9.8: Accessible Descriptions

Figure 9.1.1 Dwarf Simulation

A computer simulation showing the distribution of stars within 30 light-years of the Sun.

Image Elements

• The Sun is labeled and surrounded by a white circle at the center of the image.
• Very few bright stars like the Sun appear in the simulation.
• All known brown dwarfs are circled: those found earlier are circled in blue, and those found with the WISE infrared telescope are circled in red.
• Common, faint M-type stars are shown brighter than they would appear in reality so that they can be seen.

Credit: modification of work by NASA/JPL-Caltech

Figure 9.1.2 The Closest Stars

Three images of the Alpha Centauri star system, our nearest stellar neighbors.

Image (a)

• A wide-angle view showing Alpha Centauri A and B as a single bright object against the background stars of the Milky Way.

Image (b)

• A close-up view of Alpha Centauri A and B, with two small circles marking the individual stars: the larger Alpha Centauri A above and the smaller Alpha Centauri B below.

Image (c)

• A close-up of the lower right portion of Image (a).
• A white arrow points to the much fainter Proxima Centauri, which otherwise looks like one of the many background stars.

Credit: modification of work by ESO

Figure 9.2.1 Sagittarius Star Cloud

A Hubble Space Telescope image of a dense field of stars near the center of the Milky Way Galaxy.

Image Elements

• Thousands of stars of varying colors, including red, orange, yellow, white, and blue, are scattered across a black background.
• Star color indicates temperature: blue-white stars are hotter than the Sun, and red stars are cooler.
• The field spans about 13.3 light-years and lies roughly 25,000 light-years away.

Credit: Hubble Heritage Team (AURA/STScI/NASA)

Figure 9.3.1 Revolution of a Binary Star

A diagram showing seven observations of two stars orbiting their common center of mass.

Diagram Elements

• One star is drawn as a red dot at the center, surrounded by a blue ellipse representing the orbit of its companion.
• Red dots mark the companion star's position at seven points along the ellipse.
• A white arrow connects each red dot to an actual telescope image of the pair taken on that date.
• The observation dates, moving clockwise from lower left, are April 2000, February 2002, October 2002, February 2003, March 2003, December 2003, and January 2004.
• Some images were taken with the Hubble Space Telescope and others from the ground, which is why the pair looks different from image to image.

Credit: modification of work by ESA/NASA and Herve Bouy (Max-Planck-Institut für Extraterrestrische Physik/ESO, Germany)

Figure 9.3.2 Binary Star System

A diagram illustrating the concept of center of mass in a binary star system.

Diagram Elements

• A seesaw is shown in profile, with the plank horizontal, indicating the system is in balance.
• A red sphere labeled High-mass star sits on the left side of the plank.
• A smaller blue sphere labeled Low-mass star sits on the right side of the plank.
• The fulcrum, labeled Center of mass, is not centered under the plank but is shifted to the left, closer to the high-mass star.

Figure 9.3.3 Motions of Two Stars Orbiting Each Other and What the Spectrum Shows

Four panels, each pairing an orbit diagram with a combined spectrum, showing how Doppler shifts change as two stars (A and B) orbit their center of mass.

Panel 1

• Orbit diagram: the stars are on opposite sides of the orbit, with an arrow pointing down from star A and an arrow pointing up from star B.
• Spectrum: two separate lines are visible; star B's line is near the center of the spectrum, and star A's line is shifted slightly toward the blue (left).

Panel 2

• Orbit diagram: the stars are on opposite sides of the orbit, with an arrow pointing right from star A and an arrow pointing left from star B.
• Spectrum: the two lines merge into one, labeled A + B, because both stars are moving perpendicular to the line of sight.

Panel 3

• Orbit diagram: the stars are on opposite sides of the orbit, with an arrow pointing up from star A and an arrow pointing down from star B, the reverse of Panel 1.
• Spectrum: star B's line is near the center, and star A's line is shifted toward the red (right).

Panel 4

• Orbit diagram: the stars are on opposite sides of the orbit, with an arrow pointing left from star A and an arrow pointing right from star B, the reverse of Panel 2.
• Spectrum: the two lines merge into one again, labeled B + A.

Figure 9.3.4 Radial Velocities in a Spectroscopic Binary System

A chart of radial velocity versus time for two stars (A and B) in a spectroscopic binary system, shown below four spectra corresponding to key points on the curve.

Spectra

• Spectrum 1: two lines are visible; star A's line is near the center and star B's line is toward the right.
• Spectrum 2: the lines merge into one, labeled A + B.
• Spectrum 3: star B's line is near the center and star A's line is on the right.
• Spectrum 4: the lines merge again, labeled B + A.

Chart Elements

• The vertical axis is labeled Radial Velocity (km/s), in increments of 40 km/s.
• The horizontal axis is labeled Time (days), in increments of 2 days.
• Both curves begin at day 0 at +40 km/s.
• At day 4 (Spectrum 1), star A is at +15 km/s and star B is at +110 km/s.
• At day 9 (Spectrum 2), both stars are at +40 km/s.
• At day 13 (Spectrum 3), star A is near +65 km/s and star B is near -30 km/s.
• Near day 17 (Spectrum 4), both stars are again at +40 km/s.

Figure 9.3.5 Brown Dwarfs in Orion

Two Hubble Space Telescope images of the region surrounding the Trapezium star cluster in the Orion Nebula.

Image (a)

• Visible light image.
• Bright clouds of gas dominate the image, with only a few bright stars around the Trapezium visible below center.
• No brown dwarfs are visible; they are too faint and are hidden within the dust.
• Credit: NASA, C.R. O'Dell and S.K. Wong (Rice University)

Image (b)

• Infrared light image, which can pass through the dust.
• Much less nebulosity is visible, and many more stars cover the entire field.
• The faintest objects in this image are brown dwarfs, with masses between 13 and 80 times the mass of Jupiter.
• Credit: NASA; K.L. Luhman (Harvard-Smithsonian Center for Astrophysics) and G. Schneider, E. Young, G. Rieke, A. Cotera, H. Chen, M. Rieke, R. Thompson (Steward Observatory)

Figure 9.3.6 Mass-Luminosity Relation

A plot of stellar mass and luminosity for about 100 stars.

Chart Elements

• The vertical axis is labeled Luminosity (L_Sun) on a logarithmic scale ranging from 0 to 1,000,000.
• The horizontal axis is labeled Mass (solar masses) on a linear scale ranging from 0 to 20.
• Nearly all of the roughly 100 plotted stars lie on a straight line running from the lower left (low mass, low luminosity) to the upper right (high mass, high luminosity) corner.
• A few points lie below the lower left part of the main line; these are white dwarf stars, which do not follow the mass-luminosity relation.

Figure 9.4.1 Light Curve of an Eclipsing Binary

A light curve paired with an orbit diagram for a hypothetical eclipsing binary star system with total eclipses.

Light Curve

• The vertical axis is labeled Brightness, and the horizontal axis is labeled Time, both in arbitrary units.
• The curve is at its highest, labeled 1, then drops sharply to a low point, labeled 2.
• The curve rises back to the level of point 1, labeled 3, then drops again, but less deeply, to a point labeled 4, before rising back to the level of point 1.

Orbit Diagram

• A larger star is drawn as a red sphere, orbited by a smaller blue companion star along a blue elliptical path.
• At position 1, the companion is beside the larger star and both stars are fully visible, corresponding to maximum brightness on the curve.
• At position 2, the companion is hidden behind the larger star, corresponding to the deep dip in brightness.
• At position 3, the companion has emerged on the other side, and brightness is restored.
• At position 4, the companion passes in front of the larger star, blocking a small amount of its light, corresponding to the shallower dip in brightness.

Figure 9.4.2 Light Curve of an Edge-On Eclipsing Binary

A light curve paired with an orbit diagram for a hypothetical eclipsing binary star system viewed exactly edge-on.

Light Curve

• The vertical axis is labeled Brightness, and the horizontal axis is labeled Time, both in arbitrary units.
• The curve begins level, drops as the eclipse begins, briefly flattens at its lowest point, then rises back to its original level.
• Dashed lines connect four points on the curve to four labeled positions of the companion star above.

Orbit Diagram

• A larger star is drawn as a red sphere, with a blue arrow showing the companion star's path moving to the right through the center of the larger star.
• At position 1, first contact, the companion star just touches the left edge of the larger star, where the light curve begins to drop.
• At position 2, second contact, the companion is fully eclipsed by the larger star, where the light curve reaches its lowest brightness.
• At position 3, third contact, the companion is about to emerge from eclipse, where the light curve begins to rise.
• At position 4, last contact, the companion has fully emerged and touches the right edge of the larger star, where the light curve returns to maximum brightness.

Figure 9.5.1 Height versus Weight

A scatter plot of the heights and weights of a representative group of human beings.

Chart Elements

• The vertical axis is labeled Height, in arbitrary units, increasing upward.
• The horizontal axis is labeled Weight, in arbitrary units, increasing to the left.
• Most plotted points fall along a diagonal band, or main sequence, running from the upper left to the lower right, showing that weight generally increases as height increases.
• A few outlier points fall above and below this main band.

Figure 9.5.2 H-R Diagram for a Selected Sample of Stars

A Hertzsprung-Russell diagram plotting luminosity against spectral class and temperature for a sample of well-known, nearby stars.

Chart Elements

• The vertical axis is labeled Luminosity (L_Sun), running from 10^-4 to 10⁴ in increments of 10².
• The horizontal axis is labeled Spectral Class, divided into seven equal sections, from left to right: O, B, A, F, G, K, and M. It is also labeled Temperature (K), running from 25,000 on the left to 3,000 on the right.
• The chart's background is colored by temperature: blue for the O-B section, green for A and most of F, yellow for G, and red for the K-M section.

Star Groups

• White Dwarfs are at the lower left, between spectral types A and F.
• The Main Sequence runs diagonally across the entire diagram from upper left to lower right, and is where most stars lie.
• Giants lie on a horizontal band at about 10² L_Sun, between spectral types G and M.
• Supergiants are in the upper right.

Labeled Stars

• The Companion of Sirius is among the white dwarfs.
• Along the main sequence, from left to right: Spica, Vega, Sirius, Procyon, Alpha Centauri A, Sun, Tau Ceti, Barnard's Star, and Proxima Centauri.
• Along the giant branch, from left to right: Capella, Arcturus, and Aldebaran.
• Among the supergiants, from left to right: Rigel, Canopus, Betelgeuse, and Antares.

Figure 9.5.3 Schematic H-R Diagram for Many Stars

A schematic Hertzsprung-Russell diagram showing the five main classes of stars.

Chart Elements

• The vertical axis is labeled Luminosity (L_Sun), running from 10^-4 to 10⁶ in increments of 10².
• The horizontal axis is labeled Spectral Class, divided into seven equal sections, from left to right: O, B, A, F, G, K, and M. It is also labeled Temperature (K), running from 25,000 on the left to 3,000 on the right.

Star Groups

• White Dwarfs form an isolated group at the lower left.
• The Main Sequence runs diagonally from upper left to lower right and holds most stars.
• Red Dwarfs occupy the lower right part of the main sequence.
• Red Giants form a narrow horizontal band running from the center of the diagram to the right.
• Supergiants are a small group of stars running horizontally across the entire top of the diagram.

Figure 9.5.4 The Sun and a Supergiant

An illustration comparing the size of the Sun to the supergiant star VY Canis Majoris.

Image Elements

• At left, the supergiant VY Canis Majoris is shown as a large yellow sphere.
• A small rectangle at the lower left edge of the supergiant marks a region that is enlarged in an inset at right.
• In the inset, the Sun appears as a tiny dot surrounded by a dashed circle representing Earth's orbit, next to the curved surface of the supergiant.
• The Sun and Earth's orbit are small enough that they would fit inside VY Canis Majoris many times over.

Figure 9.5.5 Sirius and White Dwarf

Two images of the Sirius star system, showing Sirius A and its white dwarf companion, Sirius B.

Image (a)

• A visible-light image taken with the Hubble Space Telescope.
• Bright Sirius A is at the center; faint Sirius B is a tiny speck in the lower left, nearly lost in Sirius A's glare.
• Credit: modification of work by NASA, H.E. Bond and E. Nelan (Space Telescope Science Institute), M. Barstow and M. Burleigh (University of Leicester) and J.B. Holberg (University of Arizona)

Image (b)

• An X-ray image taken with the Chandra X-ray Telescope.
• Sirius B, the white dwarf, is now the bright object at the center; the fainter Sirius A appears above and slightly to the right.
• Credit: modification of work by NASA/SAO/CXC

Figure 9.6.1 Intelligent Life

A chart plotting the probability of humanity's situation being typical against the rate at which intelligent life must evolve around FGK-dwarf stars.

Chart Elements

• The vertical axis shows probability, on a logarithmic scale between about 10^-2 (1%) and 10^-1 (10%).
• The horizontal axis shows the evolution rate needed for intelligent life to emerge around FGK-dwarf stars.
• Color, ranging from blue to red, indicates the mean evolution time for intelligence, with bluer colors corresponding to slower development, more consistent with the billions of years intelligent life took to develop on Earth.

Credit: David Kipping