The sky — astronomy

The Milky Way: Our Galaxy in the Sky

The Milky Way is the barred spiral galaxy we live in, home to hundreds of billions of stars. We see it as a faint band across the night sky because we look at its disk from the inside. It is best seen from a dark site with little moonlight, and its bright core in Sagittarius rises highest on summer nights in the Northern Hemisphere.

What the Milky Way is

The Milky Way is the galaxy that contains the Sun, the Earth and every star you can see with the naked eye. Astronomers classify it as a barred spiral galaxy: a flat disk with spiral arms and a bar-shaped bulge at the center. It holds hundreds of billions of stars, and its disk is commonly estimated at around 100,000 light-years across. A light-year is the distance light travels in one year.

The name comes from ancient tradition: the Greek word galaxias (from gala, “milk”) and the Latin Via Lactea (“milky road”) both refer to the band’s milky look. In 1610 Galileo Galilei pointed a telescope at the band and found that it is made of a vast number of faint stars.

Looking at the galaxy from the inside

The solar system is not at the center. It sits in a minor spiral arm called the Orion Arm (or Orion Spur), roughly 26,000 light-years from the galactic center. That is a widely used estimate that is refined as measurements improve.

Because we are inside a flat disk, looking along its plane shows countless distant stars overlapping, and they blend into a luminous band that circles the whole sky. Looking away from the plane, there are far fewer stars and the sky looks darker. The dark lanes and patches within the Milky Way are not empty gaps: they are clouds of cosmic dust and gas that absorb the light of the stars behind them.

Every individual star you see unaided belongs to our local galactic neighborhood. ESA’s Gaia mission has measured the positions and motions of well over a billion stars, helping astronomers build a three-dimensional map of our galaxy.

The galactic center in Sagittarius

The center of the Milky Way lies in the direction of the constellation Sagittarius, near its border with Scorpius. This is the brightest and widest part of the band, rich in star clouds and dark dust lanes. The center itself is hidden from visible light by dust, but it is studied with infrared, radio and X-ray observations.

At the heart of the galaxy sits a supermassive black hole called Sagittarius A*, with a mass of about 4 million Suns. In 2022 the Event Horizon Telescope collaboration published its first image. You cannot see the black hole with your eyes; you only see the glowing region in the same direction of the sky.

Sagittarius is one of the zodiac constellations, and Ophiuchus lies nearby. For the bigger picture of how the sky is divided, see the 88 constellations.

Dark skies and the observing season

The Milky Way is a faint, extended glow, so it needs a dark sky. In cities it vanishes because of light pollution. The Bortle scale describes sky darkness from 1 (excellent dark sky) to 9 (inner-city sky). For a clear view, look for a site of about Bortle 4 or darker: away from towns, in the mountains or countryside.

Practical conditions:

• New Moon or a small Moon. A bright Moon washes out the faint glow. The week around New Moon is ideal.
• A clear, dry night. Clouds, haze and humidity reduce contrast.
• Dark adaptation. Give your eyes about 20–30 minutes in the dark and avoid looking at your phone screen. Red light disturbs your night vision less.
• Season. In the Northern Hemisphere the bright core is best seen from late spring to early autumn, especially June to August. In winter you see the fainter part of the band running through Cassiopeia, Perseus and Orion. In the Southern Hemisphere the core is high overhead during their winter months.
• Latitude. At mid-northern latitudes the core stays fairly low in the south, so you need a clear southern horizon. Closer to the equator or in the south it climbs much higher.

The summer band also passes through the Summer Triangle, a handy landmark. For general tips, see How to watch the sky.

Photographing it with a phone or camera

The Milky Way is easier to photograph than most people expect, but it needs stability and a long exposure.

With a phone:

• Put it on a tripod or a steady support.
• Use night mode or a manual mode if available.
• Aim for roughly 10 to 30 seconds of exposure.
• Focus manually on infinity or on a distant light before switching to night mode.
• Use a timer or remote to avoid shaking the phone.

With a camera:

• A wide-angle, fast lens (for example f/2.8 or faster).
• Manual mode, ISO typically between 1600 and 6400 depending on conditions.
• Exposures of about 15 to 25 seconds. A popular guide is the “500 rule”: divide 500 by your focal length to get an approximate maximum exposure in seconds before stars start to trail. It is a rule of thumb, not a law.
• Shoot in RAW to keep more detail for editing.

Stars move across the sky because the Earth rotates, so long exposures create trails. That is why serious astrophotographers use tracking mounts.

In short

The Milky Way is our closest encounter with an entire galaxy. Next time you see a faint, milky band over a dark horizon, remember: you are looking at the disk of the galaxy from the inside, from an arm about 26,000 light-years from its center.

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