Sky and Telescope - May 2012 - (Page 47)

Northern Hemisphere’s Sky

Fred Schaaf has authored 13 books on astronomy.

Fred Schaaf

How Bright Is Bright?
Nine stars and three planets shine at 1st magnitude or brighter in May.
The all-sky map on page 44 shows the sky at a time on April and May nights when bright stars and planets are very numerous. Four stars brighter than magnitude 1.5 (Sirius, Aldebaran, Rigel, and Betelgeuse) have recently set at this time. But nine more are still in the sky, with a tenth (Altair) about to rise. Added to that number are two planets near zero magnitude, Mars and Saturn — and one more, Venus, that shines immensely brighter, though it’s setting at map time. With all the bright objects visible at this hour, it’s a good time to ask a key observational question: How bright is bright? Super-bright versus bright. One way to start answering that question is to compare the objects that are currently visible. And in that endeavor we certainly know where to begin — with breathtakingly brilliant Venus. In late April and early May 2012, Venus shines at a magnitude of –4.7, its maximum brightness during this apparition. It’s also exactly 5 magnitudes (or 100 times) brighter than Saturn, at magnitude +0.3. A few stars on our all-sky map are slightly brighter than Saturn is at the start of May: Arcturus and Vega at magnitude 0.0 and Capella at magnitude 0.1. But Vega is low enough at this hour to be dimmed signiﬁcantly by atmospheric absorption — and that’s even more true for Capella, which is very low in the northwest. Mars, Saturn, and their companions. Mars shines at magnitude 0.0 as May begins, also a little brighter than Saturn. But Saturn fades substantially over the next two months, and Mars fades even faster, becoming dimmer than Saturn in June. You can get a better feel for this by comparing them to each other and to the 1st-magnitude stars they’re near. Mars was closest to 1.4-magnitude Regulus in April, but moves rapidly away in May. Saturn remains within 5° of 1.0-magnitude Spica through August. August is also when greatly faded Mars will catch Saturn and Spica, forming a dramatic close grouping with them on several nights. Check Sun, Moon, and Planets now and in the months ahead to keep track of the changing magnitudes and separations of Mars and Saturn. Brighter than 10,000 stars. But let’s come back to our original statement — that Venus is 100 times brighter than Saturn as May begins. It’s correct in terms of actual measurable light output from the two objects, but it’s deceptive in terms of our impression of the two objects —

especially if we imagine 100 separate Saturns shining in the sky to produce the light output of a single Venus. Why? Because our eye-brain system actually seems to perceive brightness logarithmically. The ancient Greeks divided stars into magnitude classes based on their intuitive impressions. And when scientists acquired the technology to measure star brightnesses, the ﬁrst class turned out to be 2.5 times brighter than the second class, second 2.5 times brighter than third, and so on down to 6th magnitude. Venus is 10,000 times brighter than a star of magnitude 5.3 — perhaps about the average naked-eye limiting magnitude in a technologically advanced country with lots of light pollution. Now let’s turn the ancient magnitude system around, so that brighter objects get higher numbers. If the faintest star visible in a typical suburb is 1, Saturn would be a 6 and Venus an 11. ✦
Sk yandTelescope.com May 2012 47

AKIRA FUJII

Arcturus, in Boötes, is the brightest star of the spring sky — about 7% brighter than Vega.

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012