Did the Sun swallow a black hole?

Title: (1) Solar evolution models with a central black hole and (2) Is there a black hole in the center of the Sun?

Authors: (1) Earl P. Bellinger, Matt E. Caplan, Taeho Ryu, Deepika Bollimpalli, Warrick H. Ball, Florian Kühnel, R. Farmer, S. E. de Mink, Jørgen Christensen-Dalsgaard; (2) Matthew E. Caplan, Earl P. Bellinger, Andrew D. Santarelli

First Author’s Institution: (1)  Max Planck Institute for Astrophysics, Garching, Germany, Department of Astronomy, Yale University, CT, USA, Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, and (2) Department of Physics, Illinois State University, Normal, 61790, IL, USA Denmark 

Status: (1) published in The Astrophysical Journal [open access] and (2) Invited article in Ap&SS

Since black holes were first theorized, physicists, astronomers, and the general public alike have used their imaginations to envision them in a variety of (often catastrophic) black hole scenarios. One popular question concerns what would happen if a black hole swallowed a star, especially the Sun. The authors of today’s paper reverse the situation and ask what would happen if the Sun, or another star, captured a black hole.

Primordial Black Holes

Black holes (BHs) come in a variety of types. Perhaps the most familiar are massive types, ranging from about the mass of the Sun to billions of times the Sun’s mass, as in the case of the supermassive BHs expected to lie at the center of every galaxy. However, much less massive BHs are also theoretically possible. Primordial black holes (PBHs), which may have formed in the first second after the Big Bang, could have masses as low as 100,000 times less than a paperclip and up to hundreds of thousands times greater than the Sun. This type of BH is particularly interesting for its potential to explain dark matter. Observational constraints on PBHs restrict the low-mass range to the scale of asteroids, at most to about 10^{18} times less massive than the Sun. Some of these PBHs could be hiding inside stars. Although this idea has not been heavily investigated, the authors of today’s paper point out that it is not new. They even suggest calling stars that harbor a central PBH “Hawking stars” since Stephen Hawking proposed them in 1971. The authors go further by investigating the evolution of Hawking stars, which may provide clues to help us search for them.

Figure 1 (Image credit: NASA). Masses of different types of compact objects, including black holes, relative to the mass of the Sun. Low-mass PBHs would exist to the far left of this diagram.

Capturing a Primordial Black Hole

How does a PBH get inside a star? If PBHs can account for dark matter, then there may be a lot of them moving around the galaxy with a range of different orbits and velocities. The slower of these have a chance of being captured by stars. However, this is highly unlikely since on average the PBH would fall toward the star too fast to become gravitationally bound to it. The chances of a PBH becoming bound to a star while the star is still forming are much greater due to the changing gravitational potential of the star-forming cloud. If a large number of asteroid-mass PBHs exist, then we can expect that at least a few were captured by stars as they formed.

Stellar Evolution

What happens to the Hawking star after formation depends on the mass of the PBH. A PBH lighter than 10^{6} times less massive than the Sun could currently exist inside of the Sun without significant changes to its present structure. In the innermost regions of the Sun, the PBH would consume nearby solar plasma, increasing the BHs mass and driving convection that would mix solar material. However, such a PBH is so small and the luminosity generated by its accretion is so much less than that from solar nuclear fusion, that the properties of the Sun would be nearly unchanged and would still agree with present-day solar observations. Furthermore, Sun-like Hawking stars may live through their entire main sequence phase– in which the star fuses hydrogen into helium– with minimal changes to their outward appearance. Examples are shown in Figure 2. After the main sequence stage things start to get more interesting.

Figure 2 (Figure 2 of Paper 2). Evolution of Sun-like hawking stars with varying PBH seed masses. In the top figure, at a given time along the horizontal axis, one can follow the vertical axis to see the structure of the star outward from its innermost regions. The structure is separated into a hydrogen fusing core (red), helium core (yellow), black hole (dark gray), the Bondi sphere– the region in which material begins to freely fall into the black hole (light gray), and the convective region (slashed). The bottom figures show the luminosity of the star due to fusion (red) and accretion on the PBH (black), as well as the total luminosity (gray shaded) over time. Note that the luminosity due to fusion drops rapidly as the accretion luminosity starts to dominate, coinciding with growth of the PBH in the top figure. A substantial helium core (yellow) accumulates only in the case of the least massive seed BH modeled here (leftmost figure).

Post-Main Sequence

A star leaves the main-sequence phase when it stops burning hydrogen in its core. In the case of a Hawking star, this is triggered by the PBH’s accretion, which starts to dominate over fusion, causing the star to expand and cool, and ultimately stops hydrogen fusion. The star becomes fully convective and puffs into a red giant. The convective mixing of the partially-fused core leads to an extremely high surface helium abundance. For those worried about the fate of Earth, the authors find that for a seed PBH of 10^{-11} times the Sun’s mass, the expansion of the Sun would halt at a maximum of about 0.03 AU, or 3% of the average distance between the Earth and Sun today. However, Earth would be warmed to a temperature of over 250˚C (480˚F), boiling off its oceans. The Hawking star would appear as a sub-subgiant star, lasting for billions of years. While subsequent stages of the star’s evolution have not been studied yet, its final fate is as a black hole. This is very different to the Sun’s expected fate if it does not contain a black hole. In the standard scenario, it would eventually fuse helium before turning into a red giant, then finally collapse into a white dwarf. Note that this sequence of events is for a particular range of PBH seed masses. BHs with even less mass have no effect on the evolution of the Sun, whereas more massive BHs would consume it faster.

How do we look for them?

If Hawking stars look outwardly the same as stars without interior BHs, then how can we find whether they exist?

Figure 3 (Credit: Penn State Astronomy & Astrophysics). HR digram of the pre-main sequence evolution of the Sun.

One clue comes from their trajectories on the Hertzsprung-Russel (HR) diagram, which is used to chart stellar life cycles. The star evolves essentially “backwards” along the standard pre-main sequence track, i.e. the evolution before the star joins the main-sequence (Figure 3). Figure 4 shows that as nuclear reactions begin to shut off, Hawking stars move toward lower luminosities (downward on the HR diagram) and cool (move to the right). After nuclear fusion ends, the star grows in luminosity and radius, but remains up to hundreds of degrees cooler than the normal red giant branch, the first part of the post-main sequence trajectory. Stars found in this region of the HR-diagram, predicted to be devoid of stars by the standard theories of stellar evolution, are known as sub-subgiants and red stragglers. By studying the composition of these types of stars using their spectra, astronomers may be able to differentiate between those with and without an interior PBH, as mixing caused by the PBH would lead to a larger abundance of helium on the surface.

Figure 4 (Figure 3 of Paper 1). HR diagram showing trajectories of Sun-like Hawking stars with different PBH masses. In normal main sequence evolution, the luminosity grows as the star migrates up the HR diagram, as shown in the case of no black hole (solid black line). In this case, the black dotted line shows the evolution of the Sun after its central hydrogen supply has been exhausted. Other color tracks represent Hawking stars with central BHs of different masses. These trajectories become dotted when the accretion luminosity exceeds the luminosity from fusion. Note that these show a migration toward lower luminosity (down) and lower temperature (right) after accretion luminosity dominates. In all cases the star expands into a red giant. Hawking stars with lower BH masses than those shown proceed through the evolutionary phases as normal. Zero-age main sequence (ZAMS) is the time when the star first joins the main sequence. Other ages are also indicated.
Figure 5 (Figure 5 of Cunha et al 2007). Sound waves moving through a cross section of the solar interior. (a) shows acoustic p-modes, and (b) shows gravity g-modes. In (a) the paths are bent by the increase in sound speed with depth, until they are refracted back towards the surface. Some stars may carry mixed modes, which have characteristics of both p- and g- modes.

Hawking stars may also be identified using asteroseismology, the study of sound waves in stars. These sound waves provide information about the internal structure of a star, since their paths depend on the sound speed, which depends on the temperature and chemical composition of the star (read more about asteroseismology in bites here, here, here, and here). Of particular relevance are observations of g-modes, which provide information about the deepest layers of a star (Figure 5). Discovering g-modes in the Sun would indicate that there is no PBH in its center. In red giants, an absence of mixed modes, which have g-mode characteristics, may also identify these stars. The typical mixed modes require stable stratification of the core, and their absence would agree with the convective mixing of the core characteristic of a Hawking star.

Outlook

Hawking stars are only hypothetical for now, but today’s paper starts to investigate their evolution and proposes ways to start searching for them. The authors note that more simulations are needed, including studies of stars of different masses and later stages of stellar evolution. More detailed asteroseismic characterization of Hawking stars would also aid in their discovery, if they do exist. While there is “probably not a black hole in the center of the sun” (Paper 2), the search for Hawking stars outside of our solar system is still worth exploring.

Astrobite edited by Sonja Panjkov

Featured image credit: black hole (NASA’s Goddard Space Flight Center/Jeremy Schnittman), solar interior (NASA Goddard)

About Emma Clarke

I am a 4th year physics PhD student at Carnegie Mellon University. My research focuses on gravitational waves from the early universe, in particular signals from MHD turbulence at phase transitions. Outside of the office, I can be found figure skating, practicing Ashtanga, reading, writing fiction, and baking.

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2 Comments

  1. Thank you for the interesting article.
    I’m puzzled, though: Why wouldn’t the black hole consume the Sun or star? I thought the black hole would eat the Sun from the inside, at an accelerating rate as the BH hole grew more massive.

    Reply
    • Thanks for your comment and question! The black hole does consume the star from the inside at a growing rate, but the time this takes depends on the seed BH mass. This article focuses on what happens in a Sun-like star with a BH massive enough to affect its evolution, but also light enough that there is some interesting, non-standard post-main sequence evolution before the BH completely consumes its host star. The featured papers themselves dive much deeper into the different scenarios and the corresponding BH masses and star lifetimes.

      Reply

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