We tend to regard our sun as an extremely large
object.
About a million Earths could fit inside of it, so it’s quite a reasonable statement
from our perspective.
But is the sun very large as far as stars go?
Actually, it isn’t.
So just how big and small can stars get?
Earlier in this astronomy series we learned about the stellar life cycle.
We talked about what happens to low mass stars like our own sun, which eventually become
white dwarfs, and we talked about what happens to high mass stars, which become neutron stars,
or even black holes above a certain mass.
So clearly, the mass of a star is the primary variable in determining its behavior and the
remnant it leaves behind.
But what is the precise range of masses, and by extension range of sizes, that a star can
exhibit?
There are so many stars out there that we can see, even just in our own galaxy, let
alone all the other billions of galaxies.
What is the smallest that a star can possibly be, and what is the biggest that a star can
possibly be?
Do these questions even have definitive answers?
Let’s take a closer look now.
The first question is much easier to answer.
There is a limit to how small a star can be, or rather, we know the minimum mass that is
required for a gas cloud such that the inward gravitational pressure is sufficient to trigger
nuclear fusion, which is the process that defines every star.
Since we are going to be dealing with masses significantly less than that of our sun, we
will no longer speak in terms of solar masses, which are multiples of our sun’s mass.
We will speak in terms of Jupiter masses, or multiples of Jupiter’s mass, since Jupiter
is a familiar planet and is less massive than any star.
The smallest type of star that is sufficiently massive so as to trigger the type of nuclear
fusion we see in familiar stars is a red dwarf star.
These begin at around 80 Jupiter masses, or 80 times the mass of Jupiter, which is about
8% of the mass of our sun.
These are compact objects, that despite their much greater mass are actually only slightly
larger than Jupiter itself due to their higher densities, sometimes as little as only 20%
larger than Jupiter.
An example of a red dwarf star would be Proxima Centauri, which we discussed when exploring
the Alpha Centauri system.
What happens if we go lower than this limit of 80 Jupiter masses?
Then we can get something called a brown dwarf.
These are technically not stars, and are thus referred to as sub-stellar objects.
But they are fascinating all the same.
They range in mass from around 13 Jupiter masses to the upper limit of around 80 where
they would be able to trigger fusion and qualify as red dwarf stars.
Because of their lower mass, inward gravitational pressure is not sufficient to sustain fusion
of ordinary hydrogen in their cores.
However, we believe they may be able to fuse heavy hydrogen, or deuterium, as well as lithium,
if their mass is on the upper end of this spectrum, above 65 Jupiter masses or so.
But the insufficient presence of these nuclides combined with the low mass of these objects
means they do not ignite, thus brown dwarfs do not glow like stars do.
We have been able to identify hundreds of brown dwarfs in our galaxy, and have even
been able to identify some with planetary systems.
So that covers the lower mass limit for stars, and an introduction to substellar objects.
The second question, identifying the upper limit for stars, is not so simple.
There is not necessarily an upper limit to how big a star can be.
Whatever mass of gas and dust happens to have accumulated to form the star, that’s the
mass of the star.
Of course, larger and larger stars become more and more statistically improbable, which
is why there is a range of masses we tend to commonly see when we look at stars around
the galaxy.
But every once in a while, a particularly enormous star will form.
It has been proposed with mathematical basis that there actually is an upper limit for
a star of around 150 solar masses, and we have indeed found plenty of stars that approach
this limit, but we have even found a few that seem to exceed it, so it’s not yet a firm
value.
But at any rate, regarding this proposed 150 solar mass limit, what size does this correspond
to for a star?
Just how big can stars get?
This is a case where citing numbers will not do justice to the true immensity of these
objects.
Let’s instead witness a series of size comparisons, starting with the earth, where objects are
shown precisely to scale.
We will zoom out from the scale of the earth until we get to one of the biggest known stars.
Prepare yourself to be astonished at just how much bigger than our sun stars can really get.
Here we go.
It is astounding to think that once this visual comparison is complete, we can no longer even
see our sun.
That’s how significant the disparity is.
We stopped at Canis Majoris, a red hypergiant star which is so huge, that if we replaced
our star with this one, it would engulf every planet in the solar system up to Saturn.
And yet, this is not even the largest star that we know of.
The current title belongs to UY Scuti, which has a radius that is even 20% larger than
Canis Majoris.
Will we ever find even larger stars?
How large is the largest star in the entire universe?
Is there some fundamental principle we can discover that can elucidate the formation
of these hypergiant stars?
Perhaps one day we might learn the answers to these questions.