What Makes a Shooting Star Glow, and Is It Really Burning?

1. A Shooting Star Is a Grain of Dust, Not a Star

NASA calls the objects behind the streaks meteoroids: space rocks anywhere from dust grains up to small asteroids. When one enters Earth's atmosphere at high speed and burns up, the flash is a meteor, and NASA notes that this is also what people call a "shooting star"[1]. The American Meteor Society adds that no actual star is involved. If a piece survives the trip and lands, it is a meteorite, but most of the particles behind meteor showers do not make it[2].

Many visible meteors come from particles between the size of a grain of sand and a small pebble[2]. Japan's National Astronomical Observatory (NAOJ) gives a similar range, from about 1 millimeter to a few centimeters[3]. A typical streak in a research study, described below, came from dust only 0.1 to 1 millimeter across[4]. Something that small makes a streak you can see from the ground, which already hints that its speed matters more than its size.

2. Why Does a Speck of Dust Glow? Fast Collisions With Thin Air

Start with the speed. Meteoroids hit the atmosphere at roughly 11 to 72 kilometers per second, or about 25,000 to 160,000 miles per hour, and the American Meteor Society notes that this speed lets even a small particle release a lot of energy[2].

Now the glow. NAOJ says the dust collides hard with the air, becomes very hot, and turns to gas, while the air and the vaporized dust give off light[3]. The team that measured meteors with the Subaru Telescope gives a more detailed picture. Above about 100 kilometers the air is extremely thin, so the dust grain simply scatters the nitrogen and oxygen atoms and molecules in its way. Those knocked-away particles bump into more air particles around them, which heats the region the dust passes through, and the heated atoms give off light[4].

So is it "burning"? Popular explanations, including NASA's, say meteors "burn up"[1], and that is fine as everyday language. But the research description we read talks about collisions and heated gas, not about a flame[4]. You may also hear that the dust glows from rubbing against the air. The two sources that explain the glow in detail[3][4] speak of collisions, not friction, and we could not tell from them whether the two words describe the same thing. What the sources do agree on is the main point: a very fast hit on thin air makes things hot enough to shine.

3. How High Up Does a Meteor Glow, and Why Does the Height Change?

Most visible meteors appear roughly 80 to 120 kilometers (50 to 75 miles) above the ground, though very fast ones can show up higher and slower, brighter ones can reach farther down[2]. That is far above the cruising height of a passenger jet, which is around 10 kilometers.

EarthSky gives a comparison. Perseid meteors, which arrive at about 61 kilometers per second, are already glowing around 100 kilometers up. The much slower Draconids, at about 23 kilometers per second, go down to about 65 kilometers before they heat up enough to glow[5]. In these two cases, the faster dust lights up higher, and EarthSky says the height where a meteor starts to glow depends on its arrival speed.

The same page points out that the length of a streak depends mostly on the angle of entry. A dust grain that comes in at a low angle heats more slowly and cuts a longer path across the sky than one that plunges steeply[5].

4. How Wide Is the Glowing Streak? A Few Millimeters

A meteor can cross a big part of the sky, so you might guess the glowing path is thick. In 2007, a Japanese team reported it is not. While photographing the Andromeda Galaxy with the Subaru Telescope in August 2004, they noticed streaks in the images. In about 19 hours of observing, there were 55 tracks, and 13 turned out to be meteors, mostly random ones rather than part of a meteor shower[4].

Because the telescope was focused on very distant objects, the meteors, which are so much closer, looked blurry, so the team could not simply measure their width in the pictures. Instead they counted the faint light given off by oxygen atoms that had been hit, which tells how many collisions happened, and worked out the width of the path from there[4]. Their result: the glowing region of a typical meteor is only a few millimeters across, about ten times the width of the dust grain. Earlier observations had only shown that it was narrower than 1 meter[4].

This applies to the typical, ordinary meteors in that study, with dust 0.1 to 1 millimeter across at about 110 kilometers up, not to every meteor[4]. Still, it is a striking picture: a line of light stretching across the sky, produced by a thread of hot gas thinner than a pencil.

5. Where Does the Dust Come From, and How Much Arrives Each Day?

Dust keeps arriving. NASA estimates that about 48.5 tons (44,000 kilograms) of meteoritic material falls on Earth every day, and that several meteors per hour can usually be seen on any clear night[1]. Random meteors not tied to a known shower are called sporadic meteors, and they tend to be more numerous before dawn than in the early evening[2].

A meteor shower happens when Earth passes through a stream of debris left behind by a comet. NAOJ explains that comets leave clouds of dust along their orbits, and when Earth crosses one, the grains arrive together, all from the same direction[3]. The Perseids in August, for example, are tiny pieces of comet Swift-Tuttle, which swings by the Sun about every 135 years[1]. Most comet debris is between a grain of sand and a pea in size and burns up before reaching the ground[1].

6. How to Watch Shooting Stars Safely

You do not need a telescope. The American Meteor Society says meteors are best watched with the unaided eye, since binoculars and telescopes narrow your view. Pick a dark spot away from lights, look at a clear part of the sky about halfway between the horizon and straight overhead, and give your eyes at least 20 to 30 minutes to adjust. Phone screens spoil that adjustment, so if you need a light, use a dim red one[2].

Go with an adult, choose a safe place such as a yard or a park, stay away from roads, and dress warmly. The meteors themselves are no danger, because the particles break up high above you[2]. When you see one, try counting "one-Mississippi." It will probably be over before you finish. How many you see depends on the night, the Moon, and the clouds.

Some questions are left for you to chase. The Subaru study looked at one kind of meteor, so how wide is the glowing path for brighter, bigger ones? And why do sources describe the heating in different words? The pages listed below are the best places to start.

Sources

Revision History