06Lab_HR-Diagram-improved_master

MONTGOMERY COLLEGE - ROCKVILLE ASTRONOMY 101 ASTR101 Laboratory 6 - The H-R Diagram* Name: A star is a delicately balanced ball of gas, fighting between two impulses: gravity, which wants to squeeze the gas all down to a single point, and radiation pressure, which wants to blast all the gas out to infinity. These two opposite forces balance out in a process called hydrostatic equilibrium, and keep the gas at a stable, fairly constant size. The radiation itself is due to the fusion of protons in the star's core - a process that produces huge amounts of energy. In previous laboratories we’'ve examined the most important properties of stars: their temperatures, colors and brightnesses. Now let's see if we can find some relationships between these stellar properties. We know that hotter stars are brighter, as described by the Stefan-Boltzmann Law, and we know that the hotter stars are also bluer, as described by Wien's Law. The H-R diagram is a way of displaying an important relationship between a star's absolute magnitude (or luminosity), and its spectral type (or temperature). Remember, absolute magnitude is how bright a star would appear to be, if it were 10 parsecs away. Luminosity is how much total energy a star gives off per second. As we studied in a previous exercise, spectral type is a system of classifying stars by temperature, from hottest (type O) to coldest (type M). Each letter in the spectral type list (O, B, A, F, G, K, and M) is further subdivided into 10 steps, numbered 0 through 9, to make finer distinctions between stars. So a B4 star is slightly hotter than a B6 star, etc. The two astronomers who figured out that there was a very interesting relationship between luminosity (or absolute magnitude) and Temperature (or spectral type) when you plotted them on a graph together were Ejnar Hertzsprung and Henry Russell. Their graph or diagram was a profound insight that has helped astronomers organize their thinking about stars since it was created in the 1930's. PART A On the next page, in Table 1, is a list of some of the brightest stars in the sky, and also some of the nearest stars in the sky. Some of these names should be familiar to you, as stars you may have seen in the sky personally. Many of these names, however, will be unfamiliar. The reason for this will become clear. 1 Last edit Spring 2023.

ASTR101 LABORATORY © We need to fill in the spectral type for each star. To do this, we'll need to search for each star in Stellarium using its Hipparcos Catalog Number. The Hipparcos Catalog is a reference list of about 100,000 stars in the sky. Every star you can see with the naked eye, and many thousands that you can't see, were all carefully organized in the Hipparcos Catalog in the 1980's and 90's by the Hipparcos spacecraft, which was built by a group of European scientists. Stellarium uses the data from the Hipparcos catalog to display stars in the sky. Using the given Hipparcos catalog numbers in Table 1 below, search and select each star in the list below by opening the search window (CTRL-F or the F3 button) and typing the letters HIP and then the Hipparcos catalog number, and then pressing Enter to select and center the star and display the information on the star. (Please note that in older versions of Stellarium you will need to use HP instead of HIP.) Look for “Absolute Magnitude” and “Spectral Type” and record these values in Table 1. For the spectral type, please keep only the first UPPER CASE letter and the subsequent number (i.e. G2, or M1) in the spectral type listing and ignore any Roman Numerals or letters after the numbers. Van Maanen 2 has been completed for you. Table 1 Nearby Stars Bright Stars # | Star Name HIP Spectr | Abs. # | Star Name HIP Spectr | Abs. Numb | al Mag. Numb | al er Type er Type Groombridge 2 1 | 34 1475 1 | Achernar 7588 F7 14.18 2 2 | Van Maanen 2 | 3829 2 | Aldebaran 21421 2 3 | Tau Ceti 8102 3 | Rigel 24436 2 4 | Epsilon Eridani | 16537 4 | Capella 24608 2 5 | Kapteyn's Star | 24186 5 | Betelgeuse 27989 2 6 | Ross 614 A 30920 6 | Canopus 30438 2 7 | Luyten’s Star 36208 7 | Sirius A 32349 2 8 | Procyon A 37279 8 | Wasat 35550 2 9 | Lalande 21185 | 54035 9 | Pollux 37826 1 3 O | Ross 128 57548 0 | Acrux 60718 1 3 1 | Alpha2 Cen 71681 1 | Mimosa 062434 1 | Alphal Cen 71683 3 | Spica 65474

ASTR101 LABORATORY © In what part of the diagram are most of the bright stars plotted? Where on your diagram are most of the stars you plotted located? Can you find a star on your diagram that is both bright and cold? What is its name? What part of the diagram are the bright and cold stars located? Can you find a star on your diagram that is both hot and dim? What is its name? What part of the diagram are the hot and dim stars located? Are main sequence stars with a larger mass generally hotter or colder? Compare the star Vega to our Sun. is it more or less massive than the Sun? How about Epsilon Eridani? The part of the H-R diagram where most of the stars are plotted is called the main sequence. The Sun, for example is on the main sequence. This part of the curve is where stars in the prime of their life are located, as they fuse hydrogen into helium. Suppose | told you | found a Main Sequence star that was type A5. Using your diagram, what should its absolute magnitude be? What about a type K3 star? What about a type M9? Are more stars on the Main Sequence or off it? List three stars that are not on the Main Sequence. In which two parts of the H-R diagram are the non-Main Sequence stars located?

ASTR101 LABORATORY © What term do we use to refer to stars in the upper right section of the H-R diagram? Stars in the upper right section of the H-R diagram are brighter than Main Sequence stars of the same temperature. In your own words, explain why. What term do we use to refer to stars in the lower left section of the H-R diagram? Stars in the lower left section of the H-R diagram are dimmer than Main Sequence stars of the same temperature. In your own words, explain why. If you were to plot an H-R diagram for all the stars in the galaxy, do you think it would look more like an H-R diagram for the nearby stars or the brightest stars? Explain. The H-R diagram is an incredibly useful tool and a brilliant insight into stars. A star's position on the diagram tells us a LOT about that star. The H-R diagram is a great 5

ASTR101 LABORATORY 6 Absolute Magnitude Sl Ol Ol- Sl- N o N 00 0d ov 0 adAJ, 1enoads 0D 0 ON