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.

More Skywatching articles

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]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

Previous Stories