LAB 06 - The HR Diagram
- Due Oct 25, 2021 at 11:59pm
- Points 100
- Questions 10
- Available after Oct 18, 2021 at 12am
- Time Limit None
- Allowed Attempts Unlimited
Instructions
LAB 06 - The H-R Diagram
The H-R Diagram can be thought of as a kind of family tree for stars. On this diagram, we can compare star temperatures / colors to star luminosity (in either solar luminosity or absolute magnitude values). You can learn more about the H-R Diagram by checking out 1403 - MODULE 04 - STELLAR PROPERTIES.pdf.
Around 1911-1913, the Danish astronomer Ejnar Hertzsprung and America's premier astronomer Henry Norris Russell compared the luminosities of various stars with their spectral types. The spectral type of a star is determined by the arrangement of lines in its absorption spectrum. It was seen that varying arrangements of these absorption lines correspond to varying temperatures (or color). The spectral classifications are summed up in the table below.
A quantity directly related to luminosity is magnitude, luminosity being the rate at which a star radiates electromagnetic energy into space (i.e., brightness). The APPARENT magnitude, denoted as mv, corresponds to the brightness of a star as viewed from Earth. The ABSOLUTE magnitude of a star, denoted Mv is the magnitude a star would have if it were 10 parsecs from Earth (There are 3.26 light- years in a parsec).
When we plot the absolute magnitude vs. spectral type for various stars we generate a graph called the Hertzsprung-Russell (H-R) diagram. Once again, Mv vs. spectral type can be thought of as a comparison of absolute brightness (i.e. a representation of the star’s actual luminosity) to temperature (i.e. color).
It is found that most stars lie on or near a certain curve. This is the main sequence curve. Stars that lie on or near this curve are referred to as main sequence dwarfs. All main sequence dwarfs burn (via thermonuclear fusion reactions) hydrogen in their cores and are maintaining hydrostatic equilibrium.
It is important to remember that the star's position on the graph has no relation to its position in the heavens. We are plotting the two stellar properties LUMINOSITY and SURFACE TEMPERATURE and a myriad of information is to be obtained by pursuing these relationships.
The following is a somewhat stylized version of the HR Diagram.
The HR Diagram is discussed in detail in Lecture 4. Here are the main things that you need to know about this diagram.
- The Main Sequence is the roughly diagonal distribution of stars that starts with blue stars in the upper left and ends with red stars on the lower right.
- The stars on the Main Sequence are all using hydrogen and their main source of fuel for the nuclear fusion processes in their cores.
- All the stars on the Main Sequence are maintaining hydrostatic equilibrium which is a balance of gravity trying to crush the star and pressure from nuclear fusion trying to blow the star apart.
- Hot stars are on the left in the chart and cool stars are on the right.
- Blue stars are the hottest stars. Red stars are the coolest.
- Small stars are towards the bottom of the chart and big stars are towards the top of the chart.
Now that you know some basic information about the HR diagram, try to answer the following questions.