Kiera Jefferson
Light pollution is a growing issue that has obscured the night sky for much of humanity. The Milky Way, a glowing band of our galaxy, is no longer visible to one-third of the world's population, including 60% of Europeans and 80% of North Americans. This is due to the artificial sky glow caused by light from streetlamps, homes, and illuminations scattering off particles and moisture droplets in the atmosphere. As a result of light pollution, most deep-sky objects cannot be seen from city centers, and even the largest telescopes may only capture a diffuse view. To observe celestial objects like galaxies, astronomers seek out dark sky sites with minimal light pollution, often in remote locations far from cities. The Bortle Scale, ranging from Class 1 (truly dark) to Class 9 (extreme light pollution), is used to estimate the amount of light pollution in a given location. At a truly dark site with minimal light pollution, faint objects like stars, galaxies, and nebulae become visible, providing a breathtaking view of the universe.
| Characteristics | Values |
|---|---|
| Bortle Scale | Developed by John E. Bortle to estimate the amount of light pollution in the sky, ranging from Class 1 (truly dark) to Class 9 (extreme light pollution) |
| Andromeda Galaxy | The only galaxy beyond the Milky Way visible to the naked eye; used to estimate the Bortle Scale |
| Light Pollution Impact | Difficulty in observing celestial objects, including galaxies, due to reduced contrast and scattered artificial "sky glow" |
| Affected Regions | North America, Europe, East Asia, Middle East, highly urbanized areas like Singapore, Italy, and large cities |
| Solutions | Travelling to dark, remote sites; using light pollution maps to find darker locations; reducing artificial lighting |
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What You'll Learn
The Bortle Scale
A Class 1 Bortle sky is characterised by the visibility of the zodiacal light in spring and autumn, and the Milky Way is bright enough to cast shadows. Examples of locations with Bortle 1 light pollution include portions of Yellowstone National Park, Steens Mountain in Oregon, and parts of Nebraska. As you move up the scale, the amount of light pollution increases, making it more difficult to observe celestial objects such as galaxies and stars.
The visibility of the Andromeda Galaxy (M31) can be used as an indicator of light pollution levels. At a Bortle 8 or 9 location, it is difficult to find the Andromeda Galaxy, and few, if any, stars are visible. At Bortle 7 to 5, the galaxy can be seen but with significant effort. At Bortle 4 or lower, the Andromeda Galaxy is immediately obvious as a diffuse "smudge" in the night sky.
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Andromeda Galaxy visibility
The Andromeda Galaxy (M31) is the closest large galaxy to the Milky Way. It is the brightest and largest galaxy in the northern hemisphere and is visible from Earth throughout much of the year, particularly in autumn. It is also the furthest object that can be seen with the naked eye.
However, light pollution can make it difficult to see the Andromeda Galaxy. Light pollution is the product of an industrialized society and can come from sources such as buildings, streetlights, headlights, and even phone screens. The light from a city center can wash out the light from stars and galaxies, hindering the performance of telescopes. Light pollution decreases the contrast around an object, making it hard to pick out faint objects like galaxies. As a result, it is impossible to see most deep-sky objects from the center of a city.
The Bortle Scale is a measure of light pollution in the sky. It ranges from Bortle 4, which is a dark sky with minimal light pollution, to Bortle 9, which is a highly light-polluted sky. The Andromeda Galaxy should be fairly obvious in the night sky in locations with low light pollution, appearing as a diffuse "smudge". In locations with higher light pollution, it will be more difficult to see, appearing as a small, grayish fuzzy patch. In highly light-polluted areas, it may not be visible at all.
To improve the visibility of the Andromeda Galaxy in light-polluted areas, one can use a telescope with a wide aperture, a light pollution filter, or a long exposure on a camera. Additionally, one can try to locate the Andromeda Galaxy by using the constellation Cassiopeia as a guide or by focusing on one of the stars of the Pegasus Square and then slewing to M31.
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Light pollution maps
The Bortle Scale is often used in conjunction with maps and websites like DarkSiteFinder.com, which provide visual representations of light pollution levels worldwide. These maps are colour-coded, with dark red representing Bortle 6, dark white or off-white for Bortle 8, and yellow for Bortle 4. By using these maps, astronomers can identify areas with minimal light pollution, increasing their chances of clear astronomical observations.
For example, when trying to observe the Andromeda Galaxy, a famous deep-sky object, light pollution can significantly impact visibility. In highly light-polluted areas (Bortle 8 or 9), finding the Andromeda Galaxy in the night sky becomes extremely challenging. However, as one moves towards darker locations (Bortle 6 and below), the galaxy becomes more discernible, appearing as a diffuse "smudge" or an oval-shaped object with a bright core.
Overall, light pollution maps are invaluable resources for astronomers, providing critical information about light pollution levels in different areas. By utilising these maps, astronomers can make informed decisions about their observation locations, ensuring they capture the awe-inspiring beauty of the universe.
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Surface brightness
The surface brightness of an astronomical object is measured in magnitudes per square arcsecond or arcminute. A lower surface brightness value indicates a dimmer object. For example, the Helix Nebula has a surface brightness of 20.8 magnitudes per square arcsecond, while the Saturn Nebula, which appears brighter due to its smaller size, has a surface brightness of 14.6 magnitudes per square arcsecond.
When observing galaxies, surface brightness is a more useful indicator of visibility than magnitude, especially for larger objects. This is because galaxies are extended objects that emit light over a large area, and their light becomes more dispersed and harder to observe against the background sky glow as light pollution increases. Therefore, galaxies with higher surface brightness, or a more concentrated light source, are easier to observe from light-polluted locations.
For example, the Cigar Galaxy (M 82) is a dim galaxy that can cut through light pollution effectively due to its high surface brightness. Its edge-on orientation also contributes to its visibility, as edge-on galaxies appear brighter than face-on galaxies of the same magnitude because their light is concentrated into a smaller area. Similarly, the Andromeda Galaxy, which is the farthest galaxy visible to the naked eye, can be seen as a "smudge" from a Bortle 8 location, but its shape and core become more defined in darker locations with less light pollution.
In summary, surface brightness is a critical factor in determining whether a galaxy can be observed from a light-polluted location. Galaxies with higher surface brightness, or a more concentrated light source, are more visible in light-polluted skies. Additionally, edge-on galaxies and those with brighter cores are generally easier to observe.
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Dark sky sites
The amount of light pollution in the sky can be estimated using the Bortle Scale, which ranges from Class 1 (truly dark) to Class 9 (heavily light-polluted). At a Bortle 8 or 9 location, it will be difficult to find the Andromeda Galaxy, the only galaxy beyond the Milky Way that can be seen easily with the naked eye. At a Bortle 6 to 9 location, it is recommended to focus on observing open clusters, planets, stars, and double stars.
To observe galaxies, it is best to travel to a dark sky site, which offers clearer views of faint celestial objects such as nebulae, galaxies, and star clusters. At a dark sky site of Bortle 4 or lower, the Andromeda Galaxy should be readily visible as a diffuse "smudge" in the sky. With a telescope, it will appear as a larger oval-shaped object with a bright core.
One such dark sky site is OM Dark Sky Park and Observatory, where visitors can observe various celestial objects with the naked eye or through a telescope. The Andromeda Galaxy, for instance, is best observed between September and February. The Pleiades, a recognisable star cluster, is another notable sight, best observed between October and February.
Other dark sky sites include portions of Yellowstone National Park, Steens Mountain in Oregon, and parts of Nebraska. These locations, rated as Bortle 1, offer pristine viewing conditions, where the zodiacal light is visible in spring and autumn, and the Milky Way is bright enough to cast shadows. Unfortunately, Bortle 1 zones are becoming increasingly rare due to light pollution.
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Frequently asked questions
Light pollution dramatically decreases the contrast around an object in your eyepiece, making it harder to see. Even an ultra-high contrast filter would still make this task very difficult. The Andromeda Galaxy, for instance, is only visible as a smudge in the sky at a Bortle 8 location, while at a Bortle 4 location, it begins to show suggestions of dust lanes and spiral shape in a telescope. So, ideally, you want to be in a Bortle 4 or lower location to see a galaxy.
The Bortle Scale was developed by John E. Bortle to estimate the amount of light pollution in the sky, ranging from Class 1 (truly dark) to Class 9 (extreme light pollution). You can determine your Bortle Scale by trying to find the Andromeda Galaxy in the night sky. If you have any significant difficulty finding it, you probably have Bortle 8 or 9 skies. If it is immediately obvious, you are likely at a dark sky site of Bortle 4 or lower.
To reduce light pollution, you need to get away from city lights and go to dark, remote sites. Dark sky advocates suggest that it is well worth the trip to travel to a location with darker skies. You can refer to a light pollution map to find nearby places that offer darker skies.
