By Yuan-Sen Ting
Black holes have been the talk of the town for the past few years. In 2015, a distinctive gravitational wave signature of two merging black holes of around 30 solar masses each was detected. Not only did these black holes send ripples down the very fabric of time and space, but they also made big waves in the scientific community. In 2017, merely two years after the said gravitational wave signature was discovered, the Nobel Committee awarded the Nobel Prize in Physics to three pioneers in the research of gravitational waves. And that is not the end of it. Shortly after, in 2019, for the first time ever, astrophysicists managed to capture the silhouette of a black hole, and it looks like a donut! [Note: Just before this article is published, it was just announced that the Nobel Prize in Physics in 2020 is awarded to the few pioneers in the study of black holes. What a fortuitous coincidence!]
You and I, along with black holes, are all children of the stars
Black holes sound mysterious, but basically, they are skeletons of stars. Even though the image from 2019 shows that black holes look like donuts, the formation process of black holes is more like that of souffles. As aforementioned, black holes are skeletons of stars. Hence, please bear with me while I tell you more about stars.
The universe started out as nothing, comprising mainly of hydrogen and helium. But because of gravitational force, these simple gases slowly coalesced into clouds of gas. Due to the lack of corresponding resistance, gravitational pull causes everything and anything to collapse into one another. This love-hate relationship continues until stars are created. At some point, the collapsed cloud of gas would become so dense that nuclear fusion is triggered, and a star is born!
Nuclear fusion is a physical process where smaller elements coalesce to form larger elements. It's just like how we can buy multiple clouds of cotton candy and knead them into a giant cloud of cotton candy. During this kneading process, mass is often lost. And this lost mass from stars would turn into energy as depicted in Einstein's famous formula, E=mc2. In short, as clouds of hydrogen in stars are being kneaded, a certain amount of mass is traded for energy. This newly formed energy is the reason behind why stars (such as our beloved Sun) shine. This energy also temporarily stops the gases that form the stars from further collapsing. Just imagine baking a souffle in an oven — it rises and puffs up because of the heat from an oven.
Of course, raw materials in a star that enable nuclear fusion is finite. After the raw materials run out, the star/souffle would no longer have enough energy to fight against gravitational pull, which means that the clouds of gases that form the star would once against start collapsing into each other. There are two possible outcomes at the end of a star's life. Stars of lower mass are equipped with outward pressure from electrons that enable them to hold up to the gravitational force even at the absence of nuclear fusion, and they become white dwarfs. White dwarfs are like piles of ashes that remain after firewood burns out. For a long time, they continue to emit a faint glow, but that glow, too, eventually fades away. Meanwhile, stars of higher mass cannot handle the cessation of nuclear fusion at all.
The sudden collapse causes the massive stars to explode, and elements that were created during nuclear fusion (giant cotton candies) are blasted out. This is just like what happens when you compress a chocolate souffle. It would burst, and the chocolate sauce would get all over the place. Moreover, the extreme heat from the explosion also sparks the formation of various other elements. For example, the oxygen we breath as well as the main elements that make up the earth such as magnesium and silicon, are ejected during this process. At the same time, the cores of these stars collapse and form black holes (Some become neutron stars, but we won't be going into details here).
The explosion of a star leads to two contrasting results. Looking inside, you would see an endless pit which is the black hole. However, it also leads to a more beautiful and varied Universe. The elements created during the explosion of a star became ingredients of life, leading to the formation of the Earth. Yes, you and I, along with black holes, are all children of the stars. And by conducting research on black holes, we are actually learning more about the elements of the Earth as well as ourselves. Where do we come from, and where will we go?
New research frontiers on black holes and its future
Since black holes are the skeletons of stars, there are obviously billions of black holes in the Milky Way. So, where are they?
The study of black holes is indeed a challenging subject for astrophysicists. After all, the main feature of black holes that we all know is the fact that even light cannot escape them. Since they do not reflect light, seeing the black hole is impossible. However, there are still ways to infer their presence. For example, after stars become black holes, gases nearby (such as those from neighboring stars) will still be pulled towards them. When this happens, the temperature of these gases can rise to the point that they emit x-rays. The detection of these x-rays contributes greatly to our research of black holes. Even before the advent of gravitational wave astronomy, we have already found more than twenty black holes via the detection of x-rays emitted by these black holes.
Here's a little bit of extra information you didn't ask for. Throughout this column, we have only talked about black holes formed after the collapse of individual stars, also known as "stellar-mass black holes". Aside from stellar-mass black holes, there are also supermassive black holes in the center of every galaxy. For example, there's a black hole that's four million times the mass of the sun in the center of the Milky Way! In fact, the groundbreaking work on discovering the supermassive black holes in our own Galaxy has led to the Nobel Prize this year for two of the Nobel laureates.
Instead of the skeletons of stars, you can think of supermassive black holes as mass graves of stars that exist in the center of every galaxy (although their formation mechanism is still vigorously debated). The donut in the photograph taken in 2019 is the "mass grave" around a few billion times the mass of the sun. Meanwhile, the black holes detected via their gravitational wave signature in 2015 are skeletons of massive stars, and they are only around 30 solar masses. The Nobel prize in Physics in 2017 was awarded to the skinny black holes, and the Nobel prize this year is awarded to the super-duper black holes!
Compared to stellar-mass black holes, supermassive black holes are easier to observe, which is why research on them has been in full swing since the past few decades. Meanwhile, research on stellar-mass black holes have just begun. Aside from gravitational wave astronomy, as mentioned a few issues ago, Gaia space observatory which can track a billion stars at the same time can again be put to great use. Even though black holes do not emit light themselves, they are often surrounded by some ordinary stars. Just imagine watching two people tango in a pitch-dark dance hall. As long as one of them is wearing fluorescent clothes, we'll be able to infer the existence of his dance partner based on his steps and movements. With the advent of gravitational wave astronomy and humanity's ability to track a massive number of stars in the Milky Way, we were finally able to kickstart research on stellar-mass black holes. Do expect information about stellar-mass black holes to blow up the internet again for the coming years.
Here comes this column's pondering session. People often ask me this: what is the point of astrophysics? I'm honestly unable to answer this question. But I feel that many things in life are pointless. The universe started out as nothing, anyway, and it will end as nothing too. In the far future, after all the energy sources in the universe are depleted, there will be nothing left but countless black holes and white dwarfs. At that time, who's to decide what's useful? Perhaps the only thing we can do is live by this quote from the sci fi novel The Three-Body Problem: "make time for civilization, for civilization won't make time".
(Yuan-Sen Ting is an astrophysicist. Yuan-Sen obtained his Ph.D. in astrophysics from Harvard University in 2017. He is currently a researcher working at the Institute for Advanced Study in Princeton, funded through a NASA Hubble Fellowship. Yuan-Sen is also an incoming faculty member at the Australian National University.)