Explanation lifetime how long a star will spend on

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Explanation: Lifetime. How long a star will spend on the main sequence is dependent on its mass, because a mass determines a) how much fuel a star has to burn b) how quickly it burns that fuel (luminosity). Note that luminosity increases much faster than mass, so a star that's half as massive as our Sun is only 1/10 as bright. A star that's ten times as massive as our Sun would be thousands of times brighter. This also means that the more massive a star, the faster it burns through its fuel and exits the main sequence. A star of one solar mass (i.e. our sun) is estimated to have a lifespan of 10 billion years on the main sequence while a massive supergiant may only last a few million years. A star is on the main sequence as long as it has enough ______________ to burn. Explanation: Hydrogen. A star becomes a main sequence star once it starts nuclear fusion. Basically, during its lifetime, a main sequence star is sitting there converting hydrogen to helium through nuclear reactions--the mass that's left over from these reactions escapes as electromagnetic radiation--light (and heat). When a star exhausts most of its core hydrogen supply, that's when it leaves the main sequence and becomes a red giant.
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Stars are born in ____________ clouds, dense clouds of gas and dust. Explanation: Molecular. A molecular cloud, sometimes referred to as a stellar nursery, is usually seen as a nebula of high density. Over time, the hydrogen in this cloud or a particularly dense portion of the cloud collapses and compacts due to its own gravity. As it compacts, it heats up, eventually forming a large and compartively cool mass of gas known as a _____________. Explanation: Protostar. A protostar is the stage before becoming an actual star. If it's too small, it may never become an actual star, instead becoming a brown dwarf. For a protostar to become an actual star, it must have enough mass to reach about 10 million degrees celsius, at which ___________ ___________ of hydrogen begins. Explanation: Nuclear fusion. It is once this nuclear reaction begins to turn hydrogen into helium that the protostar has become an actual "main sequence" star. As you learned, where it is on the main sequence will be determined by its mass. Once the star has converted just about all of the hydrogen in its core into helium, the core stars to collapse. The core will keep compressing until it reaches a temperature and density that is high enough to begin the fusion of helium to form ___________. Explanation: Carbon. Meanwhile, while the core is compressing, the outer layers of the star are expanding rapidly. What you end up with is a much bigger star with a much smaller core--a red giant. It is dark red because it is much cooler than it was as a main sequence star, but due to its sheer size, it's more luminous.
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The image below shows the difference between an average main sequence star like our sun and a red giant on the right. Note that now the same mass is in a much larger volume, so the red giant has a much lower average density.
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