The galaxy and globular clusters
Sunday morning’s switch back to Standard Time from Daylight Saving Time has the sky darkening before 6 pm. Tonight, if you have a low horizon to the southwest, you just might see the center of the Milky Way galaxy hovering 5 degrees above the horizon with the plane of the galaxy, that milky swath of sky, rising nearly straight up from the horizon to where Cygnus, the cosmic swan is winging its way south as all swans should do as winter approaches.
The center of the galaxy is just off the tip of the arrow of Sagittarius, or off the tip of the asterism of The Teapot as shown in the diagram. Saturn, now a billion miles from Earth, glows dimly just to the right (west) of the galactic equator, above the galactic center and below the Winter Solstice that lies nearly on the galactic equator. The Winter Solstice is the southernmost position on the Ecliptic, the Sun’s apparent annual path across the sky. On the diagram note that the Ecliptic passes through Sagittarius, Capricornus, Aquarius (not shown) and Pisces … the constellations of the Zodiac. These constellations designate the astrological houses precisely because they lie along the Ecliptic, allowing the Sun to pass through each (and Ophiuchus) every year. The Celestial Equator is the projection of Earth’s Equator onto the sky, showing that the solstice is when the Ecliptic is south and the Vernal Equinox in Pisces is where the Ecliptic crosses the Celestial Equator where the Sun moves into the northern sky.
The center of the Milky Way, our home galaxy, lies about 25,000 light years from us well beyond the stars of Sagittarius. There, the orbits of stars observed with radio waves that can pass through the intervening dust and gas blocking the light waves, have revealed a supermassive black hole. This speeds of the stars in their orbits show the object they orbit to be on the order of 4.3 million times the mass of our Sun crammed into the size of Mercury’s orbit, 1/3 that of the Earth. As we have learned more about other galaxies, it appears that all, or nearly all, have a supermassive black hole at their centers. It is most likely they became supermassive by merging with other black holes and massive stars in the densely populated centers of the galaxies.
But how did we discover that the center of the galaxy was in the direction of Sagittarius and not, say Cygnus, Cassiopeia, Perseus or Auriga that also live along the galactic equator? The first hint was given by star clusters … specifically globular clusters.
There are two main types of star clusters, open (or galactic) clusters and globular clusters. The open clusters are sibling groups of stars … stellar litters, if you will. Stars form from vast clouds of interstellar dust and gas that collapse under the relentless pull of gravity. During that process, different high density regions begin to collapse into separate stars. When the stars emerge from their cocoons of gas, they are clustered in the region of the original cloud. Our most familiar open cluster is the Pleiades, rising in the eastern sky as the galactic center sets this evening.
The globular clusters are much older clusters. These were some of the original structures of the galaxy and are dense, gravitationally bound spheres of hundreds of thousands of low-metal, old stars. In photographs they appear redder than open clusters since they are dominated by small, old, red stars whereas open clusters often have bright white and bluish massive young stars. The massive stars exhaust their fuel and explode within tens to hundreds of millions of years, whereas the less massive stars can shine for tens to hundreds of billions of years. Our own Sun is about half-way through its ten-billion-year life.
Harlow Shapley of Mt. Wilson Observatory began observing globular clusters in 1914. By measuring the positions and distances, by 1918 he was able to discern the rough dimensions of the galaxy and the distance to the center. Some of what he discovered is visible in the diagram. The globular clusters are represented by circles. It’s very obvious that they are not evenly distributed in the sky, but are mostly found near the galactic center. There are some in all parts of the sky, but their spherical distribution is centered on the center of the galaxy.
The orbits of the globular clusters about the center of the galaxy are also spherically distributed, completely independent of the orbits of stars in the disk of the galaxy like our own Sun. In fact, it is the “scaffolding” of these orbits that allowed astronomers to measure the Sun’s motion in its galactic orbit. The galactic year is about 225 million years, so in its 5 billion year life, the Sun has orbited the center about 22 times … “The Sun’s not much older than you!” I tell my students when we study the galaxy.
To circle the galaxy in 225 million years, the entire solar system must move at about 514,000 mph, or 143 miles per second. Look to bright Vega (VEE-guh), the “swooping eagle” of Lyra and realize that this galactic orbital motion is carrying the Sun, the Earth and you toward this star at 514 thousand miles per hour! Vega, of course, is on its own galactic orbit, so we won’t catch up, but it can bring a catch to the breath to recognize the incredible speeds at which our cosmic home is moving.
One iconic globular cluster is found in Hercules, the kneeling one, who hangs upside down in the western sky tonight. He rests a foot on the head of Draco, the dragon he has slain while he kneels with his other leg. Along the edge of his hip in The Keystone, an easily-recognized asterism, is Messier 13, or M13, the 13th uninteresting fuzzy blur in Messier’s 1771 catalogue. The interest in his day was comets so he listed the known blurry objects known to not be comets. Though he wasn’t interested in them, we now know them to be nebulae, star clusters, and galaxies that have revealed many wonders of the universe to us.
M13 shining with the combined light of 300,000 stars is barely discernable with the (sharp) unaided eye in a very dark sky. Slightly more visible in binoculars, it is best viewed through a telescope. It is 22,200 light years from Earth so the light we see tonight has been traveling since the peak of the last ice age.
The volunteer astronomers at the Adirondack Public Observatory are eager to show you M13 and other globular clusters, open clusters, galaxies and other wonders. The Roll Off Roof Observatory is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.