Among the countless galaxies in the cosmos, some remain faint from Earth. NGC 4535, located about 50 million light-years away in the constellation Virgo, illustrates this phenomenon well. Observed with modest instruments, it appears so faint that it has been nicknamed the "lost galaxy," thus hiding its true nature from amateur astronomers.
Thanks to the Hubble Space Telescope, this galaxy finally reveals its characteristics. Equipped with a 2.4-meter (7.9-foot) mirror, Hubble captures details invisible from the ground, showing well-defined spiral arms and a dense central bar.
Nearly invisible from Earth, NGC 4535 comes to life under Hubble's eye with swirling arms filled with young blue stars and bright pink nebulae.
Credit: ESA/Hubble & NASA, F. Belfiore, J. Lee and the PHANGS-HST team
Hubble's image highlights numerous young star clusters scattered along the spiral arms of NGC 4535. These stars, with an intense blue color, indicate relative youth and high temperature. Around them, pink clouds, known as H II regions, glow due to radiation from nearby massive stars, signaling ongoing star formation activity.
These massive stars profoundly influence their environment. Their energetic radiation and stellar winds push the surrounding gas, triggering the birth of new stars. At the end of their lives, they explode as supernovae, scattering heavy elements and reshaping the interstellar medium, which sets the stage for future generations of stars.
The study of NGC 4535 is part of the PHANGS program, a major research initiative aimed at mapping thousands of star-forming regions in nearby galaxies. By collecting data on these diverse environments, scientists better understand how young stars interact with cold gas. This project helps build a more complete picture of galactic evolution.
H II Nebulae
H II nebulae are gas clouds composed mainly of ionized hydrogen. They form near massive, young stars, whose intense ultraviolet radiation strips electrons from hydrogen atoms. This process creates ionization that makes the gas glow, producing characteristic pink or red hues visible in astronomical images.
These regions serve as cradles for new stars. The hot ionized gas can condense under the effect of gravity, forming stellar clusters. The presence of H II nebulae thus indicates active star formation activity, as they require a constant supply of energy from nearby stars to maintain their brightness.
In astronomy, the study of H II nebulae helps measure star formation rates in galaxies. Scientists use their brightness and distribution to estimate how many stars are born in different regions. This allows for comparing galaxies and understanding how factors such as gas density influence stellar evolution.
Furthermore, these clouds play a role in the interstellar matter cycle. When massive stars die as supernovae, they enrich the surrounding gas with heavy elements, which can then participate in the formation of new stars and planets, thus perpetuating the cosmic cycle.