The Difference Between Jovian Planets and Stars
- Jupiter and the Sun are very similiar in that they are both
spheres of highly compressed Hydrogen and Helium. Why is one a star and the other a planet?
- The difference lies in their relative masses.
- Jupiter is 1000 times less massive than the Sun.
- As a result the Sun can tap the nuclear energy
locked in its Hydrogen while Jupiter cannot.
- The Sun radiates 4x1033 ergs of energy every
- That's enough energy each second
to supply civilization's energy needs for nearly a million
- How can it sustain this rate of energy production?
- Radioactive dating of rocks indicates that the Solar
System, and thus the Sun has an age of 4.6 billion years.
- No conventional source of energy could sustain the Sun's
luminosity for billions of years.
- Chemical energy (i.e. burning) - 10,000 years
- Gravitational squeezing and contraction - 10,000,000
- Nuclear reactions provide an efficient source of energy
production. Such reactions release the energy that composes matter
itself (i.e. E=mc2).
- One gram of matter can be converted into enough energy
to supply the entire US energy need for 1 minute! A penny has a mass of 5 grams.
- 100% conversion of matter into energy can only be achieved by combining equal amounts of matter and antimatter.
- Antimatter cannot be ``mined." It must be manufactured at great energy cost, so it is not a solution to our energy needs.
- Nature takes advantage of an inefficient means of converting a small fraction of available matter into energy in order to power the stars - the conversion of hydrogen into helium via thermonuclear fusion.
- The Sun is composed mainly of hydrogen.
- Fusing hydrogen to helium releases 0.7% of the hydrogen's mass as energy.
- This form of energy release is sufficient to sustain the
Sun for 10 billion years - twice its present age.
- High temperatures are required to overcome the mutual
repulsion of protons in order to get them to fuse.
- Temperatures have to be high enough so that particles are moving fast enough to get close enough to "stick" via the strong nuclear force that binds an atomic nucleus together.
- The Sun can only achieve thermonuclear fusion if
the temperature in the interior is about 10 million degrees.
- The surface of the Sun has a temperature of only 6000
- The Sun is very massive and the weight of the outer layers
pressing down on the center make the interior very hot.
- The Sun is sufficiently massive to raise the central
temperature to 10 million degrees - just right for thermonuclear
- Jupiter is insufficiently massive to get warm enough
at its center to achieve thermonuclear fusion.
- In general, an object must be 80 times the mass of
Jupiter to fuse hydrogen into helium and maintain its
luminosity as a star.
- Objects like Jupiter begin their existence warm
from their initial contraction and gradually cool over time.
- Such objects continue to glow in the infrared
part of the spectrum and are called brown dwarfs.
- Stars all tap thermonuclear energy in a similar way to the Sun.
- Star's properties (e.g. luminosity, temperature, size...) largely
depend on the star's mass.
- At formation a star contracts until the core heats to the point where hydrogen can fuse.
- The release of energy sustains the star in this configuration (size, temperature, luminosity...) until the hydrogen fuel becomes scarce.
- When the hydrogen runs out the star begins contracting and heating again until the temperature is reached where it can burn the next nuclear fuel (helium).
- Subsequent stages of nuclear burning ( helium to carbon, carbon to
oxygen, etc.) and ultimately supernova explosions, synthesize all of the elements
in the periodic table.
Created November 27, 2000