LECTURE

Let’s return to the Hertzsprung-Russell (H-R) diagram. The diagram is a scatter graph. It ranks stars according to two types of measurement. The rst parameter is their brightness, or the amount of light they emit. This is usually measured on the vertical axis, with brighter stars higher on the graph. In effect, a star’s brightness tells us its real mass, because a large star generates more density and more energy. The second parameter is a star’s surface temperature, which can be estimated from its color. Blue stars are hot, and normally appear on the left of the graph; red stars are cooler and appear to the right. Most stars appear on a diagonal band, the so-called main sequence, running from the top left to the bottom right. Stars on the main sequence are normal mature stars doing what stars do best: turning hydrogen into helium.

The exact position of a star on the main sequence depends on one factor: its mass. Small stars generate less pressure at the center and burn at lower temperatures, so they burn their fuel slowly and may live for billions of years. They appear at the bottom right of the main sequence. Our Sun is in the middle of the main sequence. It will burn for about 9 billion years. Stars more than 10 times larger than our Sun have much higher internal temperatures and burn their fuel much faster. The biggest stars, such as Rigel in Orion, shine with a bluish light and appear at the top left of the main sequence. They may live for just a million years or so.

The death of a star begins when it runs out of hydrogen. At this point, it leaves the main sequence on the H-R diagram. When our Sun has used up all the hydrogen in its core, it will cool and collapse quite suddenly. This violent collapse will compress the star’s center so that its temperature will rise again. At about 100 million degrees Celsius, helium atoms will start to fuse, eventually forming carbon. The Sun will expand again, reaching beyond the orbit of Mercury and turning into a “red giant,” like Aldebaran in the constellation of Taurus. As it burns helium, it will move from the main sequence to the upper right of the H-R diagram.

Once all the helium in its core has been used up, the process will repeat itself. The Sun will collapse again, temperatures will rise, and it will expand again until this time it includes the orbits of Mars and Earth, vaporizing both planets as it does so. The Sun will have become a “red supergiant,” like Betelgeuse in the constellation of Orion. Eventually, it will shed its