Anne’s Image of Today: Minkowski’s Footprint

July 16, 2014

Minkowski’s Footprint, a protoplanetary nebula in Cygnus

Image Credit: ESA/Hubble & NASA

Minkowski’s Footprint [designated Minkowski 92 (M1-92) and IRAS 19343+2926] is a bipolar protoplanetary nebula, located about 8,000 light-years away from Earth in the northern constellation of Cygnus (the Swan), that features two vast onion-shaped structures on either side of a central ageing star.

Despite their name, protoplanetary nebulae have nothing to do with planets: they are clouds of dust and gas formed from material shed by an aging central star with a mass up to eight times that of the Sun. For such a star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, enveloping itself in expanding clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.

But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns that can only be truly appreciated later once the nebula begins to glow. Then the star cools down and all that is left after this process is the exposed, hot and dead core, known as a white dwarf.

A protoplanetary nebula is a relatively short-lived phenomenon, what means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently.

Finding a protoplanetary nebula is a rare opportunity for astronomers to learn more about them and to observe the beginning of the formation of planetary nebulae (hence the name protoplanetary, or preplanetary nebulae), as well as the short and poorly understood phase of stellar evolution when a red giant collapses into a white dwarf.

Technically speaking Minkowski’s Footprint is currently a reflection nebula as it is only visible due to the light reflected from the central star. In a few thousand years the star will get hotter and its ultraviolet radiation will light up the surrounding gas from within, causing it to glow. At this point it will have become a fully fledged planetary nebula.

This image was obtained with the Hubble’s Wide Field Planetary Camera 2 in the 1990s. The picture has been made from many exposures through four different color filters. Light from ionized oxygen has been colored blue (F502N), light passing through a green/yellow filter (F547M) is colored cyan, light from atomic oxygen is colored yellow (F631N) and light from ionized sulphur is colored red (F673N). The total exposure times per filter were 2080 s, 960 s, 2080 s and 1980 s respectively and the field of view is only about 36 arcseconds across.