- Stars are born and die over millions or even billions of years.
- Baryogenesis is the physical process hypothesized to have taken place during the early universe to produce baryonic asymmetry, i.e., the imbalance of matter (baryons) and antimatter (antibaryons) in the observed universe. The baryon asymmetry problem, also known as the matter asymmetry problem or the matter-antimatter asymmetry problem, is the observed imbalance in the baryonic matter and antibaryonic matter in the observable universe. Antimatter looks just like regular matter.
- Lipogenesis is the generic term for hypothetical physical processes that produced an asymmetry between leptons and antileptons in the very early universe. A lepton is an elementary particle of half-integer spin that does not undergo strong interactions. Two main classes of leptons exist charged leptons and neutral leptons. lipogenesis is the generic term for hypothetical physical processes that produced an asymmetry between leptons and antileptons in the very early universe, resulting in the present-day dominance of leptons over antileptons.
- A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion. All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin.
- stars are born in clouds of gas and dust. One such stellar nursery is the Orion Nebula, an enormous cloud of gas and dust many light-years across. Turbulence from deep within these clouds creates high-density regions called knots.
- A STAR IS BORN: Stars form in cold, dense regions of space called molecular clouds. As the solar nebula shrinks in size, its density, temperature, and shape all undergo dramatic changes. When the force of gravity pulling in on the cloud is greater than the strength of internal pressure pushing out, the cloud collapses into a protostar.
- From: 70% hydrogen, 28%helium, and 2% elements heavier than helium.
- We do not consider a protostar a star until it gets its energy from nuclear fusion instead of gravitational contraction. The core temperature of an object must reach a minimum temperature before the atoms inside will fuse. The number of low-mass stars is much higher than the number of high-mass stars.
- There are seven main types of stars. They are ranked in order of increasing temperature: M (red), K (orange), G (yellow), F (yellow-white), A (white), B (blue-white), O (blue). The coolest stars (red) are invariably the smallest, they are called red dwarfs. The hottest star is blue. Supergiants are tenuous stars, and their lifetimes are probably only a few million years, extremely short on the scale of stellar evolution. Jupiter is a failed star, with the same elements, but not enough pressure. The largest known star in the universe is UY Scuti, a hyper giant with a radius around 1,700 times larger than the sun.
- The bigger a star is, the longer it will live. The smaller a star is, the longer it will live. This is because the smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid. The very first stars likely formed when the universe was about 100 million years old, prior to the formation of the first galaxies.
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As the gases come together, they get hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy and keeps the star hot.
- During the main sequence period of its life cycle, a star is stable because the forces in it are balanced. The outward pressure from the expanding hot gases is balanced by the force of the star’s gravity. Our Sun is halfway through its 10 billion years stable phase.
- Stars live at different lengths of time, depending on how big they are. A star like our sun lives for about 10 billion years, while a star that weighs 20 times as much lives only 10 million years, about a thousandth as long.
