Milky Way facts for kids
The Milky Way is our home galaxy. It contains over 200 billion stars, including our Sun.
The Milky Way has a diameter of 100,000 light years, and is a barred spiral galaxy. The discovery of the Milky Way goes back to the Ancient Greek philosopher Democritus. The Milky Way has three main parts: a disk, in which the Solar System resides, a bulge at the core, and an all encompassing halo.
This galaxy belongs to the Local Group of three large galaxies and over 50 smaller galaxies. The Milky Way is one of the largest galaxies in the group, second to the Andromeda Galaxy. Milky Way's closest neighbour is Canis Major Dwarf, which is about 25,000 light years away from the Earth. The Andromeda Galaxy moves towards the Milky Way Galaxy, and will meet it in about 3.75 billion years. Andromeda Galaxy moves with a speed of about 1,800 kilometres per minute.
Myth
In Greek mythology, Zeus places his son (the baby Heracles) whose mother was a mortal woman on Hera's breast while she is sleeping so that the baby will drink her divine milk and become immortal. However, Hera wakes up while she is breastfeeding the baby and realizes she is nursing a baby she does not know. According to Greek mythology, she then pushes the baby away and a stream of her milk sprays the night sky, making a faint band of light known as the Milky Way.
History
Proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to refraction of the Earth's atmosphere. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, speculated (correctly) 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. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.
The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the visible sky. He produced a diagram of the shape of the Milky Way with the Solar System close to the center.
In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.
In 1904, studying the proper motions of stars, Jacobus Kapteyn reported that these were not random, as it was believed in that time; stars could be divided into two streams, moving in nearly opposite directions. It was later realized that Kapteyn's data had been the first evidence of the rotation of our galaxy, which ultimately led to the finding of galactic rotation by Bertil Lindblad and Jan Oort.
In 1917, Heber Doust Curtis had observed the nova S Andromedae within the Great Andromeda Nebula (Messier object 31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within the Milky Way. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were independent galaxies. In 1920 the Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.
The controversy was conclusively settled by Edwin Hubble in the early 1920s using the Mount Wilson observatory 2.5 m (100 in) Hooker telescope. With the light-gathering power of this new telescope, he was able to produce astronomical photographs that resolved the outer parts of some spiral nebulae as collections of individual stars. He was also able to identify some Cepheid variables that he could use as a benchmark to estimate the distance to the nebulae. He found that the Andromeda Nebula is 275,000 parsecs from the Sun, far too distant to be part of the Milky Way.
Satellite observations
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The ESA spacecraft Gaia provides distance estimates by determining the parallax of a billion stars and is mapping the Milky Way with four planned releases of maps in 2016, 2018, 2021 and 2024.
Size
The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (9×1017 km) in diameter, and is considered to be, on average, about 1000 light years thick.
It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars. The figure depends on the number of very low-mass, or dwarf stars, which are hard to detect, especially more than 300 light years from our sun. Therefore, present estimates of the total number are uncertain. This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy.
The stellar disc of the Milky Way does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the Galaxy. Beyond a radius of about 40,000 light years, the number of stars drops much faster, for reasons that are not understood.
Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12000 light years–twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if the Solar System out to the orbit of Pluto were reduced to the size of a US quarter (about an inch or 25 mm in diameter), the Milky Way would have a diameter of 2,000 kilometers. At 220 kilometers per second it takes the Solar System about 240 million years to complete one orbit of the Galaxy (a galactic year).
The Galactic halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach is at about 180,000 light years. At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.
Galactic center
The galactic disc, which bulges outward at the galactic center, has a diameter of 70–100,000 light years.
The exact distance from the Sun to the galactic center is debated. The latest estimates give distances to the Galactic center of 25–28,000 light years.
Movement of material around the galactic center shows that it has a compact object of very large mass. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is now confirmed to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.
The nature of the galaxy's bar is also actively debated, with estimates for its half-length and orientation spanning from 3,300–16,000 light years (short or a long bar) and 10–50 degrees. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.
Images for kids
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A view of the Milky Way toward the constellation Sagittarius (including the Galactic Center), as seen from a dark site with little light pollution (the Black Rock Desert, Nevada), the bright object on the lower right is Jupiter, just above Antares
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Photograph of the "Great Andromeda Nebula" from 1899, later identified as the Andromeda Galaxy
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Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known prominent components annotated including main arms, spurs, bar, nucleus/bulge, notable nebulae and globular clusters.
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Artistic close-up of the Orion Arm with the main features of the Radcliffe Wave and Split linear structures, and with the Solar System surrounded by the closest large scale celestial features at the surface of the Local Bubble at a distance of 400–500 light years.
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Diagram of the Sun's location in the Milky Way, the angles represent longitudes in the galactic coordinate system.
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The structure of the Milky Way is thought to be similar to this galaxy (UGC 12158 imaged by Hubble)
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360-degree panorama view of the Milky Way (an assembled mosaic of photographs) by ESO, the galactic centre is in the middle of the view, with galactic north up
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360-degree rendering of the Milky Way using Gaia EDR3 data showing interstellar gas, dust backlit by stars (main patches labeled in black; white labels are main bright patches of stars). Left hemisphere is facing the galactic center, right hemisphere is facing the galactic anticenter.
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Overview of different elements of the overall structure of the Milky Way.
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The long filamentary molecular cloud dubbed "Nessie" probably forms a dense "spine" of the Scutum–Centarus Arm
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Galaxy rotation curve for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter
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Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed.
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The Milky Way arching at a high inclination across the night sky, (this composited panorama was taken at Paranal Observatory in northern Chile), the bright object is Jupiter in the constellation Sagittarius, and the Magellanic Clouds can be seen on the left; galactic north is downward
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See also
In Spanish: Vía Láctea para niños
