For each of the following star descriptions, state whether the star would be a red giant, white dwarf, or main sequence star, and provide the letter(s) of a star from the H-R diagram that fits each description.
a) very bright (high luminosity) and very hot (high temperature)
b) very dim and cool
c) very dim and very hot
d) very bright and cool
The correct answer and explanation is:
a) Very bright (high luminosity) and very hot (high temperature)
- Answer: This star is a main sequence star, likely a blue giant or O-type star.
- Explanation: Stars on the main sequence of the Hertzsprung-Russell (H-R) diagram typically show a balance between temperature and luminosity due to the nuclear fusion occurring in their cores. Hotter stars tend to be more luminous. Stars with both high luminosity and high temperature are typically large and hot, such as blue giants or O-type stars. These stars have surface temperatures greater than 30,000 K, emitting a blue-white light. These stars can be found in the upper left corner of the H-R diagram.
b) Very dim and cool
- Answer: This star is a red giant or possibly a white dwarf.
- Explanation: A star that is both dim and cool typically falls into the category of red giants or white dwarfs. Red giants have cooler surface temperatures (less than 5,000 K) and are not as luminous as hotter stars, but they are still quite large in size, making them luminous in the infrared part of the spectrum. They are located in the upper right section of the H-R diagram. White dwarfs, on the other hand, are extremely small and compact remnants of low- to medium-mass stars. These stars are also dim and have temperatures around 5,000-10,000 K, but their luminosity is low because of their small size, placing them in the bottom left of the H-R diagram.
c) Very dim and very hot
- Answer: This star is likely a white dwarf.
- Explanation: White dwarfs are compact remnants of stars that were once on the main sequence. Although they are very hot (with surface temperatures exceeding 100,000 K), their small size makes them very dim in terms of luminosity. Their luminosity is low because they are not undergoing fusion, and they simply radiate their residual heat. White dwarfs are found in the bottom left of the H-R diagram.
d) Very bright and cool
- Answer: This star is a red giant.
- Explanation: Red giants are stars that are cooler (typically with surface temperatures less than 5,000 K) but much larger in size than the Sun. Their luminosity is high because their size allows them to emit large amounts of energy. These stars are located in the upper right part of the H-R diagram. They appear red because of their cooler temperature but are still bright due to their size.