Cuesta College, San Luis Obispo, CA
Students have a weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.
(The following question was asked subsequent to the lecture on Wien's law and the Stefan-Boltzmann law.)
Which star is the smallest in size? (Graded for completion.)
(A) Supergiant.
(B) Giant.
(C) The sun.
(D) White dwarf.
(E) Red dwarf.
(F) (I'm lost, and don't know how to answer this.)
Student responses (pre-instruction)
Sections 70158, 70160
(A) : 0 students
(B) : 0 students
(C) : 1 student
(D) : 29 students
(E) : 36 student
(F) : 6 students
Correct answer: (D)
According to Wien's law, a white dwarf is hotter than a red dwarf. The Stefan-Boltzmann law (luminosity is proportional to size and temperature^4) says that a white dwarf must be much smaller than a red dwarf of the same luminosity, as seen by comparison from a Hertzsprung-Russell diagram.
(Students were then prompted to fill in a text box online to justify their answer.)
Briefly explain your answer, about your choice for smallest star. (Graded for completion.)
The following are all of the student responses to this question, verbatim and unedited.
"I didn't pick one out for the reason that i'm not too sure and I don't have my text book with me to look it up!"
"because it is low in luminosity"
"A red dwarf is a star that is almost burned out."
"They have lower mass than a white dwarf, and they are cooler than a white dwarf."
"i think the sun because it depends on many things anfd it changes alot"
"it was a guess"
"We know they are so small because they have such low luminosities and therefore must have very low surface area."
"i guessed"
"im thinking the white dwarf is a star that had its matter compacted down on itself. Energy is concentrated to a tight compact ball"
"there was a red star found about the same size as jupiter"
"...why else would they call it a dwarf?"
"I am not really sure why."
"it was hopefully a lucky guess"
"a white star is smaller because it is in the last stage of the star life cycle, it has used up all of its nuclear energy...."
"I would think the word dwarf indicates a smaller side"
"The red dwarf is not only small but are also cool. The text book said so."
"I've heard someone say something along the lines of a white dwarf being small..."
"the white dwarfs are the smallest because with time they age and cool down and thats wat makes them get small."
"Red dwarfs are fragments of stars, not entire stars"
"as a sun cools down it expands, and of these options the hottest star is the white dwarf (the sun is a yellow dwarf and supergiants are the largest stars i believe so that is not an option either)."
"because it is...."
"Well giants are huge so there not small. A dwarf is a small person so it was between white and red and i've never seen a small red person."
"Because according to the H-R diagram, white dwarfs are at the bottom"
"I don't know which one is the smallesy in size; I would assume the white dwarf."
"The white dwarves have little surface area and low luminosity."
"Red dwarfs are the smallest"
"Astronomers can calculate the diameter of a star by finding its angular size and its distance from earth."
"i really dont know the answer to the question because i dont know about the sizes of the stars."
"the red dwarf is the smallest star because it is very low in luminosity."
"Dwarfs are the smallest stars."
"Well, the sun is very visible and is massive and the other stars seem to be visible as well."
"because it cannot be a giant so it must be a dwarf. The white is the smallest."
"i don't remember"
"Red dwarfs are as big as half the sun or even smaller."
"Wikipedia claimed it to be so"
"thats just the name of the star"
"The red dwarf is smaller than the white dwarf"
"The red dwarf is the coolest and faintest of the dwarf stars making it appear the smallest."
"i know a dwarf star is small, but i guessed on the color"
"the white dwarf is the smallest"
"Because they have about a .01 R."
"It is basically the last stage of a stars life after it expands to a red dwarf it, not always, becomes a white dwarf. It is smaller but just as dense."
"I am guessing, based on "dwarf"'s indication that it is a smaller star, and that a red star is less hot than a white star."
"book says white is the smallest"
"I think the white dwarf is the smallest."
"The Guiness Book of World Records lists a white dwarf named L362-81 as the smallest star in size; its diameter is estimated to be only about 3500 miles."
"'The Guiness Book of World Records lists a white dwarf named L362-81 as the smallest star in size; its diameter is estimated to be only about 3500 miles, or about the same size as the planet Mercury.' -Or so says my google search engine..."
"Sounds small"
"I just guessed"
"Gary Coleman... Pretty obvious reasons."
"becuase it has the word dwarf in it..."
"It's super small."
"I chose the Red dwarf because i looked it up online."
"A white dwarf is concentrated and new. A red dwarf is a small less concentrated star that has grown a small amount. The White dwarf would be smaller than the red dwarf."
"no idea. it sounded intuitive."
"Whites are usually smaller the reds"
"Because it doesn't generate as much nuclear fusion?"
"because thats what color my lawn gnome is"
"The book lists red and white dwarfs as the smallest, and I'd say it's about 50/50 that I'm right."
"honestly, I guessed.."
"white dwarf are the smallest in size because it ends up losing material when it is in their burning stage"
"i guessed really have no idea"
"i chose the white dwarf because it is a dead star"
"i chose white dwarf because it is the correct answer and should be picked. to pick any other answer would mean i would get the problem wrong and the objective is to get it right.....therefore, i chose white dwarf"
"Dwarfs are tiny people, so perhaps dwarf stars are the littlest stars, and red is cooler than white."
"White Dwarf star's are roughly an average size of the earth, which is not large for a star."
"It says that white dwarfs are the smallest"
"has the smallest diamete"
"becasue thats what google told me"
"A white dwarf is hotter which means its more dense."
"I'm still lost, and I still don't know how to answer that."
"less nuclear reactions"
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