Black Holes


A Black Hole is a collection of material so massive that the resulting gravity prevents any particle from escaping, including light itself.  They begin life as the remains of a dying massive star and attract other material  by gravity.  As they grow in size, the gravitational force increases and more nearby material is attracted.  Black holes will consume stars, and in fact is the likely way they get so massive.   It seems to be a never-ending process….they consume material and get larger, gravity increases, they consume more material, gravity increases, etc.  Of course there must be nearby material for the Black Hole to attract otherwise it can, and will, stop growing in size.  Once the material passes the outer boundary of the Black Hole…the Event Horizon….there is no escape.

Most Galaxies are believed to contain a Black Hole at their center.  Our Milky Way has a very large Black Hole at its center, Sagittarius A*.  This Black Hole is supermassive, containing 4 million times the mass of our sun!

How big are Black Holes?

Note that the initial size of a black hole is dependent on the size of the “parent” massive star, usually on the order of 5 to 20 times the mass of our sun.  However once the black hole is formed it begins to feed on the surrounding gas and material.  After millions of years in existence a Black Hole can easily reach the mass of millions of our sun.  Even so, the Black Hole may become inactive in the sense that once it consumes the nearby material it can cease to feed.  What size is this compared to the Galaxy it is in?  Maybe it consists of 1% of the total mass of the Galaxy!  A minor part….so don’t begin to think that the Black Hole is going to consume the galaxy …it won’t.

Physically the ultimate size of a black hole can vary a great deal:

  1. Stellar mass – this black hole is 10 miles in diameter, size of Manhattan Island.  Mass is equal to 10 suns.  This is  the initial size of Black Holes when first formed from a collapsing massive star.

  2. Intermediate mass – 40,000 miles in diameter, 5 times the size of earth.  Mass equal to 1,000,000 suns.

  3. Supermassive – 15 million miles in diameter, the size of Mercury’s orbit.  Mass ranges from a million suns to a few billion suns.

The Milky Way Black Hole is a Supermassive Black Hole.

In addition to the above nominal sizes for a Black Hole, it is also proposed by modern physicists that there may be Black Holes at either extreme from the above:

  1. sub-Stellar-mass Black Holes which would be subatomic in size,

  2. Ultra-massive Black Holes. These mammoths are 40,000 million miles in diameter and have the mass of 10 billion suns!

What is the structure of a Black Hole?

Recent studies of Black Holes (2007) indicate that they spin and in so doing they drag nearby material around.  This material, mainly gas and dust, rotates around the Black Hole in a flat  accretion disk.  If the velocity is fast enough the material may happily orbit the Black Hole and never get pulled in to the Event Horizon.  The Event Horizon is the point at the Black Hole that, once passed by an object, it will never escape.  It gets captured and is drawn in….never to be seen again!  The spin of the Black Hole has been measured by the detection of x-rays from the accretion disk.  However this creates a minor problem since the accretion disk is not hot enough to emit x-rays.  So theorists have suggested a solution….there is a highly ionized (read “hot”) corona around the Black Hole and this corona is able to radiate energy to the accretion disk, heating it up thereby allowing it to emit the x-rays, which, by the way, do actually exist.

So the structure of a Black Hole is …

  1. the Black Hole itself,

  2. the boundary of the Black Hole, the Event Horizon,

  3. an Accretion Disk,

  4. A highly ionized corona

How do they die?

Apparently, they do not die.  To date there has been no evidence that Black Holes end their life.  If they do, the mechanics, or physics, of that event would be interesting for sure!  However, it appears that Black Holes may merge, although neither this nor the remnants of such a collision have been observed.