Even though people have studied the heavens for thousands of years, we still know relatively little about the universe. While astronomers continue to explore, they learn more about the stars, planets, and galaxies in some detail and yet some phenomena remain puzzling. Whether or not scientists will be able to solve the mysteries of the universe is a mystery itself, but the fascinating study of space and all its many anomalies will continue to inspire new ideas and give impetus to new discoveries as long as humans continue to look up at the skies and wonder, "What's out there?"
Dark Matter in the Universe
Astronomers are always on the hunt for dark matter, a mysterious form of matter that can't be detected by normal means-hence its name. All of the universal matter that can be detected by current methods comprises only about 5 percent of the total matter in the universe. Dark matter makes up the rest, along with something known as dark energy. When people look at the night sky, no matter how many stars they see (and galaxies, if they're using a telescope), they're only witnessing a tiny fraction of what's actually out there.
While astronomers sometimes use the term "vacuum of space," the space that light travels through isn't completely empty. There are actually a few atoms of matter in each cubic meter of space. The space between galaxies, which was once thought to be quite empty, is often filled with molecules of gas and dust.
Dense Objects in the Cosmos
People also used to think that black holes were the answer to the "dark matter" conundrum. (That is, it was believed that the unaccounted for matter might be in black holes.) While the idea turns out not to be true, black holes continue to fascinate astronomers, with good reason.
Black holes are so dense and have such intense gravity, that nothing-not even light-can escape them. For example, should an intergalactic ship somehow get too close to a black hole and be sucked in by its gravitational pull "face first," the force on the front of the ship would be so much stronger than the force at the back, that the ship and the people inside would get stretched out-or elasticized like taffy-by the intensity of the gravitational pull. The result? No one gets out alive.
Did you know that black holes can and do collide? When this phenomenon occurs between supermassive black holes, gravitational waves are released. Though the existence of these waves was speculated to exist, they weren't actually detected until 2015. Since then, astronomers have detected gravitational waves from several titanic black hole collisions.
Neutron stars-the leftovers of the deaths of massive stars in supernova explosions-aren't the same thing as black holes, but they also collide with one another. These stars are so dense that a glass full of neutron star material would have more mass than the Moon. As gargantuan as they are, neutron stars are among the fastest spinning objects in the universe. Astronomers studying them have clocked them at spin rates of up to 500 times per second.
What's a Star and What Isn't?
Humans have a funny propensity to call any bright object in the sky a "star"-even when it's not. A star is a sphere of superheated gas that gives off light and heat, and usually has some sort of fusion going on inside it. This means that shooting stars aren't really stars. (More often than not, they're just tiny dust particles falling through our atmosphere that vaporize due to the heat of friction with the atmospheric gases.)
What else is not a star? A planet is not a star. That's because-for starters-unlike stars, planets don't fuse atoms in their interiors and they're much smaller than your average star, and while comets may be bright in appearance, they're not stars, either. As comets travel around the Sun, they leave behind dust trails. When Earth passes through a cometary orbit and encounters those trails, we see an increase in meteors (also not stars) as the particles move through our atmosphere and are burned up.
Our Solar System
Our own star, the Sun, is a force to be reckoned with. Deep inside the Sun's core, hydrogen is fused to create helium. During that process, the core releases the equivalent of 100 billion nuclear bombs every second. All that energy works its way out through the Sun's various layers, taking thousands of years to make the trip. The Sun's energy, emitted as heat and light, powers the solar system. Other stars go through this same process during their lives, which makes stars the powerhouses of the cosmos.
The Sun may be the star of our show but the solar system in which we live is full of weird and wonderful features as well. For instance, even though Mercury is the closest planet to the Sun, temperatures can drop to a frigid -280° F on the planet's surface. How? Since Mercury has almost no atmosphere, there's nothing to trap heat near the surface. As a result, the dark side of the planet-the one facing away from the Sun-gets extremely cold.
While it's farther away from the Sun, Venus is considerably hotter than Mercury due to the thickness of Venus' atmosphere, which traps heat near the surface of the planet. Venus also spins very slowly on its axis. One day on Venus is equivalent to 243 Earth days, however, Venus's year is only 224.7 days. Odder still, Venus spins backward on its axis as compared to the other planets in the solar system.
Galaxies, Interstellar Space, and Light
The universe is more than 13.7 billion years old and it is home to billions of galaxies. No one is quite sure exactly how many galaxies there are all told, but some of the facts we do know are pretty impressive. How do we know what we know about galaxies? Astronomers study the light objects emit for clues as to their origins, evolution, and age. Light from distant stars and galaxies takes so long to reach Earth that we're actually seeing these objects as they appeared in the past. When we look up at the night sky, we're in effect, looking back in time. The farther away something is, the farther back in time it appears.
For instance, the Sun's light takes almost 8.5 minutes to travel to Earth, so we see the Sun as it appeared 8.5 minutes ago. The nearest star to us, Proxima Centauri, is 4.2 light-years away, so it appears to our eyes as it was 4.2 years ago. The nearest galaxy is 2.5 million light-years away and looks the way it did when our Australopithecus hominid ancestors walked the planet.
Over the course of time, some older galaxies have been cannibalized by younger ones. For example, the Whirlpool galaxy (also known as Messier 51 or M51)-a two-armed spiral that lies between 25 million and 37 million light-years away from the Milky Way that can be observed with an amateur telescope-appears to have been through one galaxy merger/cannibalization in its past.
The universe is brimming with galaxies, and the most distant ones are moving away from us at more than 90 percent of the speed of light. One of the strangest ideas of all-and one that's likely to come true-is the "expanding universe theory," which hypothesizes that the universe will continue to expand and as it does, galaxies will grow farther apart until their star-forming regions eventually run out. Billions of years from now, the universe will be made up of old, red galaxies (those at the end of their evolution), so far apart that their stars will be almost impossible to detect.