13.5: Binary Star Systems

Binary Systems

As many as half of all stars may develop in binary systems, where two stars are born together and go through life orbiting a common center of mass. For these stars, the presence of a close-by companion can have a profound influence on their evolution. Stars can exchange material, especially during the stages when one of them swells up into a giant or supergiant, or has a strong wind. When this happens and the companion stars are sufficiently close, material can flow from one star to another, decreasing the mass of the donor and increasing the mass of the recipient. Such mass transfer can be especially dramatic when the recipient is a stellar remnant such as a white dwarf or a neutron star. In this section we will discuss the special circumstances that arise in binary systems, and how they change depending on the nature of both stars.

Novae

First, consider a system with a white dwarf where the other star is gradually transferring material onto it. As 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. As more and more hydrogen accumulates and heats up on the surface of the degenerate star, the new layer eventually reaches a temperature that causes fusion to begin in a sudden, explosive way, blasting much of the new material away.

In this way, the white dwarf quickly brightens, hundreds or thousands of times its previous luminosity. To observers before the invention of the telescope, it seemed that a new star suddenly appeared, and they called it a nova. We now know novae originate from white dwarfs, which are actually the endpoint of stellar evolution for low-mass stars. Novae fade away in a few months to a few years.

Cataclysmic variables are binary stars, with repeated novae. After an outburst, the cataclysmic variable will eventually dim back down significantly until more stellar fuel is transferred from the secondary star to the primary star. This process will go on until there is no more stellar fuel to transfer. As long as the episodes do not increase the mass of the white dwarf beyond the Chandrasekhar limit by transferring too much mass too quickly, the dense white dwarf itself remains pretty much unaffected by the explosions on its surface.

Figure \(\PageIndex{1}\) includes GK Persei, also known as the Firework Nebula, the remnant of a nova observed in 1901. NASA’s Chandra X-ray Observatory, first observed GK Persei in February 2000 and then again in November 2013. This 13-year baseline provides astronomers with enough time to notice important differences in the X-ray emission and its properties. The X-ray data show hot gas and the radio data show light from electrons that have been accelerated to high energies by the nova shock wave. The visible data reveal clumps of material that were ejected in the explosion. Over the years that the Chandra data span, the nova debris expanded at a speed of about 700,000 miles per hour. The X-ray luminosity of the GK Persei remnant decreased by about 40% over the 13 years between the Chandra observations, whereas the temperature of the gas in the remnant has essentially remained constant, at about one million K. As the shock wave expanded and heated an increasing amount of matter, the temperature behind the wave of energy should have decreased. The observed fading and constant temperature indicate that the wave of energy has swept up a negligible amount of gas in the environment around the star over the past 13 years. This suggests that the wave must currently be expanding into a region of much lower density than before, revealing clues about the stellar neighborhood in which GK Persei resides.

Figure \(\PageIndex{1}\) : GK Persei. This image of GK Persei combines X-rays from Chandra (blue), visible light from NASA’s Hubble Space Telescope (yellow), and radio data from the National Science Foundation’s Very Large Array (pink). Astronomers learned that over 13 years the nova's debris expanded expanded by 145 billion kilometers, but remained at the same temperature. (Public Domain; X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA via Wikimedia Commons) Alternative description of Figure \(\PageIndex{1}\).

Type Ia Supernovae

If a white dwarf accumulates matter from a companion star at a much faster rate, it can be pushed over the Chandrasekhar limit. The evolution of such a binary system is depicted in Figure \(\PageIndex{2}\). The more massive star evolves first to become a red giant and then a white dwarf. The white dwarf then begins to attract material from its companion, which in turn evolves to become a red giant. Eventually, the white dwarf acquires so much mass that it is pushed over the Chandrasekhar limit ,1.4 M_Sun, and begins to contract. As it does so, it heats up, and new nuclear reactions can begin in the degenerate core. After about a century, the stars internal temperature slowly increases. Then, in less than a second, an enormous amount of fusion of carbon takes place all at once, resulting in a Type Ia supernova. The fusion energy produced during the final explosion is so great 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 \(\PageIndex{2}\) : Type Ia Supernova. The transfer of material from a red giant to its white dwarf neighbor results in a type Ia supernova. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{2}\).

Type 1a supernovae produce elements up to iron on the atomic table, and also produce elements heavier than iron, like gold, silver, and uranium. These type of supernovae have the same characteristics, such as how bright they will become and length of maximum brightness. Therefore astronomers can use them as a standard candle, or a standard brightness. The very bright object in the lower left of Figure \(\PageIndex{3}\) is Supernova SN1994D in Spiral Galaxy NGC4526. This supernova is of similar brightness to its home galaxy.

Figure \(\PageIndex{3}\): SN 1994D in NGC 4526. In this Hubble Space Telescope image of the galaxy NGC 4526, the type Ia supernova SN 1994D is as bright as the core of its galaxy. (CC BY 4.0; NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team via ESA Hubble) Alternative description of Figure \(\PageIndex{3}\).

We distinguish type I supernovae from those of supernovae of type II originating from the death of massive stars discussed earlier by the absence of hydrogen in their observed spectra. Hydrogen is the most common element in the universe and is a major component of massive, evolved stars. However, as we learned earlier, hydrogen is absent 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 further refers to the presence of strong silicon absorption lines, which are absent 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 is the result of a sudden fusion of the carbon and oxygen of which the white dwarf was made.

Observational evidence now strongly indicates that SN 1006, Tycho’s Supernova, and Kepler’s Supernova 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 evidence of stellar remnants that have survived their explosions. Perhaps even more puzzling is that, so far, astronomers have not been able to identify the companion star feeding the white dwarf in any 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 alternative mechanisms of generating type Ia supernovae. All proposed mechanisms rely upon white dwarfs composed of carbon and oxygen, which are needed to meet the observed absence of hydrogen in the type Ia spectrum. And because any 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 two white dwarf stars in a binary system. The two white dwarfs may have 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 type Ia supernova explosion.

Type Ia supernovae are of great interest to astronomers in other areas of research. This type of supernova is brighter than supernovae produced by the collapse of a massive star. Thus, type Ia supernovae can be seen at very large distances, and they are found in all types of galaxies. The energy output from most type Ia supernovae is consistent, with little variation in their maximum luminosities, or in how their light output initially increases and then slowly decreases over time. These properties make type Ia supernovae extremely valuable standard bulbs for astronomers looking out at great distances, beyond the limits of our own Galaxy.

In contrast, type II supernovae are about 5 times less luminous than type Ia supernovae and are only seen in galaxies that have recent, massive star formation. Type II supernovae are also less consistent in their energy output during the explosion and can have a range a peak luminosity values.

Neutron Stars with Companions

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

Such a rapid spin could not have come from the birth of the neutron star. Recall 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 only because they have fully consumed their former partner stars through the mass transfer process.

And if you thought that a neutron star interacting with a normal star was unusual, there are also binary systems that consist of two neutron stars. It is estimated that about 5% of all neutron stars are a part of a binary system. One such system has the stars in very close orbits to one another, so much that they continually alter each other’s orbit. Another binary neutron star system includes two pulsars that are orbiting each other every 2 hours and 25 minutes. As we discussed earlier, pulsars radiate away their energy, and these two pulsars are slowly moving toward one another, such that in about 85 million years, they will actually merge.

Close binary systems of neutron stars and companion stars are similar to a white dwarf and companion binary star scenario. An accretion disk can form from material taken from the companion star by the neutron star. Neutron Stars are much hotter and more luminous than the white dwarf scenario and they become a powerful X-ray source, called an X-ray Binary. These type of objects emit bursts of energy and are called X-ray Bursters

We have now reached the end of our description of the final stages of stars, yet one piece of the story remains to be filled in. We saw that stars whose core masses are less than 1.4 M_Sun at the time they run out of fuel end their lives as white dwarfs. Dying stars with core masses between 1.4 and about 3 M_Sun become neutron stars. But there are stars whose core masses are greater than 3 M_Sun when they exhaust their fuel supplies, that end their lives as black holes.

Further Exploration: Videos

• Watch a video tour of SN 2014J a type Ia supernova discovered in the Messier 82 (M82) galaxy on January 21, 2014, as well as see brief animations of the two mechanisms by which such a supernova could form.
• Watch an interview of Dr. Scott Ransom, of the National Radio Astronomy Observatory, describer the formation of millisecond pulsars with some nice visualizations of the process.