How well do you know the Milky Way?

The Milky Way is the galaxy that contains the Solar System, and its name comes from its appearance as a hazy band of light seen from Earth. It is a barred spiral galaxy with an estimated 100-400 billion stars and at least as many planets. The Solar System is located at a radius of about 27,000 light-years from the Galactic Center, which contains Sagittarius A*, a supermassive black hole of 4.1 million solar masses. The Milky Way is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way has several satellite galaxies. Galileo Galilei first resolved the Milky Way into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies. The Milky Way is visible as a hazy band of white light, some 30° wide, arching the night sky, and is visible from rural areas when the Moon is below the horizon. Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible. The Milky Way's surface brightness is low. Its appearance has been described in mythology and history, including Enūma Eliš, Greek mythology, and Welsh mythology. Aristotle believed that the Milky Way was part of the Earth's upper atmosphere, while Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. Immanuel Kant, drawing on earlier work by Thomas Wright, speculated that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.Key Facts about the Milky Way Galaxy

• The ESA spacecraft Gaia is mapping the Milky Way and has expanded the number of observations of stars from 2 million to 2 billion.

• The Sun is located near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and is 27.14 ± 0.46 kly from the Galactic Center.

• There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, but only 64 known stars within 5 parsecs (16 ly) of the Sun.

• The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way, and the Sun passes through the Galactic plane approximately 2.7 times per orbit.

• It takes the Solar System about 240 million years to complete one orbit of the Milky Way, and the Milky Way's galactic habitable zone is where the Sun and the Solar System are located.

• The Milky Way is one of the two largest galaxies in the Local Group, and its diameter is estimated to be 26 kiloparsecs (80,000 light-years).

• The Milky Way is estimated to be 890 billion to 1.54 trillion times the mass of the Sun in total, with much of its mass being dark matter.

• The Milky Way contains between 100 and 400 billion stars and at least that many planets.

• The concentration of stars decreases with distance from the center of the Milky Way, and beyond a radius of roughly 40,000 light years (13 kpc) from the center, the number of stars per cubic parsec drops much faster with radius.

• Surrounding the galactic disk is a spherical galactic halo of stars and globular clusters that extends farther outward.

• The Milky Way is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic.

• Gaia detected a wobbling motion of the galaxy, which might be caused by "torques from a misalignment of the disc's rotation axis with respect to the principal axis of a non-spherical halo, or from accreted matter in the halo acquired during late infall, or from nearby, interacting satellite galaxies and their consequent tides".Overview of the Milky Way Galaxy

• The Milky Way is estimated to be around 13.6 billion years old and contains 100-400 billion planets.

• The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust, and stars.

• The Galactic Center is marked by an intense radio source named Sagittarius A*, which harbors a massive, compact object that is likely a supermassive black hole.

• The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of its arms.

• The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years of the Galactic Center.

• The mass of the Milky Way's hot gaseous halo is nearly equivalent to the mass of the Milky Way itself and is between 1 and 2.5 million K.

• The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location.

• The Milky Way is estimated to contain at least one planet per star, resulting in 100-400 billion planets.

• The Milky Way's central bar is larger than previously thought and may be surrounded by a ring called the "5 kpc ring."

• The Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.

• The Milky Way contains at least as many planets bound to stars as there are stars in the Milky Way, and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.

• The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure within the Milky Way that does not seem to fit within current models and may be a dwarf galaxy merging with the Milky Way.The Milky Way: Formation, Age, and Velocity

Formation:

• The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang 13.61 billion years ago.
• Nearly half the matter in the Milky Way may have come from other distant galaxies.
• Since the first stars began to form, the Milky Way has grown through both galaxy mergers and accretion of gas directly from the Galactic halo.
• The Milky Way is currently accreting material from several small galaxies, including the Large and Small Magellanic Clouds, through the Magellanic Stream.

Age:

• Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way.
• Age estimates of the oldest of these clusters give a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.
• The Milky Way Galaxy may be at least 3 billion years older than previously thought, as a result of the discovery of one of the oldest stars in the universe, 2MASS J18082002-5104378 B.
• The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238.

Velocity:

• The Milky Way is moving at approximately 630 km/s (1,400,000 mph) with respect to the Hubble flow.
• The Milky Way is moving in the general direction of the Great Attractor and other galaxy clusters, including the Shapley Supercluster, behind it.
• The Milky Way is moving at 552 ± 6 km/s (1,235,000 ± 13,000 mph) with respect to the cosmic microwave background (CMB), toward 10.5 right ascension, −24° declination (J2000 epoch, near the center of Hydra).