8.10: Accessible Descriptions

Figure 8.1.1 Planisphere

A decorative 1777 planisphere mapping the stars of the Southern celestial hemisphere.

Map Elements

• A circular star map fills the center of the page, surrounded by an ornate decorative border.
• Inset views of four Italian observatories, Pisa, Bologna, Milan, and Padova, are arranged around the border.

Figure 8.2.1 Doppler Effect

A diagram illustrating the Doppler effect using two panels of concentric wave rings from a light source.

Diagram (a)

• Even, concentric rings represent waves spreading from a stationary source, labeled S, at the center.
• From innermost to outermost, the rings are labeled 4, 3, 2, and 1, marking successive wave crests.
• An arrow points outward from the outermost ring toward an observer.

Diagram (b)

• Uneven, concentric rings represent waves from a moving source.
• The source is labeled at four positions, S1 through S4, marking where each wave crest was emitted as the source moves.
• Observer A, below the rings, sees the crests compressed together (blueshift) because the source is moving toward that observer.
• Observer C, at the top, sees the crests spread farther apart (redshift) because the source is moving away.
• Observer B, to the left, sees no change in spacing because the source's motion is perpendicular to that line of sight.

Figure 8.3.1 Large Proper Motion

Three photographs of the same star field, taken ten years apart, showing the proper motion of Barnard's Star.

Image (a)

• On the left
• Taken in 1985
• Barnard's Star is in the center of the image, surrounded by several background stars.

Image (b)

• In the center
• Taken in 1995
• The background stars have not moved, but Barnard's Star has shifted downward from the center of the image.

Image (c)

• On the right
• Taken in 2005
• The background stars have again not moved, and Barnard's Star is now near the bottom of the image.

Credit: Steve Quirk

Figure 8.3.2 Changes in the Big Dipper

Three panels illustrating how proper motion changes the shape of the Big Dipper over time.

Top Panel

• Shows the seven stars of the Big Dipper as they appeared 50,000 years ago.
• The upper right star is farther upward and to the left, giving the cup an unfamiliar shape.

Middle Panel

• Shows the Big Dipper as it appears today.
• An arrow attached to each star points in the direction of its proper motion across the sky.
• Most stars move toward the upper left, while the stars at the far left and right ends of the asterism move toward the lower right.

Bottom Panel

• Shows how the Big Dipper will appear 50,000 years from now.
• The bend in the handle is sharper, and the cup is more open, than in the middle panel.

Figure 8.3.3 Space Velocity and Proper Motion

A diagram showing how a star's radial velocity and transverse velocity combine to produce its total space velocity.

Diagram Elements

• A yellow disk at the lower left represents the Sun.
• A smaller orange disk at the upper right represents a distant star.
• A dashed line connects the Sun and the star, with a solid double-headed arrow above it labeled d for the distance between them.
• A second dashed line from the Sun, offset by a small angle labeled μ (mu) from the Sun-star line, marks the star's measured proper motion, showing the star moving toward the upper left.
• Three arrows extend from the star, showing the components of its motion:
  • An arrow pointing directly away from the Sun represents the radial velocity, along the line of sight.
  • An arrow at a right angle to the radial velocity, pointing up and to the left, represents the transverse velocity, perpendicular to the line of sight.
  • An arrow between the other two, pointing straight up, represents the total space velocity, the combination of the radial and transverse velocities.

Figure 8.4.1 Triangulation

A diagram illustrating how triangulation is used to measure the distance to an object.

Diagram Elements

• Two survey positions, A and B, are set up along a stream, connected by a line labeled Baseline.
• A tree, labeled C, sits on the far side of the stream.
• Lines connect A to C and B to C, forming triangle ABC.
• A dashed line runs from the center of the baseline to C.
• A curved arrow marks the angle between the baseline and line AC.

Figure 8.4.2 Parallax

A diagram illustrating how a nearby star's apparent position shifts as Earth orbits the Sun.

Diagram Elements

• The Sun is a yellow disk on the left, surrounded by a blue circle labeled Earth's orbit.
• Earth is shown at two positions on the orbit: A at the bottom and B at the top.
• A nearby star, shown as a red dot, sits above and to the right of the Sun.
• A group of five more distant stars appears in the upper right.
• A line from position A through the red dot points to the uppermost star in the distant group; a line from position B through the red dot points to the middle star in the group.
• A dashed line runs from the Sun to the red dot, and the parallax angle, p, is marked between this dashed line and line B.
• Two insets show the apparent position of the red dot among the background stars: "Sky as seen from A" shows it near the uppermost star, and "Sky as seen from B" shows it near the middle star.

Figure 8.4.3 H-R Diagram of Stars Measured by Gaia and Hipparcos

A Hertzsprung-Russell diagram plotting 16,631 stars with parallax accuracy of 10% or better.

Chart Elements

• The horizontal axis is labeled Spectral Type and lists, from left to right, O, B, A, F, G, K, and M.
• The vertical axis is labeled Luminosity (L_Sun) and ranges from 1/100 to 10,000, with luminosity increasing upward.
• An infrared color is plotted as a proxy for temperature, with temperature decreasing to the right.
• Color indicates the number of stars at each point, with red representing the most stars and blue the fewest.
• Most data points form a diagonal band running from high luminosity and high temperature at the upper left to low luminosity and low temperature at the lower right; these are main sequence stars.
• A separate clump of data points above the main sequence, on the right side of the diagram, is composed of red giant stars.

Credit: European Space Agency

Figure 8.4.4 Stars in Orion

Two views of the constellation Orion: a photograph and a labeled star chart.

Image (a)

• On the left
• A photograph of the constellation Orion.
• Yellow Betelgeuse is at the upper left, and blue Rigel is at the lower right.
• The three stars of Orion's belt appear just below center.
• Credit: Matthew Spinelli

Diagram (b)

• On the right
• A contemporary star chart of Orion showing the brightest stars with their proper names and Bayer Greek-letter designations.
• From top to bottom: Meissa (lambda), Betelgeuse (alpha), Bellatrix (gamma), Mintaka (delta), Alnilam (epsilon), Alnitak (zeta), Rigel (beta), and Saiph (kappa).
• The nebulae M 78, M 42, and M 43 are circled in red.
• Credit: ESO, IAU and Sky & Telescope

Figure 8.5.1 Cepheid Light Curve

A graph showing how the brightness of a typical cepheid variable star changes over time.

Chart Elements

• The vertical axis is labeled Magnitude and ranges from 4.4 at the bottom to 3.4 at the top, in increments of 0.2. Because magnitude decreases as brightness increases, the top of the axis represents the brightest values.
• The horizontal axis is labeled Time (days) and ranges from 0 to 18 in increments of 1 day.
• The curve begins near magnitude 4.1 at day 0, dips to a minimum of magnitude 4.3 at day 1.5, then rises rapidly to a maximum of magnitude 3.6 at day 3.
• The curve then slowly dips back down to magnitude 4.3 by day 7, and this rise-and-fall pattern repeats two more times through day 18, giving the plot a sawtooth shape.

Figure 8.5.2 Large Magellanic Cloud

A photograph of the Large Magellanic Cloud, a small irregular galaxy near the Milky Way.

Image Elements

• Unlike spiral or elliptical galaxies, the Large Magellanic Cloud has no distinct overall shape and is classified as an irregular galaxy.
• The main body of the galaxy is an elongated bar of stars.
• The bar is surrounded by bright star clusters and large regions of glowing gas.

Credit: ESO

Figure 8.5.3 Measuring Distance with Cepheids

A four-panel cartoon illustrating the steps an astronomer takes to measure the distance to a cepheid variable star.

Panel (a)

• Labeled "Find a cepheid variable star and measure its period."
• An observer looks through a telescope eyepiece at a star while holding a stopwatch, timing the star's period of variability.

Panel (b)

• Labeled "Use the period-luminosity law to calculate the star's luminosity."
• The observer plots her measurements on a period-luminosity graph.

Panel (c)

• Labeled "Measure the star's apparent brightness."
• The observer again observes the star, this time using electronic equipment connected to the telescope and a computer to measure its brightness.

Panel (d)

• Labeled "Compare the luminosity with the apparent brightness to calculate the distance."
• The observer performs the calculation by hand on a whiteboard.

Figure 8.5.4 Period-Luminosity Relation

A chart plotting luminosity against period for cepheid and RR Lyrae variable stars.

Chart Elements

• The vertical axis is labeled Luminosity (L_Sun) and ranges from 10 to 10,000, with each interval representing 10 times the previous value.
• The horizontal axis is labeled Period (days) and is a logarithmic scale ranging from 0.3 to 100.
• A tight cluster of data points in the lower left, between 10 and 100 L_Sun and 0.3 to 1.0 days, represents the RR Lyrae stars.
• A roughly linear grouping of data points, between about 500 and 10,000 L_Sun and 1.0 to 100 days, represents the cepheid variables.

Figure 8.6.1 Luminosity Classes

A Hertzsprung-Russell diagram showing how stars of the same temperature can belong to different luminosity classes.

Chart Elements

• The vertical axis is labeled Luminosity (L_Sun) and runs from 10^-4 to 10⁶, in increments of 10².
• The horizontal axis is labeled Spectral Class and divided into seven equal sections, from left to right: O, B, A, F, G, K, and M.
• The horizontal axis is also labeled Temperature (K), running from 25,000 on the left to 3,000 on the right, and Color Index, with values -0.4 at O, 0.0 at A, 0.6 at G, and +1.4 at M.
• Five groups of stars are plotted: an isolated group of White Dwarfs at the lower left, the diagonal band of the Main Sequence running from upper left to lower right, a horizontal band of Giants running from the center to the right, and a small number of Supergiants across the top of the graph.

Luminosity Class Curves

• Five blue curves are plotted across the diagram, from brightest to faintest:
  • Ia, crossing the entire upper part of the plot at about 10⁵ L_Sun, corresponds to the brightest supergiants.
  • Ib, parallel to and below Ia at about 10⁴ L_Sun, corresponds to the less luminous supergiants.
  • II, at about 10³ L_Sun, corresponds to the bright giants.
  • III, beginning on the main sequence near spectral type A and running horizontally at about 10² L_Sun, corresponds to the giants.
  • IV, drawn between the giants and the main sequence, corresponds to the subgiants.
• The last curve, V, traces the entire length of the main sequence and corresponds to main-sequence stars.

Figure 8.8.1 Dimming of Betelgeuse

A side-by-side comparison of the star Betelgeuse before and after its 2019 Great Dimming event.

Image Elements

• The left image, from January 2019, shows Betelgeuse at its normal brightness.
• The right image, from December 2019, shows the star noticeably dimmer, with a visible change in its apparent shape.

Figure 8.8.2 Eclipsing Binary

An artist's illustration of an eclipsing binary star system.

Image Elements

• Two reddish-yellow stars are shown close together.
• One star partially eclipses the other, as seen from the observer's point of view.