The universe is so large that ordinary numbers lose their meaning. A trip to the Moon would take a few days in a spacecraft. A trip to the nearest star would take tens of thousands of years with today's technology. And the nearest star is just next door compared with the galaxies scattered across the observable universe. How do astronomers measure such enormous distances, and how big is the universe really? This explainer takes you on a journey outward, step by step, from our own neighborhood to the edge of what we can see.
Units for cosmic distances
Kilometers and miles quickly become impractical in space. Astronomers use larger units:
- Astronomical unit (AU): The average distance from Earth to the Sun, about 93 million miles (150 million kilometers). Useful within the solar system.
- Light-year: The distance light travels in one year, about 5.9 trillion miles (9.46 trillion kilometers). Despite its name, it is a unit of distance, not time.
- Parsec: About 3.26 light-years, a unit based on the parallax method of measuring distances, explained below. Astronomers often use kiloparsecs and megaparsecs.
Because light takes time to travel, looking far into space also means looking back in time. We see the Sun as it was about 8 minutes ago, the nearest star as it was about 4 years ago and distant galaxies as they were billions of years ago.
A journey outward
Earth and the Moon
The Moon is about 239,000 miles (384,000 kilometers) from Earth on average. Light covers this distance in about 1.3 seconds. The Apollo astronauts took about three days to get there.
The solar system
The Sun is about 8 light-minutes away. Neptune, the outermost planet, is about 4 light-hours away. NASA's Voyager 1 spacecraft, launched in 1977, is the most distant human-made object, more than 160 AU from Earth, and its radio signals take nearly a day to reach us.
The nearest stars
The nearest star to the Sun is Proxima Centauri, about 4.24 light-years away, part of the Alpha Centauri system. At the speed of Voyager 1, about 38,000 miles per hour (61,000 kilometers per hour), it would take roughly 75,000 years to get there.
The Milky Way
The Sun is one of an estimated 100 to 400 billion stars in our galaxy, the Milky Way. It is a barred spiral galaxy about 100,000 light-years across. The Sun lies about 26,000 light-years from the galactic center, in one of the spiral arms, and takes about 230 million years to orbit the center once.
The Local Group
The Milky Way belongs to a small group of more than 50 galaxies called the Local Group. Its largest member is the Andromeda Galaxy, about 2.5 million light-years away, the most distant object easily visible to the naked eye from a dark site. Andromeda and the Milky Way are moving toward each other and may merge in several billion years.
Clusters and superclusters
The Local Group is part of the Virgo Supercluster, which is part of an even larger structure called Laniakea, spanning about 500 million light-years and containing around 100,000 galaxies. On the largest scales, galaxies are arranged in a vast cosmic web of filaments and walls surrounding enormous voids.
The observable universe
The observable universe is the region from which light has had time to reach us since the Big Bang, about 13.8 billion years ago. It contains an estimated two trillion galaxies, according to some estimates, although more recent studies suggest a few hundred billion galaxies are bright enough to count. Its diameter is about 93 billion light-years.
Why is the observable universe 93 billion light-years across?
This number surprises many people. If the universe is 13.8 billion years old and nothing travels faster than light, shouldn't we only be able to see 13.8 billion light-years in each direction?
The answer is that space itself has been expanding while the light traveled. Light from the most distant galaxies left them long ago when they were much closer. As the light traveled toward us, the space between us and those galaxies stretched. Today, those regions are much farther away, about 46 billion light-years in every direction. That gives a diameter of about 93 billion light-years.
The expansion of space does not mean galaxies are moving through space faster than light; rather, space itself is stretching. Einstein's rule that nothing travels faster than light applies to motion through space, not to the expansion of space itself.
How astronomers measure distances
No single method can measure all distances in the universe. Instead, astronomers use a series of methods, each calibrated with the one before. This is known as the cosmic distance ladder.
Radar and lasers
Within the solar system, distances can be measured very precisely by bouncing radar signals off planets and asteroids, and lasers off reflectors left on the Moon by Apollo astronauts, timing how long the signal takes to return.
Parallax
For nearby stars, astronomers use parallax. Hold your finger in front of your face and close one eye, then the other. Your finger seems to jump against the background. The closer your finger, the bigger the jump.
Similarly, as Earth orbits the Sun, nearby stars appear to shift slightly against more distant stars. By measuring this tiny shift six months apart and using geometry, astronomers can calculate the star's distance. A star that shifts by one arcsecond, a tiny angle, lies at a distance of one parsec. The European Space Agency's Gaia mission has measured parallaxes for more than a billion stars.
Standard candles
For greater distances, parallax becomes too small to measure. Astronomers then use standard candles: objects whose true brightness is known. By comparing how bright they appear with how bright they really are, astronomers can calculate their distance, much as you might estimate the distance of a streetlight of known wattage.
- Cepheid variable stars pulsate with a rhythm that depends on their true brightness. In 1912, Henrietta Swan Leavitt discovered this relationship, which allowed astronomers to measure distances to other galaxies. Edwin Hubble used Cepheids in the 1920s to show that Andromeda was a separate galaxy far beyond the Milky Way.
- Type Ia supernovae, exploding white dwarfs, reach a consistent peak brightness and are visible across billions of light-years.
Redshift
For the most distant galaxies, astronomers measure redshift. As space expands, light traveling through it is stretched to longer, redder wavelengths. The more distant a galaxy, the more its light is redshifted. In 1929, Edwin Hubble showed that more distant galaxies are moving away from us faster, a relationship now known as the Hubble-Lemaître law. This was key evidence that the universe is expanding.
Is the universe infinite?
The observable universe is finite, but it is not necessarily the whole universe. The universe as a whole may be much larger than what we can see, or even infinite. Measurements of the cosmic microwave background suggest that space is very nearly flat on large scales, which is consistent with an infinite universe or one so large that it appears flat to us. We cannot see beyond the observable universe because light from there has not had time to reach us, and because the universe's expansion is accelerating, some regions will never become visible to us.
The expanding and accelerating universe
In the late 1990s, two teams of astronomers studying Type Ia supernovae discovered that the expansion of the universe is speeding up. The cause is still unknown and is called dark energy. It appears to make up about 68 percent of the energy content of the universe. Another mysterious component, dark matter, makes up about 27 percent, while ordinary matter, everything we can see, accounts for only about 5 percent. Scientists are still working to understand what dark matter and dark energy are.
Questions readers ask
How big is the observable universe? About 93 billion light-years across, because space has expanded while light from distant regions traveled toward us.
What is a light-year? The distance light travels in one year, about 5.9 trillion miles (9.46 trillion kilometers).
How far is the nearest galaxy? The nearest large spiral galaxy is Andromeda, about 2.5 million light-years away. Smaller galaxies, such as the Large and Small Magellanic Clouds, are closer, around 160,000 to 200,000 light-years away.
Is there an edge of the universe? There is an edge to what we can observe, but not necessarily an edge to the universe itself. The universe may have no boundary at all.
How do we know distances to faraway galaxies? Through the cosmic distance ladder: parallax for nearby stars, standard candles like Cepheids and Type Ia supernovae for galaxies, and redshift for the most distant objects.
In short
The universe is astonishingly large. Light takes seconds to reach the Moon, minutes to reach Earth from the Sun, years to reach us from the nearest stars and millions to billions of years from other galaxies. The observable universe spans about 93 billion light-years because space has been expanding as light traveled. Astronomers measure these distances with radar, parallax, standard candles and redshift, each step building on the last. Beyond what we can see, the universe may continue much farther, perhaps forever, a reminder of how much remains to be discovered.