Taurus, the Bull
Plus the planets for January.
High in our eastern sky after it gets dark is the constellation Taurus, the Bull. Taurus is marked by the first-magnitude star Aldebaran, a reddish, spectral class K5 star that sits in front of the Hyades open star cluster, a large V-shaped grouping that is easy to recognize. Taurus is the 17th largest constellation and it has the ecliptic traveling through it, so you can sometimes find one or more of the planets within its bounds.
In mythology, the ever-amorous Zeus fell in love with Europa, the beautiful daughter of King Agenor of Phoenicia. He appeared to her and her companions as a white bull (Taurus) that was very docile. Europa and her friends were taken with the bull and made flower garlands and placed them around the bull's neck. He was so docile that the trusting Europa climbed on his back. Taurus immediately plunged into the sea and swam to Crete with the terrified Europa on his back. There he changed back into Zeus and ravaged the maiden. She bore him three sons. The oldest, Minos, brought the Cult of the Bull to Crete.
There are two major open star clusters in Taurus, both visible to the naked eye. One is the previously mentioned Hyades, which mythologically represents the five daughters of Atlas, a giant who carried the heavens on his shoulders. It is 153 light-years away (compared to Aldebaran's distance of only 65.1 light-years). It is the closest open cluster to us and covers 5.5 degrees in our sky, corresponding to a diameter of 20 light-years across for the cluster's core. Almost 200 stars have been identified with the Hyades, but not all are in the core of the cluster.
The other open cluster is the Pleiades, the seven daughters of Atlas and Pleione (half-sisters of the Hyades). This cluster is much smaller in our sky, just under two degrees across. But it is only slightly smaller than the Hyades in space, 16 light-years across versus the Hyades' 20. The difference is the distance. The Pleiades are around 400 light-years away, two and a half times the distance to the Hyades. The Pleiades has over a thousand stars, but the nine brightest are Alcyone, Atlas, Electra, Maia, Merope, Taygeta, Pleione, Celaeno and Sterope. They are called the Seven Sisters, and include their parents, Atlas and Pleione.
The stars of the Pleiades range from the hot B-class stars like the Seven Sisters down to dim brown dwarfs. Brown dwarfs are substars that are not big enough to create the hot stellar interior needed to fuse hydrogen into helium. They produce little light and heat, so they are not considered true stars. They fit in the gap between the lightest stars, about 80 times the mass of Jupiter, and the gas-giant planets. While they cannot fuse hydrogen, brown dwarfs heavier than 13 Jupiter masses can fuse the heavier deuterium (a hydrogen atom with an extra neutron in its nucleus) with another neutron to form helium-3 (two protons and one neutron in the nucleus). Larger brown dwarfs can also fuse lithium.
The first brown dwarf was discovered orbiting the red dwarf star Gliese 229 in 1994, but not confirmed until 1995. Gliese 229 is spectral class M1, though some other red dwarfs are spectral class K. Its brown dwarf companion, dubbed Gliese 229B, is between 20 and 50 times the mass of Jupiter and could fuse deuterium, but it has used up its entire supply. Gliese 229B's surface temperature is only 1,300 degrees Fahrenheit.
In 1995, Tiede 1 was discovered in the Pleiades cluster and confirmed to be a brown dwarf. The confirmation of Gliese 229B came after that of Tiede 1, so Tiede 1 is the first confirmed brown dwarf. Since that time thousands of brown dwarfs have been discovered. Most float freely in space, like stars, indicating that even on the very-low-mass end of the stellar scale, stars and planet form in very different ways.
The Planets for January
Venus departs the evening sky this month. On Jan. 1 it will be six degrees above the west-southwestern horizon as it gets dark. By 6:30 p.m., it will have set. After the first week of the month, Venus will be too close to the Sun to be seen. On Jan. 1, Venus will be a 3% illuminated crescent disc that is 60.4 seconds-of-arc across, glowing at magnitude -4.3. The Goddess of Love spends the month moving slowly westward in Sagittarius. Venus will appear in the morning sky by the end of the month.
Watch the Skies
Jan. 1, 4:14 a.m. — New Moon
Jan. 5, 2 p.m. — Jupiter at opposition
Jan. 7, 8:39 p.m. — First Quarter Moon
Jan. 15, 9:52 p.m. — Full Moon
Jan. 23, 10:19 p.m. — Last Quarter Moon
Jan. 28, 8 p.m. — Venus 2° N of Moon
Jan. 30, 2:38 p.m. — New Moon
Jan. 31, 12 p.m. — Venus stationary
Mercury puts in an appearance in the evening sky towards the end of the month. The Messenger of the Gods starts the month in Sagittarius, travels through Capricornus and ends the month in Aquarius. At the end of the month, Mercury will be magnitude -0.5 with a disc that is 7.1 seconds-of-arc across and 51% illuminated. You can find it 11 degrees above the west-southwestern horizon as it gets dark, setting just after 7 p.m.
The King of the Gods is already up in the east-northeast as it gets dark. At midmonth, Jupiter's disc is 46.5 seconds-of-arc across and the planet shines at magnitude -2.7. Jupiter is moving slowly westward in central Gemini.
Mars rises in the east just before midnight. It is moving eastward in central Virgo, shining at magnitude +0.6. Its disc gets a little larger each day, reaching 7.7 seconds-of-arc across at midmonth.
Saturn is moving slowly eastward in central Libra this month. At midmonth, the Rings are 36.6 seconds-of-arc across and tilted down 22.4 degrees with the northern face showing. Its disc is 16.1 seconds-of-arc across. The Ringed Planet shines at magnitude +0.6; it rises at 2:15 a.m. in the east-southeast.
Enjoy the cold clear nights of January and "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.