cassiopeia, Andromeda, Hercules and Perseus are all there. There are also oddballs, such as Triangulum, "The Triangle."

The southern skies are different. Mixed in with the usual mythical characters such as Centaurus, "The Centaur" is an assortment of constellations that are definitely not of mythical origin. In addition to Telescopium, there is also Microscopium, Antlia (the pump), Fornax (the furnace), Octans (the Octant – a measuring instrument). Horologium (the clock), Circinus (the compass), Mensa (the table) and many others. Triangles are obviously important, so there is one in the southern sky too – Triangulum Australe.

The southern sky has been likened to the attic of a retired scientist. Finally, to go with a collection of old instruments in a dusty attic, there is the constellation of Musca, "The Fly."

The reason for this intriguing difference is quite simple. We, along with the Greeks, Arabs and others who set up our familiar constellations, live in the Northern Hemisphere. There is a good chunk of the southern sky that never rises above the horizon in our northern mid-latitudes, and consequently never got organized. There were some gaps that were filled later, and there was a bit of "tidying up" done later, but usually by astronomers who had a classical education.

The 18th Century was a time of a great explosion in science and the quest for knowledge. Some of that need was practical; because the world was being opened up for trade, navigation was critically important. At that time French astronomer Abbé Nicolas-Louis de Lacaille wanted to measure the distances of the planets. The method he intended to use was a common surveying technique called triangulation. He wanted to make position measurements of the planets compared with the background stars, and to do this from widely separated geographic locations. He picked as an observing site the Cape of Good Hope, near the southern tip of Africa, well south of the equator.
 
As he proceeded southward, more and more unfamiliar sky emerged above the horizon, so he filled his time with organizing it into new constellations. It was he who decided to fill the southern sky with scientific instruments.

Actually though, he was not deviating from our approach to naming constellations. The ancients wrote their contemporary culture in the sky. The culture of the 18th Century was one of an enthusiastic interest in science. Lacaille did was what the Greeks did. He put the current culture in the sky.

Today, many of the devices Lacaille put in the sky, such as the octant, are no longer used, and may indeed find their way into a dusty attic. Calling a constellation "The Table" is unusual though. It  suggests Lacaille was definitely someone with an eminently practical type of mind. 

  • Venus still dominates the western sky after sunset, but is slowly sinking back into the sunset glow.
  • Mercury is there too, but is hard to see. Mars, Jupiter and Saturn lie low in the southeast before dawn.
  • Mars lies further to the left; Jupiter and Saturn lie close together, further to the right.
  • The moon will be new on the 22nd.  


How bad can sun behave?

We depend on the sun. Without it our world would be very cold, dark, and lifeless. It provides our light and heat. Since we are so dependent on it, we are also vulnerable to any fickle behaviour it might get up to.

We are familiar with its more or less regular sunspot cycle, where the level of magnetic activity and consequently the number of sunspots rises and falls over a period of 10-13 years. However, at various times in the past, such as the period 1645-1715, its behaviour changed; its magnetic activity decreased dramatically, sunspots became rare and our star got slightly dimmer, resulting in a cooling climate and weather cold enough for the River Thames in London to freeze over. This is very unusual. Then, around 1715 the cycle restarted and we returned to the behaviour pattern we are familiar with today. 

Solar magnetic activity drives solar storms, great clouds of ionized gas thrown off by the sun (better referred to as coronal mass ejections), and blasts of high energy particles and radiation can degrade or completely disrupt our technical infrastructure. So the big question is whether what we have seen the sun do since we started watching it is typical, or can it get worse.

The sun is 4.5 billion years old and about halfway through its life. What we have seen over a few hundred years and in more detail over a few decades is not really representative. However, as we exploit our planet's resources more completely, and become more and more dependent on technical infrastructure, it becomes increasingly important that we find out.

The sun is a fairly average yellow dwarf star, powered by nuclear fusion – the conversion of hydrogen into helium. Solar rotation and complex flows of extremely hot gas and magnetic fields inside the sun generate enormous electrical currents and in the process generate more magnetic fields. This process is known as the solar dynamo. These magnetic fields permeate the interior of the sun and erupt through the surface and out into space. Patches of strong magnetic field on the solar surface are known as active regions.

In some places the magnetic fields are so strong they interfere with the outward flow of energy, creating cooler patches, known as sunspots. The magnetic fields are the troublemakers. They can change the efficiency with which the sun radiates energy into space and, being elastic, enormous amounts of energy can be stored in them through stretching and twisting. Energy stored over days can be released in seconds, providing the energy for solar flares and coronal mass ejections. We need to know whether the patterns of solar behaviour we know of are typical. Can it get worse, or better? One approach being investigated is to look at sun-like stars. 

The sun is a fairly average yellow dwarf star, and our neighbourhood in the Milky Way contains a fair number of similar stars, some older, some younger, and some of these stars have been monitored for many years. Some of them show similar magnetic activity cycles, and some don't. Maybe these stars are doing what the sun did in the late 17th Century. Interestingly though, it looks as though in general those sun-like stars are more magnetically active than ours has been. 

The engine driving the magnetic activity is driven by the sun's rotation. For some reason, as yet unknown, the sun is rotating more slowly than those other sun-like stars, which would explain why they are more active, but it does not really help us with the fundamental question, namely how badly can the sun behave. We need to know.

  • Venus still dominates the western sky after sunset, but is slowly sinking back into the sunset glow.
  • Mars, Jupiter and Saturn lie low in the southeast before dawn. Mars is to the left; Jupiter and Saturn lie close together.
  • The moon will reach last quarter on the 14th.




Before the beginning

Current thinking is that the universe started just under 14 billion years ago. 

At that moment everything we see around us in the universe now, including space and time, was all scrunched into something about the size of an atom. Its density and temperature were so high our current physical knowledge cannot be applied to it. 

Then it started to expand, in an event now known as the Big Bang. As the embryo universe expanded, it cooled and became less dense, so that eventually hydrogen gas could form. This provided the raw material for making stars. The stars, in turn, made most of the other elements as waste products from their energy production. The rest were formed in the supernova explosions with which these early stars ended their lives. After a few generations of stars, the universe contained everything needed to make planets, and us. 

Having a specific start time for the universe raises some difficult questions. What was there before the Big Bang? How do we get a lot of something out of nothing? There must have been something, and where did all that something come from?

Things, including us, are made out of matter. This comes in two forms. There is "ordinary matter," which is what we, our planet, and as far as we know, most of the observable universe are made of. 

The other kind is "antimatter." This is a mirror image to ordinary matter. We know it exists; we have managed to make it using high-energy particle accelerators. If we put ordinary matter and antimatter together, they annihilate each other, producing a flash of energy. This produces a nice picture of making a universe out of nothing; we separate the nothing into ordinary matter and antimatter. As long as we keep them apart we have something. 

It is rather like having no money and borrowing, say, $1,000. You have a thousand ordinary dollars in your pocket, but you also have a debt of a thousand dollars, or if you like, a thousand antidollars. If you bring the dollars and antidollars together, paying off the debt, you will have nothing, but if you keep them apart, you can do all sorts of financial manoeuvring with both the money and debt. That's what investors do all the time. 

If this is the case, our universe should contain equal amounts of matter and antimatter. However, the visible universe is almost entirely made of ordinary matter. We can "balance the budget" if "the universe" does not in fact comprise everything. 

One idea is that the Big Bang was a two-way event, generating an ordinary matter universe expanding in one direction through time and an antimatter universe expanding in the opposite direction. The cash went one way and the debt another, keeping them apart.

One attempt to explain what happened before the Big Bang is that the universe just expands and collapses, repeating this over and over again. We have no proof of this, and the expansion of the current universe does not seem to be slowing down before starting to collapse; it is instead going faster and faster. It looks as though it is going to expand indefinitely, dying finally when the stars run out of fuel and the elementary particles making up the matter in the universe decay. This seems to fit a new idea, proposed originally by science fiction writers but getting a lot of serious discussion today. It is the idea that there is a multidimensional cosmic foam, in which universes start, expand and dissipate, like bubbles. The foam itself would be eternal, unless it is part of something even bigger.

One interesting possibility with the cosmic foam idea is that some of the universe bubbles may be touching each other, just like what we see in a bubble bath. If our universe is touching another, the interface should look a bit flatter than the rest of our bubble.  Searches are under way.

  • Venus still dominates the western sky after sunset. 
  • Left to right, Mars, Saturn and Jupiter lie close together low in the southeast before dawn. 
  • The moon will be full on the 7th.


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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