AST201H1 Lecture 9: Lecture 9-Chapter 17-Star Stuff

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copperclam445

4 Aug 2010

School

UTSG

Department

Astronomy & Astrophysics

Course

AST201H1

Professor

Stefan Mochnacki

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Chapter 17 ± Star Stuff

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x All main-sequence stars are in gravitational equilibrium ± outward push of gas

pressure balancing the inward pull of gravity.

x Because the force of gravity is greater within more massive stars, the pressure inside

must also be greater.

x To provide this greater pressure, the cores must be hotter and denser, leading to a

greater fusion rate.

x With higher core temperatures and higher fusion rates, massive stars burn through

their hydrogen gas much faster, explaining why they live shorter.

x Low-mass stars are stars born with less than about the mass of our Sun.

x Intermediate-mass stars have birth weights between about 2 and 8 solar masses.

x High-mass stars are those stars born with masses greater than about 8 solar masses.

Low-Mass Star

DIAGRAM 17.2

Red Giant Stage

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core. The star becomes out of balance, gravity shrinking its core size.

x 7KHVWDU¶VRXWHUOD\HUVKRZHYHUwill expand outward, becoming a subgiant.

x 7KHVWDU¶VOXPLQRVLW\ZLOOWKHQLQFUHDVHDQGIXUWKHUH[SDQGLQVL]HEHFRPLQJD

red giant.

DIAGRAM 17.4 ± why stars expand in size.

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vicinity still holds some hydrogen.

x Gravity then shrinks both the non-burning helium core and the surrounding shell

of hydrogen, allowing for hydrogen to sustain.

x Hydrogen burning shell will proceed at a greater fusion rate, creating a greater

luminosity.

x Size increase because energy transport within its interior cannot keep pace with

this larger energy-generation rate, so thermal pressure will build up and push the

pressure outward.

Helium Burning

x Helium fusion occurs only when nuclei slam into one another at much higher

speeds than those needed for hydrogen fusion, meaning that a higher temperature

is needed.

x The helium fusion process converts three helium nuclei into one carbon nucleus.

x Degeneracy pressure takes part in counteracting gravity, so the core does not

inflate.

x The rising temperature causes the helium fusion to soar upward, called helium

flash, releasing an enormous amount of energy into the core, allowing thermal

pressure to dominate.

x As a helium-burning star, size is smaller and is hotter, dropping its life track.

DIAGRAM 17.5

DIAGRAM 17.6

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AST201H1 Full Course Notes

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N all main-sequence stars are in gravitational equilibrium outward push of gas pressure balancing the inward pull of gravity. N because the force of gravity is greater within more massive stars, the pressure inside must also be greater. N to provide this greater pressure, the cores must be hotter and denser, leading to a greater fusion rate. N with higher core temperatures and higher fusion rates, massive stars burn through their hydrogen gas much faster, explaining why they live shorter. N low-mass stars are stars born with less than about the mass of our sun. N high-mass stars are those stars born with masses greater than about 8 solar masses. Intermediate-mass stars have birth weights between about 2 and 8 solar masses. The star becomes out of balance, gravity shrinking its core size. Diagram 17. 4 why stars expand in size.

Related Questions

1. The Jeans instability occurs when
Athe pressure in a gascloud is less than its gravitationalforce.
Bthe pressure in a gascloud is greater than its gravitationalforce.
Cthe temperature in acollapsing cloud is hot enough to fuseHydrogen.
Dthe temperature in acollapsing cloud is hot enough to fuse Helium.

2. At what stage in its life does a star pass through theprotostarphase?
AAfter condensationbut before nuclear reactions begin in itscore.
BWhile it is convertinghydrogen into helium in its core.
CAfter nuclear reactionsend in its core, but before the red giantphase.
DWhen it is expanding insize as a red giant or supergiant.

3. The characteristics of an open cluster of stars are
Aa few hundredmembers, often very young and still embedded in thegas and dustfrom which they were formed.
Bmany thousand members, ofdifferent ages.
Ca few dozen members, theremnant of a globular cluster of starsfrom which most of themembers have escaped.
Dhundreds of thousands ofmembers, all very old, and no or verylittle interstellar gas anddust.

4. What is a protostar?
AA star near the endof its life, before it explodes as asupernova.
BA sphere of gas aftercollapse from an interstellar cloud butbefore nuclear reactionshave begun.
CA small interstellarcloud, before it collapses to become astar.
DA shell of gas left behindfrom the explosion of a star as asupernova.

5. What is an H II region?
AA region of hot,ionized hydrogen around one or more O and Bstars.
BA region of molecularhydrogen inside a giant molecularcloud.
CA region of neutral,atomic hydrogen in interstellar space.
DA region of gas and dustformed by the explosion of a massive star.

6. The total time that the Sun will spend as a main sequencestaris
Aabout 4.5 millionyears.
Bat least 200 billion yearsyears.
Cabout 1 millionyears.
Dabout 10 billionyears.

7. How is a star's lifetime related to its mass?
AThe lifetimes ofstars are too long to measure, so it is not knownhow (or if) theirlifetimes depend on mass.
BA star's lifetime does notdepend on its mass.
CLower-mass stars runthrough their lives faster and have shorterlifetimes.
DHigher-mass stars runthrough their lives faster and have shorterlifetimes.

8. In the Hertzsprung-Russell diagram, how does the position ofatypical star change while it is at the main sequence phase ofitsevolution?
AA star's position onthe main sequence is determined only by itsmass and not its age,and so, stars do not move along the mainsequence duringevolution.
BMassive stars (4 solarmasses) move toward the upper left as theirluminosity increases,while lower-mass stars move toward the lowerright as theirtemperature decreases.
CStars move from upperright to lower left while they are on themain sequence.
DStars move from upper leftto lower right while they are on themain sequence.

9. Why does the core of the Sun contain more helium andlesshydrogen than does the surface of the Sun?
AHelium condenses moreeasily so, when the Sun was forming the corebecame helium-rich;vast quantities of hydrogen were added onlyafter the core becamemassive enough.
BThe hydrogen has beenlifted out of the core by the Sun's magneticfield.
CHelium is heavier thanhydrogen, and has sunk toward the center ina process of chemicaldifferentiation.
DThermonuclear reactionshave converted much of the originalhydrogen in the core intohelium.

10. In terms of nuclear reactions, what is the next stage ofastar's life after the end of hydrogen burning in thecore?
AHydrogen burning ina thin shell around the core.
BHelium burning in thecore.
CCarbon burning.
DDeath (it becomes eithera supernova or a white dwarf).

Most stars maintain an equilibrium size by balancing two forces - an inward gravitational force and an outward force resulting from the star's nuclear reactions. When the star's fuel is spent, there is no counterbalance to the gravitational force. Whatever material is remaining collapses in on itself. Stars about the same size as the sun become white dwarfs, which glow from leftover heat. Stars that have about three times the mass of the Sun compact into neutron stars. And a star with a mass greater than three times the Sun's mass collapses into a single point, called a black hole. In most cases, protons and electrons are fused together to form neutrons - this is the reason for the name neutron star. Neutron stars rotate very fast because of the conservation of angular momentum. Imagine a star of mass 7.29·1030 kg and radius 9.05·108 m that rotates once in 26.0 days. Suppose this star undergoes gravitational collapse to form a neutron star of radius 12.3 km. Determine its rotation speed.

Most stars maintain an equilibrium size by balancing two forces - an inward gravitational force and an outward force resulting from the star's nuclear reactions. When the star's fuel is spent, there is no counterbalance to the gravitational force. Whatever material is remaining collapses in on itself. Stars about the same size as the sun become white dwarfs, which glow from leftover heat. Stars that have about three times the mass of the Sun compact into neutron stars. And a star with a mass greater than three times the Sun's mass collapses into a single point, called a black hole. In most cases, protons and electrons are fused together to form neutrons - this is the reason for the name neutron star. Neutron stars rotate very fast because of the conservation of angular momentum. Imagine a star of mass 5.57·1030 kg and radius 8.85·108 m that rotates once in 26.0 days. Suppose this star undergoes gravitational collapse to form a neutron star of radius 14.9 km. Determine its rotation speed.

AST201H1 Lecture 9: Lecture 9-Chapter 17-Star Stuff

AST201H1 Full Course Notes

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