APRIL 2018 • 31
THE STARRY DOME • BERT STEVENS
Carina, the Keel E
ven here in the desert Southwest, a ship sails along our southern horizon. Carrying the legendary Jason and the Argonauts in search of the Golden Fleece, the Argo is sketched out in the southern sky. Being a large ship for its time, this constellation covers a large portion of our southern sky. Astronomers found its size so unwieldly that they broke Argo Navis up into three parts. Louis de Lacaille drew new boundaries and named the three new constellations Argûs in carina (later shortened to Carina, the keel or hull), Argûs in puppi (Puppis, the poop deck or stern) and Argûs in velis (Vela, the sails) in 1763. This allowed him to take the one hundred and sixty visible stars and assign them new Greek-letter (known as Bayer) designations that were more in line with the actual star magnitudes. While Vela and Puppis both rise completely above our southern horizon, Carina, at the bottom of Argo Navis, only manages to rise partway above the horizon. The majority of this constellation always remains below our horizon, including one of the most interesting stars in the sky, Eta Carinae, which is five million times brighter than our Sun. Eta Carinae is composed of at least two stars that orbit each other at an average distance of only fifteen times the distance from the Earth to the Sun. But these are not ordinary stars, the larger is around 200 times as massive as our Sun, while the other is around 50 solar masses. The larger one has already lost over 30 solar. These two massive stars complete an orbit every five and a half years. The binary nature became apparent in 1996 when a regular cycle of brightness variation recurring every 5.52 years was identified and later refined to 5.54 years. When they reach the closest point in their orbit, they are as close as Mars and the Sun These two stars have dramatically different stellar winds. The primary star has a massive outflow that is very dense but not especially fast, traveling around one million miles per hour. It carries away a solar mass worth of material every 1,000 years from the primary. The secondary’s stellar wind carries over a hundred times less matter but travels six times faster. The interaction of these two stellar winds creates the dramatic changes in the star’s surroundings as the secondary swings around the primary. These
changes include X-ray flares and nebular structures near the stars fading and then becoming visible again. From Earth, the brightness of the two stars cannot be measured separately, since they are around 7,500 light-years away. The total brightness of Eta Carinae changes dramatically, probably more the result of the dust thrown off by the primary star than the actual variability of the two stars. The combined system has faded below naked-eye visibility at sometimes and it has become brighter than Canopus at other times. They are currently around fourth magnitude. The most recent outburst in the star’s brightness occurred in the 1830s through 1840s, when it reached a maximum magnitude of -1.0. This period is known as the “Great Eruption”. The star’s brightness changed slowly, brightening slowly up toward the maximum with occasional peaks as the two stars reached their closest point. After reaching a peak it 1845, the star began to fade as dust built up in the system. The brightness reached a minimum in the 1920s and 1930s when the star was not visible to the naked eye. It has since begun to brighten again. The Great Eruption ejected enough dust and gas from the primary to form a planetary nebula around the star. The resulting two-lobed nebula, dubbed the Homunculus nebula, is very young, so young that its shape is due primarily to the original ejection event and not the interaction with surrounding interstellar material. The two lobes are from material ejected outward from the poles of the primary. Based on the thickness of the shell, this ejection event probably lasted around five years. Eta Carinae is an amazing star, the primary is near the maximum size that a star can be. It is also a mature star, with its extreme mass making it age very quickly. It will not be around for long, since the primary star will probably go supernova in the (astronomically) near future.
The Planets for April 2018 Venus climbs higher in the evening sky this month as it moves further from the Sun. Setting around 9:30 p.m., Venus shines at magnitude -3.9. Its disc is ninety-two percent illuminated and it is 11.0 seconds-of-arc across. During the month, the Goddess of Love travels eastward from western Aries into central Taurus.
Calendar of Events – April 2018 (MST) 02 6 a.m. Mars 1.3 degrees south of Saturn 08 1:18 a.m. Last Quarter Moon 13 10 p.m. Mercury stationary 15 7:57 p.m. New Moon 17 8 p.m. Saturn stationary 22 Noon Lyrid meteor shower peaks 22 3:46 p.m. First Quarter Moon 29 Noon Mercury farthest west of the Sun (27 degrees) 29 6:58 p.m. Full Moon
Triangulum is a small constellation that is almost overhead in our January sky. While it is small, this constellation has the most distant naked-eye object, the Pinwheel Galaxy (M33. You will need to be in a dark site away from urban areas to be able to see it. A brighter and much nearer Andromeda Galaxy (M31) is also shown on this month’s chart.
Central Libra plays host to Jupiter this month as it moves slowly westward. It rises at 8:00 p.m. in the east-southeast and it is twenty-one degrees up in the southwest as it gets light. The King of the Gods’ shines at magnitude -2.5 and its disc is 41.0 seconds-of-arc across. The Ringed Planet rises around 1:00 a.m. in north-central Sagittarius. It starts the month moving very slowly eastward. On the Eighteenth, it comes to a stop and turns back westward. It shines at magnitude +0.5 and it can be found thirty-five degrees above the southern horizon as it gets light. Saturn’s disc is 13.8 seconds-of-arc across, while the Rings are 38.6 seconds-of-arc across and they are tilted down 26.3 degrees. Moving eastward in north-central Sagittarius, the Red Planet passes Saturn on April 2. It continues eastward to the eastern end of that constellation by the end of the month. At midmonth, the God of War’s disc is 9.6 seconds-of-arc across, shining at magnitude +0.0. It rises around 1:45 a.m. and it is thirty-four degrees above the southern horizon as it gets light. Mercury makes an appearance in the morning sky but is poorly placed for Northern-hemisphere observers. It escapes the morning twilight at midmonth and reaches its furthest point from the Sun on April 29. On that date, the Messenger of the God’s disc is 8.0 seconds-of-arc across and it is forty-three percent illuminated. Mercury rises at 5:15 a.m. and it is just six degrees above the eastern horizon as it starts to get light. So, enjoy the dance of the planets as April’s warmer weather gives you plenty of opportunity to “keep watching the sky!” An amateur astronomer for more than 45 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.
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