A Black Hole Born Sans a Supernova?

August 17, 2017

It has long been believed that the lives of extremely massive stars end in an explosion of astronomical proportions, a supernova, which leaves behind a black hole.

However, recent observations may provide evidence supporting a theory that some large stars can transform into black holes without the supernova explosion usually associated with them.1

A Traditional Primer on Black Holes, Supernovae, and Red Giants

The most common stars observed are called main sequence stars. These stars generally follow a linear relationship between temperature and luminosity, as shown in the figure below. Notice along the main sequence of stars that not only does the luminosity increase with increasing temperature, but as the temperature increases the star’s lifetime decreases, and its mass increases. Hotter, more massive stars are bluer; low-mass, cooler stars are redder.

Hertzsprung—Russell diagram
Hertzsprung—Russell diagram.
Image Credit: ESO (CC BY 4.0)

Stars form within interstellar gas clouds that are made primarily of hydrogen. The force of gravity causes this hydrogen to bunch up into clumps, which pull on each other and begin to coalesce. Eventually the hydrogen atoms become densely packed together. When the mass becomes great enough (about 8% the mass of our sun, or 0.08 solar masses), the gravitational pressure at its core is strong enough to begin fusing hydrogen to helium, as the hydrogen atoms smash into each other fast enough to fuse together. Helium atoms have less overall energy (rest mass energy) than the parts they are made of, so the fusion of hydrogen into helium releases energy. This released energy causes an outward radiation pressure on the star’s material and creates the light that we see from the sun. The luminosity of a star is the amount of energy it emits from its surface per second.

Stars range in mass from about 8% of one solar mass to about 150 solar masses, although a few exceptional stars tip the scales as high as 315 solar masses.

When a low-mass star—a star of less than 8 solar masses—has fused the hydrogen in its core to helium, the helium core begins to contract, as does the hydrogen shell around it. During the contraction, the hydrogen shell becomes hot enough to fuse more hydrogen into helium, a process known as hydrogen shell fusion. The shell fuses more rapidly than the former hydrogen core did, and so it has a greater output of energy and radiation pressure, which increases the size and luminosity of the star. For example, when our sun reaches this stage it will have a radius 100 times its current size, and shine 1000 times brighter than it is now.

A star in this stage is called a red giant (with a radius 10 to 100 times that of our sun) or a red supergiant (greater than several hundred solar radii). An example of a red supergiant is Betelgeuse, which has a radius about 950 times that of our sun. Giants are cooler in temperature but more luminous (in terms of total energy emitted) because of their size. These stars are not on the main sequence. They are bigger in size, burn more luminously, and at a lower temperature, so they appear red.

Meanwhile, the core continues to collapse. Eventually the gravitational contraction of the core creates enough force to begin fusion of helium to carbon. Like helium, the rest mass energy of carbon is less than that of the parts that make it, so energy is released during the fusion and the radiation pressure continues to push the star’s exterior layers outward. The star expands and burns brighter than it had before.

Just like before, when a low-mass star fuses its helium core to carbon, the carbon core begins to contract. The outer shells also contract and heat up enough to begin fusing the helium shell along with the remaining hydrogen shell. Its outer layers expand and burn out rapidly as the core continues to collapse until the remaining outer layers are ejected outward. The outer layers become what's termed a planetary nebula, and its cooling core is called a white dwarf. The star does not have enough gravitational energy to overcome the repulsion that exists between carbon atoms (sometimes called degeneracy repulsion or Pauli repulsion), and so it slowly cools and fades. White dwarfs are very dense and cannot exceed the limit of 1.4 solar masses after ejecting their outer layers. If a white dwarf increases in mass (but remains below the limit of 1.4 solar masses), it can reignite and explode in what is called a nova. If the white dwarf reaches 1.4 solar masses, its carbon core will fuse almost instantly and the star will completely explode in a specific subtype of supernova—a “1a” explosion.

Higher-mass stars undergo a different fate, and do so in a much shorter time. When stars are greater than 8 solar masses, there is enough gravitational force to cause continuous fusion at the core. They begin forming a helium core with a hydrogen shell, which pushes the outer layers further outward creating a supergiant. Contraction of the helium core and fusion into carbon happens much more rapidly than in a low-mass star. But as the carbon core contracts, the gravitational force is still so great that fusion begins to occur again, creating heavier elements such as oxygen, neon, magnesium, sulfur, silicon, and iron. Meanwhile the star’s outer layers continue to expand as the core goes through various contractions and new fusing processes.

Iron is very stable, and doesn’t give off energy when fused—rather, iron requires more energy to fuse into something else. Similarly, iron requires energy to be split apart. When a large mass star reaches the iron stage, its core stops fusing. Eventually the inward gravitational force becomes greater than the outward radiation pressure, and the star begins to rapidly collapse. In less than a second the iron core collapses, and a huge explosion—a supernova—sends out large bursts of matter and energy, more than a billion times the energy our sun will emit in its entire lifetime. The energy is so great that it fuses atoms together to create the bulk of the heavier elements that naturally occur in the universe. The core of the star left behind turns either into a neutron star or a black hole.

Neutron stars are formed when the crushing gravitational force pushes the electrons and iron nuclei so close together in the core that all the electrons and protons combine to form neutrons. The Pauli repulsion between the neutrons balances the gravitational pull, and stops the collapse. Pulsars are rapidly spinning neutron stars that emit regular radio waves.

When the gravitational pull exceeds even the Pauli repulsion of neutrons, which occurs when the remaining collapsing core exceeds 2.7 solar masses, the collapse continues. What is left behind is a black hole, or a singularity in the fabric of space—see Reference 5 for a fun video of what would happen if you had a black hole in your pocket!

What's So Special About This Research?

The star that's challenged this traditional narrative of black hole formation is called N6946-BH1. It is about 25 solar masses, and seems to have disappeared, but with no signs of going supernova. The researchers have observational data on this star from seven different telescopes between 1995 to January 2016.

According to reference 1, this red supergiant was measured to have a constant brightness between 1999 and 2005. Between 2005 and 2008 the star faded in most of the visible light region while increasing its output at a wavelength of 3.6 micrometers (the near infrared region). In 2008 it had a slow outburst (ejection of outer layers) that appears to have lasted a number of months, but there is no evidence of a supernova. By the end of 2009 the visible output had faded, with the infrared output slowly increasing, but never reaching the full brightness of the original star. In 2015 the star was observed with the Hubble Telescope, Large Binocular Telescope, and the Spitzer Space Telescope. No observations supporting a supernova were made, and no observations of the star in the optical region were made—rather, measurements showed an emission in the near infrared, but five magnitudes fainter than the star had before its outburst in 2008.

Hubble Space Telescope images: PIA21467: Massive Star Goes Out With a Whimper Instead of a Bang.
Hubble Space Telescope images: PIA21467: Massive Star Goes Out With a Whimper Instead of a Bang.
Image Credit: NASA/ESA/C. Kochanek (OSU)

Another Theory on Black Hole Formation

Scientists think that a fraction of massive stars form a black hole without a supernova. Most scientists that believe this also believe this occurs when high mass stars have low metallicity, which means there is little in the star besides hydrogen or helium. Astronomers use this term, “metallicity” to describe stars with materials that are not considered metals; for example, stars with a lot of carbon, nitrogen, oxygen, and neon are called metal rich, even though none of these elements are metals. But it is likely that N6946-BH1 is a red super giant with solar metallicity, that is, it has elements beyond hydrogen and helium, and this star could provide evidence of a more general massive star transitioning without going supernova.

As described in the new research, no evidence has directly shown that black holes can form without a preceding supernova event for high mass stars, but the researchers point out indirect evidence such as:

  • There have not been observations of high-mass stars (> 17 solar masses) going supernova. This is unusual based on all the observed supernovae, and the number of observed stars that are greater than 17 solar masses.2
  • If there are a significant number of core-collapses that do not go supernova, it could help explain the gap between the observed masses of neutron stars and black holes.
  • Some evidence suggests that the massive star formation rate may exceed the supernova rate.
  • The detection of gravitational waves due to the merging of two black holes is likely to require the existence of a black hole that failed to go supernova.

With this indirect evidence in hand, the researchers decided to search for direct evidence of a black hole formed by a large mass star that did not go supernova. They believed that a red super giant star could transition to a black hole without going supernova and provide some evidence of this transition by way of the visible spectrum. Other researchers previously modeled the transition of a red super giant using hydrodynamic computer simulations. These simulations suggested that the red supergiant would lose mass during a core collapse by the emission of neutrinos, and that the core collapse would send out a shock wave that could unbind loosely bound hydrogen. This would result in a visible light signature with luminosity about ten million times that of our sun that would last 3 to 10 days, after which the star would cool and glow for about a year as recombination hydrogen occurred, with a luminosity of about one million times that of our sun. Dust formation could occur from the expanding ejecta, but that would occur after the original star has begun to fade notably. The original star would, in the end, become unobservable—that is, it would disappear.

The researchers utilized previous observations in their ongoing study of a red super giant candidate for transitioning to a black hole without going supernova, N6946-BH1, which was observed for four years with the Large Binocular Telescope (LBT). They also used historic data from the Hubble Space Telescope (HST), the Spitzer Space Telescope (SST), the Canada France Hawaii Telescope (CFHT), the Isaac Newton Telescope (IST), the Palomar Transient Factory (PTF), and from an avid hobbyist astronomer Ron Arbour. The researchers used new observations made with HST, shown above, which shows that N6946-BH1 is no longer there in the visible spectrum, but a faint signal in the near-infrared is present.

Based on the data and models, the scientists believe the data points toward the formation of a black hole, with the thermal emission being created by accretion material entering the black hole. To shore up this conclusion, scientists hope to collect more observations in the infrared and the X-ray regions of the spectrum. The scientists hope to use the future James Webb Telescope to prove that the star is not just obscured by surrounding cold dust.

References and Resources

1. Adams, S.M., et al., The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star, Submitted to Mon. Not. R. Astron. Soc. (2017) http://resolver.caltech.edu/CaltechAUTHORS:20161117-105756090

2. Kochanek, C.S. et al., A Survey About Nothing: Monitoring a Million Super Giants for Failed Supernovae, Astrophys. J. 684 1336 (2008) http://iopscience.iop.org/article/10.1086/590053/fulltext/

3. Collapsing star gives birth to a black hole, Phys.org, 25 May 2017 https://phys.org/news/2017-05-collapsing-star-birth-black-hole.html

4.Image, PIA21467: Massive Star Goes Out With a Whimper Instead of a Bang https://photojournal.jpl.nasa.gov/catalog/PIA21467

5. A Black Hole In Your Pocket (Video)

—H.M. Doss