Some of my readers have queried me as to why the brighter objects in the night sky have negative magnitude values, while the fainter ones have positive values, when, logically (at least to them), it should be the other way around.

For this seemingly “backward” rating system, we can thank the ancient Greek astronomer Hipparchus, who, in 129 BC, drew up the first recognized star chart. On this chart, he listed the magnitude (from Latin magnitudo or magnus meaning “great”) of the stars he could see in the night sky. Hipparchus listed the brightest stars that he could see with his naked eye as magnitude +1.0 stars, those half as bright as the magnitude +1.0 stars as magnitude 2.0 stars, and so on, until reaching magnitude +6.0, the faintest he could see.

His magnitude scale remained in use for rating the brightness of the stars (and other celestial objects by comparison) for the next 1,400 years. It wasn’t until 1609, when Italian astronomer Galileo (1564-1642) developed his first telescope and observed much fainter stars than those listed on the star charts in use at that time, that the magnitude scale was extended (with ascending positive numbers) to include the fainter stars.

In the mid-1850s, when astronomers discovered that some magnitude +1.0 stars are brighter than others, the scale was again extended outward, this time with ascending negative values to reflect the brighter stars.

The stars Rigel (Orion), Capella (Auriga), Arcturus (Bootes), and Vega (Lyra) were listed at magnitude 0.0, while stars brighter than these were given negative values. Sirius, the brightest star in the night sky, is rated at magnitude -1.43 , while our sun is rated at magnitude -26.7.

Planets and other celestial objects can also be rated on the magnitude scale. Venus, at its brightest, shines at magnitude -4.4, while the full moon beams (on average) at magnitude -12.6.

The faintest stars that the average human, naked-eye can see (under a clear sky from a dark site) is magnitude +6.0, while binoculars can boost that to magnitude +10. In contrast, the Hubble Space Telescope can see stars as faint as magnitude +30.

What does it mean?

A star’s apparent brightness or luminosity refers to the amount of light energy (from thermonuclear fusion within the star’s core) it emits, and how much of that energy passes per second through a square meter of the star’s surface area. Basically, how bright a star appears depends on how much of its light energy per second strikes the area of a light detector (in our case, the human eye). The apparent brightness we see or measure is inversely proportional to the square of our distance from the star, with the apparent brightness diminishing as the distance squares.

Astronomers use the terms “apparent magnitude” and “absolute magnitude” when denoting a star’s brightness. Apparent magnitude is how bright the star appears to an Earth-bound observer, and is directly related to a star’s apparent brightness.

Stellar measurements in the 19th century indicated that magnitude +1.0 stars are approximately 100 times brighter than magnitude +6.0 stars (i.e., it would take 100 magnitude +6.0 stars to provide as much light as a single magnitude +1.0 star). Subsequently, the stellar magnitude scale was modified so that a magnitude difference of five corresponded exactly to a factor of 100 times difference in brightness., while a difference of one magnitude equaled a difference factor of 2.512 in brightness.

This resulting stellar magnitude rating system was based on a logarithmic scale, with whole numbers, and fractions thereof, indicating varying ratios of brightness (e.g., 0 = 1 to 1; 0.2 = 1.2 to 1; 0.5 = 1.6 to 1; 1 = 2.5 to 1; 5 = 100 to 1, etc.). A star’s apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any dimness of the star’s light caused by the interference of interstellar dust along the line of sight of the observer.

When astronomers want to measure how intrinsically bright a star is regardless of its distance from Earth, they measure the star’s absolute magnitude, or its apparent magnitude if all the stars it is being compared to were placed at 10 parsecs distance from Earth. With one parsec equaling 3.26 light-years (a light-year is the distance light travels through the vacuum of space in one year; approximately 10 trillion kilometres), 10 parsecs equals 32.6 light-years, or approximately 100 trillion kms. A star’s absolute magnitude measures its true energy output (its luminosity).

As with the apparent magnitude scale, the absolute magnitude scale is also “backward”, giving less luminous stars ascending positive values, and more luminous stars ascending negative ones. For celestial objects such as comets and asteroids, the absolute magnitude scale (also with positive through negative values) is based on how bright the object would appear to an observer standing on the sun if the object were 1 AU (149,597,871 kms) away.

This week’s sky

Mercury (magnitude -0.8) is visible low (about eight degrees) above the northwest horizon shortly after 9 p.m., before dropping from view shortly after 10 p.m. This bright but small planet (heading towards its greatest eastern elongation from the sun on June 2) achieves an altitude of 18 degrees in the evening sky by May 31. It reaches its half-phase (called dichotomy) on May 29.

Venus (magnitude -4.3) appears only about 13 degrees above the western horizon shortly after 9 p.m., before setting shortly before 11 p.m.

Jupiter (magnitude -2.5) rises in the southeastern sky shortly before 1 a.m., reaching 22 degrees height in the southern sky before fading from view around 5:15 a.m.

Saturn (magnitude +0.48) follows Jupiter into the southeastern dawn sky around 1 a.m., rising to about 23 degrees above the southern horizon before it fades from sight shortly before 5 a.m.

Mars (magnitude +0.16) rises in the southeast around 2:30 a.m., reaching an altitude of about 20 degrees above the horizon before fading from view a few minutes before 5 a.m.

Currently at magnitude +4.5, Comet C/2020 F8 SWAN is now in the constellation of Perseus – the Warrior Prince. This fading comet will be difficult to see, as it reaches an altitude of only about 10 degrees above the northeastern horizon between 4 a.m. and 5 a.m., before the glow of the rising sun overtakes it. With clear skies and an unobstructed view of the northeastern horizon, it might still be seen in binoculars and small scopes.

Until next week, clear skies.

Events:

May 29 – Mercury reaches dichotomy

May 30 – First quarter moon

Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at glennkroberts@gmail.com.

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