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 from other parts of the galaxy, which are so far away that we can't see them individually without special tools.

The Milky Way is a barred spiral galaxy with a wide diameter of about 87,400 light-years. It is thought to contain between 100 and 400 billion stars, along with just as many planets. Our Solar System is located about 27,000 light-years from the center, on the edge of a spiral arm called the Orion Arm.

In the very center of the Milky Way is a very heavy object called Sagittarius A*, which scientists believe is a supermassive black hole. The oldest stars in our galaxy are almost as old as the universe itself.

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

Mythology

In ancient Babylonian stories, the Milky Way was believed to be formed from the tail of a huge salt water dragon named Tiamat. According to the epic poem Enūma Eliš, the god Marduk created the Milky Way by placing Tiamat's tail in the sky after defeating her. These tales 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. Curiously, La Voje Ladee ("The Milky Way") was also used to refer to the pilgrimage road.
• River Ganga of the Sky: this Sanskrit name (आकाशगंगा Ākāśagaṃgā) is used in many Indian languages following a Hindu belief.
• 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 who in turn borrowed it from Armenians.
• 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. It was understood to be either a guide to the pilgrims who flocked there, or a representation of the pilgrims themselves.
• 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 up of many stars and other materials that are too far away to see individually. Some parts of the band look brighter and are called star clouds, while darker areas, like the Great Rift and the Coalsack, happen where dust blocks the light from stars behind them.

The Milky Way is harder to see in places with lots of lights, like cities, because of light pollution. It is best seen in dark, rural areas when the Moon isn’t shining. From Earth, the Milky Way passes through many constellations and is brightest in the direction of Sagittarius.

Astronomical history

See also: Galaxy § Observation history

People have noticed the Milky Way for thousands of years. Early thinkers like Aristotle and Anaxagoras wondered what it was. Some believed it was just stars too far to see clearly, while others thought it was part of Earth's air.

When Galileo Galilei used a telescope in 1610, he discovered that the Milky Way is really made of many tiny stars. Later, scientists like Immanuel Kant thought the Milky Way might be a huge, spinning group of stars, like our Solar System but much bigger. Today, we know he was right! Modern tools like the ESA spacecraft Gaia help us map the Milky Way and learn even more about our home galaxy.

Astrography

The Sun is located near the inner edge of the Orion Arm in the Milky Way. It is about 27,000 to 28,000 light-years from the center of the galaxy and sits slightly above the galaxy's main flat disk. The Sun and the entire Solar System are moving through space at about 220,000 miles per hour as they orbit the center of the Milky Way.

The Milky Way is divided into four quadrants based on a coordinate system that uses the Sun as the starting point. These quadrants help scientists describe different parts 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 the stars in other parts that are too far away to see individually.

The Milky Way is a barred spiral galaxy. Scientists think its diameter is about 87,400 light-years across, but it is only about 1,000 light-years thick where the spiral arms are. There is also a lot of invisible material called dark matter around it, which might stretch almost 2 million light-years in diameter.

Contents

The Milky Way contains between 100 and 400 billion stars and at least as many planets. It also has many types of objects such as white dwarfs, neutron stars, and black holes. Between the stars, there is a disk made of gas and dust.

The Milky Way’s stars do not have a clear edge; instead, they become fewer as you move away from the center. Around the disk, there is a spherical area called the galactic halo, which holds more stars and groups of stars known as globular clusters. The Milky Way has fewer neutrinos than many other galaxies, making it a "neutrino desert."

Structure

The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust, and stars. Astronomers discovered in the 1960s that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy. This was confirmed by observations from the Spitzer Space Telescope in 2005.

The Sun is located about 25,000–28,000 light-years from the Galactic Center. The center has a dense concentration of old stars and is marked by a powerful radio source named Sagittarius A*. Observations show that this area contains a supermassive black hole, with a mass millions of times that of the Sun.

The Milky Way's disk is organized into spiral arms, which contain more gas, dust, and new stars than the average parts of the galaxy. These arms are areas where stars are actively forming. The exact structure of these arms is still being studied, but they appear to have a complex pattern that includes both older stars and younger star-forming regions.

Formation

Main article: Galaxy formation and evolution

The Milky Way began as small areas of extra mass shortly after the Big Bang about 13.61 billion years ago. These areas helped form the oldest stars and clusters, which now make up 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 about 11 billion years ago. Unlike many similar galaxies, the Milky Way hasn’t merged with large galaxies in the last 10 billion years.

Intergalactic neighborhood

Main article: Local Group

The Milky Way belongs to a group of galaxies called the Local Group, which includes about 50 galaxies. The two biggest galaxies in this group are the Milky Way and the Andromeda Galaxy. These two are like partners, orbiting each other.

The Local Group is surrounded by empty spaces called voids, where there are very few galaxies. Near the Milky Way, there are smaller galaxies, such as the Large Magellanic Cloud and the Small Magellanic Cloud, which travel along with us. Scientists think these small galaxies pull on the Milky Way, creating ripples in its shape. One day, in about 4.3 billion years, the Milky Way and Andromeda might come together to form a new galaxy.