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About the cover


Puppis, the Stern

Plus the planets for April.


What do you do when you have a constellation that is just too big? This is a dilemma that faced the uranographer (celestial map maker) Nicolas Louis de Lacaille. Along the southern spring sky was the constellation Argo Navis, representing the ship that transported Jason and the Argonauts on their epic adventure, the Argo. This was a huge constellation that was among the 48 listed by the second-century Alexandrian (Egypt) astronomer Ptolemy.


Click the image for a bigger map


Low on the southern horizon on these April evenings you can find the constellation Puppis, the stern of Argo, the ship sailed by Jason and the Argonauts. The ship was originally a huge constellation, Argo Navis, which was broken into three or four smaller constellations, including Puppis. This constellation does not have any first-magnitude stars, but Naos is a second-magnitude star that is a blue O-class star that is fairly easy to find.

Argo Navis was the largest constellation in the sky, but its exact size is unknown, since the concept of constellation boundaries would come after it was divided into at least three, possibly four, constellations. Nicolas Louis de Lacaille broke Argo Navis into Carina (the keel of the ship), Vela (the sails) and Puppis (the poop deck, or stern). These three constellations retain the original Bayer designations (alpha, beta, gamma, etc.) of the stars as they were in Argo Navis. North of Vela and east of Puppis, the constellation Pyxis (the compass, formerly the mast) is also thought to have been part of Argo Navis, but it has its own Bayer-designated stars, so that's not certain.

Puppis represents the raised deck on the back of a sailing ship. The ship's captain would often be found on the poop deck (from the French la poupe, or stern), where he could get a good view of what was going on.

The brightest star in Puppis is the second-magnitude Naos. Also known as Zeta Puppis (remember, the Bayer designations are shared with all three Argo Navis constellations), Naos is a spectral type O4 star, making it very blue and very hot. It is the only naked eye O-type star in our sky. It is almost 23 times the mass of the Sun and 14 times its diameter. Naos is 12,500 times brighter than our Sun, so it shines at magnitude 2.21 even though it is 1,090 light-years away.

Also in Puppis is one of the brightest X-ray sources in the sky, Puppis A. This X-ray source was discovered by a British Skylark rocket launched from Woomera, Australia, in October 1971. Puppis A is a supernova remnant; the gas that had been the atmosphere of the original star has been blown outward at a high rate of speed, impacting the surrounding interstellar gas to form a shock wave that gives off X-rays. The star's atmosphere is usually blown out in all directions, so the supernova remnant covers a circular area of the sky, and Puppis A is no exception. But this explosion, which would have appeared in our sky approximately 3,700 year ago, was unique in that it was lopsided.

While the cloud of gas still formed a circular shape, it is moving away from the point where the supernova explosion occurred. The core of the supernova, a neutron star called RX J0822-4300, received a kick in the opposite direction. It is now moving through space at the tremendous speed of 930 miles per second, earning it the nickname of "Cosmic Cannonball."

The original star that exploded was big enough to go supernova (1.44 times the mass of the Sun), but not so big that it would form a black hole (the remaining core would have to be more than 2-3 times the mass of the Sun). As a star gets older, it builds up a core of iron left over from the nuclear fusion processes in its atmosphere. This core is held up by what is called electron degeneracy pressure: The core's gravity tries to push electrons closer together, but the electrons cannot occupy the same energy levels and so they push each other apart, keeping the core from collapsing.

In these stars, the mass of the core becomes so large that its gravity compresses the negatively charged electrons into the positively charged protons in the atom's nucleus, transforming the protons into neutrally charged neutrons. Without the electrons keeping the atomic nuclei apart, the core collapses as the former atomic nuclei, now all neutrons, pack together, forming one giant neutrally charged atomic nucleus about six miles across with the mass of the Sun. If you stood on the surface of this star, you would be crushed flat. Your remains would join the giant nucleus and become indistinguishable from the rest of the matter of the neutron star.



The Planets for April


The King of the Gods is moving slowly eastward in central Gemini. Jupiter is almost overhead, only 14 degrees from the zenith toward the southwest as it gets dark. As we get farther from its January opposition, Jupiter is getting slightly smaller and fainter as we pull away from it. At midmonth, it is magnitude -2.2 with a disc that is 37.2 seconds-of-arc across.

Watch the Skies
(times MDT)


April 7, 2:31 a.m. — First Quarter Moon

April 8, 3 p.m. — Mars at opposition

April 14, 7 a.m. — Mars closest to Earth

April 15, 1:42 a.m. — Full Moon-Total Lunar Eclipse

April 22, 1:52 a.m. — Last Quarter Moon
11 a.m. — Lyrid meteor shower peaks

April 29, 12:14 a.m. — New Moon -
Annular Solar Eclipse in Antarctica

Mars reaches opposition on April 8 and will be visible all night. On that day it will be exactly on the opposite side of the Earth from the Sun. But both Earth and Mars are in elliptical orbits. While Earth's orbit is only slightly elliptical, Mars' orbit is quite a bit more of an oval. During April this oval shape will be bringing Mars a little closer to the Sun and also the Earth. This means that Mars will actually continue to move closer to the Earth after opposition until April 14, when it reaches its closest point. At that time it will be shining at magnitude -1.4 with a disc 15.2 seconds-of-arc across. Mars will be only 57,408,000 miles away from us at that time, drifting westward across central Virgo.

Saturn rises around 8:45 p.m. in the east-southeast. It shines at magnitude +0.1 as it moves slowly westward in central Libra. The Ringed Planet's disc is 18.5 seconds-of-arc across, while the Rings are 42.0 seconds-of-arc across and tilted down 22.0 degrees with the northern face showing.

Venus starts the month in eastern Capricornus, moving eastward quickly into Aquarius. It moves all the way through Aquarius and ends the month in southern Pisces. Venus shines at magnitude -4.2 at midmonth and its disc is 19.3 seconds-of-arc across. The disc is 61% illuminated and becoming fuller every day. Venus rises around 4:30 a.m. Mercury is too near the Sun to be seen this month.

We get to see the Moon totally eclipsed on the night of April 14 into the morning of April 15. The eclipse begins around 10:53 p.m. as the Moon's eastern limb enters the Earth's shadow. It will take about 15 minutes before you can notice the subtle shading of the penumbral eclipse. The partial phase starts at 11:58 p.m. when the Moon first enters the umbra. It is completely in the umbra beginning at 1:07 a.m., when it will be cutoff from direct sunlight. Mid-eclipse is 1:45 a.m. The Moon may appear red as it is illuminated by all the sunrises and sunsets all over the world. The Moon gets back into sunlight at 2:24 a.m. and leaves the umbra completely at 3:33 a.m. The eclipse ends at 4:37 a.m.

There is also an annular solar eclipse visible for a short time in Antarctica on April 29 as the annular shadow just nicks the Earth. Australia and the southern Indian Ocean will see a partial solar eclipse. We will not be able to see this eclipse from here, but we can still "keep watching the sky"!



An amateur astronomer for more than 40 years, Bert Stevens is co-director
of Desert Moon Observatory in Las Cruces.


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