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Founded January 1970
Julian Date: 245959.16
2019-2020: CXXIX
THE DAILY ASTRONOMER
Monday, April 20, 2020
Remote Planetarium 16: Maps and Magnitudes
Good morning!
This week starts with star charts (or star maps, if you prefer.)
We'll learn the basics of understanding them and the symbols printed on them.
Regard the following star chart segment centered on the constellation Virgo.
It looks like an indecipherable mish-mash of symbols, numbers and constructs. Small wonder that Walt Whitman stormed outside in a huff. However, please bear with us as we work through it.
- Along the top we see 15h 14h 13h 12h. Those are right ascension markers. Recall last week we learned that right ascension measures a celestial object's angular distance from the vernal equinox. The right ascension scale extends from 0 - 24 hours, both of which denote the vernal equinox point. It is the celestial equivalent to longitude.
- Along the sides we see degree marking denoting declination. Declination measures a celestial object's angular distance north or south of the celestial equator. The 0 degree declination line represents the celestial equator.
- The blue arc represents the "ecliptic," the sun's annual apparent path through the sky. Notice toward the right side the ecliptic intersects the celestial equator. That intersection, which corresponds to 12 hours right ascension, is the autumnal equinox: the Sun's position on the first day of autumn.
- The white area surrounding Virgo defines the "Virgo region." Any celestial object observed within this region is said to be "in Virgo."
- The M objects within Virgo, such as M49 and M60 are "Messier objects, named for French astronomer Charles Messier (1730-1817) an avid comet hunter who, ironically, is best known for having compiled a catalog of celestial objects that resemble comets, but aren't. We still refer to this compilation as the "Messier Catalog,"and the bodies listed within it are known as "Messier objects." M49 and M60 are both elliptical galaxies approximately 56 million and 57 million light years distant, respectively. We will encounter Messier objects and galaxies many times during this course.
- The Greek letters seen by some of the stars are part of the Bayer Nomenclature System developed by German astronomer Johann Bayer (1572-1625). In Bayer's "Uranometria." This scheme assigns the letter alpha to the constellation's brightest star, beta to the second brightest, gamma to the third brightest, et cetera. A star within the constellation would be named using this Greek letter followed by the Latin genitive of the host constellation name. For instance, Spica, Virgo's brightest star, is designated Alpha Virginis.
To understand the numbers and circles along the bottom of the star chart, we need to now discuss:MAGNITUDES
The concept of "magnitude" is actually quite straightforward. It is the system astronomers use to measure the brightness of celestial objects: stars, planets, the Moon, even comets, asteroids and meteor trails. If it's in the sky and exudes light, either self-generated or reflected, it has a magnitude value. As we shall discover throughout the course, this system is both useful as a categorizing tool and as a means of discerning stellar distances.
Historians credit the magnitude system's invention to Nicean astronomer Hipparchus (190 - 120 BCE*) His was the first catalog to include a six-category scheme indicating stellar brightness. He assigned the brightest stars the designation "magnitude 1." He labeled the faintest stars visible as "magnitude 6." He consigned the remaining stars to the four intervening categories based on their relative brightness. As each category was not calibrated to reflect variations within each one, Hipparchus' scheme was quite imprecise. Yet, it became the foundation on which the modern magnitude system was based.
British astronomer Norman Robert Pogson (1829 - 1891) formulated the magnitude system astronomers still use today. He introduced into this system a value known as the "Pogson ratio," which is 2.512. A star measuring magnitude 1.0 is 2.512 times brighter than a star of magnitude 2 which, itself, is 2.512 times brighter than a magnitude 3 star. Pogson selected this ratio because it is the fifth root of 100, so a magnitude 1 star is precisely 100 times brighter than a magnitude 6 star.
Apart from quantifying the system, Pogson also expanded its parameters to accommodate those objects that are far brighter than those stars previously denoted as magnitude 1 and those objects dimmer than magnitude 6. Today, the magnitude system extends from the Sun (magnitude -26.7) to the faintest celestial objects that the Hubble Space Telescope has observed (magnitude + 27) As examples, Sirius, the brightest night sky star, is magnitude -1.46; Venus, at maximum brightness, is magnitude -5.0.
Spica's magnitude is 0.97, making it the sky's 16th brightest star.
One important note: The International Astronomical Union has designated eighty eight different constellations. In thirty-three of them, the "alpha" star is not the brightest.
We conclude with this star chart centered on Ursa Major. We again observe the constellation boundaries, right ascension, declination markers, the Bayer system Greek letters, and the Messier objects. Included on this chart - and absent on the previous one- are Flamsteed numbers, named for Johann Flamsteed (1646-1719), the first Astronomer Royal. His system assigns numbers to each constellation according to their position relative to the vernal equinox. In other words, by increasing right ascension.
Tomorrow, we move from star charts to the a brief history of the constellations.
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