Celestial Lights—September 2009
By Phyllis Burton Pitluga


Touring the Milky Way Galaxy

Face-on Milky Way created from data using NASA’s Spitzer satellite.

The Milky Way is our home galaxy of 100 billion stars. The Sun is one of these stars and we are orbiting the Sun on planet Earth. When we look up into the dark nighttime sky, each and every individual star that we see is a nearby star. The band of light we call the Milky Way is merged light from stars farther away in our island galaxy. Our galaxy is about 10 billion years old.

Today the stars of the Milky Way are various ages. The Sun is a middle-aged star about five billion years old. Some stars are newborn stars, like in the cloud of gas and dust of the Orion Nebula or the Pleiades Star Cluster. Many are dim “dwarf” stars. Other stars are blazing a brilliant blue-white in their midlife. Yet other stars are nearing the end of life as the nuclear fuels of their blazing furnaces dwindle. The most massive stars are the hottest and consume their nuclear material at a faster rate than less massive, cooler stars.

The ultimate demise of a star depends on its amount of material or mass. The least massive take billions of years to gradually use their nuclear fuel, then they cool down and fade away. The mid-mass stars, like the Sun, undergo a long life of shining, shrinking, drawing in new nuclear fuel to their centers, re-igniting and expanding, then repeating the process until finally the atmosphere is shed in a bubble of gases and the exposed core shines for awhile as a white-dwarf star. The most massive stars burn their nuclear fuel in millions of years, then shed their atmospheres explosively in supernovae. The remaining core is a highly compressed neutron star or a stellar black hole. All of these star forms populate our Milky Way Galaxy: nebulae, newborn star clusters, stars young and old, white dwarfs, neutron stars and stellar black holes.

From above, way out in intergalactic space, the Milky Way Galaxy would look like a slowly spinning pinwheel 300,000 light years in diameter (one light year is about six trillion miles). The edge-on view of the Milky Way resembles a fried egg from the same perspective. The bulging center is like the yolk where a Super Black Hole controls its core. The surrounding “white of the egg” is the suburbs for a sea of stars. We orbit around the center at a distance of 27,000 light years in a period of 200 million years, which equals one galactic year.

On the next clear moonless night, around 9pm in mid-September, go to a place outside of town where you have a low southern horizon. To the south is the center of our galaxy. Looking up and then northward you can follow the band of the Milky Way. The individual stars to the west are stars above us in our galaxy. The individual stars to the east are below us in the plane of our galaxy. If you have binoculars or a telescope, scan along the band of the Milky Way. You may just discover a nebula or star cluster, one of our many galactic neighbors.

Edge-on Milky Way imaged using NASA’s Cosmic Background Explorer satellite.

Sky Calendar for September 2009

By following the Moon as the biggest and brightest “pointer” in the sky, during the month you can identify different planets and bright stars.

September 4, Friday: Full Moon rises at sunset. Usually the September Full Moon is the Harvest Moon; this year the Full Moon closest to the equinox will be in early October.

September 10, Thursday: The Waning Gibbous Moon passes above the Pleiades Star Cluster.

September 12, Saturday: Last Quarter Moon rises around midnight.

September 13, Sunday: The Moon passes above Mars in the dawn sky.

September 16, Wednesday: The Waning Crescent Moon, the star Regulus (of Leo the Lion) and Venus are near each other in the dawn sky.

September 16–October 2: Zodiacal Light, visible in the dawn sky away from other bright lights; this cone of light is from myriad grains of meteoric material in the plane of our solar system; each grain is like a teeny moonlet reflecting the Sun’s light. Together they create the Zodiacal Light; this time of year the cone of light is angled up nearly perpendicular so it stands up from the glow of dawn.

September 18, Friday: The day of New Moon (no Moon because its dark side faces Earth). 

September 22, Tuesday: The Autumnal Equinox is the day when the Sun is crossing the Celestial Equator from north to south. The Celestial Equator is an imaginary extension of the Earth’s equator out into space; as we orbit the Sun on our tipped planet, the Northern Hemisphere is tipped toward the Sun in June, placing the Sun north of the Celestial Equator over the northern hemisphere; in December we are tipped away from the Sun, placing the Sun south of the celestial equator and over the southern hemisphere; at the equinoxes in March and September, the Sun is on the Celestial Equator, over Earth’s equator.

September 23, Wednesday: The Crescent Moon passes above the star Antares of Scorpius the Scorpion in the evening sky

September 25, Friday: The First Quarter Moon is halfway across the sky at sunset.

September 29, Tuesday: Challenge: see if you can spot Jupiter below the gibbous Moon in the afternoon before sunset. Jupiter is six moon diameters below the Moon.