What is the life cycle of a star? (Intermediate)

In any case, have a look at http:/ /ast.star.rl.ac.uk/hr.html. This page illustrates the "Hertzsprung-Russell Diagram," a graphical plot on which astronomers plot the characteristics of stars. In the HR diagram, total brightness (known as "Luminosity" or "Absolute Magnitude") is plotted against the surface temperature of the star. As the star goes through its life cycle, it moves along the HR diagram from one place to another. The page I sent you shows tracks for stars and "failed stars" like Jupiter.

The star begins as a cool cloud of gas, well to the right (cold side) of the diagram. Under its own gravity, the cloud begins to collapse. As it collapses, it releases its gravitational energy as radiation. So a young star is cool but bright. We usually don't see these proto-stars, however, because they are generally shrouded in dust and only give off light in the far infrared part of the spectrum.

The proto-star continues to collapse, and as it does, it gets hotter and hotter, moving to the left in the HR diagram. "Stars" less than a percent of the Sun's mass eventually halt their collapse due to gas pressure. These stars are "brown dwarfs" or "giant planets," like Jupiter. They never ignite their hydrogen, and gradually dim away. These are the downward tracks you see in the diagram.

Stars larger than about 0.05 to 0.07 solar masses contract, getting hotter and hotter, until their cores are hot enough to begin burning hydrogen. When a star "turns on" its hydrogen-burning phase, we say that it has reached the main sequence, and is a true star, not a protostar, brown dwarf, or planet. The main sequence is a line of stars plotted on the HR-diagram, which represent stars which are burning hydrogen, fusing it into helium. The fusion process in the core releases heat and light, supporting the star against further gravitational collapse, and making it shine. Stars spend most of their life in one spot on the main sequence. Very massive stars live at the top of the main sequence, shining very blue and very bright, while low-mass stars are dim and red. The sun is between these two extremes.

Eventually, the hydrogen in the core whose fusion supports the star begins to run out. The core becomes mostly helium (the product of hydrogen fusion), and hydrogen burning moves out away from the core, forming a burning shell around the core. When this happens, the core begins to collapse again, but the outer regions of the star are pushed outwards. The star becomes brighter and cooler. This is the Red Giant stage. When the sun reaches the Red Giant stage, 5 billion years from now, it will likely grow to engulf Mercury, Venus, and the Earth.

If the star has little mass, it may end its life here, throwing off its outer layers, creating a planetary nebula out of its atmosphere, and a hot, dense "white dwarf" out of its core. The white dwarf shines only by residual left-over heat, and will eventually fade into a mere cinder. Cores of stars at least half as massive as the sun, however, will eventually collapse and heat enough to start burning helium in their cores. Once helium-burining starts, the star decends the giant branch again, reverses its swelling a bit, and lives happily fusing helium in its core, and hydrogen in a shell around the core.

Then the giant dance begins again. The core helium runs out, and the star once again becomes a red giant. Stars like the sun get off the bus here, and become planetary nebulae and white dwarfs. Heavier stars begin burning carbon. This happens over and over again with heavier and heavier elements. The star ascends and descends the giant branch many times. Lighter stars that don't get hot enough in their cores to burn the next element become planetary nebulae and white dwarfs. Stars that are heavy enough eventually heat their cores enough to begin the next stage of burning and descend the giant branch for a while. For each element, the process is quicker and quicker. A star might burn Hydrogen on the main sequence for billions of years, but once this process gets to, say, silicon, the star might burn silicon for only a few days.

A very massive star, more than 5-10 times solar mass, will ascend and descend the giant branch many times, until the star is ready to burn iron. But iron fusion doesn't release energy; it sucks it up. So what happens is that the star is ascending the giant branch, its iron core is collapsing and heating, until iron is ready to fuse. As soon as it does, though, it absorbs all of the heat around it, chilling the core. All fusion abrubtly stops, and the star implodes. The rebound of this implosion is the greatest explosion known in the cosmos: a supernova. A single supernova can be brighter than an entire galaxy for a few days. After the supernova, depending on the mass of the original star, the core might be left over as a white dwarf, neutron star, or black hole.

So depending on its mass, a star ends its life either in a planetary nebula or supernova, leaving its core behind as a compact object: white dwarf, neutron star, or black hole.


This page updated on June 27, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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