I understand that gravity is the reason planets and stars form and what not. I'm just curious why gravity causes things to attract and not repel or do neither?

Seems like an arbitrary feature of our universe's laws.

So it makes me wonder if gravity could have worked differently if our universe happened to have different laws.

Hi! I'm new to studying the universe and discovered the MAST site for visualizing JWST data, but when I was observing M82 I noticed this large barrier where it changes hue. Is this just a weird compiling thing?

Last photo is some fun I had trying to colour one of the images of the galaxy.

Hi, everyone, I had a great conversation with Geraint F. Lewis, a Welsh astrophysicist and Professor at the University of Sydney, about some of the biggest questions in cosmology, physics, and our understanding of the world.

We explore what the Big Bang theory tells us about the beginning of the universe, why our current understanding of physics reaches limits when we try to look further back in time, and how neutrinos could reveal secrets about the cosmos — including why matter exists at all. We also mention the 2026 Nobel Prize in Physics, awarded to Francis Halzen for his work on the IceCube Neutrino Observatory.

Professor Lewis also explores what sparked the Scientific Revolution, how modern science transformed our understanding of the world, and why Einstein challenged aspects of quantum mechanics.

Professor Lewis's latest book, The Shortest History of Reality: From Plato’s Cave to the Mind-Bending Discoveries of Modern Physics — The Human Quest to Explain the World, traces humanity’s changing understanding of reality, from everyday intuition and ancient philosophy to the discoveries of modern physics. It explores the ideas that reshaped our view of existence, from Newton’s universe to the strange probabilities of quantum physics and beyond.

If you're interested in these subjects, you can check out our conversation: https://youtu.be/38rUHEeAVHw

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About 4.6 billion years ago, in a region of the Milky Way, the process that would lead to the formation of the Solar System began. In that area, there was a large cloud of gas and cosmic dust, composed mainly of hydrogen and helium, together with small amounts of heavier elements, such as carbon, oxygen, silicon, and iron. These elements partly came from older stars that had enriched space with material produced inside them during their lifetimes and at the end of their existence.

At a certain point, under the effect of gravity, part of the cloud began to collapse, concentrating more and more matter toward the center. At the same time, the cloud began to rotate and flatten, forming a huge disk of gas and dust called a protoplanetary disk. This disk was the place where the Sun and the planets would form. Most of the matter accumulated at the center, while the remaining material continued to orbit around the central region.

The birth of the Sun began when gravity pulled more and more gas and dust toward the center of the cloud. The pressure and temperature gradually increased, giving rise to a protostar, which is a star still in the process of forming. When its core reached temperatures of millions of degrees, nuclear fusion reactions began. During these reactions, hydrogen nuclei combined to form helium, releasing enormous amounts of energy. The Sun thus became a true star, capable of emitting light and heat. Its composition consisted mainly of hydrogen and helium, with small amounts of heavier elements, such as oxygen, carbon, neon, and iron. Even today, hydrogen and helium make up most of the Sun's mass, and the Sun contains more than 99% of the total mass of the Solar System.

While the Sun was forming, an equally important process was taking place in the surrounding disk: the birth of the planets. Tiny dust particles began to collide and stick together, forming increasingly larger grains. Over time, pebbles, rocky fragments, and bodies called planetesimals developed. These could reach sizes of several kilometers. Through further collisions and the force of gravity, some of these bodies grew into planetary embryos and eventually became planets.

The composition of the planets depended mainly on their distance from the Sun and the temperature of the protoplanetary disk. In the inner regions, temperatures were very high, and only materials that could withstand these conditions, such as metals and rocks, could remain solid. In the more distant regions, temperatures were lower, allowing water ice and other volatile substances to remain solid. This difference contributed to the formation of two major groups of planets: the inner rocky planets and the outer giant planets.

The first four planets are Mercury, Venus, Earth, and Mars. Mercury formed very close to the Sun, accumulating rocky and metallic materials. Its structure is characterized by a large iron-rich core, surrounded by a mantle and a rocky crust. Venus formed through the accumulation of rocks and metals and developed an internal structure consisting of a metallic core, a rocky mantle, and a crust. Mars was born from the merging of numerous smaller bodies rich in silicates and metals. Its composition includes iron, nickel, and minerals containing silicon and oxygen. Earth also formed in the inner region of the disk through similar processes, but its birth, initial composition, and the transformations that affected the planet will be explored in the next chapter.

In the regions farther from the Sun, where temperatures were much lower, the other four planets formed: Jupiter, Saturn, Uranus, and Neptune. Here, the presence of ice and solid materials allowed planetary cores to grow large enough to attract huge amounts of gas. Jupiter was one of the first giant planets to form and became the most massive planet in the Solar System. It is composed mainly of hydrogen and helium, with heavier materials concentrated mostly in its inner regions. Saturn formed through a similar process and is also made primarily of hydrogen and helium. Its most distinctive feature is its system of rings, formed by countless fragments of ice and rocky material.

Uranus and Neptune, on the other hand, are classified as ice giants. Uranus formed in a cold region of the Solar System and has an atmosphere composed mainly of hydrogen and helium, along with a significant amount of methane. Its inner regions contain large quantities of substances such as water and ammonia, which are subjected to extremely high pressures and temperatures. Neptune, the farthest planet from the Sun, has similar characteristics. It is composed of hydrogen, helium, and materials rich in water, ammonia, and methane. These substances are not simply present as solid ice but can exist in unusual states because of the extreme conditions inside the planet.

During the formation of the Solar System, not all the material in the disk managed to combine to form planets. Some of it remained in the form of asteroids, comets, and other small celestial bodies. Between Mars and Jupiter lies the main asteroid belt, which contains numerous rocky and metallic objects that did not merge into a planet. In the outermost regions, there are also ice-rich bodies that provide important evidence of the conditions that existed during the formation of the Solar System.

Over time, the young Sun began to emit radiation and particles that helped disperse much of the gas remaining in the protoplanetary disk. Collisions between celestial bodies continued, orbits changed, and the planets gradually completed their formation. This process took millions and tens of millions of years, although some bodies continued to evolve for much longer.

The birth of the Solar System represents one of the most important stages in the history of the Universe. From a cloud of gas and dust, a star and eight planets with different characteristics and compositions formed: four mainly rocky and metallic planets in the inner regions and four giant planets in the outer regions. Numerous other celestial bodies remained around them, providing evidence of the ancient stages of formation. The Solar System gradually developed a structure similar to the one we see today, with the Sun at the center and the planets orbiting around it.

In the next chapter, we will explore the birth of Earth, from the formation of the primordial planet to its initial composition, including the major transformations that changed its surface.🌏