Science is Golden
Page 19
A galaxy (NGC 6240) that is not too far away (only 400 million light years) seems to have two supermassive Black Holes—it has two Hearts of Darkness. They are each between 10 and 100 Solar Masses and are about 3,000 light years apart. In only a few hundred million years, they will collide with each other to make an even bigger Black Hole and release an awesomely large blast of gravitational waves. (Strangely, if you smash two enormous Black Holes together, you do not destroy them—you just get one superenormous Black Hole.) Mathematicians also tell us that it’s possible for the newly formed Black Hole to be booted entirely out of the mother galaxy.
We still don’t have a good theory of how these supermassive Black Holes are made. Perhaps they are made from the repeated collisions of smaller Black Holes, or perhaps they are very hungry and eat lots of gas, dust and entire stars (a more likely theory, for various reasons). Or perhaps they were made in the so-called ‘Dark Age’, a period of time about 400 million years long that lies between the cooling down from the Big Bang and the beginning of star formation, when immense stars were thought to have lived fast and died young. Or perhaps they grow when two galaxies collide—when there are many tasty morsels of stars, gas and dust there for the eating.
How Many Black Holes Are There?
The Chandra X-Ray Telescope has given us a rough answer. Nothing can leave the Event Horizon—but as stuff falls in, it can give off X-rays as it picks up speed and crashes into other infalling material. The Chandra X-Ray Telescope looked at an unspectacular and boring part of the sky, roughly two-thirds of the size of the full Moon. Over a two-year period, it built up a total of about 23 days of staring at this tiny area of sky. (Time on any big telescope is very precious. Astronomers can only book time on a telescope in little bursts here and there.) Chandra found that most of what we thought was the fuzzy even X-ray background of the sky was actually made up of individual ‘points’ that give off X-rays. So there are at least 300 million Black Holes in the heavens—and almost certainly, a whole lot more.
Black Holes—Intermediate
These Black Holes have an average size of around 1,000 Solar Masses, with Event Horizons ranging from 1,000 to 12,000 km. They are lighter than supermassive Black Holes, but heavier than Stellar Mass Black Holes that are born in the death throes of a massive star. Until a decade ago, we were not even sure that they existed.
One of these, IRS 13E, is about three light years from Sgr A* (the supermassive Black Hole at the centre of our Milky Way galaxy). It has about 1,300 Solar Masses and is located within a rotating and moving, tightly packed cluster of six massive stars, just 24 light days across.
Some of these intermediate-size Black Holes also occur in globular clusters.
Globular clusters are tightly bound dense groups of stars, with hundreds of thousands of stars clustered close together. They spin slowly with the rest of the galaxy, but usually a fair way out from the centre. The first one was discovered in 1665. There are about 200 globular clusters in our Milky Way. In the globular cluster called M15, which is about 32,000 light years away, there’s a Black Hole of 4,000 Solar Masses. And in the Andromeda Galaxy, about 2.2 million light years away, in the globular cluster called G1, there’s a Black Hole of 20,000 Solar Masses.
Could it be that the stars in a globular cluster are so tightly packed that collisions between the stars can create these intermediate-size Black Holes?
Once again, we don’t really have a good theoretical mechanism for how these Black Holes are made. And we don’t know how they relate to the supermassive Black Hole at the core of the galaxy. Perhaps IRS 13E is one of a bunch of intermediate-size Black Holes that go out on highly elliptical orbits and bring back food for the supermassive Black Hole in the centre of the galaxy—and when Big Mother is not looking, they quietly nibble off a few stars to keep themselves running!
Binary Stars
A binary star system has two stars going around their common centre of gravity, ‘held’ together by their common gravitational attraction. It turns out that our solar system (with just one star) is quite unusual. About 75% of all stars are in multiple systems, the majority being binary! And about 10% of multiple systems have more than two stars. Back in 1802, Sir William Herschel was one of the first to define them as ‘a real double star—the union of two stars that are formed together in one system by the laws of attraction’.
Black Holes—Stellar
We do have theoretical mechanisms for ‘making’ Stellar Black Holes. They start off as stars of more than 20–40 Solar Masses and turn into Black Holes after the nuclear fires go out. They can also form from the collision of two Neutron Stars, or from matter falling onto the surface of a Neutron Star. These Black Holes have masses ranging from 2.5 to 20 Solar Masses and are about 15–120 km in diameter.
There are many Black Holes in this size range known in our, and other, galaxies. In almost all cases, they are one of the two stars in a binary star system. (This is probably because a Black Hole in a binary system is easier to find than a solitary Black Hole.)
The first Black Hole ever to be discovered was Cygnus X-1.
X-rays from Outer Space are blocked by our atmosphere. So in 1964, X-ray instrumentation was loaded into a small sounding rocket which was sent into Space. In its brief time outside the atmosphere, the X-ray instruments found something very bright in the constellation Cygnus, about 6,000 light years from Earth. Over the years since then, increasingly sophisticated X-ray telescopes have been launched. And we now know that Black Holes often show up as points of X-ray energy. Thanks to the data collected with these instruments, by 1990 scientists were pretty sure that Cygnus X-1 is a small invisible (invisible with light, but visible with X-rays) Stellar Black Hole in a binary system. It probably formed about five million years ago from a star of 40 Solar Masses.
How Do You Find Black Holes?
Here’s a question. If a Black Hole is invisible, and it’s very small as well, then how can you find it? The answer lies in the knowledge that a Black Hole has a gravitational field—and you can actually observe the effects of this gravitational field.
We have found a Black Hole at the core of virtually every galaxy that we have closely examined. A Black Hole makes up about 0.5% of the mass of its host galaxy. Stars, gas and dust are seen orbiting the core—and do so very, very quickly.
One way to find Black Holes is to look for the X-rays given off by the fast-moving gas. As gas gets pulled in towards the Black Hole, it gains huge amounts of kinetic energy and gets very hot. In turn, the heat makes the infalling matter give off a rather characteristic pattern of X-rays, which we have actually detected. We have found many cases of a fast-moving visible star orbiting very quickly around an invisible and massive object, which has lots of these characteristic X-rays coming off it, i.e. a Black Hole.
Another method was used by Rainer Schödel from the Max Planck Institute for Extraterrestrial Physics in Germany, and his co-workers in Germany, France, Israel and the USA. They used Newton’s Laws of Gravity and Kepler’s Laws (discovered some 400 years ago), to ‘weigh’ the Black Hole at the centre of our galaxy.
Schödel and his colleagues looked at a star of 15 Solar Masses, called S2, that orbits around a ‘mysterious’ object at the centre of our galaxy. Astronomers call this object Sagittarius A* (or Sgr A*), and for various reasons they have long thought it to be a Black Hole. For example, this object emits the sort of X-ray radiation that we would expect from a Black Hole.
The orbit of S2 is very elliptical—17 light hours from Sgr A* at its very closest, and 11 light days at its most distant. S2 has been monitored for many years, and we now know that it takes 15.2 years to do a complete orbit.
S2 is a lot further out from the core of our galaxy than Pluto is from the Sun, but orbits much more quickly. The reason why S2 is moving so rapidly is because it is trying to avoid being gravitationally sucked in by something that weighs 3.7 million times the mass of our Sun.
Just about the only object that could weigh so much
and be so small is a Black Hole. This kind of data makes us pretty sure that Black Holes do exist.
The other star in the binary system is a supergiant blue star, known as HDE 226868. It is very hot (with a surface temperature of 31,000 Kelvin), massive (about 30 Solar Masses) and huge (30 million km in diameter, or about 20 times bigger than our Sun). Because it is so massive, it is shedding mass at the rate of one Solar Mass every 400,000 years. Normally, it would eject this material in all directions. But the combination of the intense gravity and the closeness of Cygnus X-1 Black Hole means that most of the ejected material from the blue supergiant is captured by Cygnus X-1.
By ‘intense gravity’, I mean that Cygnus X-1 weighs in at about 8.7 Solar Masses (so it’s too massive to be a Neutron Star, which has a limit of about 2.5 Solar Masses). And by ‘closeness’, I mean that the Black Hole, Cygnus X-1, is 45 million km from the centre of the supergiant blue star, i.e. it sits practically on top of the blue supergiant. The distance between the centres of the two stars is just one-and-a-half times the size of the blue supergiant. The twostars (the Black Hole and the blue supergiant) are in a very tight orbit around each other. But because the two stars are quite massive, the time needed for a complete orbit is just 5.6 days.
Measuring Temperature
Most of us know about measuring temperature in degrees Celsius or degrees Fahrenheit.
However, scientists tend to use Kelvin (not degrees Kelvin, just Kelvin—and yes, the name is related to the Kelvinator refrigerator, and Lord Kelvin, the British physicist and mathematician who invented the Kelvin scale).
A Kelvin is the same ‘size’ as a Celsius degree. So 1 K = 1°C = 1.8°F.
But Kelvin and Celsius have different starting points. Zero Kelvin is also called ‘Absolute Zero’, and is equal to -273.15°C, while 0°C is equal to +273.15 K. Once the temperatures reach hundreds of thousands or millions of Kelvins, 273.15 does not make a lot of difference, so astronomers tend to talk about `millions of degrees’ and leave out the K or the C.
Very strange things happen to the gas that the Black Hole sucks off the supergiant blue star. As it leaves the surface of the supergiant, it gets pulled into a teardrop shape and then narrows into a thin stream, which then gets sucked into Cygnus X-1’s Event Horizon. It then spirals around the Black Hole before falling in, taking on the shape of a disc around the Black Hole. The technical name for this is an ‘accretion disc’.
But something else happens. The gas picks up huge amounts of energy as it falls towards the Event Horizon. Some of this energy is given off by enormous jets of particles, moving at significant fractions of the speed of light, and lined up at right angles to the accretion disc. The amount of power inherent in these jets is stupendous—about 1,000 times more than our Sun emits. And of course, the jet also gives off huge amounts of X-rays.
Given that there are about 400 billion stars in our galaxy, and that stars bigger than 20–40 Solar Masses end up as Black Holes, there must already be millions of stellar-size Black Holes in the Milky Way.
Black Holes—Tiny
It is theoretically possible for a tiny Black Hole to continue to exist, once it has been made.
Unfortunately, we don’t have any theoretical mechanism to say how they can be made. They might have been produced in the titanic energies of the Big Bang but would have ‘evaporated’ by now. Or perhaps they could be made in the Large Hadron Collider (LHC)—but again, they would evaporate in blindingly short times. (Read more about the Large Hadron Collider in the chapter ‘Crash Collider: LHC Destroys Universe’.)
What’s the Matter?
This diagram shows Matter being torn from the Black Hole’s companion star to form a hot, swirling accretion disc.
Low mass Black Holes can be very small indeed. A Black Hole with the mass of the Moon would be about 0.2 mm across, about half the size of the head of a pin.
The Big Conclusion
If our Sun were to be ‘magically’ turned into a Black Hole with its current mass, it would be a lot smaller than it is as a star. The Event Horizon would be about 6 km in diameter. Close to the Event Horizon—about 3 km away from the Singularity—the gravitational field would increase enormously. But far away from the Event Horizon (say, a few thousand kilometres) the Black Hole would have the same gravitational field as the Sun—because it would still have the same mass. The only real difference to us is that it would be cold and dark, because the Black Hole would not emit any heat or light.
Black Hole Bart
Black Holes are mentioned in movies, books, and even poems. But the highest praise possible in Western society is to be part of the plot of an episode of The Simpsons.
Black Holes starred heavily in the episode called ‘Treehouse of Horror’ (Season 7, Episode 6, which had three short stories, the third being Homer3). In Homer3, Homer tries to hide from Marge’s sisters when they come to visit, and ends up falling into a Black Hole. As he falls in, he yells out, ‘There’s so much I don’t know about astrophysics. I wish I’d read that book by the wheelchair guy’, referring to Stephen Hawking the British physicist (who is confined to a wheelchair).
Even more impressively, Stephen Hawking himself actually has a speaking part as a guest in the episode ‘Don’t Fear the Roofer’ (Season 16, Episode 16). Hawking suggests to Bart that a small Black Hole in front of Homer’s friend, Ray the Roofer, is absorbing all the light, so that Bart can’t see Ray.
If an object such as a comet, or even a planet, were on a collision course with the Event Horizon, yes, it would vanish inside the Black Hole. But this is just a normal property of gravity. If you are not orbiting fast enough around either a Black Hole or a regular star, you will get sucked in. So if a comet or planet started off in a stable orbit around the Sun, then it would continue in the same stable orbit after the Sun had been magically turned into a Black Hole. After all, there are hundreds of billions of stars in the Milky Way, all orbiting around the supermassive Black Hole in the centre. Apart from a very few stars very close to the centre, they will not get sucked in and will continue to orbit around this 3.7-million-Solar-Mass Black Hole.
If the Sun did become a Black Hole, the only way that the Earth could fall in would be if it lost 99.99% of its orbital angular momentum.
Black Holes are very weird, and up close to them, some of the normal rules of the Universe are turned around. But they do not suck everything into themselves.
However, as with all the Forces of Darkness, it’s better to stay out of their way.
References
Begelman, Mitchell C., ‘Evidence for Black Holes’, Science, 20 June 2003, Vol 300, No 5627, pp 1898–1904.
Cowen, R., ‘Hole in the middle: Are midsize Black Holes the missing link?’, Science News, 21 September 2001, Vol 162, No 12, p 180.
Gebhardt, Karl, ‘Into the heart of darkness’, Nature, 17 October 2002, pp 675–676.
Hellemans, Alexander, ‘X-rays show a Galaxy can have two hearts’, Science, 29 November 2002, Vol 298, No 5599, p 1698.
Iorio, Lorenzo, ‘On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system’, Astrophysics and Space Science, June 2008, 315: 1– 4, pp 335–340.
Maillard, J.P., et al., ‘The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared’, Astronomy & Astrophysics, August 2004, Vol 423, pp 155–167.
Muir, Hazel, ‘Our Black Hole may be sleeping now’, New Scientist, 5 February 2005, p 8.
Schödel, R., et al., ‘A star in a 15.2-year orbit around the supermassive Black Hole at the centre of the Milky Way’, Nature, 17 October 2002, pp 694–696.
Thomas, Vanessa, ‘Dark heart of a Globular’, Astronomy, January 2003, p 32.
Valtonen, M.J., et al., ‘A massive binary Black-Hole system in OJ 287 and a test of general relativity’, Nature, 17 April 2008, pp 851–853.
Acknowledgments
I would like to thank the various experts who were kind enough to give me some of their deep wisdom, and
read and comment on some of my little stories that touch on their field of knowledge. Of course, if any mistakes do remain (and remember the old saying, ‘If you don’t make a mistake, you don’t make anything’), I accept full responsibility and will correct them in subsequent printings of this book.
Professor Helen Muir of Cranfield University was kind enough to look at ‘Plane Truths’ (i.e. you are quite likely to survive a plane crash). All the other experts are from the University of Sydney. Professor Rosanne Taylor from the Faculty of Veterinary Science looked at ‘Sweat Like a Pig’, while Professor Geraint Lewis from the School of Physics looked at ‘Black Holes Don’t Suck’, and Dr Kevin Varvell from the School of Physics looked at ‘Crash Collider: LHC Destroys Universe’.
I would also like to thank the fine people at the Good Weekend (Saturday supplement to the Sydney Morning Herald and The Age) for helping through the 12-or-so hours that it took to write each of the original 400-word stories that appeared in my ‘Mythconceptions’ column, and Dan Driscoll from the ABC for turning them into Street Talk.
Other Dr Karl titles
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First published in Australia in 2008
This edition published in 2010
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