Watch Out: Things in the Universe are Bigger than They Appear

In about 4 billion years, the Andromeda Galaxy will collide with our galaxy, the Milky Way, unleashing a brilliant burst of star formation. This is not exactly breaking news (assuming anything that will happen billions of years in the future could be considered “news” of any kind). 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. 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. The galaxy’s bright, starry disk is about 120,000 light years in diameter, making it slightly larger than the Milky Way. In recent years, deep studies of Andromeda using the giant Keck telescopes in Hawaii revealed an extended population of stars that stretched the galaxy’s total diameter to about 200,000 light years. That’s nothing compared to the latest study, however.

Nicolas Lehner of the University of Notre Dame and his colleagues determined that Andromeda’s halo—its outer envelope of extremely thin, hot gas, kind of like a galactic atmosphere—keeps going up to 2 million light years away from its center. The AMIGA team also determined that the halo is divided into two layers, an inner one that is stirred by supernova explosions and an outer one that is much smoother and calmer. In future eons, gas from the halo will slowly rain down onto Andromeda, helping to form future generations of stars.

The Milky Way is similar to Andromeda in size and structure, and our galaxy’s halo is almost certainly similar as well. Heart to heart, Andromeda lies 2.5 million light years away from the Milky Way. If each galaxy has a halo that extends 1-2 million light years in every direction, then it’s inevitable that the two are touching. The great merger is underway.

If you could view the full extent of the Andromeda Galaxy, it would appear shockingly large in the sky. The galaxy’s disk appears as a fuzzy smudge about one-quarter of a degree wide (about half the width of the full Moon) to the naked eye, and just slightly larger through binoculars. Long-exposure images taken through telescopes reveal the faint, outer spiral arms that measure a bit over three degrees across.

Using the light from 43 distant quasars, astronomers were able to map the enormous halo that surrounds the Andromeda Galaxy, and other galaxies like it all across the cosmos. (Credit: NASA/ESA/E. Wheatley)

Using the light from 43 distant quasars, astronomers were able to map the enormous halo that surrounds the Andromeda Galaxy, and other galaxies like it all across the cosmos. (Credit: NASA/ESA/E. Wheatley)

Andromeda’s halo is not directly visible even thorough the biggest telescopes. Lehner had to study it in silhouette, by watching the light from 43 distant quasars shining through different portions of the halo. But if your eyes could pick out the diffuse glow from all of that hot gas bubbling and burbling around Andromeda, it would appear more than 60 degrees wide: Seen in full, the Andromeda Galaxy stretches more than one third of the way across the sky!

Human intuition tells you that distant astronomical objects should appear smaller in the sky than nearby objects do. But intuition tends to be a lousy guide when dealing with the unfamiliar scales and structures of the distant universe. In our solar system, only the Sun, Moon, and the occasional comet have any visible size to the naked eye. The planets are just dots. Put your eye to a telescope and you can expand the planets into small disks, but moons and asteroids remain dots. Move beyond our solar system to look at other stars, and you get more dots.

Go even farther out, though, and things begin to change. As the distance scale increases, the scale of the objects out there increases even faster. Our Milky Way galaxy is visible as a band of light that circles the entire sky. It’s a startling when you think of it that way: The remote arms of our galaxy loom much larger in the sky than any visible structure within our solar system.

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:

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.

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