Milky Way

The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System. Its name comes from how it looks from Earth: a hazy band of light in the night sky. This light is made up of many stars that are very far away.

The Milky Way is a barred spiral galaxy. It is very big, about 87,400 light-years across. It has many stars and planets—hundreds of billions of each! Our Solar System is about 27,000 light-years from the center, on the edge of a spiral arm called the Orion Arm.

In the center of the Milky Way is a very heavy object called Sagittarius A*. Scientists think it is a supermassive black hole. Some of the oldest stars in our galaxy are almost as old as the universe.

Long ago, in 1610, Galileo Galilei used a telescope to see that the hazy band of light was really made of many stars. For a long time, people thought the Milky Way was the whole universe. But in the 1920s, astronomer Edwin Hubble found that there were many other galaxies besides our own.

Mythology

In old stories from Babylon, the Milky Way was thought to be the tail of a huge salt water dragon named Tiamat. The god Marduk is said to have placed Tiamat's tail in the sky after defeating her. These stories show how people long ago tried to explain the beautiful band of light we see in the night sky.

Etymology

In Greek mythology, the name "Milky Way" comes from a story about Zeus and his son Heracles. When Heracles was very young, Zeus let him drink from Hera’s breast while she was asleep. She woke up and pushed him away, causing some of her milk to spill and form a glowing band in the sky.

The name "Milky Way" is used in many cultures around the world. For example, some people call it the "Birds’ Path" because birds seem to fly along it. In China, it is called the "Silver River," and in Japan, it is known as the "River of Heaven." Each culture has its own special name for this beautiful part of the night sky.

Other common names

• "Birds’ Path" is used in several Uralic and Turkic languages and in the Baltic languages. Northern peoples observed that migratory birds follow the course of the galaxy while migrating at the Northern Hemisphere. The name "Birds’ Path" (in Finnish, Estonian, Latvian, Lithuanian, Bashkir and Kazakh) has some variations in other languages, e.g. "Way of the grey (wild) goose" in Chuvash, Mari and Tatar and "Way of the Crane" in Erzya and Moksha.
• The Kaurna people of the Adelaide Plains of South Australia called the Milky Way wodliparri in the Kaurna language, meaning "house river".
• The Gomeroi people between New South Wales and Queensland called the Milky Way Dhinawan, the giant "Emu in the Sky" that it stretches across the night sky.
• The Milky Way was traditionally used as a guide by pilgrims traveling to the holy site at Santiago de Compostela, hence the use of "The Road to Santiago" as a name for the Milky Way.
• River Ganga of the Sky: this Sanskrit name (आकाशगंगा Ākāśagaṃgā) is used in many Indian languages.
• The Chinese name "Silver River" (銀河) is used throughout East Asia, including Korea and Vietnam (Ngân hà). In Japan and Korea, "Silver River" (Japanese: 銀河, romanized: ginga; Korean: 은하; RR: eunha) refers to any galaxy.
• The Japanese name for the Milky Way is the "River of Heaven" (天の川, Ama no gawa), as well as an alternative name in Chinese (Chinese: 天河; pinyin: Tiānhé). In Vietnamese, "River of Heaven" (Thiên hà) refers to any galaxy.
• In West Asia, Central Asia and parts of the Balkans the name for the Milky Way is related to the word for straw. Today, Persians, Pakistanis, and Turks use it in addition to Arabs. It has been suggested that the term was spread by medieval Arabs.
• In Serbo-Croatian it is interchangeably called "Kumova slama" (lit. transl. Godfather's Straw) along the "Mliječni put" (lit. transl. Milky Way).
• In England the Milky Way was called the Walsingham Way in reference to the shrine of Our Lady of Walsingham which is in Norfolk, England.
• Scandinavian peoples, such as Swedes, have called the galaxy "Winter Street" (Vintergatan) as the galaxy is most clearly visible during the winter at the northern hemisphere, especially at high latitudes where the glow of the Sun late at night can obscure it during the summer.

Appearance

The Milky Way looks like a hazy band of white light in the night sky. This band is made of many stars and other things too far away to see clearly. Some parts look brighter and are called star clouds. Darker areas, like the Great Rift and the Coalsack, happen where dust blocks the light from stars.

The Milky Way is harder to see in bright places, like cities, because of light pollution. It is best seen in dark areas far from cities when the Moon is not shining. From Earth, the Milky Way passes through many star patterns and looks brightest toward the direction of Sagittarius.

Astronomical history

See also: Galaxy § Observation history

People have seen the Milky Way for thousands of years. Early thinkers like Aristotle and Anaxagoras asked many questions about it. Some thought it was just faraway stars, while others believed it was part of Earth’s air.

When Galileo Galilei used a telescope in 1610, he found that the Milky Way is made of many tiny stars. Later, scientists like Immanuel Kant guessed that the Milky Way might be a giant, spinning group of stars, much bigger than our Solar System. Today, we know they were right! Modern tools like the ESA spacecraft Gaia help us learn more about our home galaxy.

Astrography

The Sun is in the Orion Arm of the Milky Way. It is about 27,000 to 28,000 light-years from the center. The Sun and our Solar System move through space at about 220,000 miles per hour as they go around the center of the Milky Way.

The Milky Way is split into four parts. These parts help scientists talk about different areas of the galaxy.

General characteristics

The Milky Way is a big galaxy that includes our Solar System. From Earth, it looks like a hazy band of light in the night sky. This is because we are inside the galaxy, seeing stars too far away to see one by one.

The Milky Way is a barred spiral galaxy. Scientists think it is about 87,400 light-years wide but only about 1,000 light-years thick where the spiral arms are. There is also invisible material called dark matter around it.

Contents

The Milky Way has many stars—between 100 and 400 billion. It also has planets and many other objects like white dwarfs, neutron stars, and black holes. Between the stars, there is a disk made of gas and dust.

The stars in the Milky Way do not have a sharp edge. Instead, there are fewer stars as you move away from the center. Around the disk, there is a round area called the galactic halo. This halo holds more stars and groups of stars called globular clusters. The Milky Way has fewer neutrinos than many other galaxies, so it is called a "neutrino desert."

Structure

The Milky Way has a bar-shaped center surrounded by a disk of gas, dust, and stars. In the 1960s, astronomers learned that the Milky Way is a barred spiral galaxy, not just a regular spiral galaxy. This was confirmed by the Spitzer Space Telescope in 2005.

The Sun is about 25,000–28,000 light-years from the Galactic Center. The center has many old stars and a strong radio source called Sagittarius A*. We know this area has a supermassive black hole, which is millions of times more massive than the Sun.

The Milky Way's disk has spiral arms with more gas, dust, and new stars. These arms are where stars are being born. Scientists are still learning about the exact shape of these arms, but they seem to have a mix of old and new stars.

Formation

Main article: Galaxy formation and evolution

The Milky Way started from small areas of extra mass shortly after the Big Bang. These areas helped form the oldest stars and clusters, which are now in the outer parts of the Milky Way. Over time, the Milky Way grew by combining with other small galaxies and collecting gas.

Younger stars, including our Sun, formed in a flat, spinning disk shape. The Milky Way continues to grow by pulling in material from small nearby galaxies, like the Large and Small Magellanic Clouds. Scientists think the Milky Way merged with a big galaxy called the Kraken a long time ago. Unlike many similar galaxies, the Milky Way hasn’t merged with large galaxies for a very long time.

Intergalactic neighborhood

Main article: Local Group

The Milky Way is part of a group of galaxies called the Local Group. This group has about 50 galaxies. The two largest are the Milky Way and the Andromeda Galaxy. They move around each other like partners.

The Local Group is surrounded by empty spaces with very few galaxies. Close to the Milky Way are smaller galaxies, such as the Large Magellanic Cloud and the Small Magellanic Cloud. These travel with us. Scientists believe they affect the shape of the Milky Way. One day, in about 4.3 billion years, the Milky Way and Andromeda may join together to form a new galaxy.