Massive Stars Make Their Mark In Hubble Image

This NASA/ESA Hubble Space Telescope image features a glittering blue dwarf galaxy called Markarian 178 (Mrk 178). The galaxy, which is substantially smaller than our own Milky Way, lies 13 million light-years away in the constellation Ursa Major (the Great Bear).

Mrk 178 is one of more than 1,500 Markarian galaxies. These galaxies get their name from the Armenian astrophysicist Benjamin Markarian, who compiled a list of galaxies that were surprisingly bright in ultraviolet light.

While the bulk of the galaxy is blue due to an abundance of young, hot stars with little dust shrouding them, Mrk 178 gets a red hue from a collection of rare massive Wolf-Rayet stars. These stars are concentrated in the brightest, reddish region near the galaxy's edge. Wolf-Rayet stars cast off their atmospheres through powerful winds, and the bright emission lines from their hot stellar winds are etched upon the galaxy's spectrum. Both ionized hydrogen and oxygen lines are particularly strong and appear as a red color in this photo.

Massive stars enter the Wolf-Rayet phase of their evolution just before they collapse into black holes or neutron stars. Because Wolf-Rayet stars last for only a few million years, researchers know that something must have triggered a recent burst of star formation in Mrk 178. At first glance, it's not clear what could be the cause - Mrk 178 doesn't seem to have any close galactic neighbors that may have stirred up its gas to form new stars. Instead, researchers suspect that a gas cloud crashed into Mrk 178, or that the intergalactic medium disturbed its gas as the galaxy moved through space. Either disturbance could light up this tiny galaxy with a ripple of bright new stars.