What is astrophysics? – Big Think

Whenever you take a look out at the Universe and record what you see, you’re engaging in one of the oldest sciences there is: astronomy. Similarly, whenever you investigate how a physical phenomenon in the Universe works — on quantum, classical, or cosmic scales — including by puzzling out or applying the laws that govern it, you’re engaging in the science of physics. Each of these fields, thousands of years old in their own right, were long thought to be independent of one another, with physics applying only to the mundane observations and experiments we can perform on Earth, while astronomy instead explored the realm of the heavenly.

Today, however, we generally recognize that the rules governing the Universe don’t change from one location to another; they’re the same on Earth as they are everywhere, as well as every when, in the Universe. In every way that we’ve measured them, the laws of nature appear to be identical at all points in time and in space, and do not appear to change.

Astrophysics, then, is the overlap of astronomy with physics: where we study the entire Universe, and everything within it, with the full power of the laws of physics applied to them. In a sense, it’s the primary way that we — creatures that came to life within this Universe — are able to study and know about where we all came from. Here’s the story of what astrophysics is all about.

For millennia, humans had been watching the skies, attempting to track the various objects, their daily and annual (and beyond) motions, all while looking for patterns that they might fit into. However, there was no connection to the physical laws we were discovering here on Earth, from the Babylonians to the ancient Greeks to the Persians, Romans, Ottomans and beyond. Even Galileo, famed for both his physics experiments and his astronomical observations, never managed to link the two together. When it came to the motions of heavenly objects, it was largely regarded as a philosophical, theological, or ideological concern, rather than a scientific one.

Johannes Kepler came close, as he arrived at the most precise and accurate description of the motion of bodies within our Solar System. Kepler’s three laws, that:

1. planets orbited the Sun in ellipses, with the Sun at one focus,
2. if you shaded in the area traced out by a planet in orbit around the Sun, it always traced out equal areas in equal times,
3. and that the period of a planet’s orbit, squared, was proportional to its semimajor axis, cubed,

were empirically derived, meaning that they were arrived at based on observations alone, rather than having a deeper meaning behind them. Despite their success in describing planetary motion, Kepler’s advances weren’t rooted in the physical laws that govern the Universe.

It wasn’t until Isaac Newton came along that astrophysics, as a science, was born. The motion of objects on Earth, under the influence of our planet’s acceleration-causing gravity, had been studied for around a century by the time Newton rose to prominence. The tremendous advance that Newton made, however, remarkably distinguished him from all of his contemporaries and predecessors: the “rule” that he formulated for how objects attracted one another — Newton’s law of universal gravitation — didn’t simply apply to objects on Earth. Rather, they applied to all objects, regardless of the object’s properties, universally.

When Edmond Halley approached Newton and inquired about the type of orbit that would be traced out by an object that obeyed an inverse-square force law, he was shocked to find that Newton knew the answer — an ellipse — off of the top of his head. Newton had methodically and painstakingly derived the answer over the course of multiple years, inventing calculus along the way as a mathematical tool to aid in problem-solving. His results led Halley to understand the periodic nature of comets, enabling him to predict their return. The science of astrophysics had never seemed so promising.

Two scientists that were contemporaneous with Newton, Christiaan Huygens and Ole Rømer, helped showcase the early power of applying the laws of physics to the greater Universe. Huygens, curious about the distance to the stars, made an assumption that others before him had made: that the stars in the sky were similar to our own Sun, but were simply very far away. Huygens, who was famed for both his clockmaking prowess and his experiments with light and waves, knew that if a light source was placed at double the distance it was previously at, it would only appear one-quarter as bright.

Huygens attempted to drill a series of holes in a brass disk, where he had the idea of holding the disk up to the Sun during the day. If he reduced the brightness significantly enough, he reasoned, the light that was allowed through would only be as bright as a star in the sky. Yet no matter how small he drilled his holes, the tiny pinprick of sunlight that came through vastly outshone even the brightest star. It wasn’t until he inserted a light-blocking glass bead into the smallest of the drilled holes that he could match the Sun’s reduced brightness to the night sky’s brightest star: Sirius. It required a total reduction in the Sun’s brightness of a factor of 800 million to reproduce what he saw when he looked at Sirius.

The Sun, he concluded, if it were placed ~28,000 times farther away than it presently is (about half a light-year), would appear as bright as Sirius. Hundreds of years later, we now know that Sirius is about ~20 times farther than that, but also that Sirius is about ~25 times intrinsically brighter than the Sun. Huygens, who had no way of knowing that, had truly achieved something remarkable.

Ole Rømer, meanwhile, recognized that he could use the great distances between the Sun, the planets, and their moons to measure the speed of light. As the Galilean moons of Jupiter circled behind the giant planet, they passed into and out of Jupiter’s shadow. Because Earth makes its own orbit, we can see those moons either entering or exiting Jupiter’s shadow at various times during the year. By measuring the changes in the amount of time it takes the light to travel:

• from the Sun,
• to one of Jupiter’s moons,
• and then to go from that moon back to Earth,

Rømer was able, to the best accuracy of his measurements, to infer the speed of light for the first time. Astrophysics isn’t exclusively about applying the laws of nature that we discover on Earth to the greater Universe at large, but also is about using the observations available to us in the “laboratory of the Universe” to teach us about the very laws and properties of nature itself.

Yet it would take not just many years, but centuries, before astrophysics advanced beyond the ideas of the late 1600s. Indeed, these ideas and applications encapsulated the entirety of astrophysics for the next 200 years, up through the middle of the 19th century. At that point, two additional advances occurred: the discovery of an astronomical parallax, giving us the distance to a star beyond the Sun, and the…