It’s a Big Universe
The word “astronomy” comes from Greek (astro and nomos) meaning “star arranger.” Astronomers do not exactly arrange the stars, but they do study how they are arranged by nature. In ancient times, astronomers could study only what they could see with their eyes. They had no telescopes. In modern times, astronomers have many devices which help them study the universe—telescopes, spectroscopes (to study the colors of light), satellites and much more.
And astronomers study much more than stars (for more information on stars, see, “Shoot For The Stars with Fun Facts”). They study planets, nebulae, black holes, galaxies, and the dust and gas between stars, called the “interstellar medium.”
The universe is big. To get an idea how big, imagine for a moment that the farthest known stars are in New York City and that you are in Los Angeles. Most of the night sky’s visible stars on this scale would be within 39 centimeters or about 15 inches. Don’t know the distance between New York and Los Angeles? It’s about 4000 kilometers (2400 miles). If you walked from one city to the other without stopping, it would take you about a month. You would have passed the stars visible without a telescope in the first second of your journey. In fact, on this scale you could walk across the entire Milky Way galaxy in about 20 seconds.
What’s a galaxy? It’s a large “city” of stars with anywhere from 100 million to a few trillion stars. They come in many shapes, too, including spherical, elliptical, flat disk spiral, and others. The picture, above, shows NGC 1300, a barred spiral galaxy.
Planets are large, round objects that typically orbit a star. Our Earth is a planet. And there are two kinds of planets—terrestrial (Earth-like) and gas giant. Terrestrial planets in our own star system include Mercury, Venus, Earth and Mars. “Terrestrial” really means a planet with a distinct surface (solid or liquid). The other known terrestrials have little else in common with our home planet. Mercury has no air and no oceans. Venus has too much air—a crushing atmosphere and temperatures that would melt lead. And Mars has a cold, thin atmosphere incapable of supporting life as we know it.
Gas giants don’t have solid surfaces. Their atmospheres are so thick that the air becomes a liquid before one could reach the rocky core. In the Solar system (our star system centered on Sol, our sun), the gas giants are Jupiter, Saturn, Uranus and Neptune.
Until the 1990s, we only knew of planets in our own star system. By now, scientists have discovered hundreds of planets orbiting nearby stars. Most of these are likely gas giants, because they are very massive, and because gases like hydrogen are the most common elements in the universe. The next great challenge for scientists is that of finding planets like Earth in size, temperature and atmosphere. There we might find life.
Fun Planet Facts:
- If a flying platform could be arranged above the clouds of Jupiter (since a gas giant has no solid surface), you would weigh on that platform two and a half times what you weigh on Earth. For example, if you weigh 100 pounds on Earth, you would weigh 250 on this Jupiter platform.
- On the surface of Mars, you would only weigh about 37 pounds if you weighed 100 pounds on Earth.
Stars and planets may be the most obvious objects in the universe. They may be the most important for life, but galaxies are full of other kinds of things.
Nebulae (singular, “nebula”) are large clouds of dust and gas that are loosely held together by gravity. How large? Many range in size from a dozen to several dozen light years in diameter. There are two kinds of nebulae—bright and dark. The bright nebulae glow and are very visible to astronomers in their telescopes. Dark nebulae are harder to detect, because they give off no visible light. Most dark nebulae make themselves known because they block out the light of more distant stars. They appear to be dark holes in the universe with only a handful of stars standing between us and the shadowy mass. Some nebulae show signs of local collapse. These areas are thought to be the beginnings of new star-planet systems similar to the Solar system.
Black holes are perhaps the strangest of objects. They are formed from the massive cores of exploding stars called, “supernovae.” Black holes are extremely dense objects. The gravitational pull near the object is so great that even light cannot escape. That’s why they are called “black.” Any light shown on the object will not be reflected back, but will forever be trapped within the black hole’s “event horizon”—the radius within which light is permanently trapped. Besides the remnants of individual stars, many galaxies are thought to contain super massive black holes at their centers, gobbling up stars which happen to travel too close.
More Fun Facts
- Most of the interstellar gas in the Milky Way galaxy is confined to a thin layer about 700 light years thick within the disk of our home “city” of stars. The average density of this gas is close to one particle per cubic centimeter (a cube one centimeter on a side). In other words, the space between stars is not a perfect vacuum.
- Astronomers have two scales of distance for measuring the space between stars—“light year” and “parsec.” Both of these units of measure are based on the orbit of Earth around our sun. A light year is the distance light travels in one Earth year—approximately 9.5 x 1012 kilometers (5.88 x 1012 miles), which is 9,500,000,000,000 kilometers. One parsec is about 3.26 light years.
- Pluto used to be called a planet. As more discoveries have been made, scientists felt compelled to refine their definition of a “planet.” Now, Pluto is considered to be a “dwarf planet.”
- Our sun is about 27,000 light years from the center of the Milky Way galaxy.
- The closest large galaxy is frequently called the “Andromeda galaxy” and stands over two million light years from our sun.
Robinson, J., & Muirden, James. Astronomy Data Book. John Wiley & Sons, 1979.
Shklovskii, Iosif. Stars, Their Birth, Life, and Death. W. H. Freeman and Company, 1978.
The American Heritage Dictionary of the English Language. Houghton Mifflin Company, 1970.
First and last pictures supplied by NASA. Public Domain.
Second image of a gas giant planet is a matte painting by the author for the short Hollywood film “Quest” (Pyramid Media, 1984), produced by Academy Award winning designer, Saul Bass, and written by renowned author, Ray Bradbury. All rights reserved.