Which Star Is The Smallest?

Like most of my good blog posts, this started with a question from my daughter: What is the smallest star? As much as I love answering her space questions off the top of my head, sometimes it’s even more fun when I get to say, “I don’t know; let’s find out!”

There are many different types of stars, ranging from the smallest red dwarfs to the largest red super giants. Aside from diameter, stars vary tremendously in temperature, mass, brightness (or luminosity, depending on context), color and lifespan. The Universe is home to an amazing amount of stellar variety.

So back to the question: Which star is the smallest?

Before we answer that, I feel I do need to provide a sort of disclaimer: In the context that we’re going to answer the question, when we refer to “smallest”, we’re referring to the diameter of the star, as in the size of its shape. We’re not talking about how massive (think weight) the star is1, or how bright it appears to us on Earth, but how large it would be if you lined it up next to other stars and compared its size. We’re also going to exclude neutron stars (the remnants of supernova explosions of massive stars), white dwarfs  (the remnants of dead stars that are less massive than the ones that create neutron stars), and brown dwarfs (commonly referred to as ‘failed stars’). One more thing: The Universe is huge and there are trillions of stars in it that we’ve never observed. So, our answer is only going to be based on stars that have actually been observed.

Now, without further ado. The smallest star that we have currently observed is the prosaically-named: 2MASS J0523-1403.2 The size of 2Mass J0523-1403 was listed in a 2013 paper published in The Astronomical Journal. The researchers of that paper (Sergio B. Dieterich, Todd J. Henry, Wei-Chun Jao, Jennifer G. Winters, Altonio D. Hosey, Adric R. Riedel, John P. Subasavage) set out to determine the point when a body becomes a star rather than a brown dwarf (failed star). They took data from existing sky surveys and chose 63 nearby red and brown dwarfs to study. Armed with the luminosity and temperature of these bodies, they calculated their diameters using the Stefan-Boltzmann Law. They then plotted the calculated diameters against the temperatures and were able to discover a theoretical lower limit for the diameter of a star: .086 the diameter of the Sun (roughly the size of Saturn). At that point, sat 2MASS J0523-1403, located about 40 light years from here, in the constellation Lepus.

2MASS J0523-1403 identified in its starfield

2MASS J0523-1403 identified in its starfield – Adapted from Source: Strasbourg Astronomical Data Center

And as I said before, we don’t know that this star is the smallest star in the entire universe but because of this research we know that this star is the size that marks the smallest point a star could be.

The relation between size and temperature at the point where stars end and brown dwarfs begin

The relation between size and temperature at the point where stars end and brown dwarfs begin – Image credit: P. Marenfeld & NOAO/AURA/NSF.

The above graphic shows the relationship between size and temperature, at the point where stars end and brown dwarfs begin. Interesting to note is that there are brown dwarfs that are actually larger than some stars.

8.6% is a small fraction of the Sun’s size, but compared to the Earth our new little star friend is still pretty big. You could stack 9 Earths in a row and they still wouldn’t be quite as wide as 2MASS J0523-1403. 2MASS J0523-1403 is just slightly larger than Saturn, and could easily fit inside Jupiter.

Comparison graphic between the Sun, 2MASS J0523-1403, and Earth.

Comparison graphic between the Sun, 2MASS J0523-1403, and Earth. – Source: TheStarSplitter.com

And there you have it: The smallest star in the Universe (that we know of, but this is as small as they can be, but then again there are trillions of stars we have never seen so it’s likely there’s one slightly smaller, maybe by only a few meters, but this is close enough….).

  1. You might think that the size of the star would be relative to how massive a star is, but this isn’t necessarily the case. Just as a shotput is smaller than a basketball yet has more mass, many stars have a smaller diameter yet more mass than others.
  2. The star gets its name from the sky survey that cataloged it: the Two Micron All Sky Survey (2MASS).
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