February 8 – Big Smalls

Today’s factismal: Most stars aren’t visible because they are too small and too cool.

If you go out at night and look up in the sky, you will probably see lots of stars out there. Though a couple of them are bright enough to look like they have color (e.g., Betelgeuse, Aldebaran) most of them just look like little white dots in the black velvet of night. But what you may not realize is that you are just seeing part of the picture. That’s because when you look at the night sky you can only see those stars that are big enough and bright enough to be seen – and they are just a small fraction of the stars out there!

Most stars are much smaller than the Sun (a.k.a., "Sol").

Most stars are much smaller than the Sun (a.k.a., “Sol”).

How small? Well, astronomers aren’t certain but they agree that at least 70% of all stars out there are “dwarf stars” with less than half the mass of the Sun (known as “Sol” in astronomy circles); some think that it may be as much as 85%! These stars range in size from about ten to five hundred times the mass of Jupiter (to an astronomer “size” always means “mass”). Because they are so small, they burn hydrogen very slowly. Eventually, they will run out of hydrogen and turn into white dwarfs, in a mere 500 billion years or so. In the meantime, these small stars, like Epsilon Indi BB, give off very little visible light and most of their “shining” is done in the infrared (“heat”) portion of the spectrum.

The hotter something is, the "blue-er" its color is. This is true of stars as well as planets.

The hotter something is, the “blue-er” its color is. This is true of stars as well as planets.

The rare big stars, like VY Canis Majoris, have a lot of fuel but they burn through it fast becoming super-hot and glowing a bright blue that slowly changes to red as they lose mass and cool off slightly. (And I do mean “slightly”; at its start, a hypergiant like VY Canis Majoris burns about 500,000 times as brightly as the Sun but it gradually drops to a mere 200,000 times before exploding into a nova.)

Bigger stars are much more massive and much hotter; that makes them brighter and bluer than small stars

Bigger stars are much more massive and much hotter; that makes them brighter and bluer than small stars

So there are a lot of small and dim stars that aren’t visible to the naked eye; they are the candle to the rare big star’s searchlight. But the thing is that there are a lot more candles than there are searchlights. For every star you see at night, there are at least 100 more that are too small to be seen.

A look at the Milky Way using ultraviolet and infrared light (Image courtesy NASA)

A look at the Milky Way using ultraviolet and infrared light
(Image courtesy NRAO)

Big big or small, astronomers study them all. And here’s an image of what those stars look like when we peek at them using ultraviolet and infrared light. “Ordinary” stars that we can see at night glow a greenish white, where newborn and small stars heat up the surrounding dust and make it glow a bright violet for infrared and a startling purple for radio waves. If you’d like to see more pictures like this, and maybe help find hot stars in other galaxies, then point your scope to:
https://www.zooniverse.org/projects/bridgetk/hubbles-hot-stars

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