Monday, August 4, 2008

Life - and Death - Among the Stars

Welcome back to What's Up?

Continuing from last month's topic of dwarf stars we'll look at the other end of the stellar spectrum, the giant stars.

Sometime, around four or five billion years from now our sun will start to expand becoming larger and larger until it is about the size of Earth's orbit to become a red giant. There happens to be one problem for Earth here, the orbit of the Earth will be inside the sun which means the Earth will completely melt and disintegrate, mixing into the sun.

By this time the sun will have changed to a red colour due to a lower surface temperature.

The sun will run out of hydrogen fuel and will start fusing helium for a few million years. Then it will start burning heavier and heavier elements like carbon and oxygen until it starts burning iron. In order to fuse iron more energy is needed to fuse it than the fusing process creates.

Once the sun is fusing iron it will no longer produce enough energy to keep it from contracting under its own gravity it will begin pulsating and shedding off its outer layers. Eventually, all that will be left is the core of the sun: a small dense white dwarf that may weigh half as much as the sun's original mass, yet be as small as the size of the Earth!

Larger stars live much different lives than the small yellow dwarf star we orbit around that we call the sun.Large stars live much faster paced lives. These giant stars are up to a thousand times brighter and have diameters 10 to 100 times that of the sun. Since they're so large their insides are extremely hot and this causes these stars to burn their hydrogen fuel extremely quickly.

Most stars smaller than 8 solar masses will live a few hundred million years or longer. These smaller stars will have a much longer life. Nearing the end these smaller stars' lives they will expand into a red giant star and shed off its layers just like the sun will. Stars larger than this most likely will expand to even bigger diameters and possibly become a red or blue supergiant, depending on how hot it is. These stars that are larger than 8 solar masses have life expectancies of only a few tens of millions of years.

When you look up into the night sky, many of the bright stars you see are actually giant stars.

Stars that are dozens of times the sun's mass are usually called supergiant or hypergiants depending on their physical size and luminosity. They usually are twenty to several hundred times the sun's diameter and have 10,000 to 100,000 solar luminosities. Supergiants only last a few million years or so before they explode in a supernova, an explosion where the core of the star collapses and then the star explodes leaving behind either a small, dense star called a neutron star (just a few miles across but with as much mass as two or three suns, one teaspoonful of this star would have as much mass as a mountain) or a black hole (very small dense object with gravity so strong that light can't even escape).

Finally, hypergiants, the largest of all stars, are constantly on the verge of instability since they weigh in at over 100 solar masses. They only last a few hundred thousand up to a few million years. Since they last such a short period of time they are very rare. Only about a dozen are known to exist in our galaxy.

One of the better known hypergiants is Eta Carinae. In the 1800's it was the second brightest star in the sky. But it dimmed quickly out of sight. It's now only visible with the aid of a telescope. This star could explode at any time since it is so unstable.

Since it is in the southern hemisphere's sky, most people in the northern hemisphere won't get to see it explode even if it does so within our lifetimes, so let's check out the northern sky for this month.

On Aug. 1 there was a new moon. There was a total solar eclipse visible from northern Canada and Greenland that day. Only a partial solar eclipse was visible to Islanders if they got up early enough in the morning. Don't forget to wear eye protection such as welder's goggles or use a pinhole camera (check "Hey Kids" section) if you ever plan to observe the sun.

Two weeks later on Aug. 16 there will be a full moon and this will also be a partial lunar eclipse. Don't get too excited though, the moon will barely be in the Earth's shadow so not much difference may be noticed.

Finally, on Aug. 30, there will be a new moon, the last of the summer.

Until next month, just look up!

Hey Kids...

If you want to view the sun safely without fancy goggles during solar eclipses you can make what's called a pinhole camera. All you need are two pieces of cardboard and one needle. You poke the needle through the center of the first piece of cardboard making sure that the hole isn't too big and that it's nice and round. That's all there is to making it! To use it all you have to do is face away from the sun while holding one piece of cardboard a few inches or so apart (you may want to experiment a little bit) from the first piece of cardboard the closest to the sun. You will see a little image of the sun projected on the second piece of cardboard. And there you go! If you really like to experiment you can use a cheese grater to project many little images of the sun onto a piece of cardboard or paper. Now you know how to view the next solar eclipse!


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