Content Benchmark E.8.B.6

Students know the Sun is a medium-sized star located in the Milky Way Galaxy, part of which can be seen as a glowing band of light spanning the clear night sky. W/S

A few hundred billion stars reside within the Milky Way Galaxy. We have come to understand that not all stars are created equally. Some stars are older, some are younger. Some stars are hotter, some are cooler. Some stars are larger, some are smaller. And stars come in a variety of colors (Figure 1). Astronomers are interested in all aspects of stars to help them better understand the formation and evolution of galaxies

The following NASA site “Imagine the Universe” is written in very student friendly terms and address the question, “What is a Star?” along with many others.

To access this site, visit http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html.

Figure 1. Stellar Classes (From http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml)

Size of Stars The largest stars in the Milky Way Galaxy range up to approximately 2,700,000,000 km in diameter. These are known as Supergiants and Hypergiants. Figure 2 depicts examples of stars in this category. Figure 2. Supergiant Stars and Hypergiant Stars (From http://www.angelfire.com/sc2/Trunko/starsh1.JPG)

At the other extreme, the most diminutive stars in the Milky Way Galaxy are as small as 11,000 km in diameter. Such little stars are in the category of White Dwarfs see Figure 3.

Figure 3. White Dwarf Stars (From http://www.angelfire.com/sc2/Trunko/starsd1.JPG) Our sun, Sol, is a medium-sized star 1,400,000 km in diameter. This fact and others can be found at http://www.nineplanets.org/sol.html. The Sun is a bit less than 2000 times smaller than the largest stars, and around 130 times bigger than the smallest stars. Figure 4 shows the Sun with other stars in the same evolutionary sequence within the Milky Way Galaxy. Specifically, our sun is one among billions of Main Sequence stars.

Figure 4. The Sun and Main Sequence Stars (From http://www.angelfire.com/sc2/Trunko/starsm1.JPG)

Data about the luminosity and temperature of stars is typically plotted as a Hertzsprung-Russell (HR) diagram. As evidenced in Figure 5, most stars fall within the Main Sequence of this diagram. It is where the stars spend the majority of their evolutionary history. This particular diagram also shows that the preponderance of Main Sequence stars have solar radii only a factor of 10 from the Sun. That is, most Main Sequence stars are from ten times larger than the Sun to ten times smaller than the Sun. The Sun can be located in Figure 5 as a G-class star with a luminosity of 1.

To access a description of an HR diagram, visit http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_hrintro.html. Figure 5. The Hertzsprung-Russell Diagram (From http://www.astro.psu.edu/users/saez/Class/class.html)

Though the Sun currently is a relatively average-sized star, its future holds change. The Sun is at a stage of its life where it is “burning” hydrogen. More accurately, as depicted in Figure 6, hydrogen at the core of the Sun is undergoing fusion into helium.

For more information on stellar evolution, visit http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter20.html. Figure 6. Hydrogen Fusion in the Sun (From http://www.astro.psu.edu/users/saez/Class/class.html)

There will come a time when fusion has consumed all the hydrogen in the core region of the Sun. At this point, the Sun will leave the Main Sequence as the outward pressure from the core decreases and gravitational collapse causes the Sun to contract. As the Sun contracts, the temperature at the core will increase, and when a temperature of around 100,000,000 K has been reached, fusion of helium will begin (Figure 7). The renewed fusion will cause tremendous expansion of the Sun’s outer layers. This new Red Giant Sun will have a diameter of almost 300,000,000 km, about 200 times bigger than the Sun is today. It is thought this stage will occur in around 5,000,000,000 years.

For more information on this likely change to the sun, visit http://www.astro.uiuc.edu/~kaler/sow/star_intro.html#giants.

Figure 7. Helium Fusion in the Sun (From http://www.astro.psu.edu/users/saez/Class/class.html) Stars with masses comparable to that of our Sun are unable to proceed beyond helium fusion. When all the helium at the Red Giant Sun’s core is exhausted, the Sun will again collapse. As before, the collapse will release enough energy to expand the Sun’s outer layers. There will not be enough energy to ignite carbon fusion. The outer layers will swell into a gaseous planetary nebula, leaving a White Dwarf Sun at its center. The White Dwarf Sun will be around 0.008 the current diameter of the Sun. Figure 8 is a depiction of the evolutionary move of the Sun to a position above right of the Main Sequence as it becomes a Red Giant, and then later to a position below left of the Main Sequence as the Sun becomes a White Dwarf.

For a more complete description of Red Giants and supernova, please visit http://outreach.atnf.csiro.au/education/senior/cosmicengine/stars_types.html#startyperedgiant.

Figure 8. Evolutionary Future of Sun (From http://hyperphysics.phy-astr.gsu.edu/Hbase/Astro/herrus.html#c6) Mass of Stars The Sun has a mass of 1.989 x 1030 kg, which places it in the top 10% of stars by mass in our galaxy. Astronomers believe stars in the Milky Way Galaxy have an upper limit of 150 solar masses (2.984 x 1032 kg). The least massive star thus far observed, OGLE-TR-122, is just 0.092 solar masses (1.824 x 1029 kg) as found at, http://www.newscientist.com/article.ns?id=dn7098. Figure 9 is a chart comparing the Sun’s mass to that of several other stars.

This and other information concerning the Sun and stars in the Milky Way galaxy, visit http://www.nineplanets.org/sol.html.

For more details on the mass of stars, visit http://www.space.com/scienceastronomy/050309_heavy_stars.html.

Solar Star Masses Orionis C 18.0 Becrux 16.0 Spica 10.5 Achernar 5.40 Rigel 3.50 Sirius A 2.60 Fomalhaut 2.20 Altair 1.90 Procyon A 1.35 Alpha Centauri A 1.08 The Sun 1.00 Mu Cassiopeiae 0.95 Tau Ceti 0.85 Pollux 0.83 Epsilon Eridani 0.78 Alpha Centauri B 0.68 Lalande 21185 0.33 Ross 128 0.20 Wolf 359 0.10 Figure 9. Mass of Sun as Compared to Other Stars (From http://www.essex1.com/people/speer/main.html)

Brightness of Stars The brightness of stars, including our Sun, is measured in terms of magnitude and luminosity. This measurement is somewhat complicated by the fact that nearby dimmer stars might appear brighter than really bright distant stars. The further a star is below a magnitude of zero, the brighter it is. The further above a magnitude of zero, the dimmer the star. The term “apparent magnitude” is given to the brightness of stars as they appear when viewed from Earth. A calculation is performed to create a value known as “absolute magnitude”. By doing so, computations reveal how bright the star would appear to us if it were a standard distance of ten parsecs from Earth (1 parsec equals 3.26 light years). Due to its proximity to Earth, the sun has an apparent magnitude of -26.74. But, the calculated absolute magnitude is +4.83 (Figure 10). In the Milky Way Galaxy, stars range from a brightest absolute magnitude of -8 to a dimmest absolute magnitude of +19 (negative numbers are brighter than positive numbers).

Figure 10. Magnitudes of Stars (From http://www.astronomynotes.com/starprop/s4.htm)

For more information concerning the brightness of stars, please visit http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html.

Absolute magnitude correlates with the luminosity of a star. Luminosity can be thought of as the quantity of energy released by the star per unit of time, and is proportional both to the temperature of a star and to the surface area of that star (Figure 11). The luminosities of stars are expressed in units of solar luminosity (our Sun generates one solar luminosity). One solar luminosity is equivalent to 4 × 1026 watts of power. Figure 11. Luminosity as a Factor of Temperature and Surface Area of Stars (From http://www.astronomynotes.com/starprop/s4.htm)

Color of Stars The color of a star is a measure of that star’s temperature. Cool stars are red, while hot stars are blue (Figure 12). Earth’s Sun has a surface temperature of about 5800K. This and other information about the Sun can be found at, http://www.nineplanets.org/sol.html.

Figure 12. Star Color as Compared to Star Temperature (From http://www.nightscapes.net/techniques/TechnicalPapers/StarColors.pdf) Figure 13 provides a comparison of the spectral class, temperature and color of stars. An interesting aspect of this table is that it also indicates how long a star of a given spectral class and color is expected to remain within the Main Sequence of stars in the Milky Way Galaxy. Our Sun is a yellow G-class star, and should spend from 7 to 15 billion years (Gyr) in the Main Sequence.

Figure 13. Spectral Class and Color of Stars (From http://outreach.atnf.csiro.au/education/senior/astrophysics/spectral_class.html)

Figure 14 depicts the various colors of stars in a photograph of a section of the sky in which the constellations Orion and Taurus are visible. A cross-eyed stereograph of the region of sky with the constellation Leo is presented in Figure 15. To view this image, simply cross your eyes until the two images come into focus as one image. The brightest stars will appear to be in front of the dimmest stars. Figure 14. Various Colors of Stars in Orion and Taursus (From http://www.nightscapes.net/techniques/TechnicalPapers/StarColors.pdf)

Figure 15. Cross-eyed 3D Colors of Stars around Constellation Leo (From http://www.nightscapes.net/techniques/TechnicalPapers/StarColors.pdf)

Milky Way Galaxy Our solar system lies within the plane (disk) of the Milky Way Galaxy. As such, we are unable to view the Milky Way from above or beneath this plane. When we see the Milky Way Galaxy, we are looking at it edge-on (Figure 16). This view presents itself as a dense, whitish band of light across the sky. An ancient belief that this band was made of milk spilt as the goddess Hera nursed her offspring gave us the term for galaxy. Galactose is the Greek word for milk, hence the name Milky Way Galaxy

For more information on the Milky Way Galaxy visit, http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec16.html

Figure 16. Panoramic View of Milky Way Galaxy (From http://cass.ucsd.edu/public/tutorial/MW.html)

We can see only a portion of the galactic disk of the Milky Way Galaxy when observing the night sky, as typified by this view from southern Arizona (Figure 17). In viewing images of the Milky Way Galaxy and when actually observing it in the night sky, large dark patches obscure stars from view. The patches are referred to as “dark nebulae” and are comprised of dense dust and gas more information about dark nebula can be found at http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec16.html.

Use the scrollbar to pan through a complete 360 panoramic photo of the Milky Way Galaxy, located at the following site http://canopus.physik.uni-potsdam.de/~axm/photo.cgi?Image=images/mwpan45s_full. Figure 17. Milky Way Galaxy as seen in Arizona (From http://www.ngcic.org/milkyway/default.htm) Content Benchmark E.8.B.6

Common misconceptions associated with this benchmark

1. Students have a misconception that all stars are the same size. When viewing the night sky, the stars all look about the same size. Students trust what they see with their own eyes, and decide, therefore, that the stars actually all are the same size. They do not understand that the great distances involved and the limitations in resolution of their eyes are leading them to this erroneous assumption. When they have the chance to view the stars through good, high-power telescopes, they begin to note the discrepancy.

For further information on the size of stars, please visit http://www.angelfire.com/sc2/Trunko/stars.html.

For a more complete list of astronomy misconceptions, please visit http://www.astro.uiuc.edu/~kaler/class/miscon.html

2. Students have the mistaken belief that the Sun is much smaller than most stars When students learn about the size range of stars found in the Milky Way Galaxy, they sometimes believe that since the Sun is quite small when compared to the giants and super giants, it is smaller than average. In actuality, the size of the Sun is within the range of average-sized stars for our galaxy.

The site TEN THINGS YOU THOUGHT YOU KNEW ABOUT SUN-EARTH SCIENCE has a list of misconceptions about the solar system, to view this site, please visit http://www-istp.gsfc.nasa.gov/istp/outreach/sunearthmiscons.html

Additional information on this topic can be found at http://www.astro.psu.edu/users/saez/Class/class.html

3. Students falsely believe the Sun has a very small mass compared to other stars in the Milky Way Galaxy This misconception ties in with the belief that the Sun has a below-average size (diameter) when compared to other stars. Students have been shown pictures comparing the sizes of stars, and make the conclusion that since the Sun is not one of the bigger stars, it is less than average. The reality is that when compared to all stars in the Milky Way Galaxy, our Sun is in the top 10% by mass. This and other facts about the sun can be found at http://www.nineplanets.org/sol.html.

The site TEN THINGS YOU THOUGHT YOU KNEW ABOUT SUN-EARTH SCIENCE has a list of misconceptions about the solar system, to view this site, please visit http://www-istp.gsfc.nasa.gov/istp/outreach/sunearthmiscons.html

Visit the following websites for more information of masses of the sun and other stars http://www.telescope.org/pparc/res4.html and http://outreach.atnf.csiro.au/education/senior/astrophysics/binary_mass.html

4. Students erroneously believe that all stars are white When students look at stars, they see mostly white stars in the sky. Really observant students may notice slight coloration to a few of the stars, but might discount that as atmospheric interference or optical illusion. Providing these students the experience of seeing the stars through a quality telescope will help them note that stars come in many colors.

More information pertaining to the color of stars is found at http://outreach.atnf.csiro.au/education/senior/cosmicengine/stars_colour.html and http://www.vendian.org/mncharity/dir3/starcolor/.

For a list of common astronomy misconception, please visit http://www.astro.uiuc.edu/~kaler/class/miscon.html.

5. Students have the misconception that the Sun emits only yellow light. When taught about the colors of stars, students rationalize that since our Sun is yellow, it must be transmitting yellow light only. In reality, the Sun is emitting all colors of visible light, along with many wavelengths not visible to the human eye. Due to the Sun’s temperature of 5800K, yellow is the most prevalent wavelength, but by no means the only color present.

Greater detail regarding the light given off by our Sun is found at http://www.physlink.com/Education/AskExperts/ae665.cfm.

For a list of common misconception, please see http://www.brookscole.com/astronomy_d/templates/student_resources/053439549X_pasacho ff/miscon/miscon.html

6. Students incorrectly believe that the Sun is the brightest star (highest magnitude) After students have been taught that the Sun is a star, it is easy for them to believe the Sun is the brightest star, since that appears to be what they observe. Students will need to learn that proximity of an object emitting light affects its perceived brightness, and that there is a difference between apparent magnitude and absolute magnitude.

To obtain additional information about the brightness of stars, visit http://library.thinkquest.org/26220/stars/.

For a more complete list of astronomy misconceptions, please visit http://homepage.mac.com/vtalsma/syllabi/2943/handouts/misconcept.html#astronomy Content Benchmark E.8.B.6

Sample Test Questions

Questions and Answers to follow on a separate document Content Benchmark E.8.B.6

Answers to Sample Test Questions

Questions and Answers to follow on a separate document Content Benchmark E.8.B.6

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Penn State Astronomy and Astrophysics This site provides a detailed Power Point presentation which walks the user through the relationships of temperature, luminosity, magnitude, size and mass as depicted on Hertzsprung-Russell diagrams. It does a nice job of explaining these factors, as well as addressing the mathematics involved.

To access this website, go to www.astro.psu.edu/users/kluhman/a1/Lec17.ppt.

2. How Big is That Star? A grade-appropriate activity is available at this site which helps students accomplish the following three objectives: 1) Students will be able to explain the relationship between radius and mass among a list of stars. 2) Students will understand how a binary star system's orbit can cause changes in the observed brightness of the system. 3) Students will determine the diameters of stars by analyzing data and manipulating equations.

To access this website, go to http://imagine.gsfc.nasa.gov/docs/teachers/lessons/star_size/star_size.html.

3. Color of Stars At this website, a lesson is available which has students observe colors in the flame of a burning candle to explore connections between matter, light, color, and temperature. The students then elaborate on these basic concepts as they relate to the context of astronomy and stars. When matter gets hot enough, it emits visible light.

To access this website, go to http://stardate.org/teachers/plans/plan.php?lp_id=20.

4. Star Magnitude Graphing In the lesson available at this website, students will use a spreadsheet to record data on stars, and will graphically represent stars by magnitude and location.

To access this website, go to http://www.teachervision.fen.com/stars/lesson-plan/1530.html. 5. Star Type Reading A lesson relating star masses to the types and lives of stars is presented at this web page. Students will read text passages, interpret diagrams and answer questions on this topic.

To access this website, go to http://quest.nasa.gov/projects/astrobiology/astroventure/challenge/Articles/startype.pdf.

6. The Milky Way The two activities at this site are designed to help students gain an appreciation of how the Milky Way Galaxy has been viewed by different cultures throughout history, and to understand the shape of this galaxy, as well as the general position of the Sun within the Milky Way Galaxy.

To access this website, go to http://stardate.org/teachers/activities/milkyway.html.

7. Compare the Sizes of Stars Students will create a model which compares the diameter of the Sun to that of seven other stars. The model is intended to help students grasp the vast difference in diameter of stars, from the very small to the very large, as well as providing a sense of scale of how our Sun relates in size to the other stars.

To access this website, go to www.uwosh.edu/science_outreach/comparing%20the%20sizes %20of%20stars.doc.

8. Interactive Hertzsprung-Russell Diagrams This website presents an interactive animation which depicts the evolutionary journey of a star as it first comes into existence until it dies. The graphics of the animation help students understand the color, temperature and size change stars undergo as they work their way into and out of the Main Sequence. A second interactive graphic quizzes students on the relationships of stars found in the Hertzprung-Russell diagram.

To access this website, go to http://aspire.cosmic-ray.org/labs/star_life/hr_interactive.html.