D e s e r t E x p o s u r e
January 2010
Constellation of the Month: Horologium, the Clock
After you have reached a certain age, the end of the year and the beginning of a new one seem like the end of the sweep of a pendulum in a clock, ticking off the years as they go by. So it is fitting that this month's constellation was originally named the pendulum clock (Horologium Oscillitorium) by Abb Nicolas Louis de Lacaille to honor the clock's inventor, Christiaan Huygens. When astronomers formalized the constellations, they shortened the name to Horologium, the Clock.
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Click the image for a larger map
Just east of the southern point on our horizon you can find the constellation Horologium, the Clock. There is little in this constellation to look at and it never comes completely over our southern horizon. |
Lacaille came up with this constellation between 1751 and 1752 during his stay at the Cape of Good Hope. Even though De Lacaille named the constellation for Huygens, for his invention of the regulator clock, the idea of using the pendulum actually came from Galileo. Lacaille's drawing of this constellation is very detailed, but had no real connection to the faint stars in this section of the sky.
The brightest of these faint stars is Alpha Horologii, glimmering at a dim magnitude 3.86. None of the stars in this constellation have Arabic names, only Bayer designations. The only interesting star here is the variable star R Horologii. This is a red giant star that changes its brightness in a regular cycle. It starts out at magnitude 14.3, slowly brightens to naked-eye visibility at magnitude 4.7, then fades back to its original brightness. This cycle takes 407.6 days and is the largest brightness range in variable stars of this class, called a Mira-type variable, after the prototype star of this class.
Mira-type variable stars are all red giants, near the end of their life. These stars are 2.25 solar masses or less, and have burned (through nuclear fusion) all the hydrogen in their core into helium. That helium is compressed by the weight of the star until fused in turn into carbon, when three helium nuclei slam together to form carbon-12, and in the process release energy. This energy heats up the atmosphere of the star and the atmosphere expands outward. The outer layers are now much farther from the star's core so the extra energy is spread over a much larger surface area. This causes the outer layer to be cooler, making the star appear redder than younger stars.
These stars eventually burn most of their helium, leaving a core of oxygen and carbon nuclei with a shell of helium still burning around it. The burning in the helium shell is unstable. When the energy output increases, the atmosphere of the star expands, the star becomes larger and it appears brighter. When the energy output drops, the star shrinks and becomes dimmer. In the case of R Horologii, this happens on a regular basis every 407.6 days.
These stars have a longer-term instability that lasts a few hundred years and reoccurs every 10,000 to 100,000 years. The energy of the burning helium is trapped just above the helium shell because the shell is not thick enough to lift the atmosphere above it. When the temperature gets high enough, the energy is suddenly released, causing the star to expand very rapidly in a "helium shell flash." The outer layer of the atmosphere is often pushed off into space. These wayward layers of the star's atmosphere form a cloud of gas and dust far beyond the star. We often see these clouds as planetary nebulas, such as the Ring Nebula or the Dumbbell Nebula.
In 5 billion years our Sun will become a red giant and later go through this instability phase, becoming a Mira-type variable.
The Planets for January 2010
Low in the west-southwest as it gets dark, you will find Jupiter dropping toward the horizon, setting around 8 p.m. The King of the Planets is not well placed for observing since it is only 24 degrees up as it gets dark. Mid-month will have Jupiter's disc only 34.0 seconds-of-arc across, shining at magnitude -2.1. Jupiter will travel eastward among the stars, moving from eastern Capricornus to west-central Aquarius.
Mars is visible all night, passing opposition on Jan. 29. This is the prime time to view the Red Planet since its disc will be 14.1 seconds-of-arc across, huge for tiny Mars. It is moving westward, passing from western Leo to eastern Cancer on Jan. 9. Mars shines a bright red at magnitude -1.3. This is the closest it will be to the Earth, with radio signals from our Mars probes taking only 5.5 minutes to reach Earth. Mars' northern hemisphere is tipped toward the Earth right now, and it is spring there.
Saturn is in western Virgo. It begins January moving eastward, but on the 14th, it reaches its stationary point and then turns back westward. The Ringed Planet's disc is 18.2 seconds-of-arc across, while the rings are 41.3 seconds-of-arc across, tipped down 4.9 degrees with the northern face showing. Yellowish Saturn shines at magnitude +0.8.
| Watch the Skies (all times MST) Jan. 2, 5 p.m. — Earth closest to the Sun for 2010 Jan. 7, 3:39 a.m. — Last Quarter Moon Jan. 11, 6 a.m. — Antares 0.5 degree south of the Moon Jan. 15, 12:11 a.m. — New Moon — Annular Eclipse 9 a.m. — Mercury stationary Jan. 23, 3:53 a.m. — First Quarter Moon Jan. 26, 4 p.m. — Mercury greatest elongation west (25 degrees) Jan. 27, 11:44 a.m. — Mars closest to Earth Jan. 29, 1 p.m. — Mars at opposition 11:18 p.m. — Full Moon |
Mercury makes a brief appearance in our morning sky in the last half of January. It becomes visible around Jan. 12, rising in the east-southeast about 6 a.m. The Messenger of the Gods reaches its greatest distance (25 degrees) from the Sun on Jan. 27, when Mercury will glow at magnitude -0.1, and its 61% illuminated disc will be 6.8 seconds-of-arc across. During this apparition, it will be getting smaller and fuller as the month goes on.
Venus is too close to the Sun to be visible.
The Earth passes its closest point to the Sun on Jan. 2 at 5 p.m., but our orbit is so close to circular that the tilt of the Earth has a stronger effect on our temperatures than the distance to the Sun. An annular solar eclipse will be visible from west-central Africa into southern China on Jan. 15. Even if you do not travel to the other side of the world to see the ring-of-fire eclipse this month, there is plenty of action above our heads, so "keep watching the sky"!