ACT Science: Data Representation (Drill 5) is a Science practice drill covering Data Representation. It contains 5 original questions created by Brian Stewart, a Barron's test prep author with over 20 years of tutoring experience.
ACT Data Representation questions require interpreting scientific diagrams accurately. This drill uses a Hertzsprung-Russell diagram showing stellar classification by temperature and luminosity, asking you to identify stellar types, compare properties, and classify a newly described star.
Figure 1
Astronomers classify stars by plotting their surface temperature against their luminosity (brightness relative to the Sun). This type of diagram is called a Hertzsprung-Russell (H-R) diagram. Surface temperature is plotted on the x-axis from hot (left) to cool (right), note that this axis is reversed from a conventional number line. Luminosity on the y-axis is expressed as a multiple of the Sun's luminosity (L☉). Figure 1 shows a simplified H-R diagram with several labeled stars and four major stellar groups: the Main Sequence (a diagonal band), Red Giants, White Dwarfs, and Supergiants. The Sun is included as a reference point.
Figure 1. Hertzsprung-Russell Diagram
Luminosity (L☉)
Surface Temperature (hotter left, cooler right)
10⁻⁴
10⁻²
10⁰
10²
10⁴
10⁶
30,000K
10,000K
6,000K
4,000K
3,000K
Supergiants
Red Giants
Main Sequence
White Dwarfs
Rigel
Deneb
Sirius A
Sun ☉
Arcturus
Aldebaran
Van Maanen's
Proxima Cen.
Question 1. According to Figure 1, which of the following stars has the highest surface temperature?
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A) Aldebaran
-
B) The Sun
-
C) Sirius A
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D) Rigel ✓
Explanation: In Figure 1, temperature increases to the left (as stated in the passage, the x-axis is reversed). Rigel is plotted furthest to the left of the labeled answer choices, near the 10,000–30,000 K region, making it the hottest among the options. Aldebaran is far to the right (~4,000 K, cool), the Sun is near 6,000 K, and Sirius A is near 10,000 K. Rigel's blue-white color also reflects its high surface temperature (~12,000 K).
Figure 1
Astronomers classify stars by plotting their surface temperature against their luminosity (brightness relative to the Sun). This type of diagram is called a Hertzsprung-Russell (H-R) diagram. Surface temperature is plotted on the x-axis from hot (left) to cool (right), note that this axis is reversed from a conventional number line. Luminosity on the y-axis is expressed as a multiple of the Sun's luminosity (L☉). Figure 1 shows a simplified H-R diagram with several labeled stars and four major stellar groups: the Main Sequence (a diagonal band), Red Giants, White Dwarfs, and Supergiants. The Sun is included as a reference point.
Figure 1. Hertzsprung-Russell Diagram
Luminosity (L☉)
Surface Temperature (hotter left, cooler right)
10⁻⁴
10⁻²
10⁰
10²
10⁴
10⁶
30,000K
10,000K
6,000K
4,000K
3,000K
Supergiants
Red Giants
Main Sequence
White Dwarfs
Rigel
Deneb
Sirius A
Sun ☉
Arcturus
Aldebaran
Van Maanen's
Proxima Cen.
Question 2. Based on Figure 1, Arcturus and Aldebaran are both classified as Red Giants. Compared to the Sun, both stars are:
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A) cooler and less luminous
-
B) hotter and more luminous
-
C) cooler and more luminous ✓
-
D) hotter and less luminous
Explanation: In Figure 1, both Arcturus and Aldebaran are plotted to the right of the Sun (cooler temperatures, around 3,900–4,300 K vs. the Sun's ~6,000 K) and above the Sun on the y-axis (higher luminosity, around 10²–10³ L☉ vs. the Sun's 1 L☉). Red Giants are therefore cooler but far more luminous than the Sun. They are enormously larger in physical size, which more than compensates for their lower surface temperature to produce much greater total light output.
Figure 1
Astronomers classify stars by plotting their surface temperature against their luminosity (brightness relative to the Sun). This type of diagram is called a Hertzsprung-Russell (H-R) diagram. Surface temperature is plotted on the x-axis from hot (left) to cool (right), note that this axis is reversed from a conventional number line. Luminosity on the y-axis is expressed as a multiple of the Sun's luminosity (L☉). Figure 1 shows a simplified H-R diagram with several labeled stars and four major stellar groups: the Main Sequence (a diagonal band), Red Giants, White Dwarfs, and Supergiants. The Sun is included as a reference point.
Figure 1. Hertzsprung-Russell Diagram
Luminosity (L☉)
Surface Temperature (hotter left, cooler right)
10⁻⁴
10⁻²
10⁰
10²
10⁴
10⁶
30,000K
10,000K
6,000K
4,000K
3,000K
Supergiants
Red Giants
Main Sequence
White Dwarfs
Rigel
Deneb
Sirius A
Sun ☉
Arcturus
Aldebaran
Van Maanen's
Proxima Cen.
Question 3. According to Figure 1, which of the following stars is a White Dwarf?
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A) Sirius A
-
B) Van Maanen's Star ✓
-
C) Proxima Centauri
-
D) Deneb
Explanation: In Figure 1, Van Maanen's Star is plotted within the White Dwarf ellipse, positioned at moderate temperature (near 6,000–7,000 K) but extremely low luminosity (around 10⁻³ L☉). White dwarfs are the dense, cooling remnants of stars that have exhausted their nuclear fuel. Sirius A is a Main Sequence star. Proxima Centauri is a cool, dim Main Sequence red dwarf. Deneb is a Supergiant.
Figure 1
Astronomers classify stars by plotting their surface temperature against their luminosity (brightness relative to the Sun). This type of diagram is called a Hertzsprung-Russell (H-R) diagram. Surface temperature is plotted on the x-axis from hot (left) to cool (right), note that this axis is reversed from a conventional number line. Luminosity on the y-axis is expressed as a multiple of the Sun's luminosity (L☉). Figure 1 shows a simplified H-R diagram with several labeled stars and four major stellar groups: the Main Sequence (a diagonal band), Red Giants, White Dwarfs, and Supergiants. The Sun is included as a reference point.
Figure 1. Hertzsprung-Russell Diagram
Luminosity (L☉)
Surface Temperature (hotter left, cooler right)
10⁻⁴
10⁻²
10⁰
10²
10⁴
10⁶
30,000K
10,000K
6,000K
4,000K
3,000K
Supergiants
Red Giants
Main Sequence
White Dwarfs
Rigel
Deneb
Sirius A
Sun ☉
Arcturus
Aldebaran
Van Maanen's
Proxima Cen.
Question 4. Based on Figure 1, which of the following best describes the general trend of Main Sequence stars?
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A) Hotter Main Sequence stars tend to be less luminous than cooler ones
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B) Hotter Main Sequence stars tend to be more luminous than cooler ones ✓
-
C) All Main Sequence stars have the same luminosity regardless of temperature
-
D) Main Sequence stars show no consistent relationship between temperature and luminosity
Explanation: The Main Sequence in Figure 1 runs diagonally from upper-left (hot and luminous) to lower-right (cool and dim). Stars along this band, Sirius A (hot, high luminosity), the Sun (intermediate), and Proxima Centauri (cool, low luminosity), all follow the same trend: higher temperature correlates with higher luminosity. This is the defining characteristic of main sequence stars, which generate energy through hydrogen fusion in their cores, and it holds across a luminosity range spanning many orders of magnitude.
Figure 1
Astronomers classify stars by plotting their surface temperature against their luminosity (brightness relative to the Sun). This type of diagram is called a Hertzsprung-Russell (H-R) diagram. Surface temperature is plotted on the x-axis from hot (left) to cool (right), note that this axis is reversed from a conventional number line. Luminosity on the y-axis is expressed as a multiple of the Sun's luminosity (L☉). Figure 1 shows a simplified H-R diagram with several labeled stars and four major stellar groups: the Main Sequence (a diagonal band), Red Giants, White Dwarfs, and Supergiants. The Sun is included as a reference point.
Figure 1. Hertzsprung-Russell Diagram
Luminosity (L☉)
Surface Temperature (hotter left, cooler right)
10⁻⁴
10⁻²
10⁰
10²
10⁴
10⁶
30,000K
10,000K
6,000K
4,000K
3,000K
Supergiants
Red Giants
Main Sequence
White Dwarfs
Rigel
Deneb
Sirius A
Sun ☉
Arcturus
Aldebaran
Van Maanen's
Proxima Cen.
Question 5. A newly discovered star has a surface temperature of approximately 4,000 K and a luminosity of approximately 10² L☉. Based on Figure 1, this star would most likely be classified as:
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A) a Main Sequence star
-
B) a White Dwarf
-
C) a Red Giant ✓
-
D) a Supergiant
Explanation: Plotting the coordinates: 4,000 K on the x-axis (right side, cool) and 10² L☉ on the y-axis (between the 10⁰ and 10⁴ gridlines, well above the Sun) places the star squarely in the Red Giant ellipse, near where Arcturus and Aldebaran are plotted. A Main Sequence star at 4,000 K would have a luminosity far below 10² L☉, placing it well below the Red Giant region on the diagram. White Dwarfs cluster at high temperature and very low luminosity, while Supergiants sit at extremely high luminosity (10⁴ L☉ and above).