The Hertzpsrung-Russell (H-R) diagram is a way of categorizing properties of all stars in the sky, including the Sun. The surface temperature, color, luminosity, and radius of stars can all be deduced from their location on the H-R diagram. When a star begins to condense from an interstellar dust cloud, it enters the HR diagram in the middle right. This protostar collapses toward the Main Sequence, taking tens or hundreds of millions years, until the core becomes hot enough to support hydrogen fusion. Collapse stops, hydrostatic equilibrium is attained, and the star has now become a Main Sequence star, which burns hydrogen into Helium in its core. Hot, massive O-type stars only spend a few million years on the Main Sequence; a cooler G-type star like the Sun will live on the Main Sequence for about 10 billion years; low-mass, cool M stars can survive as Main Sequence stars for 100 billion years or longer. When a star has used up most of the hydrogen in its core, it swells up and becomes a giant. The most massive stars produce supernovas; Sun-like stars die more gently and their cores become White Dwarfs.
Lab Tools
We will use the Hertzsprung-Russell Diagram Explorer developed by the University of Nebraska-Lincoln for this lab. This explorer shows a Size Comparison in the left panel between the properties of a selected star and the sun. In the panel you can adjust the temperature and luminosity of the other star and it will give you that star’s radius in solar units. The right panel shows the HR Diagram, you can adjust what the axis are under Options, but it is fine to stay with temperature versus luminosity. You can also click on and off other quantities shown on the plot. Under Plotted Stars you can select which stars are plotted on the diagram. The red x on the diagram starts at the position of the sun, you can move it by dragging it or setting its properties in the Size Comparison panel.
Assignment
First click on the brightest stars under Plotted Stars. Then click on the nearest. Why do these two choices occupy such different places in the HR diagram? Drag the red x to the coldest part of the main sequence. What is the size of a star with this temperature and luminosity. Drag the red x to the hottest value on the main sequence. What is the size of the star now?
Click on show luminosity classes. What is the range of temperatures and sizes for supergiants? How about for red giants? Click on show instability strip, this will show the region where pulsating variables live. What is the largest and smallest size stars in this region.
Questions
- If a star is the same temperature as the sun, but 10000 times more luminous what is its size?
- If a star is 10000K and one thousandths the sun’s luminosity, what is its size?
- How many times more luminous is the most massive star than the least massive star? How many times hotter is it?
Questions
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