06Lab_HR-Diagram-improved_master

M ONTGOMERY C OLLEGE – R OCKVILLE A STRONOMY 101 ASTR101 Laboratory 6 - The H-R Diagram 1 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. 1

ASTR101 L ABORATORY 6 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 Numb er Spectr al Type Abs. Mag. # Star Name HIP Numb er Spectr al Type Abs. Mag. 1 Groombridge 34 1475 M1 13.24 2 1 Achernar 7588 B6 -1.46 2 Van Maanen 2 3829 F7 14.18 2 2 Aldebaran 21421 K5 -.64 3 Tau Ceti 8102 G8 5.69 2 3 Rigel 24436 B9 -7.84 4 Epsilon Eridani 16537 K2 6.19 2 4 Capella 24608 G3 0.29 5 Kapteyn’s Star 24186 M1 10.89 2 5 Betelgeuse 27989 M1 -5.85 6 Ross 614 A 30920 M4 13.09 2 6 Canopus 30438 A9 -5.71 7 Luyten’s Star 36208 M3 11.94 2 7 Sirius A 32349 A0 1.43 8 Procyon A 37279 F5 2.66 2 8 Wasat 35550 F0 3.53 9 Lalande 21185 54035 M2 10.48 2 9 Pollux 37826 K0 1.08 1 0 Ross 128 57548 M4 13.53 3 0 Acrux 60718 B0 -3.77 1 1 Alpha2 Cen 71681 K1 5.71 3 1 Mimosa 62434 B0 3.92 1 Alpha1 Cen 71683 G2 4.38 3 Spica 65474 B1 -3.55 2

ASTR101 L ABORATORY 6 In what part of the diagram are most of the bright stars plotted? Towards the upper left Where on your diagram are most of the stars you plotted located? Throughout the chart from the upper left to the lower right Can you find a star on your diagram that is both bright and cold? What is its name? Yes, this star would be Betelgeuse What part of the diagram are the bright and cold stars located? Towards the upper right Can you find a star on your diagram that is both hot and dim? What is its name? Yes, this would be Van Maanen 2 What part of the diagram are the hot and dim stars located? Towards the lower left Are main sequence stars with a larger mass generally hotter or colder? Yes, stars with a larger mass are generally hotter Compare the star Vega to our Sun. is it more or less massive than the Sun? It is more massive than the Sun How about Epsilon Eridani? It is less massive than the Sun 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 I told you I found a Main Sequence star that was type A5. Using your diagram, what should its absolute magnitude be? 1.46 What about a type K3 star? 6.47 What about a type M9? 12.46 Are more stars on the Main Sequence or off it? More stars are on the Main Sequence List three stars that are not on the Main Sequence. Van Maanen 2, Betelgeuse, and Deneb In which two parts of the H-R diagram are the non-Main Sequence stars located? Towards the upper right and the lower left 4

ASTR101 L ABORATORY 6 What term do we use to refer to stars in the upper right section of the H-R diagram? Red Giants 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. Due to their advancing stage, known as the "Red Giant" phase, stars in the upper-right region are brighter than main sequence stars of the same temperature. Their increased size and, consequently, surface area, are the main causes of their increased brightness. These stars grow and get substantially bigger than they were in their main sequence phase as they evolve. More radiant energy is released as a result of the larger surface area. Their total luminosities are higher because, even though their surface temperatures may have dropped during this phase, the larger area more than makes up for the drop in temperature. So even though the stars in the upper-right region of the H- R diagram have colder surface temperatures, their larger sizes and greater surface areas translate into a higher total radiant energy production, making them brighter than main sequence stars of the same temperature. This expansion and increased brightness are characteristics of stars in the red giant phase of their life cycle. What term do we use to refer to stars in the lower left section of the H-R diagram? White Dwarf Stars 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. Stars in the lower-left section of the H-R diagram are dimmer than main sequence stars of the same temperature because they usually belong to a separate phase of their life cycle called the "white dwarf" phase. These stars have already gone through the stages of stellar evolution and used up all their nuclear fuel, which is mostly hydrogen. They are now compressed and very small, with high surface temperatures after collapsing. Because of these features, their temperature is comparable to certain main sequence stars, if not higher, but their luminosity is far lower. Due to their limited size and absence of active nuclear fusion, these stars have a decreased in brightness. White dwarfs no longer use core fusion processes to produce energy. Rather, over an extended length of time, they radiate away the heat energy they have stored. Because of this, they seem dimmer than main sequence stars of comparable temperature, which generate an endless supply of energy by actively fusing hydrogen and helium in their cores. 5

ASTR101 L ABORATORY 6 evolutionary stages of stars across the cosmos. They offer a structure for classifying and interpreting the great variety of stars that we see in the universe. 7