Starry Skies Above Santa Monica

December 19-26, 2000

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   This Year’s Christmas Star Is…
   … the planet Jupiter, which currently dominates the sky all night long. Only Venus can outshine Jupiter, but Venus is currently too close to the Sun in the morning sky to be easily visible. Very rarely, Mars is brighter than Jupiter, but this happens only for a few weeks around the time of its opposition to the Sun –- but Mars is currently getting close to the time of conjunction with the Sun, when it will be completely lost in the glare of the Sun, being far behind it, on the opposite side of its orbit with respect to the Earth.
   Therefore, Jupiter rules the night! Its opposition to the Sun does not come until the last day of this year, but the difference in rising and setting times is not large, so we will describe the situation on Saturday night, December 22/23. The Sun will set at 4:49 p.m. The brightest stars will become visible around 6 p.m. In the southwest, Mars will dominate, before it sets after 10 p.m. Jupiter will rise in the northeast at 5:24, and from then on, it will be the brightest “star” in the sky till the time of sunrise, when, of course, the planet will be in the northwest. It culminates high above the south (actually almost overhead for us) after midnight.
   Preceding Jupiter is Saturn, which can be seen high in the east as soon as the brightest stars become visible. Although significantly fainter than Jupiter, Saturn is quite bright, and it forms a nice pair of bright stars with the reddish Aldebaran, the “red eye of the Bull” (Taurus). You will not need any star map to recognize this pair, and, of course, you cannot miss Jupiter. The only star that approaches its brightness is Sirius, rising low in the south-east after 7 p.m.
   Actually, the brightest object in the sky will be the Moon. When our SMM week begins, on Wednesday evening, December 19, the Moon will be a fairly thick crescent, visible in the southwest after sunset. On the next evening, Thursday, the Moon will shine below Mars and fairly close to it, so that it will help the beginners to identify the planet. As the week progresses, the Moon’s shining face will become fuller. The First Quarter comes on Saturday, December 22.

   The Sun as a Star
   On December 21, the Sun will halt its southward motion (we will have the winter solstice), and will slowly begin to return to us. This is perhaps a good opportunity to say a few words about the Sun as compared to other stars. The Sun appears extremely bright because it is very close to us, measured on the scale of the universe. Its light, moving at a speed of 300,000 km/sec, reaches us in slightly more than 8 minutes. The next nearest star, Proxima Centauri, sends to us its light at the same fantastic speed, yet its rays need 4 years and 3 months to reach us.
   Please pause for a while and compare the two numbers! Perhaps this model will help you: Let’s shrink the Sun into the size of a basketball. Then the Earth will be a pinhead (albeit of the larger, colored type!), orbiting the basketball at a distance of 32 meters, and the entire solar system of all planets including Pluto would approximately fit into the UCLA campus. On the same model, the nearest star would be in Tokyo, Japan!
   Well, there would be three stars in the Tokyo Bay, on this model. To the naked eye, they appear as one star, called Alpha Centauri (the brightest star in the southern constellation of Centaurus). A good telescope shows us that it is actually a triple star. The brightest star of the three is very similar to our Sun in every respect; the second brightest star is smaller, cooler, and of course fainter – but the orientation of the triple system is such that the faintest star of the three happens to be a bit closer to us. Thus it carries the proud title “Proxima,” but there is nothing else for it to be proud of!
   Proxima emits only as much light as 45 Full Moons. If you imagine 45 Full Moons combined together, this may impress you as a lot of light. Remember, however, that our eyes record light on a logarithmic scale – and that the Sun radiates as much light as nearly half a million Full Moons (470,000)!
   OK, so what about other stars? Among the 101 nearest stars, the Sun ranks 8th by its radiation output. Thus only 7 stars out of those 100 are more luminous than our Sun! And among those seven stars, four are only just a bit brighter than the Sun, so really only three stars in our neighborhood are perceptibly larger, hotter, and brighter than the Sun.
   And our stellar neighborhood is fairly typical. Thus, while our Sun certainly holds no record in size, or mass, or brightness, a large majority of stars are cooler, smaller, reddish dwarfs.
   Life, at least as we know it, would be hardly possible on a potential planet orbiting such a faint red dwarf. On the other hand, the real stellar record holders as to light output would be hardly better where life is concerned -– except perhaps for a possible planet orbiting such a luminous star at a much larger distance than the Earth orbits the Sun.
   So, let’s appreciate our Sun and be happy with it, and, by the way, be happy to live in Southern California!