And since the “discovery” of our Milky Way–or, more accurately, the discovery of what that hazy band of stars in the sky is–we’ve come to realize just how massive our home in the cosmos really is.
That scientific journey started with the Herschels’ mapping of what was then called the “star system.” Later astronomers began to realize just how far out from the sun the stars of our galaxy really reached. Determining distances across our galaxy was the first step to discovering its size.
Later, we began to understand its structure–mapping the extraordinarily thin disk, the chaotic central bulge, and the visible part of the halo, a sphere of stars that extends beyond the plane of the galaxy.
And since then, we’ve begun to master the next critical part of understanding our galaxy: its mass.
Okay, good question. How the heck do you find an object that emits no radiation? Astronomers find—and study—just about everything in the universe using the radiation it emits or reflects. So…what happens when the object we’re looking for has such a strong gravitational pull that even light can’t escape?
Well, that’s when we need to turn to the theoretical science behind black holes. What measurable effects do they have on objects in their vicinity? Can we detect them indirectly?
Of course, some of you might be screaming at me that we’ve already photographed a black hole—in visual wavelengths! Yes, astronomers did make that achievement—we now have visual proof that what we’ve been theorizing all along is indeed real.
But that black hole was so faint, it took an interferometer the size of the Earth to image. We had to know exactly where to look in order to get that picture.
So how the heck do we find one in the first place?
We know how big stars are; they range from the size of the Earth to over a thousand times the size of the sun (which is in itself over one hundred times the size of the Earth). We know they’re huge.
But how massive are they?
Yes, that’s a different thing.
A pingpong ball and a golf ball are close to the same size, but a golf ball is much more massive—in that it has more stuff in it. A pingpong ball is hollow and easily tossed; a golf ball has more matter in it and will hit the ground with a harder thunk.
Stars are similar. They have a wide range of sizes, but nothing I’ve described thus far has told us about their masses. That is, how much stuff is in them? Are they like puffy gaseous balls, or are they more dense, like planets?
The best way to learn about stars’ masses is by studying binary stars. But what exactly are binary stars?
Welcome to my second “Science Answers” post! About a month ago, I sent out a post requesting science questions from all of you; you can find it here. This post addresses the second of the questions I was asked. If you have a question, you can ask it in the comments here or on that post, or ask it in an email. Or find me on Facebook!
Wow…great question. This is a question the greatest scientific minds have asked and tried to answer for centuries. It’s a question not even Stephen Hawking, the scientific genius of the century, has fully answered.
There are a few parts to the gravity question, and they have each been addressed one by one over time:
How does gravity work?
What is gravity?
Why does gravity work?
Isaac Newton stood on the shoulders of the giants before him—Aristotle, Ptolemy, Copernicus, and Kepler—and figured out how gravity works. But he was at a loss to explain what exactly this mysterious force was.
Einstein built on Newton’s work and came up with a theory for what gravity is—that is, distortions in space-time.
We have yet to understand why gravity works. Why is space-time warped? Why do objects distort it as if it were the material of a trampoline? What exactly is the nature of space?
But, lucky for me, the question above specifically asks what gravity is. And that, I can explain.
The best way to do that is to turn one of gravity’s oldest tricks, one that has perplexed scientists and philosophers for thousands of years: What makes the planets move?
That’s what’s diagramed above. This is a three-dimensional concept diagram of the way space sort of “clings” to an object. Notice the way it sort of tightens up when you get close to Earth? And because time is part of this whole equation…time sort of tightens up, too.
I assume that explains the “twin paradox,” as it’s called. That’s where the space-traveling twin returns home to Earth younger than their Earth bound twin.
Why? Seems to me it’s because time was tighter and passed faster on Earth, while it spread out and passed a bit slower for the traveler. (Don’t quote me on that, I just guessed that from this diagram.)
Einstein figured all this out. But scientists need evidence. Trusting Einstein’s genius wasn’t enough for them. How did they accept relativity as fact?
But Kepler never could figure out why planets orbit the sun in ellipses instead of circles. Even Isaac Newton, who at last identified gravity as the reason we stick to Earth’s surface, couldn’t explain what gravity was—only how it worked.
Einstein provided that explanation with his general theory of relativity.
If you’re from California like me, then I’m betting you have. If you’re from a place that’s not near an ocean and you’ve never been near the water all your life, then I’ll tell you a little bit about the tides.
They happen every day, twice a day. If you find yourself a nice comfortable spot overlooking the beach, you can see the waves come into the shore and then gently roll out again. If you stay for hours on end, you’ll see the water level eventually rise a bit.
And if you stay even longer, you’ll see the water level lower back down. When it’s high, it’s called high tide, and when it’s low, it’s called low tide.
The tides are partially responsible for the myth that the moon’s gravity affects you in some kind of metaphysical way. But this isn’t true at all.
Since Aristotle’s time over 2000 years ago, we have accepted that the moon orbits the Earth. We didn’t always know why, and we didn’t always accept this for the right reasons.
We used to assume that it happened just because we saw the moon move across the sky, and we believed the Earth to be the center of all motion in the solar system. But even when we realized—in the 1540s CE—that the sun was in fact the center of the solar system, the moon kept its place around the Earth.
And rightfully so. Astronomers now know that the moon orbits the Earth based on scientific observation, rather than the “logical” guesses of Aristotle’s time. And we even know why it orbits—gravity, the one force in all the universe we can’t escape.
But I can tell you, the moon’s orbit isn’t a perfect circle, and if gravity were the only reason it orbited, it would crash straight into the Earth. After all, people stay grounded on Earth’s surface because of gravity, and we don’t orbit our planet, do we?
So how does the moon orbit the Earth? For that matter, how does any satellite?