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.

Biggest mistake of Albert Einstein's life remembered

When Einstein got it wrong

A really good theory but...

How planetary systems form

Until a few years ago we were pretty confident we knew how planetary systems form around stars.

A cloud collapses into a disc. The centre of the disc forms a star, and the rest of the disc forms the planets. What's left over forms a cloud way out in the outer reaches of the system. The belts in the disc, where the would-be planets form, get wider as they get further from the star, so the outer planets tend to be bigger than the inner ones.

All the planets collect the same recipe of rocky and icy stuff, and gas, but the inner planets get a lot of that gas burned away by the young star's heat, so they tend to be rocky, while the outer planets retain their gas, forming gas giants. One planet would do well at the expense of its neighbours.

Jupiter fits the bill in our Solar System. In fact, it became massive enough for its gravity to perturb the material in the next belt inward, between Mars and Jupiter, so that a planet could not form. This left the missing planet's orbit being filled with thousands of asteroids and other debris. It all sounds fairly obvious and logical.

An early hint of problems came from studies of the moon. It came from counting and measuring craters. The moon has no atmosphere, no erosion by water, and no plate tectonics. Its surface is a history book.

When a lump of rock hits the moon, it makes a crater. The size of the crater is an indicator of the size of the impacting body. Over time, the streaks of material ejected by an impact tend to fade as they get covered by dust ejected by later impacts. In addition, when one crater is laid down overlapping another, it is easy to see which crater formed first. By measuring crater sizes and estimating ages, or at least the order of the impacts, we get a record of the early history and formation of the moon.

Our nice tidy theory for the formation of the Solar System suggests there was a lot of impacting at the beginning, with some of the impacting objects being quite big. Then, as the construction of the Solar System continued, the building material was swept up and the size and frequency of the impacts tailed off.

However, that's when scientists found something odd. There was the expected period of impacting on the moon, which gradually tailed off, and then, unexpectedly, there was another period of intense bombardment. This event is referred to as the "Late, Heavy Bombardment".

What caused that? A possible explanation appeared when we became able to detect planets orbiting other stars. Our commonsensical model for the formation of the Solar System suggested that most other stars should have planetary systems like ours. That was not the case. We have now found thousands of planets orbiting other stars, and almost all them live in systems unlike the Solar System. It seems the formation of a planetary system is more like a game of pool than a tidy process. Now we have a new idea as to how the Solar System formed.

Jupiter and Saturn formed first. They gobbled up most of the gas and dust, so later comers, such as Uranus and Neptune got less and wound up smaller.

The interplay of the gravity pulls of the two giants then catapulted Neptune and Uranus outwards, which made Jupiter move inwards towards the sun. Jupiter then swept out the belt forming a planet next out from Mars, leaving a belt of asteroids. Lots of planet-forming lumps of material were thrown out of the Solar System. Other lumps moved inwards, hitting the moon and inner planets, causing a second bombardment, and a period of additional planet building.

This sort of cosmic pool game process turns each planetary system into a unique construction project.

•••

• Venus shines very brightly in the west after sunset.

• Mars, much less bright, and reddish, lies higher in the southwest.• Saturn, golden-coloured and moderately bright, lies very low in the dawn glow.

• The Moon will reach Last Quarter on May 12.

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

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