How big is Leo II Dwarf Galaxy? | Scale of the Universe

Leo II is a small, faint dwarf galaxy that orbits the Milky Way, about 760,000 light-years from Earth in the constellation Leo. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Half of Leo II's starlight comes from a central region about 1,100 light-years across. Its faintest outer stars spread over about 4,100 light-years.

Size

A dwarf galaxy has no sharp edge. Its stars grow sparser with distance from the center, so astronomers describe its size in more than one way. One is the half-light radius, the distance from the center within which half of the galaxy's light is found.

A 2012 catalog of nearby dwarf galaxies gives Leo II a half-light radius of 2.6 arcminutes on the sky, where an arcminute is a sixtieth of a degree. At Leo II's distance of 233,000 parsecs, where a parsec is about 3.26 light-years, that is 176 parsecs. Half of its light therefore comes from a region about 350 parsecs, or 1,100 light-years, across. This figure is uncertain by about a quarter either way.

A 2007 study counted Leo II's stars over a wide area in images from the Sloan Digital Sky Survey. It fitted a standard model of how the number of stars falls with distance from the center. The model reaches zero at 632 parsecs from the center, so the galaxy is about 1,260 parsecs, or 4,100 light-years, across. Leo II is nearly round.

Size comparisons

Measured the same way, by the radius that holds half its light, the Canes Venatici I Dwarf Galaxy is about three times as wide as Leo II. Yet Leo II gives off about three times as much light.
The disk of the Milky Way, about 100,000 light-years across, is about 24 times as wide as Leo II's full extent.
If the Milky Way's disk were shrunk to the width of a basketball, Leo II would be a speck about 1 centimeter (0.4 inches) across, about 1.8 meters (6 feet) away.
Leo II is about seven and a half times as far from Earth as the Milky Way is wide.

A galaxy held together by dark matter

How fast a galaxy's stars move shows how much mass is holding them together. A 2007 survey measured the speeds of 171 stars in Leo II. Their speeds typically differ from the galaxy's average by about 6.6 kilometers per second, and the spread stays about the same all the way out to its edge. Combining those speeds with the galaxy's size, another 2007 study found that Leo II holds about 100 times as much mass, for the light it gives off, as the Sun does.

Most of that mass must be dark matter, which gives off no light and is detected only by its gravity. As expected for a galaxy so far away, the same study found no sign that the Milky Way's gravity is pulling Leo II out of shape.

Its stars and history

Leo II was discovered in 1950. It is a dwarf spheroidal galaxy: a small, rounded galaxy whose stars are spread so thinly that it looks very faint. Most of its stars formed about 8–9 billion years ago.

A 2007 study with the Subaru Telescope found that star formation then slowly shrank from the outside toward the center. It had almost stopped about 4 billion years ago, except at the very center, where a small number of younger stars formed later.