3.7: Accessible Descriptions

Figure 3.1.1 Characterizing Waves

A diagram of a wave labeled with its crest, trough, and wavelength.

Diagram Elements

• A wavy horizontal line represents the wave, with alternating high points (crests) and low points (troughs).
• The crest, the highest point of the wave, is labeled.
• The trough, the lowest point of the wave, is labeled.
• The wavelength is labeled as the horizontal distance between two successive crests.

Figure 3.1.2 Brightness and Distance

A diagram illustrating the inverse square law for light using an expanding square of area at increasing distances from a source.

Diagram Elements

• A spherical light source, such as a star, is shown on the left.
• Four arrows move radially outward from the source toward the right, each arrow marking one corner of a square cross-section.
• At step 1, one unit from the source, the arrows form a square one unit on a side, with an area of one unit squared.
• At step 2, two units from the source, the square is two units on a side, with an area of four units squared. The energy passing through this square is four times less than at step 1.
• At step 3, three units from the source, the square is three units on a side, with an area of nine units squared. The energy passing through this square is nine times less than at step 1.

Figure 3.2.1 Apparent Magnitudes

A scale comparing the apparent magnitudes of well-known astronomical objects and the faintest magnitudes observable by the unaided eye, binoculars, and telescopes.

Chart Elements

• The horizontal axis is labeled Apparent magnitude and ranges from -30 on the left, through zero in the center, to +35 on the right.
• Astronomical objects and telescopes are listed above the scale, each connected by a line to its approximate magnitude.

Labeled Objects and Limits, from Brightest to Faintest

• The Sun: -26
• The Moon: -13
• Venus, at brightest: -4.5
• Jupiter and Mars: -3
• Sirius: -1.5
• Alpha Centauri: 0
• Betelgeuse: about +0.5
• Polaris: +2
• Faintest object visible to the unaided eye: +6
• Barnard's Star: about +9
• Faintest object visible with binoculars: +10
• 1-meter telescope limit: about +19
• Faintest object visible with a 4-meter telescope: about +26
• Hale telescope limit: about +27
• Limit of the Hubble and Keck telescopes: about +30

Figure 3.3.1 The Electromagnetic Spectrum

A diagram showing the full range of the electromagnetic spectrum, ordered by wavelength and energy.

Diagram Elements

• The spectrum is arranged from left to right in order of decreasing energy and increasing wavelength.
• The regions shown, from highest to lowest energy, are gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

Figure 3.3.2 Orion in Different Wavelengths

Three images of the constellation Orion, each observed in a different band of the electromagnetic spectrum.

Image (a)

• On the left
• Visible light image of the Orion region as the human eye sees it, with dotted lines added to outline the figure of the mythical hunter Orion.
• Below the three stars of Orion's belt, at the center of the image, is the Orion Nebula.
• Credit: Howard McCallon/NASA/IRAS

Image (b)

• In the middle
• X-ray image of the same region, emphasizing the point-like X-ray sources nearby.
• Only a few of the stars seen in visible light are also seen in X-rays, so the constellation outline is omitted.
• Numerous very bright stars and other distant sources are prominent.
• Credit: Howard McCallon/NASA/IRAS

Image (c)

• On the right
• Infrared image showing mainly the glowing dust in this region.
• Some stars are visible, so the constellation outline is shown again.
• The image is nearly covered with delicate wisps of nebulosity that grow bright and dense near the Orion Nebula.
• Credit: Michael F. Corcoran

Figure 3.3.3 X-Ray Sky

A false-color map of the entire sky as seen in X-rays from above Earth's atmosphere.

Map Elements

• The map is tilted so that the disk of the Milky Way Galaxy runs horizontally across the center of the image.
• The map was constructed and artificially colored from data gathered by the European ROSAT satellite.
• Each color represents X-rays of a different frequency or energy: red, yellow, and blue.
• Red outlines the glow of a hot local bubble of gas around us, blown by one or more exploding stars in our cosmic vicinity.
• Yellow and blue show more distant sources of X-rays, such as remnants of other exploded stars or the active center of our galaxy, located near the middle of the image.
• Credit: NASA

Figure 3.4.1 Blackbody Curve

A graph showing the blackbody radiation curves for objects at four different temperatures.

Chart Elements

• The horizontal axis is labeled Wavelength and ranges from 1000 to 3000 nanometers.
• The vertical axis is labeled Intensity, in arbitrary units.
• The band of wavelengths corresponding to visible light is shown in color.
• Four curves are plotted, one for each object temperature, and the peak of each curve is marked with a dot showing the wavelength of maximum emission.

Curves, by Temperature

• 3000 K: peaks at 1200 nm, in the infrared.
• 4000 K: peaks at 900 nm, in the near-infrared.
• 5000 K: peaks at 700 nm, in the visible red.
• 6000 K: peaks at about 500 nm, in the yellow part of the visible spectrum.

Figure 3.5.1 Light Pollution Scale

A landscape view of the night sky showing the nine levels of the Bortle dark-sky scale, from the darkest skies to the most light-polluted skies.

Image Elements

• Nine panels are arranged left to right, each showing the same landscape and sky under a different Bortle level.
• The panels progress from Level 1, an excellent dark sky with the Milky Way clearly visible, on the left to Level 9, an inner-city sky where only the brightest stars are visible, on the right.
• As the level increases, the number of visible stars decreases and the sky background brightens with an orange glow from artificial light.

Figure 3.5.2 Satellites and Space Debris

A three-dimensional projection of Earth's orbits, illustrating the distribution of active satellites and space debris.

Diagram Elements

• Earth is shown at the center, surrounded by concentric orbital zones extending from low Earth orbit to highly elliptical orbit.
• Each orbital zone is labeled with its altitude range, the typical speed of objects in that orbit, and the number of active satellites it contains.
• The diagram also notes sustainable practices recommended for each orbit to prevent and mitigate damage from collisions and debris.