What are stars?

7 min read

Key facts

Nearest star	The Sun, about 93 million miles away
Next nearest star	Proxima Centauri, about 4.2 light-years
Sun's surface temperature	About 10,000°F (5,772 K)
Sun's age	About 4.6 billion years
Brightest star in the night sky	Sirius, magnitude −1.46, about 8.6 light-years away
One light-year	About 5.9 trillion miles
Stars visible to the naked eye	About 5,000 to 9,000 across the whole sky
Stars in the Milky Way	About 100 to 400 billion

In short

A star is a huge ball of hot gas that makes its own light by fusing hydrogen into helium in its core.
The Sun is an ordinary, middle-sized star; it only looks special because it is so close.
A star's color tells you its surface temperature: blue stars are hottest, red stars are coolest.
How bright a star looks depends on both its real power and its distance from us.
Small stars end as white dwarfs; the most massive explode as supernovae and leave neutron stars or black holes.

What is a star?

A star is an enormous ball of gas, mostly hydrogen and helium, held together by its own gravity. Gravity squeezes the gas inward, and the center becomes so hot and dense that atoms start to merge.

This merging is called nuclear fusion: four hydrogen nuclei combine, step by step, into one helium nucleus. A tiny bit of mass disappears in the process and turns into a lot of energy, as Einstein's E = mc² describes.

That energy pushes outward and balances the pull of gravity. As long as the star has fuel, these two forces stay in a steady standoff, and the star shines for millions or billions of years.

The Sun, our typical star

The Sun is the only star we can study up close, and it turns out to be fairly average. It is a yellow-white main-sequence star of type G, about 4.6 billion years old and roughly halfway through its stable life.

Its core reaches about 27 million°F (around 15 million K), while its visible surface is about 10,000°F. Every second it converts about 4 million tons of matter into energy.

Many stars are much smaller and dimmer than the Sun, and a few are far larger. In fact, most stars in our galaxy are red dwarfs, too faint to see without a telescope.

Colors and temperatures

Look closely at the night sky and you will notice that stars are not all white. Some are bluish, some yellow, some orange or red. The color is a thermometer: hotter objects glow bluer, cooler ones glow redder, just like a heated piece of metal.

Astronomers sort stars into spectral types using the letters O, B, A, F, G, K and M, from hottest to coolest. O stars are blue and can top 50,000°F (about 30,000 K or more). M stars are red and sit around 4,000 to 6,000°F. The Sun, a G star, falls in the middle.

Students often remember the order with the phrase "Oh Be A Fine Girl/Guy, Kiss Me."

Brightness and distance

A star that looks bright might be truly powerful, or it might just be nearby. Astronomers keep these apart with two ideas. Apparent magnitude is how bright a star looks from Earth. Absolute magnitude is how bright it would look from a standard distance of about 32.6 light-years.

The magnitude scale runs backward: smaller numbers mean brighter stars. A difference of 5 magnitudes equals a factor of 100 in brightness. The faintest stars you can see from a dark site are around magnitude 6, while Sirius shines at −1.46.

Distances are so large that miles become awkward. A light-year is the distance light travels in one year, about 5.9 trillion miles. When you look at a star 500 light-years away, you see light that left it 500 years ago.

How stars live and die

Stars are born inside nebulae, cold clouds of gas and dust. A dense clump collapses under gravity, heats up, and ignites fusion. The new star then settles onto the main sequence, the long, calm stage where it fuses hydrogen. The Sun is there now.

When a Sun-like star runs low on hydrogen in its core, its outer layers swell and cool, and it becomes a red giant. Later it gently sheds those layers into space as a glowing shell called a planetary nebula. What remains is a white dwarf: a hot core about the size of Earth that slowly cools over billions of years.

Stars born with more than about eight times the Sun's mass live fast and die violently. After fusing heavier and heavier elements, their core collapses in a fraction of a second, and the star blows apart as a supernova. The leftover core becomes a neutron star, a city-sized ball denser than an atomic nucleus, or, for the heaviest stars, a black hole.

Supernovae scatter elements such as oxygen, calcium and iron into space. Those atoms later end up in new stars, planets and people.

Why do stars twinkle?

Stars themselves do not flicker. The twinkling comes from Earth's atmosphere, where pockets of warm and cool air bend starlight slightly differently from moment to moment.

Because stars are so far away, they appear as tiny points, so these small shifts make them dance and change color. Planets look like small disks instead, and the effect averages out, which is why planets usually shine more steadily. Twinkling is strongest near the horizon, where light passes through more air.

Notable stars to find

Sirius, in Canis Major, is the brightest star in the night sky. Look for it below and to the left of Orion's Belt on winter evenings. It is about 8.6 light-years away.

Betelgeuse marks Orion's shoulder and glows a clear orange-red. It is a red supergiant, a massive star near the end of its life, roughly 550 light-years away and expected to explode as a supernova sometime in the next 100,000 years or so.

Vega, in Lyra, is a brilliant blue-white star high overhead on summer evenings in the Northern Hemisphere, about 25 light-years away.

Polaris, the North Star, is not especially bright, but it stays almost fixed above the North Pole while the sky turns around it. Find it by following the two end stars of the Big Dipper's bowl. It lies about 430 light-years away.