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An erupting, massive star in the Milky Way. NASA's Hubble Space Telescope has identified one of the most massive stars known, emitting as much as 10 million times the power of our Sun and with a radius larger than the distance between the Sun and the Earth.
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Image courtesy of NASA, Space Telescope Institute

Fusion Inside the Stars

Fusion in the core of the stars is achieved when the density and temperature arising from the gravitational pressure are high enough. There are different fusion cycles that occur in different phases of the life of a star . The first stage is the fusion of Hydrogen into Helium. This is the stage that our Sun is in.

In stars with a very high temperature (greater than 16 million degrees Kelvin) another set of fusion reactions, the so called Carbon-Nitrogen-Oxygen (CNO) cycle can take place. Here the Carbon atom is a catalyst for the reaction: it participates but it is not "burned". At still higher temperatures, Helium burning produces Carbon. Finally, at even higher temperatures the heavier elements up to Iron are formed by fusion of Carbon, Oxygen and Silicon.

To create elements heavier than Iron, neutron capture must occur. This can happen in a Supernova, when a very massive star explodes at the end of its life cycle. As a result of the fusion reactions occurring in the Sun and stars, neutrinos are emitted through the process of beta decayand reach Earth. By detecting these neutrinos, scientists learn about fusion inside the stars.

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Cool It! is the new card game from the Union of Concerned Scientists that teaches kids about the choices we have when it comes to climate change—and how policy and technology decisions made today will matter. Cool It! is available in our online store.

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Fusion Reactions

Nuclear fusion is a process where two or more nuclei combine to form an element with a higher atomic number (more protons in the nucleus). Fusion is the reverse process of nuclear fission. Fusion of light...more

The Hydrogen Fusion Process

The basic Hydrogen fusion cycle involves four Hydrogen nuclei (protons) and two electrons and yields a Helium nucleus, two neutrinos and six photons. This process occurs in three steps: the first one is...more

The Neutron Capture Process

Neutron capture can occur when a neutron approaches a nucleus close enough for nuclear forces to be effective. The neutron is captured and forms a heavier isotope of the capturing element. When the new...more

The Supernova

A Supernova is a very massive star that explodes at the end of its life cycle. The supernova is the furnace where the heavy elements (heavier than iron) are formed by neutron capture. ...more

Neutrino detectors

Neutrino interactions with matter are extremely rare, making detection difficult. Neutrino detectors are typically large tanks filled with a fluid that reacts to the passage of neutrinos. To take advantage...more

Diagnostics for the Solar Interior

The Sun, as well as other stars, releases energy in the form of radiation and particles. The processes that produce this energy are taking place in the interior of the Sun, where direct observations are...more

Fusion Inside the Stars

Fusion in the core of the stars is achieved when the density and temperature arising from the gravitational pressure are high enough. There are different fusion cycles that occur in different phases of...more


IMF stands for Interplanetary Magnetic Field. It is another name for the Sun's magnetic field. The Sun's magnetic field is enormous and is carried by the solar wind. The solar wind and magnetic field are...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA