The Life and Death of Stars Flashcards
What does the total time that a star spends as a main-sequence star depend on?
1) the amount of hydrogen in the core.
2) rate at which it is consumed
Why do more massive stars have shorter lifetimes?
Although more massive stars have more fuel (hydrogen in their cores), they fuse it at much higher rates because they are so much hotter. Thus they use up their fuel.
What is the Sun’s main-sequence lifetime?
10 to 12 billion years
Describe the relationship between mass and main sequence lifetime.
As mass increases, main sequence lifetime decreases.
OBAFGKM Classification
O stars are big and blue, and have shorter lives. M stars are smaller and redder, and have longer lives.
Amount of solar masses for a low mass star
less than 8 solar masses
Amount of solar masses for a high mass star
greater than 8 solar masses
How do red giants get so huge? (EDIT)
Gravity pushes in, squeezes and contracts, while gas pressure pushes out and expands. Gravity increases when mass increases or when radius decreases. Gas pressure increases when temperature increatures….
When is the end of main-sequence?
When there is no more hydrogen to fuse in the core.
What happens when there is no more fusion in a star’s core?
Core temperature drops, then gas pressure in core drops. Gravity wins in the combat between internal pushing outward of gas pressure and therefore the core shrinks. Because the core shrinks, it heats up again even more than before and this hotter core heats up the surrounding shell of hydrogen (the hydrogen that was just outside the core). The hydrogen shell gets hot enough to fuse hydrogen, and because the temperature is so high, the fusion rate is huge. Gas pressure increases disproportionally, beating gravity, and pushes outer layers of the star so they expand dramatically. Because of expansion, they cool down. The result is a red giant.
What happens to Earth when the sun is expected to start turning into a red giant in 5 billion years?
Oceans will boil 3-4 billion years from now. Then in 5-7 billion years the Sun will leave the main-sequence stage. About 700 million years later the red-giant Sun will envelop Earth and vaporize inner planets an evaporate the atmosphere of outer planets.
What happens to the temperature of the core of a star if the core continues to shrink?
When core temperature reaches 100 million Kelvin, Helium fusion begins. Helium fusion requires higher temperatures than Hydrogen fusion because its larger charge (two protons in each nucleus) leads to greater repulsion. Fusion of two helium nuclei doesn’t work, so helium fusion must combine three helium nuclei to make carbon. Helium-burning stars neither shrink nor grow because balance is temporarily fixed. But this phase doesn’t last very long.
What happens when the star’s core runs out of helium (i.e. when all the helium has been fused into carbon)?
Carbon only fuses above 600 million K, and since low mass star cores never get that hot, then these less than 8 solar masses stars turn into planetary nebulas.
How does a planetary nebula form?
For stars less than 8 solar masses, gravity can’t hold outer layers of gas and dusts and thus they are expelled. As debris moves away, the hot, dense core becomes visible. Hot core emits UV radiation, which heats and ionizes surrounding gas. Gas begins to glow, forming “planetary nubula”. (The core is a white dwarf, the planetary nebula is the outer layers).
What forms from the eventual fading of planetary nebulae?
When the shell spreads out far from the cooling star, it ceases to glow. Eventually, nebulae’s gases mix with interstellar medium. Later, new stars will be born from this interstellar medium.
What happens to cores that are left behind after low-mass stars die?
They become white dwarfs.
How are white dwarfs formed?
After a star expels its outer layers (planetary nebula), the core shrinks to about the size of Earth. This exposed core is no longer able to have fusion of any kind, so it slowly cools down. At first, it is bright (UV thermal radiation), but it becomes dimmer (and redder) as it cools.
What makes white dwarfs stop shrinking?
Not gas pressure: white dwarfs don’t have any kind of fusion, so they can’t keep producing heat and so gas pressure is not enough to beat gravity. They stop shrinking from degeneracy pressure.
What is degeneracy pressure?
A type of pressure that arises when electrons or neutrons are packed so tightly that the exclusion and uncertainty principles come into play. The exclusion principle says two (or more) electrons or neutrons cannot be in the same state at the same time. Therefore electrons/neutrons have to move faster to find empty states. This motion creates a pressure called degeneracy pressure.
If gas pressure depends on temperature, what does degeneracy pressure depend on?
Density (i.e. how “squeezed” matter is)
How dense are white dwarfs?
White dwarfs are a million times denser than water: one teaspoon of white dwarf material is as heavy as an elephant! Adding mass to a white dwarf increases its gravity, forcing electrons into a smaller space, forcing them to go faster.
Is there a limit to how much mass a white dwarf can support?
Einstein’s theory of relativity says that nothing can move faster than light. When electron speeds in white dwarf approach speed of light, electron degeneracy pressure can’t increase any longer, so it can no longer support the star against collapse. A white dwarf more massive than 1.4 solar masses would collapse under its own gravity.
What is the Chandrasekhar Limit?
The limit of a white dwarfs mass. A white dwarf more massive than 1.4 solar masses would collapse under its own gravity. (Increase of mass results in the decrease of the radius).
What is the difference in the stages of life and death of a high-mass star versus a low-mass star?
Remember, low mass stars cannot reach the level of heat needed for carbon fusion after helium fusion. A high-mass star can because hydrogen fuses to helium in its core at a much faster rate. This results in a red supergiant: when hydrogen fuses to Helium in a shell around inert Helium core. Helium-burning supergiant: helium fuses to carbon in core.
High-mass stars can also fuse heavier elements because the core can continue to get hotter, allowing for fusion of heavier elements.
