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Across 4. A galaxy in which the stars and gas clouds are concentrated mainly in one or more spiral arms 5. 1.A unit of astronomical distance equivalent to the distance that light travels in one year 7. A nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus 13. brightness of a celestial object as it would be seen at a distance 15. A star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass 16. 1.The effect whereby the position or direction of an object appears to differ when viewed from different positions 18. 1.A group of stars forming a pattern that is traditionally named after its apparent form or identified with a mythological figure 20. 1.A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction 21. An irregular galaxy is a galaxy that does not have a regular shape 22. A contracting mass of gas that represents an early stage in the formation of a star 23. A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars

Down 1. A fixed luminous point in the night sky that is a large 2. A small very dense star that is typically the size of a planet 3. A star showing a sudden large increase in brightness and then slowly returning to its original state over a few months 4. Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram 6. The Hertzsprung–Russell diagram is a scatter graph of stars 8. An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile 9. A cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette 10. A celestial object, thought to be a rapidly rotating neutron star, that emits regular pulses of radio waves 11. The Milky Way is the galaxy that contains our Solar System 12. The main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus 14. A system of two stars in which one star revolves around the other or both revolve around a common center 17. 1.A region of space having a gravitational field so intense that no matter or radiation can escape 19. A massive and extremely remote celestial object, emitting exceptionally large amounts of energy