CSCI 2030 - Black Holes

The Life of a Black Hole

From Birth to Death
By: Nathan Khazam

Figure 1: An artist’s rendition of a black hole

A Brief Overview

Black holes are an astrophysical phenomenon where extremely large amounts of matter
are compressed into a space so small that it collapses in on itself, creating a void-like
structure that slowly absorbs the matter around it. These structures have immense
gravitational pull and are often found at the center of galaxies keeping stars from flying out
into the cold emptiness of intergalactic space.

Birth of a Black Hole

Black holes often form from the death of a star. Near the end of their lives, stars will grow
and expand outwards up to 100 times their original size before collapsing in on themselves,
becoming a white dwarf, or in some cases, exploding as a supernova. After going
supernova, the star can be left in 1 of 2 states: a neutron star or a black hole.

Figure 2: Artist’s rendition of a supernova

Supernovae (plural for supernova) can only occur when a massive star dies and creates the
perfect conditions to make a black hole. Right before a star goes supernova, it collapses in
on itself, causing immense amounts of pressure. Couple this with the massive amount of
mass in the star and you have a good chance of creating a black hole. Once the supernova

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occurs and the gas and dust have cleared, we can see if there was enough mass and
pressure to create a black hole. If there wasn’t enough pressure or mass, a neutron star will
remain.

Neutron Stars

However, just because a black hole didn’t form initially doesn’t mean it won’t form later on.
Neutron stars are just behind black holes as the most dense objects in the universe. This
means they only require a little push to break the Tolman-Oppenheimer-Volkoff limit and
become a black hole. This limit is the theoretical amount of mass a neutron star would
need to collapse into a black hole, ranging from 2.2 to 2.9 solar masses (1 solar mass is the
mass of our sun).

To achieve this mass requirement, neutron stars have a couple of main options. The first
option can occur in a binary star system (a solar system with 2 central stars instead of 1 like
our solar system) where the neutron star siphons off material from its partner star to gain
mass. We have detected this through the X-ray emissions given off when gas makes contact
with the surface of the neutron star. If enough material is taken from its partner, the
neutron star will collapse into a black hole. The second option neutron stars have to gain
enough mass is mergers. Neutron stars can combine with one another and create even
more massive neutron stars, and if their combined mass exceeds the
Tolman-Oppenheimer-Volkoff, they will collapse and form a black hole.

White Dwarfs

White dwarfs, while they cannot become black holes easily, can eventually turn into black
holes if given enough time. First, the white dwarf must turn into a neutron star. This can be
achieved if it exceeds the Chandrasekhar limit which states the maximum possible mass of
a white dwarf is 1.4 solar masses. If it exceeds this amount, the white dwarf will do 1 of 2
things: explode in a supernova-like explosion or collapse and create a neutron star. If it
collapses into a neutron star, it follows the same path as above for neutron stars: gathering
enough mass to turn into a black hole. Otherwise, the star explodes in a type 1a supernova,
a style of supernova created by white dwarfs.

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Growing a Black Hole
Black holes can grow far larger than they started. They grow in the same style as neutron
stars: through the accretion of materials or black hole mergers. Let's start with accretion
since this is how we photographed the first black hole.

Material can be added to a black hole through its accretion disk, a plane of material
surrounding the black hole that spins faster and faster as it approaches the event horizon
(the point of no return) of the black hole. This may seem confusing to some since it is often
thought that material can just “fall” into the black hole. However, this is not the case since a
black hole manipulates the space around it, causing the material nearby to gain a lot of
energy and orbit around the black hole, creating the accretion disk. For the material to
actually “fall” into the black hole, it needs to lose enough energy to break its orbit around
the black hole and get absorbed by it. This energy is let off through friction between
materials in the accretion disk, resulting in many different wavelengths of light, from visible
up to X-ray (and maybe beyond). Through this radiation of different wavelengths of light,
we were able to take the first picture of a black hole.

The second way black holes grow is through merger events. Black holes, just like neutron
stars, can merge and create a larger black hole. However, black holes do not just have to
merge with other black holes, they can also merge with neutron stars. These merger events
have very noticeable effects on the space and gravity around them, often sending out
massive gravity waves across the universe when a merge occurs. These waves require a
huge amount of energy to be created, and for a split second, are more powerful than all of
the stars in the visible universe combined! This often leaves a black hole with less mass
than we expect after a merger event due to how much mass is converted into energy and
used to propagate the gravitational waves.

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Figure 3: Image of a computer simulation where 2 black holes are preparing to merge.

Death of a Black Hole

While black holes may seem endless, they could eventually die, although we have not
witnessed the death of any black holes. The death of a black hole would be due to Hawking
radiation, a theoretical phenomenon based on quantum mechanics. It uses the fact that
particles can randomly pop into existence as pairs, with 1 matter particle and 1 anit-matter
particle, which almost immediately collide and annihilate each other. Near a black hole, the
anti-matter particle could be absorbed into the black hole, causing it to lose mass because
the anti-matter particle would annihilate some of the matter within the black hole. This
would cause the black hole to slowly fizzle out and the regular matter particle of the broken
pair be emitted as radiation. Over an insanely large amount of time, this could cause a
black hole to slowly shrink until it eventually disappears. However, these time scales are far
larger than we could ever comprehend so it is unlikely a black hole has met this fate yet.

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Figure 4: Diagram of Hawking radiation in action

Closing Thoughts
Black holes are incredible feats of nature, pushing the boundaries of what we thought was
possible. They may unlock secrets of the universe we never imagined and be the key to
future civilizations. Until then, we will continue to study them and discover more about
these hidden giants of the universe.

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Citations:

Image links:
● Figure 1:
https://www.colorado.edu/today/sites/default/files/styles/hero/public/article-image/
729665main_a-blackholeart-pia16695_full.jpeg?itok=izF9X2I8
● Figure 2: https://spaceplace.nasa.gov/supernova/en/supernova1.en.jpg
● Figure 3:
https://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/BBH_gravitational_le
nsing_of_gw150914.webm/1200px--BBH_gravitational_lensing_of_gw150914.webm.j
pg
● Figure 4:
https://c02.purpledshub.com/uploads/sites/48/2021/07/Hawking-radiation-33a5ec1.
jpg

Research links:
● Star lifecycle: https://www.schoolsobservatory.org/learn/astro/stars/cycle
● Neutron stars:
https://en.wikipedia.org/wiki/Neutron_star#:~:text=If%20the%20remnant%20star%2
0has,and%20form%20a%20black%20hole.
● Chandrasekhar limit:
https://en.wikipedia.org/wiki/Chandrasekhar_limit#:~:text=The%20currently%20acce
pted%20value%20of,resist%20collapse%20through%20thermal%20pressure.
● Hawking radiation:
https://en.wikipedia.org/wiki/Hawking_radiation#:~:text=Hawking%20radiation%20is
%20the%20theoretical,event%20horizon%2C%20it%20cannot%20escape.
● And: https://www.youtube.com/watch?v=fqnmUNzn5N8
● Any information I have not linked here is from class (either memorized/remembered
or gathered from slides)