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Name: Beth S.
Status: other
Grade: other
Location: NV
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How can we tell a star's age?


Estimating the age of a star depends on our understanding of the evolution and process within a star. All stars are expected to begin their lives as a collapsing mass of mostly hydrogen and helium with a few trace elements. When the nuclear process begins, the hydrogen is steadily converted to helium and heavier elements.

Main sequence stars (those that are not very massive and much like our Sun) spend most of their life converting hydrogen to helium. As the hydrogen is exhausted, the star expands (becoming a red giant). Helium fusion then begins and helium becomes the main fusion reactant. The star will have a higher luminosity and surface temperature.

Main sequence stars eventually run out of fusion fuel, shed most of their elements as part of the solar wind, creating a nebula of gases that can eventually form a new solar system. The compressed mass at the center of the star is not massive enough to continue to fuse elements and it is now known as a white dwarf and eventually fade out, their nuclear fires extinguished.

More massive stars can continue fusing elements heavier than helium. At this stage the fusion reaction is fueled by elements such as oxygen, etc. This will progress until the iron core (iron does not fuse since it is at the thermodynamic well of energy) becomes so massive that the star collapses on itself and explodes in a supernova. Most of the star matter is blown away and what remains is a neutron star (or in very massive stars - a black hole). The blown out material can be recycled to form new planetary systems.

Thus, since we have an expectation of how stars mature, we simply need to look at the spectrum of light emitted from stars to know what atoms exists on the star. If it is mostly hydrogen, then it must be a young star. If it has quite a bit of helium than it is in its second stage of development. If it has even heavier elements, than it must be a massive star at its later stages. The ratio of elements can tell us at what stage the star is.

Greg (Roberto Gregorius)

A very simple way is by seeing the star's color. A red star is cooler than a blue or yellow one, and tends to be an older star as well! Blue stars tend to be the youngest.

Good question!

David H., Levy

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