This view of the Perseus cluster of galaxies shows over 1000 galaxies all clustered together some 240 million light-years away, with many tens of thousands more identifiable in the background portion of the image. In the centers of large galaxy clusters, Milky Way-like galaxies with spiral structures, new stars, and gas-rich disks are rare, as cluster centers are instead dominated by gas-free, red-and-dead giant elliptical galaxies. (Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi, CC BY-SA 3.0 IGO)
If our Milky Way were located in the Virgo cluster instead of the Local Group, chances are we’d already be a “red and dead” galaxy.
What makes a galaxy dead or alive is simple: internal stores of gas.
The low-mass, dusty, irregular galaxy NGC 3077 is actively forming new stars, has a very blue center, and has a hydrogen gas bridge connecting it to the nearby, more massive M81. As one of 34 galaxies in the M81 Group, it’s an example of the most common type of galaxy in the Universe: much smaller and lower in mass, but far more numerous, than galaxies like our Milky Way. The young stars within it have formed from gas reservoirs still present within this galaxy, indicating an “alive” galaxy. (Credit: ESA/Hubble and NASA)
Inside living galaxies, gas is required to enable the formation of new stars.
The enormous bar at the core of galaxy NGC 1300 spans many tens of thousands of light-years, nearly the full width of the galaxy. While many spiral galaxies contain large, prominent bars such as this one, our Milky Way’s central bar is far more modest, extending only about a third of the way out to the Sun’s position. The pink regions found along the spiral arms are evidence of new star formation, triggered by the interaction of internal gas and the density waves of the internal structure. (Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: P. Knezek (WIYN))
When massive gas clouds gravitationally collapse, new stars inevitably arise.
Star-forming regions, like this one in the Carina Nebula, can form a huge variety of stellar masses if they can collapse quickly enough. Inside the ‘caterpillar’ is a proto-star, but it is in the final stages of formation, as external radiation evaporates the gas away more quickly than the newly-forming star can accrue it. Within the first ~2 million years of this star’s birth, protoplanets should already begin arising within the accompanying protoplanetary disk. (Credit: NASA, ESA, N. Smith, UC Berkeley, and the Hubble Heritage Team (STScI/AURA))
As matter fragments, the various clumps grow rapidly, forming new stars and massive star clusters.
The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses and shines brighter than more than 8 million suns, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image, although they have the shortest lifetime, living for between 1–10 million years only. Within a cloud of gas, the process of core fragmentation leads to enormous populations of large numbers of stars. (Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team)
Many events trigger galactic star-formation, including:
The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way. (Credit: CTIO/NOIRLab/DOE/NSF/AURA; Acknowledgment: M. Soraisam (University of Illinois); Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin)
The spiral galaxy UGC 12158, with its arms, bar, and spurs, as well as its low, quiet rate of star formation and hint of a central bulge, may be the single most analogous galaxy for our Milky Way yet discovered. It is neither gravitationally interacting nor merging with any nearby neighbor galaxies, and so the star-formation occurring inside is driven primarily by the density waves occurring within the spiral arms in the galactic disk. (Credit: ESA/Hubble & NASA)
