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.
