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The ultimate fate of white dwarfs

If the birth of a main-sequence star is defined by the onset of fusion reactions, then we must consider the end of all fusion reactions to be the time of a star’s death. As the core is stabilized by degeneracy pressure, a last shudder of fusion passes through the outside of the star, consuming the little hydrogen still remaining. Now the star is a true white dwarf: nuclear fusion in its interior has ceased. [link] shows the path of a star like the Sun on the H–R diagram    during its final stages.

Evolutionary track for a star like the sun.

In this plot the vertical axis is labeled “Luminosity (LSun)”, and goes from 0.1 at bottom to 10,000 at top, in increments of 10 times the previous value. The horizontal scale is labeled “Surface Temperature (K)”, and goes from 1000 at right to 100,000 at left, in increments of 10 times the previous value. The “Main sequence” is drawn as a straight red line beginning at around 12,000 K and 6000 LSun, and ends near 6,000K and 0.1 LSun. The evolutionary curve is plotted in black. It begins at point A near 4,000 K and 6000 LSun, moves horizontally above the main sequence (B) until about 70,000 K and 10,000 LSun and then curves downward to point C near 60,000 K and 10 LSun.
This diagram shows the changes in luminosity and surface temperature for a star with a mass like the Sun’s as it nears the end of its life. After the star becomes a giant again (point A on the diagram), it will lose more and more mass as its core begins to collapse. The mass loss will expose the hot inner core, which will appear at the center of a planetary nebula. In this stage, the star moves across the diagram to the left as it becomes hotter and hotter during its collapse (point B). At first, the luminosity remains nearly constant, but as the star begins to cool off, it becomes less and less bright (point C). It is now a white dwarf and will continue to cool slowly for billions of years until all of its remaining store of energy is radiated away. (This assumes the Sun will lose between 46–50% of its mass during the giant stages, based upon various theoretical models).

Since a stable white dwarf can no longer contract or produce energy through fusion, its only energy source is the heat represented by the motions of the atomic nuclei in its interior. The light it emits comes from this internal stored heat, which is substantial. Gradually, however, the white dwarf radiates away all its heat into space. After many billions of years, the nuclei will be moving much more slowly, and the white dwarf will no longer shine ( [link] ). It will then be a black dwarf —a cold stellar corpse with the mass of a star and the size of a planet. It will be composed mostly of carbon, oxygen, and neon, the products of the most advanced fusion reactions of which the star was capable.

Visible light and x-ray images of the sirius star system.

Sirius in Visible Light and X-rays. In panel (a), at left, shows Sirius A and B in visible light. Sirius A is the overexposed mass of light at center and Sirius B is the faint speck at lower left. Panel (b), at right, shows the same system in X-ray light. The bright object at center is Sirius B, and the fainter object above and to the right is Sirius A.
(a) This image taken by the Hubble Space Telescope shows Sirius A (the large bright star), and its companion star, the white dwarf known as Sirius B (the tiny, faint star at the lower left). Sirius A and B are 8.6 light-years from Earth and are our fifth-closest star system. Note that the image has intentionally been overexposed to allow us to see Sirius B. (b) The same system is shown in X-ray taken with the Chandra Space Telescope. Note that Sirius A is fainter in X-rays than the hot white dwarf that is Sirius B. (credit a: modification of work by NASA, ESA, H. Bond, M. Barstow(University of Leicester); credit b: modification of work by NASA/SAO/CXC)

We have one final surprise as we leave our low-mass star in the stellar graveyard. Calculations show that as a degenerate star cools, the atoms inside it in essence “solidify” into a giant, highly compact lattice (organized rows of atoms, just like in a crystal). When carbon is compressed and crystallized in this way, it becomes a giant diamond-like star. A white dwarf star might make the most impressive engagement present you could ever see, although any attempt to mine the diamond-like material inside would crush an ardent lover instantly!

Evidence that stars can shed a lot of mass as they evolve

Whether or not a star will become a white dwarf depends on how much mass is lost in the red-giant and earlier phases of evolution. All stars that have masses below the Chandrasekhar limit when they run out of fuel will become white dwarfs, no matter what mass they were born with. But which stars shed enough mass to reach this limit?

One strategy for answering this question is to look in young, open cluster    s (which were discussed in Star Clusters ). The basic idea is to search for young clusters that contain one or more white dwarf stars. Remember that more massive stars go through all stages of their evolution more rapidly than less massive ones. Suppose we find a cluster that has a white dwarf member and also contains stars on the main sequence that have 6 times the mass of the Sun. This means that only those stars with masses greater than 6 M Sun have had time to exhaust their supply of nuclear energy and complete their evolution to the white dwarf stage. The star that turned into the white dwarf must therefore have had a main-sequence mass of more than 6 M Sun , since stars with lower masses have not yet had time to use up their stores of nuclear energy. The star that became the white dwarf must, therefore, have gotten rid of at least 4.6 M Sun so that its mass at the time nuclear energy generation ceased could be less than 1.4 M Sun .

Astronomers continue to search for suitable clusters to make this test, and the evidence so far suggests that stars with masses up to about 8 M Sun can shed enough mass to end their lives as white dwarfs. Stars like the Sun will probably lose about 45% of their initial mass and become white dwarfs with masses less than 1.4 M Sun .

Key concepts and summary

During the course of their evolution, stars shed their outer layers and lose a significant fraction of their initial mass. Stars with masses of 8 M Sun or less can lose enough mass to become white dwarfs, which have masses less than the Chandrasekhar limit (about 1.4 M Sun ). The pressure exerted by degenerate electrons keeps white dwarfs from contracting to still-smaller diameters. Eventually, white dwarfs cool off to become black dwarf s, stellar remnants made mainly of carbon, oxygen, and neon.

Questions & Answers

What kind of weather does Venus experience?
Michael Reply
why are the hyperlinks not working?
Gregory Reply
If the atmosphere blocks the view of Venus' surface, what am I seeing that looks like craters?
Michael Reply
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters when view the planet?
Michael Reply
I don't Know! perhaps the mountains?
my question is if the surface of Venus is shining y not our earth
when we look at venus we can't see any crater like things... check it once again
and venus shines because of the clouds that are made up sulpher dioxide and sulphuric acid droplets. and the clouds are so dense. the case of earth is different
also because of Venus's 70% albedo phenamenon
and plus it reflects 70 percent of its light back into space earth don't shine because it's not a cloudy planet and its farther than the sun
can I become an astronaut without taking mathematics as a subject in 11th class
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters? Are these atmospheric storms?
can a death of a massive star be the new big bang
Neeta Reply
No it might be a supernova. The big bang was much more massive.
It created the universe as we know it
the big bang is just a new beginning
the death of a gigantic star makes it possible for a new begging
Only in the local area of space
so many smaller universes are created and destroyed
i think it created a supernova!
you still think? I am sure I created one supernova of my own
I'm sure you did
neeta the death of a massive star could lead to an explosion that leads to a super nova and when the supernova explodes it becomes a nebula like messier 1
a supernova remnant
what is means by earthbound
Satyam Reply
you are earthbound, arnt we all earthbound? except the ISS etc.
I don't agree we may be earthbound because of our gross body/physical body...but there is also a subtle body which does not limit us to earthly existence
good questions, as humans our habitat is earth, we are bound to this and have to alter ourselves to stay alive off land and outside the atmosphere, do you not agree? I'd guess you could also say the moon is earthbound. anything earthbound is restricted to earth. hope this helps, love your question
how do you describe astral projection
Astral projection is spiritual. I'm assuming Satyam was talking about the physical world.
ah yes spiritually we are bound only to what we chose, like nothing, I like to think that way
anyone here also participating in seti@home?
what is seti?
what has extraterrestrial entity got to do with this why @seti project..we are discussing human existence on earth and beyond earth
please join me at the cosmic Diner where all your intergalactic dreams come true.
I was just thinking what if somebody ignorantly mistaken a cosmic microwave Background with a conventional microwave we be in a lot of trouble.
don't kno
You mean the Restaurant at the end of the Universe...right we can come back and visit again but not sure of the mode of payment
yeah I'm already heading there
how to read stars
Amresh Reply
you don't
astrology i guess
astrologer is the study of star systems and constellations...but astronomy is the study of formation of universes multiverses birth and evolu of stars
what is astronomy
babul Reply
study of formation evolution and the death of star star systems and galaxies
astronomy is the study of universe
astronomy and physics are basically the same except physics is the study of the motion and behavior of the universe
What is 12 constellation of zodiac and why it is important to study astrology
And also what is main purpose of these 12 constellation of zodiac in astronomy
astronomy and astrology with two different things
it Is the study of universe and of life speaking generally! i think!
universe is born 13,7 miliardi of years with big Bang if i remember well! but there are also other theories for the universe, speaking generally! i remember so!
universe is actually about 13.7 billion years old
is astrology like astronomy
debjani Reply
Astrology is a lower dimension.. astronomy is much more vast and multidimensional
yes.. astrology is about constellations only, and astronomy is about all stars, galaxies, gravity, dark matter, dark energy.. etc everything including astrology
can you become an astronaut without taking mathematics as a subject in 11th class
I was reading the chapter on Cosmic Microwave Background. And, I can not seem to find it now. If anyone could help me find that portion of the Astronomy Textbook I would really appreciate it. AZ
Alba Reply
Where or which chapter discusses Cosmic Microwave Background?
29 the big bang ch 29.4
what's cold dark matter?
pratham Reply
Dark matter even the science community is not sure what it is...!!! That's why they have some of the smartest math wizards around to try to figure out that puzzle!
How are Roche Worlds formed?
AlteredEdge Reply
In need to read some books about Astronomy so how can in get it actually leg live in Ethiopia can uh help me with that?
Mom Reply
I am sri Sharan .m .what is my best favourable numbers
Madhesh Reply
how can we know it
bahut hard
Practice Key Terms 2

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