By Talya Klinger, Homeschooler
Contrary to popular opinion, black holes do not exist solely to swallow up your socks, keys, and the last scoop of Rocky Road you were saving for a late night snack. Rather, a black hole is an object with such a large mass in such a small volume that nothing, not even light, can escape its gravitational pull. The black hole’s gravitational pull absorbs whatever is in its reach.
The outer limit of a black hole is an imaginary surface
called its event horizon, where the black hole's gravitational pull is just
strong enough that not a single photon can escape, creating a large dark space.
According to Einstein's theory of general relativity, even light rays that pass
by the event horizon are bent and distorted by the black hole's gravity in a
process called gravitational lensing.
This simulation of a spinning supermassive black hole from the movie Interstellar is approximately what a black hole would look like, according to general relativity.
As black holes absorb more and more objects, their mass
grows. Not all black holes grow to a similar size, however. Depending on their
mass, black holes generally fall into two radically different size categories:
stellar mass and supermassive. Most stellar mass black holes, which are 10 to
24 times the size of the sun, are isolated and difficult to detect. Supermassive
black holes, on the other hand, are millions or billions of times the size of
the sun and are found at the center of most large galaxies, such as the Milky
Way. Even when supermassive black holes are not absorbing matter, scientists
can observe the effects such black holes have on the stars and gases around
them. While stellar mass black holes are more difficult to detect unless they
are in the process of absorbing matter, scientists know more about how they
form than they do about the formation of supermassive black holes.
A simulation of gravitational lensing around a black hole and a galaxy
Many of the properties of black holes are well documented, yet
the formation and growth of supermassive black holes are on the cutting edge of
astrophysics. Black holes usually form out of supernovas – the explosions at
the end of a star's lifespan. In young or middle-aged stars, the energy created
by nuclear fusion counteracts gravity, and keeps a star from collapsing into a
black hole. When a massive star reaches the end of its lifespan (when it has
burned all the fuel inside of it), it explodes in a phenomenon known as a
supernova. Because fusion cannot occur in the remnants of a supernova, when there
is not enough energy for the supernova to counteract gravity, there is nothing
to prevent the remaining matter from collapsing into a dense object, such as a
black hole. By astronomical standards, only supermassive stars have enough
matter to become black holes, so small stars, including our sun, merely compact
into white dwarfs or neutron stars. (Spoiler alert: the sun will eventually
become a white dwarf, so there is no danger of it becoming a black hole.) Scientists
know more about the creation of stellar-mass black holes than about the creation
of supermassive black holes, but
there is a possibility that stellar-mass black holes can grow to a supermassive
size by rapidly consuming the matter around them.
Once a black hole forms, it can continue to grow by
absorbing more and more matter. The following is theoretical. For example, in
binary star systems containing two large stars, the first star to become a
black hole will absorb matter from its companion star until the younger star
vanishes. When black holes are too far from stars to absorb their matter, they
consume the dust and gas floating around them. When two black holes collide, it
has been hypothesized that they merge together to become an even larger black
hole, producing a whopping amount of energy and sending ripples known as gravitational waves through the universe.
A stellar-mass black hole in a binary star system
So far, the only observations of gravitational waves have
been contradicted by other, more detailed observations. However, as pairs of
supermassive black holes at the centers of distant galaxies spiral closer and
closer to each other, the chances are good that we will eventually be able to
observe and study such dramatic black hole growth.
In the upcoming Marin Science Seminar, "Snacking,
Gorging, and Cannibalizing: The Feeding Habits of Black Holes,"
astrophysicist Steve Croft, Ph.D. will discuss how innovative telescope
technologies make it possible to observe the growth of black holes in a new
way, and perhaps, track
disappearances from laundry baskets, tables, and refrigerators once and for all.
For more information, come to the next Marin Science Seminar at Terra Linda High School from 7:30-8:30 p.m. on March 11th, 2015.
Sources:
http://news.discovery.com
http://faculty.mtsac.edu
For urban legends: http://mysticinvestigations.com
http://faculty.mtsac.edu
For urban legends: http://mysticinvestigations.com
Image Credits:
MSS
interns attend and assist with a minimum of 6 science seminars per
academic year (there are 12 per year) during which they meet the
speakers and assist with various logistical duties. Sessions take place
on Wednesday evenings at Terra Linda High School, Room 207, during the
school year. Interns arrive evening of a session at 7 pm and are free to
leave once breakdown is completed (between 8:30 and 9 pm). 
