What’s Up There?
What do we see when we look up at the night sky? Lots of stars? What else? Let’s start with our Solar System. The most frequently visible, brightest object is our nearest neighbor, our Moon. There are the planets, appearing regularly according to their own patterns of movement. Occasionally we catch a glimpse of an asteroid, a comet, or some meteors whiz by. And then there are a few pieces of Earth that we’ve tossed up there, the International Space Station, the weather satellites, in all about 2,500 of them.
But as important as these objects are to us, they’re a very small part of what’s there. If our eyes are good and the nights are clear, we can see about 6,000 different spots of light as they move across the sky over the year. There are single stars and multiple stars, clusters, nebulae, and galaxies.
Many of these stars are like our Sun; some are younger (the bluish ones like Bellatrix in Orion), some older than ours (the reddish ones like Antares in Scorpius). Some of these have planetary systems. In the last ten years, 156 such systems have been found, almost all with only one planet identified. None so far seems to have a beautiful blue Earth-like planet, potentially harboring some kind of life.
Centuries ago, for convenience, people grouped the stars into named patterns or constellations such as Pegasus, Orion, and the Northern Cross.
The patterns have been useful in helping us locate them and their stars, and also ourselves, for centuries. We also know that because of our Earth’s changing location relative to the stars, and because of the stars’ own movements, these familiar patterns will be distorted over the millennia.
Cepheid variables are single stars whose period of variability correlates with their absolute luminosity. They were named for the first that was discovered, Delta Cephei, which is close enough to Earth that astronomers could also determine its distance by measuring parallax.
Because astronomers can determine distance by comparing absolute and apparent luminosity (what we actually see), Cepheids are a huge help in determining distances to host clusters and galaxies.
Most of the objects that appear to be single sources of light are actually two or more sources. Our single Sun is in a minority. When we use binoculars to look at Sirius for instance, we can see his companion, “the Pup.” Sirius, a double, or binary, shines at a magnitude -1.5, while the Pup is a fainter magnitude 8.5. Recently the North Star has been found to be a trinary (triple) system. Astronomers have a special interest in multiple stars because as they revolve around each other their luminosity fluctuates so, like Cepheids, they’re useful in calculating distances in space.
Gllobular cluster G1 in Galaxy M31. Courtesy Michael Rich, Kenneth Mighell, James D. Neill (Columbia University), Wendy Freedman (Carnegie Observatories), and NASA.
Two other groupings of stars are called clusters. Open clusters such as the Pleiades are stars which are gravitationally bound to each other. While these stars look a little like constellations, they actually do belong together and have a number of factors in common:
They’re about the same distance from us, they‘re about the same age as each other, and they have the same chemical composition. Because of their similarities, open clusters are useful in studying the evolution of stars.
The second grouping of clusters, globular clusters, make up a more complex grouping than open clusters.
Through a telescope they look a bit like a hundred thousand bees swarming around a hive. The absolute magnitude (how bright they would appear if they were at a standard distance of 10 parsecs) of the individual stars in each cluster would be many times that of our Sun. But because they’re so far away, their apparent magnitude taken all together is less than we can see with our naked eyes. So it wasn’t until telescopes were invented that any of them were found.
Astronomers have been particularly interested in the age and composition of the large clouds of gasses or nebulae. Their gasses and bits of dust are thinly spread over a great range. Newly forming stars are sometimes found in their dense areas. These stars may be bright enough to excite the surrounding gasses so that they shine, such as in the Horsehead nebula and the Crab nebula, well known from the spectacular photographs taken from the Hubble Space Telescope.
Moons and planets and comets, all the stars and clusters and nebulae—millions and billions and trillions of them—still our sky appears to our naked eyes to be about 99% empty. Of course we don’t see everything. That 99% has a dilute mix of hydrogen and helium, along with a tiny dusting of metals.
Up to now all these objects that I’ve listed except some of the Cepheids are in our own galaxy, our humongous, swirling system of stars.
But in the Northern Hemisphere we can see one other object with our naked eyes that is way out and beyond. This is M31, the Andromeda galaxy, about 100,000 light years (ly) away. When it’s a clear night, you might be able to spot it by tracing a line from the two western most stars of Pegasus, Algenib and Alpheratz, towards the North Star. Shortly beyond Alpherzta and a little to the east will be the bright center of M31 which is a spiral galaxy like ours.
The galaxies astronomers have identified appear to have all been formed about 10-15 billion years ago. With the Hubble telescope, and using the knowledge gained from Cepheids, astronomers recently have measured the distance of a galaxy in Virgo and found it to be at 56 million ly. Galaxies have helped determine the age of the universe, thought to have begun with the “Big Bang.”
Just as we once believed that our Earth was the center of the Universe (everything that turns, all space), and then slowly accepted the counterintuitive concept that our Sun didn’t “rise” every morning, later we were sure that our galaxy (the word refers to “milk,” as in the Milky Way) was everything in the sky. With more and more powerful telescopes, we’ve seen that our Milky Way Galaxy is only one of billions of galaxies. Space just gets bigger and bigger.
Is our “universe” still expanding? Or is it our understanding of space? Is there any end to expanding universal space? What else will we find “up there?”