Starry Skies Above Santa Monica

March 20-27, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   First Week of Spring
   Spring is here, this time it is official! (Although the wind in our canyon is still pretty cold…). The Sun crossed the celestial equator this afternoon, Wednesday, March 20. By Sunday, March 24, the Sun will already be 1.7 degrees above the equator, moving in front of the stars of the constellation Pisces (the Fishes).
   The Sun now culminates above the south when our clocks and watches show 12 noon, rises at 5:51 a.m. and sets at 6:09 p.m.

   Our Four Evening Planets
   Last week. I counted five of them, since I included Mercury, although it was not placed favorably for easy observation. This week, Mercury is too close to the Sun, so we are left with four evening planets. Among them, Mars is about to disappear rather soon, and will be followed by Saturn and Jupiter -– while Venus is approaching the time of a real evening glory.
   On Sunday, Venus will set by 7:26, which is 77 minutes after the Sun. Venus is still projected fairly close to the Sun in the zodiac -– both are in Pisces, but at the opposite ends of that large constellation. You will be able to spot Venus at about 6:40, low above the western horizon, a little to the north of the place where the Sun has set.
   Higher up along the ecliptic is Mars, in Aries (the Ram), climbing higher along the ecliptic fairly fast, thus delaying its time of setting, which comes by 9:30 p.m. Mars is now only barely as bright as a star of the first magnitude, and is still fairly conspicuous in the evening western sky only because there is no bright star in that region of the sky.
   Saturn is already to the west of the meridian when the sky becomes sufficiently dark, and then sets in the northwest after 11 p.m. Saturn still forms a fairly close and fairly conspicuous pair with Aldebaran, the brightest star in Taurus (the Bull). Saturn is brighter, lies a bit to the north of the reddish Aldebaran, and the loose star cluster of the Hyades is located between them.
   The evening sky is still dominated by Jupiter, which shines at the western edge of Gemini, close to the boundary with Taurus. Jupiter culminates above the south shortly after sunset, then moves to the western sky and sets shortly before 1:30 a.m.

   The Moon Rules
   This coming week, the Moon will grow in brightness, as the illuminated part of its disk increases. Tonight, Wednesday, March 20, the Moon will shine close to Saturn. In its First Quarter, tomorrow, March 21, the Moon will pass close to Jupiter, in the westernmost part of the large constellation of Gemini. Then it will travel through Gemini for two days, and on the evening of March 23, Saturday, it will shine below the pair of stars that gave the name of that constellation: Castor and Pollux.
   The next constellation of the zodiac is Cancer, where there are no bright stars. On the evening of March 25, Monday, the Moon will shine above the first magnitude star Regulus in Leo, then pass below that prominent constellation, and reach the phase of Full Moon during the day of March 28.

   A Very Hot Star You Cannot See
   The amount of radiation that a star emits increases rapidly with its surface temperature. The star I want to introduce to you actually holds a record in this respect: its surface temperature is 250,000 degrees C! Nevertheless, it is extremely difficult to spot it even with the world’s biggest telescopes. But, I must confess, I’ve told you only half of the truth. The radiation from unit surface of the star (say, from one square inch) does indeed increase very rapidly with its surface temperature, but the amount of the star’s total radiation naturally also depends on the star’s size.
   And that very hot star is an extremely small ball of hot gases; its radius is only 10 km. Our Earth is certainly a tiny object compared with normal stars; nevertheless, its radius is 6,378 km, much larger!
   That weird hot star is a neutron star, a tiny compact remnant of a much bigger star that once exploded as a supernova. That paternal supernova exploded about 340,000 years ago in the upper part of the constellation of Orion. The tremendous blast virtually destroyed the original star, and its tiny remnant, the neutron star, was ejected in the direction of Gemini. Hot gases were expelled in all directions into the surrounding cold and extremely rarefied interstellar gas.
   That explosion occurred almost in our neighborhood, only some 100 light years from us. If it been closer, its effects on life on the Earth could have been very, very bad. As it happened, though, this particular explosion was actually beneficial, at least for astronomers: it cleared our cosmic environment. Without it, we would be probably living in a fairly dense cosmic fog. The blast removed the interstellar gas and dust from a fairly large region around us, so that we can enjoy a very good view of the universe.