3.7: Accessible Descriptions

Figure 3.1.1 Characterizing Waves

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

Diagram Elements

• The wave, represented by a blue line that repeatedly curves up and down, is bisected by a horizontal black line.
• The highest point of the wave above the center line is the crest.
• The lowest point of the wave below the center line is the trough.
• The repeating crests and troughs are all at the same maximum heights and are equally spaced apart.
• The wavelength is 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 light source. This represents how the same amount of energy is spread over a larger area, becoming less concentrated with distance.

Diagram Elements

• The light source is a star on the left side of the figure.
• 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 concentrated 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 concentrated 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 frequency.
• The regions shown, from highest to lowest energy, are gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.
• The diagram includes a wave that starts with a long wavelength and steadily decreases in wavelength moving left to right. The represents the decreasing wavelength and increasing frequency of each category of light.
• The figure includes illustrations of objects that are on the same scale for each type of wavelength. It also two scales, one for wavelength and the other for frequency. The values for each category of light is in Table \(\PageIndex{1}\).

Table \(\PageIndex{1}\): Categories of Light

Type of Light	Wavelength Range (in meters)	Illustration for Scale	Frequency (in Hz)
Radio	longer than 1	Buildings	10^4–10^8
Microwave	10^-3 – 1	Humans to Honey Bee	10^8 – 10^11
Infrared	7 x 10^-7 – 10^-3	Pinpoint	10^11 – 10^14
Visible	4 x 10^-7 – 7 x 10^-7	10^3–10^4 K	Stars
Ultraviolet	2 x 10^-8 - 4 x 10^-7	2 x 10^–108 K	Gas in clusters of galaxies, supernova remnants, solar corona
X-rays	10^-11 – 2 x 10^-8	10^6–10^8 K	Gas in clusters of galaxies, supernova remnants, solar corona
Gamma rays	Shorter than 10^-11	More than 10^8 K	Produced in nuclear reactions; require very high-energy processes

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.

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.

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.

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.

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.