3. Atomic and Subatomic Particles

    • The positively charged nucleus of an atom is enveloped by a swarm of negatively charged electrons.

    • The nucleus of an atom contains protons. The positive charge of each proton is equal to the negative charge of an electron.

    • The nucleus of an atom also contains neutrons, without any electrical charge, that help hold the protons together.

    • A hydrogen atom contains one proton, one electron, and no neutrons.

    • The nucleus of a helium atom contains two protons and two neutrons, and the helium atom has two electrons.

    • The size of the nucleus of an atom is about 100,000 times smaller than the size of the atom.

    • The proton is 1,836 times more massive than an electron, a neutron is 1,839 times more massive than an electron, and most of an atomís mass resides in its nucleus.

    • The motion of tiny unseen particles, such as the molecules in a gas, produces heat, and more heat is generated by hotter particles.

    • Hotter particles of a given mass move at faster speeds, with greater thermal energy and thermal velocity.

    • At a given temperature, particles with smaller mass move at faster speeds.

    • The temperature determines the average speed of a large number of particles, but there are a smaller number of particles that move faster and slower than the average speed.

    • The Maxwell speed distribution specifies the fraction of gas particles moving at a particular speed at any given temperature and particle mass.

    • Moving gas particles produce outward gas pressure, which holds our atmosphere above the ground.

    • The Sun is so hot inside that there are no atoms, just protons and electrons separated from their atomic bonds. Such an electrically neutral collection of charged particles is known as plasma.

    • The motion of the protons inside the Sun support the star, and protons closer to the Sun center have to be hotter and move faster to support greater amounts of overlying material.

    • The temperature at the center of the Sun is 15.6 million K.

    • The visible solar disk is known as the photosphere, and it has an effective temperature of 5,780 K.

    • The spectrum of sunlight contains dark Fraunhofer absorption lines whose wavelengths specify the chemical elements in the Sun.

    • Helium was discovered on the Sun before it was found on the Earth.

    • Spectral lines are emitted or absorbed when electrons change orbits, within the Bohr model of the atom.

    • The most abundant element in the Sun and other stars is hydrogen, and the second-most abundant element is helium.

    • The dominant spectral lines in starlight depend on the effective temperature of the stellar disk.

    • Radial motion produces a shift in the wavelength of a spectral line known as the Doppler shift. Outward motion along the line of sight produces a redshift to longer wavelengths.

    • A gravitational redshift of radiation is produced when the radiation escapes the gravitational attraction of a star.

    • Spectral lines can be broadened by the thermal motions of the particles that produce the lines or by the rotation of the radiation source.

    • The wavelength of a spectral line can be split into three components in the presence of a magnetic field. Known as the Zeeman effect, the size of the splitting can be used to measure the strength of the magnetic field and the polarization of the split lines can be used to determine the direction of the magnetic fields.

Copyright 2010, Professor Kenneth R. Lang, Tufts University