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Lecture 38
Basic Properties of Stars (continued). Stellar Lives.
• Stellar Temperatures
• The Hertzsprung-Russell Diagram
• Groups of Stars and Their Lives
Chapter 17.12 17.16
Surface Temperature
Surface temperature determines a star’ color.
The coolest stars are red, the hottest ones are blue.
Only the brightest star colors can be recognized by the naked eye.The color can be determined better by comparing a star’s brightness in different filters.
Betelgeuse has a temperature of ~3,400 K,Sirius ~9,400 K, the hottest stars – up to 100,000 K.
Spectral Type
The surface temperature also determines the line spectrum of a star.
Hot stars display lines of highly ionized elements, while cool stars show molecular lines.
Stars are classified by assigning a spectral type.The hottest stars are called spectral type O, followed by B, A, F, G, K, M as the surface temperature declines.
Oh Be A Fine Girl, Kiss Me
Stellar Masses
It is harder to measure stellar masses.The best method is to apply Kepler’s third law in combination with Newton’s law of gravity.
This procedure can only be applied to orbiting objects:Visual binary – a resolved pair of stars (Mizar)Eclipsing binary – a pair orbiting in the plane of our line of sightSpectroscopic binary – an object with regularly moving spectral lines or with 2 line systems.
The Hertzsprung-Russell Diagram
Invented by Ejnar Hertzsprung (Denmark) and Henry Norris Russell (USA) in 1912.
The diagram is a plot of stellar luminosities against their surface temperatures.
Temperature increases leftward.Luminosity increases upward.
H-R diagram
Patterns in the H-R diagram
Main sequence – location of the most stars(from upper left to lower right corner)Luminosity class V
Supergiant branch – along the top (class I)Giant branch – just below the supergiants (class III)
White dwarfs – near left corner (small size, high temperature)
Groups of Stars by Mass
Low-Mass Stars: birth mass < 2 Msun
Intermediate-Mass Stars: 8 < M/Msun < 2
High-Mass Stars: > 8 Msun
Low- and Intermediate-mass stars evolve into red giants and ultimately become white dwarfs.
High-mass stars pass through a supergiant phase and end their lives in violent explosions.
Star Birth
A star’s life begins in an interstellar cloud.Star-forming clouds are dense and cold (10-30 K).These clouds are called molecular clouds.
The conditions in molecular clouds allow gravity to overcome thermal pressure and begin the gravitational collapse.
Gravitational contraction increases the cloud’s thermal energy, which is radiated into interstellar space as long-wavelength infrared radiation.
The Protostar Stage
A collapsing cloud fragment starts with some angular momentum, which increases the spin rate of the fragment as it collapses.
As a result, a protostellar disk is formed.The disk slows down the protostar’s rotation.The rotation generates magnetic field.The field lines transfer some of the angular momentum to the disk.The magnetic field also generates a strong protostellar wind.
The Protostar Stage
A Star’s Infancy
A star is born when its core temperature exceeds 10 million K hydrogen fusion begins.
The star’s interior stabilizes: thermal energy generated by fusion maintains the balance between gravity and pressure.
The star becomes a main-sequence star.
Life Tracks
The transitions that occur during star birth can be shown with a special H-R diagram.Such a diagram is called an evolutionary track.
A solar-mass protostar path
Low-Mass Star at Main-Sequence
Low-mass stars produce helium from hydrogen through the proton-proton chain during their main-sequence lifetime.
The energy moves outward from the core through random walk and convection.
The number of particles in the core reduces, the core keeps shrinking, and the luminosity increases over time.
Red Giant Stage
When the core hydrogen depletes, nuclear fusion ceases in the core.The core with no energy source shrink faster.
The star’s outer layers expand and the luminosity rises.The stars becomes a red giant through a subgiant.
The radius increases >100 times.The luminosity increases thousands times.
Red Giant Stage
Why does the star grow bigger when the core is shrinking?
The core is now made of helium, but the surrounding layers contain plenty of hydrogen.Gravity shrinks everything, so fusion begins around the core (in a shell).
The fusion rate in the shell is higher than in the core during the main-sequence stage.The newly produced helium is added to the core.
Switching Energy Sources
The core and the shell keep shrinking, while thermal pressure keeps pushing upper layers outward.
This cycle breaks down when the core reaches a temperature of ~100 million K.At this point helium starts to fuse together.
Helium atoms have 2 protons and a higher positive electric charge than hydrogen atoms.Helium fusion occurs at higher temperatures.The process converts 3 He nuclei (alpha-particle) into 1 C nucleus + energy according to E=mc2.
Helium Burning
The total energy production rate falls from its red giant phase peak.This reduces the star’s luminosity and decreases the star’s radius, making its surface hotter.
In the H-R diagram, the star goes down and to the left.All low-mass stars fuse helium into carbon at nearly the same rate they have almost the same luminosity, but differ in temperature.
Helium fusion inflates the core, which pushes out the hydrogen-burning shell; the shell burning rate drops.
What Will Happen to Earth?
The Sun keeps increasing its luminosity.In ~5 billion years from now the hydrogen burning will stop in its core.
The Sun will then expand to a subgiant.It will become 23 times brighter.
The Earth’s temperature will rise, the oceans will be evaporated, the life may not survive.The Earth may be destroyed, when the Sun becomes planetary nebula.
