How big is NGC 1232?

NGC 1232 is a large spiral galaxy in the constellation Eridanus, about 60 million light-years away. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Its disk is about 130,000 light-years across, roughly 1.3 times the width of the Milky Way's. It faces Earth almost head-on, so its spiral arms are in full view.

Size

Seen from Earth, NGC 1232 covers a patch of sky about 6.7 by 7.8 arcminutes, about a quarter of the width of the full Moon. An arcminute is a sixtieth of a degree. The Third Reference Catalogue of Bright Galaxies, which measures galaxies out to a set level of faintness, gives a width of 7.4 arcminutes.

Turning that angle into a width needs the galaxy's distance. A 2013 study used the average of nine published estimates: 18.7 million parsecs, where a parsec is about 3.26 light-years. That is about 61 million light-years, which makes the galaxy about 120,000 to 140,000 light-years across. The distance is uncertain by about 14%, so the width is too.

A 1998 press release from the European Southern Observatory (ESO) gave a width of about 200,000 light-years, twice the size of the Milky Way. That was the width of the whole photograph, not of the galaxy, at a distance of 100 million light-years used at the time. At about 60 million light-years, the same photograph is about 120,000 light-years wide.

Size comparisons

NGC 1232 is about 1.3 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
It is about the same width as the bright disk of the Andromeda Galaxy and the bright main body of the giant galaxy Virgo A, when all three are measured to the same faintness.
The Pinwheel Galaxy, measured the same way, is about 1.4 times as wide as NGC 1232.
If the Milky Way's disk were shrunk to the width of a basketball, 24 centimeters (9.4 inches), NGC 1232 would be about 32 centimeters (12 inches) across and about 150 meters (480 feet) away.

The 1998 photograph

ESO photographed NGC 1232 on September 21, 1998, with FORS1, a new instrument on its Very Large Telescope. The picture combines three exposures, in ultraviolet, blue and red light. Its colors show different kinds of stars: older, reddish stars fill the center, while young blue stars and many regions of star birth line the spiral arms.

A smaller galaxy, NGC 1232A, appears beside it in the picture. It lies farther away than NGC 1232 and is probably not interacting with it.

A collision seen in X-rays

Images from NASA's Chandra X-ray Observatory, published in 2013, revealed a giant cloud of gas in NGC 1232 at nearly 6 million degrees Celsius. Gas that hot glows only in X-rays. The likely cause is a dwarf galaxy crashing into NGC 1232 and setting off a shock wave, like a sonic boom. The brightest part of the cloud is about 23,600 light-years across, about a quarter of the width of the Milky Way's disk.

If confirmed, this would be the first collision of its kind found only in X-rays. The cloud holds between 40,000 and 3 million times the mass of the Sun, depending on its shape. The collision should last about 50 million years, and the hot gas should keep glowing for tens to hundreds of millions of years.