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What is a star? | A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth. | |
Where do stars come from? | According to current star formation theory, stars are born as clumps within gigantic gas clouds that collapse in on themselves. The cloud’s material heats up as it falls inward under the force of its own gravity. When the gas reaches about 10 million K (18 million °F), hydrogen nuclei begin to fuse into helium nuclei, ... | |
What is the definition of the universe? | The universe is everything. It includes all of space, and all the matter and energy that space contains. It even includes time itself and, of course, it includes you. Earth and the Moon are part of the universe, as are the other planets and their many dozens of moons. The physical universe is defined as all of space an... | |
How did the universe begin? | The Big Bang theory says that the universe came into being from a single, unimaginably hot and dense point (aka, a singularity) more than 13 billion years ago. It didn’t occur in an already existing space. Rather, it initiated the expansion—and cooling—of space itself. The best-supported theory of our universe's origin... | |
What is the size of the universe? | In 2013, the European Space Agency's Planck space mission released the most accurate and detailed map ever made of the universe's oldest light. The map revealed that the universe is 13.8 billion years old. Planck calculated the age by studying the cosmic microwave background. 'The cosmic microwave background light is a... | |
What is the shape of the universe? | The theory of general relativity, under which space itself can curve, allows for the universe to take one of three forms: flat like a sheet of paper, closed like a sphere, or open like a saddle. This astronomical geometry is no trivial matter — the fate of the cosmos depends on it. As Princeton University cosmologist D... | |
How old is the universe? | About 93 billion light-years. The proper distance—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years (14 billion parsecs), making the diameter of the observable universe about 93 billion light-years (28 billion pars... | |
What is beyond the observable universe? | The observable Universe is bounded by a 'cosmic horizon', much like the horizon at sea. Just as we know there's more ocean over the horizon, we know there are more galaxies (possibly an infinite number) beyond the cosmic horizon. Their light simply hasn't had time to reach us yet. Roughly 13.75 billion years ago, our u... | |
What is dark matter? | Galaxies in our universe seem to be achieving an impossible feat. They are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together; they should have torn themselves apart long ago. The same is true of galaxies in clusters, which leads scientists to believe th... | |
What is dark energy? | Dark energy makes up approximately 68% of the universe and appears to be associated with the vacuum in space. It is distributed evenly throughout the universe, not only in space but also in time – in other words, its effect is not diluted as the universe expands. The even distribution means that dark energy does not ha... | |
How does the theory of relativity relate to the universe? | A century ago, physicist Albert Einstein unveiled a theory that would change the world — general relativity. It would cement his place at the top of the pantheon of scientific minds, and see him transformed into a modern icon. But what is general relativity, and why does it matter? Ask someone on the street about Einst... | |
What is the cosmological principle? | The cosmological principle states that the universe, on the average, looks the same from any point. It is motivated by the Copernican argument that the Earth is not in a central, preferred position. If the universe is locally isotropic, as viewed from any point, it is also uniform. Therefore, the cosmological principle... | |
What is a black hole? | A black hole is an astronomical object with a gravitational pull so strong that nothing, not even light, can escape it. A black hole’s “surface,” called its event horizon, defines the boundary where the velocity needed to escape exceeds the speed of light, which is the speed limit of the cosmos. Matter and radiation fa... | |
What is the role of quantum mechanics in cosmology? | Quantum cosmology refers to the study of the Universe as a quantum system. There are two main aspects to this field:
1. **Quantum Dynamics of the Universe**: This involves modeling the dynamics of the universe using quantum mechanics, especially when simplifying assumptions are made, such as assuming homogeneity. Clas... | |
Are there other universes (multiverse theory)? | Multiverse theory proposes that our universe may not be the only one. Instead, there could be an entire ensemble of universes, each with its own laws of physics, collections of stars and galaxies, and possibly even intelligent civilizations. This idea suggests that our universe is just one of potentially infinite unive... |
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