Conceptos de los estudiantes y preguntas
Ideas comunes de los estudiantes
La expansión del Universo se debe a la expansión del espacio.
Puente para el aprendizaje: El espacio no se expande como una tela o algo que pueda estirarse (idea errónea introducida por la mayoría de los modelos). Del mismo modo, el Universo no se expande "hacia" nada, dado que no existe espacio "fuera" de él. En su lugar, la escala que se utiliza para definir el espacio cambia. Por esta razón, puede ser útil hablar de que el espacio-tiempo del Universo se expande en lugar de decir que el espacio se expande.
En este video del JPL sobre la Expansión del Universo, se dibujan una serie de ondas sobre una banda elástica. Observa que hay puntos en cada extremo de las ondas dibujadas. Imagina que hay un término que define la distancia entre esos dos puntos. A medida que la banda se estira, el término sigue siendo el mismo, pero su escala ha cambiado.
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
What causes a star to blow up?
There are two different mechanisms. White dwarfs that accumulate too much material from their partner star heat up so much that they begin to fuse carbon, creating a thermal runaway fusion reaction which leads to a Type Ia supernova. Another possible scenario is the merger of two white dwarfs in a binary system.
Massive stars undergo core collapse when they reach a point where the forces generated by fusion are unable to overcome gravitational force. This results in the violent outward explosion of the outer layers of the star, producing what’s called a core collapse supernova. Type IIp supernovae are the example we use in this investigation, but there are other types (e.g., Type Ib and Type IIL). Type Ia is the only class of supernovae not caused by core collapse.
What other kinds of supernovae are there?
Other kinds of supernovae are classified based on slight variations in the shapes of their light curves, and differences in which elements are observed in their spectrum.
Some very energetic supernovae are called hypernovae.
https://en.wikipedia.org/wiki/Supernova#Classification
Could the supernova of a nearby star destroy the Earth?
Probably not. The average “safe distance” from a supernova is estimated to be 160 light years away, and at present there are no known stars within that distance that could one day form a supernova.
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.
http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/#.WpR-BRPwa-o
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.