Question

section vocabulary
absolute magnitude the brightness that a star would have at a distance of 32.6 light - years from earth
apparent magnitude the brightness of a star as seen from earth
light - year the distance that light travels in one year; about 9.46 trillion kilometers
parallax an apparent shift in the position of an object when viewed from different locations
spectrum the band of colors produced when white light passes through a prism

1. identify what are the two main elements that make up most stars?
2. apply concepts put the following star classes in order from hottest to coolest: a, b, g, k, o.
3. analyze why do scientists use light - years to measure the distances between stars and earth?
4. explain why do stars seem to move in the sky?
5. compare what is the difference between apparent magnitude and absolute magnitude?
6. explain why is the actual movement of stars hard to see?

Explanation:

1. Hydrogen and helium are the two main elements that make up most stars. Hydrogen is the most abundant, and through nuclear fusion in the star's core, hydrogen fuses into helium, releasing energy.
2. The order from hottest to coolest is O, B, A, G, K. The spectral - class sequence is based on temperature, with O - type stars being the hottest and K - type being cooler.
3. Scientists use light - years to measure distances between stars and Earth because the distances are extremely large. Using kilometers or miles would result in very large numbers that are difficult to handle, while a light - year provides a more convenient and manageable unit for expressing these vast cosmic distances.
4. Stars seem to move in the sky mainly due to the Earth's rotation on its axis. As the Earth rotates, the apparent position of stars changes, creating the illusion of movement. Also, over long periods, the proper motion of stars (their actual movement through space) can be observed, but this is much slower and harder to notice.
5. Apparent magnitude is the brightness of a star as seen from Earth, which depends on the star's actual luminosity, its distance from Earth, and any interstellar matter that might dim its light. Absolute magnitude is the brightness a star would have at a standard distance of 32.6 light - years from Earth, which allows for a more direct comparison of the intrinsic brightness of stars, independent of their distance from us.
6. The actual movement of stars is hard to see because stars are extremely far away. Even though they may be moving at high speeds through space, their great distance from Earth means that the change in their position as seen from Earth is very small over short periods. It takes many years or even centuries to observe significant changes in their positions due to proper motion.

Answer:

1. Hydrogen and helium.
2. O, B, A, G, K.
3. Because cosmic distances are extremely large, and light - years provide a more convenient unit.
4. Due to Earth's rotation and, over long periods, proper motion.
5. Apparent magnitude is as seen from Earth, affected by distance and interstellar matter; absolute magnitude is at a standard distance, showing intrinsic brightness.
6. Stars are very far away, so changes in position are small over short periods.