How big is Pistol Star? | Scale of the Universe

The Pistol Star is a massive, extremely luminous star near the center of the Milky Way, about 26,000 light-years from Earth. A light-year is the distance light travels in one year. It is a luminous blue variable, a rare kind of unstable star, and thick clouds of dust hide it from view in visible light. A 2009 study estimated its radius at about 306 times the Sun's, which makes it about 430 million kilometers across.

Size

Dust blocks almost all of the Pistol Star's visible light: only about one in a trillion particles of that light reaches Earth. Astronomers therefore study the star in infrared light. Its size is calculated, not measured directly, from the star's total light output and its surface temperature. Both come from comparing its infrared spectrum, its light spread out by wavelength, with computer models of the star.

In 2009 a team led by Francisco Najarro found that the star gives off about 1.6 million times as much light as the Sun. Its surface is about 11,800 kelvins (degrees measured from absolute zero), roughly twice as hot as the Sun's. Together these give a radius of 306 times the Sun's. That is measured to the layer where the star's light escapes through the dense wind of gas around it.

The figure is uncertain. The temperature could be off by 1,500 kelvins either way, and the star's brightness depends on how much the dust is assumed to dim it. In 1997, NASA described the star as big enough to fill the width of Earth's orbit, about 300 million kilometers, somewhat smaller than the 2009 figure.

Size comparisons

The Pistol Star is about 300 times as wide as the Sun.
If the Pistol Star took the Sun's place, its surface would reach past the orbit of Earth, but not as far as the average distance of Mars from the Sun.
At its largest, S Doradus, another luminous blue variable in the explorer, is about one-quarter wider than the Pistol Star.
Betelgeuse, a red supergiant, is about 2.5 times as wide.
If the Sun were shrunk to the size of a basketball, the Pistol Star would be a ball about 73 meters (240 feet) across, about two-thirds the length of an American football field.

The Pistol Nebula

The star is named after the Pistol Nebula, a cloud of glowing gas and dust around it that is shaped like a pistol. The star threw off this material in two large eruptions, about 6,000 and 4,000 years ago. The nebula measures about 4.6 by 3.9 light-years and holds about nine times the Sun's mass in gas.

The Pistol Star belongs to the Quintuplet cluster, a group of massive young stars near the center of the galaxy. Without the dust in the way, it would be visible to the naked eye.

A changing estimate

After the Hubble Space Telescope studied the star, NASA announced in 1997 that it might be the most luminous star known. It seemed to give off up to 10 million times the Sun's power. A 1998 analysis could not decide between a brighter and a dimmer version of the star. In 2009, improved computer models settled on the dimmer one, cutting the brighter estimate's light output in half.

Even so, the Pistol Star gives off as much energy in about 20 seconds as the Sun does in a year. The 2009 study estimated that it was born with about 100 times the Sun's mass. It has since lost much of that mass, though its present mass is uncertain.