Student Ideas and Questions
Common Student Ideas
The H-R Diagram (or a color-magnitude diagram) is an actual representation of the physical arrangement of stars in space. Each point on the diagram represents the location of a star.
Bridge to learning:
Direct students to compare the patterns of stars in the image of a star cluster to the patterns seen on an H-R Diagram or CMD. Ask questions such as “Where are the blue stars on the H-R Diagram? (Answer: top left.) Where are the blue stars in the cluster image? (Answer: randomly dispersed). Are star clusters all shaped like CMDs?
The Sun is the (biggest/hottest/brightest) star OR The Sun is an average star.
Bridge to learning:
This will be addressed by working through the investigation and exploring the ranges of star temperatures, luminosities, and sizes. Students will discover that the Sun is neither extreme nor average because it is not located at the midpoint of the range of star characteristics like size or temperature. Point out that the Sun only appears to be the hottest and brightest star because of its very close distance to Earth.
The Sun will blow up sometime in the future.
Bridge to learning:
Although the Sun will become a giant star in the future, it will collapse and end its life as a white dwarf star. Single stars can produce a supernova only if their mass is about eight times greater than the mass of the Sun.
Almost all stars are white.
Bridge to learning:
This idea results from the fact that humans see all but the brightest stars as white, because the cones (color receptors) in our eyes need a considerable amount of light in order to discriminate colors. By examining both the data on the CMDs and the star cluster images, students will develop an appreciation for the range of colors.
Common Student Questions
Why are there so few blue stars?
The energy radiated by massive blue stars as they are forming becomes so great that it halts the accretion of additional gas. So in a sense, the forming star works against its own development. The stars that do succeed at attaining high masses are likely to have been located in optimal star-forming regions where a plentiful supply of material is channeled on to the forming star. Blue stars also have relatively short lifetimes when compared to other stars.
http://physicsworld.com/cws/article/news/2009/jan/15/how-massive-stars-form
Why are not all white dwarf stars white? What does it mean if a white dwarf is not white?
Star color is directly related to temperature. When white dwarfs initially form, they may appear blue-white. The colors of white dwarfs change as they cool. The coolest white dwarf star detected has a surface temperature of about 4900K, making it orange in color. But as their temperature drops, so does luminosity, so seeing color in these faint, cool white dwarfs is not so easy.
Why aren’t there any violet stars?
It’s possible for a star to have a peak electromagnetic radiation curve in the violet range, but this sort of star would also produce high amounts of blue light. Since our eyes are more sensitive to blue, it would appear blue to us.
https://parade.com/295384/marilynvossavant/can-stars-be-green-or-purple/
Why do we see white stars instead of green? What are there no green stars?
Our Sun actually outputs more light in green wavelengths than any other color, but since it is emitting all the other colors in relatively even amounts, they all mix together to look white to our eyes. It’s only when there’s a great imbalance of color (for example, much more blue than red) that the stars appear to be blue or red, etc.
Why is it that the stars form groups and are not randomly scattered over the H-R Diagram?
Main sequence stars are stable, hydrogen-fusing stars. For any given initial mass, a star must fuse hydrogen at a certain rate in order to balance the gravitational force. This fusion rate controls the temperature and luminosity of the star, so the main sequence is a well-defined region in the diagram. Likewise, the properties of giants and white dwarfs are constrained by the different internal physical processes that create an equilibrium state, so there are groupings of temperatures and luminosities that result.
http://www.atnf.csiro.au/outreach/education/senior/cosmicengine/stars_hrdiagram.html
Why are most of the stars on the Main Sequence?
A star spends about 90% of its lifetime in this initial hydrogen fusing phase, so that’s where most of the stars are located. Although the life spans of white dwarfs are considerably long, the Universe is still so young that large quantities of white dwarfs have not yet formed.
Why is the temperature axis on the H-R Diagram backwards (bigger numbers near the origin)?
The original diagrams published by Hertzsprung and Russell arranged the stars along the x-axis by spectral classes from B through M. A star of spectral class B had a visual spectrum that peaked in short (blue) wavelengths, while the spectrum of a class M star peaked longer (red) wavelengths. The (peak) wavelengths increased from left to right. Later H-R Diagrams began listing temperatures along the x-axis, but the temperature trend is the opposite to the trend of peak wavelength. A blue star has a large temperature value but a small peak spectral wavelength.
How do we know the age of the Sun?
We assume that the Sun formed at about the same time as its planetary system. We can make an estimate of the Sun’s age by applying what we know about nuclear fusion rates to the Sun’s physical characteristics. The oldest unaltered rocks in the solar system are meteorites with radiometric dates that average 4.6 billion years, and that is in agreement with the age predicted by nuclear physics.
How do we know the temperature of the Sun?
The Sun’s surface temperature may be found in the same way as all other stars, by plotting a electromagnetic radiation curve of its energy output. Wien’s Law can then be used to estimate its temperature. The temperature estimate can be refined by analyzing absorption lines in the Sun’s spectrum.
How do we know the mass of the Sun?
Newton’s Law of Universal Gravitation can be used to find the Sun’s mass. To solve for this, it is necessary to know the distance of a planet (such as Earth) from the Sun, and its orbital velocity. Kepler’s Third Law of Planetary Motion provides this information.
White dwarfs are no longer powered by nuclear fusion. What prevents them from collapsing into a black hole?
In a normal hot gas, the electrons of atoms can occupy higher energy levels and leave some unfilled spaces in the lower energy levels. White dwarf stars collapse until they are so compressed that electrons are forced inward towards the nucleus of the atoms and there are no empty spaces in the lower energy levels. This creates a degenerate gas, which cannot be compressed any further, and opposes the force of gravity.