when a white dwarf star collects matter from a neighboring star, fusion reactions on the surface of the white dwarf cause
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Study with Quizlet and memorize flashcards terms like the exhaustion of hydrogen at its core., swells up and becomes a red giant., dropping temperature and constant brightness. and more.
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the exhaustion of hydrogen at its core.
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The first red giant phase of a star is caused by
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swells up and becomes a red giant.
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When the hydrogen fuel runs out at the center of a main sequence star, the star
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Terms in this set (20)
the exhaustion of hydrogen at its core.
The first red giant phase of a star is caused by
swells up and becomes a red giant.
When the hydrogen fuel runs out at the center of a main sequence star, the star
dropping temperature and constant brightness.
The red subgiant stage of a star is best described by
red subgiant.
A star that is cooling and swelling just enough to keep the same total brightness could be a
decreases but not to its main sequence size.
When a red giant star begins to burn helium, its diameter
helium at their centers.
Stars on the horizontal branch of the HR diagram are burning
an explosion in the helium core.
The first red giant stage of a one solar-mass star's life usually ends with
the explosive ignition of a star's helium core.
The 'helium flash' refers to
the exhaustion of helium at its core.
The red supergiant phase of a star is caused by
swells up and becomes a red supergiant.
When the helium fuel runs out at the center of a horizontal branch star, it
electrons touch each other.
The core of a red supergiant star stops shrinking because its
prevents carbon-burning from starting.
The formation of electron-degenerate matter in the carbon core of a solar-mass red super giant
a star becomes a white dwarf.
A planetary nebula forms when
planetary nebula.
The formation of a new white dwarf is usually accompanied by a
the Earth.
The size of a typical white dwarf star is comparable to the size of
white dwarf star.
A star that is approximately the size of the Earth is probably a
decreases from millions of degrees K to zero.
The surface temperature of a white dwarf star
lower left corner.
On a HR diagram, a visible white dwarf star is in the
a white dwarf steals fuel from a neighbor.
A nova occurs when novas
When a white dwarf star collects matter from a neighboring star, fusion reactions on the surface of the white dwarf cause
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Verified questions
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Imagine the Universe!
This site is intended for students age 14 and up, and for anyone interested in learning about our universe.
Advanced Basic
White Dwarf Stars
A white dwarf is what stars like the Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, this type of star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains. This core becomes a very hot white dwarf, with a temperature exceeding 100,000 Kelvin. Unless it is accreting matter from a nearby star (see Cataclysmic Variables), the white dwarf cools down over the next billion years or so. Many nearby, young white dwarfs have been detected as sources of soft, or lower-energy, X-rays. Recently, soft X-ray and extreme ultraviolet observations have become a powerful tool in the study the composition and structure of the thin atmosphere of these stars.
An Artist's conception of the evolution of our Sun (left) through the red giant stage (center) and onto a white dwarf (right).
A typical white dwarf is half as massive as the Sun, yet only slightly bigger than Earth. An Earth-sized white dwarf has a density of 1 x 109 kg/m3. Earth itself has an average density of only 5.4 x 103 kg/m3. That means a white dwarf is 200,000 times as dense. This makes white dwarfs one of the densest collections of matter, surpassed only by neutron stars.
What's inside a white dwarf?
Because a white dwarf is not able to create internal pressure (e.g. from the release of energy from fusion, because fusion has ceased), gravity compacts the matter inward until even the electrons that compose a white dwarf's atoms are smashed together. In normal circumstances, identical electrons (those with the same "spin") are not allowed to occupy the same energy level. Since there are only two ways an electron can spin, only two electrons can occupy a single energy level. This is what's known in physics as the Pauli Exclusion Principle. In a normal gas, this isn't a problem because there aren't enough electrons floating around to fill up all the energy levels completely. But in a white dwarf, the density is much higher, and all of the electrons are much closer together. This is referred to as a "degenerate" gas, meaning that all the energy levels in its atoms are filled up with electrons. For gravity to compress the white dwarf further, it must force electrons where they cannot go. Once a star is degenerate, gravity cannot compress it any more, because quantum mechanics dictates that there is no more available space to be taken up. So our white dwarf survives, not by internal fusion, but by quantum mechanical principles that prevent its complete collapse.
Degenerate matter has other unusual properties. For example, the more massive a white dwarf is, the smaller it is. This is because the more mass a white dwarf has, the more its electrons must squeeze together to maintain enough outward pressure to support the extra mass. However, there is a limit on the amount of mass a white dwarf can have. Subrahmanyan Chandrasekhar discovered this limit to be 1.4 times the mass of the Sun. This is appropriately known as the "Chandrasekhar limit."
With a surface gravity of 100,000 times that of Earth, the atmosphere of a white dwarf is very strange. The heavier atoms in its atmosphere sink, and the lighter ones remain at the surface. Some white dwarfs have almost pure hydrogen or helium atmospheres, the lightest of elements. Also, gravity pulls the atmosphere close around it in a very thin layer. If this occurred on Earth, the top of the atmosphere would be below the tops of skyscrapers.
Scientists hypothesize that there is a crust 50 km thick below the atmosphere of many white dwarfs. At the bottom of this crust is a crystalline lattice of carbon and oxygen atoms. Since a diamond is just crystallized carbon, one might make the comparison between a cool carbon/oxygen white dwarf and a diamond.
Last Modified: December 2010
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Source : imagine.gsfc.nasa.gov
White Dwarfs Facts, Information and Photos
Find out more about the Solar System's aging stars.
PHOTOGRAPH COURTESY NASA/TOD STROHMAYER (GSFC)/DANA BERRY (CHANDRA X-RAY OBSERVATORY)
SCIENCEREFERENCE
White Dwarfs
Find out more about the Solar System's aging stars.
4 MIN READ
When they reach the end of their long evolutions, smaller stars—those up to eight times as massive as our own sun—typically become white dwarfs.
These ancient stars are incredibly dense. A teaspoonful of their matter would weigh as much on Earth as an elephant—5.5 tons. White dwarfs typically have a radius just .01 times that of our own sun, but their mass is about the same.
Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.
Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass. When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself.
Red Giants
As the star condenses and compacts, it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward. At this stage, the star becomes a large red giant.
Because a red giant is so large, its heat spreads out and the surface temperatures are predominantly cool, but its core remains red-hot. Red giants exist for only a short time—perhaps just a billion years–compared with the ten billion the same star may already have spent burning hydrogen like our own sun.
The brightest star in the nighttime sky, Sirius, or the Dog Star, greatly outshines its white dwarf companion, Sirius B. At 8.6 light-years away, Sirius B is the nearest known white dwarf star to Earth.
PHOTOGRAPH COURTESY NASA/ESA/H. BOND (STSCL)/M. BARSTOW (UNIVERSITY OF LEICESTER)
Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop.
Further Incarnations
Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius).
Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe's oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs—yet.
Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe.
Ancient white dwarf stars shine in the Milky Way galaxy. Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel.
PHOTOGRAPH COURTESY HUBBLESITE
But not all white dwarfs will spend many millennia cooling their heels. Those in a binary star system may have a strong enough gravitational pull to gather in material from a neighboring star. When a white dwarf takes on enough mass in this manner it reaches a level called the chandrasekhar limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star will detonate in a thermonuclear supernova.
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