In about 4 billion years, the Andromeda Galaxy will collide with our own galaxy, the Milky Way. This is not exactly breaking news (assuming anything that will happen billions of years in the future could be considered news). Astronomers have known about the impending collision for decades, many popular stories have discussed it, and a team working with the Hubble Space Telescope even put together pretty illustrations of what the impending conflagration will look like.
But there’s an unexpected twist to the story.
Earlier this week, researchers working on a sky-mapping project called AMIGA reported that the early stages of the Andromeda-Milky Way collision will happen long before the main event. You don’t have to wait 4 billion years to watch a galaxy smash-up, after all. With a little vision enhancement you can see it happening right now…because the Andromeda-Milky Way collision has already begun.
The reason the collision is happening so soon is that the Andromeda Galaxy is much bigger than it appears. So is our Milky Way. And so, it turns out, are almost all of the things in the universe around us. What does it mean when people say that comet C/2019 Y4 (ATLAS) was “half the size of the Sun”? That number refers to the total size of the comet’s tail and coma—which is to say, that is the size of its entire environmental footprint in the solar system.
If you measure other things in the solar system using the same standard, you get some really unfamiliar answers. For instance, how big is the Earth if you measure its environmental footprint in the solar system? Earth’s magnetotail (the elongated bubble created by the interaction of our planet’s magnetic field with the solar wind) can stretch 5 million miles long, perhaps even 10 million miles. See: Earth’s Magnetosphere
So the Earth is 10 times the size of the Sun.
Based on measurements from the Voyager spacecraft, it seems that Jupiter’s magnetotail may extend about 300 million miles beyond the planet. See: http://www.igpp.ucla.edu/people/mkivelson/Publications/279-Ch24.pdf
So Jupiter is 300 times the size of the Sun.
But wait! The Sun itself creates a huge bubble of magnetic plasma around it; that bubble eventually bumps against the material of the interstellar medium. The outer edge of this bubble, called the heliopause, extends about 10 billion miles (give or take) in all directions.
So the Sun is 20,000 times the size of the Sun.
These enormous structures are real and meaningful, but they are not at all what non-scientists mean when we ask how large something is in space. Normally we’re talking about the discrete structure (solid, liquid, gas or plasma) of the object itself. In the case of Comet ATLAS, its solid nucleus was probably less than 1 mile wide before it began fragmenting.
