Do Stars Within A Galaxy Touch One Another?

Many body problems with contact forces, like grains of sand on a beach, are quite hard to solve satisfactorily with a computer. Is there a similar problem with galaxies where the stars are in gravitational contact, and could that be a factor in the problem with the galactic rotation curves not turning out as expected?
The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird’s-eye view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor. Though the galaxy is over 2 million light-years away, the Hubble telescope is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy’s pancake-shaped disk. It’s like photographing a beach and resolving individual grains of sand. Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird’s-eye view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor. Though the galaxy is over 2 million light-years away, the Hubble telescope is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy’s pancake-shaped disk. It’s like photographing a beach and resolving individual grains of sand. Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

Originally posted on Forbes!

Grains of sand are still pretty tricky to model as convincing-looking sand. And as much as we use grains of sand as a metaphor for the number of stars in a galaxy, stars and sand can be a pretty poor comparison if you take the metaphor too far. Galaxies may appear to be solid objects in our skies, with tightly packed clusters of stars, but in fact, galaxies contain huge amounts of empty space. The problem is that for most galaxies, we don’t have the observing power required to distinguish the individual stars, and so the stellar multitudes blur their light together.

If we start to look around within our own galaxy - for instance around our own solar system - we can get a handle on just how much empty space we’re dealing with. Our Sun lives about two thirds of the way out from the center of our galaxy, so while we’re certainly not in the densest part of a galaxy, we’re by no means in a particularly vacant neighborhood either. The nearest stars to us, Alpha Centauri and Proxima Centauri, are 4 light years away. And looking at the diagram, you can see that that 4 light year distance is about as closely packed as the stars get in our neck of the galactic woods.

A diagram of Earth’s location in the Universe, in the Solar Interstellar Neighborhood. Image credit: Andrew Z. Colvin, CC A-SA 3.0

A diagram of Earth’s location in the Universe, in the Solar Interstellar Neighborhood. Image credit: Andrew Z. Colvin, CC A-SA 3.0

Now, Alpha Centauri raises an interesting point - Alpha Centauri is actually two stars. This is not uncommon in a galaxy. The two stars orbit each other as a binary system, and those two stars together travel around the galaxy’s core. However, the two stars are really not very close to each other at all. The two stars in Alpha Centauri are typically somewhere between 1 billion miles and three billion miles apart.  That’s enough space to fit at least half our solar system between the two stars. At their narrowest, the two stars are separated by the distance between the Sun and Saturn.

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Image credit: Wikimedia user Skatebiker, CC A-SA 3.0

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Image credit: Wikimedia user Skatebiker, CC A-SA 3.0

Contact binaries between stars - where the surfaces of the two stars are actually touching - do exist. Stars are not solid objects, like grains of sand, so instead of wobbling unstably against each other’s surfaces, their atmospheres are pulled together into an irregular, double-lobed star blob. Contact binary stars are relatively rare. Not because it’s so hard to have two stars so close together, but because this is a very unstable arrangement, and is likely to rapidly morph into a single star, or to violently detonate itself in some kind of supernova.

This artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either with the formation of a single giant star or as a future binary black hole. Image credit: ESO/L. Calçada

This artist’s impression shows VFTS 352 — the hottest and most massive double star system to date where the two components are in contact and sharing material. The two stars in this extreme system lie about 160 000 light-years from Earth in the Large Magellanic Cloud. This intriguing system could be heading for a dramatic end, either with the formation of a single giant star or as a future binary black hole. Image credit: ESO/L. Calçada

The main force governing the behavior and orbits of stars around the galaxy’s core is plain old gravity.  Your typical wide binary stars do not dominate the population of stars in the galaxy, and contact binaries, as the most extreme type of binary, are even less common, and not expected to stick around long enough to really change the galaxy as a whole. If the orbits of stars around the center of the galaxy look weird, and we can rule out other stars causing perturbations to their behaviors, the rotation curves of galaxies must look odd for another reason. Dark matter surrounding the galaxies, adding additional mass to the galaxy, and changing the shape of the gravitational distortion that each galaxy sits within, fits our requirements the best.

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