Celestial Lights
By Phyllis Pitluga, Feb 2, 2007

Three planets and the zodiacal light

As February begins, at about 45 minutes after sunset, Mercury is visible low but very bright in the west-southwest glow of twilight, and the much brighter Venus can be seen about 14 moon-diameters above it. By mid-month, Saturn rises in the eastern sky at twilight. Adding to the drama of the night sky, start watching for the growing darkness in the eastern sky as the Earth’s own shadow falls upon our atmosphere. At the same time, to the west, a huge upright cone of light extends above the twilight glow. This is the zodiacal light—the merged glow from a vast cloud of dust in the inner Solar System. These dust motes are reflecting sunlight.

To many using telescopes, Saturn is the queen of the planets because beautiful icy rings envelop it. We are looking at the southern face of the rings during 2007. This year, Saturn passes in front of the stars of Leo-the-Lion. Saturn takes 30 years to complete an orbit around our Sun. Between now and April 20, Earth will pass by Saturn, so the ringed planet will appear to move backward, or westward, among the stars. Saturn is to the right (west) of the bright star Regulus now. Continue to notice the widening space between Saturn and Regulus in the coming weeks because of our motion.

Star death in our galaxy and beyond

Last month, we explored the realm of clouds of gas and dust where stars are currently being born in our Milky Way Galaxy. Once a star is born, it continues shining for millions, billions or trillions of years. The bigger the star (the more gas, or mass), the hotter it is and the quicker it burns, thus the shorter its life. Our Sun is a medium-mass star that has been shining for almost five billion years and has enough gas in its center to continue for another five billion years. It is in the centers of stars where the temperature is high enough to fuse hydrogen into helium and release energy in the form of starlight. This outpouring of starlight is what keeps a star puffed up into a sphere.

Eventually, a star begins to run low on hydrogen in its center to fuse into helium and starlight. As the starlight subsides, the star contracts, draws in new fuel and re-ignites. Hotter stars begin burning helium and then carbon. Each of these new phases causes the star’s outer atmosphere to puff up and brighten, then dim and shrink, and it becomes a variable star. In the case of Sun-sized stars, the outer atmosphere becomes so tenuously held that it floats away as a big bubble around the exposed star core, called a “white dwarf” star. 
It is the more massive stars that have the dramatic, explosive star deaths. Ever heavier atomic nuclei are fused in the centers of these stars (hydrogen, helium, carbon, oxygen, silicon and iron). With the creation of an iron core, the fusion reaction stops, the iron disintegrates into helium, energy is absorbed instead of being released, and the overlying material suddenly collapses into the core—creating a neutron star core in less than a second! The outer atmosphere of the star falls and rebounds off the super-dense neutron star into a shock wave. The implosion is followed by an explosion that blows the star apart, except for the neutron-star core.

The Crab Nebula photograph shows the fragmented remains of a star explosion that was seen by Chinese astronomers amid the stars of Taurus-the-Bull in 1054 AD. Now, almost 1,000 years later, we see it with space telescopes as a beautiful and complex nebula. It is profound to contemplate that, at the beginning of our Universe, the only gases were mostly hydrogen, some helium and a sprinkling of lithium. It was only in the furnaces of the exploding supernovae that the rest of the 92 elements of nature were created. These elements became ensnared in clouds of gas and dust that were forming new stars—a process that continues to this day. Our Sun, Earth and we are made of ancient stardust from exploded stars.

Sky Calendar, February 2007 

By following the Moon as the biggest and brightest “pointer” in the sky, during the month you can identify different planets and bright stars. On following nights you can relocate them but without the Moon—the Moon moves about 25 times its own diameter from one night to the next. The Moon is much closer than the planets of our Solar System, and the stars are even farther. So, when the Moon appears close to a celestial light, they are actually separated by millions, billions or trillions of miles. 

Feb. 1, Thursday: Full Moon rises at sunset. For the next two weeks, the Moon rises about 50 minutes later each night, making it also visible in the morning sky as it orbits toward the direction of the Sun. The Moon passes two moon-diameters above Saturn.

Feb. 2–3 Friday–Saturday: Waning Gibbous Moon (oval-shaped) passes just above the bright star Regulus in the zodiacal constellation of Leo-the-Lion. 

Feb. 7, Wednesday: Mercury is highest up in the evening sky before it begins orbiting back to pass in front of, but above, the Sun on Feb. 22. Also, the Waning Gibbous Moon passes two moon-diameters beneath the bright star Spica in the zodiacal constellation of Virgo-the-Maiden.

Feb. 10, Saturday: Saturn, rising opposite the Sun, is closest to Earth for the year (thus brightest) and is up all night. Last Quarter Moon rises after midnight.

Feb. 12, Monday: Waning Crescent Moon passes 12 moon-diameters beneath Jupiter.

Feb.14, Wednesday: Waning Crescent Moon passes 8 moon-diameters beneath Mars.

Feb. 17, Saturday: New Moon (the Moon is not visible because it has no sunlight shining upon the side that always faces the Earth). 

Feb. 19, Monday: Waxing Crescent Moon passes above Venus.

Feb. 23, Friday: Waxing Crescent Moon passes just above the Pleiades Star Cluster.

Feb. 24, Saturday: First Quarter Moon, at sunset, is halfway across the sky from west to east and will continue orbiting eastward each evening.

Phyllis Burton Pitluga is Astronomer Emerita of the Adler Planetarium and Astronomy Museum, Chicago, and is now a San Miguel resident.