What’s Up in the Night Sky – February 2011

Fred Barrett | Feb 03, 2011

By Fred Barrett

Supernovas, planetary nebulas, white dwarfs, neutron stars, black holes. What are they all anyway?

It was all over the news recently that 10-year-old Kathryn Gray of Fredericton, New Brunswick, became the youngest person ever to discover an exploding star called a supernova. She spotted it on New Year’s Eve.

That got me thinking that it might be interesting to discuss what causes a supernova. Last month I wrote that scientists don’t understand 95% of the universe. Of the remaining 5%, all but a tiny amount, 0.03%, is stars, hydrogen, helium and neutrinos. We and everything around us is made up of that remaining 0.03%.  Where did that 0.03% come from? Those ingredients came from exploding stars - supernovas. We are literally made up of star stuff blown out into the galaxy from countless generations of exploding stars!

All stars make energy by nuclear fusion. Fusion happens when there is enough hydrogen in a newly forming star to create pressures high enough in the centre to cause temperatures to go higher than 10 million degrees Celsius. At this temperature, hydrogen nuclei are forced together and fuse into helium. This releases energy.

Stars come in all sizes, depending on the nebular cloud they formed in and how much material their increasing gravity can draw in. They vary from about 0.08 solar masses to giants of 120 solar masses (a solar mass is the standard unit of mass in astronomy and is equal to the size of our sun). The size of the star dictates whether it will become a white dwarf or a supernova. Stars below 8 solar masses will become white dwarfs, and produce planetary nebulas as they die. In my next column I will discuss these.

It is the stars above 8 solar masses that become first supernovas and then either neutron stars or black holes.

The higher gravity of these more massive stars causes higher pressures at their centers, which result in temperatures high enough to fuse helium into carbon. The fusion rate and energy output increases with the size of the star. This occurs because greater mass crushes inward to make the core temperature much higher, which in turn leads to more rapid fusion. A star with 10 times the mass of our sun emits 10,000 times more energy per second than the sun – but it uses up all its fuel much faster too. Our sun is half way through its lifespan of 10 billion years, whereas the massive stars may last only 250 million years or less.

A star above 8 solar masses has enough gravity to compress and heat its core high enough to fuse its carbon into nitrogen and oxygen, but by now time is running out for it. The next conversion is to silicon. It has taken about 10 million years for this star to convert its hydrogen to helium. The next fusion step to carbon took about a million years, then about a thousand years to convert carbon into oxygen. It now will take about a year to fuse oxygen into silicon. Converting silicon to iron takes less than a day! This is the end of the star. Iron created in the core absorbs energy when particles fuse with it. That is catastrophic! There is no longer any outward pressure to balance with the inward squeeze of gravity. When the core is all iron, fusion stops and gravity compresses the core to such a density that it no longer obeys the normal laws of physics. As the core collapses electrons fuse with protons to become neutrons and neutrinos. The neutron-rich core collapses from a diameter of 2 thousand kilometers to about 30 kilometers at roughly a quarter of the speed of light (the speed of light is 299,792 km/second). The neutrons pack closer and closer together until a density limit called neutron degeneracy is reached. That is at about 10 trillion times the density of water! It is a repulsive force and is powerful enough to overcome the inward rush. The core rebounds violently and the resulting explosion is called a supernova. All this happens within seconds of core collapse. The energy released is so great that the new supernova outshines all the stars in a galaxy! But the supernova event isn’t quite over yet. The core re-collapses and if it is between 1.4 and 3 solar masses, nuclear degeneracy pressure halts any further collapse and a neutron star forms. If the neutron core is greater than 3 solar masses nuclear degeneracy is overcome and the neutron core collapses further and forms a Black Hole.

As Joni Mitchell wrote: “We are stardust..”. And everything we interact with is made from the products of those great supernova explosions that have been occurring since the first stars formed.

The two weeks starting roughly February 20th is the best time to view the Zodiacal light, which is made up of fine dust particles in the plane of the solar system. Scattered sunlight from the bits of dust makes its glow. It can be seen in the east in October too. These times make for the best viewing. This is because the dust particles are in the plane of the ecliptic and the ecliptic is at its steepest angle to the horizon in late February and October. Find a nice dark spot and look to the western horizon after evening twilight. Be careful not to confuse it with twilight. Look for a cone of light tilting slightly to the left.

Jupiter is still with us. Look for this beautiful planet in the southwest and west in early evening. It will be close to the crescent moon on the 6th. Saturn can be found rising in the east around midnight and it moves towards the southwest by sunrise. Its rings are tilted close to 10 degrees from edge on.  Don’t forget to look for Venus about 2 hours before sunrise in the southeast. It is rising closer to sunrise as February progresses.

During February between 8 and 9 PM the great constellation Orion is high in the south. Go out after supper and have a look at it and all the beauty around it. Your eyes alone can see plenty of detail after they are dark-adapted. For more detail and to look at interesting little bits, use binoculars. The Milky Way spreads from the southeast to the northwest. In the southeast is the constellation Canis Major with its star Sirius, the brightest star in the northern night sky. It continues through Orion and its spectacular Orion Nebula. There is Gemini above Orion with the open star cluster Messier 35 separating it from the house shape of Auriga. To Auriga’s side and above it to the northeast is beautiful Perseus with the Double Cluster at its head. If we go a little farther, we arrive at Cassiopeia and the open cluster Messier 52 on the side opposite Perseus. You will need a telescope for this one. The star chart accompanying this column should help your navigation.

By the way, the bright star Rigel, which is the left foot of Orion below and to the right of the belt of Orion will be going supernova soon - relatively speaking. That will be in a million years, give or take a few. Its core is at the helium stage. It is about 850 light years from us, so we’ll be safe if we’re around when it happens. It is roughly 74 times the diameter of the Sun and shines with the light of 85,000 suns. It is about 10 million years old.

If you have questions or suggestions, Fred Barrett may be contacted at This email address is being protected from spambots. You need JavaScript enabled to view it.

The Beginner’s Observer’s Guide by Leo Enright is available at the Sharbot Lake Pharmacy or by contacting the Royal Astronomical Society of Canada www.rasc.ca/publications, subscriptions for our very own excellent Canadian astronomy magazine, Sky News, are also available from RASC..