Dwarfs in the Cosmos

What They Say About Themselves

Last week, I was a guest speaker at StarFest, a Denver-based Sci-Fi Con that regularly draws several thousand folks. I usually talk about astronomy topics and this year was no exception.  My main talk was about Pluto (and its dwarf planet status), and I also participated in a panel discussion about hoaxes — astronomical, planetary, and paranormal.  The Pluto talk went really well, and the crowd was really into the whole story of  its meandering progress through planetary status.

Pluto is a dwarf planet, a status that is not a bad thing. I’ve said before that giving it the “dwarf” nomenclature tells us something about its evolution, its place in the solar system hierarchy, and even gives us a clue about our own understanding (or lack thereof) about the details of Pluto’s composition and history.  It’s not terribly different from looking at a dwarf galaxy and wondering what that “dwarf-ness” means. A dwarf galaxy is NOT a wanna-be galaxy. It’s not a consolation prize. It’s a status that helps astronomers understand the evolutionary state of such collections of stars, as well as other characteristics such as the metal content of their constituent stars (and the materials those stars formed from). Dwarf galaxies are small, usually containing up to a few billion stars, and they are implicated in the evolutionary process that forms larger galaxies.  Right now, dwarf spheroidal galaxies are being sucked into our own Milky Way Galaxy or are orbiting nearby.

There are also dwarf stars which comprise the main sequence (a classification scheme that lumps stars together by their color and brightness). The Sun is a dwarf star, for example.  There are red dwarfs, yellow dwarfs (the Sun), blue dwarfs, white dwarfs, and so on.  The most fascinating ones (to me, anyway) are the brown dwarfs.  These are not actually stars like the Sun, but are really sub-stellar objects. They’re not massive enough to fuse hydrogen into helium as most other stars do, but they do have enough  mass to fuse deuterium (an isotope of hydrogen) in their cores.  The masses of brown dwarfs range from about 0.08 solar masses and more than about 13 Jupiter masses.

Where do brown dwarfs come from?  Their origins are still really not well understood. Whereas astronomers can trace the beginnings of dwarf galaxies in the early universe, and we think we kind of know where dwarf planets come from in the evolutionary history of the solar system, the formation of brown dwarf substellar objects is still a hot topic in astronomy.  Some astronomers think that they are born much like stars are born, through the collapse of interstellar gas clouds. Low-mass clouds might be yielding l0w-mass objects.  Others suspect that brown dwarfs form in larger clouds along with stars of various masses, and that the brown dwarfs are ejected from their birth places in gravitational interactions with their higher-mass siblings.

Only a few hundred brown dwarfs have actually been observed, so as astronomers find more of these objects that are too cool to be stars and too hot to be planets, they should get a better handle on the environments in which they formed.  And that will tell them more about brown dwarf formation throughout the history of the cosmos. So, as with Pluto — which is going to help planetary scientists understand the worlds of the extreme outer solar system — brown dwarfs may help shed light (no pun intended) on what is still a little-understood population of objects that form in interstellar gas clouds.

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