Astronomy-Chapter 8
The Death of Stars
Nova: from the Latin, meaning “new”, a sudden and temporary brightening of a star
making it appear as a new star in the sky, evidently caused by an explosion of nuclear
fuel on the surface of a white dwarf
Supernova: a “new star” in the sky that is roughly 4000 times more luminous than a
normal nova and longer lasting, evidently the result of an explosion of a star
Giant Star: large, cool, highly luminous star in the upper right of the H-R diagram,
typically 10 to 100 times the diameter of the Sun
Supergiant Star: exceptionally luminous star whose diameter is 100 to 1000 times that
of the Sun
Horizontal Branch: The location in the H-R diagram of giant stars fusing helium
Planetary Nebula: an expanding shell of gas ejected from a medium-mass star during
the latter stages of its evolution
Open Cluster: a cluster of 100 to 1000 stars with an open, transparent appearance,
usually relatively young and located in the disk of the galaxy
Globular Cluster: a star cluster containing 100 000 to 1 million stars in a sphere about
75 ly in diameter, generally old, metal-poor, and found in the spherical component of the
galaxy
Turnoff Point: the point in an H-R diagram at which a cluster’s stars turn off the main
sequence and move toward the red-giant region, revealing the approximate age of the
cluster
Degenerate Matter: extremely high-density matter in which pressure no longer depends
on temperature due to quantum mechanical effects
Compact Object: one of the three final states of stellar evolution, which generates no
nuclear energy and is much smaller and denser than a normal star
Chandrasekhar-Landau: the maximum mass of a white dwarf, about 1.4 solar masses.
Awhite dwarf of greater mass cannot support itself and will collapse
Roche Lobe: the volume of space a star controls gravitationally within a binary system
Angular Momentum: a measure of the tendency of a rotating body to continue rotating.
Mathematically the product of mass, velocity, and radius Accretion Disk: the rotating disk that forms in some situations as matter is drawn
gravitationally toward a central body
Synchrotron Radiation: radiation emitted when high-speed electrons move through a
magnetic field
Supernova Remnant: the expanding shell of gas and dust marking the site of a
supernova explosion
Type I Supernova: a supernova whose spectrum contains no hydrogen lines
Type II Supernova: a supernova explosion caused by the collapse of a massive star
Neutron Star: a small, highly dense star, with radius about 10km, composed almost
entirely of tightly packed neutrons
Pulsar: a source of short, precisely timed radio bursts, understood to be spinning neutron
stars
Lighthouse Model: the explanation of a pulsar as a spinning neutron star sweeping
beams of electromagnetic radiation around the sky
General Theory of Relativity: Einstein’s theory that describes gravity as due to
curvature of space-time
Gravitational Radiation: expanding waves in a gravitational field that transport energy
through space at the speed of light, as predicted by general relativity
Millisecond Pulsar: a pulsar with a pulse period of only a few milliseconds
Singularity: an object of zero radius and infinite density
Black Hole: a mass that has collapsed to such a small volume that its gravity prevents the
escape of all radiation. Also, the volume of space from which radiation may not escape.
Event Horizon: the boundary of the region of a black hole from which no radiation may
escape. No event that occurs within the event horizon is visible to a distant observer
Schwarzschild Radius (Rs): the radius of the event horizon around a black hole
Time Dilation: the slowing of moving clocks or clocks in strong gravitational fields
Gravitational Redshift: the lengthening of the wavelength of a photon as it escapes
from a gravitational field
Gamma-ray Burst: a sudden, powerful burst of gamma rays Hypernova: produced when a very massive star collapses into a black hole. Thought to
be a possible source of gamma-ray bursts
• Massive stars use up their nuclear fuel at a furious rate
• Low-mass stars use their fuel sparingly
GIANT STARS
• Amain sequence star generates its energy by nuclear fusion reactions that
combine hydrogen to make helium
• The giant star is the first step in the death of a star
Expansion Into AGiant
• The nuclear reactions in a main-sequence star’s core produce helium
o Helium accumulates at the star’s center
• When the temperature of the surrounding hydrogen becomes high enough,
hydrogen fusion begins in a spherical layer, called a shell, surrounding the
exhausted core of the star
• Stars like the sun become giant stars 10 to 100 times the present diameter of the
Sun and the most massive stars become supergiant stars as much as 1000 times
larger than the sun
• The expansion of a star to giant or supergiant size cools the star’s outer layers,
and so the stars move toward the upper right in the H-R diagram
• Because the core is not hot enough to fuse helium, gravity squeezes it to a
relatively tiny size
Helium Fusion
• As a star becomes a giant, fusing hydrogen in a shell, the inert core of helium ash
contracts ad grows hotter
• The helium in the core changes the structure of the star. The star now makes
energy in 2 locations by 2 different processes:
o Helium fusion in the core
o Hydrogen fusion in the surrounding shell
• The point that represents the star on the H-R diagram moves downward,
corresponding to lower luminosities, responding to higher surface temperatures to
a region above main sequence called the horizontal branch
• Helium fusion produces carbon and oxygen that accumulate in an inert core
• The approximate rule is that if the core of a post-main-sequence star is “dead” the
star is a red giant, and if the star is “alive”, the star is a yellow giant
o Giants are rare
Star Clusters
• The differences among stars in one cluster must arise from differences in their
masses
1. There are two kinds of star clusters 2. You can estimate the age of a star cluster by observing the distribution of
the points on the H-R diagram
3. By comparing clusters of different ages, you can visualize how stars
evolve
THE DEATHS OF LOW-MASS STARS
• Low-mass stars have little gravitational energy, they cant get very hot
• Structural differences divide the lower-main-sequence stars into 2 groups:
o Very-low-mass red dwarfs
o Medium-mass stars (like the Sun)
Red Dwarfs
• They have very small masses, and very little weight to support
• Their pressure-temperature thermostats are set low, and consume their hydrogen
fuel very slowly
• Have very long lives
• Are completely convective
o Stirred by circulating currents of hot gas rising from the interior a
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