Stars

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But planets like Jupiter and Saturn are mostly gas, like a star. The difference is that they aren't big enough to cause the nuclear reactions, so they aren't so hot and don't emit much light (certainly none visible to the eye, what we see is the reflection of the sun).

Back 1

About the galaxy: no human being has been past the Earth's moon. So we're stuck in one corner of our solar system (our sun and it's planets). The solar system is just a tiny part of the galaxy (which has billions of stars). And there are many galaxies in the known universe. The universe is a very big place

Front 2

Planets can vary a bit, and I don't know the exact definition. Planets like the earth and Mars are rocky with a metallic center -- in other words, they're solid. But planets like Jupiter and Saturn are mostly gas, like a star. The difference is that they aren't big enough to cause the nuclear reactions, so they aren't so hot and don't emit much light (certainly none visible to the eye, what we see is the reflection of the sun).

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stars form inside relatively dense concentrations of interstellar gas and dust known as molecular clouds. These regions are extremely cold (temperature about 10 to 20K, just above absolute zero). At these temperatures, gases become molecular meaning that atoms bind together. CO and H2 are the most common molecules in interstellar gas clouds. The deep cold also causes the gas to clump to high densities. When the density reaches a certain point, stars form.Since the regions are dense, they are opaque to visible light and are known as dark nebula. Since they don't shine by optical light, we must use IR and radio telescopes to investigate them.Star formation begins when the denser parts of the cloud core collapse under their own weight/gravity. These cores typically have masses around 104 solar masses in the form of gas and dust. The cores are denser than the outer cloud, so they collapse first. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 millions years

Front 3

We're all pretty familiar with stars. We see them on most clear nights as tiny, twinkling pinpricks of light in the sky. Stars are the topic of countless poems, stories, and nursery rhymes alike. But just what is a star, exactly?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.

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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 degrees F), hydrogen nuclei begin to fuse into helium nuclei, and the star is born. Energy from nuclear fusion radiates outward from the center of the burgeoning star, and gradually halts the gas cloud’s collapse.