Celestial Lights
Lunar eclipse at sunset on March 2, 2007
This image, taken by NASA’s Hubble Space Telescope, shows the colorful “last hurrah” of a star like our Sun. The star is ending its life by casting off its outer layers of gas, which formed a cocoon around the star’s remaining core. Ultraviolet light from the dying star makes the material glow. The burned-out star, called a white dwarf, is the white dot in the center. Our Sun will eventually burn out and shroud itself with stellar debris, but not for another 5 billion years.
A very dramatic and different moonrise will occur on Saturday evening, March 3. As the Sun sets in the west, turn your back to the Sun and look to the east to search for the rising Moon. Because the edge of the Moon rising first is in total eclipse, you may see a red ball rising—or it may be a much darker color. You can view this eclipse from anywhere you have a clear eastern horizon or join us at El Charco del Ingenio Botanical Garden where many activities and telescopes will be available.
The sequence of this celestial event is:
6:48pm: The Moon rises while still about 1/3rd covered in shadow.
7:12pm: The Moon moves out of Earth’s deep shadow.
8:23pm: The Moon moves completely out of the much fainter penumbral shadow.
Things to notice during the eclipse are: the shape of Earth’s shadow on the Moon (which shows the shape of our planet); the color of the shadow (which is affected by the clarity of the air above the sunrise and sunset “edges” of Earth); the Moon is moving left—or eastward—out of the shadow (as the Moon orbits Earth) while the whole eastern sky is rising (because of Earth’s rotation).
Unbelievably dense cores after stars explode
Last month we explored star death, when a star no longer is undergoing nuclear fusion at its center that produces the starlight (February 2 issue of Atención, page 75). We saw that medium-mass stars, like the Sun, end up surrounded by a bubble of glowing gas (caused by the greatly swollen star no longer able to hold onto its tenuous outer gases) with the center of the star stripped of its outer atmosphere. More massive stars end with an explosion that violently strips the atmosphere off the core and blasts it out into space as a raggedy cloud. In its mid-life, a star is balanced between the inward pull of gravity and the outward pressure of starlight, keeping it round and steady.
When the Sun and similar stars stop their fusion reactions, the outward pressure of starlight ceases and gravity takes over, squeezing out the spaces from between the electrons. Thus, the stellar corpse that started out roughly the size of the Sun (100 times the diameter of Earth) gets squeezed down to about the size of Earth. No new energy is being created and only the heat left from the squeezing leaks away—making a “white dwarf star.” More than 1000 white-dwarf stars have been identified—mostly nearby because they are so dim. The brightest star in the night sky, Sirius, has a white-dwarf companion star (difficult to see even with a telescope). Sirius is high in our southern sky as darkness descends.
A star core that has one-and-a-half to three times more gas than the Sun and stops nuclear fusion is crushed even more. Now the electrons are merged into the protons to create a sphere of neutrons—resulting in a “neutron star.” The end result is a stellar corpse, more massive than the Sun, squeezed into the size of San Miguel! The neutron star is so dense that a spoonful weighs a billion tons. Their magnetic fields get squeezed so strongly that electrons get blasted of their surfaces and swept into an escaping jet of radiation. As the neutron star spins, this jet of radiation sweeps across space like a turning lighthouse beacon. When a neutron star beacon happens to be aimed at Earth, we see pulses of light from the rapidly spinning neutron star. These cases of neutron stars have been dubbed pulsars. The pulsar embedded in the Crab Nebula (photograph in last month’s article) pulses 30 times a second. That is how fast the crushed star core is spinning! The Crab Nebula is visible in medium-sized telescopes.
Larger stellar corpses are squeezed further still—into black holes. The immense amount of unsupported gases merge past electrons touching electrons (white dwarf), past electrons merging into protons to create neutrons (neutron star) to overwhelming all repulsive forces so that the matter is crushed into an infinitely small black-hole star core. It is black because light has to travel faster than the speed of light to escape. It is a hole because the immense gravity on and near the surface pulls nearby matter into it. The best example is in the constellation of Cygnus, which rises in the east before sunrise. It is called Cygnus X-1. With x-ray sensitive space telescopes, we can detect x-rays coming from beside a visible star whose gases are being ripped off and accelerated into the black hole. These collapsed star-core black holes are much different in scale than the super-massive black holes that lurk at the centers of the giant galaxies (for a future article).
Sky calendar: March 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 truly separated by millions, billions or trillions of miles.
Fri, Mar 2: The nearly Full Moon passes just above the star “Regulus”—the brightest star in the zodiacal constellation of Leo-the-Lion.
Sat, Mar3: The Full Moon rises, partially eclipsed by Earth’s shadow 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 the Moon orbits toward the direction of the Sun.
Wed, Mar 7: Waning Gibbous Moon (oval-shaped) passes two moon-diameters beneath the bright star “Spica” in the zodiacal constellation of Virgo-the-Maiden.
Sun, Mar 11: Waning Gibbous Moon passes below the star “Antares” in the zodiacal constellation of Scorpious. Last Quarter Moon rises after midnight.
Fri, Mar 16: Waning Crescent Moon passes just beneath Mars.
Sat, Mar 17: Waning Crescent Moon passes just beneath Mercury.
Sun, Mar 18: New Moon (The Moon is not visible because it has no sunlight shining upon the side that always faces the Earth.). A partial solar eclipse will be visible in Asia and Eastern Europe.
Tues, Mar 20: Spring Equinox, when the midday Sun will be higher over the northern hemisphere and the hours of daylight will increase for the next three months.
Wed, Mar 21: Waxing Crescent Moon passes above Venus.
Fri, Mar 23: Waxing Crescent Moon passes just above the Pleiades Star Cluster.
Sun, Mar 25: First Quarter Moon, at sunset, is halfway across the sky from west to east and will continue orbiting eastward each evening.
Wed, Mar 28: Waxing Gibbous Moon passes above Saturn.
Thurs, Mar 29: Moon passes above the star “Regulus”—notice the relative motion of Saturn to Regulus in the coming months.
Phyllis Burton Pitluga is Astronomer Emerita of the Adler Planetarium and Astronomy Museum, Chicago, and is now a San Miguel resident..