This year there really will be a Christmas star - or to be more accurate, an artificial comet. It will be a glowing cloud of barium now scheduled for release early Christmas Day 70,000 miles above the Galapagos Islands in the Pacific. It should suddenly appear as a moderately bright greenish star, acquire a bluish hue, expand rapidly to at least a sixth the size of the full moon, and grow a brilliant comet-like tail.
The experiment is part of an ongoing three-nation, triple-satellite program. Scientists involved literally want to paint the sky with a marker dye to gain what US project manager Gilbert W. Ousley calls ''a deeper understanding of the basic physical processes which are common in the universe.''
These are processes that involve the action of electrically charged atoms and magnetic force fields. Physicists call a mixture of such charged particles and electrons a plasma.
The Sun and stars are great, glowing balls of plasma. The ''solar wind'' which sweeps past the planets is a stream of hot plasma expanding outward from the Sun in all directions at a speeds of around a million miles an hour.
The Christmas comet is designed to help scientists use a natural plasma laboratory which surrounds Earth - the so-called magnetosphere. This is an invisible envelope of charged particles whose motions are channeled by magnetic forces.
The magnetosphere's outer edge is the interface between near-earth space and the interplanetary realm where the solar wind dominates. It is an ideal place to study the action of cosmic plasmas as the solar wind blows past our planet. Dr. Ousley notes that ''a fundamental question of space physics'' asks how much of the solar wind leaks into the magnetosphere and how the entering particles are accelerated to thousands or millions of times the energy with which they enter.
To search for answers, Britain, West Germany, and the United States sent a trio of satellites into orbit last Aug. 16. Germany's Ion Release Module (IRM) carried 16 ejectable canisters containing barium and lithium. It entered an elongated orbit with a high point of about 70,000 miles, just outside the magnetosphere. Britain's satellite accompanied it into orbit and will remain nearby to monitor magnetic fields and particles. The US satellite is in a lower orbit, inside the magnetosphere, to trace the course of barium and lithium released by the IRM.
This program, called AMPTE (Active Magnetospheric Particle Tracer Explorers), is like the experiments made on Earth when scientists drop colored dyes into streams to trace water flow. Mario H. Acuna, AMPTE project scientist at the NASA Goddard Space Flight Center, explains: ''The solar wind contains little natural lithium and it becomes, in effect, a marker dye . . . a very dilute dye which paints a large area of the magnetosphere.''
Two lithium releases were made in September to trace penetration of particles from the solar wind into the magnetosphere. More will be made next year.
The Christmas comet barium release is meant to show how the solar wind sweeps material past the magnetosphere. Its tail should be visible for about 10 minutes as it grows to a length of 5 degrees or about 10 times the diameter of the full Moon. Observers in the western parts of North and South America and across the Pacific should have a good view. The release is scheduled for sometime between 3 :30 and 5:30 a.m. PST. Dec. 27 is the backup date.
After Dec. 1, up-to-date information can be obtained from the National Aeronautics and Space Administration by calling (301) 344-0470. This is a toll call.