Sky at a Glance May 12 – 19

Photo showing Apparent Magnitude of various celestial objects.

This Week’s Sky at a Glance, May 12 – 19 ~by Curt Nason

In the second century BC the Greek astronomer Hipparchus of Nicaea ranked the stars according to their brightness in six categories called magnitudes (for greatness). The 20 brightest stars were rated first magnitude and the faintest stars were sixth magnitude. This system was retained for two millennia and standardized in the 19th century when much fainter stars were being detected by astrophotography. English astronomer Norman Pogson devised a logarithmic system whereby five magnitudes was a difference in star brightness of exactly 100 times. With this system, a magnitude 1 star is about 2.5 times brighter than a magnitude 2 star, and that one is 2.5 times brighter than a magnitude 3 star.

For many of us, the faintest star we can detect with the naked eye in a reasonably dark sky is sixth magnitude (commonly just called mag 6). Vega, the fifth brightest star, is mag 0, as is slightly brighter Arcturus. With the ability to measure the exact brightness of stars, their magnitudes are often recorded to one or two decimal places, and negative values are used for very bright objects. Sirius is mag -1.4; Jupiter is currently mag -2.5 and Venus is -3.8. The full Moon is mag -12.6, approximately 400,000 times fainter than the Sun at -26.7. A first magnitude star is brighter than mag 1.5, a second magnitude star shines between mag 1.5 and 2.5, and so on.

These brightness values are for the apparent magnitude of a star, as we see them when they are highest in the sky. At lower altitudes the atmosphere will absorb some of the starlight, making them appear dimmer. Astronomers call this effect extinction. The apparent magnitude of a star depends on its size and temperature, and also on its distance from us. A doubling of distance reduces the brightness by a factor of four, and ten times the distance by a factor of 100. Therefore, if one star is ten times farther than a mag 3 star of equal size and temperature, it would be at mag 8 and we would require binoculars to see it.

Just a little astronomy lesson to brighten your day.

This Week in the Solar System

Saturday’s sunrise in Moncton is at 5:50 am and sunset will occur at 8:41 pm, giving 14 hours, 51 minutes of daylight (5:57 am and 8:43 pm in Saint John). Next Saturday the Sun will rise at 5:42 am and set at 8:49 pm, giving 15 hours, 7 minutes of daylight (5:50 am and 8:51 pm in Saint John).

The Moon is new on Tuesday and the crescent Moon pairs with Venus in the evening sky on Thursday. Jupiter teeters upward in the southeast as they totter toward the northwestern horizon. Jupiter’s famous Red Spot storm is prominent in a telescope this year and it is facing in our direction at 11:15 pm on Tuesday. Saturn and Mars offer great viewing in the morning sky, with Mars passing near the faint globular cluster M75 early in the week.

Questions? Contact Curt Nason.

The Apparent Magnitude chart shown above was made by the European Space Agency.

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