Ch. 20-22 Key Terms

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Across
  1. 2. large, cold interstellar clouds with diameters of dozens of light-years and typical masses of 105 solar masses; found in the spiral arms of galaxies, these clouds are where stars form
  2. 4. a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared
  3. 6. luminous knots of gas in an area of star formation that are set to glow by jets of material from a protostar
  4. 8. a loose group of young stars whose spectral types, motions, and positions in the sky indicate a common origin
  5. 10. one of about 150 large, spherical star clusters (each with hundreds of thousands of stars) that form a spherical halo around the center of our Galaxy
  6. 12. a planet orbiting a star other than our Sun
  7. 15. a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star
  8. 16. a large, dense, cold interstellar cloud; because of its size and density, this type of cloud can keep ultraviolet radiation from reaching its interior, where molecules are able to form
  9. 20. location in the H–R diagram where stars begin to leave the main sequence
  10. 22. the reddening of starlight passing through interstellar dust because dust scatters blue light more effectively than red
  11. 23. a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus
  12. 25. a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet
  13. 26. the outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star
  14. 27. the attenuation or absorption of light by dust in the interstellar medium
  15. 28. a line denoting the main sequence on the H–R diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered substantially by nuclear reactions
Down
  1. 1. a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star)
  2. 3. the building up of heavy elements from lighter ones by nuclear fusion
  3. 5. the region of ionized hydrogen in interstellar space
  4. 7. the cycling of mass in and out of the interstellar medium, including accretion of gas from intergalactic space, loss of gas back into intergalactic space, and conversion of interstellar gas into stars
  5. 9. tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices
  6. 11. a comparatively loose cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of our Galaxy; sometimes referred to as a galactic cluster
  7. 13. a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune)
  8. 14. a very young star still in the process of formation, before nuclear fusion begins
  9. 17. the gas and dust between the stars in a galaxy
  10. 18. when one astronomical object moves in front of another
  11. 19. atomic nuclei (mostly protons) and electrons that are observed to strike Earth’s atmosphere with exceedingly high energies.
  12. 21. a slightly denser cloud inside the Local Bubble, inside which the Sun also lies
  13. 24. a region of low-density, million degree gas in which the Sun and solar system are currently located