Amazing Astronomy Facts

Image of the Week for February 10, 2006 - Black Hole Gobbles a Star
Black holes gobble up material that strays too near them.They're among the oddest, weirdest things in the universe. NASA/CXC/M.Weiss

Even though humans have studied the heavens for thousands of years, people still know very little about what's out there in the universe. As astronomers continue to explore, they learn more about the stars, planets, and galaxies in some detail, although some processes remain puzzling. The mysteries will eventually be cleared up because that's how science works, but understanding them will take a long time. 

Dark Matter in the Universe 

Astronomers are always on the hunt for dark matter. This is a mysterious form of matter that can't be detected by normal means (which is why it's called dark matter). All the matter that can be detected comprises only about 5 percent of all the matter in the universe. The dark matter makes up the rest, along with something known as dark energy. So, when people look out at the sky at night and see all the stars (and galaxies, if they're using a telescope), they're only witnessing a tiny fraction of what's actually out there.

Dense Objects in the Cosmos

People used to think that black holes were the answer to the "dark matter" problem. That is, they thought that the missing matter might be in black holes. The idea turns out not to be true, but black holes continue to fascinate astronomers. These are objects so dense and have such intense gravity, that nothing—not even light—can escape them. 

If a ship somehow got too close to a black hole and sucked in by its gravitational pull "face first," it would pull harder on the front part of the ship than the back. The ship and the people inside would get stretched out—or spaghettified—by the intense pull. Nobody gets out alive.

It turns out that black holes can and do collide. When that happens with supermassive ones, gravitational waves are released. These waves were known to exist and were finally detected in 2015. Since then, astronomers have detected gravitational waves from other titanic black hole collisions. 

There are also objects that aren't quite black holes that also collide with each other. These are the neutron stars, the leftovers of the deaths of massive stars in supernova explosions. These stars are so dense that a glass full of neutron star material would have more mass than the Moon. They are among the fast-spinning objects astronomers have studied, with spin rates up to 500 times per second.

Our Star Is the Bomb

Not to be outdone in the strange and weird, our Sun has a few tricks inside as well. Deep inside, in the core, the Sun fuses hydrogen 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 various layers of the Sun, 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. 

What's a Star and What Isn't?

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. Humans have a funny propensity to call anything in the sky a "star," even when it's not. For example, shooting stars really aren't stars. They are usually just tiny dust particles falling through our atmosphere that vaporize due to the heat of friction with the atmospheric gases. Earth sometimes passes through cometary orbits. As comets travel around the Sun, they leave behind dust trails. When Earth encounters that dust, we see an increase in meteors as the particles travel through our atmosphere and are burned up.

Planets aren't stars either. For one thing, they don't fuse atoms in their interiors. For another, they're much smaller than most stars. Our own solar system has interesting worlds with amazing properties.  Even though Mercury is the closest planet to the Sun, temperatures there can reach -280 degrees F on its surface. How can this happen? Since Mercury has almost no atmosphere, there is nothing to trap heat near the surface. So, the dark side of Mercury (the side facing away from the Sun) gets very cold.

Venus is considerably hotter than Mercury, even though it is farther away from the Sun. The thickness of Venus’ atmosphere traps heat near the surface of the planet. Venus also spins very slowly on its axis.

A day on Venus is 243 Earth-days long, while Venus's year is only 224.7 days. Even weirder, Venus spins backward on its axis compared to the other planets in the solar system.

Galaxies, Interstellar Space, and Light

There are billions of galaxies in the universe. No one is quite sure exactly how many. The universe is more than 13.7 billion years old, and some older galaxies have been cannibalized by younger ones. The Whirlpool galaxy (also known as Messier 51 or M51) is a two-armed spiral that lies between 25 million and 37 million light-years away from the Milky Way. It can be observed with an amateur telescope, and appears to have been through one galaxy merger/cannibalization in its past. 

How do we know what we know about galaxies? Astronomers study their light for clues to their origins and evolution. That light also gives hints about an object's age. Light from distant stars and galaxies takes so long to reach Earth that we are actually seeing these objects as they appeared in the past. As we look up at the sky, we are really looking back in time.

For example, the Sun's light takes almost 8.5 minutes to travel to Earth, so we see the Sun as it looked 8.5 minutes ago. The nearest star to us, Proxima Centauri, is 4.2 light-years away, so it appears as it was 4.2 years ago. The nearest galaxy is 2.5 million light-years away, and it looks as it did when the australopithecus hominid ancestors walked the planet. The farther away something is, the further back in time it appears.

The space that light travels through isn't completely empty. Astronomers sometimes use the term "vacuum of space," but it turns out that there are a few atoms of matter in each cubic meter of space. The space between galaxies, which was also once thought to be quite empty, can often be filled with molecules of gas and dust.

The universe is filled with galaxies, and the most distant ones are moving away from us at more than 90 percent of the speed of light. In one of the weirdest ideas of all, that will likely come true, the universe will continue to expand. As it does, galaxies will be farther apart. Their star-forming regions will eventually run out, and billions upon billions of years from now, the universe will be filled with old, red galaxies, so far apart that their stars will be tough to detect. That's called the "expanding universe" theory, and as of right now, it's how astronomers understand the universe will exist. 

Edited and updated by Carolyn Collins Petersen.