18) (10 pts.) the logic of science thaorios and observations about stars and galaxies below we have listed a series of scientific ideas. half of these are theories and half are observations. every theory has a set of observations that supports it, and every set of observations has a theory to explain it. for each one you must do two things: (1) identify it as theory or observation. (2) write down the number of its pair 1. when gas is caught by the gravity of a black hole, it forms an accretion disk, where friction makes the gas very hot, and slows it down, allowing it to spiral into the black hole. 2. a minority of stars called red giants are very luminous and reddish in color. 3. there are hundreds of x-ray binaries in our galaxy: systems with two stars that have very hot objects that give off large quantities of x-rays. 4. occasionally a star will disappear in a supernova, becoming brighter than a billion normal stars for a few weeks. 5. when a star builds up a core of iron, this causes the core to collapse, making the star explode. 6. most of the mass in our galaxy is a mysterious unseen "dark matter." 7. when a star runs out of hydrogen in its core, it burns hydrogen in a shell around this core. the star's fusion runs very fast, and the outer layers of the star expand and cool. 8. spiral galaxies are currently forming new stars, while star formation in elliptical galaxies ended long ago. 9. today our universe is expanding at a faster rate than it was a few billion years ago, so that on average galaxies are moving away from each other faster and faster. 10. there is a mysterious substance which we call "dark energy" that that fills all of space and makes up about 2/3 of the total mass-energy of our universe. 11. spiral galaxies tend to look blue in color, while elliptical galaxies tend to be orange or red. 12. stars in our galaxy move faster than we would expect if they were just being pulled by the gravity of other stars.