Classifying Stars by Luminosity and Size

Background:

Astronomers use the Hertzsprung Russell diagram above to plot stars according to their surface temperature and luminosity. The vertical axis of the H-R diagram represents a star’s luminosity or absolute magnitude. Luminosity is technically the amount of energy a star radiates in one second, but you can think of it as how bright or how dim the star appears. Depending upon the textbook you use, the labels on the HR diagram could be a little different. Luminosity is a common term, as is absolute magnitude to describe the brightness of a star. In either case, the scale is a "ratio scale" in which stars are compared to each other based upon if they are brighter or dimmer than our Sun. Notice how our Sun has a value of 1.

Luminosity (absolute magnitude) is different than the apparent brightness of a star that we see with the naked eye. This is because the brightness that we interpret can be affected by many variables such as the strength of the light emanating from the star, the distance from us to the star and the amount and kind of obstacles between us and the star (such as clouds.)

Also note that there is a predictable relationship between the brightness and size of a star. This shows up on the HR diagram. We know that hotter things are brighter. A hotter temperature means that more energy is radiated into space. Bigger stars are brighter. A bigger surface area means that more energy is radiated into space.

BIG!

Bright!

Dim

small!

Classifying Stars by Luminosity and Size

Directions:

1) Use the cards to arrange stars labeled on the H-R diagram in decreasing size order by placing them on the appropriate spot on the flow chart. Record your answers on your lab record sheet.

2) Label each star with its Luminosity. Is Luminosity also decreasing or is it increasing?

3) Use the H-R diagram to answer the conclusion questions.

4) Restack the cards neatly for the next group.

Classifying Stars by Luminosity and Size

Rigel

Polaris

Sirius

Sun

Procyon B

Betelgeuse

Proxima Centauri

Spica

Classifying Stars by Luminosity and Size

Conclusion Questions:

1) Measurements indicate that a certain star has a very high luminosity (100,000 times that of our sun) and yet has a temperature that is cooler than the sun. What can you conclude about this observation?

A) It could be a main sequence star. B) It may be quite large.

C) This is a typical characteristic of stars. D) There must be an error in measurement.

2) Two stars of the same color are plotted on an H-R diagram. Star A is more luminous than star B. Which one of the following statements could explain this?

A) Star A is hotter than star B. B) Star A is more distant than star B.

C) Star A appears brighter in the sky than star B. D) Star A is larger than star B.

3) If we plot many stars on an H-R diagram, all with the same luminosity but different temperatures, they

A) would all lie on the main sequence B) would be all over the diagram

C) would form a horizontal line D) would form a vertical line

4) The apparent brightness of an object such as a star does not depend on

A) how fast the star is moving

B) the strength of the light emanating from the star

C) the distance from us to the star

D) the amount and kind of obstacles between us and the star

5) Which of the following stars is least bright?

A) the sun B) a blue supergiant

C) a white dwarf D) a red giant

6) Which factor does not affect a stars absolute magnitude (Luminosity)?

A) The star's temperature. B) The star's size.

C) The star's distance. D) The star's shape

7) The smallest stars on a H-R diagram are found

A) at the upper left end of the main sequence

B) at the lower right end of the main sequence

C) at the upper right corner of the H-R diagram

D) at the lower left corer of the H-R diagram

8) Red giant stars have greater luminosity than our sun mainly because they are

A) hotter B) farther away

C) larger D) older

Classifying Stars by Temperature and Color:

Background: A Star’s color that we see with the naked eye depends upon its surface temperature. As materials become hotter, their color changes from:

Astronomers use the Hertzsprung Russell diagram above to plot stars according to their surface temperature and luminosity. The horizontal axis represents the star’s surface temperature (not the star’s core temperature – we cannot see into the core of a star, only its surface)! Usually this is labeled using the Kelvin temperature scale. But notice: In most graphs and diagrams, zero (or the smaller numbers) exist to the left on the diagram. This is not the case here. On this diagram, the higher (hotter) temperatures are on the left, and the lower (cooler) temperatures are on the right. Some HR diagrams include the color of stars as they can be seen through filters on spectroscopes. We can use this diagram to acquire information about stars found in familiar constellations.

Classifying Stars by Temperature and Color

Directions:

1) Use the cards to arrange stars labeled on the H-R diagram in increasing temperature order by placing them on the appropriate spot on the flow chart. Record your answers on your lab record sheet.

2) Use the colored pencils to shade in the appropriate color for each star.

3) Use the H-R diagram to answer the conclusion questions.

4) Restack the cards neatly for the next group.

Classifying Stars by Temperature and Color

Rigel

Polaris

Sirius

Sun

Procyon B

Betelgeuse

Proxima Centauri

Spica

Station #4: Classifying Stars by Temperature and Color

Conclusion Questions:

1) A Red giant star would most likely have a temperature of

A) 5,000ºK

B) 10,000ºK

C) 20,000ºK

D) 30,000ºK

2) An astronomer can estimate the temperature of a star by observing its

A) size

B) shape

C) color

D) brightness

3) Which star color indicates the hottest star surface temperature?

A) blue

B) white

C) yellow

D) red

16) Small cool stars would most likely appear to be

A) blue

B) red

C) yellow

D) white

4) Which of the following stars is hottest?

A) a red giant

B) a white dwarf

C) the sun

D) a red dwarf

5) Barnard's Star has a surface temperature of about

A) 300 ºK

B) 3000 ºK

C) 5000 ºK

D) 10,000 º