Life and Death of Stars Flashcards
The brightness and color of a star depend on its ____________ ______________.
Surface temperature. The hottest stars are blue, moving down the spectrum to the coolest stars, which are red
The surface temperature may determine the brightness and color of a star, but the surface temperature itself is basically determined by the star’s _______.
Mass. Given the mass of a star, we can tell a star’s radius, luminosity and temperature
To summarize (for stars in the main sequence):
More mass means higher temperature
More mass means higher luminosity
More mass means bigger
Because a star’s mass determines how much fuel it has to burn and how fast it will burn that fuel, a star’s mass also determines the _____________ of a star.
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
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.
Stars are born in ____________ clouds, dense clouds of gas and dust.
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 _____________.
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.
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 ___________.
arbon. 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.
So to summarize up to this point:
1> the hydrogen in a molecule cloud collapses to form a weakly glowing protostar.
2> A protostar collapses until it gets hot and dense enough to start nuclear fusion to begin turning its hydrogen into helium.
3> As soon as this nuclear fusion begins, the core stops collapsing and stabilizes–it is now a ______ ____________ star.
Main sequence. Some protostars don’t have enough mass to ever reach the 10 million Celsius required for nuclear fusion to start. These little protostars end up becoming brown dwarfs instead–too big to be a planet and too small to be a star.
To continue with the summary:
4> the main sequence star burns until it’s used up all of the hydrogen in its core.
5> Once the hydrogen in the core has all been converted into a core of helium, the ___________ ____________ stops. As a result, the core destabilizes and resumes its collapse.
Nuclear fusion. The core of a star maintains its stability only while nuclear fusion is going on to counter the pressure of gravity. If there’s no nuclear fusion, then the core is going to keep compressing until it gets hot and dense enough for nuclear fusion to occur.
Now that the star is no longer burning hydrogen, its time as a main sequence star is done. Continuing with the sequence:
6> While the core is collapsing, the outer envelope is expanding rapidly.
7> Once the core collapses enough, the pressure is great enough to start up nuclear fusion to change the helium into carbon. Once this nuclear fusion starts, the core is stable again and now you have a red giant.
8> Of course, eventually, the red giant will burn through all of its helium, and what happens at that point is going to depend on the _______ of the star.
Mass. Generally, at this stage, stars are split into low-mass stars and high-mass stars. Low mass stars and high-mass stars are born the same way and go through the same steps up to the red giant stage (high-mass stars just go through the steps much quicker), but after the red giant stage, their paths diverge.
If it’s a low-mass star, the red giant’s core will collapse into a tiny, hot and superdense core while it sheds the outer layers into a beautiful planetary nebula. At this point, the red giant has become a ________ _________.
White dwarf. A white dwarf is so dense that it’s considered to be one of the densest forms of matter in the universe, surpassed only by neutron stars. Although it’s hot and superdense, it’s not very luminous because it’s so small. The white dwarf is a dying star in its final stages–it has no source of energy so it may start out very hot when it’s initially formed, but it will gradually radiate away all of its heat and cool down.
A white dwarf will eventually cool to a point where it’s just a dead lump of carbon in space–a ________ __________.
Black dwarf. It’s unknown if any black dwarves actually exist–scientists speculate that the universe probably isn’t old enough yet for any white dwarves to have finished cooling off and dying.
In high-mass stars, the red giant (technically a red supergiant) doesn’t become a white dwarf. Yes, its core of carbon starts contracting, but it’s so massive that as it contracts, it can cause further nuclear reactions, converting carbon into oxygen, neon, silicon, sulphur, and finally to _______.
Iron. The more mass a star has, the higher the gravitational pressure in its core. Most stars can’t generate enough pressure to burn carbon–that’s why once their core is converted into carbon, they turn into white dwarves and die.
With a high-mass star, it keeps undergoing successive nuclear reactions, each time forming a heavier and heavier element until finally it ends with iron. Iron is the most stable element and there is no energy to be gained from converting it into a heavier element.
Once the core of a red supergiant is essentially all iron, the core collapses even further and what you get is a mighty explosion known as a ______________.
Supernova. Right before the fully mature red giant explodes in a supernova, it has the structure of an onion. That’s because each nuclear reaction produces a heavier element which sinks to the center, becoming fuel for the subsequent reaction.
Note: If you’re feeling overwhelmed by all of these details, keep in mind that on the actual test, you really just need the “big picture” to do well. In other words, a low mass red giant becomes a white dwarf; a high mass red giant eventually explodes in a supernova.
