Our universe is filled with some pretty bizarre objects. Many originate from the exposed cores of dead stars. Below are just a few of them.
A white dwarf is the dead core of a low mass star. Any star with an initial mass of about 4 solar masses will leave behind a white dwarf when it dies. In a little over 5 billion years, our sun will blow off its outer layers (planetary nebula) and expose its carbon core to cold space as a white dwarf. This object will be about the same size as the earth and slowly radiate heat into space, cooling the object to a black dwarf over a very long period. Electron pressure prevents a white dwarf from collapsing. S. Chandrasekhar showed that the maximum mass a white dwarf can have is 1.4 solar masses, because any mass over that will overcome that electron pressure (compelling it to collapse into something even smaller and denser).
The surface gravity of a white dwarf is fairly high. One spoonful of white dwarf material would weight more than an elephant. This might sound impressive, .... but you ain't seen nothing yet.
The word nova translates to "new" in Latin. On occasion, a star flares up so it becomes visible to the naked eye. This pattern can repeat on a fairly regular pattern. Astronomers now have a way of explaining this event. It involves a binary system where one star is a white dwarf and the other star is a red giant. When this occurs, material is systematically stripped away from the red giant and drawn (by gravity) to the white dwarf.
Recall that the white dwarf is really just a ball of hot carbon (and not producing any fusion reactions), ... but not in this case. As material (mostly hydrogen) is added to the surface of the white dwarf, it eventually reaches a critical point and begins to fuse (H → He). This produces a sudden brightening which we detect as a nova. You can see why this process can repeat as long as more material is drawn to its surface from the red giant companion.
As long as this process continues, more mass is added to the white dwarf. But Chandrasekhar showed us that a white dwarf will collapse if it exceeds 1.4 solar masses. When this happens, it rebounds with an incredible explosion astronomers call a Type 1a Supernova. These explosions do not produce the abundance of heavier elements found in Type II supernovae, but they are fairly important to astronomers for a different reason. Records show that a Type 1a supernova will brighten suddenly and then taper off in light output which follows a consistent pattern. Why shouldn't it? All Type 1a supernovae are made the same way and implode when they reach the same mass (Chandrasekhar Limit). It was found that at their brightest, they have an absolute magnitude of -19!!!! That is a lot of light, which means they can be seen from just about anywhere in this universe. It gets better ...
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If you recall, astronomers use Cepheid variables as distance indicators because we have a way of determining the absolute magnitude indirectly. The same situation applies to Type 1a supernovae. When one of these explosions occur, all you need to do is measure the apparent magnitude (which is easy) and the distance to the explosion can be estimated. Using Cepheid variable stars has its limits because there comes a point where you no longer are able to make out individual stars in a galaxy. However, a Type 1a supernovae can be seen from a few billion of light years away.
Both Type II and Type 1a Supernovae also leave behind a collapsed remnant. Theory predicts that if you are able to overcome electron pressure, the collapse continues until you literally push electrons into the nucleus of the atom, ... forming a ball of solid neutrons. Therefore, astronomers named this object a neutron star. Observational astronomers look at the remnants of supernovae and discover objects which emit bursts of energy in pulses (much like a lighthouse). Astronomers called these objects pulsars (short for pulsating star). The first pulsar was discovered in 1967 and it wasn't until later that the connection between pulsars and neutron stars was made. There are now over 1000 pulsars which have been identified. The pulsar in the crab nebula is flashing about 30 times every second. Pulsars which flash at 1000 times a second have been found, but most pulse at a slower rate (about 1 second). The slowest pulsar takes about 10 seconds to pulse. What causes this? These objects are spinning at incredible speeds and you really don't need to be a physics major to understand why. Any ice skater knows that if you want to spin faster, you draw your arms into your body and you spin like a top (see animations below). A physicists say that they are conserving angular momentum but we can just call it the "ice skater effect".
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Spread mass outward and you spin slower (animation) |
Pull the mass to the axis of rotation and you spin faster (animation) |
Now imagine a large object that has some small initial spin and it collapses (draws its mass closer to the axis of rotation). The ice skater effect demands that it spins much faster.
Neutron stars must have a mass greater than the Chandrasekhar limit of 1.4 solar masses. However, they are no bigger than a small city (perhaps 12 miles across). This is almost beyond comprehension. Imagine how strong the surface gravity of a pulsar is? Some scientists have calculated that a marshmallow falling onto the surface of a neutron star will release the energy of a thousand hydrogen bombs. Marshmallows don't fall on neutron stars, but gas certainly does (especially if the neutron star has a binary companion).
Why do pulsars flash? The collapse of the star also means that the magnetic field is also concentrated and incredibly strong. As charged particles fall toward the neutron star, they are directed towards the magnetic poles where they are rapidly accelerated and emit a beam of radiation (red lines). Most pulsars emit most of their energy in radio waves, but some emit a lot of x-rays. The magnetic pole (where the radiation comes out) and axis of rotation are not necessarily the same. The second image below shows that the radiation beam will sweep through space (like a lighthouse beam). If the earth happens to lie along this beam, we see it as a pulse of radiation.
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As stated above, the magnetic field of a neutron star is pretty intense. Toss in a few extra charged particles and the magnetic field gets mind blowing huge .... 100 trillion times the strength of the Earth's magnetic field. Now you can call it a magnetar. If the gravitational field isn't enough to impress you, now the magnetic field alone could rip apart any atom that gets too close. Keep away please!
Black holes have to be the most bizarre thing this universe could possibly produce. The thought that the universe could actually create them was contemplated when Einstein's concepts of warped space was introduced. What is this concept? Imagine an object that is so compact and massive that its gravity will not even let light escape from it. It would, therefore, appear like a "black hole" to us. That is, we could never see it. As a student (in the early 1970's), astronomers were still not sure whether black holes really existed, but had several potential candidates. The consensus (at that time) was that they probably do exist because astronomers observed strange things happening in binary systems. They were observing binary stars, ... only ... they couldn't see one of the stars in the system. In addition, calculations showed that the hidden companion was very massive and material was being drawn off the visible companion, and drawn to a "hole". As material was peeling off the companion star, it was funneled into a smaller and smaller volume. This caused it to emit x-rays, ... and then it disappeared! Freaky! Today, astronomers don't question whether black holes exist, they wonder how many there are.
Astronomers still debate the cutoff points but imagine acquiring enough mass so that even a neutron star will collapse. To overcome "neutron pressure" you will need somewhere around 3 solar masses. Suppose you have just collapsed 3 times the mass of the sun into a volume the size of a neutron star. The laws of physics say that it would continue to collapse, ... and collapse, ... and collapse, ... to a point of zero volume. Since there is no known force in nature to prevent the collapse, it never does! This point is known as a singularity, .. a point of zero volume. It is hard for anyone to imagine this because everything we see takes up space (has volume). A black hole doesn't! Any object which gets near it, will fall in to it, ... including a photon of light. Physicists imagine a boundary between us and the black hole called the event horizon. Anything (including light) which is in inside of this boundary has no choice, ... it is going in. However, a photon just outside the event horizon will still be able to escape, and we will be able to see it. We can only observe phenomena related to a black hole as long as it occurs outside the event horizon.
Astronomers believe that black holes are made when the cores of very massive stars collapse. We are talking about stars with masses slightly over 100 times the mass of our sun!!! Click here to see a list. It was traditionally assumed that some Type II supernovae may produce black holes. However, astronomers have recently found stellar explosions which release 10 times the energy of a supernovae. Called hypernovae, these are the most likely conditions needed to make a black hole.
There may be several ways to make a black hole. Recall that a neutron star must have a mass at least 1.4 times the mass of the sun. What happens if a neutron star has a binary companion? As the neutron star accretes mass from the companion, it may eventually exceed a critical mass (3 solar masses??) and collapse to form a black hole. You could even make a black hole by merging two objects such as a neutron star and a white dwarf. Nature may make black holes in ways astronomers haven't even thought of. For example, Stephen Hawking believes that "mini black holes" (ones with very low masses) may have been created when the universe started (The Big Bang).
Update: LIGO detected gravitational waves from the merger of two neutron stars on August 17, 2017. The result has since been determined to be a black hole. Observation has now caught up with theory!
Feeding time for Black Holes
Once a black hole is made, it will gain mass as it "eats" anything that gets too close to it. A lot of my students ask me if the sun were to become a black hole (don't worry ... it won't), would the earth get pulled into it? No, because the mass is the same. At a distance of 1 AU, from the "black hole sun" (sorry Soundgarden) we would simply orbit in the exact same orbit. It would not be pleasant because the earth would die for lack of sunlight but we would still be OK orbit wise. The event horizon for a black hole the mass of the sun is about 6 km (less than 4 miles). Get that close and in you go and in doing so, the black hole now has more mass (and the event horizon moves further out). If a star gets too close, the black hole can gobble up planets and even stars. In 2019, such an event was even captured by the TESS satellite - click here. Think about this paragraph when you read about supermassive black holes (below) and quasars.
The easiest way for a black hole to reveal itself to us is when it has a binary companion star. This way we can observe material which is systematically fed into the black hole (before it crosses the event horizon). What is observed is a hot accretion disk (which emits radiation) as well as mysterious "jets" of material which is ejected near the poles (much like the way protostars and neutron stars do).
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V4641 Sgr: The Closest Black Hole Candidate |
Astronomers may be able to detect black holes by observing a gravitational lens effect (covered in a previous unit).
Another way black holes may reveal themselves to us is by observing something known as "Hawking radiation". Dr. Hawking predicted that the conditions just outside the event horizon may be able to create particles which we may be able to detect someday.
I had to delete a statement I had here that once said, we have not directly imaged a black hole yet. But that is old news. See the picture of the supermassive black hole below taken in 2019.
Astronomers peer in the center of most galaxies (covered in the next section) and they find extremely massive objects which are smaller than our solar system. These objects have been dubbed "supermassive black holes". Masses on the order of billions of solar masses have been observed in other galaxies. Even our own galaxy, the Milky Way, harbors a supermassive black hole (known as Sag A) with an estimated mass of a 4.6 million times the mass of the sun. Astronomers observe active jets of matter blasting from the region and rapidly swirling accretion disks that suggest that nature can actually make such peculiar objects.
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Jets from Radio Galaxy 3C296 |
X-Ray Jet From Centaurus A |
The Event Horizon Telescope (EHT) has now imaged matter at the event horizon of a 6 billion solar mass supermassive black hole in the center of the distant galaxy M87. The image below was taken in 2019 which shows the event horizon surrounding the black hole. You will never see the black hole itself but this is the best image anyone can get of a black hole. Way cool!!!!
First image of matter around a supermassive black hole in a distant galaxy
How do you make a "supermassive black hole"? Simple, ... make a black hole and let it eat! What is there for a black hole to eat? Gas, dust, and even stars! The centers of galaxies are fairly crowded. Lots of matter is concentrated there, and if a black hole is around, it can have a feeding frenzy. In the process, it devours anything that gets near it. Astronomers call this an active galactic nuclei (AGN). However, if matter is far enough away, it is perfectly safe (by virtue of the inverse square law). Most nearby galaxies show evidence of a supermassive black hole (in the center), but the damage has already been done (no active galactic nuclei). By that, I mean any matter near the black hole has already been swept up by the black hole (making it supermassive in the process). The region is not currently active because there is not a lot of matter falling in. In rare cases, we see some nearby galaxies where the process is not finished (matter is still feeding the black hole .... making it active) See the "jet" images above. These "active galaxies" are the exception, not the rule. In the past, the central regions of most galaxies were extremely active as the supermassive black hole devoured surrounding material. It must have been quite a sight. Imagine the incredible energies that had to be displayed to the universe when these objects were being formed, ... enormous quantities of gas swirling toward the black hole, forming a huge accretion disk, ... monstrous jets of matter spewing from the poles, ... x-rays and gamma rays flooding space as it superheats, ... and then gone, ... as it slips past the event horizon and into the black hole. Our own galaxy must have been pretty spectacular long ago, ... when this was happening. The center of our galaxy is no longer active yet the supermassive black hole still remains.
Some nearby galaxies still have very active centers. They have been observed emitting intense amounts of electromagnetic radiation. This implies that they are still feeding the supermassive black holes in their center. In the next unit, we will discuss the possibility that one galaxy can collide, merge, or cannibalize another nearby galaxy. When that happens, a new supply of "fuel" is available for the central black hole to consume, and the nucleus becomes very active again. This idea received a boost when two supermassive black holes were discovered in the nucleus of a nearby galaxy.
In 1963, Allan R. Sandage discovered a strange object. It looked (visually) like a star but didn't have the familiar spectral lines of normal stars. This and others found were called "quasi-stellar radio sources" which quickly became shortened to quasars. Quasars all showed huge Doppler red shifts which means they were receding from us with velocities close to the speed of light. This fact suggests that quasars are extremely distant objects (because the universe is expanding, ... something we will discuss very soon). If quasars are actually that distant, and appear to us as stars, they must be extremely luminous. This puzzled astronomers for decades because it was believed that nothing could emit the required energies. A solution to this problem is now realized.
Not only are quasars very distant, ... they are very old. By that, I mean they are objects found in a very young universe. The most distant quasars are 13+ billion light years away. This means we are now viewing something that was happening in our universe 13+ billion years ago. It took that much time for the light to reach us. If this is true, the early universe must have been much different from the universe today. At this early time, galaxies were just forming (and perhaps colliding). This could also be the time when supermassive black holes were forming in the center of these galaxies .... making the nucleus extremely active. In other words, the quasars we see today is the radiation produced from an active galactic nuclei as supermassive black holes were forming long, long ago in a galaxy far, far away. Isn't it great when one mystery solves another mystery?
Ever since the first gamma rays detectors were built (late 1960's), astronomers have been recording daily events known as gamma ray bursts. These gamma rays come from random directions, but their source was an enigma. Gamma rays are produced by the most energetic events. Recently astronomers have pinpointed the source of an event known as GRB 130427A. It was a supernova. Gamma ray bursts are likely produced when high mass stars go supernova or hypernova ... producing neutron stars or black holes. Perhaps some of these bursts are also produced when two neutron stars merge.
ŠJim Mihal 2004, 2014, 2018, 2019 - all rights reserved
