For the first time in several years, the role of the first Christmas Eve star will not be played by bright planets, but by real stars. Of the relatively bright planets, only Saturn will be visible shortly after sunset, but its 1.16-magnitude glow is slightly fainter than that of several stars.
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Which facilities will compete for the title of the first star?
How bright is Vega?
When can you start looking for the first star?
What role will Jupiter play in the sky on December 24?
On December 24, the sun will set around 3:30 p.m., but the exact moment of this phenomenon differs by several or even several minutes for individual places in Poland. About half an hour later we can start looking for the first star.
When we look at the sky after sunset on Christmas Eve, we will see the Moon on the southern side of the sky. It will be a narrow crescent, four days after the new moon, heading towards the first quarter. To the east (to the left) of the Moon we find Saturn, the second largest planet in the solar system. Saturn will shine at magnitude 1.16, or in practice slightly less (about magnitude 1.40) because it is about 30 degrees above the horizon, which means we are looking at the sky through a thicker layer of atmosphere than if it were visible closer to the zenith.
Vega and Kapella. Which of them will be the “first star”?
The brightest object in the sky after the Moon, just after sunset, will be the star Vega, with a magnitude of 0.00 (real magnitude 0.17). It will shine high above the western horizon. Therefore, we may be slightly disturbed by the remnants of the Sun's light just hidden behind the horizon. Therefore, it may be easier for us to spot Kapella on the opposite side of the sky (in the northeast). This object has a stellar magnitude of 0.05 magnitude (in practice, after taking into account the distance from the zenith, it will be 0.27 magnitude).
If we wait a while longer, more stars will start to appear. For example, in the area where Vega is, we will also see Altair and Deneb, which together with Vega form a large triangle in the sky, called the Summer Triangle. In turn, in the area where Kapella is located, Aldebaran will appear, and around 5 p.m. Jupiter will begin to rise, which with a glow of minus 2.65 will take over the role of the brightest object after the Moon.
Scale of “stellar magnitudes”. It determines the brightness of stars and planets
Let us also explain what the “stellar magnitude” scale is, used to determine the brightness of stars and planets in the sky. Its unit is magnitude. The smaller the number, the brighter the object. The faintest stars visible to the naked eye (with good eyesight, dark sky and no lights) are about magnitude 6. The brightest ones have magnitudes around one and zero (the exception is Sirius, which has minus 1.45 magnitude). Very bright planets can be brighter than Sirius, the leader being Venus, whose luminosity can reach even magnitude minus 4.7. The division into stellar magnitudes comes from ancient times and, after modifications, is still used in astronomy today.
The first star in the Christmas Eve sky is a reference to the Star of Bethlehem. The Bible contains information about an astronomical object that was supposed to announce the birth of Jesus Christ and helped the Wise Men from the East (the Three Kings) reach the place of his birth. In current culture, the object is commonly associated with a comet because that is how it is most often depicted in illustrations. However, if we look scientifically, there are several hypotheses about what the Star of Bethlehem was. In this context, we also talk about multiple conjunctions (close-ups in the sky) of bright planets, a supernova explosion, and even the Andromeda Galaxy (haze visible to the naked eye in the Andromeda constellation).
