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 \(\PageIndex{1}\).
• The scales in this figure are not exact but are meant to represent the average scale of the wavelength and frequency ranges. The numbers reported in the Table \(\PageIndex{1}\) are the accepted wavelength ranges, and they sometimes disagree with what is in the figure.

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

Type of Light	Wavelength Range (in meters)	Illustration for Scale	Frequency (in Hz)
Radio	longer than 1	Buildings	less than 10^8
Microwave	10^-3 – 1	Humans to Honey Bee	10^8 – 10^11
Infrared	7 x 10^-7 – 10^-3	Pinpoint	10^11 – 4 x 10^14
Visible	4 x 10^-7 – 7 x 10^-7	Protozoans	4 x 10^14 – 7 x 10^14
Ultraviolet	2 x 10^-8 - 4 x 10^-7	Molecules	7 x 10^14 – 1 x 10^16
X-rays	10^-11 – 2 x 10^-8	Atoms	1 x 10^16 – 3 x 10^19
Gamma rays	Shorter than 10^-11	Atomic Nuclei	greater than 3 x 10^19

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 left to right, starting with Level 1, representing a dark sky, to Level 8/9, representing a sky with high light pollution.
  • Level 1: Excellent Dark Sky Site
  • Level 2: Dark Sky Site
  • Level 3: Rural Sky
  • Level 4: Suburban/Rural Transition
  • Level 5: Suburban Sky
  • Level 6: Bright Suburban Sky
  • Level 7: City/Suburbia Transition
  • Level 8/9: City/Inner City Sky
• 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 not included in the figure, but would be in the center, surrounded by thousands of tiny black dots representing space debris and active satellites. The cardinal direction label N for North is located directly above where Earth would be indicating that Earth and the satellites are oriented with the north pole at the top, and Earth's spin axis is vertical.
• The scale representing the density of space debris and active satellites ranges from white for low density through gray for medium density to black for high density.
• There are 9 areas where the density of satellites is higher that are labeled orbital zones.
• There is a scale representing the urgency for action that ranges from green for low urgency through yellow-green for medium, orange for urgent, to red for very urgent.

Orbital Zones

The 9 orbital zones are listed from highest to lowest urgency.

Low Earth Orbit (LEO)

• This zone is located close to Earth and has a black line marking the area. The overall shape of the zone is circular, except that the areas near the north and south pole are not included.
• The satellites in the zone form a circular shape that is nearly black around the edge of the circle, representing very high density. The rest of the circle is a little less black.
• Height: 200-2,000km
• Typical Object Speed: 27,500km/hr
• Number of Active Satellites: 10,000 +2,400/year
• Urgency for action is red. This zone is rapidly getting crowded with satellites that have collision speeds of 9-14 km/s and needs immediate protection.

Sun-Synchronous Orbit (SSO)

• This zone is marked by a black oval with the longest part of the oval aligned with the spin axis of Earth, or vertically. This zone directly overlaps with Low Earth Orbit zone. The numbers below indicate that it has a much lower population than the LEO zone, but the region appears dense due to the overlap.
• The satellites in this zone are mostly for Earth observation.
• Height: 600-1000 km
• Typical Object Speed: 27,000km/hr
• Number of Active Satellites: 800 sats +150/year
• Urgency for action is orange. The danger is when satellites cross zones near the poles. Conjunctions, predicted close approach between satellites, are steadily increasing. This zone is also used as a graveyard orbit, or someplace that active satellites are moved to and deactivated.

Superspreader Debris Ring

• This zone does not represents the location of space debris, instead of active satellite orbits. The zone is a ring of material that is a large distance from Earth in orbits that have inclinations of roughly 30 degrees relative to Earth's equator
• Urgency for action is orange. There is a large rocket body in this zone.

Medium Earth Orbit (MEO)

• This zone covers a large range of heights, but appears to be in the same plane as Earth's equator which has a medium density of objects.
• Height: 2,000-32,000km
• Typical Object Speed: 25,000-18,000km/h
• Number of Active Satellites: 160 +12/year
• The satellites in this zone should be de-orbited because their orbits will naturally decay in about 1000 years.
• Urgency for action is yellow-green because there is no significant congestion. Collision speeds range from 5-7 km/s. The zone should be better monitored because multi-orbit satellite constellations will start using it with no regulation.

GPS, GLONASS, BeiDou, Galileo

• This zone includes the navigation satellites systems for the Unites States (GPS), Russia (GLONASS), China (BeiDou), and Europe (Galileo). The zone is a low density shell around the at a large distance.
• Height: 19,000-23,000km
• Typical Object Speed: 14,000 km/h
• Number of Active Satellites: 110
• Urgency for action is yellow-green because there are very few satellites and debris.

Geosynchronous Orbit (GSO)

• This zone includes a region around the Earth's equator that covers a range of 8 degrees extending above and below the equator. It is medium to low density and represents the largest orbits in this figure.
• This zone includes mostly big, trackable, inactive satellites. Their orbits are stable, so they will remain in their orbit indefinitely.
• There is one superspreader debris ring with a medium density just below this zone.
• Height: 35,000-36,000 km
• Number of Active Satellites: 800 sats +20/year
• Urgency for action is yellow-green because there is one 50 m close approach per year. The inactive satellites should be moved to graveyard orbits.

GEO Graveyard Orbit

• This zone is full of satellites that were moved out of geostationary orbits and deactivated. The one is medium to low density.
• Height: 36,100km
• Typical Object Speed: 11,000km/hr
• Urgency for action is yellow-green.

Geostationary Orbit (GEO)

• Geostationary orbits are timed with the rotation of the Earth. The satellite is also positioned above the equator, so it stays over the same point on Earth. This means that the orbits are at a fixed height and speed.
• The zone is a ring of medium density dots far from the Earth in the same plane as the equator.
• Height: 35,786 km
• Typical Object Speed: 11,050 km/hr
• Number of Active Satellites: 600 +15/year
• Urgency for action is green because the zone is fairly regulated and organized. The collision speeds are 0-2.5 km/s collision speeds. service for extending life is needed
• These satellites require service because to remain in orbit, they must regularly activate thrusters to compensate for gravitational forces from the Sun and Moon. When they run out of fuel they need a resupply to stay in orbit.

Highly Elliptical Orbit (HEO)

• Satellites with highly elliptical orbits will change their height and speed throughout their orbit. This means that their orbit covers a large area that can't really be represented as a dot for an individual satellite. However, it means that the satellite can spend most of its time in low-density zones, away from the high density zones.
• Height: 2,000-40,000 km
• Number of Active Satellites: 60 sats +2/year
• Urgency for action is green because there are very few satellites or debris the probability, or chance, of collision is nearly zero.

Inner Van Allen Radiation Belt

• This zone represents areas where Earth's magnetic field concentrates high energy particles.
• Height: 1,500-10,000km
• Urgency for action is green. High energy particles interfere with and damage electronic equipment, so there are few active satellites here.

Key Takeaways

• The Low Earth orbit zone has the highest density and requires immediate action.
• The orbital zones become less dense with distance from Earth, and have lower levels of urgency.