If we were in a place like the Coma Cluster instead of the Local Group, we’d already be dead.
In a living spiral galaxy, like the Milky Way, the rich gas inside enables the ongoing formation of new stars.
The brightest, closest galaxy confirmed to be beyond the local group is NGC 300, at just 6 million light years distant. 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. (ESO / WIDE FIELD IMAGER (WFI))
When enough gas gets concentrated in a single location, it collapses under its own gravity.
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. (NASA, ESA, N. SMITH, UNIVERSITY OF CALIFORNIA, BERKELEY, AND THE HUBBLE HERITAGE TEAM. STSCI/AURA)
Various matter clumps will grow, faster and faster, leading to new stars and star clusters.
Hubble space telescope image of the merging star clusters at the heart of the Tarantula Nebula, the largest star-forming region known in the local group. The hottest, bluest stars are over 200 times the mass of our Sun. The new star cluster, shown here, is less than 2 million years old. (NASA, ESA, AND E. SABBI (ESA/STSCI); ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WIDE FIELD CAMERA 3 SCIENCE OVERSIGHT COMMITTEE)
This can be triggered by internal dynamics, an external gravitational influence, or a merger with another galaxy.
Zw II 96 in the constellation of Delphinus, the Dolphin, is an example of a galaxy merger located some 500 million light-years away. Star formation is triggered by these classes of events, and can use up large amounts of gas within each of the progenitor galaxies, rather than a steady stream of low-level star formation found in isolated galaxies. (NASA, ESA, THE HUBBLE HERITAGE TEAM (STSCI/AURA)-ESA/HUBBLE COLLABORATION AND A. EVANS (UNIVERSITY OF VIRGINIA, CHARLOTTESVILLE/NRAO/STONY BROOK UNIVERSITY))
Galaxies that are relatively isolated form new stars at a slow, constant rate: for much longer than the Universe’s current age.
The isolated galaxy MCG+01–02–015, all by its lonesome for over 100,000,000 light years in all directions, is presently thought to be the loneliest galaxy in the Universe. The features seen in this galaxy are consistent with it being a massive spiral that formed from a long series of minor mergers, but having been relatively quiet on that front for billions of years. Still, new stars are forming inside at a low but steady rate. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)
But once a galaxy’s gas is gone, star formation ceases, as there’s no material left to fuel future stellar generations.
This is a Hubble Space Telescope image of galaxy NGC 1277. The galaxy is unique in that it is considered a relic of what galaxies were like in the early universe. The galaxy is composed exclusively of aging stars that were born 10 billion years ago. But unlike other galaxies in the local universe, it has not undergone any further star formation. These ‘red-and-dead’ galaxies are found most often stripped of gas as they speed through dense galaxy clusters, like NGC 1277 is doing here. It also contains a supermassive black hole thousands of times the mass of the one at the center of our Milky Way. There are still mysteries to be solved about this galaxy, as well as the Perseus Cluster it inhabits. (NASA, ESA, M. BEASLEY (INSTITUTO DE ASTROFÍSICA DE CANARIAS), AND P. KEHUSMAA)
When a galaxy enters a rich, massive cluster, it has to contend with two murderous factors.
The night sky of Earth, showing the Milky Way, Andromeda and how they’ll look from our perspective over the next 7 billion years, as they merge. Note the transformation from two spirals to a massive, star-forming galaxy, to a red-and-dead elliptical. (NASA; ESA; Z. LEVAY AND R. VAN DER MAREL, STSCI; T. HALLAS, AND A. MELLINGER)
A single major merger can use up all the gas in both progenitor galaxies, leading to a red-and-dead elliptical galaxy.
A Hubble (visible light) and Chandra (X-ray) composite of galaxy ESO 137–001 as it speeds through the intergalactic medium in a rich galaxy cluster, becoming stripped of stars and gas, while its dark matter remains intact. (NASA, ESA, CXC)
Even without one, the intracluster medium is rich in matter, and speeding through it can strip out a galaxy’s gas.
One of the fastest known galaxies in the Universe, speeding through its cluster (and being stripped of its gas) at a few percent the speed of light: thousands of km/s. Trails of stars form in its wake, while the dark matter continues on with the original galaxy. (NASA, ESA, JEAN-PAUL KNEIB (LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE) ET AL.)
Without that gaseous presence, new stars can no longer form.
The galaxy shown at the above right is a member of the Coma Cluster: the largest cluster of galaxies in our local Universe. The central feature is evidence of ram pressure stripping as the galaxy speeds through the intracluster medium, quickly losing its capacity to form new stars. The galaxy next to it is simply an older version, having already become ‘red-and-dead’ many billions of years ago by a likely similar process. (NASA, ESA, AND W. CRAMER AND J. KENNEY (YALE UNIVERSITY))
Gas-free galaxies are most commonly found in clusters, with the pile-up of matter being the culprit.
A map of neutral hydrogen (in red) overlaid on this galaxy in the Coma Cluster shows how much gas is being quickly stripped from this galaxy as it travels through the cluster. Galaxies found in environments like this one become ‘red-and-dead’ far more quickly than galaxies in less dense regions of space. (NASA, ESA, AND W. CRAMER AND J. KENNEY (YALE UNIVERSITY))
Mostly Mute Monday tells the astronomical story of an object or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.