An attempt to give some non-technical explanation (this web page is a work in progress)

Martin Gaskell

Astronomy Department, University of Texas, Austin, TX 78712

What do I do my research on?

The two-word answer is: “black holes”! For some curious reason though, that answer doesn’t seem to stop questions! So I’ll attempt to answer some of the more common questions here.  But first, since we need to talk about huge masses and huge distances, let’s try to understand the scale of what is going on. The best way to do this is to try to relate astronomical scales to scales we are familiar with on earth.

Distances in astronomy

In this age of air travel and weather satellite photos we probably all have a sense of what the earth is like.   The nearest astronomical body to the earth is normally the moon.  The moon is about 400,000 km away (about 250,000 miles).  That’s a distance we can sort of relate to.  If you maintain your car well it should make it to 400,000 km (although it won’t be worth much by then!)  So we can think of the moon as being within car-driving distance. 

As most people know, the earth is one of eight planets (sorry Pluto!) orbiting a star we call the sun.  The sun is about 150 million km away (93 million miles).  Already in describing distances I’m into big numbers.  150 million km is hundreds of times further than a car lasts so this not such an easy number to relate to.  That’s further than an airline pilot will fly in his or her entire career.   I think it is much easier to think in terms of how long it takes to get somewhere if you travel at the speed of light, the fastest speed possible, (about 300,000 km/sec).  Starting with the moon, light takes less than one and half seconds to get to us.  Light from the sun takes over 8 minutes to get to us.  The nearby terrestrial (“earth-like”) planets are about the same distance away as the sun, so radio waves (which also travel at the speed of light) take of the order of 5 to 15 minutes to get to Mars depending on how far away Mars is.  Light from the most distant giant planet (Neptune) takes over four hours to reach us.  

Our sun is just one of many stars.  Compared to distances to the planets orbiting the sun, it is a big jump to the nearest star.  The nearest star we can see from most of the northern hemisphere is Sirius, the brightest star in the sky (to the lower left of Orion if you are north of the tropics).  When you see Sirius you are seeing light that has taken almost 9 years to reach us.  What were you doing 9 years ago?!  Astronomers commonly give distances in light units such as “light years” or “light days”.  One light year is how far light travels in one year.  Sirius is thus almost 9 light years away.  I hope it is obvious that when you look at astronomical objects you are always looking back into the past.  The Joint Astronomy Center of the University of Hawaii has a fun web site where you can enter your date of birth and discover your birthday star – a star which, on the day you were born, was sending out the light we are now seeing!  Most of the stars in the sky, however, even bright ones, are so far away that the light we are now seeing started on its journey long before you were born.

Stars are mostly found in galaxies.  There are something like 100 billion stars orbiting together in our own galaxy, the Milky Way.  Light from the center of the Milky Way has taken about 25,000 years to get to us.  That is an amazingly long time ago!  The earliest city known to archaeologists (Çatalhöyük in Turkey) only dates back to about 9500 years ago.  25,000 years ago was during the last ice age in the heyday of woolly mammoths!

The nearest large galaxy outside our own is the Andromeda galaxy.  From a dark place (even some city suburbs) you can see this with the unaided eye.  It is an easy target in binoculars or any small telescope.  If you do see it, that light hitting your retina has travelled through intergalactic space for two and a half million years!   According to my paleontologist wife, the big event on earth around that time was the joining of North and South America.  As a result of this lots of animals went extinct because new predators were crossing the Isthmus of Panama.  2.5 million years is only how long it took light from the nearest other galaxy to get to us!

The universe is believed to have been created 13.73 +/- 0.12 billion years ago, and astronomers are now seeing galaxies back to within one billion years of creation.  By contrast with this, our earth has only been around for 4.5 billion years.   In recent evolution of the universe, astronomers do not consider a mere billion years to be long, but in the context of life on the earth, enormous changes have taken place in the last billion years.  For example, the “Cambrian Explosion” when multi-cellular life as we know it suddenly develops was only 530 million years ago.


Masses in astronomy also involved huge numbers so astronomers like to work in terms of the mass of our sun.  This is just over 300,000 times more massive than the earth.  The mass of an entire big galaxy like our own can easily be over 100 billion solar masses.

OK, so what exactly is a black hole?

A black hole is a region of space where gravity is so strong that even if you go at the speed of light you can’t escape.  This can happen to anything if you squeeze it down small enough so there can be very massive black holes as well as very low mass ones.   Whatever the mass, if you get too close, you’re really stuck!  Now something you fall into or get stuck in gets called a “hole”, and if there is no light being radiated or reflected from something we say it is “black”.  Thus we get the name “black hole” for these regions of space.  Although he did not originally coin the phrase, the term black hole was popularized by former University of Texas professor John Archibald Wheeler.  The point of no return is called the “Schwarzschild radius” or the “event horizon” because we don’t know about events that take place inside it.

Who first came up with the idea that there are black holes?

A rather remarkable English minister, the Rev. John Michell proposed the idea in 1784.  He called them “dark stars”.

If light cannot escape, then how do we know that black holes exist?

That’s easy: by their gravitational influence.  Black holes might not emit light from inside the event horizon, but they still pull on things!  This has two consequences.  First, we can tell that a black hole is there because we can see things orbiting it.  The best illustration of this provided by the black hole in the center of our own galaxy (the Milky Way).  We can see the stars orbiting around the black hole.  If you follow this link you can see a neat movie of this.  The second way the gravitational attraction reveals the presence of a black hole is when the black hole is swallowing things.  As things spiral in towards a black hole, gravity speeds up the infalling matter.  The energy gets dissipated as heat so matter falling into a black hole can get extremely hot.  This makes them glow very brightly and give off X-rays and ultraviolet radiation.   Most black holes that are actively swallowing material are initially detected through their X-ray and ultraviolet radiation.

So black holes really exist?

Yes!  There are two main types of black holes that we know about: ones with masses comparable massive stars (roughly 10 – 20 times the mass of our sun), and then the ones I study, supermassive black holes in the centers of galaxies.  Supermassive black holes can have masses up to 10,000 million times the mass of the sun.  The smaller (“stellar mass”) black holes that we know of are all in binary star systems.  This means that there is a normal companion star (not a black hole) orbiting them.  The companion provides the matter that falls onto the black hole and enables us to know that the black hole is there.   We refer to these systems as “X-ray binaries”. All the supermassive black holes we know of are in the centers (nuclei) of galaxies.   When they are actively swallowing material we call them “active galactic nuclei” (AGNs).  When they are swallowing material at the maximum rate the material around them gets heated up and shines brighter than all the stars in the galaxies they are in!  This makes them the most powerful steady energy sources in the universe.  Black holes are now the most conservative explanation of X-ray binaries and active galactic nuclei.  If black holes do not exist, then something even more bizarre is needed!

Could there be black holes wandering through space that we don’t know about?

Certainly!  We only know about stellar mass black holes when they are in binary systems and many stars (like our sun) are not in binary systems.  So many black holes will not have a companion and there is no way to detect them unless something gets eaten by them.

How close is the nearest black hole?

The nearest stellar mass one that we know about is called Cygnus X-1.  You can see the star which is orbiting it and emission from material falling into the black hole with a pair of binoculars if you know where to look in the constellation of Cygnus.  The nearest supermassive black hole that we know of is in the center of our galaxy.  It has a mass of about 2 million solar masses.

How big are black holes?

The Schwarzschild radius (radius of the event horizon) is simply directly proportional to the mass of the black hole.  If our sun were to become a black hole (which we do not think will happen), then the Schwarzschild radius would be 3 km.  A 10 solar mass black hole would have a 30 km radius, and so on.  The supermassive black hole in the center of the Milky Way is thus bigger than the moon’s orbit, but it is small enough to easily fit between the sun and the earth or between any of the planets.   The most massive black holes known are about 10 billion times the mass of the sun.  That makes them much bigger than the orbit of the Neptune but still a small fraction of the distance to the nearest star.

Do I study what is going on inside black holes?

No.  I don’t study what is going on inside black holes, or even what happens as you cross the event horizon.   I am interested in studying how supermassive black holes grow and in trying to explain how the matter spiraling in towards them generates the radiation we see.

How do black holes form?

We know that stellar mass black holes form when massive stars die.  How a star dies depends on its mass.  A star like our sun is going to die peacefully as a white dwarf star.  Its innermost parts will collapse down to a white dwarf of about the size of earth (right now the sun is about 100 times the diameter of the earth).  Stars with masses of more than a few solar masses undergo violent supernova explosions during which the core collapses down to neutron star only 24 km (15 miles) in diameter.  The cores of the more massive stars still form black holes.  We believe we are seeing the formation of these black holes in some events called g-ray bursters

How do supermassive black holes form?

They probably start off as stellar mass black holes.  Once it has formed, a black hole will grow as matter falls into it.  So long as there is a supply of matter a black hole will keep growing.  However, there is a limit to how fast a black hole can grow because the higher the rate of matter falling towards a black hole the greater the intensity of the radiation generated.  The pressure of that radiation will stop the inflow of matter.  If 10% of the energy of the matter going into the black hole goes into radiation then a black hole doubles in mass in 30 million years.  At this rate of growth an initial 10 solar mass black hole grows to 10 billion solar masses in less than a billion years.

What do black holes eat?

Although a black hole can swallow anything, it is rather hard to feed it compact objects like stars or planets.  Black holes mostly eat gas instead.  Compact objects simply orbit in, swing around, and continue off in their orbits back away from the black hole.  It’s a bit like Halley’s comet coming close to the sun from the outer Solar System every 76 years and then orbiting out.  To spiral into the black hole matter needs to slow down to avoid flying off into space again.  Gas can do this because viscosity provides friction which slows it down.

What is a quasar?

A quasar, or as researchers tend to call them, AGNs (for “active galactic nuclei”), are actively accreting black holes in the centers (“nuclei”) of galaxies.  Because they are accreting matter they are putting out a lot of energy. The brightest ones far outshine the galaxies in which they are located.

Why is so much energy given out when gas spirals into black holes?

When friction slows something down it generates heat.  If you rub your hands together they warm up.  The greater the velocity the more impressive the results.  For example, a piece of metal rubbing against a grind wheel can generate a shower of sparks.  The brilliant streak of a meteor at night is just due to tiny meteoroid about the size of a grain of sand heating up the air as it is slowed down.  The gas spiraling into a black hole is going a 100 to a 1000 times faster than a meteor, so, not surprisingly, the slowing down of the gas generates a lot of heat.  What is perhaps surprising is that the amount of energy liberated per kilogram is typically more than 10 times the energy released by a nuclear bomb!  (That makes a nuclear bomb pretty wimpy compared with dropping the same amount of mass into a black hole!)  This means that supermassive black holes are the most powerful energy sources in the universe.

Does every galaxy have a supermassive black hole?

As far as we know, every galaxy more massive than our Milky Way has a supermassive black hole in the center. 

Would I like to discover a new black hole?

It is now well established that there is a supermassive black hole in the center of every massive galaxy so finding a black hole is no big deal. What is more interesting is finding how massive a black hole is. Actually, what would be really exciting would be to find a massive galaxy that doesn't have a supermassive black hole in it!

Are there inactive supermassive black holes in the centers of some galaxies?

Yes. Nowadays most supermassive black holes are inactive. The black hole in the center of the Andromeda galaxy and the one in the center of its small companion galaxy called M32, are very quiet. We only know the black holes are there because of their gravitational effect on stars near them. The black hole in the center of the Milky Way is pretty inactive too.

So what specific things do I study about supermassive black holes?

I am especially interested in the distribution and motions of the gas close to black holes. This gas is called the “broad-line region.” I am also interested in how the energy is produced.

How do I study the gas close to black holes?

The gas glows because of the intense radiation ionizing hitting it. This produces what we call an “emission-line” spectrum. The spectrum itself tells us about the physical conditions of the gas and about the nature of the radiation hitting it. In 1972 two Russian astronomers, Victor Lyuty and Anatoly Cherepashchuk, demonstrated that as the brightness of an AGN changed, the strength of the emission lines from the gas changed but with a time delay because of how long it took light to travel to the broad-line region. Since we know how fast light travels this time delay tells us how far the gas clouds are from the source of ionizing radiation. In the mid-1980s Linda Sparke and I showed how to use the method to easily get the sizes of broad-line regions. We found that broad-line regions were typically a few light days to a few light weeks across (i.e., bigger than the Solar System) This has now led to a major industry called “reverberation mapping”. In a 1988 paper I showed how the same light-echo technique could be used to show the direction of motion of the “broad-line region” gas. I showed that the gas was spiraling in towards the black hole under the influence of the black hole's gravity. This might seem obvious, but up until then the favored model had been that the gas was being blasted out of the AGN.

If you could get close to an AGN, what would it look like?

I believe it would look like a slowly-rotating bird's nest with a brilliant light inside it! On the outside there is dust which blocks your view of the black hole unless you are looking in from above or below. Inside there is hot glowing gas. On the right is a computer rendition by our son Daniel of what I think an AGN looks like:

Why does what this gas is doing matter?

If we understand what the broad-line region gas is doing then we can calculate the mass of the black hole and how fast it is accreting matter.

How do astronomers get the masses of black holes?

Getting the mass of a black hole is fairly easy so long as we see something orbiting it and know how close the something is to the black hole. The orbital speed then tells us how strong the gravity of the black hole is and hence how massive the black hole is. For stellar-mass black holes (X-ray binaries) we get the mass by seeing how fast the companion star orbits. For “nearby” supermassive black holes (closer than about 100 million light years counts as nearby!) we can measure the motions of stars near the black holes. For further away supermassive black holes we can only measure the masses by measuring the speed that gas is moving at close to the black hole. This was first done by Russian astronomer Ernst Dibai in 1977. Getting black hole masses is a problem I have worked on quite a bit. My 1988 paper showing that gas was spiraling into black holes was important because it showed that it was possible to get black hole masses from the gas near the black hole by the reverberation-mapping technique. This has now been done for several dozen supermassive black holes. This has shown that masses can be estimated without actually having to observe the light echoes. I have recently shown that black hole masses can be determined as accurately from the broad-line region as from the speed stars are moving at.

How does a supermassive black hole affect the galaxy it is in?

Obviously the gravity of the black hole influences the motions of stars near it, but over the last 40 years astronomers have realized that black holes have influenced much more than the immediate vicinity of the black hole (within a few light years). A remarkable result, first published by UT astronomer John Kormendy in 1993, is that the mass of the central black hole is tightly correlated with the mass of what is called the “bulge” of the galaxy.  This suggests that the formation of galaxies and their central supermassive black holes are intimately connected.  One popular theory about how this happens is that the energy released during black hole growth has such a strong effect on the galaxy that it regulates how many stars can form. This is a “hot” topic right now and I am working on how to get accurate black holes masses for large numbers of black holes.

Do supermassive black holes ever swallow stars?

Although black holes mainly eat gas, we do expect that on rare occasions an unfortunate star will get too close to a supermassive black hole and get swallowed. The most massive black holes swallow stars whole; smaller ones disrupt the star before it gets swallowed. Although an encounter between a supermassive black hole and a star is very rare, there are lots of galaxies and black holes known and sometimes there is a flare up around an inactive black hole in the center of a galaxy. We suspect that some of these events are due to stars being disrupted.

Can you get more than one supermassive black hole in a galaxy?

Yes. One of the main ways galaxies grow is through merging with other galaxies. If two galaxies merge and each has a black hole in it then we end up with a galaxy with two black holes in it! Several examples are known of galaxies with two black holes in them.

Can black holes collide?

Yes! This must certainly happen because when galaxies merge we get more than one black hole, yet the vast majority of galaxies only have one black hole. When two black holes merge there will be a tremendous pulse of “gravitational radiation” Scientists are building ingenious experiments on the earth to try to detect gravitational waves.

Are there answered questions about black holes and active galaxies?

You bet!

Answers to more questions to come! . . .


 [Updated: 2009 October 30]