Stars & Galaxies Exam 3 Chapters 20-22

Front 1

Core-hydrogen-burning

Back 1

The energy burning stage for main-sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning.

Front 2

Hydrogen-shell-burning

Back 2

Fusion of hydrogen in a shell that is driven by contraction and heating of the helium core. Once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there.

Front 3

Subgiant branch

Back 3

The section of the evolutionary track of a star corresponding to changes that occur just after hydrogen is depleted in the core, and core hydrogen burning ceases. Shell hydrogen-burning heats the outer layers of the star, which causes a general expansion of the stellar envelope.

Front 4

Red-giant branch

Back 4

The section of the evolutionary track of a star corresponding to intense hydrogen shell burning, which drives a steady expansion and cooling of the outer envelope of the star. As the star gets larger in radius and its surface temperature cools, it becomes a red giant.

Front 5

Electron degeneracy pressure

Back 5

The pressure produced by the resistance of electrons to further compression once they are squeezed to the point of contact.

Front 6

Helium flash

Back 6

an explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again.

Front 7

Horizontal branch

Back 7

Region of the H-R diagram where post-main-sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core and fusing hydrogen in a shell surrounding the core.

Front 8

Asymptotic-giant branch

Back 8

Path on the H-R diagram corresponding the changes that a star undergoes after helium burning ceases in the core. At this stage, the carbon core shrinks and drives the expansion of the envelope, and the star becomes a swollen red giant for a second time.

Front 9

Planetary nebula

Back 9

The ejected envelope of a red-giant star, spread over a volume roughly the size of our solar system.

Front 10

Black dwarf

Back 10

The end point of the evolution of an isolated, low-mass star. After the white-dwarf stage, the star cools to the point where it is a dark “clinker” in interstellar space.

Front 11

Main-sequence turnoff

Back 11

Special point on the H-R diagram for a cluster, indicative of the cluster’s age. If all the stars in the cluster are plotted, the lower mass stars will trace out the main sequence up to the point where stars begin to evolve off the main sequence toward the red giant branch. The point where stars are just beginning to evolve off is the main-sequence.

Front 12

Roche lobe

Back 12

An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a teardrop-shaped zone of gravitational influence. Any material within the Roche lobe of a star can be conserved to be part of that star. During evolution, one member of the binary system can expand so that it overflows its own Roche lobe and begins to transfer matter onto the other star.

Front 13

Nova

Back 13

A star that suddenly increases in brightness, often by a factor of as much as 10,000, then slowly fades back to its original luminosity. This is the result of an explosion on the surface of a white-dwarf star, caused by matter falling onto its surface from the atmosphere of a binary companion.

Front 14

Accretion disk

Back 14

Flat disk of matter spiraling down onto the surface of a neutron star or black hole. Often, the matter originated on the surface of a companion star in a binary-star system.

Front 15

Core-collapse supernova

Back 15

One possible explosive death of a star, in which the highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star.

Front 16

Supernovae

Back 16

Explosive death of star, caused by the sudden onset of nuclear burning (type I), or an enormously energetic shock wave (Type II). One of the most energetic events of the universe, a supernova may temporarily outshine the rest of the galaxy in which it resides.

Front 17

Type I supernovae

Back 17

One possible explosive death of a star. A white dwarf in a binary-star system can accrete enough mass that it cannot support its own weight. The star collapses and temperatures become high enough for carbon fusion to occur. Fusion begins throughout the white dwarf almost simultaneously and an explosion results.

Front 18

Type II supernovae

Back 18

One possible explosive death of a star, in which the highly evolved stellar core rapidly implodes and then explodes, destroying the surrounding star.

Front 19

Carbon-detonation supernova

Back 19

One possible explosive death of a star. A white dwarf in a binary-star system can accrete enough mass that it cannot support its own weight. The star collapses and temperatures become high enough for carbon fusion to occur. Fusion begins throughout the white dwarf almost simultaneously and an explosion results.

Front 20

Supernova remnant

Back 20

The scattered glowing remains from a supernova that occurred in the past. The Crab Nebula is one of the best-studied supernova remnants.

Front 21

Stellar nucleosynthesis

Back 21

The formation of heavy elements by the fusion of lighter nuclei in the hearts of stars. Except for hydrogen and helium, all other elements in our universe resulted from this event.

Front 22

Helium capture

Back 22

The formation of heavy elements by the capture of a helium nucleus. For example, carbon can form heavier elements by fusion with other carbon nuclei, but it is much more likely to occur by helium capture, which requires less energy.

Front 23

Neutron capture

Back 23

The primary mechanism by which very massive nuclei are formed in the violent aftermath of a supernova. Instead of fusion of like nuclei, heavy elements are created by the addition of more and more neutrons to existing nuclei.

Front 24

Supermassive black holes

Back 24

Black hole having a mass a million to a billion times greater than the mass of the Sun; usually found in the central nucleus of a galaxy.

Front 25

Remnant

Back 25

The object left behind after a supernova explosion. Can refer to (1) the expanding and cooling shell of glowing gas resulting from the event, or (2) the neutron star or black hole that remains at the center of the explosion.

Front 26

Neutron star

Back 26

A dense ball of neutrons that remains at the core of a star after a supernova explosion has destroyed the rest of the star. Typical neutron stars are about 20 km across, and contain more mass than the Sun.

Front 27

Lighthouse model

Back 27

The leading explanation for pulsars. A small region of the neutron star, near one of the magnetic poles, emits a steady stream of radiation that sweeps past Earth each time the star rotates. The period of t he pulses is the star’s rotation period.

Front 28

Pulsar

Back 28

Object that emits radiation in the form of rapid pulses with a characteristic pulse period and duration. Charged particles, accelerated by the magnetic field of a rapidly rotating neutron star, flow along the magnetic field lines, producing radiation that beams outward as the star spins on its axis.

Front 29

X-ray burster

Back 29

X-ray source that radiates thousands of times more energy than our Sun in short bursts lasting only a few seconds. A neutron star in a binary system accretes matter onto its surface until temperatures reach the level needed for hydrogen fusion to occur. The result is a sudden period of rapid nuclear burning and release of energy.

Front 30

Millisecond pulsar

Back 30

A pulsar whose period indicates that the neutron star is rotating nearly 100 times each second. The most likely explanation for these rapid rotators is that the neutron star has been spun up by drawing in matter from a companion star.

Front 31

Gamma-ray bursts

Back 31

Object that radiates tremendous amounts of energy in the form of gamma rays, possibly due to the collision and merger of two neutron stars initially in orbit around one another.

Front 32

General theory of relativity

Back 32

Theory proposed by Einstein to incorporate gravity into the framework of special relativity.

Front 33

Spacetime

Back 33

Single entity combining space and time in special and general relativity

Front 34

Black hole

Back 34

A region of space where the pull of gravity is so great that nothing, not even light, can escape. A possible outcome of the evolution of a very massive star.

Front 35

Schwarzschild radius

Back 35

The distance from the center of an object such that, if all the mass were compressed within that region, the escape speed would equal the speed of light. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible.

Front 36

Event horizon

Back 36

Imaginary spherical surface surrounding a collapsing star, with radius equal to the Schwarzschild radius, within which no event can be seen, heard, or know about by an outside observer.

Front 37

Gravitational redshift

Back 37

A prediction of Einstein’s general theory of relativity. Photons lose energy as they escape the gravitational field of a massive object. Because a photon’s energy is proportional to its frequency, a photon that loses energy suffers a decrease in frequency, which corresponds to an increase, or redshift, in wavelength.

Front 38

Time dilation

Back 38

A prediction of the theory of relativity, closely related to the gravitational redshift. To an outside observer, a clock lowered into a strong gravitational field will appear to run slow.

Front 39

Singularity

Back 39

A point in the universe where the density of matter and the gravitational field are infinite, such as at the center of a black hole.