Welcome to your first lesson on how black holes are formed! Your goal is to understand the physical process behind the birth of these fascinating objects. To get there, we need to start with the single most important ingredient in this cosmic recipe: gravity. Without gravity, stars wouldn't form, and without stars, there would be no black holes.
In this introductory lesson, we will focus on the most basic, yet most profound, aspect of gravity. By the end, you will be able to describe gravity as a fundamental force that attracts any two objects with mass. We'll start with what you already know from daily life and expand that idea to the entire universe.
What is Gravity?
You experience gravity every moment of every day. It's the force that keeps your feet on the ground, that pulls a dropped apple to the floor, and that gives you a sense of "down." At its core, gravity is a force of attraction.
To get a simple and clear definition, let's turn to a resource from NASA.
What Is Gravity? | NASA Space Place – NASA Science for Kids
This article from NASA's Space Place provides a great, non-technical introduction to the concept of gravity.
Please read the first few paragraphs. Start with the section titled What Is Gravity?, and continue through the section What else does gravity do? Focus on how it describes gravity as an "invisible force that pulls objects toward each other."
As the article explains, gravity isn't just a property of the Earth. It's a property of mass. Anything that has "stuff" in it—what physicists call mass—has gravity. This includes you, this device you're reading on, the Moon, the Sun, and even the most distant stars.
To get a more intuitive feel for the concept of mass, the following short video clip offers a simple but effective definition.
The Gravity of the Situation: Crash Course Astronomy #7
This video from the "Crash Course Astronomy" series provides a very clear, conceptual definition of mass.
Watch the segment from this timestamp. Pay attention to how the host describes mass as "how much stuff makes up an object" and connects it directly to gravity.
The key idea here is that because you have mass, you exert a gravitational pull on the Earth, just as it pulls on you. Of course, since the Earth is so much more massive, its pull completely overpowers yours, and you don't notice the Earth moving toward you. But the force is there, and it's mutual.
A Universal Force of Attraction
This leads us to the central point of today's lesson: gravity is universal. It doesn't just act between you and the Earth. It acts between any two objects in the universe that have mass. The Sun pulls on the Earth, the Earth pulls on the Moon, and your computer pulls on the book sitting next to it.
This mutual attraction between everyday objects is incredibly weak, so we don't notice it. But it does exist. The following video demonstrates this universal principle.
Understanding Universal law of Gravitation!
This video, titled "Understanding Universal law of Gravitation!", provides excellent visual demonstrations of gravity's universal nature.
Watch the following three short segments: This first part shows a tabletop experiment where small masses are attracted to larger ones. Next, watch the segment from this timestamp, which explains how Newton generalized the idea of a falling apple to the Moon and the Earth. Finally, watch the brief statement from this point, which summarizes the universal law perfectly.
The idea that "all the objects in the universe attract each other" is the foundation of our entire understanding of the cosmos. This simple rule governs the orbits of planets, the structure of galaxies, and, as we'll see, the birth and death of stars.
This can be summarized with a simple visual.
The Cosmic Role of Gravity
While the gravitational pull between two small objects is negligible, the effect becomes monumental when you consider truly massive objects, like stars and galaxies. Gravity's power lies in its infinite range and its cumulative effect. Over millions of years, the gentle gravitational pull between countless tiny particles of gas and dust in space can draw them together.
As more material gathers, the total mass increases, and so does its gravitational pull, attracting even more material. This is the process that forms stars.
DOE Explains...Gravity - Department of Energy
This article from the Department of Energy explains how gravity's features allow it to shape the universe.
Read the paragraph that begins here. Focus on the description of how gravity's infinite range allows it to "coalesce" small masses into larger ones, eventually creating stars and planets.
This creative power of gravity is the first step on our journey to understanding black holes. A star is born from gravity pulling matter together, and as we will discover, a black hole is born when a very massive star's gravity becomes so strong that it collapses in on itself.
Conclusion
In this lesson, we established the fundamental nature of gravity. Here are the key takeaways:
- Gravity is a fundamental force of attraction. It always pulls objects together; it never pushes them apart.
- Any object with mass has gravity. This property is not unique to planets and stars; it applies to everything, including you.
- Gravity is universal. The force of gravity acts between any two objects with mass, anywhere in the universe.
We have begun our journey by understanding what gravity is. In our next lesson, we will explore what determines its strength. We'll answer the questions: Why is the Sun's gravity so much stronger than the Earth's? And why does the Moon orbit the Earth instead of crashing into it? We will find that the strength of gravity depends on just two things: the mass of the objects and the distance between them.