Why do Cepheids pulsate?
What will be an ideal response?
Studies using stellar models show that variable stars in the instability strip pulsate like beating hearts because of an energy-absorbing layer in their outer envelopes. This layer is the region where helium is partially ionized. Above this layer, the temperature is too low to ionize helium, and below the layer it is hot enough to ionize all of the helium. Like a spring, the helium ionization zone can absorb energy when it is compressed, and release it when the zone expands, and that is enough to keep the star pulsating.
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Which statement best describes the surface of Ganymede?
a. dark and dirty ice, heavily pocked with craters b. old and dark on some portion, but cratered with a young bright grooved terrain c. icy crust free of craters with a liquid water ocean beneath d. covered in active sulfur-based volcanoes e. mostly water ice mixed with methane
Earth's magnetic field has reversed itself
Indicate whether the statement is true or false
Which is the correct order of the planets, moving from closest to the Sun to farthest from the Sun?
a. Mercury, Venus, Earth, Saturn, Mars, Jupiter, Neptune, Uranus b. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune c. Venus, Mercury, Mars, Earth, Saturn, Jupiter, Neptune, Uranus d. Mars, Venus, Earth, Mercury, Jupiter, Saturn, Uranus, Neptune e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus
The resistance of bacteria to certain antibiotics is a current example of
A) natural variety B) artificial selection C) un-natural selection D) natural selection