Content Benchmark E.12.B.1
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Students know common characteristics of stars. I/S

As astronomers study stars, there are a number of characteristics that can be investigated: temperature, composition, luminosity, mass, motion, and more. Some characteristics are directly observable (such as temperature and some motions), while others (such as mass) require inference from other data. Of these characteristics, the most important are color, temperature, mass, and luminosity.

Although most appear white to our eyes, most stars have a predominant color that is dependent upon their surface temperature. The hotter the star, the more blue light it emits; conversely, cooler stars emit more red light. The predominant “color” can also be outside of the visible range of wavelengths, for very hot (> 20,000K) or very cool (<4500K) stars. If different colors are emitted with each about the same intensity, the star will appear white; this can occur for stars whose surface temperature is moderate. Note that these temperatures, though much hotter than what we encounter in our lives, are still quite small compared to the temperature at the star’s core (which can be tens to hundreds of millions of degrees!)

Figure 1. Blackbody curves of three stars showing the relationship between temperature and color of maximum brightness. (from http://www.astro.washington.edu/larson/Astro101/LecturesFraknoi/astro101s04.html).

The mass of a star, though much more difficult to measure, determines how the star changes, including its rate of change and ultimate demise. As a matter of convenience, stellar masses are compared to that of the Sun, rather than using kilograms. The Sun is 1 MSun (“one solar mass”), which is roughly equal to about 2 × 1030 kilograms. The smallest stars are a little less than 10% of the mass of the Sun, or 0.08 MSun, while the most massive star yet measured is about 150 MSun. The mass of the star will influence most of its other properties, including diameter, temperature, and lifespan.

To learn more about how astronomers determine the mass of a star, go to http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970609f.html.

A star’s mass determines the strength of its gravitational attraction, which in turn influences the temperature and density at the core of the star. These factors will impact the rate at which hydrogen is fused into helium (and later, other nuclear reaction rates as well), thus determining the star’s lifespan. The most massive stars exist for the shortest amount of time, while the low-mass stars can last hundreds or even thousands of times longer. Our Sun is expected to have a main sequence lifespan (when it is fusing hydrogen into helium) of 10 billion years. A star with a mass of 15 MSun has a main sequence lifespan of only 15 million years, whereas a star with 0.5 MSun has a main sequence lifespan of 200 billion years.

Figure 2. This graph shows the main sequence lifespan of a star is related to the star's mass (dotted line). The graph also shows how the star's luminosity is related to its mass (solid line). (From http://btc.montana.edu/ceres/html/Rare/rareearthactivity1.htm)

The star’s mass will also determine how it will “die,” whether as a planetary nebula and white dwarf (in the case of low mass stars), or as a supernova which leaves behind a neutron star or black hole (the most massive stars). Humans have not been able to observe stellar evolution directly, of course: rather, computer and theoretical models are supported by observations of thousands of individual stars.

To learn more about stars’ lifespans and the main sequence, go to http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html.

A star’s diameter is also determined by its mass – more massive stars, during their main sequence period, have larger diameters. The diameter and temperature together determine the star’s luminosity, or the total amount of energy emitted by the star every second. The luminosity of stars ranges from about 0.0001 to more than 100,000 solar units (or LSun). Unfortunately, we cannot measure luminosity directly – how bright the star appears to us depends upon its distance as well as its luminosity.

To learn more about a star’s luminosity, go to http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_luminosity.html.

A star’s luminosity and temperature are frequently plotted on a graph called the Hertzsprung-Russell, or H-R, Diagram (named after two astronomers who independently developed it). Temperature (or sometimes its spectral class, which is related) is plotted on the horizontal axis, with the values decreasing to the right. Luminosity is plotted on the vertical axis, using a logarithmic scale. When the corresponding values for a large number of stars are then plotted on this graph, groupings of stars can be easily identified, including the main sequence.

Figure 3. The H-R Diagram shows different groupings of stars that have been interpreted as different stages of a star's lifespan. (From http://cse.ssl.berkeley.edu/segwayed/lessons/startemp/l6.htm)

To learn more about the H-R Diagram, go to
http://aspire.cosmic-ray.org/labs/star_life/hr_diagram.html.

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Content Benchmark E.12.B.1

Students know common characteristics of stars. I/S

Common misconceptions associated with this benchmark:

1. Students incorrectly think that all stars are white.

Stars emit all colors of light, but not necessarily the same amount of each. Depending upon the star’s temperature, it may emit more of one color of light than of other colors. In extreme cases, this color can even be viewed from Earth (for example, the star Betelgeuse in the constellation Orion can appear reddish).

To learn more about the different colors of stars, go to http://www.madsci.org/posts/archives/may97/859923240.As.r.html.

2. Students incorrectly think that blue corresponds to lower temperatures and red corresponds to higher temperatures.

When we observe the human body and the technology around us, we see that cold is represented by blue and hot is represented by red. Our skin takes on a bluish tint at extremely cold temperatures and is reddened by sunburns and exertion, both of which are seen more frequently at hotter temperatures. Likewise, the temperature controls in our cars, homes, and the like use blue for cold and red for hot. However, the emission of light works opposite of this: objects at lower temperatures emit more red light and objects at higher temperatures emit more blue light. This phenomenon can also be seen in chemistry, when using a Bunsen burner, where the hottest part of the flame is blue.

To learn more about how temperature relates to color, go to
http://amazing-space.stsci.edu/resources/qa/ems.php.p=Astronomy+basics
%40%2Ceds%2Castronomy-basics.php&a=%2Ceds (in particular, see Questions 5 and 6).

3. Students incorrectly believe that more massive stars live longer than less massive stars.

Since more massive stars contain more hydrogen, the possibility that it will take longer for that hydrogen to be fused into helium seems perfectly reasonable. However, this is not the case. More massive stars have a larger gravitational attraction pulling the gas particles together, resulting in higher temperatures and pressures in the core. This leads to a higher rate of fusion (the transformation of hydrogen into helium), so the hydrogen is depleted faster than in a less massive star. The more massive stars therefore have shorter lifespans than less massive stars.

To learn more about how a star’s mass relates to its lifespan, go to http://www.astronomynotes.com/evolutn/s2.htm.

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Content Benchmark E.12.B.1

Students know common characteristics of stars. I/S

 Sample Test Questions

1st Item Specification: Identify the processes of stellar evolution.

Depth of Knowledge Level 1

1. Which of the following determines most characteristics and future events of a star’s existence?

A. Size

B. Temperature

C. Color

D. Mass

2. When a star is on the main sequence, it is

A. converting hydrogen into helium through nuclear fusion.

B. converting uranium into helium through nuclear fission.

C. burning hydrogen and oxygen to make heavier elements.

D. burning radioactive elements to create lighter elements.

A. convection of stellar material from the core.

B. immense pressure created by energy in the core.

C. coronal mass ejections originating from the core.

D. endothermic cooling of hydrogen fuel in the core.

 

A. Main sequence, planetary nebula, red giant, white dwarf

B. Red giant, white dwarf, black hole, main sequence

C. Main sequence, red giant, planetary nebula, white dwarf

D. Red giant, main sequence, black hole, planetary nebula

 

5. Which of the following lists the stages of life of a high-mass star in correct order?

A. Main sequence, red giant, supernova, neutron star

B. Red giant, main sequence, planetary nebula, neutron star

C. Main sequence, black hole, supernova, planetary nebula

D. Red giant, white dwarf, black hole, main sequence

Depth Of Knowledge Level 2

6. Use the diagram below to answer the following question.

 
(From http://emsc32.nysed.gov/osa/reftable/esp10-16.pdf)

Which of these stars is considered a main sequence star?

A. Aldebaran

B. Alpha Centauri

C. Betelgeuse

D. Procyon B

 

7. What happens to cause a star to leave the main sequence? The gravitational pull inward becomes

A. greater than the outward pressure, so the core of the star shrinks while the outer layers expand, creating a red giant.

B. greater than the outward pressure, so the core and the outer layers of the star both shrink, creating a black hole.

C. less than the outward pressure, so the core and the outer layers of the star both expand, creating a red giant.

D. less than the outward pressure, so the core and the outer layers of the star both expand, creating a black hole.

2nd Item Specification: Recognize that stars change in temperature, size, color, and brightness as they age.

Depth of Knowledge Level 1

8. A star’s brightness

A. remains the same over its lifetime.

B. increases as it gets older.

C. decreases as it gets older.

D. fluctuates during its lifetime.

9. A star’s temperature

A. fluctuates during its lifetime.

B. decreases as it gets older.

C. increases as it gets older.

D. remains the same over its lifetime.

Depth of Knowledge Level 2

10. As a star leaves its main sequence stage, it gets

A. cooler, smaller, bluer, and dimmer.

B. hotter, larger, redder, and brighter.

C. cooler, larger, redder, and brighter.

D. hotter, smaller, bluer, and dimmer.

11. As a star leaves its red giant stage and goes toward a white dwarf, it gets

A. cooler, smaller, bluer, and dimmer.

B. hotter, larger, redder, and brighter.

C. cooler, larger, redder, and brighter.

D. hotter, smaller, bluer, and dimmer.

12. Compared to a red giant, a white dwarf is

A. older, hotter, smaller, and brighter.

B. older, hotter, smaller, and dimmer.

C. younger, cooler, smaller, and brighter.

D. younger, hotter, smaller, and dimmer.

3rd Item Specification: Demonstrate the general relationship between the color and temperature of stars.

Depth of Knowledge Level 1

13. What color are the hottest stars?

A. Red

B. Blue

C. Yellow

D. Green

14. The color of a star mainly depends on

A. its location within the galaxy.

B. its position within a constellation.

C. the temperature of the star.

D. the composition of the star.

15. If two stars, one red and one blue, are both on the main sequence, what is known about their temperatures?

A. The blue star is hotter.

B. The red star is hotter.

C. They are the same temperature.

D. Nothing is known about their temperatures.

Depth of Knowledge Level 2

16. Rank the order of a blue, a yellow, and a red star from COOLEST to HOTTEST.

A. Red -> blue -> yellow

B. Red -> yellow -> blue

C. Yellow -> red -> blue

D. Yellow -> blue -> red

17. Use the diagram below to answer the following question.

(From http://emsc32.nysed.gov/osa/reftable/esp10-16.pdf)

What is the approximate temperature of Rigel?

A. 6,000 °C

B. 8,000 °C

C. 10,000 °C

D. 12,000 °C

Content Benchmark E.12.B.1.

Students know common characteristics of stars. I/S

Answers to Sample Test Questions

1. D, DOK level 1
2. A, DOK level 1
3. B, DOK level 1
4. C, DOK level 1
5. A, DOK level 1
6. B, DOK level 2
7. A, DOK level 2
8. D, DOK level 1
9. A, DOK level 1
10. C, DOK level 2
11. D, DOK level 2
12. B, DOK level 2
13. B, DOK level 1
14. C, DOK level 1
15. A, DOK level 1
16. B, DOK level 2
17. D, DOK level 2

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Content Benchmark E.12.B.1

Students know common characteristics of stars.

Intervention Strategies and Resources

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

1. Amazing Space’s “Star Light, Star Bright”
This website explores the electromagnetic spectrum through several interactive animations. In particular, the activities “Heating Up” and “Stellar Encounters” focus on the relationship between a star’s temperature and color.

The program can be found at:
http://amazing-space.stsci.edu/resources/explorations/light/

2. Sloan Digital Sky Server
The SDSS contains two detailed activities relating star color to temperature through the use of real astronomical data.

The basic activity can be found at http://cas.sdss.org/dr5/en/proj/basic/color/ and an advanced one is at http://cas.sdss.org/dr5/en/proj/advanced/color/.

3. Windows to the Universe “ExploraTour: A Peek into the Lives of Stars”
Although this web-based tour includes much more than just light and temperature, there are several pages of interest.

Start with the link “Colors of the night sky” at: http://www.windows.ucar.edu/tour/link=/cool_stuff/tourstars_1a.html.

4. Planetarium at the College of Southern Nevada (CSN)
The Planetarium presents programs to the general public and the local schools onto a domed screen.  The screen features an Evan & Southerland Digistar 3 hemispheric video projection system that creates virtual realities on the dome surrounding the audience.  A complete list of available programs along with their descriptions can be accessed at http://www.csn.edu/planetarium/

The planetarium at CSN also operates the NASA Regional Educator Resource Center (NRERC) for the State of Nevada.  Countless NASA resources are available through the following link,
http://sites.csn.edu/planetarium/RTRC.html

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