GLOSSARY FOR EARTH/SPACE SCIENCE: S

1. SCIENTIFIC METHOD

   --The investigative technique used by all natural scientists throughout the world. In general, some data or ideas are first gathered, then a theory is proposed to explain them, and finally an controlled investigative experiment is devised to test the theory.

2. SECONDARY ATMOSPHERE

   --Gases that a planet exhales from its interior after having lost its primary or primordial atmosphere.

3. SECONDARY MIRROR

   --In a reflecting telescope, a small mirror mounted in the beam of radiation that strikes the primary mirror, and from which radiation is reflected and brought to a focus.

4. SECOND-GENERATION STAR

   --A star having some heavy elements and which is thus made of matter that has been previously processed through other stars.

5. SECOND OF ARC

   -- See "arc second."

6. SIESMIC WAVE

   --A systematic vibration emanation from the site of an earthquake and traveling through Earth's interior.

7. SEYFERT GALAXY

   --A spiral-type galaxy having an unusually bright, compact nucleus, and that seems to have properties midway between normal galaxies and the most violent of the active galaxies.

8. SHOCK WAVE

   --A rapidly rushing shell of gas that tends to push aside and sometimes implode matter in its wake.

9. SINGULARITY

   --A superhot, superdense state of matter, where the known laws of physics are likely to break down.

10. SOLAR CONSTANT

    --The amount of energy received from the Sun per unit area per unit time

11. SOLAR CYCLE

    --The cyclical change, repeated about every 11 years, in the strength of the Sun's magnetic field, the number of sunspots and the number of particles emitted in the solar wind; see also "sunspot cycle."

12. SOLAR ECLIPSE

    --A blockage of light from the Sun when the Moon is positioned precisely between the Sun and the Earth.

13. SOLAR INTERIOR

    --The region of the Sun between its core and convection zone.

14. SOLAR MAXIMUM

    --The midst of a sunspot cycle when the numbers of sunspots are substantial, and the output of particles and radiation is maximized.

15. SOLAR MINIMUM

    --The beginning and end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.

16. SOLAR SYSTEM

    --The collection of 1 star, 9 planets, 60+ moons, and innumerable smaller objects (asteroids, comets, and meteoroids) orbiting about the Sun; both the Sun and Earth are members of the Solar System.

17. SOLAR WIND

    --A stream of energetic, charged particles of matter (mostly electrons and light-element ions) that constantly escape the Sun, thereby bathing much of the Solar System in an extended solar atmosphere or "heliosphere."

18. SPACE INFRARED TELESCOPE FACILITY (SIRTF)

    --NASA's planned Great Observatory for infrared astronomy, scheduled to orbit Earth around the turn of the century.

19. SPACE SHUTTLE

    --The principal vehicle, consisting of a manned orbiter powered by liquid-fuel rocket engines, an external tank for liquid fuel and two solid-fuel rockets, use by the United States to place payloads into orbits as high as several hundred miles above Earth's surface.

20. SPACE TELESCOPE

    -- See "Hubble Space Telescope."

21. SPACE TELESCOPE SCIENCE INSTITUTE (STScI)

    --An international research center operated by AURA for NASA and located on the Homewood campus of the Johns Hopkins University in Baltimore, Maryland and from which The Hubble Space Telescope's science mission is designed, conducted, and data archived.

22. SPACETIME

    --A synthesis of the three dimensions of space and of a fourth dimension, time; a hallmark of relativity theory.

23. SPECTRAL CLASS

    --A classification scheme that groups stars according to their spectral lines or surface temperatures.

24. SPECTRAL LINE

    --A radiative feature observed in emission (bright) or absorption (dark) at a specific frequency or wavelength.

25. SPECTROGRAPH (OR SPECTROMETER)

    --An instrument that spreads electromagnetic radiation into its component frequencies and wavelengths for detailed study at each individual frequency and wavelength.

26. ,H3>SPACTROSCOPIC PARALLAX--The third rung of the distance ladder, whereby a star's spectrum is used to estimate its absolute brightness and thus its distance.

27. SPECTROSCOPY

    --An observational and analysis technique designed to disperse radiation into itS component wavelengths in order to study in fine detail the way that matter emits or absorbs radiation.

28. SPECTRUM

    -- See "electromagnetic spectrum."

29. SPICULE

    --A short-lived jet or spike of gas that regularly expels particles of matter from near the surface of the Sun.

30. SPIRAL ARM

    --Part of a pinwheel structure of young stars and interstellar clouds usually winding out from a galaxy's center.

31. SPIRAL GALAXY

    --A disk-shaped galaxy having a spiral or pinwheel pattern, some more than others, and composed of a mixture of old and young stars as well as loose interstellar matter.

32. S PROCESS

    --Element building that occurs in highly evolved stars when a neutron is "slowly'" captured by a nucleus.

33. STAR

    --A gaseous, self-luminous object held together by its own gravity, and so hot that its core releases energy by fusing lighter nuclei into heavier nuclei; the Sun is one such star.

34. STAR CLUSTER

    --A group of stars born at almost the same time and place, capable of remaining a unit for billions of years because of the mutual gravitational attraction of its member stars; see also "galactic cluster" and "globular cluster."

35. STELLAR EPOCH

    --A period in the history of the Universe (including now) when the stars form.

36. STELLAR EVOLUTION

    --The changes experienced by stars as they originate, mature, and grow old.

37. STELLAR NUCLEOSYNTHESIS

    --Element building that occurs in stars when nuclei collide and fuse with one another.

38. STELLAR PARALLAX

    --The apparent shift in the position of a star caused by Earth's yearly motion in orbit around the Sun.

39. STRONG FORCE

    --See "nuclear force."

40. SUNSPOT

    --Relatively cool, dark, disturbed regions on the surface (photosphere) of the Sun.

41. SUNSPOT CYCLE

    --The regular increase and decrease in the number of sunspots over the course of roughly 11 years; see also "solar cycle."

42. SPUERMASSIVE BLACK HOLE

    --A black hole housing millions of times the Sun's mass; see al;so "black hole."

43. SUPERNOVA

    --An explosive death of a massive star whose tremendous energy output causes its expanding debris to glow brightly for weeks or months thereafter.

44. SUPERNOVA REMNANT

    --The remains of a supernova, namely glowing debris scattered over a light-year or more.

45. SYNCHRONOUS ORBIT

    --See "geosynchronous orbit."

    ---