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Become a nitrogen atom in the nitrogen cycle in our Traveling Nitrogen Classroom Activity Kit/Game. See all our games, activity kits and classroom activities.

ExploraTour: A Peek into the Lives of the Stars

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Why does nuclear fusion in stars stop after an iron core is created?

The diagram at the left gives the answer to that question. It shows the excess energy per nucleon plotted against the atomic mass number of an element.

To illustrate what that means let's consider one particular example. The carbon nucleus (atomic mass number of 12) is made up of 6 protons and 6 neutrons. If we add up the mass of these particles, the carbon nucleus should have a mass of 12.098940. Yet the actual mass of the carbon nucleus is only 12.00000. There is a difference in the mass of 0.098940.

This mass was converted into energy and released when the carbon nucleus was created. If we divide this energy by the number of nucleons (protons and neutrons), we get the quantity plotted on the vertical axis in the figure.

Less and less energy is released as heavier and heavier elements (moving toward Iron-56) are created by fusing lighter elements together. In fact, when we attempt to fuse iron to produce heavier elements, no energy is released. Energy must be added.

When a star's core is converted to iron, nuclear fusion is no longer an energy source. With nothing to oppose gravity, the core collapses with blinding speed.

Shop Windows to the Universe Science Store!

Our online store includes issues of NESTA's quarterly journal, The Earth Scientist, full of classroom activities on different topics in Earth and space science, ranging from seismology, rocks and minerals, oceanography, and Earth system science to astronomy!

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ExploraTour - Looking at the World in a Different Light

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ExploraTour - Looking at the World in a Different Light

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ExploraTour - Looking at the World in a Different Light

Your eye is a wonderful detector of visible light. Different frequencies of light produce different sensations in the eye which we interpret as colors. Our eyes detect light by using light sensitive components...more

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ExploraTour - Looking at the World in a Different Light

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF