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Skywatching

The time when our fundamental understanding of the universe changed

Celestial discoveries

A few centuries ago, many believed our planet was the centre of creation and everything we see in the sky orbits around us.

Then, we found our world is just one of a number of planets orbiting the sun. When, in 1785, William Herschel made the first ever map of the Milky Way, he assumed we live in the centre. However, by the time of the “Great Debate,” we were further demoted to the suburbs of the Milky Way, well out from the centre, with our sun just one star out of billions.

Our next demotions started to come together in 1920. Before then most astronomers believed the Milky Way was alone in the universe. As telescopes improved, astronomers found an increasing number of fuzzy, spiral things. It was widely accepted this was the spiralling in of material in the process of forming new stars and planetary systems, and was located within the Milky Way. However, by 1920, a substantial minority of astronomers suggested those spirals were other galaxies, lying well outside ours.

The discussion of the two ideas culminated in what became known as the "Great Debate", when, on April 28, 1920, the two factions got together to argue it out. Some, including Harlow Shapley, hung onto the idea those spirals lay within our galaxy. Others, such as Heber Curtis, maintained the spirals were other galaxies, "island universes" in their own right.

Over the following decade, the evidence of the "other galaxies idea" proved correct and became conclusive. The universe was filled with galaxies, some larger than ours, some smaller, extending out into space as far as we could see at the time. Our concept of the universe had fundamentally changed. We now orbited a star that was one of billions, in a galaxy that is just one of billions. The picture changed again in the early 1930s, when evidence started to appear that the matter making up us, and the observable universe, is actually a very minor ingredient. The main ingredient is invisible.

Astronomer Fritz Zwicky found himself a puzzle. It was 1933, and he was observing a distant cluster of galaxies known as the Coma Supercluster. The cluster lies about 321 million light years away, far outside our galaxy. Zwicky measured the speeds the galaxies were orbiting around each other, and how far apart they were. He estimated the numbers of stars and from these he obtained a value for the masses of the galaxies.

Newton's theory of gravitation ties masses, separations and orbital speeds together. If we have two of these quantities, we can calculate the other. So Zwicky used the orbital speeds and separation distances to calculate the masses. In measurements and calculations like this we expect there to be errors or discrepancies. However, the estimated mass of the cluster was about ten times larger than he estimated from what he could see.

The galaxies were moving so fast that the gravitational attraction of the visible matter could not hold the cluster together. This discrepancy was far too big to be an error. There was something fundamentally wrong. He concluded there was a lot of matter - some 90% of the matter in the cluster - that was invisible. Being a native German speaker, he called this unseen matter "Dunkle Materie" , which in English means "Dark Matter".

Since the 1930s, we have found countless galaxies extending out into space and back in time to around a billion years after the Big Bang, which happened just under 14 billion years ago. Dark matter is a major ingredient in galaxies. Moreover, it seems that without this massive fraction of dark matter, the galaxies would not even have formed.

That decade between the early 1920s and 1930s, between the two World Wars, fundamentally changed our view of the universe.

•••

• Venus and Mars lie close together in the west after sunset.

• Saturn rises in the early hours, and Jupiter appears low in the sky before dawn.

• Mercury lies very low and is hard to see in the dawn glow.

• The Moon will reach its last quarter on June 10.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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





Long, long ago, in a distant galaxy

Death of stars from the past

Twenty-one million years ago, a star in a distant galaxy exploded.

The galaxy, known as Messier 101, is located in the constellation of Ursa Major, "The Great Bear". At the time of the explosion, the earth was in the Miocene period. Huge flows of lava were in the process of covering much of the southern interior of British Columbia, forming a great plateau. Volcanic activity was intense.

As time passed, the light from that stellar explosion spread out in all directions into intergalactic space. It so happens that M101 is 21 million light years from earth, so the light from that explosion - a supernova - has just arrived here.

If you have a telescope, you might be able to see it for yourself. Find the Big Dipper, which is the brightest part of Ursa Major. The star at the end of the handle is Alkaid, and the next star heading towards the bowl is Mizar, with its close partner Alcor. From a point halfway between Alkaid and Mizar, scan upwards about two-thirds the distance between the stars.

This supernova marks the end of a giant star. Stars obtain energy by fusing small atoms, like hydrogen, into bigger ones, like carbon, oxygen and so on. This happens in the cores of the stars where pressures and temperatures are high.

The end comes when there are no atoms left in the core that can be used to produce energy. At that point, the star collapses. Ironically, at that final moment, the star still has lots of fuel available, but it lies in the cooler, much less compressed outer layers.

Extensive study and observations have shown that there is almost no mixing between the core and outer layers, so that fuel is not available for energy production. However, in the collapse it tumbles down into core region and gets compressed and heated by the fall. Runaway nuclear fusion takes place, releasing a huge pulse of energy that helps blow the star apart.

Ironically though, this lack of mixing is extremely useful to astronomers. It means the surface layers of a star, which we can observe, are preserved samples of the material from which the star formed. In other words, each star tells us something about the evolution of the universe.

In the beginning the only elements were hydrogen and helium. These were in the form of giant clouds which over billions of years provided the raw materials to make new stars. Over time, these clouds have been increasingly enriched or polluted by the elements forged in stars as by-products of their energy production.

That means the surface layers of the very oldest stars would contain nothing other than hydrogen and helium. Succeeding generations would have surface layers containing increasing amounts of elements formed by earlier generations. It means we can put together a sort of chronology of generations of stars. Our sun's surface layers show it is not an old star, but not a teenager either. This is good because both very young and very old stars can be unstable. The young ones require time to settle down and stabilize. The older ones are unstable too, because they are running out of fuel.

That supernova in M101 has ejected its material into nearby clouds of gas and dust. We can see nearby parts of the cloud material glowing in the radiation from newly born stars. Of course we are seeing the situation as it was 21 million years ago. By now, that material could be helping to make new stars and planets.

Life on Earth seems to have appeared around half a billion to a billion years after the Solar System formed. So by now, 21 million years later, any new planets forming from the elements released will still be far too young. However, clearly the process of world creation and the possible appearance of living things go on.

•••

• Venus and Mars lie close together in the west after sunset. Saturn rises in the early hours, and Jupiter appears low in the sky before dawn.

• Mercury lies very low and hard to see in the dawn glow.

• The Moon will be full on June 3.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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



How a planet comes to the end of its life

When a star eats a planet

Imagine a planet slowly spiralling in towards its star.

As it gets closer the increasing heat, together with the star's equivalent of the solar wind, blast away the atmosphere. Then, as what is left of the planet continues its inward spiral, it disintegrates into a shower of falling fragments.

For the first time, astronomers have actually observed this happening. The star in question, poetically named ZTF SLRN-2020, lies in the constellation of Aquila, "The Eagle", at a distance of some 12,000 light years. It might be a sobering thought that the Earth will share a similar fate in a few billion years.

Imagine a system of planets orbiting another star. For most of its life, the star, like our sun, behaves itself, shining reasonably steadily, and keeping any life-bearing planets comfortable. Then, like all stars, it starts to run low on fuel. This, paradoxically, causes it to expand into a red giant, and to radiate more energy. Planets that orbit close in find themselves having to shoulder their way through the gas and dust making up the star's extended envelope, instead of moving more or less frictionlessly through a near vacuum.

The drag sucks energy from the planet and its orbit turns into an inward spiral. Ploughing through the gas and dust, together with the increasing heat from the star, gradually strips away the planet's atmosphere and boils away any oceans it might have had, turning it into a dead world. Intriguingly though, the story does not end there.

When we swing a weight on a piece of string in circles around our heads, we feel an outward pull on the string. This is actually the inertia of the weight resisting being pulled into a curved path rather than doing what it really wants to, which is fly off in a straight line. We have come to refer to this outward pull, not completely accurately, as "centrifugal force".

This "force" is related to two things: the speed the object is moving and the diameter of the circle in which it moves. The smaller the circle, or the higher the speed, the stronger the force.

A planet orbiting a star in a more or less circular orbit is moving so that the inward-directed gravitational pull of the star is balanced by the outward-directed centrifugal force.

Actually, the balance of forces occurs only at the planet's centre of gravity. Half the planet is closer to the star than the centre of gravity. The other side is further away. Since the planet is a solid object, all parts of it are moving at the same speed.

Therefore, the outer part of the planet is moving faster than is needed, and the centrifugal force is bigger than the pull of the star's gravity, pulling that part of the planet outwards. Similarly, the half of the planet closest to the star is not moving fast enough, and there is a net inward force due to gravity being stronger than the centrifugal force.

The result is the part of the planet closest to the star is being pulled inward, and the outer part pulled outwards. This stretching force is often referred to as a tidal force, because it is the reason we have ocean tides here on earth.

Normally, as we can see with the planets in the Solar System, this tidal force is too small to endanger the planet. However, the gravitational attraction rises rapidly as we get closer to the star, quadrupling each time we halve the distance.

The result is that, as our ill-fated planet spirals in closer and closer to its star, the tidal force increases rapidly.

Rock is really good at resisting compression, which is why we make pyramids out of it. However, it is not very good at handling stretching. Eventually, for our planet, the tidal forces become too great and the planet disintegrates, with its fragments falling down into the star.

This will probably be the ultimate fate of our planet, although not for billions of years.

•••

Venus shines very brightly in the west after sunset. Mars, much less bright, and reddish, lies a little higher. Saturn, golden coloured and moderately bright, lies low in the dawn glow.

The Moon will reach last quarter on May 27.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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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Biggest mistake of Albert Einstein's life remembered

When Einstein got it wrong

If you have ever seen videos of astronauts in training on the "Vomit Comet,” ridden on a particularly exciting roller-coaster or maybe even tried skydiving, you will have noticed there is something about the force of gravity that is different.

When you are in an aircraft during its takeoff run, or have been a passenger when your friend is showing off the acceleration in his or her electric car, you feel firmly pushed back in your seat.

During the acceleration of a spacecraft heading for space, passengers can feel pushed back in their seats by a force several times their weight. When falling freely, allowing gravity to accelerate you, there is no sensation of weight at all. If your spaceship has no windows, you won't be able to tell if you are floating around in the remote reaches of space or falling earthward at high speed.

That is one of the things that led Albert Einstein to decide gravity was not a force like all the others. One of the main intentions behind his General Theory of Relativity was to come up with a better idea of what gravity might be. He proposed gravity is the curvature of the fabric of space-time by massive objects. Imagine bowling balls and cannonballs sitting on a trampoline.

Since gravity plays a major role in defining the structure of the universe, Einstein applied his concept of gravity to the universe and found something he did not like. Along with many others at the time, he believed the universe as a whole is unchanging and eternal, with planets, stars and galaxies coming and going within it.

His calculations described a universe that wanted to either expand or collapse. The only time his universe would be stationary is the moment between when expansion ceases and contraction begins. He then made what he later described as the biggest mistake of his life. He fudged his calculations to fit his opinion. He added a fudge factor, which he impressively named "Cosmological Constant.” He could adjust the value of this to make his universe static.

At almost the same time, a senior Jesuit priest, Georges Lemaitre, was doing the same calculation. However, he believed the results. He concluded the universe was truly expanding, and if it was, the expansion could be tracked back in time to when everything was concentrated in one tiny lump, which he called the Primaeval Atom.

This, then, started to expand rapidly, in an event we now call the Big Bang, leading to the universe we see around us today.

In 1927, Lemaitre had a chance to present his ideas at an international science conference. When he had a chance to talk to Einstein about them, the Great Man said to Lemaitre "Your calculations are correct, but your grasp of physics is abominable."

This rude and crushing response might very well have been due to Einstein having years of work questioned, and maybe a consequence of too much fame and adulation.

Astronomer Edwin Hubble had been comparing the speeds galaxies are receding from us with their measured distances, and in 1929 he presented his results. The universe is expanding, and the farther away a galaxy lies, the faster it is receding. This result independently suggested there was a point in the past when it was all confined to one small lump. Lemaitre was right. Einstein would have been right too if he had not added his fudge factor.

In the light of these observations, Einstein publicly apologized to Lemaitre and the two men became friends. There is a moral to this story, think carefully about the results of your calculations before fudging them to give the result you want.

•••

• Venus shines very brightly in the west after sunset. Mars, much less bright, and reddish, is a little higher in the sky.

• Saturn, golden coloured and moderately bright, lies low in the dawn glow.

• The Moon will be new on May 19.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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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