Can a White Dwarf Become a Star Again Can a Binary White Dwarf Become a Star Again

Learning Objectives

By the end of this section, you volition be able to:

• Describe the kind of binary star organization that leads to a nova event
• Describe the blazon of binary star system that leads to a blazon Ia supernovae result
• Indicate how type Ia supernovae differ from type 2 supernovae

The discussion of the life stories of stars presented so far has suffered from a bias—what we might call "unmarried-star chauvinism." Because the human being race developed around a star that goes through life alone, nosotros tend to call back of most stars in isolation. Merely every bit we saw in The Stars: A Angelic Demography, it now appears that as many equally half of all stars may develop in binary systems—those in which two stars are born in each other's gravitational embrace and go through life orbiting a common heart of mass.

For these stars, the presence of a close-by companion can accept a profound influence on their development. Under the right circumstances, stars tin exchange material, especially during the stages when one of them swells upwards into a behemothic or supergiant, or has a potent wind. When this happens and the companion stars are sufficiently shut, material tin can catamenia from one star to another, decreasing the mass of the donor and increasing the mass of the recipient. Such mass transfer can be peculiarly dramatic when the recipient is a stellar remnant such as a white dwarf or a neutron star. While the detailed story of how such binary stars evolve is beyond the scope of our book, we exercise want to mention a few examples of how the stages of evolution described in this chapter may change when there are two stars in a arrangement.

White Dwarf Explosions: The Mild Kind

Let'south consider the following system of ii stars: i has become a white dwarf and the other is gradually transferring textile onto it. Every bit fresh hydrogen from the outer layers of its companion accumulates on the surface of the hot white dwarf, it begins to build up a layer of hydrogen. Every bit more and more hydrogen accumulates and heats upwardly on the surface of the degenerate star, the new layer eventually reaches a temperature that causes fusion to brainstorm in a sudden, explosive way, diggings much of the new textile away.

In this way, the white dwarf quickly (but only briefly) becomes quite bright, hundreds or thousands of times its previous luminosity. To observers before the invention of the telescope, information technology seemed that a new star suddenly appeared, and they chosen it a nova.^[2] Novae fade away in a few months to a few years.

Hundreds of novae accept been observed, each occurring in a binary star system and each later showing a shell of expelled cloth. A number of stars have more than one nova episode, as more than fabric from its neighboring star accumulates on the white dwarf and the whole process repeats. As long as the episodes do not increase the mass of the white dwarf beyond the Chandrasekhar limit (by transferring besides much mass too apace), the dense white dwarf itself remains pretty much unaffected by the explosions on its surface.

White Dwarf Explosions: The Violent Kind

If a white dwarf accumulates affair from a companion star at a much faster rate, information technology can be pushed over the Chandrasekhar limit. The evolution of such a binary system is shown in Figure 1. When its mass approaches the Chandrasekhar mass limit (exceeds 1.iv 1000 _Sun), such an object can no longer support itself every bit a white dwarf, and it begins to contract. Every bit it does so, it heats upwards, and new nuclear reactions can brainstorm in the degenerate core. The star "simmers" for the adjacent century or so, building up internal temperature. This simmering phase ends in less than a second, when an enormous amount of fusion (particularly of carbon) takes identify all at once, resulting in an explosion. The fusion free energy produced during the last explosion is so smashing that it completely destroys the white dwarf. Gases are blown out into space at velocities of about 10,000 kilometers per second, and afterward, no trace of the white dwarf remains.

Figure 1: Evolution of a Binary Organization. The more massive star evolves showtime to become a blood-red giant and so a white dwarf. The white dwarf then begins to attract material from its companion, which in turn evolves to become a cherry giant. Eventually, the white dwarf acquires so much mass that it is pushed over the Chandrasekhar limit and becomes a type Ia supernova.

Such an explosion is also called a supernova, since, similar the devastation of a high-mass star, information technology produces a huge amount of free energy in a very short time. Still, different the explosion of a loftier-mass star, which can go out behind a neutron star or black hole remnant, the white dwarf is completely destroyed in the process, leaving behind no remnant. We call these white dwarf explosions type Ia supernovae.

Nosotros distinguish blazon I supernovae from those of supernovae of type 2 originating from the death of massive stars discussed earlier by the absence of hydrogen in their observed spectra. Hydrogen is the nigh common chemical element in the universe and is a major component of massive, evolved stars. However, as we learned earlier, hydrogen is absent-minded from the white dwarf remnant, which is primarily composed of carbon and oxygen for masses comparable to the Chandrasekhar mass limit.

The "a" subdesignation of type Ia supernovae farther refers to the presence of strong silicon absorption lines, which are absent-minded from supernovae originating from the collapse of massive stars. Silicon is one of the products that results from the fusion of carbon and oxygen, which bears out the scenario we described above—that there is a sudden onset of the fusion of the carbon (and oxygen) of which the white dwarf was fabricated.

Observational prove now strongly indicates that SN 1006, Tycho's Supernova, and Kepler's Supernova (meet Supernovae in History from Supernova Observations) were all type Ia supernovae. For instance, in contrast to the case of SN 1054, which yielded the spinning pulsar in the Crab Nebula, none of these historical supernovae shows any prove of stellar remnants that have survived their explosions. Mayhap even more puzzling is that, so far, astronomers have not been able to identify the companion star feeding the white dwarf in whatsoever of these historical supernovae.

Consequently, in order to address the mystery of the absent companion stars and other outstanding puzzles, astronomers have recently begun to investigate culling mechanisms of generating type Ia supernovae. All proposed mechanisms rely upon white dwarfs equanimous of carbon and oxygen, which are needed to see the observed absence of hydrogen in the type Ia spectrum. And because whatever isolated white dwarf below the Chandrasekhar mass is stable, all proposed mechanisms invoke a binary companion to explode the white dwarf. The leading alternative mechanism scientists believe creates a type Ia supernova is the merger of ii white dwarf stars in a binary system. The two white dwarfs may accept unstable orbits, such that over time, they would slowly move closer together until they merge. If their combined mass is greater than the Chandrasekhar limit, the result could also be a blazon Ia supernova explosion.

You can sentry a curt video about Supernova SN 2014J, a blazon Ia supernova discovered in the Messier 82 (M82) galaxy on January 21, 2014, every bit well as see brief animations of the two mechanisms by which such a supernova could form.

Type Ia supernovae are of great interest to astronomers in other areas of enquiry. This type of supernova is brighter than supernovae produced by the collapse of a massive star. Thus, type Ia supernovae can exist seen at very big distances, and they are found in all types of galaxies. The energy output from near type Ia supernovae is consistent, with piffling variation in their maximum luminosities, or in how their light output initially increases and and so slowly decreases over time. These properties make type Ia supernovae extremely valuable "standard bulbs" for astronomers looking out at great distances—well across the limits of our own Galaxy. You'll learn more about their apply in measuring distances to other galaxies in The Large Blindside. In dissimilarity, type 2 supernovae are near v times less luminous than type Ia supernovae and are merely seen in galaxies that have contempo, massive star germination. Type II supernovae are besides less consistent in their free energy output during the explosion and tin can accept a range a peak luminosity values.

Neutron Stars with Companions

Now let's look at an even-more mismatched pair of stars in action. It is possible that, under the right circumstances, a binary system can even survive the explosion of one of its members as a type II supernova. In that case, an ordinary star can eventually share a system with a neutron star. If material is and then transferred from the "living" star to its "expressionless" (and highly compressed) companion, this fabric will be pulled in by the strong gravity of the neutron star. Such infalling gas will be compressed and heated to incredible temperatures. It will chop-chop become and so hot that it will experience an explosive flare-up of fusion. The energies involved are so great that we would expect much of the radiation from the flare-up to emerge as X-rays. And indeed, high-free energy observatories higher up Earth's atmosphere (see Astronomical Instruments) accept recorded many objects that undergo but these types of 10-ray bursts.

If the neutron star and its companion are positioned the right way, a meaning amount of material can be transferred to the neutron star and can set it spinning faster (equally spin energy is also transferred). The radius of the neutron star would also decrease as more mass was added. Astronomers accept establish pulsars in binary systems that are spinning at a rate of more 500 times per second! (These are sometimes called millisecond pulsars since the pulses are separated past a few thousandths of a 2d.)

Such a rapid spin could not take come from the birth of the neutron star; it must accept been externally caused. (Call back that the Crab Nebula pulsar, one of the youngest pulsars known, was spinning "only" 30 times per second.) Indeed, some of the fast pulsars are observed to be part of binary systems, while others may be alone but because they accept "fully consumed" their former partner stars through the mass transfer procedure. (These have sometimes been called "black widow pulsarsouthward.")

View this curt video to come across Dr. Scott Ransom, of the National Radio Astronomy Observatory, explain how millisecond pulsars come about, with some nice animations.

And if you lot thought that a neutron star interacting with a "normal" star was unusual, there are also binary systems that consist of two neutron stars. One such arrangement has the stars in very shut orbits to one another, so much that they continually alter each other's orbit. Some other binary neutron star system includes two pulsars that are orbiting each other every ii hours and 25 minutes. As nosotros discussed earlier, pulsars radiate away their energy, and these two pulsars are slowly moving toward ane another, such that in about 85 million years, they will actually merge.

We take now reached the cease of our description of the final stages of stars, notwithstanding one slice of the story remains to be filled in. Nosotros saw that stars whose core masses are less than one.4 M _Sun at the time they run out of fuel end their lives as white dwarfs. Dying stars with cadre masses betwixt 1.4 and nearly three M _Sunday become neutron stars. But there are stars whose cadre masses are greater than iii Grand _Sun when they exhaust their fuel supplies. What becomes of them? The truly bizarre result of the death of such massive stellar cores (called a blackness hole) is the subject field of our side by side affiliate. Only first, we will look at an astronomical mystery that turned out to be related to the deaths of stars and was solved through clever sleuthing and a combination of observation and theory.

Key Concepts and Summary

When a white dwarf or neutron star is a fellow member of a close binary star system, its companion star can transfer mass to it. Textile falling gradually onto a white dwarf can explode in a sudden burst of fusion and make a nova. If cloth falls rapidly onto a white dwarf, information technology can push information technology over the Chandrasekhar limit and crusade information technology to explode completely as a type Ia supernova. Another possible mechanism for a type Ia supernova is the merger of 2 white dwarfs. Textile falling onto a neutron star can crusade powerful bursts of X-ray radiations. Transfer of cloth and angular momentum tin speed up the rotation of pulsars until their periods are just a few thousandths of a 2d.

Glossary

nova: the cataclysmic explosion produced in a binary system, temporarily increasing its luminosity by hundreds to thousands of times

millisecond pulsar: a pulsar that rotates so quickly that it can requite off hundreds of pulses per second (and its period is therefore measured in milliseconds)

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