Leo I (dwarf galaxy)

Leo I is a small, round galaxy found in the sky area known as the constellation Leo. It is very far away, about 820,000 light-years from us. This places it in a group of galaxies called the Local Group, which includes our own Milky Way galaxy. Scientists think Leo I might be one of the farthest objects orbiting our Milky Way.

Leo I was first seen in 1950 by a scientist named Albert George Wilson. He noticed it while looking at pictures taken with a big camera at Palomar Observatory. These pictures were part of a project to map the night sky, and they helped discover many interesting objects far away from Earth.

Visibility

Leo I is found just 12 arc minutes away from Regulus, the brightest star in the constellation Leo. Because of this, it is sometimes called the Regulus Dwarf. The light from Regulus makes it harder to study the galaxy, and it wasn’t until the 1990s that people could see it with their eyes.

Because Regulus is so bright and Leo I doesn’t shine very strongly, it can be tricky to spot. You usually need a medium-sized telescope, like one that is 15 cm or bigger, and a very dark sky to see it. However, in April 2013, someone reported seeing Leo I with a smaller telescope—just 11 cm—or even a 7 cm refractor, but only under very dark sky conditions.

Mass

Scientists studied the movement of some bright stars in Leo I and found that its mass is at least (2.0 ± 1.0) × 10⁷ M_☉. We are not sure if there is a lot of invisible material, called dark matter, around the galaxy. One study thought there might be a very big black hole in the center, but another study did not find this. Among the small galaxies close to the Milky Way, Leo I has a lot of dark matter.

Some people think Leo I might be part of the outer edge of the Milky Way, but this is not certain.

Star formation

Leo I has very few heavy elements, only about one percent of what our Sun has. Observations from the Hubble Space Telescope show that most of its stars formed between 6 and 2 billion years ago. Before that, there were almost no stars at all.

About a billion years ago, star formation in Leo I slowed down a lot. This may have happened because it passed close to the Milky Way, losing the gas it needed to make new stars. Some small amount of star formation might have continued until 200 to 500 million years ago. Because of this, Leo I is considered one of the youngest small galaxies orbiting our Milky Way. It might also be surrounded by a cloud of glowing gas about as heavy as the galaxy itself.